Hsp90 functions to balance the phosphorylation state of Akt during C2C12 myoblast differentiation

The function of the 90-kDa heat shock protein (Hsp90) is essential for the regulation of a myriad of signal transduction cascades that control all facets of a cell's physiology. Akt (PKB) is an Hsp90-dependent serine-threonine kinase that plays critical roles in the regulation of muscle cell physiology, including roles in the regulation of muscle differentiation and anti-apoptotic responses that modulate cell survival. In this report, we have examined the role of Hsp90 in regulating the activity of Akt in differentiating C2C12 myoblasts. While long-term treatment of differentiating C2C12 cells with the Hsp90 inhibitor geldanamycin led to the depletion of cellular Akt levels, pulse-chase analysis indicated that geldanamycin primarily enhanced the turnover rate of newly synthesized Akt. Hsp90 maintained an interaction with mature Akt, while Cdc37, Hsp90's kinase-specific co-chaperone, was lost from the chaperone complex upon Akt maturation. Geldanamycin partially disrupted the interaction of Cdc37 with Akt, but had a much less significant effect on the interaction of Hsp90 with Akt. Surprisingly, short-term treatment of differentiating C2C12 with geldanamycin increased the phosphorylation of Akt on Ser473, an effect mimicked by treatment of C2C12 cells with okadaic acid or the Hsp90 inhibitor novobiocin. Furthermore, Akt was found to interact directly with catalytic subunit of protein phosphatase 2A (PP2Ac) in C2C12 cells, and this interaction was not disrupted by geldanamycin. Thus, our findings indicate that Hsp90 functions to balance the phosphorylation state of Akt by modulating the ability of Akt to be dephosphorylated by PP2Ac during C2C12 myoblast differentiation.