Antibodies to extracellular regions of G protein-coupled receptors and receptor tyrosine kinases as one of the causes of autoimmune diseases
Albumin endocytosis in renal proximal tubule cells is a clathrin- and receptor-mediated mechanism that, in several pathophysiological conditions, is involved in initiating or promoting tubule-interstitial disease. Although much work has been done on this pathway, the regulation of albumin endocytosis in proximal tubule cells is not well understood. Here, we study the modulation by angiotensin II (Ang II) of albumin endocytosis in LLC-PK1, a model of proximal tubule cells. We observed that Ang II increases albumin endocytosis by approximately 100% at 10(-9) M. This effect is completely reversed by 10(-9) M PD123319, a specific AT(2) receptor antagonist, but not by losartan, a specific AT(1) receptor antagonist, at concentrations up to 10(-7) M. The Ang II effect on albumin endocytosis is also reversed by: phosphoinositide 3-kinase inhibitors LY294002 (2.5 x 10(-6) M) or wortmannin (10(-7) M), the protein kinase B inhibitor (2 x 10(-5) M), and staurosporine (2 x 10(-6) M), an inhibitor of 3'-phosphoinositide-dependent kinase 1. Ang II induced the selective phosphorylation of protein kinase B (PKB) at the Thr-308 residue without a change in Ser-473 phosphorylation, a combination that leads to an increase in PKB activity. These effects were completely abolished by 3 x 10(-6) M staurosporine or 10(-8) M PD123319. Our experiments also showed that PKB is present in the membrane fraction in overnight-starved LLC-PK1 cells. Taken together, these data show that Ang II increases albumin endocytosis through an AT(2) receptor mediated by activation of PKB in the plasma membrane, which depends on the basal activity of the phosphatidyl-inositol 3-kinase.