Okadaic acid stimulated TRE binding activity in a papilloma producing mouse keratinocyte cell line involves increased AP-1 expression.

The effects of the non-phorbol ester type tumor promoter okadaic acid, a serine-threonine phosphatase inhibitor, on activator protein 1 (AP-1) DNA binding activity were studied in papilloma producing 308 mouse keratinocytes. Okadaic acid increased AP-1 binding to a consensus TPA responsive element (TRE) within 2 h; maximum stimulation was observed at 6 h followed by a gradual decrease to basal levels within 24 h. Jun B, Jun D and Fos B proteins were identified as the major components of the AP-1 complex binding to the TRE element at 6 h. Inhibition of transcription with actinomycin D and inhibition of protein synthesis with cycloheximide abrogated the okadaic acid effect on AP-1 DNA binding, indicating that transcription and translation are required for okadaic acid increased TRE binding activity. Northern and Western blot analyses revealed a correlation between increased AP-1 binding activity and accumulation of jun B, jun D and fos B mRNAs and proteins. These data suggest increased AP-1 expression as principal mechanism of okadaic acid stimulated AP-1 activation in the mouse keratinocytes studied.