Yin-tak Woo

Development of structure-activity relationship rules for predicting carcinogenic potential of chemicals

Since the inception of Section 5 (Premanufacturing/Premarketing Notification, PMN) of the Toxic Substances Control Act (TSCA), structure-activity relationship (SAR) analysis has been effectively used by U.S. Environmental Protection Agencys (EPA) Structure Activity Team (SAT) in the assessment of potential carcinogenic hazard of new chemicals for which test data are not available. To capture, systematize and codify the Agencys predictive expertise in order to make it more widely available to assessors outside the TSCA program, a cooperative project was initiated to develop a knowledge rule-based expert system to mimic the thinking and reasoning of the SAT. In this communication, we describe the overall structure of this expert system, discuss the scientific bases and principles of SAR analysis of chemical carcinogens used in the development of SAR knowledge rules, and delineate the major factors/rules useful for assessing the carcinogenic potential of fibers, polymers, metals/metalloids and several major classes of organic chemicals. An integrative approach using available short-term predictive tests and non-cancer toxicological data to supplement SAR analysis has also been described.

Chemical Induction of Cancer

ASPERGER, H. (1932), Arch. f. kinderheilk., 97, I67. BEHREND, G. (I88o), Ibid., I, 138. BELL, D., and WOODS, A. W. (1942), Arch. Dis. Childh., 17, I57. CASPARY, J. (I884), Arch. Dermat. u. Syph., I6, I22. FERRI, U. (I928), La Pediatria, 36, 843. GOEDHART, C. (1930), Nederl. Tijdschr. v. geneesk., 74, 51I47. GOVAN, C. D., COTTON, A. L., and RYDEEN, J. O. (I950), U.S. Armed Forces M.J., x, 682. HALLEZ, G.-L. (1932), Nourrisson, 20, 270. KAPOSI, M. (z887) cit. Govan et al. LEINER, C. (I929), Monatschr. f. kinderheilk., 62, 33I. MAcGILLIVRAY, A. G. (x942), Brit. med. J., ii, 340. RITTER VON RITTERSHAIN, G. (I878), Centralzt.f. kinderheilk., 2, 3. RITTER VON RITTERSHAIN, G. (I88o), Arch. kinderheilk., I, 53.

Structural identification of p-dioxane-2-one as the major urinary metabolite of p-dioxane.

SummaryAnalysis by gas chromatography (GC) of the volatile compounds present in the urine from rats administered dioxane, a hepatic carcinogen to this species, revealed a major metabolite. The appearance of the metabolite was pH-dependent, undetectable at high pH; reacidification of the urine sample brought about the reappearance of the metabolite. The amount excreted was dose-dependent and time-dependent, reaching a maximum between 20 and 28 h after dioxane administration. Diethylene glycol administered to rats gave rise to the same metabolite. When isolated and purified from lyophilized urine by preparative GC, the metabolite exhibited an intense carbonyl band at 1750 cm−1 in the infrared spectrum. Nuclear magnetic resonance spectrum showed two triplets and one singlet with equal intensity at δ 3.85, 4.48 and 4.37, respectively. GC-mass spectrometric studies indicated a parent peak at m/e 102. The metabolite was identified as p-dioxane-2-one. Synthetic reference compound exhibited identical IR, NMR, and GC-mass spectra as the metabolite. The tentative pathway and the biological significance of dioxane metabolism are discussed.

Indirect Modification of Chemical Carcinogenesis by Nutritional Factors Through Regulation of the Mixed-Function Oxidase System

The nutritional status of the host may significantly affect chemical carcinogenesis through modification of its microsomal mixed-function oxidase (MFO) system in the target tissue(s). The MFO system, a multifunction monooxygenase enzyme system—consisting of cytochrome P-450 (as the terminal oxidase) and cytochrome P-450 reductase—is the key enzyme system involved in the activation and/or detoxification of most, if not all, chemical carcinogens. The overall effect of nutritional modification of chemical carcinogenesis via MFO is dependent on: (i) the type of nutritional factor, (ii) the metabolic activation/detoxification profile of the chemical carcinogen, (iii) the specific enzymic form of cytochrome P-450 affected, and (iv) the specific target tissue involved.

Brief Overview of the Endocrine System

The endocrine system is involved in virtually all aspects of the mammalian organism, its biochemical, physiological, locomotor, reproductive, and psychologic functions. At the cellular level, hormones participate in gametogenesis, fertilization, implantation, organogenesis, and differentiation. In the total organism hormones mediate, for example, muscular activity, respiration, digestion, hematopoiesis, thought, mood, and behavior. There are few, if any, organs or tissues in mammalian systems that are not affected either directly or indirectly by the endocrine system. Therefore, the concept of specific target organs or tissues for hormones is probably only valid insofar as the degree of sensitivity and the extent at which organs or tissues are affected by a given hormone. The involvement and complexity of the endocrine system in mammalian organisms are astonishing, pervasive, and downright awesome.

Editors’ Note Added in Proof: Stress Proteins: Heat-Shock Proteins/Molecular Chaperones

Investigations in the last few years led to the discovery of stress proteins and their catalytic roles in the folding of nascent proteins, as well as in the maintenance of/refolding native functional protein conformations in the face of stress-related insult. Insult to the cell environment activates a set of genes which express proteins that can repair stress-induced molecular damage to proteins and reestablish normal homeostasis. The standardizeh form of stress that has been used in studies with prokaryotic and eukaryotic cell cultures is exposure to unusually elevated temperatures (e.g., 42°C). This evokes a heat-shock response by inducing the dramatically enhanced expression of heat-shock-proteins (Hsp) to stabilize and reestablish partially denatured protein conformations. Under normal conditions Hsp’s are expressed at relatively low levels.

Computerized Data Management as a Tool to Study Combination Effects in Carcinogenesis

Humans and animals are exposed, either sequentially or simultaneously, to combinations of a variety of chemical agents which include low doses of naturally occurring and synthetic chemical carcinogens as well as carcinogenesis-modifying agents such as inhibitors and promoters. Assessment of the potential cancer hazard of exposure to environmentally occurring complex chemical mixtures/combinations is a difficult and challenging toxicological problem and a subject of major current concern to both the scientific and regulatory communities (e.g., 1–3). Besides the usual problems associated with risk assessment of individual chemicals, there are three major obstacles associated with mixtures: (a) the impossibility of testing myriads of possible combinations of chemicals, (b) the lack of a universally accepted index for quantitative measurement of cancer risk of chemical carcinogens, and (c) the uncertainty of the possible outcomes of interactions among the various constituents of the mixture.

On Evidence for Preventive Significance of Dietary Supplementation

Chapter 4 and Sections III and IV of the present chapter review the tumor inhibitory and/or carcinogenesis chemopreventive effects in experimental animals of certain vitamins and minerals, in particular vitamin A-like compounds (retinoids and carotenoids), vitamin C, vitamin E, vitamin D3 (as 1,25-dihydroxycholecalciferol), selenium, calcium, and copper.

Editors’ Note Added in Proof: On the Significance of Environmental Xenoestrogens

Because of the well-established connection between estrogens and neoplasia, a widely publicized 1993 epidemiological study [M. S. Wolff et al.: J. Natl. Cancer Inst. 85, 648 (1993)] focused attention on the cancer risk of exposure to environmentally distributed chemical agents, which are either estrogenic themselves or affect estrogen production and metabolism (i.e., are xenoestrogens). This case-control study of 58 prospectively gathered cases from a cohort of 14,000 women found that women having the highest serum levels of DDE (a metabolite of the pesticide DDT) had a fourfold greater risk of breast cancer than women with the lowest levels. However, a second epidemiological study which revisited the topic one year later [N. Krieger et al.: J. Natl. Cancer Inst. 86, 589 (1994)] came to a diametrically opposite conclusion and found no association between risk of breast cancer and serum levels of DDE in 150 women who developed breast cancer in an average of 14 years after the samples have been collected. Despite the overall inconclusive outcome of these two recent epidemiological studies, other results support the association between chlorinated organics and breast cancer. In one study the breast lipids of women with cancer at biopsy had about 40% more of certain chlorinated pesticides, such as metabolites of DDT and higher levels of PCBs [F. Falck, Jr., et al.: Arch. Environ. Health 47, 143 (1992)]. In another study 50% more hexachlorocyclohexane was detected in pooled blood of breast cancer cases compared to a pooled reference group [H. Mussalo-Rauhamaa et al.: Cancer 66, 2124 (1990)].

Effect of Hormones on Carcinogenesis by Nonhormone Chemical Agents

An early clue that the endocrine system has a significant influence on the induction of tumors by nonhormone chemical agents came from the study of Korteweg and Thomas (1) in 1939. They found that hypophysectomy increases the latent period of skin tumor induction in mice by epithelial application of benzo[a]pyrene (BaP) (108 to 184 days in operated mice versus 44 to 100 days in intact mice). However, Smith et al. (2) reported 3 years later that hypophysectomy has little or no effect on carcinogenesis by subcutaneous administration. Later results in the 1950s lent credence to the above divergent findings of Korteweg and Thomas (1) and Smith et al. (2). Thus, pituitary dwarf mice (which lack growth hormone [GH] and some other pituitary hormones) respond poorly to skin tumor induction by 3-methylcholanthrene (3-MC) (3), but respond normally to carcinogenesis by subcutaneous administration of this compound (4, 5).