[en] The tumor promoter, okadaic acid (OA), an inhibitor of protein phosphatases, stimulates the activity of the human PRL (hPRL) proximal promoter. We analyzed in detail the effects of OA on transcription factor binding to elements P1 and P2 of this promoter, sequences known to contain at least one Pit-1 binding site each. OA treatment induces binding of an AP1-related transcription factor to the P1 site. This effect is specific, as protein binding to the P2 site is not altered by the treatment. Specific antibodies were used to confirm that the OA-induced complex is related to AP1 and to show that it contains JunD and c-fos, but not Pit-1. The increase in AP1 binding to P1 and to a canonical AP1 site correlates to an increase in cellular JunD and c-fos content. Transient transfection experiments showed that both AP1 and Pit-1 are involved in the regulation of basal and OA-stimulated promoter activity. Our results demonstrate that a member of the AP1 family, containing JunD and c-fos, can bind to the proximal element P1 within the hPRL promoter. In addition, they show that AP1 is involved in both basal and OA-stimulated expression of the hPRL gene.