Early Passage Syrian Hamster Embryo Research Articles

Mechanisms Involved in Responses to the Peroxisome Proliferator WY-14,643 on Gap Junctional Intercellular Communication in V79 Hamster Fibroblasts

WY-14,643, a potent hepatic peroxisome proliferator, decreased gap junctional intercellular communication when used at low and intermediate concentrations (1 nM to 10 μM) in the V79 Chinese hamster fibroblast cell line. It did not decrease intercellular communication in early passage Syrian hamster embryo fibroblasts. The mechanism of WY-14,643-induced suppression of intercellular communication was studied by preexposure of V79 cells to inhibitors of protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, or mammalian target-of-rapamycin before addition of WY-14,643. Only protein kinase C, particularly the δ isoform, appeared involved in the inhibition of communication by WY-14,643. Also clofibrate-induced suppression of GJIC in V79 cells appeared to involve protein kinase Cδ. Furthermore, WY-14,643 did not cause any activation of the mitogen-activated protein kinases p44/42, p38, or Jun N-terminal kinase. When WY-14,643 was used at a higher concentration (100 μM), intercellular communication was increased both in V79 and Syrian hamster embryo cells. This effect was inhibited by preexposure of V79 cells to brefeldin A. Thus, there may be a pool of connexins in the Golgi complex that could be recruited to the plasma membrane upon exposure to high concentrations of WY-14,643.

Exposure of mammalian cell cultures to benzo[a] and light results in oxidative DNA damage as measured by 8-hydroxydeoxyguanosine formation

Carcinogenic polycyclic aromatic hydrocarbons induce DNA damage through direct covalent interactions with nucleotides of the DNA in cells in which they are activated to 'ultimate carcinogenic metabolites'. To determine whether they also induce oxidative damage to DNA under the same circumstances, early passage Syrian hamster embryo and human mammary carcinoma cell line MCF-7 cultures were treated for 24 h with 0-5 micrograms/ml benzo[a]pyrene (BaP) or for 1 h with 0-100 microM methylene blue (as a positive control for oxidative damage). The cells were then exposed to fluorescent light for 1 or 4 h or retained in darkness. After cell harvest, DNA isolation and enzymatic digestion of the DNA to deoxyribonucleosides, the amounts of 8-hydroxy-2'deoxyguanosine (8-OH-dGuo) and unmodified deoxyguanosine present were determined by reverse-phase HPLC with electrochemical and UV detection respectively. Cultures treated with methylene blue for 1 h followed by light exposure for 1 h contained 5-fold (10 microM) and 8- to 28-fold (100 microM) higher 8-OH-dGuo levels than cells treated with methylene blue not exposed to light or untreated cells with methylene blue not exposed to light or untreated cells exposed to light. There was no significant change in 8-OH-dGuo levels in cultures treated with 1-5 micrograms/ml BaP for 24 h in the absence of light. However, both the human and hamster cell cultures treated with BaP and then exposed to fluorescent light for 4 h contained 3-fold (1 micrograms/ml) and 8- to 10-fold (5 micrograms/ml) higher 8-OH-dGuo levels than those not exposed to light or not treated with BaP. These results indicate that BaP treatment does not cause 8-OH-dGuo formation in DNA of cells maintained in darkness. Exposure of BaP-treated cells to fluorescent light causes formation of significant amounts of oxidative DNA damage as measured by 8-OH-dGuo formation. These findings suggest that oxidative damage of DNA could be involved in tumor induction by BaP in tissues, such as skin, in which exposure to BaP can occur in the presence of light.

TPA inhibition of gap junctional intercellular communication in Syrian hamster embryo cells through two different pathways

Exposure of early passage Syrian hamster embryo cells to high TPA concentrations (8 or 16 μ m) decreased gap junctional intercellular communication (GJIC) to about 20% of the control during the first hour. Subsequently, the GJIC capacity recovered to 70–80% after 4–10 hr. Exposure to lower TPA concentrations also reduced GJIC, but did not result in the subsequent rapid enhancement of communication. Treatment of the cells to different TPA concentrations down-regulated protein kinase C (PKC), but this down-regulation did not seem to explain fully the induced recurrence of GJIC following high TPA concentrations. The maximal down-regulation of PKC took place at TPA concentrations above 0.16 μ m. Addition of 8 μ m TPA to cells pretreated with 0.016 and 8 μ m for 10 hr reduced GJIC from 80 to 40%, whereas pretreatment with 0.16 or 1.6 μ m TPA made the cells refractory to further TPA-induced inhibition. The PKC inhibitor staurosporine suppressed the effect of TPA on GJIC in cells exposed to 0.016 μ m TPA, whereas no suppression was observed in cells depleted of PKC. The results indicate that TPA is capable of inhibiting GJIC through two different mechanisms, a staurosporine-sensitive mechanism at low TPA concentrations (EC 50 = 0.18 μ m) and a staurosporine-insensitive mechanism at higher concentrations (EC 50 = 7 μ m).

Effects of five phorbol esters on gap junctional intercellular communication, morphological transformation and epidermal growth factor binding in Syrian hamster embryo cells.

The effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), 12-deoxyphorbol-13-phenylacetate (DOPP), 12-deoxyphorbol-13-phenylacetate-20-acetate (DOPP A), sapintoxin D (SAP D) and sapintoxin A (SAP A) on the decrease in [125I]epidermal growth factor (EGF) binding (indicating protein kinase C activation), suppression of gap junctional intercellular communication (GJIC) and induction of morphological cell transformation (MCT) in Syrian hamster embryo (SHE) cells were investigated. All five phorbol esters were found to reduce [125I]EGF binding in early passage SHE cells at comparable concentrations. DOPP A was approximately 10-fold less potent in decreasing GJIC compared to the other phorbol esters in early passage SHE cells, while the compounds showed less difference in suppressing GJIC in the phorbol ester sensitive SHE cell line BPNi. The decreases in [125I]EGF binding and GJIC were found to be transient in the continuous presence of phorbol esters. All phorbol esters induced MCT in early passage SHE cells, but DOPP and DOPP A were approximately 10-fold less potent than TPA, SAP D and SAP A. Thus, there seems to be some degree of correlation, but not to a full extent, between the ability of the phorbol esters to activate PKC, decrease GJIC and to induce MCT. The results do not suggest a simple relationship between PKC activation, inhibition of GJIC and the reported tumor-promoting activities of the compounds.

Phosphatase inhibitors, gap junctional intercellular communication and [125I]-EGF binding in hamster fibroblasts.

A number of phosphatase inhibitors (okadaic acid, calyculin A, aluminium fluoride, sodium molybdate, sodium orthovanadate, pervanadate and vanadyl sulphate) were investigated for their effects on gap junctional intercellular communication (GJIC) and [125I]-epidermal growth factor (EGF) binding in early passage Syrian hamster embryo cells (mainly fibroblast-like cells) and in V79 Chinese hamster lung fibroblasts. Only pervanadate decreased GJIC significantly. After the initial pervanadate-induced decrease the GJIC recovered rapidly. Only pervanadate was able to change the band pattern of the gap junction protein connexin43 (cx43) in Western blots. Together this may indicate either that there is a low turnover of phosphate groups in cx43 under basal conditions or that the putative phosphatases are not sensitive to most of the phosphatase inhibitors applied. In contrast, pervanadate, orthovanadate and molybdate decreased [125I]-EGF binding. 12-O-Tetradecanoylphorbol-13-acetate (TPA) is able to induce the phosphorylation of both cx43 and the EGF receptor, concomitantly with a decrease in GJIC and [125I]-EGF binding. These effects are reversible after removal of TPA. It could be imagined that other phosphatases would act on cx43 and the EGF receptor after the forced phosphorylation of the two molecules. Thus TPA was used to downregulate GJIC and [125I]-EGF binding and phosphatase inhibitors were applied in the upregulation phase. Only pervanadate affected the upregulation of GJIC, and pervanadate, orthovanadate and molybdate affected the upregulation of [125I]-EGF binding. Thus it is not an identical complement of phosphatases that act on cx43 and the EGF receptor. All the downregulating agents are assumed to be phosphotyrosine phosphatase inhibitors.

12-O-tetradecanoylphorbol-13-acetate-induced inhibition of gap junctional communication is differentially regulated in a transformation-sensitive Syrian hamster embryo cell line compared to early passage SHE cells.

The transformation-sensitive cell-line BPNi was more susceptible to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inhibition of gap junctional intercellular communication (GJIC) than early passage Syrian hamster embryo (SHE) cells, while the potency of TPA to down-regulate EGF-binding was similar in the two cell types. The kinetics of TPA-induced inhibition of GJIC suggested that different mechanisms may operate at high and low TPA concentrations. The initial inhibition after exposure to high TPA concentrations was followed by a recovery of GJIC. The recovery was much more pronounced in SHE than in BPNi cells. This effect could not be explained by differences in down-regulation of protein kinase C. Removal of high TPA concentrations also resulted in a faster recovery of GJIC in SHE than in BPNi cells. In addition, although forskolin induced a similar protection against the inhibitory effect of TPA on GJIC, forskolin restored GJIC blocked by TPA much faster in SHE than in BPNi cells. Thus, BPNi cells are more sensitive to TPA induced inhibition of GJIC than SHE cells, and have reduced capability to recover from down-regulated GJIC as compared to SHE cells.

Modification of gap junctional intercellular communication by changes in extracellular pH in Syrian hamster embryo cells

Studies were conducted to determine the effect of culture medium pH on gap junctional intercellular communication (GJIC) in early passage Syrian hamster embryo (SHE) cells. Previous studies have demonstrated that SHE cells cultured at a clonal density at pH 6.70 are morphologically transformed by carcinogens at a significantly higher frequency than cells cultured in media of higher pH. Several other cell characteristics consistent with promotion-like effects are observed with pH 6.70 culture of SHE cells. It was postulated that the promotion-like effects observed in SHE cells cultured at acidic pH are mediated in part by a reduction of GJIC. In this study, we evaluated GJIC in SHE cells by fluorescent dye coupling. Results from this study indicate that GJIC decreased as a function of decreased extracellular pH. Cells cultured at pH 6.70, 7.15 or 7.35 exhibited 47, 75 and 85% coupled cells respectively. The decrease in dye coupling occurred by 24 h after switching the cells from pH 7.15 to 6.70 medium. The decreased GJIC observed at pH 6.70 was not due to changes in cell proliferation and was reversible within 24 h when pH 6.70 cultures were refed with pH 7.15 medium. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate inhibited SHE cell GJIC in a pH-dependent manner with cells at pH 6.70 exhibiting the greatest inhibition by TPA and cells at pH 7.35 being unresponsive. The effect of pH on GJIC in SHE cells is consistent with the pH-dependent response to chemically induced morphological transformation and may be mechanistically related to this phenomenon.

Effects of ionizing radiation on expression of genes encoding cytoskeletal elements: Kinetics and dose effects

We examined the modulation in expression of genes encoding three cytoskeletal elements (beta-actin, gamma-actin, and alpha-tubulin) in Syrian hamster embryo (SHE) cells following exposure to ionizing radiations. Early-passage SHE cells were irradiated in plateau phase with various low doses (12-200 cGy) of neutrons, gamma-rays, or x-rays. RNA samples were prepared from cells at different times postexposure and were analyzed for levels of specific transcripts by northern blots. The results revealed that alpha-tubulin was induced by both high-linear energy of transfer (LET) (neutrons) and low-LET (gamma-rays and x-rays) radiations with similar kinetics. The peak in alpha-tubulin mRNA accumulation occurred between 1 and 3 h postexposure; for gamma-actin mRNA, accumulation was similarly induced. For both gamma-actin and alpha-tubulin, the higher the dose during the first hour postexposure (up to 200 cGy gamma-rays), the greater the level of mRNA induction. In contrast, mRNA specific for beta-actin showed decreased accumulation during the first hour following radiation exposure, and remained low up to 3 h postexposure. These results document the differential modulation of genes specific for cytoskeletal elements following radiation exposure. In addition, they demonstrate a decrease in the ratio of beta-actin:gamma-actin mRNA within the first 3 h following gamma-ray exposure. These changes in mRNA accumulation are similar to those reported in some transformed cell lines and in cells treated with tumor promoters, which suggests a role for changes in actin- and tubulin-mRNA expression in radiation-mediated transformation.

An interlaboratory comparison of enhanced morphological transformation of Syrian hamster embryo cells cultured under conditions of reduced bicarbonate concentration and pH

Initial studies performed in our laboratory indicated that early passage Syrian hamster embryo (SHE) cells exhibit optimal clonal proliferation when cultured in medium with a sodium bicarbonate concentration of 8.9 mM and pH of 6.70 instead of 44 mM and pH 7.35 as used previously by others. Subsequent studies indicated that morphological transformation frequency induced by benzo[ a]pyrene (BP) was also enhanced at pH 6.70 compared to 7.35 and the level of enhancement was affected by cell density and duration of culture. With optimal conditions identified, the carcinogens BP, 3-methylcholanthrene, N-methyl- N′-nitro- N-nitrosoguanidine, 2-acetylaminofluorene and the non-carcinogen anthracene were tested at pH 6.70 and 7.35 in our laboratory and at Microbiological Assoc. Inc. under code. Additionally, the non-carcinogens 4-acetylaminofluorene, and caprolactam were tested in our laboratory. Results from these studies indicate that all carcinogens tested caused a significant increase in morphological transformation frequency compared to controls at pH 6.70. In contrast, only BP caused a significant increase in the morphological transformation frequency at pH 7.35. The non-carcinogens did not significantly increase the morphological transformation frequency compared to controls. Interlaboratory comparisons were in qualitative agreement despite the fact that different lots of serum and hamster cell isolates were used by the two laboratories. However, different dose-response curves for the various chemicals were observed between the two labs. It was also demonstrated that the enhanced morphological transformation frequency is not due to a decrease in culture medium osmolality or Na concentration, a condition which accompanies media with a reduced bicarbonate concentration and pH. These results demonstrate that with the chemicals tested, low pH transformation of SHE cells agrees with carcinogenic potential and that assay variability is minimized. The implications of these results regarding use of the SHE cell assay as a short-term test for predicting the carcinogenic potential of chemicals are discussed.

Glucolimnanthin, a plant glucosinolate, increases the metabolism and DNA binding of benzo[a]pyrene in hamster embryo cell cultures.

Glucosinolates are common components of cruciferous vegetables that can be hydrolyzed during food processing to yield isothiocyanates, some of which have been shown to inhibit the induction of mammary tumors in rats by 7,12-dimethyl-benz[a]anthracene. To determine how intact glucosinolates affect the metabolism of benzo[a]pyrene (B[a]P) in mammalian cells in culture, the effects of a series of glucosinolates on the metabolism and DNA binding of B[a]P were investigated in early passage Syrian hamster embryo cell cultures. Glucolimnanthin, a glucosinolate from Limnanthes douglasii increased the amount of B[a]P metabolized by the hamster embryo cell cultures during a 24 h exposure. The glucolimnanthin-treated cultures contained a higher proportion of B[a]P-phenol glucuronides and other water-soluble metabolites than control cultures. Cotreatment with glucolimnanthan and [3H]B[a]P for 24 h resulted in a greater than 2-fold increase in the amount of B[a]P bound to DNA and a 3-fold increase in the amount of deoxyguanosine adduct formed by reaction of 7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydroB[a]P [(+)-anti-B[a]PDE]. The glucolimnanthin was metabolized essentially completely within 24 h. An increase in B[a]P metabolism similar to that caused by glucolimnanthin was induced by cotreatment of hamster embryo cell cultures with m-methoxybenzyl isothiocyanate, a metabolite that can be formed from glucolimnanthin by enzymatic hydrolysis. These results indicate that the glucosinolate glucolimnanthin can increase the metabolic activation of B[a]P in mammalian cells in culture.

Enhanced morphological transformation of early passage Syrian hamster embryo cells cultured in medium with a reduced bicarbonate concentration and pH.

Recent studies from our laboratory have demonstrated that clonal cell proliferation of early passage Syrian hamster embryo (SHE) cells is optimal at a bicarbonate concentration in the culture medium of 8.9 mM (pH 6.65-6.75) under the experimental conditions reported. The purpose of the studies reported here was to examine whether morphological transformation induced by benzo[a]pyrene (BP) was enhanced under optimal culture conditions for SHE cell proliferation. Culture media of pH 6.70, 7.11 and 7.34 under incubator conditions of 10% CO2 in air were obtained by the addition of 0.75 (8.9 mM), 2.25 (26.8 mM) and 3.75 g/l (44.6 mM) of NaHCO3 respectively to a modified formulation of Dulbecco's modified Eagles medium. The frequency of morphological transformation of SHE cells was increased at 8.9 mM bicarbonate (pH 6.70) relative to media containing 26.8 or 44.6 mM bicarbonate (pH 7.11 and 7.34 respectively). Additionally, the isolate of embryo cells and lot of fetal bovine serum used supported transformation induced by BP at 8.9 mM bicarbonate (pH 6.70), but did not with media of higher bicarbonate concentration and pH. The duration of cell culture and the no. of colonies per plate influenced the amount of increase of morphological transformation observed at 8.9 mM bicarbonate relative to media of higher bicarbonate concentration. Initial studies have shown that a fraction of morphologically transformed colonies generated at reduced bicarbonate concentration were tumorigenic in newborn hamsters. These results are discussed in terms of the potential utility of low bicarbonate concentration cultured SHE cells for transformation studies.

An early, nonrandom karyotypic change in immortal Syrian hamster cell lines transformed by asbestos: Trisomy of chromosome 11

Cytogenetic studies were performed on eight early passage Syrian hamster embryo cell lines independently derived following asbestos exposure. The modal chromosome number of all the immortal cell lines was near-diploid. At the earliest passage examined, six of eight cell lines had only numerical chromosome changes. Cells in each of these six cell lines had an extra chromosome #11, either as a sole karyotypic change or with other numerical changes. The remaining two cell lines displayed both numerical and structural chromosome changes, but without involvement of chromosome #11. Common abnormalities were −X or −Y, +3, and 8p− in one cell line, and −13 and t(13;21) in the other cell line. A nonrandom gain of chromosome #8 was also found in four cell lines. In three of the four cell lines, trisomy of chromosome #8 seems to have occurred during karyotypic progression. The observation that nonrandom changes in chromosome number are an early karyotypic change after carcinogen treatment supports our hypothesis that induction of aneuploidy by asbestos is mechanistically important in the transformation of Syrian hamster embryo cells in culture and, further, suggests that trisomy 11 plays a major role in the early steps of immortalization and neoplastic progression.

The induction of transformed-like morphology and enhanced growth in Syrian hamster embryo cells grown at acidic pH.

The effect of the pH, Na+ concentration and osmolality of the culture medium on early passage Syrian hamster embryo (SHE) clonal cell proliferation was examined. The pH of the medium was adjusted from 6.49 to 7.45 by addition of different amounts of NaHCO3 to the medium and incubating the cell cultures in a fixed atmosphere of 10% CO2/90% air. Our results indicate that clonal SHE cell proliferation is optimal at pH 6.65-6.80 while plating efficiency is independent of pH between 6.65 and 7.45. Adjustment of Na+ to that concentration in the medium (3450 p.p.m., 0.15 M) of the greatest NaHCO3 addition caused a moderate depression of cell proliferation over the entire pH series. Adjusting the osmolality of the culture medium to a constant value of 338 mOsm/kg did not alter the pH effect on cell proliferation. The pH of the medium also affected cellular and colony morphology. Below pH 6.90 there was an increase in the number of colonies which exhibited a transformed-like morphology ('altered' colonies). The 'altered' phenotype was characterized by a multilayered, criss-cross pattern of growth throughout the colony. This phenotype was stable upon sub-cloning into pH 6.65 medium but was reversible if sub-cloned into pH 7.36 medium. The induction of 'altered' colonies at low pH could be partially suppressed by Na+ or osmolality adjustment. These results are discussed in terms of optimizing growth conditions for SHE cells in order to enhance their usefulness for cell transformation studies. The induction of 'altered' colonies by low pH is also discussed relative to the involvement of pH regulation in tumor-promoter and growth-factor action on cells in culture.

Neoplastic transformation of normal and carcinogen-induced preneoplastic Syrian hamster embryo cells by the v-src oncogene.

The ability of cloned Rous sarcoma virus (RSV) DNA encoding the v-src oncogene to neoplastically transform normal, diploid Syrian hamster embryo (SHE) cells was examined. Transfection of RSV DNA into early passage SHE cells resulted in a low but significant number of tumors when treated cells were injected into nude mice. Tumors formed with a low frequency (two tumors out of ten sites injected) and only after a long latency period (14 weeks). In contrast to the normal SHE cells, several different carcinogen-induced preneoplastic immortal SHE cell lines were highly susceptible to transformation by the v-src oncogene to the neoplastic phenotype. Tumors formed with high efficiency and a short latency period (less than 3 weeks). Further studies were performed to determine the basis for the inefficient transformation of the normal SHE cells. NeoR clones isolated after cotransfection of SHE cells with pSV2-neo and RSV DNAs were neither morphologically altered nor immortal and did not contain detectable levels of the v-src gene product. These results suggest that neoplastic transformation by v-src DNA in the normal cells is initially suppressed. However, cells from a v-src-induced tumor expressed v-src RNA, and antibody to v-src protein precipitated from the tumor cells a 60,000-molecular-weight protein which displayed protein kinase activity. Karyotypic analyses confirmed that the tumor was derived from Syrian hamster cells and suggested that it was clonal in nature. These results indicate that the v-src oncogene was primarily responsible for neoplastic transformation of SHE cells. In contrast to the results with the v-src oncogene, our previous studies showed that v-Ha-ras oncogene alone is unable to induce neoplastic transformation of SHE cells. Furthermore, the v-myc oncogene was able to compliment v-Ha-ras to neoplastically transform SHE cells, while cotransfection with v-src plus v-myc did not increase the incidence of tumors.

Neoplastic transformation of normal and carcinogen-induced preneoplastic Syrian hamster embryo cells by the v-src oncogene.

The ability of cloned Rous sarcoma virus (RSV) DNA encoding the v-src oncogene to neoplastically transform normal, diploid Syrian hamster embryo (SHE) cells was examined. Transfection of RSV DNA into early passage SHE cells resulted in a low but significant number of tumors when treated cells were injected into nude mice. Tumors formed with a low frequency (two tumors out of ten sites injected) and only after a long latency period (14 weeks). In contrast to the normal SHE cells, several different carcinogen-induced preneoplastic immortal SHE cell lines were highly susceptible to transformation by the v-src oncogene to the neoplastic phenotype. Tumors formed with high efficiency and a short latency period (less than 3 weeks). Further studies were performed to determine the basis for the inefficient transformation of the normal SHE cells. NeoR clones isolated after cotransfection of SHE cells with pSV2-neo and RSV DNAs were neither morphologically altered nor immortal and did not contain detectable levels of the v-src gene product. These results suggest that neoplastic transformation by v-src DNA in the normal cells is initially suppressed. However, cells from a v-src-induced tumor expressed v-src RNA, and antibody to v-src protein precipitated from the tumor cells a 60,000-molecular-weight protein which displayed protein kinase activity. Karyotypic analyses confirmed that the tumor was derived from Syrian hamster cells and suggested that it was clonal in nature. These results indicate that the v-src oncogene was primarily responsible for neoplastic transformation of SHE cells. In contrast to the results with the v-src oncogene, our previous studies showed that v-Ha-ras oncogene alone is unable to induce neoplastic transformation of SHE cells. Furthermore, the v-myc oncogene was able to compliment v-Ha-ras to neoplastically transform SHE cells, while cotransfection with v-src plus v-myc did not increase the incidence of tumors.

Benzo[e]pyrene-induced alterations in the binding of benzo[a]pyrene to DNA in hamster embryo cell cultures.

Benzo[e]pyrene (B[e]P), a weakly carcinogenic polycyclic aromatic hydrocarbon (PAH) modifies tumor induction in mouse skin and the induction of mutation in mammalian cells by carcinogenic PAH. To determine how B[e]P alters the metabolic activation of the carcinogen benzo[a]pyrene (B[a]P), early passage Syrian hamster embryo cell cultures were exposed to [3H]B[a]P or [3H]trans-7,8-dihydro-7,8-dihydroxyB[a]P (B[a]P-7,8-diol) in the presence of various concentrations of B[e]P for 24 h. The DNA was isolated, degraded to deoxyribonucleosides and the B[a]P-deoxyribonucleoside adducts were analyzed by h.p.l.c. As the dose of B[e]P increased, the amount of B[a]P bound to DNA decreased and the ratio of anti-B[a]P-7,8-diol-9,10-epoxide (B[a]PDE)-deoxyguanosine adduct to syn-B[a]PDE-deoxyguanosine adduct decreased. B[e]P treatment inhibited the binding of B[a]P-7,8-diol to DNA to a greater extent than it inhibited the binding of B[a]P and decreased the ratio of anti- to syn-B[a]PDE-deoxyguanosine adducts formed from the 7,8-diol. These results indicate that B[e]P decreases the activation of B[a]P to DNA-binding intermediates in these cells; especially the oxidation of B[a]P-7,8-diol to a diol-epoxide. The B[e]P-induced alterations in the ratio of DNA adducts formed from the syn- and anti-isomers of B[a]PDE suggest that B[e]P selectively inhibited certain pathways of metabolic activation of B[a]P. Thus, B[e]P-induced modifications in the biological activity of PAH may result from alteration in both the amounts and the relative proportions of various isomeric forms of the ultimate carcinogenic metabolites formed from PAH.