AMPK protects proximal tubular cells from stress-induced apoptosis by an ATP-independent mechanism: potential role of Akt activation

Abstract

We examined the role of AMP-activated protein kinase (AMPK) in modulating the viability of cultured kidney proximal tubular cells subjected to metabolic stress induced by either dextrose deprivation, inhibition of glycolysis, or inhibition of mitochondrial respiration. We used BU.MPT cells, a conditionally immortalized kidney epithelial cell line derived from the proximal tubules of transgenic mice bearing a temperature-sensitive mutation of the simian virus 40 large-tumor antigen. All three forms of metabolic stress increased the phosphorylation and activity of AMPK. Activation of AMPK led to changes in the phosphorylation of two downstream targets of AMPK, acetyl coenzyme A carboxylase and p70 S6 kinase. Inhibition of AMPK, either pharmacologically with compound C (CC) or by gene silencing, significantly increased the amount of apoptosis in response to all three forms of metabolic stress. Although the amount of apoptosis was directly related to the severity of ATP depletion, inhibition of AMPK had no effect on cellular ATP levels. Notably, metabolic stress increased the phosphorylation and activity of Akt. Furthermore, inhibition of AMPK, with CC or gene silencing, abrogated the ability of metabolic stress to activate Akt. The augmentation of apoptosis induced by inhibition of AMPK was comparable to that induced by inhibition of Akt. We conclude that activation of AMPK following acute metabolic stress plays a major role in promoting the viability of cultured proximal tubular cells. Protection by AMPK appears to be due not to AMPK-mediated conservation of cell energy stores, but rather, at least in part, to AMPK-mediated activation of Akt.