Regulatory mechanisms of monofunctional and bifunctional anticarcinogenic enzyme inducers in murine liver

Abstract

Anticarcinogenic enzyme inducers are of two types: (a) bifunctional inducers [2,3,7,8-tetrachlorodibenzo-p-dioxin, polycyclic aromatics, azo dyes, beta-naphthoflavone] that elevate both Phase II enzymes [e.g., glutathione S-transferases, UDP-glucuronosyltransferases, and NAD(P)H:(quinone-acceptor) oxidoreductase] and certain Phase I enzymes [e.g., aryl hydrocarbon hydroxylase (AHH)]; and (b) monofunctional inducers [e.g., diphenols, thiocarbamates, 1,2-dithiol-3-thiones, isothiocyanates] that elevate primarily Phase II enzymes without significantly affecting AHH. Since Phase I enzymes such as AHH may activate precarcinogens to ultimate carcinogens whereas Phase II enzyme induction suffices to achieve chemoprotection, an understanding of the molecular mechanisms that regulate these enzymes is critical for devising methods for chemoprotection. We report a systematic analysis of the inductions of aryl hydrocarbon hydroxylase (AHH) and NAD(P)H:quinone reductase (QR) by seven monofunctional and eight bifunctional inducers, singly or in combination, in a murine hepatoma cell line (Hepa 1c1c7) and two mutants defective in either Ah (Aryl hydrocarbon) receptor function (BPrc1) or in AHH expression (c1). We have also examined such inductions in genetically defined mouse strains with high affinity (C57BL/6J) and low affinity (DBA/2J) Ah receptors. The combination of our earlier model for the induction of Phase I and Phase II enzymes (H. J. Prochaska, M. J. De Long, and P. Talalay, Proc. Natl. Acad. Sci. USA, 82: 8232, 1985) with mechanism(s) for autoregulation of AHH (O. Hankinson, R. D. Anderson, B. W. Birren, F. Sander, M. Negishi, and D. W. Nebert, J. Biol. Chem., 260: 1790, 1985) is compatible with our results. Thus, induction of QR by monofunctional inducers does not depend on a competent Ah receptor or AHH activity and appears to involve an electrophilic chemical signal. In contrast, bifunctional inducers require competent Ah receptors to induce both AHH and QR, although the latter process appears to be regulated by more than one mechanism. It is our view that bifunctional inducers bind to the Ah receptor thereby enhancing transcription of genes encoding both AHH and QR. Metabolizable bifunctional inducers are then converted by the induced AHH to products that resemble monofunctional inducers and are capable of generating the aforementioned chemical signal. The existence of mechanism(s) for AHH autoregulation that also affect Phase II enzyme expression would account for the high basal activities of QR in the AHH-defective mutant (c1).