Induction of cytochrome P-450IA1 gene expression in human breast tumour cell lines

Transient induction of increased aldehyde dehydrogenase 3A1 levels in cultured human breast (adeno)carcinoma cell lines via 5'-upstream xenobiotic, and electrophile, responsive elements is, respectively, estrogen receptor-dependent and -independent

Transient up-regulation of ALDH3A1, CYP1A1 and CYP1B1 transcription by transient exposure to aryl hydrocarbon receptor (AhR) ligands, e.g. 3-methylcholanthrene, is via transient transactivation of xenobiotic responsive elements (XRE) present in the 5'-upstream regions of these genes. Others have shown that AhR ligand-mediated induction of increased CYP1A1 levels in cultured human breast (adeno)carcinoma cell lines is apparently estrogen receptor (ER)-dependent, i.e. it was observed in ER(+) cell lines but not in ER(-) cell lines, whereas AhR ligand-mediated induction of increased CYP1B1 levels is ER-independent, i.e. it was observed in both ER(+) and ER(-) cell lines. The present investigation established that transient, AhR ligand/XRE-mediated induction of increased ALDH3A1 levels in human breast (adeno)carcinoma cell lines was, like that of CYP1A1 and unlike that of CYP1B1, apparently ER-dependent. Thus, transient exposure to 3-methylcholanthrene induced increased levels of ALDH3A1 in five cultured human breast (adeno)carcinoma cell lines that were documented as being ER(+), viz., MCF-7/0, MCF-7/OAP, T-47D, ZR-75-1 and MDA-MB-468, but failed to induce increased levels of this enzyme in four cultured human breast (adeno)carcinoma cell lines that have been historically viewed as being ER(-), viz., MDA-MB-231, SK-BR-3, HS-578-T and MDA-MB-435. Somewhat at odds with the foregoing, transient exposure to 3-methylcholanthrene also induced increased levels of ALDH3A1 and CYP1A1 in cultured, essentially ER(-), human breast epithelial MCF-10A cells. These cells, like cultured human breast (adeno)carcinoma cells, are immortal, but unlike the latter, are not tumorigenic. Transient induction of increased ALDH3A1 levels can also be effected by agents that are not AhR ligands, viz., electrophiles such as catechol, and thus, cannot up-regulate ALDH3A1 transcription via transactivation of a 5'-upsteam region XRE. Rather, they are thought to up-regulate ALDH3A1 transcription via transient transactivation of an electrophile responsive element (EpRE) that is putatively also present in the 5'-upstream region of this gene. Electrophile-initiated/EpRE-mediated induction of increased ALDH3A1 levels was found to be ER-independent. Thus, catechol transiently induced increased levels of ALDH3A1 in the five ER(+) human breast (adeno)carcinoma cell lines, the four ER(-) human breast (adeno)carcinoma cell lines, and the ER(-), immortal but not tumorigenic, human breast epithelial cell line.

Induction of 7-ethoxyresorufin-O-deethylase activity by planar chlorinated hydrocarbons and polycyclic aromatic hydrocarbons in cell lines from the rainbow trout pituitary

The induction of 7-ethoxyresorufin-o-deethylase (EROD) activity was examined in three rainbow trout pituitary cell lines: RTP-91E, RTP-91F and RTP-2. RTP-91E and RTP-91F were developed from the pituitary of a male and have epithelial-like and fibroblast-like morphologies, respectively. RTP-2, which was described previously, was developed from the pituitary of a female and has an epithelial-like shape. In all cell lines EROD activity was induced by 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD). Immunoblotting with the polyclonal antibody, anti-trout CYP1A1(277-294)/KLH, confirmed induction of a 58-kDa polypeptide. Potential inhibitors of the aryl hydrocarbon receptor, geldanamycin and alpha-naphthoflavone, inhibited EROD induction by TCDD. Other compounds inducing EROD activity were 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and 3-methylcholanthrene (3MC). When judged by the concentration eliciting 50% of the maximal response (EC50), induction was similar in RTP-2 and RTP-91E, and less effective in RTP-91F. Regardless of the cell line, the rank order from most to least potent inducer on the basis of EC50 value was TCDD> or =PCDD>TCDF>PCB 126>>3MC. When induction potencies were expressed relative to TCDD, the values obtained with the pituitary cell lines were similar to previously published values derived with a rainbow trout liver cell line.

The expression and regulation of drug metabolism in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears restricted. Maternal cigarette smoking or PCB exposure increase the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene into DNA-bound adducts, both in vitro and in vivo. Studies with RT-PCR technique have demonstrated that first trimester placentae express at the mRNA level CYP1A1, 1A2, 2C, 2D6, 2E1, 2F1, 3A4, 3A5, 3A7 and 4B1 and at full term CYP1A1, 2E1, 2F1, 3A3/4, 3A5 and 3A7. However, more detailed studies on cDNA probes or with specific antibodies or 'diagnostic' substrates for other than CYP1A1, 2E1 and 3A gene products have yielded negative results. Studies on human placenta and a chorioncarcinoma cell line, JEG 3 cells, boulster the concept that placental CYP1A1 and 1B1 - although their expression is Ah receptor and ARNT mediated - is controlled by distinct mechanisms. Aromatase, CYP19, and cholesterol side-chain cleaving, CYP11B, genes, proteins and activities are catalytically active in human placentae throughout the pregnancy and those parameters do not seem to be affected by maternal cigarette smoking but rather maternal health status. However, the substrate binding pocket of aromatase accepts as its substrate several xenobiotics and is responsible for constitutive xenobiotic biotransformations.Functional placental glutathione S-transferase, N-acetyl transferase and epoxide hydrolase are expressed via one gene each and their function reflects the placenta as an endocrine organ rather than a xenobiotic-metabolizing unit. However, markers for oxidative stress can be detected in decreased glutathione S-transferase activities.Because human placenta has quite well defined metabolic characteristics, and obtaining placental samples will not meet any drastic ethical difficulties, it could be used more intensively as a source of metabolizing enzymes in in vitro studies during the course of a drug development program. The human placenta, or its subcellular organelles, could serve as a real alternative model for an extrahepatic tissue in replacing recombinant expression systems especially if CYP11, 19, 1A1 or potentially 2E1 are target enzymes for potential metabolic interactions.

Aryl hydrocarbon (Ah) nonresponsiveness in estrogen receptor-negative MDA-MB-231 cells is associated with expression of a variant arnt protein

Several studies have reported a correlation between expression of the estrogen receptor (ER) and aryl hydrocarbon (Ah) responsiveness in human breast cancer cell lines. MDA-MB-231 cells are ER-negative and Ah-nonresponsive; however, initial studies showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin induced CYP1A1 mRNA levels (5.8-fold) and chloramphenicol acetyltransferase activity (2.6-fold) in high passage (Hp, >50 passages) cells transiently transfected with an Ah-responsive plasmid. In contrast, no induction responses were observed in low passage (Lp, <20 passages) cells. The Ah responsiveness of Hp compared to Lp MDA-MB-231 cells was associated with a >2-fold increased expression of the Ah receptor in Hp cells. Further analysis revealed that the apparent molecular weight of the Ah receptor mRNA transcript and immunoreactive protein were comparable in Lp MDA-MB-231 and Ah-responsive human HepG2 cells. In contrast, RT-PCR analysis of the Ah receptor nuclear translocator (Arnt) protein showed that HepG2 cells expressed the expected 2.6-kb transcript, whereas a 1.3-kb transcript was the major product in MDA-MB-231 cells. Western blot analysis confirmed that HepG2 cells primarily expressed a 97-kDa wild-type form of Arnt, whereas a dominant 36-kDa variant was expressed in MDA-MB-231 cells. Complete sequence analysis of the variant form of Arnt revealed a major deletion of the C-terminal region of the protein (aa 330 to 789). Like HepG2 cells, the wild-type 2.6-kb transcript was detected in ER-positive (Ah-responsive) MCF-7 cells, whereas the low-molecular-weight variant Arnt was dominant in ER-negative MDA-MB-231, MDA-MB-435, and Adriamycin-resistant MCF-7 cells. These results suggest that expression of this protein may be useful as a prognostic factor in breast cancer.

Protein kinase C modulates regulation of the CYP1A1 gene by the aryl hydrocarbon receptor

Transcriptional activation of the human CYP1A1 gene by halogenated and polycyclic aromatic hydrocarbons is mediated by the aryl hydrocarbon receptor (AhR) complex, a ligand-dependent transcription factor. A competent AhR comprises at least two components following nuclear translocation and DNA binding, the AhR and the AhR nuclear translocator (Arnt) protein, whose combined action on human CYP1A1 gene transcription is shown to be dependent upon functional protein kinase C (PKC). In the present study, we examined the effects of phorbol 12-myristate 13-acetate, a potent PKC activator, on the ligand-induced transcriptional activation of the CYP1A1 gene and cellular function of the AhR in human HepG2 101L cells. The 101L cells carry a stable transgene consisting of 1800 bases of 5'-flanking DNA and the promoter of the human CYP1A1 gene linked to the firefly luciferase structural gene (Postlind, H., Vu, T. P., Tukey, R. H. & Quattrochi, L. C. (1993) Toxicol. Appl. Pharmacol. 118, 255-262). Pretreatment of cells with 12-myristate 13-acetate enhanced ligand-induced CYP1A1 gene expression 2-3-fold. Inhibition of PKC activity blocked directly the transcriptional activation and the transactivation of the CYP1A1 gene, indicating a role for PKC in the AhR-mediated transcriptional activation process. However, the DNA binding activities of the in vitro activated and the induced nuclear AhR as measured by electrophoretic mobility shift analysis were not affected when CYP1A1 transcription was inhibited, indicating the actions of PKC to be a nuclear event that works in concert with or precedes AhR binding to the gene. These results illustrate that PKC is absolutely essential for the cellular and molecular events that control induction of CYP1A1 gene transcription.

Effect of transient expression of the oestrogen receptor on constitutive and inducible CYP1A1 in Hs578T human breast cancer cells

Hs578T human breast cancer cells are an oestrogen receptor (ER)-negative cell line. Treatment of these cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in formation of a 6.9 S nuclear aryl hydrocarbon (Ah) receptor complex, which bound to a [32P]dioxin-responsive element in a gel electrophoretic mobility shift assay. However, TCDD does not induce CYP1A1 gene expression or chloramphenicol acetyl transferase (CAT) activity in cells transiently transfected with pRNH11c or pMCAT5.12, which are Ah-responsive plasmids derived from the 5'-flanking region of the human and murine CYP1A1 genes respectively. Restoration of Ah responsiveness was investigated by co-transfecting Hs578T cells with pRNH11c or pMCAT5.12 and plasmids that express the ER (hER), Ah receptor (AhR) and AhR nuclear translocator (Arnt) proteins. ER expression resulted in significantly increased basal CAT activity; however, TCDD did not induce CAT activity in the transiently transfected cells. Expression of the AhR or Arnt proteins did not alter basal or inducible CAT activity. Expression of N- or C-terminal truncated ER in Hs578T resulted in differential regulation of Ah responsiveness. In Hs578T cells transiently expressing the ER, which contains C-terminal deletions (amino acids 282-595), basal CAT activity was also increased; however, Ah responsiveness was not restored. In contrast, transient expression of N-terminal-deleted (amino acids 1-178) ER resulted in a marked decrease in basal CAT activity but a restoration of Ah responsiveness. These results suggest that basal and inducible CAT activity in Hs578T cells transiently transfected with pRNH11c is modulated differentially by ER domains that are present in the N- and C-terminal regions of the ER.

Comparative properties of the nuclear aryl hydrocarbon (Ah) receptor complex from several human cell lines

The aryl hydrocarbon (Ah) responsiveness of the T-47D, Hep G2, LS180, MCF-7, A431, C-4II and MDA-MB-231 human cancer cell lines was determined by the induction of CYP1A1 mRNA levels and ethoxyresorufin O-deethylase activity. With the exception of teh MDA-MB-231 breast cancer cell line, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) significantly induced CYP1A1 mRNA levels and ethoxyresorufin O-deethylase activity in the remaining six cell lines and, based on their EC50 values, for ethoxyresorufin O-deethylase induction, their Ah responsiveness followed the order T-47D > C-4II > MCF-7 > LS180 > HEP G2 > A431. In contrast, all the cell lines expressed the nuclear Ah receptor complex (167.1-24.5 fmol/mg protein) which bound to a 32P-labeled consensus dioxin responsive element (DRE) in a gel mobility shift assay. The results of gel permeation chromatography a sucrose density gradient centrifugation studies showed that the calculated Mr values for the nuclear Ah receptor complex varied from 175 kDa (MDA-MB-231 cells) to 221 kDa and the apparent molecular weight of the nuclear Ah receptor complex cross-linked to a bromodeoxyuridine-substituted DRE was 200 kDa. The data show that the molecular properties and levels of the nuclear Ah receptor complex from seven different human cancer cell lines do not predict Ah responsiveness.

Effect of a negative regulatory element (NRE) on the human CYP1A1 gene expression in breast carcinoma MCF-7 and hepatoma HepG2 cells

The expression of the cytochrome P4501A1 gene, CYP1A1, is induced by e.g. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mainly by transcriptional mechanisms. The inducers mediate their effect upon binding and activation of the aryl hydrocarbon receptor (AHR) transcription-factor complex. Utilizing chimeric CYP1A1/CAT constructs transient gene expression experiments indicate that the putative negative regulatory element (NRE) of CYP1A1 influence the relative TCDD induced CAT activity in HepG2 cells, whereas this effect was not observed in MCF-7 cells. Differences in the formation of cell-specific protein-DNA complexes were demonstrated by gel retardation assays suggesting a functional difference of NRE in these two cell lines.

Regulation of cytochrome P450 gene expression in human colon and breast tumour xenografts

It is extremely difficult to identify the factors which regulate the expression of drug-metabolising enzymes in man. To address this problem, we have developed a model involving the use of human tumours grown as xenografts in immune deficient mice. Mice bearing human colon or breast tumours as xenografts were challenged with a range of compounds, known from animal studies to be inducers of cytochrome P450s from a variety of gene families. Almost all of the compounds tested could induce human tumour P450 expression, measured either by Western blot or immunohistochemical analysis. Indeed, the levels of P450s from several distinct gene families or subfamilies including CYP2A, CYP2B, CYP2C, CYP3A and CYP4A were induced. Of particular interest was the profound induction of human P450s by 1,4 bis 2-(3,5dichloro-pyridyloxybenzene)(TCPOBOP), a compound which exhibits a marked species specificity in its ability to induce P450 expression in experimental animals. Induction of a human CYP2B protein by this compound was confirmed by Northern blot analysis and in situ hybridisation for mRNA, indicating that induction occurred at the level of transcription. These studies have a variety of implications: they provide a method for approaching the previously intractable problem of how environmental, hormonal and metabolic factors regulate human P450 genes and other genes involved in drug metabolism; they demonstrate that human tumours express P450s constitutively and that the levels of these proteins can be modulated by exogenous agents.

Evidence for the mechanism of action of the 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated decrease of nuclear estrogen receptor levels in wild-type and mutant mouse Hepa 1c1c7 cells

Treatment of wild-type Hepa 1c1c7 cells with 1 nM [3H]-17 beta-estradiol resulted in the rapid accumulation of the nuclear estrogen receptor complex whose levels were maximized within 1 hr. Cotreatment of the cells with 10 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and [3H]-17 beta-estradiol did not affect the nuclear estrogen receptor levels 1 hr after addition of the radioligand; however, pretreatment of the cells for 1, 6, 24 or 42 hr with 10 nM TCDD prior to the addition of the radiolabeled hormone caused a greater than 50% decrease in nuclear estrogen receptor levels (determined by velocity sedimentation analysis) 1 hr after the addition of [3H]-17 beta-estradiol. In parallel experiments in which 10 nM TCDD was added 6 hr prior to the radiolabeled hormone, TCDD caused a 63 and 74% decrease in immunodetectable cytosolic and nuclear estrogen receptor protein levels, respectively, in the wild-type Hepa 1c1c7 cells. The nuclear estrogen receptor was also detected in two Hepa 1c1c7 mutant (class 1 and class 2) cell lines which have been characterized previously as TCDD non-responsive due to either decreased aryl hydrocarbon (Ah) receptor levels or a defect in the accumulation of transcriptionally active nuclear Ah receptor complexes, respectively. Treatment of these mutant cell lines with TCDD and [3H]-17 beta-estradiol (as described above) caused only a minimum (class 1) or non-detectable (class 2) decrease in nuclear estrogen receptor binding activity or immunodetectable protein levels. These results, coupled with the structure-dependent differences in the activities of TCDD (a strong Ah receptor agonist) and 2,8-dichlordibenzo-p-dioxin (a weak Ah receptor agonist) in this assay system, support a role for the Ah receptor in the TCDD-mediated decrease of the nuclear estrogen receptor in mouse Hepa 1c1c7 cells. In addition, actinomycin D and cycloheximide both inhibited the TCDD-mediated decrease of nuclear estrogen receptor levels in the Hepa 1c1c7 wild-type cells, and these results suggest that TCDD may induce specific gene products which are involved in this process.

Differences in 2,3,7,8-tetrachlorodibenzo-p-dioxin-inducible CYP1A1 expression in human breast carcinoma cell lines involve altered trans-acting factors

Differences in expression of the CYP1A1 gene have previously been observed in human breast carcinoma cell lines exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using an expression vector containing the functional 5'-regulatory region of human CYP1A1 (up to -1140) fused to the reporter gene CAT (for chloramphenicol acetyltransferase), the breast carcinoma cell lines, MCF-7, T47-D and ZR-75-1, classified as highly responsive to TCDD, were highly responsive to TCDD in the chloramphenicol acetyltransferase assay as well. Gel mobility shift assays have shown that these cell lines express a nuclear protein that binds the aryl hydrocarbon (Ah) receptor responsive element. The low or non-responsive cell lines, AL-1, BT-20 and CAMA-1, were low or non-responsive to TCDD in the chloramphenicol acetyltransferase assay, suggesting that the low-responsive phenotype is caused by altered trans-acting factors. However, the mechanism appears to differ among the cell lines. Whereas no induction was observed in AL-1, a fivefold induction in activity was observed in BT-20 and CAMA-1. The TCDD concentration giving half-maximum induction differed greatly between CAMA-1 and BT-20. The gel mobility shift assay showed the presence of a protein that bound specifically to the Ah responsive element in the non-responsive cell line AL-1, as well as the low-responsive cell lines, BT-20 and CAMA-1. The high basal activity but low induction observed in CAMA-1 may be due to an Ah receptor constitutively bound to the Ah responsive element.

Is aryl hydrocarbon hydroxylase activity a new prognostic indicator for breast cancer?

Aryl hydrocarbon hydroxylase (AHH) activity was measured in the breast tumours of 153 primary and 17 recurrent cancer patients, and in 18 patients with benign breast tumour. All operations were carried out in 1983-84. The cytosolic fraction was collected for steroid receptor determination, and microsomes were separated for AHH assay from the same tissue samples. The AHH distribution was wide and highly skewed in all groups. About 10% of the samples showed activities below detection limit. The medians and ranges for primary cancers were 34 (less than 5-2683), for recurrent cancers 40 (20-239) and for benign tumours 11 (less than 5-37) fmol min-1 mg-1 protein. After logarithmic transformation, the mean AHH activities of cancer samples differed significantly from those of benign tumours. The logarithm of AHH activity (log AHH) correlates positively with axillary lymph node status, and negatively with steroid receptor levels. The development of the disease and the survival of the patients were followed for 4 years. The survival and the disease-free interval of the cancer patients who had low AHH activity was significantly higher than that of the high AHH group. The multivariate analysis with Cox's proportional hazad model showed primary tumour size, progesterone receptor concentration, nodal status and log AHH to be the most important independent prognostic factors for survival, while the occurrence of metastases, log AHH and tumour size were the equivalent factors for the disease-free interval in primary breast cancers. We conclude that AHH activity may reflect the overall malignant potential of breast cancer tissue.

Comparison between cobalt and pyrazole in the increased expression of coumarin 7-hydroxylase in mouse liver

The data in this report show that administration of both cobalt and pyrazole results in an elevation in the amount of hepatic mRNA encoding for microsomal P45015 alpha/P450Coh, an increase in the amount of P450Coh protein, and an activation of COH and to a lesser extent testosterone 15 alpha-hydroxylase in two inbred strains of mice. Considerable quantitative differences between the two compounds and the two mouse strains in the response suggest that the effects of cobalt and pyrazole are mediated, at least partly, through different mechanisms. It is of interest that human hepatic COH resembles very closely that in the mouse liver.

Species variation in the response of the cytochrome P-450-dependent monooxygenase system to inducers and inhibitors

1. In the safety evaluation of drugs and other chemicals it is important to evaluate their possible inducing and inhibitory effects on the enzymes of drug metabolism. 2. While many similarities exist between species in their response to inducers and inhibitors, there are also important differences. Possible mechanisms of such variation are considered, with particular reference to the cytochrome P-450 system. 3. Differences in inhibition may be due to differences in inhibitory site of the enzyme involved, which is not always the active site of the enzyme, in competing pathways or in the pharmacokinetics of the inhibitor. 4. Differences in induction could be due to differences in the nature of the induction mechanism, in the isoenzyme induced, in tissue- or age-dependent regulation, in competing pathways for the substrate or its products, or in the pharmacokinetics of the inducing agent. 5. Examples of each of these possible differences are considered, often from our own work on the P450 IA subfamily, and results in animals are compared with those in humans, where possible. 6. At present, the differences between species in their response to inducers and inhibitors make extrapolation to humans from the results of animal studies difficult, so that ultimately such effects should be studied in the species of interest, humans.

Extracts of airborne particulates collected at different locations in the Copenhagen area induce the expression of cytochrome P-450IA1

Acetone extracts of airborne particulates collected at different sites in the greater Copenhagen area were tested for their ability to induce the expression of cytochrome P-450IA1 RNA in a human breast cancer cell line, T47-D. The induction efficiency was expressed as an benz[a] anthracene equivalents, that is, the amount of benz[a]anthracene required to give the same level of induction. A significantly higher level of induction of P-450IA1 RNA was seen with samples collected on days with a smog alert. The inducibility of samples collected in rural areas was lower, but no significant difference in inducibility was found between samples collected in urban and suburban areas. Lack of correlation between the mutagenic activity in the Ames assay and the P-450IA1-inducing activity of the samples suggests that the complex mixture of compounds found in airborne particulates may have different biological activities in the two short-term test systems. Measurements of P-450IA1 inducibility provide a new, sensitive approach to assess the biological activity of material present in air pollution. The presence in airborne particulates of chemical compounds that induce cytochrome P-450IA1 an enzyme responsible for the metabolism of ubiquitous chemical carcinogens, suggests that the general environment may change an individual's response to the impact of exogenous chemicals, including the carcinogens present in cigarette smoke.

Immunochemical and molecular biological studies on human placental cigarette smoke-inducible cytochrome P-450-dependent monooxygenase activities

The induction of specific forms of cytochrome P-450 and P-450-associated xenobiotic-metabolizing monooxygenase activities by maternal cigarette smoking was characterized in human placenta employing polyclonal and monoclonal antibodies and recombinant DNA probes. The anti-BNF-B2 (prepared against rat liver P-450 induced by beta-naphthoflavone) inhibited about 60 per cent of aryl hydrocarbon hydroxylase (AHH) and 7-ethoxyresorufin O-deethylase activities (ERDE) in placental tissues from smoking mothers, whereas the anti-PB-B2 (to phenobarbital-induced rat liver P-450) was without significant inhibitory effect. Inhibition of 7-ethoxycoumarin O-deethylase (ECDE) by the anti-BNF-B2 was dependent on maternal smoking: the enzyme from non-smokers was not significantly inhibited, whereas the enzyme from smokers was variably inhibited by 15-60 per cent. The monoclonal antibodies towards the major 3-methylcholanthrene-inducible and phenobarbital-inducible rat liver P-450s (Mab 1-7-1 and 2-66-3, respectively) behaved similarly, except the inhibition was somewhat stronger if present. Antibody raised against rat liver NADPH-cytochrome P-450 oxido-reductase did not inhibit any activity studied. In immunoblotting experiments, the anti-reductase recognized the protein in human placental microsomes. However, neither anti-BNF-B2, anti-PB-B2 or Mab 1-7-1 or Mab 2-66-3 detected any proteins in human placental microsomes, regardless of smoking status. Northern blot hybridization analysis of placental RNA samples showed that only P-450IA1 mRNA existed in the placentas of smoking mothers with detectable ERDE activity. Despite the discrepancy between protein blotting and immunoinhibition data all other findings support the conclusion that maternal cigarette smoking induces the expression of the CYPIA1 gene (and not CYPIA2), resulting in an increased synthesis of P-450IA1 protein and increased AHH, ERDE and ECDE activities in human placenta.