Cytochrome P450-based cancer gene therapy: recent advances and future prospects

Cytochrome P450-based cancer gene therapy is a novel prodrug activation strategy for cancer treatment that has substantial potential for improving the safety and efficacy of cancer chemotherapeutics. The primary goal of this strategy is to selectively increase tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating P450 enzyme. This strategy has been exemplified for the alkylating agents cyclophosphamide and ifosfamide, which are bioactivated by select P450 enzymes whose expression is generally high in liver and deficient in tumor cells. Transduction of tumors with a prodrug-activating P450 gene, followed by prodrug treatment, greatly increases intratumoral formation of activated drug metabolites. This leads to more efficient killing of the transduced tumor cells without a significant increase in host toxicity. P450 gene therapy is accompanied by substantial bystander cytotoxicity which greatly enhances the therapeutic effect by extending it to nearby tumor cells not transduced with the therapeutic P450 gene. Although endogenous P450 reductase is not expected to be a limiting factor in prodrug activation in tumor cells that express moderate levels of an exogenous P450 gene, P450 reductase transduction has recently been found to substantially enhance intratumoral prodrug activation and its associated therapeutic effects. Using this gene combination, an overall 50- to 100-fold increase in tumor cell kill in vivo over that provided by hepatic drug activation alone has been observed. Striking improvements in therapeutic effects can thus be achieved using an established anticancer drug in an intratumoral prodrug activation strategy based on the combination of a cytochrome P450 gene with the gene encoding NADPH-P450 reductase. This strategy is readily extendable to several other widely used P450-activated cancer chemotherapeutic prodrugs, as well as to prodrugs that undergo P450 reductase-dependent bioreductive activation and which may exhibit synergy when combined with P450-activated prodrugs in a P450/P450 reductase-based cancer gene therapeutic regimen.

Distinctive roles of STAT5a and STAT5b in sexual dimorphism of hepatic P450 gene expression. Impact of STAT5a gene disruption

Stat5b gene disruption leads to an apparent growth hormone (GH) pulse insensitivity associated with loss of male-characteristic body growth rates and male-specific liver gene expression (Udy, G. B., Towers, R. P., Snell, R. G., Wilkins, R. J., Park, S. H., Ram, P. A., Waxman, D. J., and Davey, H. W. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 7239-7244). In the present study, disruption of the mouse Stat5a gene, whose coding sequence is approximately 90% identical to the Stat5b gene, resulted in no loss of expression in male mice of several sex-dependent, GH-regulated liver cytochrome P450 (CYP) enzymes. By contrast, the loss of STAT5b feminized the livers of males by decreasing expression of male-specific CYPs (CYP2D9 and testosterone 16alpha-hydroxylase) while increasing to female levels several female-predominant liver CYPs (CYP3A, CYP2B, and testosterone 6beta-hydroxylase). Since STAT5a is thus nonessential for these male GH responses, STAT5b homodimers, but not STAT5a-STAT5b heterodimers, probably mediate the sexually dimorphic effects of male GH pulses on liver CYP expression. In female mice, however, disruption of either Stat5a or Stat5b led to striking decreases in several liver CYP-catalyzed testosterone hydroxylase activities. Stat5a or Stat5b gene disruption also led to the loss of a female-specific, GH-regulated hepatic CYP2B enzyme. STAT5a, which is much less abundant in liver than STAT5b, and STAT5b are therefore both required for constitutive expression in female but not male mouse liver of certain GH-regulated CYP steroid hydroxylases, suggesting that STAT5 protein heterodimerization is an important determinant of the sex-dependent and gene-specific effects that GH has on the liver.

In vivo modulation of alternative pathways of P-450-catalyzed cyclophosphamide metabolism: impact on pharmacokinetics and antitumor activity

The widely used anticancer prodrug cyclophosphamide (CPA) is activated in liver by a 4-hydroxylation reaction primarily catalyzed by cytochrome P-4502B and P-4502C enzymes. An alternative metabolic pathway involves CPA N-dechloroethylation to yield chloroacetaldehyde (CA), a P-4503A-catalyzed deactivation/neurotoxication reaction. The in vivo modulation of these alternative, competing pathways of P-450 metabolism was investigated in pharmacokinetic studies carried out in the rat model. Peak plasma concentrations (Cmax) for 4-OH-CPA and CA were increased by 3- to 4-fold, and apparent plasma half-lives of both metabolites were correspondingly shortened in rats pretreated with phenobarbital (PB), an inducer of P-4502B and P-4503A enzymes. However, PB had no net impact on the extent of drug activation or its partitioning between these alternative metabolic pathways, as judged from AUC values (area-under-the-plasma concentration x time curve) for 4-OH-CPA and CA. The P-4503A inhibitor troleandomycin (TAO) decreased plasma Cmax and AUC of CA (80-85% decrease) without changing the Cmax or AUC of 4-OH-CPA in uninduced rats. In PB-induced rats, TAO decreased AUCCA by 73%, whereas it increased AUC4-OH-CPA by 93%. TAO thus selectively suppresses CPA N-dechloroethylation, thereby increasing the availability of drug for P-450 activation via 4-hydroxylation. By contrast, dexamethasone, a P-4503A inducer and antiemetic widely used in patients with cancer, stimulated large, undesirable increases in the Cmax and AUC of CA (8- and 4-fold, respectively) while reducing the AUC of the 4-hydroxylation pathway by approximately 60%. Tumor excision/in vitro colony formation and tumor growth delay assays using an in vivo 9L gliosarcoma solid tumor model revealed that TAO suppression of CPA N-dechloroethylation could be achieved without compromising the antitumor effect of CPA. The combination of PB with TAO did not, however, enhance the antitumor activity of CPA, despite the approximately 2-fold increase in AUC4-OH-CPA, suggesting that other PB-inducible activities, such as aldehyde dehydrogenase, may counter this increase through enhanced deactivation of the 4-hydroxy metabolite. Together, these studies demonstrate that the P-4503A inhibitor TAO can be used to effectively modulate CPA metabolism and pharmacokinetics in vivo in a manner that decreases the formation of toxic metabolites that do not contribute to antitumor activity.

Down-regulation of liver JAK2-STAT5b signaling by the female plasma pattern of continuous growth hormone stimulation

The suppression of male-specific, GH pulse-induced, liver transcription in adult female rats has been linked to the down-regulation of STAT5b activation by the female plasma pattern of near-continuous GH exposure. The mechanism underlying this down-regulation was studied in the rat liver cell line CWSV-1, where continuous GH suppressed the level of activated (tyrosine- phosphorylated) STAT5b to approximately 10-20% of the maximal GH pulse-induced STAT5b signal within 3 h. In contrast to the robust JAK2 kinase-dependent STAT5b activation loop that is established by a GH pulse, JAK2 kinase signaling to individual STAT5b molecules was found to be short lived in cells treated with GH continuously. Moreover, maintenance of the low-level STAT5b signal required ongoing protein synthesis and persisted for at least 7 days provided that GH was present in the culture continuously. Increased STAT5b DNA-binding activity was observed in cells treated with the proteasome inhibitor MG132, suggesting that at least one component of the GH receptor (GHR)-JAK2-STAT5b signaling pathway becomes labile in response to continuous GH treatment. The phosphotyrosine phosphatase inhibitor pervanadate fully reversed the down-regulation of STAT5b DNA-binding activity in continuous GH-treated cells by a mechanism that involves both increased STAT5b activation and decreased STAT5b dephosphorylation. Moreover, the requirement for ongoing GH stimulation and active protein synthesis to maintain STAT5b activity in continuous GH-treated cells were both eliminated by pervanadate treatment, suggesting that phosphotyrosine dephosphorylation may be an obligatory first step in the internalization/degradation pathway for the GHR-JAK2 complex. Finally, the sustaining effect of the serine kinase inhibitor H7 on GH pulse-induced JAK2 signaling to STAT5b was not observed in continuous GH-treated cells. These findings suggest a model where continuous GH exposure of liver cells down-regulates the STAT5b pathway by a mechanism that involves enhanced dephosphorylation of both STAT5b and GHR-JAK2, with the latter step leading to increased internalization/degradation of the re-ceptor-kinase complex.

Cross-talk between janus kinase-signal transducer and activator of transcription (JAK-STAT) and peroxisome proliferator-activated receptor-alpha (PPARalpha) signaling pathways. Growth hormone inhibition of pparalpha transcriptional activity mediated by stat5b

Hepatic peroxisome proliferation induced by structurally diverse non-genotoxic carcinogens is mediated by the nuclear receptor peroxisome proliferator-activated receptor (PPARalpha) and can be inhibited by growth hormone (GH). GH-stimulated Janus kinase-signal transducer and activator of transcription 5b (JAK2/STAT5b) signaling and the PPAR activation pathway were reconstituted in COS-1 cells to investigate the mechanism for this GH inhibitory effect. Activation of STAT5b signaling by either GH or prolactin inhibited, by up to 80-85%, ligand-induced, PPARalpha-dependent reporter gene transcription. GH failed to inhibit 15-deoxy-Delta12, 14-prostaglandin-J2-stimulated gene transcription mediated by an endogenous COS-1 PPAR-related receptor. GH inhibition of PPARalpha activity required GH receptor and STAT5b and was not observed using GH-activated STAT1 in place of STAT5b. GH inhibition was not blocked by the mitogen-activated protein kinase pathway inhibitor PD98059. STAT5b-PPARalpha protein-protein interactions could not be detected by anti-STAT5b supershift analysis of PPARalpha-DNA complexes. The GH inhibitory effect required the tyrosine phosphorylation site (Tyr-699) of STAT5b, an intact STAT5b DNA binding domain, and the presence of a COOH-terminal trans-activation domain. Moreover, GH inhibition was reversed by a COOH-terminal-truncated, dominant-negative STAT5b mutant. STAT5b must thus be nuclear and transcriptionally active to mediate GH inhibition of PPARalpha activity, suggesting an indirect inhibition mechanism, such as competition for an essential PPARalpha coactivator or STAT5b-dependent synthesis of a more proximal PPARalpha inhibitor. The cross-talk between STAT5b and PPARalpha signaling pathways established by these findings provides new insight into the mechanisms of hormonal and cytokine regulation of hepatic peroxisome proliferation.

Termination of growth hormone pulse-induced STAT5b signaling

STAT5b (signal transducer and activator of transcription 5b) is a key mediator of the effects of plasma GH pulses on male-specific liver gene expression. STAT5b is activated in liver cells in vivo by physiological pulses of GH and then is rapidly deactivated. Investigation of the cellular events involved in this activation/deactivation cycle using the rat liver cell line CWSV-1 established that a brief exposure to GH and the associated activation of JAK2 (Janus kinase 2) tyrosine kinase activity are both necessary and sufficient to initiate all of the downstream steps associated with STAT5b activation by tyrosine phosphorylation and the subsequent deactivation of both JAK2 kinase and STAT5b. JAK2 signaling to STAT5b at the conclusion of a GH pulse could be sustained by the protein synthesis inhibitor cycloheximide or by the proteasome inhibitor MG132, indicating that termination of this JAK2-catalyzed STAT activation loop requires synthesis of a labile or GH-inducible protein factor and is facilitated by the proteasome pathway. This factor may be a phosphotyrosine phosphatase, since the phosphatase inhibitor pervanadate both sustained GH pulse-induced JAK2 signaling to STAT5b and blocked the rapid deactivation of phosphorylated STAT5b (t(1/2) = 8.8 +/- 0.9 min) seen in its absence. Finally, the serine kinase inhibitor H7 blocked down-regulation of JAK2 signaling to STAT5b in a manner that enabled cells to respond to a subsequent GH pulse without the need for the approximately 3-h interpulse interval normally required for full recovery of GH pulse responsiveness. Termination of GH pulse-induced STAT5b signaling is thus a complex process that involves multiple biochemical events. These are proposed to include the down-regulation of JAK2 signaling to STAT5b via a cycloheximide- and H7-sensitive step, proteasome-dependent degradation of a key component or regulatory factor, and dephosphorylation leading to deactivation of the receptor-kinase signaling complex and its STAT5b substrate via the action of a phosphotyrosine phosphatase.

Retroviral transfer of human cytochrome P450 genes for oxazaphosphorine-based cancer gene therapy

Cyclophosphamide (CPA) and ifosfamide (IFA) are widely used anticancer prodrugs that are bioactivated in the liver by specific cytochrome P450 enzymes (CYPs). The therapeutic activity of these antitumor agents can be compromised by a low therapeutic index that is, in part, due to the systemic distribution of activated drug metabolites. Here, recombinant retroviruses were used to deliver six different CPA- or IFA-metabolizing human CYP genes to 9L gliosarcoma cells: 2B6, 2C8, 2C9, 2C18 (Met385 and Thr385 alleles), 2C19, and 3A4. Intratumoral cytochrome P450 expression conferred substantial sensitivity to CPA cytotoxicity, with the most dramatic effects seen with CYP2B6. Strong CPA chemosensitivity was also seen following transduction of CYP2C18-Met, despite a very low level of CYP protein expression (>60-fold lower than that of 2B6). In contrast to CPA, the cytotoxicity of IFA was greatest toward tumor cells transduced with CYP3A4, followed by CYPs 2B6 and 2C18-Met. A substantial further increase in chemosensitivity was achieved upon transduction of 2B6 or 2C18-Met-expressing tumor cells with P450 reductase, which provided for more efficient intratumoral prodrug activation and cytotoxicity at lower drug concentrations. With 2B6- plus P450 reductase-transduced tumor cells, CPA but not IFA conferred a strong cell contact-independent bystander cytotoxic effect on non-P450-expressing 9L cells. CPA treatment of tumors that were transduced with 2B6 or 2C18-Met together with P450 reductase and were grown s.c. in immunodeficient mice resulted in a large enhancement of the liver P450-dependent antitumor effect seen with control 9L tumors, with no apparent increase in host toxicity (growth delay of >25-50 days in P450-expressing tumors versus approximately 5-6 days without P450). CYP2B6 plus P450 reductase and CYP2C18-Met plus P450 reductase thus appear to be excellent gene combinations for use with CPA in P450/prodrug activation-based cancer gene therapy.

Activation of peroxisome proliferator-activated receptors by chlorinated hydrocarbons and endogenous steroids

Trichloroethylene (TCE) and related hydrocarbons constitute an important class of environmental pollutants whose adverse effects on liver, kidney, and other tissues may, in part, be mediated by peroxisome proliferator-activated receptors (PPARs), ligand-activated transcription factors belonging to the steroid receptor superfamily. Activation of PPAR induces a dramatic proliferation of peroxisomes in rodent hepatocytes and ultimately leads to hepatocellular carcinoma. To elucidate the role of PPAR in the pathophysiologic effects of TCE and its metabolites, it is important to understand the mechanisms whereby PPAR is activated both by TCE and endogenous peroxisome proliferators. The investigations summarized in this article a) help clarify the mechanism by which TCE and its metabolites induce peroxisome proliferation and b) explore the potential role of the adrenal steroid and anticarcinogen dehydroepiandrosterone 3beta-sulfate (DHEA-S) as an endogenous PPAR activator. Transient transfection studies have demonstrated that the TCE metabolites trichloroacetate and dichloroacetate both activate PPAR alpha, a major liver-expressed receptor isoform. TCE itself was inactive when tested over the same concentration range, suggesting that its acidic metabolites mediate the peroxisome proliferative potential of TCE. Although DHEA-S is an active peroxisome proliferator in vivo, this steroid does not stimulate trans-activation of PPAR alpha or of two other PPAR isoforms, gamma and delta/Nuc1, when evaluated in COS-1 cell transfection studies. To test whether PPAR alpha mediates peroxisomal gene induction by DHEA-S in intact animals, DHEA-S has been administered to mice lacking a functional PPAR alpha gene. DHEA-S was thus shown to markedly increase hepatic expression of two microsomal P4504A proteins associated with the peroxisomal proliferative response in wild-type mice. In contrast, DHEA-S did not induce these hepatic proteins in PPAR alpha-deficient mice. Thus, despite its unresponsiveness to steroidal peroxisome proliferators in transfection assays, PPAR alpha is an obligatory mediator of DHEA-S-stimulated hepatic peroxisomal gene induction. DHEA-S, or one of its metabolites, may thus serve as an important endogenous regulator of liver peroxisomal enzyme expression.

Modulation of P450-dependent ifosfamide pharmacokinetics: a better understanding of drug activation in vivo

The anti-cancer prodrug ifosfamide (IF) is metabolized by liver P450 enzymes by two alternative pathways. IF is activated to 4-hydroxy IF (4-OH-IF), which ultimately yields the alkylating mustard isophosphoramide, whereas IF N-dechlororethylation inactivates the drug and produces the neurotoxic metabolite chloroacetaldehyde (CA). Both reactions are catalysed by multiple liver P450 enzymes in vitro in isolated rat liver microsomes. The present pharmacokinetic study investigates the potential for modulation of these alternative pathways of IF metabolism in vivo using the adult male Fischer 344 rat model. Rats were treated with IF alone or in conjunction with various P450 inducers and inhibitors in an effort to improve the balance between drug activation and drug inactivation. Plasma concentrations, areas under the curve (AUC) and half-lives were calculated for 4-OH-IF and CA, allowing estimations of the extent of IF activation and deactivation/toxification. Induction of liver P450 2B enzymes by 4-day high-dose phenobarbital (PB) pretreatment significantly decreased the fraction of IF undergoing 4-hydroxylation (AUC(4-OH-IF)/AUC(4-OH-IF)+AUC(CA)), from 37% to 22% of total metabolism (P < 0.05), consistent with in vitro findings that the PB-inducible P450 enzyme 2B1 plays a major role in IF N-dechloroethylation. Pretreatment with the P450 3A inducer dexamethasone proportionally decreased the AUC for both IF metabolites, without any net impact on the fraction of IF undergoing metabolic activation. By contrast, the P450 2B1 inhibitor metyrapone preferentially increased the AUC for the 4-hydroxylation pathway in 3-day low-dose PB-induced rats, thereby increasing the total fraction of IF metabolized via the activation pathway from 36% to 54% (P < 0.05), whereas the P450 inhibitors orphenadrine and troleandomycin had no significant affect on AUC values. These findings demonstrate specific roles for P450 2B and 3A enzymes in catalysing these pathways of IF metabolism in vivo, and demonstrate the potential for modulation of IF's alternative metabolic pathways in a therapeutically useful manner. These studies also highlight several clinically relevant drug interactions that may occur during concomitant administration of IF with drugs and other compounds that modulate hepatic P450 enzyme levels.

Potentiation of cytochrome P450/cyclophosphamide-based cancer gene therapy by coexpression of the P450 reductase gene

Intratumoral expression of cytochrome P450 2B1 sensitizes tumor cells to the cytotoxic action of the alkylating agent prodrug cyclophosphamide (CPA) and provides a novel strategy for cancer gene therapy that may enhance the selectivity and the effectiveness of this class of antitumor drugs [L. Chen and D. J. Waxman, Cancer Res., 55: 581-589, 1995]. P450-catalyzed drug metabolism is obligatorily dependent on electron input from the flavoenzyme NADPH-P450 reductase (RED), which is widely expressed in many cell types, including tumor cells. Here, we investigate the potential utility of combining RED gene transfer with CPA-based P450 gene therapy. Rat 9L gliosarcoma cells stably expressing either basal or elevated (up to 10-fold increase) levels of RED, in the presence or absence of P450 2B1, were selected and characterized. RED overexpression substantially increased the sensitivity of these cells to CPA, but only when combined with P450 2B1 expression. An enhanced cytotoxic response was also obtained when recombinant adenovirus encoding P450 2B1 was used to deliver the P450 gene to RED-overexpressing tumor cells. CPA cytotoxicity was substantially decreased by the RED inhibitor diphenyleneiodonium chloride or by the P450 inhibitor metyrapone, providing evidence of its dependence on the catalytic contributions of both protein components of the P450 metabolic pathway. Conditioned media from P450 2B1-expressing and RED-overexpressing tumor cells treated with CPA exhibited increased formation of the primary 4-hydroxy metabolite and greater cell contact-independent bystander cytotoxic potential compared to tumor cells containing P450 2B1 and basal levels of RED. Evaluation of the impact of P450/RED combination gene therapy using a s.c. solid tumor model/tumor excision assay revealed a dramatic 50-100-fold increase in tumor cell kill in vivo over that provided by liver drug activation alone. These findings establish the importance of endogenous RED levels as a determinant of the sensitivity of tumor cells to CPA/P450 gene therapy and demonstrate the striking therapeutic effectiveness of an anticancer prodrug activation strategy based on the combination of a cytochrome P450 gene with the gene encoding RED.

Human cytochrome P4502B6: interindividual hepatic expression, substrate specificity, and role in procarcinogen activation

The level of expression and interindividual variation in human hepatic microsomal cytochrome P450 (CYP) 2B6 was characterized using a polyclonal antibody (WB-2B6) raised against rat CYP2B1. Immunoblot analysis using cDNA-expressed human CYPs revealed strong cross-reactivity of this antibody with CYP2B6 (limit of detection < 0.05 pmol) and only minor cross-reactivities with human CYP2A6, CYP2D6, and CYP2E1, all of which could be resolved from CYP2B6 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of human liver microsomes using this antibody revealed immunodetectable CYP2B6 protein in a majority of individual liver samples, with levels up to 74 pmol/mg protein in the CYP2B6-positive samples. Kinetic analysis of cDNA-expressed CYPs identified many of these enzymes as catalysts of 7-ethoxy-4-trifluoromethylcoumarin (7EFC) O-deethylation, but with significantly different apparent K(M) values (CYP1A2 < CYP2B6 approximately CYP1A1 < CYP2C19 < CYP2C9 < CYP2E1 < CYP2A6). By assaying liver microsomal 7EFC O-deethylase activity at a low 7EFC concentration (5 microM) and preincubating human liver microsomes with anti-CYP1A, anti-CYP2C, and anti-CYP2E1 antibodies, we were able to monitor CYP2B6-dependent 7EFC O-deethylase activity in a panel of 17 human liver microsomes and observe a significant correlation (r2 = 0.80) between this activity and CYP2B6 protein content. The ability of CYP2B6 to activate prodrugs and procarcinogens was examined using gene locus mutation assays in CYP2B6-expressing human lymphoblast cells. CYP2B6-expressing cells were found to be more sensitive than control cells to the cytotoxicity and mutagenicity of cyclophosphamide, aflatoxin B1, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. CYP2B6 is thus a widely expressed human liver microsomal CYP that can contribute to a broad range of drug metabolism and procarcinogen activation reactions.

Interaction of growth hormone-activated STATs with SH2-containing phosphotyrosine phosphatase SHP-1 and nuclear JAK2 tyrosine kinase

Growth hormone (GH) rapidly stimulates tyrosine phosphorylation followed by serine/threonine phosphorylation of multiple cytoplasmic STAT transcription factors, including one, STAT5b, that is uniquely responsive to the temporal pattern of plasma GH stimulation in rat liver and is proposed to play a central role in the activation of male-expressed liver genes by GH pulses in vivo (Waxman, D. J., Ram, P. A., Park, S. H., and Choi, H. K. (1995) J. Biol. Chem. 270, 13262-13270). We now show that JAK2, the GH receptor-associated tyrosine kinase, is present both in the cytosol and in the nucleus in cultured liver cells and in rat liver in vivo and that GH-activated STAT3 but not STAT5b becomes associated with nuclear JAK2. GH is also shown to activate by 3-4-fold SHP-1, a phosphotyrosine phosphatase that contains two src homology 2 (SH2) domains. GH also induces nuclear translocation and binding of SHP-1 to tyrosine-phosphorylated STAT5b, suggesting that this GH-activated phosphatase may play a role in dephosphorylation leading to deactivation of nuclear STAT5b following the termination of a plasma GH pulse in male rat liver in vivo. No such association of SHP-1 with GH-activated STAT3 was detected, a finding that could help explain the marked desensitization of STAT3, but not STAT5b, to subsequent GH pulses following an initial GH activation event.

Requirement of STAT5b for sexual dimorphism of body growth rates and liver gene expression

The signal transducer and activator of transcription, STAT5b, has been implicated in signal transduction pathways for a number of cytokines and growth factors, including growth hormone (GH). Pulsatile but not continuous GH exposure activates liver STAT5b by tyrosine phosphorylation, leading to dimerization, nuclear translocation, and transcriptional activation of the STAT, which is proposed to play a key role in regulating the sexual dimorphism of liver gene expression induced by pulsatile plasma GH. We have evaluated the importance of STAT5b for the physiological effects of GH pulses using a mouse gene knockout model. STAT5b gene disruption led to a major loss of multiple, sexually differentiated responses associated with the sexually dimorphic pattern of pituitary GH secretion. Male-characteristic body growth rates and male-specific liver gene expression were decreased to wild-type female levels in STAT5b-/- males, while female-predominant liver gene products were increased to a level intermediate between wild-type male and female levels. Although these responses are similar to those observed in GH-deficient Little mice, STAT5b-/- mice are not GH-deficient, suggesting that they may be GH pulse-resistant. Indeed, the dwarfism, elevated plasma GH, low plasma insulin-like growth factor I, and development of obesity seen in STAT5b-/- mice are all characteristics of Laron-type dwarfism, a human GH-resistance disease generally associated with a defective GH receptor. The requirement of STAT5b to maintain sexual dimorphism of body growth rates and liver gene expression suggests that STAT5b may be the major, if not the sole, STAT protein that mediates the sexually dimorphic effects of GH pulses in liver and perhaps other target tissues. STAT5b thus has unique physiological functions for which, surprisingly, the highly homologous STAT5a is unable to substitute.

Thyroid regulation of NADPH:cytochrome P450 oxidoreductase: identification of a thyroid-responsive element in the 5'-flank of the oxidoreductase gene

The current study demonstrates that T3-activated transcription of the NADPH:cytochrome P450 oxidoreductase (P450R) gene is dependent on the thyroid hormonal status of the animal, with both transcriptional and post-transcriptional pathways being important in regulating the cellular P450R mRNA level. The region required for transcriptional activation of the P450R gene by T3 has been identified. Nuclear run-on experiments demonstrated that the effects of T3 on P450R transcription are dependent on thyroid status, with a transcriptional enhancement obtained in T3-treated hypothyroid rat liver (1.8-fold increase) but not in T3-treated euthyroid animals. Transient cotransfection of P450R promoter/chloramphenicol acetyl transferase (CAT) constructs and the thyroid hormone receptor beta1 (TR beta1) expression plasmid into rat hepatoma H4IIE cells resulted in a 2.4-fold induction of promoter activity that was both T3 and TR beta1 dependent. Analysis of promoter deletion constructs identified a P450R-thyroid response region (P450R-TRE; bases, -564 to -536) containing three imperfect direct repeats of the thyroid response motif, AGGTCA. Mutational analysis further established that T3 induction was dependent only on the upstream direct repeat, having the sequence AGGTGAgctgAGGCCA. Footprint analysis showed that all three motifs were protected by proteins present in rat liver nuclear extracts, and a direct interaction between P450R-TRE and T3 receptors TR alpha1 and TR beta1 was demonstrated by gel-shift analysis. In vitro binding studies with P450R-TRE revealed the formation of heterodimeric complexes when TR alpha1 was coincubated with either the retinoic X receptor alpha or nuclear extract from rat liver, COS, or H4IIE cells. In addition, placement of the P450R-TRE upstream of the T3-nonresponsive heterologous thymidine kinase promoter resulted in a 2.7-fold transcriptional enhancement that was both T3 and TR beta1 dependent. Previous studies have demonstrated that T3 augments P450R mRNA levels approximately 20-30-fold and approximately 12-fold, respectively, in hypothyroid and euthyroid rats. Hence, for the hypothyroid state, transcriptional and post-transcriptional events contribute to the T3-induced mRNA increases; however, the marked increase in message level in T3-treated euthyroid animals depends primarily on post-transcriptional pathways.

Identification of the polymorphically expressed CYP2C19 and the wild-type CYP2C9-ILE359 allele as low-Km catalysts of cyclophosphamide and ifosfamide activation

Cyclophosphamide and ifosfamide are alkylating agent prodrugs that require activation by cytochrome P450 (CYP) to manifest their cancer chemotherapeutic activity. The present study investigates the activity of four individual human CYP2C enzymes and their allelic variants in cyclophosphamide and ifosfamide activation as an initial attempt to gain insight into the underlying basis for the large interpatient differences in the clinical pharmacokinetics and metabolism of these anticancer drugs. Recombinant CYP2C8, CYP2C19, two allelic variants of CYP2C18, and six variants of CYP2C9 expressed in a yeast cDNA expression system were each enzymatically active, as judged by the ability of the isolated microsomes to catalyse 7-ethoxycoumarin O-deethylation after reconstitution with purified NADPH-cytochrome P450 reductase and cytochrome b5. With cyclophosphamide as substrate, CYP2C19 had the lowest apparent Km, followed by CYP2C9, CYP2C18 and CYP2C8, whereas in the case of ifosfamide, the rank order was: Km CYP2C19 < CYP2C18 < CYP2C9 < CYP2C8. CYP2C18 had the highest in vitro intrinsic clearance/catalytic efficiency (apparent Vmax/Km) in cyclophosphamide and ifosfamide activation, followed by 2C19 > 2C9 approximately 2C8. Examination of a panel of CYP2C allelic variants revealed that CYP2C18-Thr385 had both a higher Vmax and a higher apparent Km toward cyclophosphamide than CYP2C18-Met385 with no difference in catalytic efficiency, whereas with ifosfamide the Thr385 allele exhibited a strikingly lower apparent Km resulting in a six-fold higher catalytic efficiency. In the case of CYP2C9, a Ile359 to Leu mutation associated with poor metabolism of the hypoglycemic drug tolbutamide decreased catalytic efficiency toward cyclophosphamide by increasing the apparent Km, whereas the same mutation reduced the efficiency of this P450 toward ifosfamide by decreasing the Vmax. Substitution of CYP2C9-Gly417 by Asp resulted in a two-fold lower catalytic efficiency for cyclophosphamide metabolism but a three-fold higher efficiency for ifosfamide metabolism. A His276 to Gly substitution resulted in an increase in both Vmax and apparent Km with no net change in catalytic efficiency for either oxazaphosphorine. Mutations at CYP2C9 residues 144 and 358 had little or no effect. Thus (a) wild type CYP2C19 and CYP2C9 are relatively low Km catalysts of cyclophosphamide and ifosfamide activation, and (b) all four human CYP2C enzymes activate these two anticancer prodrugs with varying efficiencies and with striking differences among naturally occurring allelic variants in the case of CYP2C9 and CYP2C18.

Enhanced cyclophosphamide and ifosfamide activation in primary human hepatocyte cultures: response to cytochrome P-450 inducers and autoinduction by oxazaphosphorines

The anticancer oxazaphosphorine prodrugs cyclophosphamide and ifosfamide are activated in human liver by a 4-hydroxylation reaction catalyzed by multiple cytochrome P450 (CYP) enzymes. In the present study, we used a cultured human hepatocyte model to identify possible inducers of the CYP-catalyzed activation of these two anticancer prodrugs. Treatment of primary cultures of human hepatocytes with phenobarbital, dexamethasone, or rifampin elevated hepatocyte microsomal oxazaphosphorine 4-hydroxylation by up to 200-400% of control for both drug substrates. These inductions were associated with corresponding increases in immunoreactive CYP2B6, CYP2C8, CYP2C9, and CYP3A4, all previously shown to catalyze oxazaphosphorine activation. Rifampin (1 microM, 96-h exposure) was a particularly potent inducer of ifosfamide and cyclophosphamide 4-hydroxylation, as well as of CYP3A protein levels and CYP3A-dependent testosterone 6beta-hydroxylation. CYP3A4, CYP2C8, and CYP2C9 protein levels were also increased by exposure of the hepatocytes to cyclophosphamide or ifosfamide (50 microM), which thereby enhanced their own rates of 4-hydroxylation in the cultured hepatocytes. In one human hepatocyte culture that contained the polymorphically expressed CYP3A5 in addition to the more widely expressed CYP3A4, only CYP3A4 was induced by cyclophosphamide, ifosfamide, and rifampin. These studies: (a) demonstrate an underlying metabolic basis for the clinically important oxazaphosphorine autoinduction pharmacokinetics seen with these drugs in cancer patients; and (b) identify rifampin and other CYP inducers as potentially useful for increasing the rates of cyclophosphamide 4-hydroxylation and ifosfamide 4-hydroxylation in human liver in a manner that could favorably impact the clinical pharmacokinetics of these anticancer prodrugs.

Regulation of signal transducer and activator of transcription (STAT) 5b activation by the temporal pattern of growth hormone stimulation

Plasma GH profiles, intermittent in adult male and continuous in adult female rats, respectively, activate unique patterns of gene transcription in male and female rat liver. Pulsatile, but not continuous, GH exposure activates liver STAT5 (signal transducer and activator of transcription-5) by tyrosine phosphorylation, leading to nuclear translocation, and is proposed to play a key role in GH pulse-regulated male-specific liver gene expression. The mechanisms underlying the GH pattern dependence of STAT5 activation are presently investigated using a rat hepatocyte-derived cell line. Rat GH stimulated tyrosine phosphorylation followed by serine or threonine phosphorylation, leading to activation of the DNA-binding activity of STAT5b, the major STAT5 form present in these cells. Maximal STAT5b activation required a full 20 min at a receptor-saturating GH concentration of 50 ng/ml, suggesting that hormone binding leading to receptor dimerization is a relatively slow process. Repeat cycles of GH pulsation led to repeat cycles of STAT5b activation followed by deactivation, similar to rat liver in vivo. Full responsiveness to succeeding GH pulses required a minimum GH off-time of > or = 2.5 h, but was independent of new protein synthesis. Continuous GH exposure led to down-regulation of activated STAT5b, consistent with the desensitization of this GH pulse-activated pathway observed in female rat liver. The rapid deactivation of STAT5b after termination of a GH pulse involved phosphotyrosine dephosphorylation as a key first step and could be blocked by pervanadate, a phosphotyrosine phosphatase inhibitor. Unexpectedly, serine/threonine kinase inhibitors also inhibited STAT5b deactivation. These studies establish that STAT5b is responsive to the temporal pattern of GH stimulation and demonstrate a role for both a tyrosine phosphatase and a serine/threonine kinase in resetting this JAK/STAT signaling apparatus so that it may respond to subsequent rounds of GH pulse activation.

Interaction of a novel sex-dependent, growth hormone-regulated liver nuclear factor with CYP2C12 promoter

CYP2C12 is a steroid hydroxylase cytochrome P450 whose female-specific expression in adult rat liver is transcriptionally activated by the continuous plasma growth hormone (GH) profile characteristic of adult female rats. DNase I footprinting and gel mobility shift analysis of the 5'-flank of the CYP2C12 gene were carried out to identify cis-acting elements and trans-acting factors that may contribute to the GH-regulated, sex-dependent transcription of this P450 gene. DNase I footprinting analysis revealed sex- and GH-regulated DNase I hypersensitivity sites at the boundaries of several protein binding sites detected along a 1560-nucleotide upstream segment of CYP2C12. Five distinct sites bound a novel continuous GH-regulated nuclear factor, GHNF, which is enriched in adult female and continuous GH-treated male liver nuclear extracts compared to untreated male liver nuclear extracts. Two other footprinted sites correspond to binding sites for the liver transcription factors C/EBP and albumin D element-binding protein and a third to an HNF1 binding site. A specific binding site for GHNF was also found in the 5'-proximal promoter of CYP2C11, an adult male-specific liver P450 gene, suggesting that GHNF may contribute to the down-regulation of that gene by continuous GH. GHNF was distinguished from the nuclear factors that bind to a GH response element upstream of the rat Spi 2.1 gene and is also distinct from the GH-activatable latent cytoplasmic transcription factors STAT 1, STAT 3, and STAT 5. These findings support the hypothesis that continuous GH-activated transcription of CYP2C12 in adult female rat liver (a) involves the activation of a novel GH-regulated nuclear factor which binds to multiple sites along the 5'-flank of this cytochrome P450 gene, and (b) proceeds via a signaling pathway distinct from the GH pulse-activated STAT5 pathway proposed to induce CYP2C11 and other male-expressed liver genes.

Role of cytochrome P450 in oxazaphosphorine metabolism. Deactivation via N-dechloroethylation and activation via 4-hydroxylation catalyzed by distinct subsets of rat liver cytochromes P450

The roles of individual liver cytochrome P450 (P450) enzymes in N-dechloroethylation leading to deactivation and neurotoxification of the isomeric alkylating agent prodrugs ifosfamide (IF) and cyclophosphamide (CPA) were investigated using an in vitro rat liver model. Rats were pretreated with a panel of drugs, including phenobarbital (a strong inducer of liver P450 2B1/2B2) and dexamethasone (a strong inducer of P450 3A enzymes), to examine the effects of these P450-inducing agents on IF and CPA N-dechloroethylation catalyzed by rat hepatic microsomes. The P450 3A-specific inhibitor troleandomycin and inhibitory monoclonal antibodies reactive with P450 2B and 2C enzymes were used to identify the individual P450 subfamilies involved in microsomal N-dechloroethylation of IF and CPA. It was found that dexamethasone pretreatment preferentially elevated microsomal CPA N-dechloroethylation activity (12-fold increase) and that P450 3A enzymes catalyzed up to > 95% of this reaction in both uninduced and drug-induced liver. In contrast, IF N-dechloroethylation activity was stimulated (approximately 8-fold increase) in liver microsomes by phenobarbital pretreatment, and P450 2B1/2B2 were responsible for the majority of this activity. In addition, P450 2C11 catalyzed approximately 50% of IF N-dechloroethylation in uninduced male rat liver microsomes. inducers of P450 1A and 4A enzymes had no effect on N-dechloroethylation of IF or CPA. These P450 enzyme patterns for the N-dechloroethylation reaction are distinct from those previously determined for IF and CPA activation via 4-hydroxylation. In accord with this observation, the balance between oxazaphosphorine activation (4-hydroxylation pathway) and deactivation/neurotoxication (N-dechloroethylation pathway) could be modulated by P450 form-selective inducers and inhibitors. Thus, dexamethasone pretreatment substantially decreased the extent of IF N-dechloroethylation, from 47% to 24% of total metabolism, whereas it increased CPA N-dechloroethylation from 29% to 84% of total metabolism. Moreover, troleandomycin selectively inhibited CPA N-dechloroethylation, thereby increasing net metabolism of the drug via the therapeutically productive 4-hydroxylation pathway. Oxazaphosphorine activation and deactivation/neurotoxication are thus catalyzed by distinct subsets of liver P450 enzymes, in a manner that may allow for improvements in therapeutic indices for this class of drugs by using P450 form-selective modulators.

Growth hormone receptor cytoplasmic domain differentially promotes tyrosine phosphorylation of signal transducers and activators of transcription 5b and 3 by activated JAK2 kinase

GH-induced activation of JAK2, a GH receptor (GHR)-associated tyrosine kinase, leads to tyrosine phosphorylation and activation of STATs (signal transducers and activators of transcription) 1, 3, and 5. The present study investigates the importance of the GHR cytoplasmic domain in the activation of STAT3 and STAT5b. As the perimembranous Box1 region of the GHR cytoplasmic domain is necessary for activation of wild-type (WT) JAK2 by GH, we examined this question using GHR/JAK2 chimeras that have an activatable JAK2 kinase domain replacing the GHR cytoplasmic domain. STAT5b and STAT3, when each was coexpressed in COS-7 cells with WT GHR and WT JAK2, were both strongly tyrosine phosphorylated in response to GH. Coexpression of STAT3 with GHR/ JAK2 chimeras resulted in a strong GH-independent tyrosine phosphorylation of STAT3 that was 40% as active as that seen with WT GHR plus WT JAK2, whereas STAT5b was more minimally phosphorylated (13% of WT GHR plus WT JAK2) when coexpressed with chimeras devoid of the GHR cytoplasmic domain. Transient coexpression of each STAT together with WT JAK2 and GHR COOH-terminal truncation mutants indicated that a GH-induced STAT3-DNA binding complex, but not a STAT5b-DNA binding complex, was detectable when a GHR devoid of 85% of the cytoplasmic domain COOH-terminus (but eliciting significant JAK2 tyrosine phosphorylation) was expressed. In vitro binding experiments using GST/GHR cytoplasmic domain fusions demonstrated that both STATs could interact at a low basal level with GHR regions distal to residue 317. Phosphorylation of tyrosine residues in those distal regions greatly enhanced the receptor's interaction with STAT5b, but not STAT3. We conclude that GH induces activation of STAT3 and STAT5b by two different pathways: one primarily dependent on activation of JAK2 (STAT3) and another that is additionally reliant on the presence of an intact and tyrosine-phosphorylated GHR cytoplasmic domain (STAT5b).

Role of metabolism in the activation of dehydroepiandrosterone as a peroxisome proliferator

The adrenal steroid dehydroepiandrosterone (DHEA) stimulates a dramatic increase in both the size and the number of peroxisomes present in liver when given at pharmacological doses to rodents. Structurally diverse chemicals including many fatty acids, hypolipidemic drugs and other foreign chemicals, can also induce such a peroxisome proliferative response. This response is associated with a dramatic induction of perosisomal fatty acid beta-oxidation enzymes and microsomal cytochrome P450 4A fatty acid hydroxylases and, long-term, can lead to induction of hepatocellular carcinoma. This review examines the underlying mechanisms by which DHEA induces peroxisome proliferation and evaluates the possible role of peroxisome proliferator-activated receptor (PPAR) in this process. Like DHEA, the 17 beta-reduced metabolite 5-androstene-3 beta. 17 beta-diol (ADIOL) is an active peroxisome proliferator when administered in vivo, whereas androgenic and estrogenic metabolites of DHEA are inactive. In primary rat hepatocytes, however, DHEA and ADIOI are inactive as inducers of P450 4A and peroxisomal enzymes unless first metabolized by steroid sulfotransferase to the 3 beta-sulfates, DHEA-S and ADIOL-S. Investigations as to whether DHEA utilizes the same induction mechanism employed by classic, foreign chemical peroxisome proliferators, namely, activation of the intracellular receptor molecule PPAR, have shown that DHEA-S and ADIOL-S are ineffective with respect to PPAR activation in transient transfection/trans-activation assays. This inactivity of DHEA-S in vitro suggests a requirement for specific cellular transport or for further metabolism of the steroid which is only met in liver cells. Alternatively, the action of DHEA-S may require accessory proteins or other nuclear factors that modulate the activity of PPAR, such as retinoid X receptor (RXR), hepatocyte nuclear factor-4 (HNF-4) or chick ovalbumin upstream promoter transcription factor (COUP-TF). Investigations using Ca(2+)-channel blockers such as nicardipine suggest that there are important mechanistic similarities between the foreign chemical- and DHEA-S-stimulated induction responses, and support the hypothesis that these two classes of peroxisome proliferators both activate Ca(2+)-dependent signaling pathways. Further studies are required to ascertain whether this potential of DHEA and its sulfated metabolites to serve as physiological modulators of fatty acid metabolism and peroxisome enzyme expression contributes to the striking anti-carcinogenic and other useful chemoprotective properties that DHEA is known to possess.

Peroxisome proliferator-activated receptor alpha required for gene induction by dehydroepiandrosterone-3 beta-sulfate

Peroxisome proliferator-activated receptor alpha (PPAR alpha) mediates the effects of foreign chemical peroxisome proliferators on liver and kidney, including the induction of peroxisomal, mitochondrial, and microsomal enzymes involved in beta-oxidation of fatty acids. However, the role of this receptor in the peroxisome proliferative effects of the endogenous steroid dehydroepiandrosterone (DHEA) is not known. DHEA-3 beta-sulfate fd(DHEA-S) is shown to induce a liver peroxisome proliferative response in rats in vivo at a dose at which DHEA is much less active, which is consistent with cultured hepatocyte studies indicating a requirement for sulfation for the activation of DHEA. Transient transfection experiments demonstrated that in contrast to the prototypic foreign chemical peroxisome proliferator pirinixic acid, DHEA-S and its 17 beta-reduced metabolite, 5-androstene-3 beta, 17 beta-diol-3 beta-sulfate, are inactive in mediating trans-activation by PPAR alpha in COS-1 cells. Two other mammalian PPAR isoforms, gamma and delta/Nucl, were also inactive with respect to DHEA-S trans-activation. To test whether PPAR alpha mediates peroxisomal gene induction by DHEA-S in intact animals, we administered DHEA-S or clofibrate to mice lacking a functional PPAR alpha gene. Both peroxisome proliferators markedly increased hepatic expression of two microsomal cytochrome P450 4A proteins as well as six mRNAs known to be associated with the peroxisomal proliferative response in wild-type mice. In contrast, neither DHEA-S nor clofibrate induced these hepatic proteins and mRNAs in PPAR alpha-deficient mice. Clofibrate-induced expression of kidney CYP4A mRNAs was also blocked in the PPAR alpha gene knockout mice. Thus, despite its unresponsiveness to steroidal peroxisome proliferators in transfection assays, PPAR alpha is obligatory for DHEA-S-stimulated hepatic peroxisomal gene induction. DHEA-S, or one of its metabolites, may thus serve as an important endogenous regulator of liver peroxisomal enzyme expression via a PPAR alpha-mediated pathway.

Sensitization of human breast cancer cells to cyclophosphamide and ifosfamide by transfer of a liver cytochrome P450 gene

The cancer chemotherapeutic agent cyclophosphamide (CPA) and its isomer ifosfamide (IFA) are alkylating agent prodrugs that require metabolism by liver cytochrome P450 (P450) enzymes for antitumor activity. The therapeutic effectiveness of these oxazaphosphorines is limited by the hematopoietic, renal, and cardiac toxicity that accompanies the systemic distribution of liver-derived activated drug metabolites. Transfer of a liver cytochrome P450 gene, CYP2B1, into human breast MCF-7 cancer cells is presently shown to greatly sensitize these cells to oxazaphosphorine toxicity as a consequence of the acquired capacity for intratumoral CPA and IFA activation. Thus, CPA and IFA were highly cytotoxic to MCF-7 cells following stable transfection of CYP2B1 but exhibited no toxicity to parental tumor cells or to a beta-galactosidase-expressing MCF-7 transfectant. This cytotoxicity could be appreciably blocked by the CYP2B1 inhibitor metyrapone. Cell cycle analysis revealed that CPA arrested the CYP2B1-expressing cells, but not CYP2B1-negative cells, at G(2)-M phase. A strong bystander cytotoxicity effect that does not require direct cell-cell contact was mediated by CYP2B1-expressing MCF-7 cells on non-CYP2B1 cells. Intratumoral CYP2B1 expression conferred a distinct therapeutic advantage when treating MCF-7 tumors grown in nude mice with CPA, as revealed by a 15-20-fold greater in vivo cytotoxicity, determined by tumor excision/colony formation assay, and by the substantially enhanced antitumor activity, monitored by tumor growth delay, for CYP2B1-e xpressing MCF-7 tumors as compared to CYP2B1-negative control tumors. These enhanced therapeutic effects were obtained without any apparent increase in host toxicity. To evaluate the extent to which a CPA/P450 gene therapy strategy may be generally applicable to other tumor cell types, a replication-defective recombinant adenovirus carrying the CYP2B1 gene driven by the cytomegalovirus (CMV) promotor ad.CMV-2B1 was constructed and used to infect a panel of human tumor cell lines. Ad.CMV-2B1 infection rendered each of the cell lines highly sensitive to CPA and IFA cytotoxicity, with substantial chemosensitization seen at multiplicities of infection as low as 10. The CPA/P450 prodrug activation system may thus serve as a useful paradigm for further development of novel cancer gene therapy strategies that utilize drug susceptibility genes to significantly potentiate the antitumor activity of conventional cancer chemotherapeutic agents.

Growth hormone activation of Stat 1, Stat 3, and Stat 5 in rat liver. Differential kinetics of hormone desensitization and growth hormone stimulation of both tyrosine phosphorylation and serine/threonine phosphorylation

Intermittent plasma growth hormone (GH) pulses, which occur in male but not female rats, activate liver Stat 5 by a mechanism that involves tyrosine phosphorylation and nuclear translocation of this latent cytoplasmic transcription factor (Waxman, D. J., Ram, P. A., Park, S. H., and Choi, H. K. (1995) J. Biol. Chem. 270, 13262-13270). We demonstrate that physiological levels of GH can also activate Stat 1 and Stat 3 in liver tissue, but with a dependence on the dose of GH and its temporal plasma profile that is distinct from Stat 5 and with a striking desensitization following a single hormone pulse that is not observed with liver Stat 5. GH activation of the two groups of Stats leads to their selective binding to DNA response elements upstream of the c-fos gene (c-sis-inducible enhancer element; Stat 1 and Stat 3 binding) and the beta-casein gene (mammary gland factor element; liver Stat 5 binding). In addition to tyrosine phosphorylation, GH is shown to stimulate phosphorylation of these Stats on serine or threonine in a manner that either enhances (Stat 1 and Stat 3) or substantially alters (liver Stat 5) the binding of each Stat to its cognate DNA response element. These findings establish the occurrence of multiple, Stat-dependent GH signaling pathways in liver cells that can target distinct genes and thereby contribute to the diverse effects that GH and its sexually dimorphic plasma profile have on liver gene expression.

P450 superfamily: update on new sequences, gene mapping, accession numbers and nomenclature

We provide here a list of 481 P450 genes and 22 pseudogenes, plus all accession numbers that have been reported as of October 18, 1995. These genes have been described in 85 eukaryote (including vertebrates, invertebrates, fungi, and plants) and 20 prokaryote species. Of 74 gene families so far described, 14 families exist in all mammals examined to date. These 14 families comprise 26 mammalian subfamilies, of which 20 and 15 have been mapped in the human genome and the mouse genome, respectively. Each subfamily usually represents a cluster of tightly linked genes widely scattered throughout the genome, but there are exceptions. Interestingly, the CYP51 family has been found in mammals, filamentous fungi and yeast, and plants-attesting to the fact that this P450 gene family is very ancient. One functional CYP51 gene and two processed pseudogenes, which are the first examples of intronless pseudogenes within the P450 superfamily, have been mapped to three different human chromosomes. This revision supersedes the four previous updates in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene, we recommend that the italicized root symbol "CYP' for human ("Cyp' for mouse and Drosophila), representing "cytochrome P450', be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen is no longer recommended in mouse gene nomenclature. "P' ("ps' in mouse and Drosophila) after the gene number denotes a pseudogene; "X' after the gene number means its use has been discontinued. If a gene is the sole member of a family, the subfamily letter and gene number would be helpful but need not be included. The human nomenclature system should be used for all species other than mouse and Drosophila. The cDNAs, mRNAs and enzymes in all species (including mouse) should include all capital letters, and without italics or hyphens. This nomenclature system is similar to that proposed in our previous updates.

Growth hormone regulation of male-specific rat liver P450s 2A2 and 3A2: induction by intermittent growth hormone pulses in male but not female rats rendered growth hormone deficient by neonatal monosodium glutamate

Growth hormone (GH) secretory patterns regulate the expression of several sex-dependent liver cytochrome P450 (CYP) genes. Studies using the hypophysectomized rat model have established that the intermittent plasma GH secretory pattern associated with adult male rats markedly stimulates liver expression of the male-specific CYP 2C11, a testosterone 2 alpha- and 16 alpha-hydroxylase, but is not required for expression of other male-specific liver enzymes, including CYP 2A2, a testosterone 15 alpha-hydroxylase, and CYP 3A2, a testosterone 6 beta-hydroxylase. In the present study, the effects of intermittent GH treatment on liver CYP expression were studied in adult rats rendered GH deficient by neonatal administration of monosodium glutamate (MSG), which depletes circulating adult GH without the global loss of other pituitary-dependent hormones that is associated with hypophysectomy. Restoration of the normal masculine circulating GH profile of six daily pulses (180-225 ng GH/ml/peak) in MSG-treated male rats by the use of an external pumping apparatus led to a substantial (30-50%) restoration of normal male levels of CYP 2A2 and CYP 3A2 activity, protein, and mRNA. GH pulsation at the nonphysiological frequencies of two or four times per day was less effective unless given at a dose that resulted in supraphysiological plasma GH levels. Although intermittent GH treatment can induce male-specific P450 expression in hypophysectomized female rats, the same hormone treatment did not stimulate CYP 2A2 or CYP 3A2 expression in MSG-treated female rats. Liver GH receptor mRNA levels at adulthood were not significantly altered by neonatal MSG treatment, suggesting that the unresponsiveness of MSG-treated females and the previously reported low responsiveness of MSG-treated males to GH-induced CYP 2C11 expression are not due to the absence of GH receptor. Moreover, normal liver IGF-1 mRNA levels were expressed in the MSG-treated female rats, suggesting that the liver GH receptor is functional in these animals. The present findings establish that the adult male-specific enzymes CYP 2A2 and CYP 3A2 can be positively regulated by intermittent GH pulsation despite their GH-independent expression in hypophysectomized rats. Moreover, neonatal MSG treatment, particularly in female rats, may lead to the loss of factors other than GH that are required for full expression of the pulsatile GH-stimulated CYP 2A2, 3A2, and 2C11 genes.

Complete reversal by thaliblastine of 490-fold adriamycin resistance in multidrug-resistant (MDR) human breast cancer cells. Evidence that multiple biochemical changes in MDR cells need not correspond to multiple functional determinants for drug resistance

The emergence of drug resistance is a major obstacle to effective cancer chemotherapy. The identification of novel agents that serve as selective, potent and nontoxic modulators of drug resistance is thus an important goal for improving the success of cancer treatment. Thaliblastine (TBL), a plant alkaloid and P-glycoprotein (P-gp) inhibitor, is presently shown to fully reverse 490-fold resistance to Adriamycin (AdR) in a multidrug-resistant (MDR) human breast cancer cell line (MCF/AdR) that overexpresses P-gp, whereas the same treatment had no effect on AdR cytotoxicity in the drug-sensitive parental MCF-7 cells. Mechanistic studies showed that this striking resistance reversal was achieved without alteration of cellular levels of glutathione and without inhibition of glutathione S-transferase, glutathione peroxidase or P450 reductase by TBL, each of which is significantly altered in MCF/AdR cells, and each of which has been proposed to contribute to AdR resistance in this MDR line. Rather, resistance reversal by TBL can be entirely explained by this drug's capacity to restore the intracellular accumulation of AdR in the resistant cells. These results establish that MDR associated with P-gp overexpression can be fully reversed by the potent P-gp inhibitor TBL. They further indicate that although changes in multiple drug-metabolizing enzymes may accompany the development of MDR, these multiple biochemical alterations need not correspond to multiple functional determinants for drug resistance.

Arachidonic acid metabolism by human cytochrome P450s 2C8, 2C9, 2E1, and 1A2: regioselective oxygenation and evidence for a role for CYP2C enzymes in arachidonic acid epoxygenation in human liver microsomes

The membrane-bound endogenous fatty acid arachidonic acid can be released from membranes by phospholipases and then metabolized to biologically active compounds by cyclooxygenases, lipoxygenases, and cytochrome P450 (CYP) enzymes. In the liver the CYP pathway is the most significant. Liver CYP arachidonate products include epoxyeicosatrienoic acids (EETs) and monohydroxylated products (HETEs). We examined metabolism of [1-14C]arachidonic acid by a panel of 10 human CYP enzymes expressed in HepG2 cells. In the absence of expressed CYP enzymes, control HepG2 cell microsomes generated only small amounts of omega- and omega--1-OH arachidonic acid (ratio 2:1). Microsomes from HepG2 cells expressing CYP2C8, 2C9, 1A2, and 2E1 were 7-21 times more active than microsomes from the HepG2 controls. CYP2C8, 2C9, and 1A2 principally generated epoxygenase products; 36 to 48% were in the form of EET-diols, reflecting host HepG2 microsomal epoxide hydrolase activity. CYP2C8 and 2C9 formed more 14,15- and 11,12-EET than did CYP1A2, while CYP1A2 formed more 8,9-EET. CYP2C9 also generated a peak with the retention time of 12-HETE. CYP2E1 generated omega--1-OH arachidonic acid and, to a lesser extent, omega-OH arachidonic acid (ratio 2:1). A small amount of epoxygenase activity was also detected for CYP2B6; its overall activity, however, was only about twice control levels. Activities of CYP2A6, 3A3, 3A4, and 3A5 were low and limited to the omega-/omega--1-OH arachidonic acid peak; CYP2D6 was inactive. Microsomes prepared from three individual human livers varied threefold in total arachidonic acid metabolism. For all three livers omega-OH arachidonic acid was the major product (up to 74% of total metabolites). Epoxygenase products constituted 14 to 28% of the total products; 60 to 83% of those were EET-diols, indicating that the human liver microsomes have substantial EET-epoxide hydrolase activity. 11,12-EET was the major EET for two livers and 14,15-EET for the third. The CYP2C inhibitor sulfaphenazole depressed human liver microsomal epoxygenase activity by 50% at 50 microM, while alpha-naphthoflavone inhibited arachidonic acid epoxygenase activity by 27% at 2 microM and by 32% at 10 microM. Collectively, these findings suggest that human liver microsomal arachidonic acid metabolism is catalyzed principally by CYP2C enzymes. CYP1A2, CYP2E1, and possibly CYP2B6 are likely to play more minor roles, though their contribution may be enhanced by exposure to inducers of those enzymes. CYP2A6, CYP2D6, and CYP3A enzymes are unlikely to make any significant contribution.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of thiotepa antitumor activity in vivo by alteration of liver cytochrome P450-catalyzed drug metabolism

The anticancer drug and alkylating agent thiotepa is metabolized by oxidative desulfuration to yield the alkylating metabolite N,N',N"-triethylenephosphoramide (TEPA) in a reaction that is catalyzed by specific liver cytochrome P450 (CYP) enzymes, including CYP2B1, the major phenobarbital-inducible P450 of rat liver, and CYP2C11, a constitutively expressed, male-specific form. The present study investigates the potential for modulating the cytotoxicity and antitumor activity of thiotepa by prior treatment of tumor-bearing rats with the CYP2B1 inducer phenobarbital or the CYP2C11 inhibitor 2-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF-525A) and examines the role of TEPA in the cytotoxicity of thiotepa in vivo. Administration of thiotepa to adult male rats bearing 9L gliosarcoma, grown s.c., resulted in dose-dependent cytotoxicity (ED90 approximately 12 mg/kg i.v., single dose), as determined by a tumor excision/in vitro colony formation assay carried out 24 hr after drug treatment. Tumor growth delay experiments revealed that thiotepa (5 mg/kg) inhibited 9L tumor growth over a 5- to 7-day period after alkylating agent treatment and this effect was accompanied by moderate body weight loss. Pretreatment with phenobarbital, under conditions in which liver CYP2B1 levels and liver microsomal thiotepa desulfuration to yield TEPA are both markedly increased, did not alter thiotepa's short-term (24-hr) cytotoxicity, as judged by a tumor excision assay, nor did it affect the extent of bone marrow toxicity associated with drug treatment. However, phenobarbital did block the tumor growth delay effect of thiotepa and it also attenuated the body weight loss that occurred during the first 5 days after drug treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Intermittent plasma growth hormone triggers tyrosine phosphorylation and nuclear translocation of a liver-expressed, Stat 5-related DNA binding protein. Proposed role as an intracellular regulator of male-specific liver gene transcription

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription that are regulated by the temporal pattern of pituitary GH release, which is intermittent in male rats and nearly continuous in females. To investigate the influence of these GH secretory patterns on intracellular hepatocyte signaling, we compared the pattern of liver nuclear protein tyrosine phosphorylation in male and female rats. An M(r) approximately 93,000 polypeptide, p93, was found to be tyrosine phosphorylated to a high level in male but not female rats. GH, but not prolactin, rapidly stimulated p93 tyrosine phosphorylation in hypophysectomized rats. Intermittent plasma GH pulses triggered repeated p93 phosphorylation, while continuous GH exposure led to desensitization and a dramatic decline in liver nuclear p93. p93 was cross-reactive with two monoclonal antibodies raised to mammary Stat 5, whose tyrosine phosphorylation is stimulated by prolactin. Intermittent GH pulsation translocated liver Stat 5/p93 protein from the cytosol to the nucleus and also activated its DNA binding activity, as demonstrated using a mammary Stat 5-binding DNA element derived from the beta-casein gene. p93 is thus a liver-expressed, Stat 5-related DNA binding protein that undergoes tyrosine phosphorylation and nuclear translocation in response to intermittent plasma GH stimulation and is proposed to be an intracellular mediator of the stimulatory effects of GH pulses on male-specific liver gene expression.

Identification of glutathione S-transferase as a determinant of 4-hydroperoxycyclophosphamide resistance in human breast cancer cells

Aldehyde dehydrogenase (ALDH) is well known for its involvement in the resistance of tumor cells to cyclophosphamide (CPA) and its activated derivatives, such as 4-hydroperoxy-CPA (4HC). The role of other drug-metabolizing enzymes such as glutathione S-transferase (GST) in CPA resistance is, however, less certain. In the present study of a human breast cancer cell line (MCF-7) exhibiting about 6-fold resistance to 4HC (MCF/HC), cellular levels of glutathione (GSH) were increased 1.4-fold, while cytosolic GST and ALDH activities were increased 2.7- and 7.2-fold, respectively, relative to the MCF-7 parental line. No significant changes in glutathione peroxidase and NADPH cytochrome P450 reductase activity, and no increase in microsomal GST and GST pi mRNAs were found in the resistant cells. Treatment with the ALDH substrate octanal sensitized the cells to the cytotoxic effects of 4HC to a modest extent in both MCF-7 and MCF/HC cells [dose modification factor (DMF) of 1.4 and 1.6, respectively]. Depletion of GSH by treatment with the GSH synthesis inhibitor buthionine sulfoximine (BSO) enhanced the cytotoxic effect of 4HC to a similar extent in both cell lines. By contrast, ethacrynic acid, which inhibited GST activity by > 85% in MCF-7 and MCF/HC cell extracts without depletion of GSH, sensitized the resistant but not the parental cells to 4HC cytotoxicity, indicating the importance of GST as a determinant of 4HC resistance in these cells. This conclusion is supported by the observation that in MCF/HC cells, ethacrynic acid in combination with BSO increased the DMF 3-fold higher than did BSO or EA alone, while in the parental MCF-7 cells ethacrynic acid with BSO had no significant chemosensitization effect over BSO alone. These studies establish that in addition to ALDH, GST overexpression can contribute to acquired resistance of tumor cells to 4HC and, furthermore, suggest that modulators that target the GSH/GST system could be useful in overcoming CPA resistance in the clinic.

Intratumoral activation and enhanced chemotherapeutic effect of oxazaphosphorines following cytochrome P-450 gene transfer: development of a combined chemotherapy/cancer gene therapy strategy

Cyclophosphamide and its isomer ifosfamide are cell cycle-nonspecific alkylating agents that undergo bioactivation catalyzed by liver cytochrome P-450 enzymes. The therapeutic efficacy of these oxazaphosphorine anticancer drugs is limited by host toxicity resulting from the systemic distribution of activated drug metabolites formed in the liver. Since tumor cells ordinarily do not have the capacity to activate oxazaphosphorines, we examined whether introduction into tumor cells of a cDNA encoding CYP2B1, a major catalyst of oxazaphosphorine activation, sensitizes the cells to the cytotoxic effects of cyclophosphamide and ifosfamide. Here we show that 9L gliosarcoma cells stably transfected with a cDNA encoding rat CYP2B1 are highly sensitive to cyclophosphamide and ifosfamide cytotoxicity as compared to parental 9L cells or 9L cells transfected with an Escherichia coli beta-galactosidase gene. The CYP2B1 enzyme inhibitor metyrapone protects the CYP2B1-expressing 9L cells from oxazaphosphorine cytotoxicity, demonstrating that the chemosensitivity of these cells is a direct consequence of intracellular prodrug activation. Moreover, CYP2B1-expressing 9L cells potentiate the cytotoxic effects of cyclophosphamide and ifosfamide toward cocultured CYP2B1-negative 9L tumor cells. This "bystander effect" does not require cell-cell contact, and therefore may have the therapeutic advantage of distributing cytotoxic drug metabolites to a wide area within a solid tumor mass. In vivo experiments using Fischer 344 rats implanted s.c. with CYP2B1-expressing 9L tumor cells demonstrated that intratumoral expression of the CYP2B1 gene provides a substantial therapeutic advantage over that provided by liver cytochrome P-450-dependent drug activation alone; cyclophosphamide treatment resulted in complete growth inhibition of CYP2B1-positive tumors, whereas only a modest growth delay effect was obtained with CYP2B1-negative tumors. These studies establish that drug-activating CYP genes may be useful for the development of novel combined chemotherapy/gene therapy strategies for cancer treatment utilizing established cancer chemotherapeutic agents.

Markedly enhanced cytochrome P450 2E1 induction and lipid peroxidation is associated with severe liver injury in fish oil-ethanol-fed rats

We evaluated the role of changes in cytochrome P-450 2E1 (CYP 2E1) and lipid peroxidation in relation to development of severe liver injury in fish oil-ethanol-fed rats. The experimental animals (male Wistar rats) were divided into 5 rats/group and were fed the following diets for 1 month: corn oil and ethanol (CO+E) or corn oil and dextrose (CO+D), and fish oil and ethanol (FO+E) or fish oil and dextrose (FO+D). For each animal, microsomal analysis of CYP 2E1 protein, aniline hydroxylase activity, fatty acid composition, and conjugated dienes was conducted. Also, evaluation of severity of pathology was done for each rat. The mean +/- SD of the pathology score was significantly higher (p < 0.01) in the FO+E (6.0 +/- 1.3) group than in the CO+E group (3.0 +/- 0.5). No pathological changes were evident in the dextrose-fed controls. The CYP 2E1 protein levels (mean +/- SD) were significantly higher (p < 0.01) in the FO+E group (13.1 +/- 2.0) compared with the CO+E (4.7 +/- 1.2) and FO+D (1.8 +/- 0.5) groups. Higher levels of eicosapentaenoic and docosahexaenoic acids and lower levels of arachidonic acid were detected in liver microsomes from rats fed fish oil compared with corn oil. A significant correlation was obtained between CYP 2E1 protein and conjugated diene levels (r = 0.78, p < 0.01). Our results showing markedly increased CYP 2E1 induction and lipid peroxidation in the FO+E group provides one possible explanation for the greater severity of liver injury in this group.

1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) modulates rat liver microsomal cyclophosphamide and ifosphamide activation by suppressing cytochrome P450 2C11 messenger RNA levels

The alkylating anticancer drug 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU; lomustine) is frequently administered to cancer patients as part of a combination chemotherapy regimen. Previous studies have indicated that CCNU treatment of adult male rats leads to prolonged decreases in liver cytochrome P450 (CYP)-mediated enzyme activities. Because the alkylating agent prodrugs cyclophosphamide and ifosphamide are known to be activated by liver cytochrome P450 enzymes, the potential for interaction between CCNU and these oxazaphosphorines was examined. Treatment of adult male rats with a single dose of CCNU (30 mg/kg i.p.) resulted in a progressive loss of liver microsomal cyclophosphamide and ifosphamide hydroxylation activities in vitro (30-60% decrease after 7-27 days). The individual liver P450 forms modulated by CCNU were then identified using P450 form-specific microsomal testosterone hydroxylase assays. CCNU treatment was found to decrease substantially CYP2C11-dependent testosterone 2 alpha-hydroxylase activity (80-90% decrease after 14 or 27 days), but it did not affect CYP3A2-dependent testosterone 6 beta-hydroxylase activity. It only modestly decreased CYP2A2-mediated testosterone 15 alpha-hydroxylase activity. The reduction in CYP2C11 activity was not associated with a decline in liver microsomal NADPH-cytochrome P450 reductase activity, but rather was caused by a complete suppression of CYP2C11 mRNA levels. In contrast to other alkylating agents, such as cisplatin, which is known to feminize the overall expression profile of gender-specific liver enzymes, CCNU did not increase levels of the female-predominant liver enzymes steroid 5 alpha-reductase and CYP2C7, nor did it deplete circulating testosterone levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in cytochromes P-450, 2E1, 2B1, and 4A, and phospholipases A and C in the intragastric feeding rat model for alcoholic liver disease: relationship to dietary fats and pathologic liver injury

The influence of dietary fat and alcohol on hepatic microsomal levels of cytochromes P-450 2E1, 2B, and 4A; phospholipases A and C; and UDP-glucuronosyltransferase was studied in the intragastric feeding rat model for alcoholic liver injury. Eight groups of animals were evaluated. Control and ethanol fed rats received either saturated fat or corn oil and were killed after 2 weeks and 1 month of feeding. All animals were pair-fed by continuous infusion of liquid diet through permanently implanted gastric cannulas. Alcoholic liver injury developed only in the corn oil-ethanol-fed groups and was manifest by 1 month. Livers were subjected to the following analyses: pathologic evaluation of liver injury; levels of cytochromes P-450 2E1, 2B, and 4A protein and mRNA; aniline hydroxylase activity; and phospholipase A and C and UDP-glucuronosyltransferase activities. Ethanol-induced increases in cytochromes P-450 2E1 and 2B protein determined by Western blotting were greatest in the corn oil-ethanol-fed group, which developed pathologic changes in the liver. Cytochromes P-450 2E1 and 2B1 mRNA levels were unaffected, suggesting that posttranscriptional mechanisms are responsible for the increase in the corresponding P-450 proteins. In contrast, cytochrome P-450 4A levels were higher in the saturated fat-ethanol groups compared with the corn oil-ethanol groups. Phospholipase A and phospholipase C levels were higher in the corn oil-ethanol groups compared with pair-fed dextrose controls and the saturated fat-ethanol groups. UDP-glucuronosyltransferase levels declined with time in the ethanol-fed groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Dehydroepiandrosterone 3 beta-sulphate is an endogenous activator of the peroxisome-proliferation pathway: induction of cytochrome P-450 4A and acyl-CoA oxidase mRNAs in primary rat hepatocyte culture and inhibitory effects of Ca(2+)-channel blockers

The role of steroids related to the adrenal androgen dehydroepiandrosterone (5-androstene-3 beta-ol-17-one; DHEA) in regulating the expression of peroxisomal and cytochrome P-450 4A (CYP4A) enzymes active in fatty acid metabolism was assessed using a primary rat hepatocyte culture system. Exposure of hepatocytes to the peroxisome proliferator, clofibric acid (10-250 microM), for 48-96 h led to substantial increases in CYP4A protein, CYP4A1, CYP4A2 and CYP4A3 mRNAs, and the mRNAs encoding both forms of peroxisomal acyl-CoA oxidase (ACOX-I and ACOX-II), as judged by Northern-blot analysis using gene-specific oligonucleotide probes. Although DHEA treatment in vivo is effective in inducing these mRNAs in rat liver, it had no effect in the cultured hepatocytes. In contrast, treatment of the cells with DHEA 3 beta-sulphate (DHEA-S; 10-250 microM) stimulated major increases in CYP4A and ACOX mRNA levels. Examination of several analogues indicated a preference for 3 beta-sulphate over 17 beta-sulphated steroids and the inactivity of a 3 alpha-hydroxy-17 beta-sulphate derivative (DHEA-S > 5-androstene-3 beta,17 beta-diol 3-sulphate approximately 5 alpha-androstene-3 beta-ol-17-one 3-sulphate > 5-androstene-3 beta, 17 beta,17 beta-diol 17-sulphate approximately 5 beta-androstane-3 alpha-ol-17-one 3-sulphate >> 5 alpha-androstane-3 alpha, 17 beta-diol 17-sulphate). Induction of CYP4A mRNAs by either DHEA-S or clofibric acid was partially blocked by structurally diverse Ca(2+)-channel antagonists (nicardipine, nifedipine and diltiazem; 50 microM), suggesting that both the steroidal and fibrate classes of CYP4A inducers stimulate peroxisomal-proliferative responses via a Ca(2+)-dependent pathway. Retinoic acid alone slightly induced CYP4A mRNAs but did not enhance the induction by clofibrate or DHEA-S. As DHEA-S corresponds to a physiologically important major circulating androgen, these findings suggest that it may serve as an endogenous regulator of hepatic peroxisome enzyme levels. They further suggest that Ca(2+)-channel blockers may be useful pharmacological tools for the further study of the underlying cellular mechanism whereby endogenous steroids and fibrate drugs induce peroxisome proliferation, and the relationship of these events to activation of the peroxisome proliferator-activated receptor.

Rat liver cytochrome P450 2B3: structure of the CYP2B3 gene and immunological identification of a constitutive P450 2B3-like protein in rat liver

The cytochrome P450 2B subfamily in the rat contains an estimated eight to eleven members at the genomic level. Synthesis in the liver of the prototypic forms P450 2B1 and P450 2B2 is dramatically induced by phenobarbital. The 1.9-kb mRNA for P450 2B3, a third member of the P450 2B subfamily, is constitutively present in rat liver but is not inducible by phenobarbital. We have now cloned and sequenced exonic sequences corresponding to the entire 2B3 mRNA and determined their exon-intron structure, which is identical to that of CYP2B1/CYP2B2 and other CYP2B genes. A putative CYP2B3 transcription start site was identified and CYP2B3 5'- and 3'-flanking sequences were compared to those of CYP2B1 and CYP2B2. CYP2B3, like CYP2B1 and CYP2B2, has a modified TATA box preceding the transcription start site and lacks the canonical polyadenylation signal preceding the poly(A) site. A 2B3 expression vector, pMT2-2B3, directed the synthesis in COS-1 cells of an approximately 50-kD protein detectable on Western blots with a polyclonal antibody and with one of four monoclonal antibodies raised against 2B1 but not with a polyclonal antibody raised against P450 PB6. The 2B3 protein migrated with a slightly higher electrophoretic mobility than 2B1 and comigrated with a protein detected by anti-2B1 antibodies in liver microsomes from untreated rats. The results indicate that a 2B3-like protein is present in rat liver and that it is distinct from P450 PB6 and other known constitutive rat hepatic P450s.

Experimental tumor therapy in mice using the cyclophosphamide-activating cytochrome P450 2B1 gene

Most malignant tumors of the central nervous system do not respond well to chemotherapy. The anticancer drug cyclophosphamide (CPA) is largely ineffective against these neoplasms as its conversion to DNA-alkylating, cytotoxic metabolites is restricted primarily to the liver and these metabolites do not readily cross the blood-brain barrier. Here, we show that brain tumor cells can be sensitized to the cytotoxic effects of CPA, both in culture and in vivo, by introduction of the hepatic enzyme responsible for the activation of CPA, cytochrome P450 2B1. Stable transfection of rat C6 glioma cells with the P450 2B1 gene rendered the cultured tumor cells sensitive to CPA. Further, C6 cells bearing this gene were more sensitive than parental cells to the cytotoxic action of CPA when grown subcutaneously in the flanks of athymic mice. Murine fibroblasts producing a retrovirus vector encoding P450 2B1 and expressing this enzyme were then prepared and grafted into the brains of athymic mice seeded with rat C6 gliomas. Intrathecal administration of CPA prevented the development of meningeal neoplasia and led to partial regression of the parenchymal tumor mass. By contrast, C6 glioma-bearing mice receiving fibroblasts expressing the Escherichia coli lacZ gene and CPA exhibited extensive meningeal tumors and parenchymal solid brain tumors. The in situ activation of CPA by cytochrome P450 2B1 provides a novel approach not only for brain tumor gene therapy, but also for negative, drug-conditional selection of other defined cell populations.

Growth hormone-dependent and -independent sexually dimorphic regulation of phenobarbital-induced hepatic cytochromes P450 2B1 and 2B2

Sexually dimorphic regulation of phenobarbital-induced cytochromes P450 2B1 and 2B2 (collectively referred to as P450 2B) as well as P450 2B-dependent monooxygenase activities was studied in multi-hormone-depleted hypophysectomized rats and in growth hormone (GH)-deficient monosodium glutamate (MSG)-treated rats. Our results indicate that endogenous GH suppresses phenobarbital induction of P450 2B and that the feminine pattern of continuous GH secretion is more suppressive than the masculine profile of episodic secretion. Moreover, we have found that it is the height of the GH pulse, and not necessarily its frequency nor the interpulse trough periods, that signals the suppressive effects of GH on P450 2B expression in male rats. Last, irrespective of the presence or absence of circulating GH, the magnitude of phenobarbital induction of P450 2B and associated monooxygenases was consistently lower in female rats, suggesting the presence of some degree of sex-dependent, but GH-independent regulation.

Induction of microsomal and peroxisomal enzymes by dehydroepiandrosterone and its reduced metabolite in rats

Dehydroepiandrosterone (DHEA) given to rodents in pharmacological doses induces several hepatic enzymes including cytochromes P4504A, NADPH:P450 oxidoreductase, palmitoyl coenzyme A oxidase, and other enzymes associated with the peroxisomal beta-oxidation pathway, leading to peroxisome proliferation and development of hepatocellular carcinoma in rodents. Comparison of the inductive potency of DHEA and other intermediates of the steroid biosynthetic path demonstrated that only DHEA, 5-ene-androstene-3 beta,17 beta-diol (ADIOL), and to a lesser extent, 17 alpha-hydroxypregnenolone, a precursor of DHEA, induce cytochromes P4504A protein and other enzymes associated with the peroxisome proliferative response in vivo. ADIOL exerted its inductive response at a somewhat lower dosage than DHEA, whereas ADIOL and DHEA both induced the microsomal enzymes (P4504A and its oxidoreductase) at somewhat lower dosages than those required to induce peroxisomal enzymes. Northern analysis demonstrated increases in the mRNAs encoding the cytochromes P4504A (> 20-fold) and NADPH:P450 oxidoreductase (> 10-fold) in the livers of DHEA- and ADIOL-treated rats. Run-on transcription analysis demonstrated that DHEA induces CYP4A gene expression 11-fold at the level of transcription initiation. Comparison of the responsiveness of individual rat CYP4A genes (4A1, 4A2, and 4A3) to DHEA and ADIOL in immature versus mature male rats revealed 2-3-fold higher levels of induced CYP4A1 and 4A3 mRNAs in immature rat livers. In contrast, hepatic CYP4A2 mRNA was induced to 6-10-fold higher levels in mature rats. No basal or significant inducible expression of mRNA for CYP4A1 and 4A3 was noted in rat kidney. Significant basal levels of kidney CYP4A2 mRNA were observed only in mature animals, where they were inducible by ADIOL and DHEA to a 3-5-fold greater extent than in the kidneys of immature rats. These studies demonstrate developmental differences in the responsiveness of CYP4A mRNA levels to DHEA and ADIOL in rat kidney and liver. Moreover, the striking inducibility of CYP4A protein and mRNAs, together with the increased rates of synthesis of nascent CYP4A mRNA transcripts in hepatic nuclei from DHEA-treated rats, establish that DHEA increases the expression of these microsomal enzymes at the transcriptional level.

Evaluation of triacetyloleandomycin, alpha-naphthoflavone and diethyldithiocarbamate as selective chemical probes for inhibition of human cytochromes P450

A variety of chemicals, including triacetyloleandomycin (TAO), alpha-naphthoflavone (ANF), and diethyldithiocarbamate (DDC), are widely used as inhibitory probes for select individual human cytochrome P450 (CYP) enzymes, despite the fact that the selectivity of these inhibitors has not been rigorously evaluated. In the present study we take advantage of recent advances in cDNA-directed human P450 expression to evaluate directly the P450 form selectivity of TAO, ANF, and DDC, using a panel of 10 individual cDNA-expressed human P450s. Under experimental conditions known to yield maximal TAO complexation with P450 hemoproteins, TAO (20 microM) inhibited the catalytic activity of expressed CYPs 3A3, 3A4, and 3A5, whereas it did not affect CYPs 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, or 2E1 activity. ANF inhibited not only CYPs 1A1 and 1A2 (IC50 = 0.4-0.5 microM), but it was also similarly effective against CYPs 2C8 and 2C9. Increasing the concentration of ANF to 10 microM led to inhibition of CYP2A6 and CYP2B6. Although a previous study suggested that DDC is a selective inhibitor of CYP2E1, the present investigation shows that at concentrations required to inhibit CYP2E1 (IC50 approximately 125 microM when preincubated with NADPH), DDC also inhibited CYPs 1A1, 1A2, 2A6, 2B6, 2C8, 3A3, and 3A4. Decreasing the concentration of DDC to 10 microM, however, led to inhibition of CYP2A6 (65% inhibition) and CYP2B6 (50% inhibition), but none of the other P450s examined, including CYP2E1. Overall, these results establish that (a) TAO is a selective inhibitor of the human CYP3A subfamily; (b) ANF potently inhibits CYP2C8 and CYP2C9, in addition to CYPs 1A1 and 1A2; and (c) DDC cannot be employed as a diagnostic inhibitory probe for CYP2E1.

Use of reverse transcription-polymerase chain reaction to evaluate in vivo cytokine gene expression in rats fed ethanol for long periods

We evaluated the expression of interleukin-1 alpha, interleukin-1 beta, tumor necrosis factor-alpha and transforming growth factor-beta mRNAs in the intragastric-feeding rat model of alcoholic liver disease. Rats were fed different diets for periods of 2 or 4 wk. Animals fed saturated fat and ethanol and the corn oil-dextrose control group had no liver injury, whereas animals fed corn oil and ethanol showed pathologic changes. RNA was extracted from the livers at the time of killing, reverse-transcribed and amplified; polymerase chain reaction products were subjected to electrophoresis on agarose gel. Interleukin-1 alpha mRNA was present in all groups at 2 and 4 wk; interleukin-1 beta and transforming growth factor-beta mRNAs were present in all groups at 4 wk. Tumor necrosis factor-alpha mRNA was absent in all groups at 2 wk but was present in the corn oil-ethanol group only at 4 wk. Because pathological liver injury was evident in the corn oil-ethanol group by 4 wk, the presence of tumor necrosis factor-alpha mRNA at this time suggests a pathogenetic role for tumor necrosis factor-alpha in alcohol-induced liver injury.

Multiple, functional DBP sites on the promoter of the cholesterol 7 alpha-hydroxylase P450 gene, CYP7. Proposed role in diurnal regulation of liver gene expression

Hepatic cytochrome P450 cholesterol 7 alpha-hydroxylase, CYP7, is regulated in vivo at the protein and the mRNA level in response to multiple physiological factors, including liver cholesterol synthesis, bile acid feedback inhibition, and diurnal rhythm. In the present study we investigated whether the liver transcription factor DBP (albumin promoter D-site binding protein), which undergoes a striking diurnal rhythm in rat liver (DBP levels during evening/morning approximately 100:1), contributes to the diurnal regulation of CYP7 gene expression. DNase I footprinting analysis using bacterially expressed DBP and a cloned 5'-flanking DNA segment of the rat CYP7 gene revealed five distinct DBP-binding sites, designated A-E, distributed between nucleotides (nts) -41 and -295 relative to the CYP7 transcription start site. CYP7-directed gene transcription in HepG2 cells transfected with a 5'-CYP7 promoter-chloramphenicol acetyl-transferase reporter was activated up to 12-fold upon cotransfection of a DBP expression vector, whereas an HNF-1 alpha expression vector did not stimulate CYP7 gene activity. 5'-Deletion analyses and site-specific mutagenesis revealed that this stimulating effect of DBP can in part be ascribed to its functional interaction with DBP binding sites B (nts -115/-125), C (nts -172/-195), and D (nts -214/-230). C/EBP beta (LAP), another liver-enriched basic-leucine zipper transcription factor, bound to these same sites but effected a more modest increase in CYP7-directed gene transcription (up to 3-4-fold) when expressed in HepG2 cells. Competition for CYP7 promoter-binding sites between C/EBP, which undergoes < or = 2-fold diurnal change in rat liver, and the diurnally regulated DBP is proposed to determine the relative rates of basal versus diurnally regulated CYP7 gene transcription and thus may be a primary mechanism for setting the 3-6-fold amplitude that characterizes the circadian rhythm of liver CYP7 expression. Moreover, since DBP is first expressed in rat liver 3-4 weeks after birth, these findings may account for both the enhanced expression and the onset of the diurnal pattern of CYP7 enzyme levels at this stage of development.

Cimetidine prevents alcoholic hepatic injury in the intragastric feeding rat model

Cytochrome P450 induction is believed to be important in the pathogenesis of alcoholic hepatic disease. Because cimetidine is a general inhibitor of cytochrome P450 enzymes, it was hypothesized that it could be useful in preventing alcoholic hepatic injury. An intragastric feeding model was used these studies. Experimental animals were divided into groups of four to five rats/group and fed the following diets: corn oil+dextrose, corn oil+ethanol (CE) and corn oil+ethanol+cimetidine (250 mg kg-1 day-1) (CEC). The rats in each group were sacrificed at the following time intervals: 2 weeks, 1 month and 2 months. For each animal, the severity of the pathologic findings and relative protein levels of cytochromes P450 2E1, 2B and 4A were measured. In addition, plasma levels of thromboxane B2, 6-ketoprostaglandin F1 alpha and 8-isoprostane were also measured. The most significant finding was that cimetidine completely prevented alcoholic hepatic injury in this model system. The pathologic scores (an indication of the severity of injury) were significantly lower in the CEC groups compared with the CE group. There was however, no significant difference in cytochrome P450 2E1, 2B or 4A protein levels between CE and CEC groups. Thromboxane B2 and 8-isoprostane levels were significantly lower and 6-ketoprostaglandin F1 alpha, significantly higher in the CEC group than in the CE group. These results indicate that possible mechanisms involved in the protective action of cimetidine include inhibition of thromboxane production and lipid peroxidation.

Role of cellular glutathione and glutathione S-transferase in the expression of alkylating agent cytotoxicity in human breast cancer cells

Glutathione (GSH) and glutathione S-transferases (GSTs) play an important role in the protection of cells against toxic effects of many electrophilic drugs and chemicals. Modulation of cellular GSH and/or GST activity levels provides a potentially useful approach to sensitizing tumor cells to electrophilic anti-cancer drugs. In this study, we describe the interactions of four representative alkylating agents (AAs), melphalan, 4-hydroperoxy-cyclophosphamide (4HC), an an activated form of cyclophosphamide, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and cisplatin, with GSH and GST in the human breast cancer cell line MCF-7. Depletion of cellular GSH pools by approximately 80% by treatment of the cells with the GSH synthesis inhibitor buthionine sulfoximine (BSO) sensitized the tumor cells to each AA to a different extent, with dose-modifying factors of 2.39, 2.21, 1.64, and 1.27 observed for melphalan, 4HC, cisplatin, and BCNU, respectively. Treatment of the cells with the GST inhibitor ethacrynic acid (EA) failed to show any significant effects on the cytotoxicity of these AAs. However, EA did potentiate the cytotoxicity of melphalan when given in combination with BSO, an effect that may be due to a more complete depletion of cellular GSH levels by the combined modulator treatment. Following a 1-hr exposure to cytotoxic-equivalent concentrations of these AAs, GSH levels decreased substantially in the case of 4HC and BCNU, but increased by 30-50% in the case of cisplatin and melphalan. BSO pretreatment largely blocked this effect of cisplatin and melphalan on cellular GSH, while it further enhanced the GSH-depleting activity of both 4HC and BCNU. On the basis of these results, it is concluded that (a) GSH affects the cytotoxicity of different AAs to different extents, (b) basal GST expression in MCF-7 cells does not play a major role in AA metabolism, (c) EA can potentiate the enhancing effect of BSO on melphalan cytotoxicity in MCF-7 cells, and (d) depletion of cellular GSH by pretreatment with BCNU or cyclophosphamide may correspond to a useful strategy for enhancing the anti-tumor activity of other AAs given in a sequential combination.

Differential activation of cyclophosphamide and ifosphamide by cytochromes P-450 2B and 3A in human liver microsomes

The present study identifies the specific human cytochrome P-450 (CYP) enzymes involved in hydroxylation leading to activation of the anticancer drug cyclophosphamide and its isomeric analogue, ifosphamide. Substantial interindividual variation (4-9-fold) was observed in the hydroxylation of these oxazaphosphorines by a panel of 12 human liver microsomes, and a significant correlation was obtained between these 2 activities (r = 0.85, P < 0.001). Enzyme kinetic analyses revealed that human liver microsomal cyclophosphamide 4-hydroxylation and ifosphamide 4-hydroxylation are best described by a 2-component Michaelis-Menten model composed of both low Km and high Km P-450 4-hydroxylases. To ascertain whether one or more human P-450 enzymes are catalytically competent in activating these oxazaphosphorines, microsomal fractions prepared from a panel of human B-lymphoblastoid cell lines stably transformed with individual P-450 complementary DNAs were assayed in vitro for oxazaphosphorine activation. Expressed CYP2A6, -2B6, -2C8, -2C9, and -3A4 were catalytically competent in hydroxylating cyclophosphamide and ifosphamide. Whereas CYP2C8 and CYP2C9 have the characteristics of low Km oxazaphosphorine 4-hydroxylases, CYP2A6, -2B6, and -3A4 are high Km forms. In contrast, CYP1A1, -1A2, -2D6, and -2E1 did not produce detectable activities. Furthermore, growth of cultured CYP2A6- and CYP2B6-expressing B-lymphoblastoid cells, but not of CYP-negative control cells, was inhibited by cyclophosphamide and ifosphamide as a consequence of prodrug activation to cytotoxic metabolites. Experiments with P-450 form-selective chemical inhibitors and inhibitory anti-P-450 antibodies were then performed to determine the contributions of individual P-450s to the activation of these drugs in human liver microsomes. Orphenadrine (a CYP2B6 inhibitor) and anti-CYP2B IgG inhibited microsomal cyclophosphamide hydroxylation to a greater extent than ifosphamide hydroxylation, consistent with the 8-fold higher activity of complementary DNA-expressed CYP2B6 with cyclophosphamide. In contrast, troleandomycin, a selective inhibitor of CYP3A3 and -3A4, and anti-CYP3A IgG substantially inhibited microsomal ifosphamide hydroxylation but had little or no effect on microsomal cyclophosphamide hydroxylation. By contrast, the CYP2D6-selective inhibitor quinidine did not affect either microsomal activity, while anti-CYP2A antibodies had only a modest inhibitory effect. Overall, the present study establishes that liver microsomal CYP2B and CYP3A preferentially catalyze cyclophosphamide and ifosphamide 4-hydroxylation, respectively, suggesting that liver P-450-inducing agents targeted at these enzymes might be used in cancer patients to enhance drug activation and therapeutic efficacy.

Denitrosation of the anti-cancer drug 1,3-bis(2-chloroethyl)-1-nitrosourea catalyzed by microsomal glutathione S-transferase and cytochrome P450 monooxygenases

The alkylating agent BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea] can be inactivated through denitrosation reactions catalyzed by both cytosolic and microsomal enzymes. While previous studies have identified a class mu glutathione S-transferase [rat transferase 4-4 (Yb2)] as a major catalyst of the cytosolic denitrosation reaction, the enzymatic catalysts of BCNU denitrosation in microsomal membranes have not been identified. In the present study, both NADPH and glutathione (GSH) were found to support BCNU denitrosation catalyzed by isolated rat liver microsomes. Treatment of rats with the microsomal enzyme inducers phenobarbital and dexamethasone increased NADPH-dependent liver microsomal BCNU denitrosation up to fivefold without major effect on the GSH-dependent denitrosation activity. Although the NADPH-dependent activity was fully inhibited by antibody to NADPH-P450 reductase, purified NADPH-P450 reductase catalyzed BCNU denitrosation at rates that could only account for approximately 2-3% of the microsomal activity. Other experiments, including selective inhibition of NADPH-dependent microsomal BCNU denitrosation by chemical and antibody inhibitors of cytochrome P450, competitive inhibition of P450-catalyzed cyclophosphamide and ifosfamide activation by BCNU, and reconstitution of the denitrosation reaction by purified P450 enzyme 2B1 (major phenobarbital-inducible P450 form), established an important role for cytochrome P450 in BCNU denitrosation. By contrast, GSH-dependent microsomal BCNU denitrosation was unaffected by cytochrome P450 inhibitors, but was inhibited, with varying degrees of selectivity, by the microsomal glutathione S-transferase inhibitors ethacrynic acid, bromosulfophthalein, and indomethacin. These studies establish that BCNU inactivation can be catalyzed by two independent microsomal enzyme systems and suggest that therapeutically useful improvements in BCNU antitumor activity might be achieved through differential inhibition of these enzyme systems in tumor as compared to extratumoral sites.