Differential ion
Differentiation 13, 65-66 (1979)
0 Springer-Vcrlag 1979
Cell Regulation and Cancer I. B. WEINSTEIN Columbia University College of Physicians and Surgeons. New York, N Y 10032, USA Abstract written by M. Radman
Cancer epidemiology and experimental carcinogenesis have established that the carcinogenic process is multifactor in its causation and multistep in its development. Classical studies by Berenblum [ 11 have defined the socalled two-stage mouse skin carcinogenesis system, which has since been shown to apply to induced carcinogenesis in several other organs and organisms. This system demonstrates at least two qualitatively different phases of the carcinogenic process, which are elicited by two distinctly separate classes of chemical agents, initiators and promoters of carcinogenesis. The characteristics of initiating agents which differentiate them from the promoting agents are: (1) initiators are carcinogenic at high doses, i.e., they are solitary carcinogens, (2) in the two-stage experimental carcinogenesis they must be given before the promoting agent, (3) single exposure is sufiicient and their action is irreversible, (4) initiators are electrophiles that bind to nucleic acids and other macromolecules, ( 5 ) they are mutagenic: strong carcinogens tend to be strong mutagens, weak carcinogens tend to be weak mutagens. Promoting agents are not carcinogenic by themselves, but when given after an initiating agent they greatly enhance the frequency, and shorten the lag-time of induced tumors. Promoting agents must be applied repeatedly for long periods of time because their action is reversible at early stages. There is no evidence of their covalent binding to nucleic acids, nor is there evidence that they are mutagenic. Many potent carcinogens (initiators) must be metabolically activated to yield the active electrophile. Dr. Weinstein reviewed the molecular nature of the active derivatives of benzo(a)pyrene (7,8dihydroxy, 9,lOepoxy-7,8,9,10 tetrahydro benzo(a)pyrene [2] and of N2-acetylaminofluorene I3, 41. Obviously, different mutagenic carcinogens produce very different molecular
adducts with various DNA constituents and, except for their electrophilicity, there is little molecular resemblance among different carcinogens. However, their lesions to DNA, when not correctly repaired by the excision repair system, cause genetic (i.e., mutations, such as base substitutions, frameshifts, deletions, and chromosomal rearrangements) and possibly also epigenetic (i.e., SOS induction in bacteria and perhaps alterations in chromatin structure, DNA methylation, feedback loops, etc.) cellular changes [SI. Dr. Weinstein also reviewed the work done in his and other laboratories on the cellular effects of the most efficient known tumor promoter 12-O-tetradecanoylphorbol- 13-acetate (TPA). TPA causes both in vivo and in cell culture two distinct classes of events: earb, reversible events and late, irreversible events. In vivo, initiated cells can grow to produce benign papillomas, which may revert when TPA application is discontinued. In cell culture, a mimicry of transformation [61 is observed including (1) cell surface and membrane changes (altered morphology, altered NdK ATPase, increased phospholipid synthesis, altered fucose-glycopeptides, decreased LETS proteins, altered receptors, increased uptake of 32P,“Rb, deoxyglucose, and altered membrane fluidity) [ 5 , 61, (2) growth properties (increased cell saturation density, altered cell-cell orientation, decreased serum requirement) [61, (3) enzyme induction (increased plasminogen activator [ 71, ornithine decarboxylase [81, and prostaglandin syntheses [91) and inhibition of terminal differentiation in various cell culture systems (types of differentiations studied: erythroid, myogenesis, chondrogenesis, lipocytes, neurite, and melanogenesis) [5, 61. All these effects are early, inducible, and reversible upon removal of TPA, and hence do not readily explain why prolonged applications of TPA on mouse skin or in cell culture eventually result in the for0301-4681/79/0013/0065/1 01.00
66 mation of malignant tumors and oncogenically transformed cells respectively, that do not revert upon removal of TPA. How a cellular response mechanism that is normally inductive is converted to one which is constitutive or autonomous, remains one of the major dilemmas in carcinogenesis. It seems likely that the answer to this question relates to the nature of the irreversible change in cells produced by the initiating agent. Two hypotheses have been presented. One, proposed already by Berenblum [ l l and accentuated lately by Weinstein [SI and by Cairns [lo], postulates initiation as an aberrant differentiation program which remains dormant until expression of related genes are induced by the promoting agent. An alternative theory proposed by Kinsella and Radman [l 11 is that a late and irreversible step involves a further change in the cellular genome induced by the tumor promoter. Specifically, they have proposed that the initiating event results in a cell carrying (a) recessive chromosomal change($, probably mutations, and that tumor promoters cause their segregation to homozygosity or hemizygosity by inducing aberrant mitotic segregations (e.g., through mitotic recombination, non-disjunction and chromosomal rearrangements). This speculation was based on their finding that TPA induces sister chromatid exchanges, probably by inducing recombination enzymes, and that antipromoters inhibit such induction. Dr. Weinstein concluded with an optimistic view, that a number of cell culture systems, genetic a p proaches, and molecular technologies are now available for specifically testing the above as well as other theories. Such fundamental research would not only profoundly influence our approaches to cancer prevention and treatment but should also provide new insights into the mechanisms of cellular growth control, development, and differentiation.
I. B. Weinstein: Cell Regulation and Cancer
References 1. Berenblum. 1.: Carcinogenesis and tumor pathogenesis. Adv.
Cancer Res. 2, 129 (1954) 2. Weinstein, I. B., Jeffrey, A. M.,Jennette, K. W., Blobstein, S. H., Harvey, R. G., Harris, C., Autrup, H.. Kasai, H., Nakanishi, K.: Bcnzo(a)pyrene diolcpoxides as intermediates in nucleic acid binding in vitro and in vivo Science 193, 592 (1976) 3. Fuchs, R. P. P., Lcfevre, J. F., Pouyet, J., Daune, M.P.: Orientation of the fluorene residue in native DNA modified by Nacetoxy-N2-acetylaminofluoreneand two 7-halogen derivatives. Biochemistry IS, 3347 (1976) 4. Grunbcrgcr, D., Weinstein, I. B.: The base displacement model: An explanation of the conformational and functional changes in nucleic acids modified by chemical carcinogens. In: Biology of radiation carcinogenesis. Yuhas, J. M.,Tennant, R. W., Regan, J. D. (eds.), pp. 175-187. New York: Raven Press 1976 5. Weinstein, I. B., Yamasaki, H.,Wigler, M.,Lee, L.-S.,Fisher, P. B., Jeffrey, A., Grunberga, D.: Molecular and cellular events associated with the action of initiating carcinogens and tumor promoters. In: Carcinogens: Identification and mechanisms of action. GMin, A. C., Shaw, C. R. (eds.), pp. 399-418. New York: Raven Press 1979 6. Wcinstan, I. B., Wigler, M.:Cell culture studies provide new information on turnout promoters. Nature, 270, 659 (1978) 1. Wigler, M.,Weinstein. I. B.: Tumour promoter induces plasminogen activator. Nature 259, 232 (1976) 8. Yuspa, S. H., Lichti, U., Ben, T., Patterson, E., Hennings, H., Slaga, T. J., Colburn, N., Kelsey, W.: Phorbol esters stimulate DNA synthesis and ornithine decarboxylase activity in mouse epidermal cell cultures. Nature 262, 402 (1976) 9. Levine. L., Hassid, A.: Effects of phorbol-l2,13diesters on prostaglandin production and phospholipase activity in canine kidney (MDCK) cells. Biochem. Biophys. Res. Commun. 79, 417 (1977) 10. Cairns, J.: Some thoughts about cancer research in lieu of a summary. In: Origins of human cancer. Hiatt, H., Watson, J. D., Winsten, J. A. (cds.). pp. 1813-1820. Cold Spring Harbor Laboratory, Cold Spring Harbor N.Y. 1977 11. KinseUa, A., Radman, M.:Tumor$romoter induces sister chromatid exchanges: relevance to mechanisms of carcinogenesis. Proc. Natl. Acad. Sci. USA 75, 6149 (1978)
Differentiation 13, 65-66 (1979)
0 Springer-Vcrlag 1979
Cell Regulation and Cancer I. B. WEINSTEIN Columbia University College of Physicians and Surgeons. New York, N Y 10032, USA Abstract written by M. Radman
Cancer epidemiology and experimental carcinogenesis have established that the carcinogenic process is multifactor in its causation and multistep in its development. Classical studies by Berenblum [ 11 have defined the socalled two-stage mouse skin carcinogenesis system, which has since been shown to apply to induced carcinogenesis in several other organs and organisms. This system demonstrates at least two qualitatively different phases of the carcinogenic process, which are elicited by two distinctly separate classes of chemical agents, initiators and promoters of carcinogenesis. The characteristics of initiating agents which differentiate them from the promoting agents are: (1) initiators are carcinogenic at high doses, i.e., they are solitary carcinogens, (2) in the two-stage experimental carcinogenesis they must be given before the promoting agent, (3) single exposure is sufiicient and their action is irreversible, (4) initiators are electrophiles that bind to nucleic acids and other macromolecules, ( 5 ) they are mutagenic: strong carcinogens tend to be strong mutagens, weak carcinogens tend to be weak mutagens. Promoting agents are not carcinogenic by themselves, but when given after an initiating agent they greatly enhance the frequency, and shorten the lag-time of induced tumors. Promoting agents must be applied repeatedly for long periods of time because their action is reversible at early stages. There is no evidence of their covalent binding to nucleic acids, nor is there evidence that they are mutagenic. Many potent carcinogens (initiators) must be metabolically activated to yield the active electrophile. Dr. Weinstein reviewed the molecular nature of the active derivatives of benzo(a)pyrene (7,8dihydroxy, 9,lOepoxy-7,8,9,10 tetrahydro benzo(a)pyrene [2] and of N2-acetylaminofluorene I3, 41. Obviously, different mutagenic carcinogens produce very different molecular
adducts with various DNA constituents and, except for their electrophilicity, there is little molecular resemblance among different carcinogens. However, their lesions to DNA, when not correctly repaired by the excision repair system, cause genetic (i.e., mutations, such as base substitutions, frameshifts, deletions, and chromosomal rearrangements) and possibly also epigenetic (i.e., SOS induction in bacteria and perhaps alterations in chromatin structure, DNA methylation, feedback loops, etc.) cellular changes [SI. Dr. Weinstein also reviewed the work done in his and other laboratories on the cellular effects of the most efficient known tumor promoter 12-O-tetradecanoylphorbol- 13-acetate (TPA). TPA causes both in vivo and in cell culture two distinct classes of events: earb, reversible events and late, irreversible events. In vivo, initiated cells can grow to produce benign papillomas, which may revert when TPA application is discontinued. In cell culture, a mimicry of transformation [61 is observed including (1) cell surface and membrane changes (altered morphology, altered NdK ATPase, increased phospholipid synthesis, altered fucose-glycopeptides, decreased LETS proteins, altered receptors, increased uptake of 32P,“Rb, deoxyglucose, and altered membrane fluidity) [ 5 , 61, (2) growth properties (increased cell saturation density, altered cell-cell orientation, decreased serum requirement) [61, (3) enzyme induction (increased plasminogen activator [ 71, ornithine decarboxylase [81, and prostaglandin syntheses [91) and inhibition of terminal differentiation in various cell culture systems (types of differentiations studied: erythroid, myogenesis, chondrogenesis, lipocytes, neurite, and melanogenesis) [5, 61. All these effects are early, inducible, and reversible upon removal of TPA, and hence do not readily explain why prolonged applications of TPA on mouse skin or in cell culture eventually result in the for0301-4681/79/0013/0065/1 01.00
66 mation of malignant tumors and oncogenically transformed cells respectively, that do not revert upon removal of TPA. How a cellular response mechanism that is normally inductive is converted to one which is constitutive or autonomous, remains one of the major dilemmas in carcinogenesis. It seems likely that the answer to this question relates to the nature of the irreversible change in cells produced by the initiating agent. Two hypotheses have been presented. One, proposed already by Berenblum [ l l and accentuated lately by Weinstein [SI and by Cairns [lo], postulates initiation as an aberrant differentiation program which remains dormant until expression of related genes are induced by the promoting agent. An alternative theory proposed by Kinsella and Radman [l 11 is that a late and irreversible step involves a further change in the cellular genome induced by the tumor promoter. Specifically, they have proposed that the initiating event results in a cell carrying (a) recessive chromosomal change($, probably mutations, and that tumor promoters cause their segregation to homozygosity or hemizygosity by inducing aberrant mitotic segregations (e.g., through mitotic recombination, non-disjunction and chromosomal rearrangements). This speculation was based on their finding that TPA induces sister chromatid exchanges, probably by inducing recombination enzymes, and that antipromoters inhibit such induction. Dr. Weinstein concluded with an optimistic view, that a number of cell culture systems, genetic a p proaches, and molecular technologies are now available for specifically testing the above as well as other theories. Such fundamental research would not only profoundly influence our approaches to cancer prevention and treatment but should also provide new insights into the mechanisms of cellular growth control, development, and differentiation.
I. B. Weinstein: Cell Regulation and Cancer
References 1. Berenblum. 1.: Carcinogenesis and tumor pathogenesis. Adv.
Cancer Res. 2, 129 (1954) 2. Weinstein, I. B., Jeffrey, A. M.,Jennette, K. W., Blobstein, S. H., Harvey, R. G., Harris, C., Autrup, H.. Kasai, H., Nakanishi, K.: Bcnzo(a)pyrene diolcpoxides as intermediates in nucleic acid binding in vitro and in vivo Science 193, 592 (1976) 3. Fuchs, R. P. P., Lcfevre, J. F., Pouyet, J., Daune, M.P.: Orientation of the fluorene residue in native DNA modified by Nacetoxy-N2-acetylaminofluoreneand two 7-halogen derivatives. Biochemistry IS, 3347 (1976) 4. Grunbcrgcr, D., Weinstein, I. B.: The base displacement model: An explanation of the conformational and functional changes in nucleic acids modified by chemical carcinogens. In: Biology of radiation carcinogenesis. Yuhas, J. M.,Tennant, R. W., Regan, J. D. (eds.), pp. 175-187. New York: Raven Press 1976 5. Weinstein, I. B., Yamasaki, H.,Wigler, M.,Lee, L.-S.,Fisher, P. B., Jeffrey, A., Grunberga, D.: Molecular and cellular events associated with the action of initiating carcinogens and tumor promoters. In: Carcinogens: Identification and mechanisms of action. GMin, A. C., Shaw, C. R. (eds.), pp. 399-418. New York: Raven Press 1979 6. Wcinstan, I. B., Wigler, M.:Cell culture studies provide new information on turnout promoters. Nature, 270, 659 (1978) 1. Wigler, M.,Weinstein. I. B.: Tumour promoter induces plasminogen activator. Nature 259, 232 (1976) 8. Yuspa, S. H., Lichti, U., Ben, T., Patterson, E., Hennings, H., Slaga, T. J., Colburn, N., Kelsey, W.: Phorbol esters stimulate DNA synthesis and ornithine decarboxylase activity in mouse epidermal cell cultures. Nature 262, 402 (1976) 9. Levine. L., Hassid, A.: Effects of phorbol-l2,13diesters on prostaglandin production and phospholipase activity in canine kidney (MDCK) cells. Biochem. Biophys. Res. Commun. 79, 417 (1977) 10. Cairns, J.: Some thoughts about cancer research in lieu of a summary. In: Origins of human cancer. Hiatt, H., Watson, J. D., Winsten, J. A. (cds.). pp. 1813-1820. Cold Spring Harbor Laboratory, Cold Spring Harbor N.Y. 1977 11. KinseUa, A., Radman, M.:Tumor$romoter induces sister chromatid exchanges: relevance to mechanisms of carcinogenesis. Proc. Natl. Acad. Sci. USA 75, 6149 (1978)
