A CONSTITUTIVELY ACTIVE FORM OF CREB CAN ACTIVATE EXPRESSION OF THE RAT PROLACTIN PROMOTER IN NONPITUITARY CELLS

The pituitary cell-specific transcription factor Pit-1 has been show t o trans-activate expression of the prolactin (PRL) promoter in non-pit uitary cells. However, the cyclic AMP response element (CRE)-binding p rotein CREB is known to play a major role in cell-specific expression of hepatocyte-specific genes. Since the PRL promoter contains an asymm etrical form of a cyclic AMP response element (termed the CLE), we inv estigated whether CREB could also induce PRL promoter activity in non- pituitary cells. Transient expression in rat glial C6 cells of a const itutively active CREB-VP16 fusion protein strongly trans-activated exp ression of a co-transfected rat PRL promoter construct, (-187)PRL-CAT. Analysis by 5'-deletion showed that this response requires PRL promot er sequences between positions -113/-75. CREB-VP16 did not stimulate e xpression in C6 cells of any of three control promoter-CAT constructs, implying that the strong response of the PRL promoter to activated CR EB is both promoter-specific, and is not due to non-specific transcrip tional effects of the potent VP16 moiety of CREB-VP16. Surprisingly, m utations in the CLE only slightly reduced activation by CREB-VP16 of c onstruct (-204)PRL-CAT, implying that the major action of CREB-VP16 on the PRL promoter; does not involve a direct interaction with the CLE. CREB-VP16 stimulated PRL-CAT activity in C6 cells as strongly as, and synergistically with, Pit-1. These results imply that CREB can strong ly and specifically activate expression of the PRL promoter in non-pit uitary cells, via a mechanism different from that employed by Pit-1.