INVOLVEMENT OF P38 MITOGEN-ACTIVATED PROTEIN-KINASE SIGNALING PATHWAYIN THE RAPID INDUCTION OF THE 78-KDA GLUCOSE-REGULATED PROTEIN IN 9L RAT-BRAIN TUMOR-CELLS

We have previously shown that treatment with okadaic acid (OA) followe d by heat shock (HS) (termed OA --> HS treatment) leads to rapid trans activation of the 78-kDa glucose-regulated protein gene (grp78) in 9L rat brain tumor cells, A cAMP-responsive element-like (CRE-like, TGACG TGA) promoter sequence and a protein kinase A signaling pathway are in volved in this induction, and activation of both CRE binding protein ( CREB) and activating transcription factor-2 (ATF-2) is required in the above process, Herein, we report that transactivation of grp78, as we ll as phosphorylation/activation of ATF-2, can be completely annihilat ed by SB203580, a highly specific inhibitor of p38 mitogen-activated p rotein kinase (p38(MAPK)). Activation of p38(MAPK) by OA --> HS is als o substantiated by its own phosphorylation as well as the phosphorylat ion and activation of MAPK activating protein kinase-2 in cells subjec ted to this treatment, The involvement of p38(MAPK) in the activation of ATF-2, which leads to the transactivation of rat grp78, is confirme d by electrophoretic mobility shift assay using a probe containing the CRE-like sequence as well as by transient transfection assays with a plasmid containing a 710-base pair stretch of the grp78 promoter, Toge ther with our previous studies, these results led us to conclude that phosphorylation/ activation of CREB upon OA --> HS treatment is mediat ed by cAMP-dependent protein kinase, whereas that of ATF-2 is mediated by p38(MAPK), The transcription factors may bind to each other to for m heterodimers that in turn transactivate grp78 by binding to the CRE- like element, This suggests that distinct signaling pathways converge on CREB-ATF-2, where each subunit is individually activated by a speci fic class of protein kinases, This may allow modulation of grp78 trans activation by diverse external stimuli.