OKADAIC ACID, SPHINGOSINE, AND PHORBOL ESTER REVERSIBLY MODULATE HEATINDUCTION 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 simila r to 200% of control level. However, when cells were first treated wit h 500 nM tumor promoter phorbol ester TPA at 37 degrees C for 30 min t o activate cellular protein kinase C (PKC) or with 400 nM okadaic acid at 37 degrees C for 30 min to inhibit cellular protein phosphatases f ollowed by heat shock at 46 degrees C for another 30 min, the heat ind uction on kinase FA/GSK-3 alpha was found to be completed blocked. In sharp contrast, when cells were first treated with 1 mu M TPA at 37 de grees C for 24 h or with 5 mu M 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 similar to 250% of cont rol level, demonstrating that kinase FA/GSK-3 alpha may not represent a constitutively active/mitogen-inactivated protein kinase as previous ly conceived. Taken together, the results provide initial evidence tha t TPA/sphingosine and okadaic acid could reversibly modulate the heat induction on kinase FA/GSK-3 alpha. in A431 cells, suggesting that pho sphorylation/dephosphorylation mechanisms are involved in the regulati on of the heat-shock induction of kinase FA/GSK-3 alpha, representing a new mode of signal transduction for the regulation of this multisubs trate protein kinase and a new mode of signaling pathway modulating th e heat-induction process. (C) 1996 Wiley-Liss, Inc.