DIFFERENTIAL REGULATION BY CALCIUM REVEALS DISTINCT SIGNALING REQUIREMENTS FOR THE ACTIVATION OF AKT AND P70(S6K)

Activation of the phosphatidylinositol 3-kinase (PI3K) plays an import ant role in the mitogenic response of many cell types. Recently, two s erine/threonine kinases Akt and p70(S6k) have been identified as physi ological targets of FI3K. Observations that expression of activated fo rms elf Akt led to the activation of p70(S6k) implied Akt might mediat e mitogenic signaling through activation of p70(S6k). TO clarify the r elationship between signaling through these two kinases, we have exami ned their regulation by various mitogenic stimuli. In this study we ha ve focused on the role of calcium in the regulation of each kinase in Balb/c-3T3 fibroblasts. Depletion of intracellular calcium stores by E GTA pretreatment has no effect on growth factor-induced Akt activation but completely abolishes p70(S6k) stimulation. Increase of intracellu lar calcium induced by ionomycin or thapsigargin results in a full act ivation of p70(S6k), whereas little or no activation of Akt is observe d. Furthermore, although PI3k in anti-phosphotyrosine immunoprecipitat es is only very weakly activated by ionomycin, the calcium-induced sti mulation of p70(S6k) is completely inhibited by the specific FISH inhi bitor wortmannin. We conclude Akt and p70(S6k) Ii, on separate signali ng pathways, Activation of signaling to Akt is insufficient for the ac tivation of p70(S6k), which can be achieved independently of Akt. p70( S6k) requires a separate calcium-dependent and wortmannin-sensitive pr ocess that is likely to be independent of type, I-A PI3K family member s.