RECEPTOR-MEDIATED ENDOCYTOSIS OF ALBUMIN BY KIDNEY PROXIMAL TUBULE CELLS IS REGULATED BY PHOSPHATIDYLINOSITIDE 3-KINASE

Receptor-mediated endocytosis of albumin is an important function of t he kidney proximal tubule epithelium. We have measured endocytosis of [I-125]-albumin in opossum kidney cells and examined the regulation of this process by phosphatidylinositide 3-kinase (PI 3-kinase). Albumin endocytosis was inhibited by both wortmannin (IC50 6.9 nM) and LY2940 02 (IC50 6.5 mu M) at concentrations that suggested the involvement of PI 3-kinase in its regulation. Recycling rates were unaffected. We tr ansfected OK cells with either a wild-type p85 subunit of PI 3-kinase, or a dominant negative form of the p85 subunit (Delta p85) using the LacSwitch expression system, Transfects were screened by immunoblottin g with anti-PI 3-kinase antibodies, Under basal conditions, transfects demonstrated no expression of p85 or Delta p85, but expression was br iskly induced by treatment of the cells with IPTG (EC50 13.7 mu M). In hibition of PI 3-kinase activity by Delta p85 was confirmed by in vitr o kinase assay of anti-phosphotyrosine immunoprecipitates from transfe cted cells stimulated with insulin. Expression of Delta p85 resulted i n marked inhibition of albumin endocytosis, predominantly as a result of reduction of the V-max of the transport process. Expression of p85 had no significant effect on albumin uptake. The results demonstrate t hat PI 3-kinase regulates an early step in the receptor-mediated endoc ytosis of albumin by kidney proximal tubular cells.