The function and mechanism of promoters of carcinogenesis

Induction of epidermal hyperplasia, hyperkeratosis, and papillomas in transgenic mice by a targeted v-Ha-ras oncogene

The regulatory elements of the human keratin K1 gene have been used to target expression of the v-Ha-ras oncogene exclusively in the epidermis of transgenic mice. We developed 12 transgenic mouse lines that express the HK1.ras transgene, producing epidermal hyperplasia in neonates and hyperkeratosis in juveniles. Eventually this skin phenotype diminished but with time adult animals developed papillomas that could persist or regress. The rate and frequency of tumorigenesis appeared to be limited, which suggests that v-Ha-ras requires a second or even third event to elicit and maintain a benign phenotype in transgenic mice. Since in certain transgenic lines papillomas appeared at wound sites, it appears that the promotion stimulus from wounding may be a second event. We envision that such transgenic mice that express v-Ha-ras in the epidermis will become a powerful model for assessing how environmental and molecular factors affect the process of multistage skin carcinogenesis in vivo, as well as a model for evaluating novel therapeutic protocols.

Vitamin E and cancer prevention: recent advances and future potentials

Many animal and in vitro experiments have shown that the supplementation of diet with vitamin E within a certain dose range reduced the risk of chemical- and radiation-induced cancers. In vitro studies revealed that alpha-tocopheryl succinate (TS) induced differentiation and growth-inhibition in certain animal and human tumor cells in culture, whereas alpha-tocopherol (alpha-T), alpha-tocopheryl acetate (alpha-TA) and alpha-tocopheryl nicotinate (alpha-TN) were ineffective, alpha-TS also reduced basal and ligand-stimulated adenylate cyclase activity, and expression of c-myc and H-ras oncogenes in certain tumor cells in culture. The relative efficacy of various forms of vitamin E in cancer prevention in animal or human models has not been evaluated. Human epidemiologic studies utilizing retrospective and prospective case-control experimental designs are not suitable for evaluating the role of vitamin E in cancer prevention due to several inherent problems associated with these methodologies. Intervention trials utilizing vitamin E with appropriate biological and statistical rationales are most suitable for testing the role of vitamin E in cancer prevention in humans. Some human trials utilizing vitamin E alone or in combination with other nutrients are in progress.

Detection of mutant Ha-ras genes in chemically initiated mouse skin epidermis before the development of benign tumors

An activated Ha-ras oncogene has been consistently found in chemically initiated benign and malignant mouse skin tumors, and an activated ras oncogene has been shown to initiate the process of mouse skin carcinogenesis. However, the exact timing of mutational activation of the Ha-ras gene relative to application of the chemical carcinogen is not known. A sensitive mutation-specific PCR technique was used to experimentally address the timing of Ha-ras gene mutational activation. This technique can detect mutant Ha-ras alleles in the presence of a very large excess of normal ras alleles. Activated Ha-ras genes with 61st codon A----T mutations were found in the epidermis of mice 1 week after topical initiation with 7,12-dimethylbenz[a]anthracene or urethane by using this assay. These results were confirmed by Xba I restriction fragment length polymorphism analysis and direct DNA sequencing. One week after initiation is 1-2 months before the appearance of benign papillomas that harbor activated Ha-ras oncogenes when the initiated mice are promoted with the tumor promoter phorbol 12-myristate 13-acetate. Our data support the hypothesis that initiated epidermal cells containing an activated Ha-ras gene can remain dormant in the skin until a tumor promoter induces regenerative hyperplasia that allows for outgrowth of these cells with an activated ras oncogene to give rise to a benign papilloma.

Expression of alpha 6 beta 4 integrin increases during malignant conversion of mouse epidermal keratinocytes: association of beta 4 subunit to the cytokeratin fraction

The expression of alpha 6 beta 4 integrin has been analyzed in several keratinocyte cell lines representative of various stages of mouse epidermal carcinogenesis. The immunological analyses carried out show that alpha 6 beta 4 is expressed at the cell surface of the cell lines which exhibit an epithelial or epithelioid morphology. The relative levels of alpha 6 beta 4 expressed at the cell surface increase noticeably from premalignant to malignant cells, as detected by fluorescence flow cytometry. This increase also correlates with the abundance of soluble beta 4 subunit detected by Western immunoblotting in the different cell lines. However, complete absence of alpha 6 beta 4 has been found in spindle carcinoma cells showing a fibroblast-like phenotype in culture. The integrin remains associated to detergent- and high salt-insoluble cytoskeletal components, organized in stable anchoring contacts, as in human keratinocytes (Carter et al., J. Cell Biol., 111, 3141, 1990). In addition, a significant fraction of the beta 4 subunit is detected associated to highly purified cytokeratin fractions. These results, together with those regarding the organization of both cellular components in drug-treated cells, support the existence of a close association between alpha 6 beta 4 and the intermediate filaments of the cytoskeleton.

Effect of benzoyl peroxide on two-stage oral carcinogenesis and gamma-glutamyl transpeptidase in hamsters

Hamster buccal pouches were treated with 7,12-dimethylbenz(a)anthracene (DMBA) triweekly for 3 wk and subsequently with 40% benzoyl peroxide (BP) in acetone for up to 27 wk. BP treatment resulted in a marked hyperplasiogenic effect and a weak tumor promoting effect. Whereas most carcinogens and tumor promoters induce gamma-glutamyl transpeptidase (GGT) activity, BP diminished its activity as compared to controls. Comparable results have also been noted in the liver, where a group of newly isolated hepatocarcinogens, peroxisome proliferators (PP), also characteristically deplete the GGT activity and placental glutathione S-transferase (GST-P), another tumor marker.

Two protein kinase C activators, bryostatin-1 and phorbol-12-myristate-13-acetate, have different effects on haemopoietic cell proliferation and differentiation

Primary B lymphocytes can be induced to proliferate and certain haemopoietic cell lines such as HL60 and U937 can be induced to differentiate by the addition of phorbol esters, which have been shown to activate protein kinase C. Several non-phorbol esters, such as the bryostatins, have also been shown to bind to and activate protein kinase C. Although bryostatin-1 and 12-O-tetradecanoylphorbol-13-acetate (TPA) compete for and activate protein kinase C to the same degree and with similar kinetics and also induce similar levels of expression of the CD23 cell-surface antigen, bryostatin-1 is a weak mitogen for B lymphocytes and fails to induce the differentiation of both HL60 and U937 cells. Such an outcome suggests that these two activators have different binding properties for the enzyme that have a physiological consequence which may be useful for analysing the role that protein kinase C plays in both differentiation and proliferation. Analysis of competition assays between bryostatin-1 and TPA leads us to put forward a model where protein kinase C is required to be constantly reactivated and recycled during proliferation and differentiation which can be accomplished by TPA but not by bryostatin, although we cannot exclude the differential activation of some of the sub-species of the kinase by the two agonists.

Effects of acute vs. chronic phorbol ester exposure on transepithelial permeability and epithelial morphology

In previous experiments we have shown that acute (30 minutes) exposure to phorbol esters or other protein kinase C activators causes increased transepithelial permeability, specifically by the increased paracellular permeability through tight junctions. However, the role of protein kinase C activators in carcinogenesis is predicted upon a chronic exposure of an effective dose at frequent intervals for a prolonged period of time. We therefore sought to determine the effect of chronic phorbol ester exposure on transepithelial permeability by exposing cells of the polar renal epithelial cell line, LLC-PK1, to phorbol esters for time periods as long as 16 weeks. The following changes ensued: (1) after the initial drop in transepithelial resistance due to phorbol ester exposure, i.e., an increase in transepithelial permeability (in the acute phase of exposure), an adaptive response occurs as transepithelial resistances in chronically exposed cultures recover to approximately 50% of control values, (2) the cell sheets in chronically exposed cultures lose their acute responsiveness of transepithelial permeability to phorbol ester exposure, (3) cell sheet architecture changes as cells occasionally multilayer and actual polyp-like cell masses appear at high frequency, and (4) cytosolic protein kinase C activity decreases to 50% of control level with acute exposure and then is further decreased to less than 1% of control level in chronically treated cells; membrane-associated PKC activity is not as sharply decreased. The possible role of transepithelial permeability in carcinogenesis and the value of chronically treated epithelial cell cultures as a model for two-stage carcinogenesis are discussed.

Modifications in tissue histamine levels in mice treated with chronic epicutaneous application of phorbol-12-myristate-13-acetate

Chronic epicutaneous application of phorbol-12-myristate-13-acetate (PMA) on the right flank of C 57 BL/6 mice induces an inflammatory reaction and a selective increase in skin histamine level at the site of treatment. Tissue histamine levels were not increased at other sites such as contralateral skin, stomach rumen and skeletal muscle, and were decreased in stomach fundus and dorsal skin. These results are in agreement with the hypothesis that mast cells play a significant role in cutaneous inflammation induced by PMA.

Physiological and clinical aspects of vitamin A and its metabolites

Retinoids, including retinol and retinoic acid (RA), are a group of naturally occurring and synthetic compounds that exhibit vitamin A-like biological activity. They achieve their effects by binding to intracellular proteins. Important sites of action are the nuclear retinoic acid receptors (RAR). These receptors, namely, RAR alpha, RAR beta, and RAR gamma, function as transcription factors by binding to RA-responsive elements (RARE) of multiple genes. Retinoids play a role in vision, embryogenesis, immune modulation, growth and differentiation of normal, premalignant and malignant tissues, the suppression of carcinogenesis, and the inhibition of tumor growth in experimental systems and humans. Reports of the significant antitumor effect of all-trans-RA in acute promyelocytic leukemia and the synthesis of new, less toxic, and more potent retinoids has generated renewed interest in these compounds. Retinoids may have an important role to play in the chemoprevention and therapy of cancer.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Mechanisms of X-ray-mediated protooncogene c-jun expression in radiation-induced human sarcoma cell lines

c-jun is a protooncogene associated with neoplastic transformation and is transcriptionally induced by ionizing radiation. To examine the possible mechanisms of radiation-induced c-jun transcription, we analyzed RNA from human tumor cell lines RIT-3 and STSAR-5 following x-irradiation in the presence of protein kinase inhibitors, or the absence of serum and calcium. Protooncogene c-jun expression increased several fold following irradiation of these radiation-induced human sarcoma cell lines. The expression of c-jun was not altered following irradiation in conditioned medium containing serum as compared to that of cells in serum free medium. Depletion of PKC by prolonged TPA treatment resulted in inhibition of c-jun expression. In addition, nonspecific protein kinase inhibitors, staurosporin and H7 attenuated c-jun expression, whereas the analogue of ATP (sangivamycin) did not. Furthermore, the selective inhibitor of cAMP dependent protein kinase HA 1004 did not alter radiation-mediated c-jun induction. These data indicate that ionizing radiation exposure results in c-jun induction which is dependent upon the activation of PKC. Protein kinase C activation and the subsequent expression of the protooncogene c-jun by ionizing radiation may further define the molecular mechanisms of radiation-induced neoplastic transformation.

A role for the E-cadherin cell-cell adhesion molecule during tumor progression of mouse epidermal carcinogenesis

The expression of the cell-cell adhesion molecules E- and P-cadherin has been analyzed in seven mouse epidermal keratinocyte cell lines representative of different stages of epidermal carcinogenesis. An inverse correlation between the amount of E-cadherin protein and tumorigenicity of the cell lines has been found, together with a complete absence of E-cadherin protein and mRNA expression in three carcinoma cell lines (the epithelioid HaCa4 and the fibroblastoid CarB and CarC cells). A similar result has been detected in tumors induced in nude mice by the cell lines, where induction of E-cadherin expression takes place in moderately differentiated squamous cell carcinomas induced by HaCa4 cells, although at much lower levels than in well-differentiated tumors induced by the epithelial PDV or PDVC57 cell lines. Complete absence of E-cadherin expression has been observed in spindle cell carcinomas induced by CarB or CarC cells. P-cadherin protein was detected in all cell lines that exhibit an epithelial (MCA3D, AT5, PDV, and PDVC57) or epithelioid (HaCa4) morphology, as well as in nude mouse tumors, independent of their tumorigenic capabilities. However, complete absence of P-cadherin was observed in the fibroblast-like cells (CarB and CarC) and in spindle cell carcinomas. The introduction of an exogenous E-cadherin cDNA into HaCa4 cells, or reactivation of the endogenous E-cadherin gene, leads to a partial suppression of the tumorigenicity of this highly malignant cell line. These results suggest a role for E-cadherin in the progression to malignancy of mouse epidermal carcinogenesis. They also suggest that the loss of both E- and P-cadherin could be associated to the final stage of carcinogenesis, the development of spindle cell carcinomas.

Inhibition of intercellular communication in Chinese hamster V79 cells by fractionated asphalt fume condensates

Asphalt fume condensate is a skin carcinogen in mice, yet this complex mixture contains relatively low levels of known carcinogenic initiators. Consequently, its biological activity has been attributed to the presence of cocarcinogenic or tumor-promoting agents. One of several proposed mechanisms of tumor promotion is inhibition of intercellular communication. In an attempt to determine if asphalt fume has tumor-promoting potential inhibition of intercellular communication was measured in V79 cells exposed to fractionated asphalt fume condensate. Fume from air-blown Arabian crude asphalt was trapped and separated into five fractions by preparative-scale high-pressure liquid chromatography. The parent fume condensate and the five fractions inhibited intercellular communication in a concentration-dependent fashion, with a minimum effective concentration of 2.5 microgram/ml for the most potent fraction. Cytotoxicity assays were performed at the same time and concentrations as the metabolic cooperation assays. Cytotoxic responses paralleled the inhibition of intercellular communication.

Involvement of protein kinase C in the regulation of ornithine decarboxylase mRNA by phorbol esters in rat hepatoma cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulates a rapid increase in ornithine decarboxylase (EC 4.1.1.17; ODC) activity in target cells. Here we demonstrate that this process involves a rapid accumulation of ODC mRNA, which is maximal 3 h after treatment (three- to eightfold greater than control cells) and decays to control levels within 18 h. Stimulation of ODC mRNA by TPA is blocked by phorbol dibutyrate down-regulation of protein kinase C (PKC). ODC mRNA was also induced by the PKC activators, phospholipase C and 1-oleoyl-2-acetyl-rac-glycerol, and blocked by kinase inhibitors (trifluoroperazine, H7, and palmitoyl-L-carnitine), consistent with a requirement for PKC activation in the induction mechanism. However, the non-PKC-specific protein kinase inhibitor HA1004 also suppressed expression of ODC mRNA in response to TPA, under conditions where it did not inhibit PKC, suggesting that additional kinases may be involved in the intracellular signalling process. The stability of the ODC mRNA (control value = 6.2 +/- 1.6 h) is not significantly changed by either TPA (5.7 +/- 0.8 h) or by cycloheximide (6.0 h). These results are inconsistent with any contribution from altered mRNA half-life towards the accumulation of ODC mRNA following treatment with phorbol ester tumor promoters.

An overview of the relationship between oxidative stress and chemical carcinogenesis

Substantial experimental evidence exists which implicates both oxygen- and organic-free radical intermediates in the multiple stages of chemical carcinogenesis. This overview summarizes some of the biochemical and molecular interactions which could result from a state of oxidative stress following the generation of free radical intermediates from chemical carcinogens in target cells. Evidence for a relationship between carcinogenesis and inflammation as a mediator of oxidative stress is also discussed. Understanding the interactions of radical intermediates with target biomolecules should lead to the development of relevant biomarkers of these interactions as well as rational chemoprotective strategies with antioxidants or other radical detoxifiers for the prevention of neoplasia.

v-Ha-ras transgene abrogates the initiation step in mouse skin tumorigenesis: effects of phorbol esters and retinoic acid

Experimental carcinogenesis has led to a concept that defines two discrete stages in the development of skin tumors: (i) initiation, which is accomplished by using a mutagen that presumably activates a protooncogene, and (ii) promotion, which is a reversible process brought about most commonly by repeated application of phorbol esters. We have created a transgenic mouse strain that carries the activated v-Ha-ras oncogene fused to the promoter of the mouse embryonic alpha-like, zeta-globin gene. Unexpectedly, these animals developed papillomas at areas of epidermal abrasion and, because abrasion can also serve as a tumor-promoting event in mutagen-treated mouse skin, we tested these mice for their ability to respond to phorbol ester application. Within 6 weeks virtually all treated carrier mice had developed multiple papillomas, some of which went on to develop squamous cell carcinomas and, more frequently, underlying sarcomas. We conclude that the oncogene "preinitiates" carrier mice, replacing the initiation/mutagenesis step and immediately sensitizing them to the action of tumor promoters. In addition, treatment of the mice with retinoic acid dramatically delays, reduces, and often completely inhibits the appearance of promoter-induced papillomas. This strain has use in screening tumor promoters and for assessing antitumor and antiproliferative agents.

Subpopulations of primary adult murine epidermal basal cells sedimented on density gradients

Epidermal cells were harvested from the dorsal skin of adult mice by trypsinization and were sedimented through continuous density gradients of Percoll, formulated to separate basal cells of different buoyant density. Five fractions from the gradients were characterized with regard to the number of cells present, their viability and morphology and their basal origin. Suprabasal keratinocytes remained primarily at the top of the gradient; basal keratinocytes sedimented throughout. With increasing density, a relative enrichment was observed: (i) for [3H]-thymidine and [3H]-benzo[alpha]pyrene label-retaining (slowly cycling) keratinocytes; (ii) for keratinocytes that could proliferate in vitro in the continuous presence of 0.1 micrograms ml-1 of 12-O-tetradecanoylphorbol-13-acetate; (iii) for cells from untreated as well as initiated epidermis able to proliferate under conditions where calcium induces terminal differentiation; and (iv) for primary in vitro clonogenic keratinocytes from normal epidermis. The relative enrichment for epidermal basal cells having characteristics thought to be associated with immaturity and with the initiation and promotion of skin carcinogenesis suggests that density gradient sedimentation could be used in conjunction with other methods for the eventual purification of epidermal progenitors.

Inhibition of growth of established human glioma cell lines by modulators of the protein kinase-C system

The protein kinase-C (PKC) second messenger system contributes to regulation of cell growth and differentiation. This study was undertaken to examine the effects of modulators of the PKC enzyme system on the state of differentiation and proliferation rates of human gliomas in vitro. The administration of the PKC-activating phorbol esters 4-beta-phorbol-12,13-dibutyrate (PDB) and phorbol-12-myristate-13-acetate (PMA) resulted in a dose-related inhibition of growth of human glioma cell lines in vitro as measured by 3H-thymidine uptake. The synthetic nonphorbol PKC activator (SC-9) produced an even more pronounced decrease of 3H-thymidine uptake. Diacylglycerol, an endogenous activator of the system, applied externally, transiently decreased the proliferation, in concordance with its short-lived existence in vivo. Conversely, the administration of 4-alpha-phorbol-12,13-didecanoate (alpha-PDD), a phorbol ester that binds but does not activate the enzyme, had no effect on the proliferation rate. At the dosages that maximally decreased proliferation, there was no evidence of direct glioma cell lysis induced by these agents as measured by a chromium-release assay. Immunocytochemical analysis and cytofluorometric measurement of glial fibrillary acidic protein (GFAP) staining in the treated cultures revealed an increase in GFAP staining over control cultures. In contrast to the response of glioma cells, nonmalignant human adult astrocytes treated with the PKC activators responded by increasing their proliferation rate. The authors postulate that the diametrically opposed effects of PKC activators on nonmalignant astrocytes versus glioma growth may be due to a high intrinsic PKC activity in glioma cells, with resultant down-regulation of enzyme activity following the administration of the pharmacological activators.

Skin hyperkeratosis and papilloma formation in transgenic mice expressing a ras oncogene from a suprabasal keratin promoter

The promoter region of the suprabasal keratin 10 gene has been used to direct expression of a mutant human Harvey-ras oncogene to the differentiating cells of the mouse epidermis. Transgenic animals develop hyperkeratosis of the skin and forestomach--the two sites known to express high levels of the keratin 10 polypeptide in vivo. Papillomas subsequently develop on the skin surface, initially at sites subject to biting or scratching such as the base of the tail or behind the ears. The results suggest that the "second event" involved in tumor development in these transgenic animals is the local induction of a mild wounding stimulus. Furthermore, because the H-ras transgene is expressed in suprabasal cells, it appears that cells which have left the stem cell compartment can be induced to form at least benign tumors in vivo.

Effect of phorbol esters on the susceptibility of a glioma cell line to lymphokine-activated killer cell activity

The mechanisms by which lymphokine-activated killer (LAK) cells exert their cytotoxic effects are not well understood. This study demonstrates that phorbol ester pretreatment of a LAK cell-sensitive glioma cell line (SNB-19) induced a significant decrease in the susceptibility of cells to LAK cell-mediated lysis. This effect was produced by low concentrations of the tumor-promoting phorbol ester, phorbol-12,13-myristate acetate (PMA), and was reversible. Protein kinase C (PKC) inhibitors failed to block this phenomenon. No apparent alteration in the ability of LAK cells to bind to their targets was observed. Thus, PMA may have exerted its effects by a mechanism that does not require PKC, or these glioma cells may possess an isozyme of PKC which is insensitive to the inhibitors used in these studies.

Response of malignant glioma cell lines to activation and inhibition of protein kinase C-mediated pathways

To evaluate the role of protein kinase C-mediated pathways in the proliferation of malignant gliomas, this study examined the effect of a protein kinase C (PKC)-activating phorbol ester (12-O-tetradecanoyl-13-phorbol acetate or TPA) and a protein kinase C inhibitor (polymyxin B) on deoxyribonucleic acid (DNA) synthesis of malignant glioma cells in vitro. A serum-free chemically defined medium, MCDB 105, was employed for all studies. Two established human malignant glioma cell lines (T98G and U138), two rat glioma lines (9L and C6), and two low-passage human glioma lines (obtained from surgical specimens) were studied. With the exception of the C6 line, all tumors responded in a dose-dependent fashion to nanomolar concentrations of TPA with a median effective dose that varied from 0.5 ng/ml for the U138 glioma to 1 ng/ml for the T98G glioma. At optimal concentrations (5 to 10 ng/ml), TPA produced a two- to five-fold increase in the rate of DNA synthesis (p less than 0.05) as assessed by incorporation of 3H-thymidine. However, TPA had no additive effect on the mitogenic response produced by epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). Inhibition of PKC using the antibiotic polymyxin B (20 micrograms/ml) abolished the TPA-induced mitogenic response in the five responsive lines tested. In two tumors (U138 and 9L), polymyxin B also eliminated EGF-, PDGF-, and serum-induced DNA synthesis as well as abolishing baseline DNA synthesis. These cells remained viable, however, as assessed by trypan blue exclusion; after removal of polymyxin B from the medium, they were able to resume DNA synthesis in response to TPA and serum. In the three other tumors (T98G and the two low-passage human glioma lines), growth factor-induced and serum-induced DNA synthesis were inhibited by approximately 25% to 85%. It is concluded that PKC-mediated pathways affect DNA synthesis in the human malignant glial tumors studied. The response of the glioma cells to TPA is similar to the responses seen in fetal astrocytes, but differs significantly from those reported for normal adult glial cultures. Because the response of the 9L glioma to TPA is similar to the responses seen in the human tumors, the 9L rat glioma model may prove useful for examining the role of PKC-mediated pathways in controlling glioma growth in vivo.

Zinc increases EGF-stimulated DNA synthesis in primary mouse hepatocytes. Studies in tumor promoter-treated cell cultures

To investigate factors influencing cell proliferation, cells are often cultured in serum-free medium. In the present study it is shown that addition of zinc chloride (40 microM) to primary mouse hepatocytes, cultured in Dulbecco's minimal essential medium, markedly enhanced growth factor (EGF)-stimulated [3H]thymidine incorporation into DNA. Treatment of cell cultures with phenobarbital or 3,4,3',4'-tetrachlorobiphenyl (enzyme inducers and tumor promoters in vivo) or with 12-O-tetradecanoylphorbol-13-acetate (the classical skin tumor promoter) further increased EGF-stimulated DNA synthesis. The results emphasize the need to adequately substitute zinc in serum-free cultured cells.

Effect of glycerol on cell kinetics and tumorigenesis in mouse lung following urethan administration

Extending previous studies, we hypothesized that glycerol modulates pulmonary tumorigenesis in mice through metabolic activation in bronchiolar cells. To test this, we examined the effects of glycerol on pulmonary cell kinetics and tumorigenesis induced by urethan. Male ddY mice were given 1 mg/g urethan and/or 5% glycerol solution for a fixed period. [3H]Thymidine autoradiography revealed that glycerol administration significantly suppressed the urethan-induced facilitation of cell kinetics of the bronchiolar cells (p less than 0.05), but not that of the alveolar wall cells. Furthermore, glycerol did not affect urethan-induced pulmonary tumorigenesis. These findings suggest that glycerol modifies the metabolism of carcinogens in the bronchiolar epithelium.

Cellular and molecular biological aspects of human bronchogenic carcinogenesis

This is a time of rapid progress in the field of human bronchogenic carcinogenesis due to recent advances in cellular and molecular biology. Important developments over the last 10 years include establishment of methods for culturing NHBE cells under defined conditions, and molecular biological and biochemical epidemiological techniques for identifying genetic changes that are associated with malignant transformation of these cells. Most progress in defining genes associated with human carcinogenesis has been due to discoveries related to oncogenes and more recently, tumor suppressor genes. As was described in Section II.B.3.a, we now know that oncogene products serve as growth factors, growth factor receptors, and cytosolic and nuclear regulatory proteins. In addition, although the actions of putative tumor suppressor genes are less well understood, the first isolated tumor suppressor gene Rb, interacts with the products of DNA viruses which, in turn, are involved in regulation of transcription as was described in Section II.B.3.b. Thus, not surprisingly, both oncogenes and tumor suppressor genes code for classes of proteins that are known to play an important role in regulation of cell proliferation. Recently, a second gene that appears to possess tumor suppression activity (p53) has been identified on the short arm of chromosome 17 (17p). The initial data suggesting a possible tumor suppressor gene on chromosome 17p came from cytogenetic and RFLP studies associating loss of heterozygosity in the chromosome 17p13 region with tumor cells and tissues. Since the p53 gene is located in this region it was evaluated and found to be frequently or always altered in several types of tumor cells. Recently, it was determined that introduction of the wild-type p53 gene into NIH3T3 cells will inhibit subsequent malignant transformation. Thus, the preponderance of evidence now supports the hypothesis that while mutated p53 acts as an oncogene, the wild-type p53 gene codes for a tumor suppressor function. The role of balance between oncogenes and tumor suppressor genes in control of proliferation is presently an active area of investigation. As discussed, introduction of a chromosome containing a tumor suppressor gene will suppress tumorigenicity of a malignant cell line, even though that cell line possesses an active c-Ha-ras oncogene. Whether or not the level of expression of an activated oncogene is related to tumorigenicity is presently being investigated.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic and cellular basis of multistep carcinogenesis

Experimental evidence indicates that cancer development is a multistep process, and that multiple genetic changes are required before a normal cell becomes fully neoplastic. These genetic changes involve oncogenes, tumor suppressor genes, and possibly senescence genes. From studies in vivo using several different animal models, the stages are broadly defined as initiation, progression, and clearly involve both genetic and epigenetic events. Studies in vitro using cell culture systems have allowed the multistep process to be dissected in greater detail at both the cellular and molecular genetic level. In the Syrian hamster embryo cell culture model, neoplastic progression requires four heritable changes, involving activation of two oncogenes and loss of two tumor suppressor genes. Like the experimental systems, a limited number of studies of human tumors suggest that the multistep paradigm is also applicable, and that similar genetic events are involved in the development of cancer in humans.

Free radicals and carcinogenesis

The role of free radicals and active states of oxygen in human cancer is as yet unresolved. Various lines of evidence provide strong but inferential evidence that free radical reactions can be of crucial importance in certain carcinogenic mechanisms. A central point in considering free radical reactions in carcinogenesis is that human cancer is really a group of highly diverse diseases for which the initial causation and the progression to clinical disease occur through a wide variety of mechanisms. Furthermore, for many human cancers it appears that there are alternate pathways capable of tumor initiation and tumor progression. While for certain of these pathways free radical reactions appear necessary, it is unlikely that there are human cancers for which free radicals, or any other mechanism, are sufficient for the entire process beginning with the genetic alteration leading to a somatic mutation and eventually resulting in clinically overt disease. It is crucial that we view free radical reactions as among a panoply of mechanisms leading to human cancer, and consider research about the role of free radicals in cancer as opportunities to prevent the initiation or progression of human cancer.

Stem cell theory of carcinogenesis

Our present understanding of the carcinogenic process, involving complex interactions of genetic, developmental, sex, dietary and environmental factors during the multistage initiation/promotion/progression process of carcinogenesis, would lead us to reject simplistic non-biologically based risk assessment models. This understanding, plus recent results of the National Toxicology Bioassay program and of the studies of short-term tests for genotoxicity, has challenged the primary paradigm of 'carcinogens as mutagens' which governs our current risk assessment models. The concepts of the stem cell theory of cancer, of oncogenes/tumor suppressor genes, of gap junctional intercellular communication, and of mutagenic and epigenetic mechanisms must be integrated into a biologically-based model of the multistage nature of carcinogenesis. Current understanding of the complex interactions during this process prevents us from believing that a simple and accurate, biologically-based risk assessment model will be developed soon, if ever.

The enzyme-activated irreversible inhibitor of ornithine decarboxylase, DL-alpha-difluoromethylornithine: a chemopreventive agent

DL-alpha-Difluoromethylornithine (DFMO) is an enzyme-activated irreversible inhibitor of mammalian ornithine decarboxylase. DFMO, when administered in drinking water, precludes increases in the levels of intracellular putrescine and spermidine and also inhibits the induction of skin, breast, colon, urinary bladder, and intestinal cancers in experimental animal models. DFMO may be a useful drug for cancer prevention in humans; however, long-term medication with higher doses (9 g/m2/day) of DFMO has resulted in several toxic side effects such as thrombocytopenia and reversible ototoxicity. Smaller doses (less than 1 g/m2/day), selected by our in vitro human skin punch biopsy assay, may be given for a longer period without appreciable toxicity. Further evaluation in human cancer prevention trials is indicated.

Amplification of bovine papillomavirus DNA by N-methyl-N'-nitro-N-nitrosoguanidine, ultraviolet irradiation, or infection with herpes simplex virus

Treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or irradiation with ultraviolet light (uv254 nm) induces amplification of integrated as well as episomal sequences of bovine papillomavirus (BPV) type 1 DNA in BPV-1-transformed mouse C127 cells (i.e., ID13 cells). This is shown by filter in situ hybridization and Southern blot analysis of cellular DNA. Similarly, infection of ID13 cells with herpes simplex virus (HSV) type 1 which has been shown to be mutagenic for host cell DNA leads to amplification of BPV DNA sequences. In contrast to this induction of DNA amplification by initiators, treatment of ID13 cells with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) does not result in increased synthesis of BPV DNA nor does TPA treatment modulate the initiator-induced DNA amplification. Similar to other cell systems infection with adeno-associated virus (AAV) type 2 inhibits BPV-1 DNA amplification irrespective of the inducing agent. In contrast to initiator-induced DNA amplification, treatment with carcinogen (MNNG) or tumor promoters or combination of MNNG and promoter of C127 cells prior to transformation by BPV-1 does not lead to an increase in the number of transformed foci. The induction of amplification of papillomavirus DNA by initiating agents possibly represents one of the mechanisms by which the observed synergism between papillomavirus infection and initiators in tumorigenesis might occur.

Critical parameters in the quantitation of the stages of initiation, promotion, and progression in one model of hepatocarcinogenesis in the rat

Critical parameters in the quantitation of altered hepatic foci (AHF) developing during multistage hepatocarcinogenesis in the rat include: 1) the enumeration of AHF induced by test agents as well as those AHF occurring spontaneously in livers of untreated animals; 2) the volume percentage or fraction of the liver occupied by all AHF as a reflection of the total number of altered cells within the liver and the degree of tumor promotion which has occurred; and 3) the phenotype of individual AHF as determined by multiple markers with serial sections. These parameters, especially the number of AHF, should be corrected by the presence of spontaneous AHF which increase with the age of the animal, more so in males than females. While accurate estimation of the background level of spontaneous AHF can be important in demonstrating that a carcinogenic agent does not possess the ability to increase the numbers of AHF above the background level, a better method to distinguish the effectiveness and relative potencies of agents as initiators or promoters is reviewed. The relative effectiveness of four different markers--gamma-glutamyltranspeptidase (GGT), a placental form of glutathione S-transferase (GST), canalicular ATPase, and glucose 6-phosphatase (G6Pase)--was described for the chemicals C.I. Solvent Yellow 14 and chlorendic acid as promoting agents in males and females. C.I. Solvent Yellow 14 is a more effective promoting agent in females than males, and AHF exhibit extremely low numbers scored by GGT. On the other hand, the numbers of AHF present in livers of male rats promoted by this agent are more than twice those seen in livers of female animals, possibly owing to the effectiveness of this agent as an initiator in the male but not the female. Very few AHF, especially in the male, are scored by GGT during chlorendic acid promotion. The distribution of phenotypes with these markers also differs in the spontaneous AHF appearing in the livers of animals fed 0.05% phenobarbital on either a crude NIH-07 or AIN-76 purified diet. Such studies emphasize the extreme dependence of the promoting stage of hepatocarcinogenesis on environmental factors of sex, diet, and the molecular nature of the promoting agent itself. The hallmark of the final stage of progression in the development of hepatocellular carcinomas is aneuploidy, which may be reflected by phenotypic heterogeneity within individual AHF, termed foci-in-foci. The implications of such quantitative analyses during hepatocarcinogenesis induced by specific agents in relation to the specific action of the agent at one or more of the stages of hepatocarcinogenesis are discussed.

Synergistic inhibition of metabolic cooperation by oleic acid or 12-0-tetradecanoylphorbol-13-acetate and dichlorodiphenyltrichlorethane (DDT) in Chinese hamster V79 cells: implication of a role for protein kinase C in the regulation of gap junctional intercellular communication

The effects of TPA and/or DDT and oleic acid and/or DDT on gap junction-mediated intercellular communication (i.e. metabolic cooperation) between Chinese hamster V79 cells was examined. Addition of TPA, DDT or oleic acid alone to cocultures of 6-thioguanine-resistant (6-TGR) and 6-thioguanine-sensitive (6-TGS) V79 cells significantly increased the recovery of 6-TGR cells indicating inhibition of metabolic cooperation. In the presence of TPA and DDT or oleic acid and DDT the observed recovery of 6-TGR cells was significantly greater than the expected (calculated) additive 6-TGR cell recovery. No synergistic increases in 6-TGR cell recovery were observed when co-cultures of V79 cells were exposed to dieldrin and DDT. These results indicate that TPA and DDT or oleic acid and DDT can act synergistically to inhibit metabolic cooperation. These data suggest a role for protein kinase C in the regulation of gap junction-mediated intercellular communication.

Histologic alterations produced by chrysarobin (1,8-dihydroxy-3-methyl-9-anthrone) in SENCAR mouse skin: relationship to skin tumor promoting activity

Histologic changes induced in SENCAR skin following a single treatment with chrysarobin (1,8-dihydroxy-3-methyl-9-anthrone) exhibited differences in time course from that observed with 12-O-tetradecanoylphorbol-13-acetate (TPA). Although not significantly different, maximum elevations in epidermal thickness, total number of nucleated epidermal cells, and dark basal keratinocytes (DCs) induced by 220 nmol chrysarobin occurred at 96 h after treatment, while those induced by 3.4 nmol TPA occurred at 48 h. Both compounds elicited comparable inflammatory responses. Twice-weekly applications of chrysarobin for 2.5 weeks induced a moderate hyperplasia, increase in total nucleated epidermal cells, and increased DCs at 48 and 96 h after the last treatment, with a higher value for these parameters occurring at 48 h. Interestingly, the magnitude of these changes was similar to that observed after a single application. In contrast, twice-weekly applications of TPA induced a dramatic, potentiated induction of epidermal hyperplasia and DCs. Once-weekly applications of chrysarobin led to a potentiated induction of both hyperplasia and DCs compared to the twice-weekly treatment regimen and also more effectively promoted epidermal papillomas in previously initiated SENCAR mice. Skin sections from mice treated with chrysarobin displayed overt signs of epidermal toxicity including altered basal cell morphology and a decreased number of basal cells per 125 micron of basement membrane. Hyperplasia induced by multiple but not single treatments with chrysarobin and TPA correlated quantitatively with their papilloma promoting activity. In addition, the data suggest that epidermal toxicity may play a role in tumor promotion by anthrones.

Activation of the cellular Harvey ras gene in mouse skin tumors initiated with urethane

Mouse skin tumors, benign papillomas, and squamous cell carcinomas (SCCs) were initiated by a single topical application of urethane followed by repeated promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). Using the NIH 3T3 focus forming assay, dominant transforming activity was detected in DNA isolated from SCC samples. Rearranged and amplified copies of the c-Ha-ras gene were detected in NIH 3T3 transformant cell lines, indicating that an activated Ha-ras gene had been transferred to the NIH 3T3 recipient cells. Analysis of p21ras from the transformant cell lines suggested that the activating ras mutation was present in codon 61. Ultimately, the Ha-ras gene was shown to be activated by a specific A----T transversion at the second position of codon 61. This mutation was detected in both benign papillomas and SCCs, suggesting the activation occurred early in tumor development. The results demonstrate a highly consistent activation of the Ha-ras oncogene by a specific point mutation, suggesting a functional role for an activated ras gene in the initiation of mouse skin tumors by urethane.

Antioxidants and multistage carcinogenesis in mouse skin

The two-step initiation-promotion protocol for the induction of skin tumors in mice is a convenient model to elucidate what molecular events are involved in the multistage process of carcinogenesis and how they can be modulated. The current theories concerning the mechanisms of skin tumor initiation, stages 1 and 2 of tumor promotion, and tumor progression are reviewed. Because chemical carcinogens and tumor promoters may, directly or indirectly, generate reactive oxygen species (ROS) and because various antioxidants inhibit effectively some of the biochemical and biological events linked to tumor initiation, promotion and/or progression, it is conceivable that different sequences and levels of free radical-induced macromolecule damage may contribute to the evolution of the epidermal target cells from the preneoplastic stage to the malignant stage.

Effects of 5-azacytidine, sodium butyrate, and phorbol esters on amino acid transport system A in a kidney epithelial cell line, MDCK: evidence for multiple mechanisms of regulation

Neutral amino acid transport by system A was investigated in the epithelial cell lines MDCK and MDCK-T1. The latter line is a chemically induced, oncogenically transformed line derived from MDCK. Inducers of differentiation, sodium butyrate and 5-azacytidine, and a tumor promoter, TPA, were used as probes to delineate pathways of regulation involved in system A response to a variety of physiological conditions and agents. Azacytidine, an inhibitor of DNA methylation, and butyrate, an enhancer of histone acetylation, inhibited expression of system A, had little effect on system ASC, and slightly stimulated system L. Inhibition of system A expression by butyrate and azacytidine occurred under different conditions. Increases in system A activity due to amino acid starvation or transformation were inhibited by butyrate but not by azacytidine. Repressed system A activity, normally observed in the presence of high levels of amino acids, was more sensitive to azacytidine than to butyrate. The tumor promoter, TPA, stimulated system A activity in MDCK cells under normal growth conditions but did not stimulate activity in amino acid-starved MDCK cells or in MDCK-T1 cells. Stimulation of system A activity by TPA was prevented by prior exposure to butyrate but not to azacytidine. These results suggest 1) that system A expression observed in growing amino-acid-repressed MDCK cells is modulated by an azacytidine-sensitive mechanism and 2) that the elevated expression of system A activity induced by amino acid starvation, by chemical transformation to MDCK-T1, and by TPA is modulated by a butyrate-sensitive mechanism.

Characterization of the morphological, growth, and steroidogenic effect of TPA on mouse Y-1 adrenal cortical tumor cells in culture

The tumor-promoting agent 12-0-tetradecanoyl-phorbol-13-acetate (TPA) caused a time- and dose-dependent morphological change in Y-1 adrenocortical tumor cells. The morphological alteration was apparent 2 hr following addition of 1 microgram/ml TPA to cell cultures and became more striking with longer treatment times. Smaller doses of TPA took a longer time to produce an effect. Cultures grown in the presence of TPA exhibited more rounding and piling up of cells than similar cultures maintained in medium lacking TPA. These TPA-stimulated morphological changes were reversible, and after 24 hr in TPA-free media, the cultured cells began to flatten. After 96 hr in TPA-free media they resembled the control cultures. The reversibility of the morphological change was also dose dependent: cells treated with 1 microgram/ml TPA took a longer time to resume the typical control morphology than did cultures treated with 0.01 microgram/ml TPA. In addition, TPA treatment resulted in a decrease in cell growth rate, an increase in steroid production, and an increase in the localization of free catalytic units of cAMP-dependent protein kinase in the cytoplasm. The steroidogenic effect of ACTH on the cell population was inhibited in cultures maintained in TPA. The results of this study indicate that TPA induces morphological changes in the Y-1 adrenocortical tumor cell population while increasing steroidogenesis and the activation of cAMP-dependent protein kinase and decreasing cell growth rate.

Acetic acid, a potent agent of tumor progression in the multistage mouse skin model for chemical carcinogenesis

Acetic acid, a very weak tumor promoter in the multistage mouse skin model for chemical carcinogenesis, was found to be very effective at enhancing cancer development, when applied during the progression phase of the model. Papilloma-bearing mice when repeatedly treated with acetic acid had a greater carcinoma incidence and a greater conversion of papillomas to carcinomas than vehicle treated mice. Selective cytotoxicity is discussed as a possible mechanism.

Isolation and characterization of ras-transfected BALB/3T3 clone showing morphological transformation by 12-O-tetradecanoyl-phorbol-13-acetate

The transformation frequency of mouse BALB/3T3 cells was significantly enhanced after transfection with an activated ras oncogene (v-Ha-ras) followed by treatment with a tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), suggesting that the ras oncogene acted as an initiator in two-stage carcinogenesis. A cell clone (Bhas42) containing the ras oncogene was isolated from the ras-transfected BALB/3T3 cells. Bhas42 cells were flat and showed contact inhibition, but the addition of TPA to quiescent Bhas42 cultures resulted in a dramatic change of cell morphology to spindle shape, doubling of the cell population, and increased DNA synthesis.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.