Epithelial and fibroblast cell lines cultured from the transgenic BigBlue rat: an in vitro mutagenesis assay

We have isolated, cultured, and immortalised three new BigBlue transgenic rat cell lines for the study of mutation induction in vitro. The two epithelial cell lines, from the mammary gland and oral cavity, were designated BBR/ME and BBR/OE, respectively, and the third is a mammary fibroblast line designated BBR/MFib. We have characterised these cell lines with respect to chromosome number and the expression of some cell-specific antigens. The clonogenic survival and cII transgene mutation induction responses of these three cell lines to N-ethyl-N-nitrosourea (ENU) treatment were determined. Both epithelial cell lines were much more sensitive to ENU toxicity than was the fibroblast cell line. However, all cell lines showed similar ENU dose-dependent increases in mutant frequency. We hope that cell lines such as these will extend the power of the BigBlue assay to in vitro studies.     

Heat-induced phosphorylation of NBS1 in human skin fibroblast cells

NBS1 is known to be involved in DNA damage-induced cellular responses after exposure to ionizing radiation (IR). Phosphorylation of NBS1 contributes to cell-cycle checkpoints. The aim of this study was to determine whether heat exposure induces or stimulates cellular responses mediated by the phosphorylation of NBS1 in human skin fibroblast cell lines. The results of immunofluorescent staining and Western blot analysis showed that NBS1 proteins are phosphorylated after exposure to heat in the nucleus of a normal skin fibroblast cell line (82-6 cells). This suggests that the NBS1-mediated signal transduction could be induced by heat. We further examined whether a deficiency in the NBS1 protein modifies heat sensitivity in human skin fibroblast cell lines. A skin fibroblast cell line (Gmtert), derived from a Nijmegen breakage syndrome (NBS) patient containing mutant NBS1, showed higher sensitivity to heat than the same cell line transfected with the wild-type copy of the NBS1 gene. We also showed that transfection of a DNA cassette expressing small interference RNA (siRNA) targeted to NBS1 into 82-6 cells enhanced cell sensitivity to heat. These results suggest that NBS1 is involved in cellular responses to DNA damage which is induced by heat exposure as well as by radiation exposure in human skin fibroblast cells.     

Similar repair of O6-methylguanine in normal and ataxia-telangiectasia fibroblast strains. Deficient repair capacity of lymphoblastoid cell lines does not reflect a genetic polymorphism

The ability of human fibroblast strains to repair the mutagenic DNA adduct O6-methylguanine (O6-MeG) induced by brief exposure to N-methyl-N'-nitroso-N-nitrosoguanidine (MNNG) was investigated. The repair reaction proceeded rapidly during the first hour after alkylation, followed by a slow, continuous phase of repair, and both processes were saturated by low doses of carcinogen. This was similar to what had previously been found in human lymphoblastoid lines. Three fibroblast strains from healthy donors and six strains from patients with ataxia telangiectasia were all proficient in their capacity to repair O6-MeG and had the same sensitivity to the cytotoxicity of MNNG and methyl methanesulphonate as normal cells. Three of these cell strains were derived from individuals whose lymphoblastoid lines were deficient in their ability to repair O6-MeG. These lymphoblastoid lines were also extremely hypersensitive to killing by methylating carcinogens. Because non-transformed cells from the same donors behaved normally with regard to both parameters, we concluded that the repair deficiency accompanied by carcinogen hypersensitivity of the lymphoblastoid lines does not indicate a genetic deficiency in the donor. These findings imply that lymphoblastoid lines may not always be the appropriate cell type for investigating genetic susceptibility to chemical mutagens.     

Mutagen detection with a mouse line containing 3 distinct mutations conferring sensitivity

A mouse-cell mutant sensitive to methyl methanesulfonate (MMS), X-rays, ultraviolet light (UV), and crosslinking agents was selected using the replica plating and cell suspension spotting methods. This mutant (XUM1) is a mitomycin C-sensitive derivative of previously reported XU1, a mutant sensitive to MMS, X-rays and UV. Since XU1 is highly susceptible to the lethal effect of 4-nitroquinoline-1-oxide (4NQO), XUM1 is also hypersensitive to 4NQO. Growth inhibition area tests showed that low concentrations of mutagens were detected with the multiple mutagen-sensitive mutant XUM1. Hence XUM1 cells will be useful in detecting with high sensitivity a wide range of mutagens and carcinogens which mimic X-rays, UV and crosslinking agents.     

Hsp/Hsc70相互作用蛋白CHIP在大鼠肾纤维化组织中的高表达

利用DNA重组技术 ,将编码人Hsp Hsc70相互作用蛋白CHIP(carboxylterminusofHsc 70 interactingprotein ,CHIP)全长、N端TPR及C端U box结构域的DNA片段分别构建于融合表达质粒pGEX 5X 1中 ,转化大肠杆菌 .纯化GST CHIP融合蛋白 ,制备了特异性识别人、小鼠及大鼠组织细胞内CHIP蛋白的多克隆抗体 .利用该抗体 ,对多种组织、细胞的内源性CHIP蛋白进行了检测 .发现在肾组织中 ,CHIP主要表达于皮质、髓质小管上皮细胞的胞浆中 ,且在单侧结扎输尿管所至的大鼠肾纤维化模型 (UUO)组织中 ,CHIP的表达较假手术组有显著增强趋势     

Cell fusion corrects the 4-nitroquinoline 1-oxide sensitivity of Werner syndrome fibroblast cell lines

We have shown that Werner syndrome (WRN) fibroblast cell lines are unusually sensitive to the DNA-damaging agent 4-nitroquinoline 1-oxide (4NQO), though not to gamma radiation or to hydrogen peroxide. The fusion of 4NQO-sensitive WRN and 4NQO-resistant control fibroblast cell lines generated proliferating WRN × control cell hybrids that expressed WRN protein and were 4NQO-resistant. These results establish the recessive nature of 4NQO sensitivity in WRN cell lines and provide a cellular assay for WRN protein function. Electronic Publication     

Alleviation of benzo[a]pyrene-diolepoxide-DNA damage in human lung carcinoma by glutathione S-transferase M2

Cellular detoxification is important for the routine removal of environmental and dietary carcinogens. Glutathione S-transferases (GST) are major cellular phase II detoxification enzymes. MRC-5 cells have been found to exhibit significantly higher GST activity than human H1355 cells. This study investigates whether GST-M2 activity acts as a critical determinant of the target dose of carcinogenic benzo[a]pyrene-diolepoxide (BPDE) and whether it has an effect on MDM2 splicing in the two cell lines. We used RT-PCR to clone Mu-class GST cDNA. Two forms of GST coming from the cell lines were characterized as GST-M2 (from MRC-5 cells) and GST-M4 (from H1355 cells). Nested-PCR showed that BPDE-induced MDM2 splicing had occurred in the H1355 cell line but not in normal MRC-5 cells. Furthermore, using nested-PCR and competitive ELISA, we found that in H1355 cells modified to stably overexpress GST-M2, splicing was abolished and BPDE adducts appeared in low abundance. In conclusion, exogenously overexpressed GST-M2 was effective in reducing BPDE-induced DNA damage in H1355 cells. The catalytic activity of GST-M2 may play an important future role in lowering the incidence of BPDE-induced DNA damage.     

The inter-relationship between anchorage independence and tumorigenicity in early cultures of oral keratinocytes

This study examines the expression of anchorage independence and tumorigenicity in early cultures of oral rat keratinocytes. The epithelial cell lines originated from the palatal and the lingual mucosa of rats that had been painted with the carcinogen 4-nitroquinoline N-oxide. The colony forming efficiency (CFE) in gel culture of the cell lines derived from five squamous cell carcinomas of the tongue and palate predominantly increased with passage in culture. Carcinoma-derived cell lines that had a relatively high CFE (greater than 2.5%) formed tumours when transplanted to athymic mice, but cells in which the CFE was less than 2.5% were non-tumorigenic. Keratinocytes from a dysplastic palatal lesion were immortal, anchorage dependent and non-tumorigenic. A lingual papilloma cell line consistently expressed a very low CFE but was tumorigenic at the higher culture passages. The results show that the routine passage of cells in culture leads to the emergence of the anchorage independent and tumorigenic phenotypes in keratinocytes of malignant origin and, further, suggest that anchorage independence and tumorigenicity may exist as distinct phenotypes, with anchorage independence preceding tumorigenicity.     

Characterization of a UV-resistant mutant of CHO-K1 with normal repair activity

The biological and repair responses of Mut 8–16, an ultraviolet radiation (UV)-resistant derivative of CHO-K1, were characterized with respect to UV and to the active chemical carcinogen, benzo[a]pyrene-4,5-oxide. In comparison to the parent, the UV-survival response curve of this mutant showed a significantly larger shoulder but little or no difference in the slope of the exponential survival region. In addition, the mutant cell line demonstrated significantly larger mutation frequencies at high survival UV fluences, but smaller mutation frequencies at high survival equitoxic concentrations of the carcinogen benzo[a]pyrene-4,5-epoxide relative to the parent cell. However, these relative differences in mutation frequencies between parent and mutant appeared to decrease as survival decreased. Despite these observations there were no measurable differences in excision-repair, or in post-replication repair although the mutant appeared to show a nominal reduction (not an enhancement) of replication-repair activity following the UV exposure. These data imply there is another lesion recognition system in CHO cells whose effects on survival and mutation are best observed at low doses of carcinogen and/or radiation but which are masked at higher doses where major repair processes dominate. The dissimilar relationship of cytotoxicity to mutation induction frequency observed in UV and carcinogen treated mutant vs. parent cell lines, imply that the probabilities for lethality and mutation are independent of one another in the presence of otherwise unrepaired (residual) damage.     

Synthesis of basic fibroblast growth factor upon differentiation of rat mammary epithelial to myoepithelial-like cells in culture

Acidic fibroblast growth factor (aFGF) mRNA was detected in a rat mammary fibroblastic cell line, but not in rat mammary epithelial cell lines or myoepithelial-like cell lines. Basic FGF (bFGF) mRNA was detected in both the fibroblasts and the myoepithelial-like cells, but was absent from the epithelial cells. A series of cell lines representing stages in the differentiation pathway of epithelial cells to a myoepithelial-like morphology showed an increase in the amount of bFGF mRNA and activity present and the FGF from the myoepithelial-like rat mammary 29 cells was able to displace [125I]-bFGF specifically bound to rat mammary fibroblasts. FGF activity was also present in an extract of rat mammary gland. Analysis of cell extracts and conditioned medium indicated that FGF activity was cell-associated. The cell-associated bFGF was resistant to degradation by trypsin. Extraction of myoepithelial-like cells with Triton X-100 and 2 M NaCl showed that 50-65% of the cell-associated bFGF was in a detergent-resistant but 2 M NaCl-labile structure. Thus, the synthesis of bFGF is developmentally regulated in rat mammary cell lines, and at least 50% is present in the extracellular matrix.     

NAD(P)H:Quinone oxidoreductase activity is the principal determinant of beta-lapachone cytotoxicity

beta-Lapachone activates a novel apoptotic response in a number of cell lines. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) substantially enhances the toxicity of beta-lapachone. NQO1 expression directly correlated with sensitivity to a 4-h pulse of beta-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significantly protected NQO1-expressing cells from all aspects of beta-lapachone toxicity. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality after beta-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of beta-lapachone by NQO1 leads to a futile cycling between the quinone and hydroquinone forms, with a concomitant loss of reduced NAD(P)H. In addition, the activation of a cysteine protease, which has characteristics consistent with the neutral calcium-dependent protease, calpain, is observed after beta-lapachone treatment. This is the first definitive elucidation of an intracellular target for beta-lapachone in tumor cells. NQO1 could be exploited for gene therapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during neoplastic transformation.     

Chromosomal instability in mutagen-sensitive mutants isolated from mouse lymphoma L5178Y cells. II. Abnormal induction of sister-chromatid exchanges and chromosomal aberrations by mutagens in an ionizing radiation-sensitive mutant (M10) and an alkylating agent-sensitive mutant (MS1)

To determine the mutual relationships between cell survival and induction of sister-chromatid exchanges (SCEs) as well as chromosomal aberrations (CAs), mutagen-induced SCEs and CAs were analyzed in an ionizing radiation-sensitive mutant (M10) and an alkylating agent-sensitive mutant (MS 1) isolated from mouse lymphoma L5178Y cells. The levels of CA induction in both mutants strictly corresponded to the sensitivity to lethal effects of mutagens, except that caffeine-induced CAs in M10 are considerably lower than those in L5178Y. The results clearly indicate that except for caffeine-induced CAs in M10, mutagen-induced lethal lesions are responsible for CA induction. In contrast, SCE induction in mutants was complicated. In M10, hypersensitive to killing by gamma-rays, methyl methanesulfonate (MMS), and 4-nitroquinoline 1-oxide (4NQO), but not sensitive to UV or caffeine, the frequency of SCEs induced by gamma-rays was barely higher than that in L5178Y, and the frequencies of MMS- and UV-induced SCEs were similar to those in L5178Y, but 4NQO- and caffeine-induced SCEs were markedly lower than those in L5178Y. MS 1, which is hypersensitive to MMS and caffeine, but not sensitive to UV or 4NQO, responded to caffeine with an enhanced frequency of SCEs and had a normal frequency of MMS-induced SCEs, but a reduced frequency of UV- and 4NQO-induced SCEs. Thus, susceptibility to SCE induction by mutagens is not necessarily correlated with sensitivity of mutants to cell killing and/or CA induction by mutagens. Furthermore, the spontaneous levels of SCEs are lower in M10 and higher in MS 1 than that in L5178Y (Tsuji et al., 1987). Based on these results, we speculate that M10 may be partially defective in the processes for the formation of SCEs caused by mutagens. On the other hand, MS 1 may modify SCE formation-related lesions induced by UV and 4NQO to some repair intermediates that do not cause SCE formation. In addition, MMS-induced lethal lesions in MS 1 may not be responsible for SCE induction whereas caffeine-induced lethal lesions are closely correlated with SCE induction. Thus, the lesions or mechanisms involved in SCE production are in part different from those responsible for cell lethality or CA production.     

Application of an epithelial liver cell line, metabolically competent, for mutation studies of promutagens.

Recently numerous attempts have been made to reduce the use of vertebrate animals in laboratory experiments to evaluate general and acute toxicity, mutagenesis and teratogenesis of new drugs or chemicals. One common approach is to use established, proliferating cell lines that preserve differentiated functions such as the competence to metabolize xenobiotics. To this end a continuous Chinese hamster epithelial liver cell line (CHEL cells) was established, cultured as used for mutagenesis studies. Structurally different promutagens, such as 7,12-dimethylbenz[a]anthracene (7,12-DMBA), benzo[a]pyrene (B(a)P), aflatoxin B1 (AB1) and cyclophosphamide (CP), were used in order to check and validate the test system. anti-Chrysene-1,2-diol 3,4-epoxide (CDE) and mitomycin C (MMC) were taken as representatives of direct mutagens. The genetic change induced by the mutagens was quantified by measuring mutation frequencies at the HGPRT locus. Several parameters, such as mutant expression time for each chemical, cell density for selection of mutants and enzymatic characterization for HGPRT phenotype, were examined to establish the optimal assay conditions. All promutagens analyzed significantly affected either the cloning efficiency and/or the mutant frequency of CHEL cells after 24 h of exposure. In addition, various enzyme activities involved in the metabolism of the promutagens were determined in CHEL cells, under the experimental conditions of chemical exposure used in the mutagenesis assay. The enzyme activities were compared with those found in uninduced Chinese hamster liver.     

Characterization and co-culture of novel nontransformed cell lines derived from rat endometrial epithelium and stroma

Summary Normal and neoplastic growth of epithelial cells depends on mutual interactions between epithelial and stromal cells. As a tool for the study of the underlying molecular mechanisms, we have developed temperature-sensitive, nontransformed cell lines derived from rat uterine epithelium and stroma by transfecting primary cultures with a temperature-sensitive mutant of the SV40 large T antigen. The epithelial and stromal cell lines obtained shared relevant morphological characteristics with the primary cells from which they were derived. Immunocytochemical analysis showed that the epithelial cell lines expressed the intermediate filament cytokeratin, whereas the stromal lines expressed the intermediate filament vimentin. Alkaline phosphatase activity was present in all cell lines examined. All cell lines were anchorage dependent and did not form foci. One epithelial cell line expressed oxytocin mRNA, a gene product recently shown to be highly expressed in vivo in the uterine epithelium at term. If grown on Matrigel, this cell line formed domelike structures, a further characteristic of its differentiated phenotype. In an attempt to reconstitute an endometrium in vitro, epithelial cells were seeded on top of a layer of stromal cells. Paraffin cross sections showed that this in vitro system consisted of a bilayer structure. Four to five cuboidal epithelial cells were typically anchored atop one stromal cell, forming an endometriumlike tissue. The present in vitro system should provide a useful model for further studies on endometrial functions and epithelial/stromal cell interactions at a molecular level.     

ATP independent proteasomal degradation of NQO1 in BL cell lines

Human NAD(P)H: quinone oxidoreductase 1 (NQO1) catalyzes the obligatory two-electron reduction of quinones. For this peculiar catalytic mechanism, the enzyme is considered an important cytoprotector. The NQO1 gene is expressed in all human tissues, unless a polymorphism due to C609T point mutation is present. This polymorphism produces a null phenotype in the homozygous condition and reduced enzyme activity in the heterozygous one. We previously demonstrated that two cell lines of haematopoietic origin, HL60 and Raji cells, possess the same heterozygous genotype, but different phenotypes; as expected for a heterozygous condition the HL60 cell line showed a low level of enzyme activity, while the Raji cell line appeared as null phenotype. The level of NQO1 mRNA was similar in the two cell lines and the different phenotype was not due to additional mutations or to expression of alternative splicing products. Here we show that in Raji BL cell line with heterozygous genotype the null NQO1 phenotype is due to 20S proteasome degradation of wild type and mutant protein isoforms and is not directly linked to C609T polymorphism. This finding may have important implications in B-cell differentiation, in leukaemia risk evaluation and in chemotherapy based on proteasome inhibitors.     

Analysis of glutathione transferase P gene regulation with liver cells in primary culture.

Glutathione transferase P (GST-P) gene is specifically and highly activated during rat chemical hepatocarcinogenesis. We have previously cloned the GST-P gene and have identified putative regulatory regions. To further explore regulatory mechanisms, deletion constructs of the GST-P gene fused to the chloramphenicol acetyltransferase (CAT) structural gene were introduced into primary cultured rat hepatocytes by electroporation, and their activity was determined. The expression of the GST-P-CAT fusion gene is quite low in these cells as compared to that in both a rat fibroblast cell line, 3Y1 cells, and a rat hepatoma cell line, dRLh84. The presence of the strong enhancer GPEI did not elicit any enhancing activity at its original position, or when it was located 3' of the CAT gene, although this element does enhance CAT activity significantly when located adjacent to the promoter. Cotransfection of neither c-jun nor c-fos expression vector, nor both vectors, could enhance the CAT activity, even though GPEI consists of two phorbol ester response element-like sites. Furthermore, the expression of jun family gene was not correlated with GST-P gene expression either in primary cultured hepatocytes or in hepatoma cell lines.     

Different growth factors stimulate cell division of rat mammary epithelial, myoepithelial, and stromal cell lines in culture

A number of single-cell-cloned cell lines have been used to examine the growth-promoting effects of putative mammotrophic agents on the various cell types in normal and neoplastic rat mammary glands. A partially purified novel pituitary-derived growth factor stimulates only cuboidal epithelial cells to divide whereas fibroblast growth factor (FGF) stimulates the growth of stromal and myoepithelial-like cells. Epidermal growth factor (EGF) has a widespread but variable growth-stimulating action, but prolactin and growth hormone are essentially inactive when added alone at a concentration of 5 micrograms/ml. Phosphoethanolamine stimulates the growth of one epithelial cell line and a derivative myoepithelial-like cell line, but is inactive on the other cell lines tested. The use of defined cloned cell lines provides a direct and reproducible assay for the identification and purification of inducers of mammary growth.     

Modulation of GST P1-1 activity by polymerization during apoptosis

Glutathione S-transferases (GSTs, EC 2.5.1.18) belong to a large family of functionally different enzymes that catalyze the S-conjugation of glutathione with a wide variety of electrophilic compounds including carcinogens and anticancer drugs. Drug resistance may result from reduction in apoptosis of neoplastic cells when exposed to antineoplastic drugs. The c-Jun N-terminal Kinase (JNK) belongs to the family of stress kinases and has been shown to be required for the maximal induction of apoptosis by DNA-damaging agents. Recently, an inhibition of JNK activity by GST P1-1, which was reversed by polymerization induced by oxidative stress, has been reported in 3T3-4A mouse fibroblast cell lines. The finding that GST P1-1 might inhibit JNK activity and that it is frequently highly expressed in tumor tissues suggests its possible implication in "apoptosis resistance" during antineoplastic therapy. We investigated the modulation of GST P1-1 during apoptosis in a neoplastic T-cell line (Jurkat) induced by hydrogen peroxide and etoposide. Apoptosis was paralleled by the appearance of a dimeric form of GST P1-1 on western blotting, associated with an increase in the Km(GSH) and a reduction in GST P1-1 specific activity toward 1-chloro-2,4-dinitrobenzene, which reached statistical significance only in H(2)O(2)-treated cells. Our data seem to suggest that H(2)O(2) and etoposide may partly act through a process of partial inactivation of the GST P1-1, possibly involving the "G" site in the process of dimerization, and thus favoring programmed cell death.     

Towards an understanding of the malignant transformation of diploid human fibroblasts

This paper reviews the major reports of the spontaneous or carcinogen-induced transformation of human fibroblasts to the malignant state, to infinite lifespan, or to anchorage independence. In some cases, the transformed cells and the parent cell with which the work began were made available to us to be tested to determine whether the cells shared common isozymes, HLA antigens, restriction-fragment length polymorphisms, marker chromosomes, etc., as one would expect. When we examined the normal fibroblastic cell line KD for these markers, and the transformed HuT cell lines developed from it by Kakunaga (Proc. Natl. Acad. Sci. (U.S.A.), 75, 1334, 1978) for these markers, we found marked differences, indicating that KD cells and HuT cells are derived from different individuals. When we applied these techniques to the 3 human fibroblast cell lines transformed by Namba to acquire infinite lifespan in culture (Gann, 27, 221, 1981), it became clear that KSMT-6 was derived from the parent cell, KMS-6, but that both cell lines CT-1 and SUSM-1 were derived from the same parental cell line, AD387. Similar studies with other sets of cell lines are also reported. In the light of these studies, it appears that there is no example of the malignant transformation of human fibroblasts by carcinogen treatment. However, neoplastic transformation and transformation to infinite lifespan by carcinogen treatment have been achieved by a number of workers. We speculate as to how malignant transformation might be obtained by carcinogen treatment.     

Mutagen-sensitive cell lines are obtained with a high frequency in V79 Chinese hamster cells

A replica-plating technique has been adopted for the isolation of mutagen-sensitive mutants of Chinese hamster V79 and CHO cell lines. After the mutagenic treatment (ENU) clones derived from these cell lines were replica plated into micro wells and replicas were treated with UV (254 nm), X-ray, MMC, EMC or MMS. Clonal cell lines which demonstrated mutagen sensitivity were retested by the determination of survival. Only one UV-sensitive line was obtained in 1500 clonal lines derived from CHO cells. This mutant appeared also sensitive to 4NQO and MMC. The sensitivity to UV and MMC was 2-3-fold enhanced, while the increase in sensitivity to 4NQO was 4-5-fold. In V79 cells 9 mutagen-sensitive lines were found after screening of 500 clonal lines; six of them showed increased sensitivity towards UV, two towards MMC, and one cell line was found to be X-ray sensitive. A considerable cross-sensitivity for the various agents was found among the isolated mutants. When a 2-fold increase is taken as a minimum to indicate mutagen sensitivity 6 mutants were sensitive to UV, 8 mutants were sensitive to MMC, 6 mutants were sensitive to 4NQO and 4 mutants were sensitive to X-rays. The difference in sensitivity to UV versus 4NQO makes it unlikely that 4NQO can be considered as a UV-mimetic agent. The sensitivity to MMC appears to fall into 2 classes: a class with moderate sensitivity (2-8-fold) and a class with high sensitivity (30-100-fold). The presence of similar classes is indicated for UV. Except for the two lines V-E5, V-B7 and the two lines V-H11, V-H4 all obtained mutants have a different spectrum of mutagen sensitivities which suggests that different genetic alterations underly these effects. The observed high frequency of mutagen-sensitive mutants in V79 cells, although unexpected and substantially higher than those published for CHO cells and L5178Y cells, can still be explained by the presence of functionally hemizygous loci.