Abstract
Albumin in the glomerular filtrate is normally retrieved by concerted efforts of clathrin, LDL-type receptor megalin- and clathrin-associated sorting proteins. In glomerular diseases, albumin overload triggers a proapoptotic and inflammatory response contributing to tubulointerstitial fibrosis and tubular atrophy. The relationship between albumin overload-induced proximal tubule injury and albumin endocytosis remains to be discovered. We investigated presence of a possible overlap between endocytosis and cell survival. We showed a novel interaction between prosurvival protein, protein kinase B (PKB/Akt), and adaptor protein, disabled 2 (Dab2), with coimmunoprecipitation. Further delineation of this interaction by GST pull-down experiments utilizing different Dab2 constructs identified proline-rich domain as the interacting partner. Expression of Dab2 and PKB/Akt was downregulated at high concentrations of albumin associated with apoptosis. We then examined the physiological relevance of this interaction with functional studies. Overexpression of PKB/Akt increased albumin uptake in human proximal tubule cells. Conversely, inhibition of PKB/Akt with a nonselective Akt/PKB signaling inhibitor-2 and a dominant negative construct of PKB/Akt resulted in a decrease in albumin uptake. Inhibition of Dab2 by silencing RNA abolished PKB/Akt-induced albumin uptake demonstrating the physiological importance of this novel interaction. We concluded that PKB/Akt is part of an endocytic machinery and it mediates albumin uptake through its interaction with Dab2. The role that PKB/Akt plays in the endocytic cascade may dictate its decreased expression in proteinuric states in an attempt to limit albumin endocytosis that may tilt the balance between cell survival and apoptosis toward cell death.