Angiotensin II Stimulates Activation Of SGK1 Though mTORC2, But Not Akt, In an mSIN1-Dependent Manner In Renal Proximal Tubule Cells

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: MON 723-757-Renin-Angiotensin-Aldosterone System/Endocrine Hypertension
Bench to Bedside
Monday, June 17, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board MON-755
Catherine E Gleason*1 and David Pearce2
1UCSF, San Francisco, CA, 2Univ of CA - San Francisco, San Francisco, CA
In addition to its well-established roles in blood pressure regulation and ion homeostasis, the kidney also plays an important role in glucose homeostasis. Renal gluconeogenesis is restricted to proximal tubule cells of the kidney cortex where it is stimulated by hormones such as angiotensin II (AngII) and glucocorticoids. In contrast to AngII and glucocorticoids, insulin inhibits gluconeogenesis in the kidney, as it does in liver. Multiple lines of evidence support the idea that two related kinases, SGK1 and Akt, mediate these opposing effects. SGK1 has been implicated as a mediator of AngII effects on ion balance in various renal cell types, including the proximal tubule where it has been shown to stimulate NHE3 expression and activity. Akt, on the other hand, mediates insulin-stimulated inhibition of gluconeogenesis. Both of these kinases require phosphorylation of a critical homologous residue in their hydrophobic motif (HM) by the same upstream kinase, mTORC2, for activation.  How mTORC2 is able to modulate activity of these related kinases in such a way that allows them to mediate distinct effects on electrolyte balance and energy metabolism is not understood. This is a particularly important and interesting question to address in the proximal tubule considering that both AngII and insulin have been shown to stimulate SGK1 and Akt HM phosphorylation in other cell types yet produce opposing physiological effects on renal glucose production. Previously, we determined that SGK1, but not Akt, forms a direct interaction with the mammalian stress-activated map kinase interacting protein, mSIN1. mSIN1, a defining protein of the mTORC2 complex, is required for mTORC2 complex formation and stability. Disruption of the mSIN1/SGK1 interaction by a point mutation, Q68H, within the binding region impairs SGK1 activity, but not Akt. To further understand the molecular mechanisms underlying mTORC2 substrate specificity, we studied the affect of AngII on SGK1 and Akt HM phosphorylation in HEK-293-AT1R cells and proximal tubule-derived cells lines, OKP and LLC-PK1. We found that AngII stimulation triggers SGK1 HM phosphorylation in a PI3K- and mTORC2-dependent manner. However, in the same experiments, Akt HM phosphorylation was not altered. Interestingly, AngII induces phosphorylation of mSIN1 and we have identified the key residues modified during stimulation. AngII-induced mSin1 phosphorylation does not regulate the interaction of mSIN1 with SGK1 but is required for SGK1 HM phosphorylation. Together, our results suggest that via modification of mSIN1, mTORC2 can respond uniquely to AngII in a manner that achieves specific activation of SGK1 but not Akt. These findings have potentially important implications for defining molecular mechanisms that specify selective control of ion balance and glucose metabolism in proximal tubule cells.

Nothing to Disclose: CEG, DP

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