Regulation and function of the multidrug resistance genes in liver.

The P-glycoproteins are integral membrane proteins that function as ATP-dependent transporters. The multidrug resistance genes which encode P-gp comprise a small gene family, with 2 members in humans and 3 in rodents. The P-gp encoded by the mdr1 gene functions as a drug efflux pump to remove drugs from cells and may serve as a barrier to protect cells from cytotoxic agents. In normal tissues, P-gp is localized on the luminal surface of transporting epithelia in the liver, kidney, small intestine, testes, and blood-brain barrier. Transient exposure to drugs transcriptionally increases the level of expression of the mdr1 genes, however, the cellular pathways critical to this regulation are yet unknown. This observation may have some implications on the level of expression in tumors and response to chemotherapy. Examination of the basal level of MDR expression in tumors may not be a reliable predictor of the effect of P-gp on chemotherapy. Induction of MDR transcription by drugs may further impede the effectiveness of anti-cancer agents. This is most obvious for drugs which are substrates for P-gp transport, however, it also applies significantly to compounds which are not themselves substrates but affect the response to other drugs simultaneously or subsequently administered. A clear understanding of the mechanisms that regulate basal and drug-induced mdr transcription will facilitate development of novel agents which circumvent this obstacle or permit targeted modification of mdr expression. Expressed on the bile canalicular surface of the liver, P-gps represent the first ATP-dependent biliary transporters to be characterized. The P-gp encoded by mdr2 is the major form of P-gp expressed in normal liver and transports phospholipids into bile. A defect in this protein leads to severe liver disease caused by chronic inflammation of the biliary system that results from high concentrations of free bile salts. The cellular origin and molecular basis of the ensuing liver tumors in these mice are unclear. It is possible that the chronic damage to the biliary ductules causes an increased growth rate of the surrounding cells, including putative stem cells in the liver. Thus, these mice may serve as a model for carcinogenesis in which the liver is under constant promotion placing the proliferating cells at increased risk to further genetic alterations or expansion of preexisting, but normally quiescent, mutations. Mdr2-deficient animals may also provide a model for human chronic inflammatory liver disease. Clearly, these exciting results indicate that further characterization of the P-gps as normal physiologic canalicular membrane transporters is necessary.