OKADAIC ACID INDUCES ACTIVATOR PROTEIN-1 ACTIVITY AND IMMEDIATE-EARLYGENE-TRANSCRIPTION IN RAT PHEOCHROMOCYTOMA CELLS - MECHANISM OF ACTION

The serine/threonine protein phosphatase inhibitor okadaic acid (OA) w as found to enhance mRNA transcripts of c-fos and of the jun family of proto-oncogenes including c-jun, jun B and jun D in cultured pheochro mocytoma PC12 cells. This expression remained elevated for more than 8 hr. An increase in the binding of the transcription factor activator protein 1 (AP1) to its DNA consensus sequence that occurred prior to e arly gene transcription was observed. Enhanced AP1 activity was still observed when OA was added to the cells together with the transcriptio n inhibitor actinomycin D, or with the protein synthesis inhibitor cyc loheximide, indicating that it is actually AP1 activation due to postt ranslational modifications that triggers transcription of the fos and jun genes. AP1 was activated through serine/threonine phosphorylation since its activation was abolished when nuclear extracts of OA-treated cells were incubated with protein phosphatase-l or, to a lesser exten t, with protein phosphatase-2A. C-Jun and Jun D proteins are likely ca ndidates for being phosphorylated, since they were shown to constitute the AP1 complex at the time when it was activated (2 hr after OA addi tion).