Presensitization of human cells with extrinsic signals to induced chemical carcinogenesis

Metabolism and binding of benzo[a]pyrene in randomly-proliferating, confluent and S-phase human skin fibroblasts

The metabolism of benzo[a]pyrene in randomly proliferating and confluent cultures of human skin fibroblast cells was compared with cell cultures in early S phase of the cell cycle after a G1 block. When each cell population was exposed to [G-3H]benzo[a]pyrene for 24 hours and the organic soluble metabolites in the extracellular medium and intracellular components were analyzed by HPLC, a quantitative increase in metabolism was observed in the confluent cell populations. The amount of organic soluble metabolites in the extracellular medium of the confluent dense cultures was 2.7 times the amount found in randomly proliferating cultures and 1.5 times that of the synchronized cultures. The trans-7,8- and 9,10 dihydrodiols and 3-hydroxy benzo[a]pyrene were the major metabolites formed. Small amounts of the sulphate conjugate, 9-hydroxy-benzo[a]pyrene and the tetrols were also detected. Cytoplasmic as well as nuclear extracts from the confluent cell cultures also contained higher amounts of metabolites compared to those from the randomly proliferating and S-phase cells. The levels of DNA modification by metabolically activated benzo[a]pyrene did not differ among the randomly proliferating, confluent and S-phase cells. However, the S-phase cells exhibited approximately 50-fold increase in the frequency of transformation compared to the randomly proliferating cells. Confluent cells were not transformed by benzo[a]pyrene. These data suggest that factors other than random modification of DNA by the carcinogen might have a significant role in the expression of a transformed phenotype and that metabolism and transformation are not directly related. Furthermore, confluent dense cultures with a heightened capability for metabolism of benzo[a]pyrene were more active in the detoxification of benzo[a]pyrene than in the production of the metabolites associated with cellular transformation.

Cocarcinogenicity of saccharin and N-alkylnitrosoureas in cultured human diploid fibroblasts

Previous attempts to transform human foreskin fibroblasts in vitro with N-methylnitrosourea (MNU) or N-ethylnitrosourea (ENU) have been unsuccessful, and concurrent treatment with cocarcinogens or tumor promotors and either MNU or ENU have also failed to produce a neoplastic response. The present study was undertaken to test the effect of sodium saccharin on MNU- or ENU-induced cell transformation. Saccharin alone was not effective in inducing the growth of colonies in soft agar (anchorage-independent growth). However, concurrent treatment with saccharin (50 micrograms/ml, nontoxic dose) and MNU or ENU (29 micrograms/ml or 44 micrograms/ml, respectively) was effective in inducing transformation (greater than 300 colonies/10(5) cells), but only when the cells were treated with saccharin after being released from a G1 block (amino acid deprivation) and followed by MNU or ENU treatment in early S phase. In contrast to results obtained with other chemical carcinogens, transformation frequencies induced by saccharin and MNU or ENU were only slightly decreased in the absence of insulin, which is normally required for growth in this system. Saccharin-MNU- or saccharin-ENU-treated cells that exhibited growth in soft agar also exhibited cellular invasiveness in 9-d-old embryonic chick skin in vitro. In addition, these cells reacted with a monoclonal antibody prepared against a molecular weight 115,000 sarcoma-cell surface-associated glycoprotein and also developed tumors in nude mice. These data demonstrate the cell-cycle-dependent cocarcinogenic potential of saccharin and MNU or ENU in cultured human skin fibroblasts.

Comparison of features of carcinogen-transformed human cells in vitro with sarcoma-derived cells

To define characteristics of chemically transformed phenotypes during and after progression to neoplasia and to assess their relationship to those phenotypes expressed by surgically removed sarcoma lesions, we compared the characteristics in the following manner. We investigated: (1) alterations in growth patterns; (2) anchorage-independent growth; (3) reactivity with monoclonal antibodies directed against surface antigen; (4) invasiveness in embryonic chick skin; (5) tumorigenicity in nude mice; and (6) karyology. Fifty different sarcoma cell lines were examined which exhibited different rates and absolute numbers of population doublings. With one exception, all sarcoma cell lines exhibited a finite life span ranging from 60 to 100 population doublings. Populations of these cells that exhibited anchorage-independent growth in soft agar also reacted positively with a monoclonal antibody (MoAb) 345.134S directed against a 115K-GP cell surface glycoprotein. Similarly, chemically transformed cells that grew in soft agar also reacted with the MoAb 345.134S, whereas cells with an inability to grow in soft agar did not. Cell lines established from human sarcoma and from chemically transformed human fibroblasts that reacted positively with the MoAb 345.134S were invasive for embryonic chick skin and formed tumors in nude mice. The selection medium used during culture of the carcinogen-treated cells resulted in the appearance of an altered phenotype that after at least 16 population doublings exhibited characteristics common to those cells derived from human sarcomas.

A comparative study of dimethylhydrazine regioisomers and the methylazoxymethanol metabolite of 1,1- and 1,2-dimethylhydrazine in relation to transformation in human fibroblasts

Comparative analysis of the cytotoxicity, transformation efficiency, induction of alkali labile sites (ALS) and DNA methylation in human foreskin fibroblasts was carried out with two dimethylhydrazine (DMH) regioisomers (1,1-DMH and 1,2-DMH) and the acetate (A) derivative of the metabolite methylazoxymethanol (MAM) of 1,2-DMH. Effective ED50 cytotoxic doses for MAMA, 1,1-DMH and 1,2-DMH were 0.056, 6.83 and 6.30 mM, respectively. MAMA and 1,1-DMH were more effective transformers than 1,2-DMH. However, methylation of purines accounted for less than 1% of the total radiolabel associated with DNA for all 3 agents. 1,2-DMH, 1,1-DMH and MAMA induced O6MeGua/N7MeGua ratio of 0.04, 0.32 and 0.18, respectively. Only MAMA induced measurable alkali labile lesions at transforming doses. These results suggest that other mechanisms may play a role in the initiation of transformation events by hydrazine analogues.

Preferential binding of benzo[a]pyrene diol epoxide to the linker DNA of human foreskin fibroblasts in S phase in the presence of benzamide

Addition of benzamide (BZ) at the onset of S phase inhibited expression of the neoplastic phenotype in human foreskin fibroblasts treated in vitro with (+/-)-7 alpha,8 beta-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P diol epoxide) in early S phase. Analysis of the specific B[a]P diol epoxide-DNA adducts revealed that ca. 65% of the total adducts in BZ and non-BZ carcinogen-treated cells was the B[a]P diol epoxide-deoxyguanine adduct. Limited micrococcal nuclease digestion of the early S phase nuclei from cells treated with B[a]P diol epoxide indicated that the carcinogen binds equally to linker and core DNA. However, when the cells were predominantly in S phase, in the presence of BZ, there was ca. three times more binding of B[a]P diol epoxide to the linker DNA compared to the core region. The confluent cells in G1 cell arrest treated only with B[a]P diol epoxide also bound the carcinogen preferentially to the linker region. These data indicate that pretreatment of the cells with BZ at the onset of S phase established a preferential binding pattern in the linker DNA similar to that observed in the cells treated with B[alpha]P diol epoxide in G1 arrest.

Cell cycle-dependent intervention by benzamide of carcinogen-induced neoplastic transformation and in vitro poly(ADP-ribosyl)ation of nuclear proteins in human fibroblasts

Human fibroblasts were subjected to nutritionally induced G1 block, followed by release and subsequent entry into S phase, and exposed to nontoxic concentrations of carcinogens in early S phase. Cell transformation occurred as determined by early morphologic cell alterations, anchorage-independent colony formation, cell invasiveness, and augmentation of Ab 376 human malignancy-specific cell-surface antigenic determinant. Methylazoxymethanol acetate was the most potent transforming agent at doses that were negative in toxicity tests. Benzamide (10 microM intracellular concentration), a specific inhibitor of poly(ADP-ribose) polymerase, prevented transformation in a cell cycle-specific manner, maximal prevention coinciding with early S phase, also characteristic of maximal susceptibility to transformation. Neither an interference of carcinogen deoxyguanosine nucleoside adduct formation nor a chemical reaction between benzamide and carcinogens was detected. Methylazoxymethanol acetate at transforming but nontoxic dose partially inhibited poly(ADP-ribosyl)ation to about the same extent as benzamide. However, simultaneous exposure of cells to both agents in early S phase, resulting in the prevention of transformation, augmented poly(ADP-ribosyl)ation above the controls. Enzymatic activities ran parallel with the formation of DNA-associating polymer-nonhistone protein adducts that are assumed to regulate the physiological function of chromatin at the structural level.

A comparison of expression of neoplastic potential of carcinogen-transformed human fibroblasts in nude mice and in chick embryonic skin

Human foreskin fibroblasts transformed by representative chemicals from five different classes of chemical carcinogens, some requiring enzymatic activation and direct acting carcinogens, produced cell populations that exhibited anchorage-independent growth and expression of neoplastic potential in either nude mice or chick-embryonic skin (CES). There is a high degree of correlation between tumor incidence and invasiveness of CES. The unique feature of CES is the rapidity of expression of cellular neoplasia and interpretation of the simulated tumor in 4 d as a simulated fibrosarcoma. This method represents a system that can be used to evaluate human carcinogens in vitro in 6 to 10 wk.

Characterization of human cells transformed by chemical and physical carcinogens in vitro

Several different classes of chemical carcinogens induced the transformation of human fibroblasts grown in vitro. Characteristics of the events that occur from time of treatment through the expression of neoplastic transformation are presented. The S-phase appeared to be the portion of the cell cycle most vulnerable to insult. Staging of the cells by blocking them in G1 before releasing them to proceed through scheduled DNA synthesis (S) was required to induce reproducible transformation. Compounds such as insulin were added to the cells upon release from the block to sensitize the cells to the carcinogen that was added during S. Growth of the transformed cells as distinct from nontransformed cells was promoted by growth in medium supplemented with 8X nonessential amino acids. Carcinogen-treated cells in the early stage of transformation exhibited abnormal colony morphology and were able to grow at 41 degrees C, in air atmosphere, and in medium supplemented with only 1% serum. In addition, the transformed cells were insensitive to KB cell lysate and exhibited density independent, as well as anchorage independent, growth (i.e., growth in 0.33% agar). Cells that grew in soft agar also produced undifferentiated mesenchymal tumors in preirradiated nude mice.

Characteristics of benzo[a]pyrene and A-ring reduced 7,12-dimethyl benz[a]anthracene induced neoplastic transformation of human cells in vitro

The polynuclear aromatic hydrocarbons (PAH) benzo[a]pyrene (BP) and the A-ring reduced analogue of 7,12-dimethylbenz[a]anthracene (DMBA), 1,2,3,4-tetrahydro-7,12-dimethylbenz[a]anthracene (TH-DMBA) are carcinogenic to human cells. The unsaturated PAH, DMBA exhibits no carcinogenic activity on human cells as measured by growth in soft agar. The TH-DMBA and BP treated cells exhibit a colony frequency in soft agar of 84 and 86, respectively. These anchorage independent cells, when seeded on the chick embryonic skin (CES) organ cultures, are invasive and form a fibrosarcoma. It is highly unlikely that TH-DMBA, which does not contain an aromatic A-ring, can undergo metabolism in human cells in culture to form a bay region 3,4-dihydrodiol-1,2-epoxide. These results suggest that an alternate mechanism for the induction of carcinogenesis is appropriate to explain the absence of bay region diol-epoxide metabolite as the ultimate form of the carcinogen in TH-DMBA induced carcinogenesis in human diploid cells.