CYP1B1 and endothelial nitric oxide synthase combine to sustain proangiogenic functions of endothelial cells under hyperoxic stress

We have recently shown that deletion of constitutively expressed CYP1B1 is associated with attenuation of retinal endothelial cell (EC) capillary morphogenesis (CM) in vitro and angiogenesis in vivo. This was largely caused by increased intracellular oxidative stress and increased production of thrombospondin-2, an endogenous inhibitor of angiogenesis. Here, we demonstrate that endothelium nitric oxide synthase (eNOS) expression is dramatically decreased in the ECs prepared from retina, lung, heart, and aorta of CYP1B1-deficient (CYP1B1(-/-)) mice compared with wild-type (CYP1B1(+/+)) mice. The eNOS expression was also decreased in retinal vasculature of CYP1B1(-/-) mice. Inhibition of eNOS activity in cultured CYP1B1(+/+) retinal ECs blocked CM and was concomitant with increased oxidative stress, like in CYP1B1(-/-) retinal ECs. In addition, expression of eNOS in CYP1B1(-/-) retinal ECs or their incubation with a nitric oxide (NO) donor enhanced NO levels, lowered oxidative stress, and improved cell migration and CM. Inhibition of CYP1B1 activity in the CYP1B1(+/+) retinal ECs resulted in reduced NO levels and attenuation of CM. In contrast, expression of CYP1B1 increased NO levels and enhanced CM of CYP1B1(-/-) retinal ECs. Furthermore, attenuation of CYP1B1 expression with small interfering RNA proportionally lowered eNOS expression and NO levels in wild-type cells. Together, our results link CYP1B1 metabolism in retinal ECs with sustained eNOS activity and NO synthesis and/or bioavailability and low oxidative stress and thrombospondin-2 expression. Thus CYP1B1 and eNOS cooperate in different ways to lower oxidative stress and thereby to promote CM in vitro and angiogenesis in vivo.

Cyp1b1 exerts opposing effects on intestinal tumorigenesis via exogenous and endogenous substrates

Cytochrome P450 1B1 (Cyp1b1) metabolism contributes to physiologic functions during embryogenesis but also to carcinogenic activation of polycyclic aromatic hydrocarbons (PAH). We generated Cyp1b1-deficient mice carrying the Min allele of the adenomatous polyposis coli gene. These Cyp1b1-deficient Min mice developed twice as many tumors as Min controls, which, however, remained similar in size and histology. Tumors from older (130 days) Cyp1b1-deficient Min mice selectively exhibited focal areas of nuclear atypia associated with less organized epithelia. The metabolism of endogenous substrates by Cyp1b1, therefore, suppresses tumor initiation but also affects progression. Treatment of Min mice with 7,12-dimethylbenzanthracene (DMBA) doubled both tumor multiplicity and size within 20 days but not when mice lacked Cyp1b1. This was paralleled by an abnormal staining of crypts with beta-catenin, phospho-IkappaB kinase, and RelA, which may represent an early stage of tumorigenesis similar to aberrant crypt formation. Cyp1b1 deletion did not affect circulating DMBA and metabolites. Cyp1b1 expression was higher in the tumors compared with normal small intestines. Increased tumorigenesis may, therefore, arise from generation of DMBA metabolites by Cyp1b1 in the developing tumors. Benzo(a)pyrene (BP), which is similarly activated by Cyp1b1 in vitro, did not affect tumorigenesis in Min mice. By contrast, BP and DMBA each suppressed tumor multiplicity in the absence of Cyp1b1. Cyp1b1 metabolism of DMBA and endogenous oxygenation products may each affect a tumor-promoting nuclear factor-kappaB activation, whereas Ah receptor activation by PAH affects suppression. Tumorigenesis may, therefore, depend on activation of PAH by Cyp1b1 and on offsetting suppression by Cyp1b1 of endogenous tumor-enhancing substrates.

Binding of steroidogenic factor-1 to the regulatory region might not be critical for transcriptional regulation of the human CYP1B1 gene

Cytochrome P450 (CYP) 1B1, which catalyzes 17beta-estradiol 4-hydroxylation, is expressed in steroid-related tissues including ovary, testis, and adrenal gland. Generally, the expressions of steroidogenic CYPs are transcriptionally regulated by steroidogenic factor-1 (SF-1) and cAMP response element (CRE) binding protein (CREB). In the present study, we examined the possibility that the human CYP1B1 gene might be regulated by SF-1 and CREB. Gel shift analyses revealed that in vitro translated SF-1 can bind to the putative SF-1 binding sites, SF-1a (at -1722) and SF-1b (at -2474), on the CYP1B1 gene. In vitro translated CREB barely binds to the putative SF-1 binding sites. Luciferase analysis revealed that a reporter plasmid, pGL3 (-2623/+25), containing the SF-1a and SF-1b elements is transactivated by the concomitant co-expression of SF-1 and protein kinase A (PKA). However, the transcriptional activity is induced by PKA alone. Mutations in the SF-1a and SF-1b elements did not affect the luciferase activity. Thus, the binding of SF-1 to the putative SF-1 binding sites of the human CYP1B1 gene might not be essential for transcriptional regulation. Interestingly, deletion and mutation analyses indicated that the PKA signaling pathway is involved in the xenobiotic responsive element (XRE)-mediated transactivation of the human CYP1B1 gene.

The predominant cAMP-stimulated 3 x 5 kb StAR mRNA contains specific sequence elements in the extended 3'UTR that confer high basal instability

cAMP stimulation of rodent steroidogenic cells produces two StAR transcripts, a major 3.5 kb and a minor 1.6 kb mRNA, differing only in their 3' untranslated regions (3' UTR). They exhibit very different responses to stimulation and removal of 8-Br-cAMP, with the 3.5 kb form increasing and declining much more rapidly than the 1.6 kb form. The 3' end of the 3.5 kb StAR mRNA contains three conserved AU-rich element (AURE) motifs that mediate fast mRNA turnover in over 900 genes in the human genome. In this paper, we explore post-transcriptional regulation in steroidogenic and non-steroidogenic cells using expression vectors containing StAR or luciferase with different StAR 3' UTRs. We show that the basal steady-state levels of StAR or luciferase protein and mRNA are five to eight times lower with the 3' UTR of 3.5 kb StAR compared with that of the 1.6 kb 3' UTR. Examination of transcript stability by direct mRNA transfection showed only a 1.5-fold increase in the rate of cytoplasmic decay of the 3.5 kb mRNA relative to the 1.6 kb mRNA. However, the long 3' UTR caused a fivefold decrease in the rate of appearance of mature cytoplasmic mRNA despite transcription from the same promoter. This is attributed to less efficient nuclear processing of immature transcripts prior to export to cytoplasm. Selective 3' UTR sequence substitutions, deletions, and mutations showed that this loss of expression is produced additively by specific sequences in a 700-base basal instability region and by non-specific length effects. These mechanisms are selectively enhanced in steroidogenic cells. The AURE contribute a smaller basal destabilization effect selective for steroidogenic cells that is removed by their mutations. Inclusion of introns in the 3.5 kb StAR vector enhances StAR expression, suggesting the effects of introns complexes on nuclear processing. Br-cAMP provides an additional means to rapidly modulate StAR expression independent of transcription by attenuating the nuclear and cytoplasmic instability mechanisms within the extended 3' UTR.

Mitochondrial processing of bovine adrenal steroidogenic acute regulatory protein

Steroidogenic acute regulatory (StAR) protein is an important regulatory protein in steroidogenesis and rapidly undergoes proteolysis after import into the mitochondria. In this study, we determined the proteolytic cleavage sites and investigated the effects on the stimulation of steroidogenic activity of the blockage of these sites by mutation. The cleaved StAR proteins, which were purified using an anti-StAR immobilized column, reacted with antiserum against the StAR C-terminal oligopeptide. The molecular weights of the purified proteins were determined by MALDI-TOF mass spectrometry, and were found to be identical to those of the 40-285 and 55-285 amino-acid-regions of the StAR protein. To confirm the identification of the cleavage sites, we constructed site-directed mutants of bovine StAR cDNA, which contained the amino acids R37A/R38A/L40A and/or R53A/R54A/R55A. These mutant StAR proteins expressed in COS-1 cells were not cleaved at positions 39-40 and 54-55, and were processed at sites different from those in the wild-type StAR protein. These mutant proteins stimulated pregnenolone formation at almost the same rate as the wild-type StAR protein in COS-1 cells, which suggests that the cholesterol transfer activity was not affected by the mutation.

Liver X receptor opens a new gateway to StAR and to steroid hormones

Liver X receptors (LXRs) broadly limit cholesterol accumulation by regulating expression of genes involved in cholesterol efflux and storage. In this issue of the JCI, Cummins et al. report that LXRalpha is involved in similar regulation in the adrenal cortex, but it also substantially modulates glucocorticoid synthesis (see the related article beginning on page 1902). LXRalpha deletion in mice increases the availability of adrenal cholesterol for steroid synthesis by decreasing the expression of cholesterol efflux transporters. Glucocorticoid synthesis requires intramitochondrial cholesterol transport mediated by the steroidogenic acute regulatory protein (StAR). Surprisingly, LXR deletion and stimulation by an agonist each increase glucocorticoid synthesis. This parallels increased expression of StAR and several other steroidogenic genes.

Benzo(a)pyrene and 7,12-dimethylbenz(a)anthrecene differentially affect bone marrow cells of the lymphoid and myeloid lineages

Polycyclic aromatic hydrocarbons (PAHs) are common environmental contaminants that are carcinogenic and immunosuppressive. Benzo(a)pyrene (BP) and 7,12-dimethylbenz(a)anthracene (DMBA) are two prototypic PAHs known to impair the cell-mediated and humoral immune responses. We have previously shown that, in C57BL/6J mice, total bone marrow (BM) cellularity decreased two-fold following intraperitoneal DMBA treatment but not BP treatment. Here, we have used flow cytometry to demonstrate that BP and DMBA differentially alter the lymphoid and myeloid lineages. Following DMBA treatment, the pro/pre B-lymphocytes (B220(lo)/IgM(-)) and the immature B-lymphocytes (B220(lo)/IgM(+)) significantly decreased, while the mature B-lymphocytes (B220(hi)/IgM(+)) remained unaffected. In contrast, BP treatment decreased the pro/pre B-lymphocytes, and did not affect the immature B-lymphocytes or mature B-lymphocytes. The Gr-1(+) cells of the myeloid lineage were depleted 50% following DMBA treatment and only minimally depleted following BP treatment. Interestingly, the monocytes (7/4(+)1A8(lo)) and neutrophils (7/4(+)1A8(hi)) within this Gr-1(+) population were differentially affected by these PAHs. Monocytes and neutrophils were depleted following DMBA treatment whereas neutrophils decreased and monocytes increased following BP treatment. Although TNFalpha and CYP1B1 are implicated as essential mediators of hypocellularity, the similar induction of TNFalpha mRNA and CYP1B1 mRNA in the BM by BP and DMBA suggests that they are not limiting factors in mediating the different effects of these PAHs. Given that similar amounts of BP and DMBA reach the BM when administered intraperitoneally, their differential effects on the lymphoid and myeloid lineages probably stem from differences in reactive metabolites such as PAH quinones and PAH-dihydrodiol-epoxides.

Microarray analysis of early adipogenesis in C3H10T1/2 cells: cooperative inhibitory effects of growth factors and 2,3,7,8-tetrachlorodibenzo-p-dioxin

C3H10T1/2 mouse embryo fibroblasts differentiate into adipocytes when stimulated by a standard hormonal mixture (IDMB). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), via the aryl hydrocarbon receptor (AhR), inhibits induction of the key adipogenic gene peroxisome proliferator-activated receptor gamma (PPARgamma) and subsequent adipogenesis. This TCDD-mediated inhibition requires activation of the extracellular signal-regulated kinase (ERK) pathway, which can be accomplished by serum, epidermal growth factor (EGF), or fibroblast growth factor (FGF). In the absence of serum or growth factors, IDMB induced adipogenesis without mitosis. Microarray analysis identified 200 genes that exhibited expression changes of at least twofold after 24 h of IDMB treatment. This time precedes most PPARgamma stimulation but follows the period of TCDD/ERK cooperation and periods of increased cell contraction and DNA synthesis. Functionally related gene clusters include genes associated with cell structure, triglyceride and cholesterol metabolism, oxidative regulation, and secreted proteins. In the absence of growth factors TCDD inhibited 30% of these IDMB responses without inhibiting the process of differentiation. A combination of EGF and TCDD that blocks differentiation cooperatively blocked a further 44 IDMB-responsive genes, most of which have functional links to differentiation, including PPARgamma. Cell cycle regulators that are stimulated by EGF were substantially inhibited by IDMB but these responses were unaffected by TCDD. By contrast, TCDD and EGF cooperatively reversed IDMB-induced changes in cell adhesion complexes immediately prior to increases in PPARgamma1 expression. Changes in adhesion-linked signaling may play a key role in TCDD affects on differentiation.

Differentiation of pluripotent C3H10T1/2 cells rapidly elevates CYP1B1 through a novel process that overcomes a loss of Ah Receptor

Stimulation of C3H10T1/2 cells by an adipogenic hormonal mixture (IDM) consisting of insulin (I), dexamethasone (D), and methylisobutylxanthine (M) substantially induces cytochrome P450 (CYP) 1B1 expression. This stimulation represents up to 40% of the level produced by maximum activation of the arylhydrocarbon receptor (AhR) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dexamethasone and methylisobutylxanthine in combination produced near maximum elevation of CYP1B1 along with a subsequent decline in AhR that paralleled the rise in peroxisome proliferator-activated receptorgamma1 (PPARgamma1). Inhibitors of AhR activity, which block TCDD induction, did not affect this increase of CYP1B1 expression, which was, therefore, independent of AhR activity. These responses were unaffected by inhibition of DNA synthesis, which was required for PPARgamma1 induction and terminal differentiation. Induction of CYP1B1 mRNA was paralleled by increased CYP1B1 promoter-luciferase reporter activity. The initial 0.8kb of promoter region, which was sufficient for 24h near maximum stimulation, did not contain either the key AhR-responsive elements that mediate the TCDD response or CREB and SF1 elements that mediate cAMP stimulation of rat CYP1B1 in steroidogenic cells. This reporter response to IDM stimulation, but not to TCDD, was maintained in AhR-null fibroblasts. CYP1B1 expression, unlike TCDD induction, was stimulated by IDM in only about half the cells. CYP1B1 expression partially overlapped with PPARgamma expression, which was also inversely related in clonal sub-lines. CYP1B1 expression may, therefore, represent an early stage of differentiation that requires factors associated with DNA synthesis to subsequently generate PPARgamma1.

Rodent StAR mRNA is substantially regulated by control of mRNA stability through sites in the 3'-untranslated region and through coupling to ongoing transcription

The steroidogenic acute regulator (StAR) gene is transcribed to 1.6 kb and 3.5 kb mRNAs that differ only through the length of the 3'-untranslated region (3'-UTR). These forms result from alternative polyadenylation sites in exon 7. These sites are utilized similarly in unstimulated adrenal cells whereas Br-cAMP selectively stimulates 3.5 kb mRNA. After removal of Br-cAMP, 3.5 kb mRNA declines rapidly (t(1/2) = 2 h) while 1.6 kb mRNA responds more slowly. This selective degradation is more evident in testis MA10 cells and is seen even in the presence of Br-cAMP. Transfection of Y-1 cells with CMV promoted StAR vectors confirmed that the 3.5 kb form is less stable and that Br-cAMP slowly increases this instability. Basal instability resides solely in the extended 3'-UTR which contains AU-rich elements. Br-cAMP enhances this degradation of 3.5 kb mRNA but additionally requires translated and 5'-UTR sequences. Degradation of both forms is arrested by inhibitors of transcription or translation, indicating that mRNA stability is coupled to these processes independent of the extended 3'-UTR. Br-cAMP stimulates substantial selective synthesis of 3.5 kb StAR mRNA despite this instability. The preferential generation of the unstable form may facilitate rapid increases and decreases of StAR activity in response to external changes.

Role of cytochrome p4501 family members in the metabolic activation of polycyclic aromatic hydrocarbons in mouse epidermis

Polycyclic aromatic hydrocarbons (PAHs) are known to be activated by the cytochrome P450 (P450) 1 family. However, the precise role of individual P4501 family members in PAH bioactivation remains to be fully elucidated. We therefore investigated the formation of PAH-DNA adducts in the epidermis of Cyp1a2(-/-), Cyp1b1(-/-), and Ahr(-/-) knockout mice. A panel of different PAHs was used, ranging in carcinogenic potency. Mice were treated topically on the dorsal skin with the following tritium-labeled PAHs: dibenzo[a,l]pyre-ne (DB[a,l]P), 7,12-dimethylbenz[a]anthracene (DMBA), benzo[a]pyrene (B[a]P), dibenzo[a,h]anthracene (DB[a,h]A), benzo[g]chrysene (B[g]C), and benzo[c]phenanthrene (B[c]P). At 24 h after treatment, mice (two male and two female mice per group) were sacrificed, and epidermal DNA was isolated and hydrolyzed with DNase I; subsequently, DNA adducts were quantitated by liquid scintillation counting. In the DB[a,l]P-treated mice, levels of DNA adducts were significantly lower in Cyp1a2(-/-) and Cyp1b1(-/-) mice by 57 and 46%, respectively, as compared to wild-type (WT) mice (C57BL/6 background). The levels of DB[a,l]P DNA adducts formed in Ahr(-/-) mice were 26% lower, but this was not statistically significant. The levels of DMBA-DNA adducts in Cyp1a2(-/-) mice were not different than the WT mice but were significantly lower in Cyp1b1(-/-) and Ahr(-/-) mice by 64 and 52%, respectively. DMBA-DNA adduct samples were further analyzed by HPLC following further digestion to deoxyribonucleosides. HPLC analysis of individual DMBA-DNA adducts revealed differences in the ratio of syn-DMBA-diol epoxide- to anti-DMBA-diol epoxide-derived adducts in the Ahr(-/-) and Cyp1b1(-/-) mice. The ratio of syn-/anti-derived adducts in WT mice was 0.49. This ratio was 0.23 in the Cyp1b1(-/-) mice and 0.87 in the Ahr(-/-) mice. In contrast to the results with DB[a,l]P and DMBA, the levels of B[a]P-, DB[a,h]A-, B[g]C-, and B[c]P-DNA adducts were significantly lower in Ahr(-/-) mice by 73, 75, 50, and 81%, respectively, as compared to WT mice but were not significantly lower in the Cyp1a2(-/-) or Cyp1b1(-/-) mice. Collectively, these and other results support a role for both P4501A1 and P4501B1 in the bioactivation of DMBA; P4501A2, P4501B1, and possibly P4501A1 in the bioactivation of DB[a,l]P; and P4501A1 in the bioactivation of B[a]P, DB[a,h]A, B[g]C, and B[c]P in mouse epidermis. Furthermore, in the metabolic activation of DMBA in mouse epidermis, P4501B1 shows a preference for the formation of syn-DMBA-diol epoxide adducts, whereas P4501A1 shows a preference for the formation of anti-DMBA-diol epoxide adducts.

Identification of novel TCDD-regulated genes by microarray analysis

TCDD exposure of multipotential C3H10T1/2 fibroblasts for 72 h altered the expression of over 1000 genes, including coordinated changes across large functionally similar gene clusters. TCDD coordinately induced 23 cell cycle-related genes similar to epidermal growth factor (EGF)-induced levels but without any affect on the major mitogenic signaling pathway (extracellular signal-regulated kinase, ERK). TCDD treatment also decreased glycolytic and ribosomal clusters. Most of these TCDD-induced changes were attenuated by the presence of EGF or an adipogenic stimulus, each added during the final 24 h. TCDD prevented 10% of EGF-induced gene responses and 40% of adipogenic responses. Over 100 other genes responded to TCDD during adipogenesis. This group of responses included complete suppression of three proliferins and stimulations of several cytokine receptors. Despite these varied secondary effects of TCDD, direct AhR activation measured by integrated AhR-responsive luciferase reporters was similar under quiescent, EGF-stimulated or adipogenic conditions. Only 23 genes were similarly induced by TCDD regardless of conditions and 10 were suppressed. These 23 genes include: 4 genes previously recognized to contain AhR response elements (cytochrome P450 (CYP) 1B1, CYP1A1, NAD(P)H quinone reductase 1 (NQO1), and aldehyde dehydrogenase 3A1); two novel oxidative genes (alcohol dehydrogenase 3 and superoxide dismutase 3); and glypican 1, a plasma membrane proteoglycan that affects cell signaling. Further experiments demonstrated that TCDD maximally induced NQO1, glypican 1 and alcohol dehydrogenase 3 by 6 h. Glypican 1 activates the actions of many growth factors and therefore may contribute to secondary effects on gene expression.

Induction of CYP1A1 and CYP1B1 in liver and lung by benzo(a)pyrene and 7,12-d imethylbenz(a)anthracene do not affect distribution of polycyclic hydrocarbons to target tissue: role of AhR and CYP1B1 in bone marrow cytotoxicity

Polycyclic aromatic hydrocarbons (PAHs) (50 mg/kg, i.p.) selectively deplete mouse bone marrow (BM) hematopoietic cells through a process that is dependent on CYP1B1. 7,12-dimethylbenz(a)anthracene (DMBA), which forms greater amounts of dihydrodiol-epoxide-DNA adducts in BM, is much more effective in depleting BM cells than benzo(a)pyrene (BP). BM toxicity by BP is restored in congenic mice expressing a weakly responsive aryl hydrocarbon receptor (AhR(d) replaces AhR(b)). BP strongly induces CYP1A1 around the hepatic vein whereas DMBA produces a weaker diffuse response, paralleling differences in CYP1A1 protein. These responses are absent in AhR(d) mice. BP and DMBA broadly and equally induce CYP1A1 in the lung, while CYP1B1 is induced in bronchial blood vessels. In sternum, CYP1B1 is induced in BM and white fat, whereas CYP1A1 is induced only in brown fat. BP and DMBA levels were similar within blood, lung, and BM and did not rise in AhR(d) mice. In liver, selective decrease of BP was consistent with induced metabolism via CYP1A1, which nevertheless does not determine the blood levels and distribution to BM. Effective delivery of BP to BM is indicated by formation of BP-quinone DNA adducts and the effective induction of CYP1B1. The low formation of BP-dihydrodiol-epoxide-DNA adducts suggests effective AhR induction of BM detoxifying reactions that prevents their formation from dihydrodiols. These findings contrast with the substantial hepatic CYP1A1 contribution for PAHs previously seen for intragastric administration where first pass elimination limits the amount of PAHs reaching the BM.

Close proximity, but not VLA-4-dependent adherence between pre-B cells and bone marrow stromal cells, is required for DMBA-induced apoptosis of pre-B cells in vitro

We have previously reported that 7,12-dimethylbenz[a]anthracene (DMBA) induced apoptosis in precursor B lymphocytes (pre-B cells) only when they were co-cultured with bone marrow stromal (BMS) cells. The goal of this research was to determine whether this process was dependent on the adherence of the pre-B cells and stromal cells. Conditioned media from DMBA-treated BMS cells induced apoptosis in pre-B cells, but only when the pre-B cells were co-cultured with stromal cells. This result suggested that stromal cells may release a soluble factor that initiates apoptosis, but their presence was still required for apoptosis. When the stromal cells and pre-B cells were separated with a membrane filter insert, DMBA-induced apoptosis of the pre-B cells was blocked suggesting that contact with or close proximity to stromal cells was required for apoptosis. The addition of an anti-VLA-4 Mab disrupted adherence of pre-B cells to the stromal cell monolayer, but did not diminish the numbers of apoptotic pre-B cells. The results of this study support the hypothesis stromal cells and pre-B cells must be in close proximity for apoptosis to occur, but direct interaction via VLA-4 and VCAM-1 is unlikely to be required for this response.

Steroidogenic factor-1 interacts with cAMP response element-binding protein to mediate cAMP stimulation of CYP1B1 via a far upstream enhancer

CYP1B1 activates polycyclic aromatic hydrocarbon carcinogens in cAMP-regulated tissues such as the adrenal, ovary, and testis. A 27-fold cAMP stimulation of the CYP1B1-luciferase reporter in Y-1 adrenal cells depends entirely on a far upstream enhancer region (FUER; -5298 to -5110). Cooperative participation of multiple steroidogenic factor 1 (SF-1) elements with the downstream cAMP response element (CRE) in FUER is essential for both basal and cAMP-stimulated activities of FUER. Basal and induced activities were similarly lowered by DAX-1, an SF-1 suppressor, and raised by steroid receptor coactivator 1, an SF-1 coactivator. cAMP response element-binding protein (CREB)-binding protein (CBP) that interacts preferentially with the phosphorylated-CREB increased the cAMP-induced FUER. 10T1/2 cells and human embryonic kidney (HEK)293 cells do not express SF-1. Introduction of exogenous SF-1 generated cAMP stimulation of the FUER in 10T1/2 fibroblasts. The same transfection only increased basal activity of FUER in HEK293 cells, despite presence of active CREB in cells. HEK293 cells therefore remain deficient in additional factor(s) critical to the cAMP stimulation of CYP1B1. Mutations of the protein kinase A (PKA) and the mitogen-activated protein kinase phosphorylation sites (Ser-430 and Ser-203) on SF-1 had no effect on the SF-1-dependent FUER stimulation in Y-1 and 10T1/2 cells. This contrasts with loss of activity with mutation of CREB at PKA phosphorylation site (Ser-133). SF-1 phosphorylation at these sites is therefore not essential for the cAMP stimulation and the cooperation with CREB. cAMP-enhanced activation protein 1 (AP-1) and stimulatory protein 1 (Sp1) complexes in the proximal promoter region contributed substantially to both basal and cAMP-stimulated FUER activity. Chromatin immunoprecipitation from primary rat adrenal cells demonstrated cAMP stimulation of histone acetylation proximal to, respectively, the FUER and AP-1 sites of CYP1B1.

Disruption of cell-cell contact maximally but transiently activates AhR-mediated transcription in 10T1/2 fibroblasts

The aryl hydrocarbon receptor (AhR) is activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but activation without an exogenous ligand also occurs when normal cell-cell contact is prevented. Suspension of several C3H10T1/2 fibroblast clonal sub-lines that contain an integrated AhR-responsive reporter produced a time course and level of reporter activation and CYP1B1 induction that paralleled TCDD stimulation in confluent monolayer culture. Suspension activation was, however, more transient. Loss of cell-cell contact at low density also activated these reporters independent of cell cycle changes to levels comparable to TCDD stimulation of confluent cells. Loss of cell-cell contact may, therefore, activate AhR. Suspension and TCDD activations exhibited comparable nuclear translocation of AhR and then AhR/ARNT complex formation. Each AhR activation process was equally attenuated by inhibition of, respectively, HSP90 ATPase, the 26S proteosome, and by depletion of intracellular Ca2+. By contrast, the AhR antagonist alpha-naphthoflavone (alphaNF) blocked ligand-stimulated AhR activity, but not activation through loss of cell-cell contact. Suspension-induced reporter activation was selectively enhanced by LiCl, which prevented GSK-3beta effects on the simultaneously released beta-catenin. The effects of suspension and LiCl on reporters were reversed by Ro-31-8220, which did not affect beta-catenin, TCDD-activation processes, or AhR turnover. Neither LiCl nor Ro-31-8220 altered suspension-induced AhR/ARNT complex formation. Loss of cell-cell contact permits nuclear translocation and AhR activation that is largely replicated after TCDD binding, but with activity differences due to contact-sensitive factors functioning after AhR/ARNT complex formation.

Multiple contributions from long-chain fatty acid metabolism in Y-1 and MA-10 cells

Acute steroidogenesis in either Y-1 or MA-10 cells is sensitive to different effects of fatty acids compare to a chronic stimulation. A 3-h stimulation of StAR expression in both cell types was completely blocked by NDGA and AA861, each functioning as lipoxygenase inhibitors. However, the acute 15-min stimulation in Y-1 cells was inhibited by these agents by distinct mechanisms. The inhibition by NDGA was reversed by arachidonic, linoleic, and oleic acids. The inhibition by AA861 was insensitive to reversal by these acids. StAR expression was not affected by these short-term inhibitor treatments. These more rapid fatty acid reversible effects of NDGA are consistent with previously reported inhibition of mitochondrial Acyl CoA ligase. This may function in cooperation with StAR and PBR in providing fatty acid regulation of mitochondrial cholesterol transport. The acute effect of AA861 supports the involvement of an NDGA-insensitive lipoxygenase in the acute stimulation of mitochondrial cholesterol metabolism. The activity of MA-10 cells during prolonged treatments with cAMP appears to utilize each of these processes, which depend on different metabolism of fatty acids.

Linked expression of Ah receptor, ARNT, CYP1A1, and CYP1B1 in rat mammary epithelia, in vitro, is each substantially elevated by specific extracellular matrix interactions that precede branching morphogenesis

Cytochrome P4501B1 (CYP1B1), the major constitutively expressed CYP in the rat mammary gland, is induced by Ah-receptor (AhR) ligands, while CYP1A1 is predominantly expressed only after induction. These CYPs contribute to carcinogenic activation of polycyclic aromatic hydrocarbons (PAHs). AhR, ARNT, and CYP1B1 were only weakly expressed, even after 2,3,7,8-tetrachlorodibenzo-p-dioxin induction, when rat mammary epithelial cells (RMEC) were cultured on plastic. RMEC cultured on the extracellular matrix (ECM), Matrigel, or on a floating gel of collagen I demonstrated branching morphogenesis and substantially increased basal CYP1B1 and induced CYP1A1 expression, in parallel with large increases in AhR and ARNT expression. Branching was more pronounced in the Wistar Kyoto than in the Wistar Furth rat strain. Although EGF enhanced branching, neither strain nor growth factor treatment substantially impacted CYP expression. Increased AhR and ARNT expression is observed within 24 h of dispersal on Matrigel, substantially prior to branch formation. Culture on thin layers of collagen I, collagen IV, and laminin, respectively, failed to reproduce the branching morphogenesis or increases in AhR, ARNT, or CYP expression. However, adherent, gelled collagen I recapitulated the increased protein expression, without supporting branching. This increased protein expression was closely paralleled by enhanced expression of beta-catenin and E-cadherin, components of cell-cell adhesion complexes. A synthetic peptide that selectively antagonizes integrin-ECM interactions reduced branch formation, without diminishing AhR, ARNT, and CYP expression. These data demonstrate that early ECM surface adhesion interactions mediate AhR and ARNT expression, which enhances CYP expression, independent of branching morphogenesis.

7-12 Dimethylbenz[a]anthracene-induced bone marrow hypocellularity is dependent on signaling through both the TNFR and PKR

In addition to being carcinogenic, polycyclic aromatic hydrocarbons (PAHs) are known to cause deleterious effects on the immune system, including a marked reduction in bone marrow granulocytes and B lymphocytes. The molecular mechanisms underlying bone marrow hypocellularity are incompletely understood. Hematopoiesis is governed by the production of cytokines and the resultant signaling pathways that they initiate. Our hypothesis was that PAHs may disrupt cytokine production in the bone marrow resulting in the perturbation in bone marrow cellularity observed after PAH administration. TNF-alpha and IFN-gamma are two cytokines that are involved in the regulation of hematopoiesis. Based on observations made in previous research, we sought to determine if the effects of 7-12 dimethylbenz[a]anthracene (DMBA) on the murine bone marrow were mediated through the actions of these molecules. Transgenic mice that were null for either IFN-gamma or TNF-alpha receptors were injected with DMBA and the resulting bone marrow cellularity compared with wild-type mice. We observed that tumor necrosis factor alpha receptor (TNFR) null mice were protected against DMBA-induced bone marrow hypocellularity, while IFN-gamma null mice were not. In addition, we found that dsRNA-dependent protein kinase (PKR) null mice were also protected from DMBA-induced hypocellularity. PKR is an intracellular signaling molecule that has been demonstrated to be activated by TNFR-mediated signaling. Furthermore, we observed upregulation of PKR in the bone marrow after DMBA administration that was dependent on signaling through TNFR. These results point to a role for TNFR-dependent signaling, operating at least in part via PKR activation, as a mechanism for DMBA-induced bone marrow toxicity.

TCDD administration after the pro-adipogenic differentiation stimulus inhibits PPARgamma through a MEK-dependent process but less effectively suppresses adipogenesis

Hormone (IDMB)-induced adipogenesis in C3H10T1/2 cells is suppressed by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the aryl hydrocarbon receptor (AhR). We have previously reported that TCDD addition 48 h before the hormonal stimulation of IDMB suppresses a key mediator of adipogenesis, the peroxisome proliferator-activated receptor (PPARgamma), by a MEK/ERK dependent mechanism. Here we add to previous evidence that this synergism functions after IDMB addition but before increased PPARgamma1 transcription. Suppression remains effective and MEK/ERK dependent when TCDD is added 6-12 h after IDMB addition but not when delayed to 16-24 h, thus preceding the rise in PPARgamma mRNA. TCDD suppression of the number of committed adipocytes and of triglyceride formation is less effective with the delayed addition. TCDD therefore does not directly suppress the expression of the key mediator PPARgamma1. An alternative mediation of adipocyte commitment is apparently less sensitive to the 6-12 h of delayed TCDD addition. TCDD suppression potencies (EC(50) = 50 pM) match the potencies for stimulation of CYP1B1 protein and AhR-sensitive reporters. The AhR antagonist 3'-methoxy-4'-nitroflavone (3-MNF) inhibited both TCDD-mediated CYP1B1 induction and inhibition of PPARgamma protein expression. This antagonism was only effective when 3-MNF was present in the 24-h period after IDMB addition. TCDD activation of AhR in conjunction with MEK/ERK therefore generates PPARgamma1 suppression activity before the increase of PPARgamma1 synthesis. The potency and inhibition data are consistent with induction of one or more gene products that sustain suppression through the extended period of PPARgamma1 transcription.

PPAR?1 synthesis and adipogenesis in C3H10T1/2 cells depends on S-phase progression, but does not require mitotic clonal expansion

Adipogenesis is typically stimulated in mouse embryo fibroblast (MEF) lines by a standard hormonal combination of insulin (I), dexamethasone (D), and methylisobutylxanthine (M), administered with a fresh serum renewal. In C3H10T1/2 (10T1/2) cells, peroxisome proliferator-activated receptor gamma1 (PPARgamma1) expression, an early phase key adipogenic regulator, is optimal after 36 h of IDM stimulation. Although previous studies provide evidence that mitotic clonal expansion of 3T3-L1 cells is essential for adipogenesis, we show, here, that 10T1/2 cells do not require mitotic clonal expansion, but depend on cell cycle progression through S-phase to commit to adipocyte differentiation. Exclusion of two major mitogenic stimuli (DM without insulin and fresh serum renewal) from standard IDM protocol removed mitotic clonal expansion, but sustained equivalent PPARgamma1 synthesis and lipogenesis. Different S-phase inhibitors (aphidicolin, hydroxyurea, l-mimosine, and roscovitin) each arrested cells in S-phase, under hormonal stimulation, and completely blocked PPARgamma1 synthesis and lipogenesis. However, G2/M inhibitors effected G2/M accumulation of IDM stimulated cells and prevented mitosis, but fully sustained PPARgamma1 synthesis and lipogenesis. DM stimulation with or without fresh serum renewal elevated DNA synthesis in a proportion of cells (measured by BrdU labeling) and accumulation of cell cycle progression in G2/M-phase without complete mitosis. By contrast, standard IDM treatments with fresh serum renewal caused elevated DNA synthesis and mitotic clonal expansion while achieved equivalent level of adipogenesis. At most, one-half of the 10T1/2 mixed cell population differentiated to mature adipocytes, even when clonally isolated. PPARgamma was exclusively expressed in the cells that contained lipid droplets. IDM stimulated comparable PPARgamma1 synthesis and lipogenesis in isolated cells at low cell density (LD) culture, but in about half of the cells and with sensitivity to G1/S, but not G2/M inhibitors. Importantly, growth arrest occurred in all differentiating cells, while continuous mitotic clonal expansion occurred in non-differentiating cells. Irrespective of confluence level, 10T1/2 cells differentiate after progression through S-phase, where adipogenic commitment induced by IDM stimulation is a prerequisite for PPARgamma synthesis and subsequent adipocyte differentiation.

Bone marrow cytotoxicity of benzo[a]pyrene is dependent on CYP1B1 but is diminished by Ah receptor-mediated induction of CYP1A1 in liver

We have previously used CYP1B1-null mice to demonstrate that dimethylbenz(a)anthracene (DMBA) requires CYP1B1 for bone marrow (BM) toxicity. Benzo(a)pyrene (BP), a much more potent Ah receptor ligand, shows very different responses that nevertheless depend on CYP1B1. Wild-type (AhR(b)) mice treated with DMBA for 48 h exhibit a large loss in BM cellularity and disruption of marrow structure that is not seen for BP treatment. In congenic mice with a low affinity AhR (AhR(d)), DMBA and BP are equally toxic to the BM whereas AhR(d) x CYP1B1-null mice are fully protected. In situ hybridization demonstrates that CYP1B1 mRNA is constitutively expressed in marrow cells and is induced by PAHs according to their AhR affinity (BP>DMBA), including lower levels in AhR(d) mice. Importantly, expression of CYP1A1 mRNA was undetectable in BM. In wild-type mice, BP treatment leads to a fivefold greater induction of hepatic CYP1A1 than that of DMBA treatment. Neither induction occurs in AhR(d) mice. Thus, hepatic metabolism may prevent BP from reaching the BM, where it can be bioactivated by CYP1B1. Flow cytometric analyses of BM cells showed that there were decreases in granulocytes and lymphocytes following DMBA treatment, but not after BP treatment. These data suggest that there is an inverse relationship between liver metabolism and BM toxicity resulting from limitations on the delivery of PAH to CYP1B1 present in BM, where only very low constitutive levels are needed.

Ah receptor regulation of mouse Cyp1B1 is additionally modulated by a second novel complex that forms at two AhR response elements⋆

Cyp1B1 is expressed constitutively in many extrahepatic cells and is induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). An enhancer region (AhER-810 to -1075 of the mouse Cyp1B1 promoter), which mediates aryl hydrocarbon receptor (AhR) regulation of transcription, contains three consensus XRE sequences (designated XRE1, XRE4, and XRE5) and a central Ebox. XRE5 is essential for both basal and induced activity in C3H10T1/2 cells. AhR/ARNT binding to XRE1, XRE4, and the Ebox complex function in combination to support the AhR/ARNT complex at XRE5. The identical 12 base cores of XRE1 and XRE4 differ from the core of XRE5 by two bases outside of the consensus XRE. These sites bind a constitutive complex slightly smaller than AhR/ARNT (anomalous complex; anC), which is not formed at XRE5 or six Cyp1A1 XREs. Exchange of these bases (m3 mutations) restores selective AhR/ARNT binding at XRE1/XRE4 and introduces anC binding at XRE5. The activities of multimeric XRE1 and XRE5 luciferase reporters responded in parallel to the extent of AhR/ARNT binding. The consensus anC binding sequence ((C/T)GCG(C/T)GCGC(C/A)GC) overlaps the XRE1/XRE4 AhR/ARNT element. Gel mobility analyses show that anC binds to XRE1/XRE4 under basal conditions, while AhR/ARNT partially displaces anC following TCDD induction. Selective depletion of anC with biotin-oligonucleotides increases AhR/ARNT binding. M3-mutations at, respectively, XRE1 and XRE4 of the AhER sequence, had opposite effects on luciferase reporters. Activities increased for the XRE1 mutation and decreased for the XRE4 mutation, but also depended on the level of AhR transfected into AhR -/- fibroblasts. AnC compete with AhR at XRE1 while playing an activating role at XRE4. This positive effect of constitutive anC binding at XRE4 may contribute to the characteristic basal Cyp1B1 expression in embryo fibroblasts, which is mediated by low constitutive activities of AhR.

Cell selective cAMP induction of rat CYP1B1 in adrenal and testis cells. Identification of a novel cAMP-responsive far upstream enhancer and a second Ah receptor-dependent mechanism

CYP1B1 is unique among P450 cytochromes in exhibiting inductive responses mediated by both the Ah receptor (AhR) and cAMP. cAMP induction was mediated either by a 189bp far upstream enhancer region (FUER, -5110 to -5298) or by a 230bp AhR-responsive enhancer region (AhER) (-797 to -1026). CYP1B1 luciferase reporters respond selectively to cAMP and TCDD in adrenal Y-1 cells (only cAMP), testis MA10 cells (cAMP>TCDD), and C3H10T1/2 mouse embryo fibroblasts (only TCDD). In Y-1 cells, which lack AhR, cAMP induction is totally dependent on the FUER, including absolute requirements for upstream and downstream halves of this region, and for CREB activity at a CRE sequence located at the 3(')-end. cAMP stimulation of the FUER was remarkably high (27-fold) and equally effective when linked to an HSV-TK promoter, indicating direct cAMP activation of the FUER. Binding of CREB to the essential CRE was demonstrated along with dominant negative effects of functionally impaired mutants. cAMP induction in MA10 cells was partially mediated by the FUER mechanism but was regulated additionally by AhER through AhR activity. MA10 cells also exhibit cAMP-dependent AhR down-regulation and AhR/Arnt complex formation. Mutations in AhER including XRE5 were similarly inhibitory to cAMP stimulation in MA10 cells and to TCDD stimulation in C3H10T1/2 cells. Transfection of AhR into the AhR-deficient Y-1 cells did not introduce this second mechanism, which indicated a need for additional components that are present in MA10 cells.

7,12-Dimethylbenz[a]anthracene-induced bone marrow toxicity is p53-dependent

Polycyclic aromatic hydrocarbons (PAHs) are known immunotoxins and carcinogens. Our laboratory and others have demonstrated that metabolism of these compounds by CYP1B1 is required for carcinogenicity and immunotoxicity to occur. Previously, our laboratory reported significantly decreased bone marrow cellularity in mice following 7,12-dimethlybenz[a]anthracene (DMBA) administration. In addition, we have observed that DMBA causes apoptosis via activation of both caspase-8 and -9 in pre-B cells co-cultured with bone marrow stromal cells in vitro. In this study, we investigated the importance of the p53 protein in the bone marrow response to DMBA. Through the use of p53 gene knockout mice, we demonstrated that the effect of DMBA on bone marrow cellularity is p53-dependent. In addition, apoptosis of primary cultures of progenitor B cells cultured with bone marrow stromal cells and DMBA is also p53-dependent. The results of this study provide evidence for the importance of p53 in the signaling pathways by which PAHs cause immunotoxicity.