Nm. Conus et al., DIFFERENTIAL REGULATION BY CALCIUM REVEALS DISTINCT SIGNALING REQUIREMENTS FOR THE ACTIVATION OF AKT AND P70(S6K), The Journal of biological chemistry, 273(8), 1998, pp. 4776-4782
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.