P-glycoprotein functions and substrates: possible roles of MDR1 gene in the kidney.

There is a renewed attention on the multidrug resistance genes and their products, P-glycoproteins, since recent molecular and functional studies revealed unexpected functions in normal tissues. There are two types of human P-glycoprotein: Type I, encoded by the MDR1 gene, present in excretory organs and in non-polarized cells; and Type II, encoded by MDR2, present in the canalicular membrane of hepatocytes. MDR1 Pgp transports xenobiotics, peptides, steroids, and phospholipids, and is also a regulator of swelling-activated chloride channels. MDR2 Pgp is exclusively a phosphatidylcholine translocase. In the kidney, the MDR1 gene and protein are expressed in mesangial, proximal tubule, thick loop of Henle, and collecting duct cells. In mesangial and proximal tubule cells Pgp transports xenobiotics. Concomitant exposure of kidney cells to two Pgp substrates results in increased cell toxicity. Extracts from supernatants of mesangial cell cultures inhibit Pgp-mediated transport, suggesting that a mesangial-cell metabolite could be a substrate of Pgp. Active vitamin D3 and platelet activating factor inhibit Pgp transport and are possible endogenous substrates in proximal tubule and mesangial cells, respectively. Pgp could be also a regulator of swelling-activated chloride channels present in the kidney.