The primary focus of our study is to investigate the biochemical and molecular changes associated with carcinogen resistance. We developed a series of carcinogen-resistant cells by continuous exposure to BP. These cells are a model for studying the early biochemical and molecular changes that may prevent cancer due to environmental carcinogens and xenobiotics. Our BP resistant cells are also co-resistant to DMBA. To our knowledge, this is the first systematic examination of the overall mechanisms involved in carcinogen resistance, including carcinogen efflux, activation, detoxification and DNA repair. Our results suggested that the biochemical changes of carcinogen resistance are distinctively different from those for drug resistance. We first studied detoxification enzymes and P-gp, the putative efflux pump. Comparing BP resistant cells to WT, we found no major changes in detoxification enzymes and found no detectable expression of P-gp by both Western and Northern analysis. Although detoxification enzymes were unchanged, a significant increase in reduced glutathione levels was observed in BP resistant cells. Carcinogen activation enzyme, P450 reductase, was also measured in both WT and BP cells and was undetectable. Our observation suggested a redox mediated pathway cascade may be involved in carcinogen resistance. Therefore, we examined the major pathway enzyme of the pentose phosphate shunt, glucose-6-phosphate dehydrogenase (G6PD) and the redox coupling enzyme from the proline cycle, pyrroline 5-carboxylate reductase (P5CR). We found both G6PD and P5CR activities were markedly increased in BP resistant cells compared to WT. We proposed that the major changes in carcinogen resistant cells are in redox mediated DNA repair mechanisms. Our hypothesis was further supported by molecular studies of RNA expression. The expression of G6PD, P5CR, and TOPO II was increased with increasing resistance, and HGPRT expression was markedly enhanced at an early stage of resistance but did not increase with further resistance.
