Okadaic acid, sphingosine, and phorbol ester reversibly modulate heat induction on protein kinase FA/GSK-3 alpha in A431 cells.

Exposure of A431 cells to a rapid and sudden increase from 37 degrees C to 46 degrees C for 30 min could induce an increase in protein level and cellular activity of protein (kinase FA/GSK-3 alpha) up to approximately 200% of control level. However, when cells were first treated with 500 nM tumor promoter phorbol ester TPA at 37 degrees C for 30 min to activate cellular protein kinase C (PKC) or with 400 nM okadaic acid at 37 degrees C for 30 min to inhibit cellular protein phosphatases followed by heat shock at 46 degrees C for another 30 min, the heat induction on kinase FA/GSK-3 alpha was found to be completed blocked. In sharp contrast, when cells were first treated with 1 microM TPA at 37 degrees C for 24 h or with 5 microM sphingosine at 37 degrees C for 30 min to down-regulate cellular PKC, the heat induction on kinase FA/GSK-3 alpha was found to be reversely promoted up to approximately 250% of control level, demonstrating that kinase FA/GSK-3 alpha may not represent a constitutively active/mitogen-inactivated protein kinase as previously conceived. Taken together, the results provide initial evidence that TPA/sphingosine and okadaic acid could reversibly modulate the heat induction on kinase FA/GSK-3 alpha in A431 cells, suggesting that phosphorylation/dephosphorylation mechanisms are involved in the regulation of the heat-shock induction of kinase FA/GSK-3 alpha, representing a new mode of signal transduction for the regulation of this multisubstrate protein kinase and a new mode of signaling pathway modulating the heat-induction process.