Albumin endocytosis is regulated by heterotrimeric GTP-binding protein G alpha i-3 in opossum kidney cells.

Proteinuria is an adverse feature in patients with renal disease, possibly due to toxicity of albumin to proximal tubular cells. Albumin is reabsorbed from tubular fluid by receptor-mediated endocytosis. The mechanism of regulation of the endocytosis is unknown. The large quantities of G proteins in proximal tubular cell apical membranes suggests that they may have a regulatory role in endocytosis. 125I-labeled albumin uptake was measured in opossum kidney (OK) cells. This is a saturable process with high-affinity [apparent dissociation constant (Kd) = 24.3 mg/l] and low-affinity (Kd = 15.9 g/l) components. The endocytic uptake of gold-albumin into OK cells was confirmed by electron microscopy. 125I-albumin endocytosis in OK cells was inhibited by pertussis toxin, but cholera toxin had no effect. Pertussis toxin also inhibited uptake of [3H]inulin. OK cells were stably transfected with a cDNA for the G protein subunit G alpha i-3 and transfectants were screened by immunoblotting. Several G alpha i-3-overexpressing clones were detected. OK cells overexpressing G alpha i-3 demonstrate increased 125I-albumin uptake, which is abolished by pertussis toxin, in both a concentration- and time-dependent manner. These results suggest that albumin endocytosis in OK cells is regulated by the G protein G alpha i-3.