EXPRESSION OF CONSTITUTIVELY ACTIVE G(S) ALPHA-SUBUNITS IN GH3 PITUITARY-CELLS STIMULATES PROLACTIN PROMOTER ACTIVITY

Although G protein alpha subunits are known to regulate such cellular functions as growth and enzymatic activity, the ability of these prote ins to regulate target gene expression has not yet been directly inves tigated. Transient expression in GH3 pituitary cells of a target rat p rolactin promoter-chloramphenicol acetyltransferase construct, (-1957) PRL-CAT, was increased by coexpressed constitutively active alpha(s) m utant Q227L-alpha(s) but not by wild-type alpha(s). Thus activated alp ha(s) but not basal state alpha(s) can stimulate prolactin promoter ac tivity. Q227L-alpha(s) also stimulated expression of construct (-187)P RL-CAT, showing that only the proximal prolactin promoter region is re quired for a response to activated alpha(s). The promoter specificity of the transcriptional influence of activated alpha(s) was demonstrate d by the inability of either Q227L-alpha(s) or wild-type alpha(s) to s timulate expression of control target constructs containing either the rat growth hormone promoter or the thymidine kinase promoter. Previou s studies have shown that the most proximal prolactin promoter binding site for the pituitary-specific transcription factor pit-1, site 1P, can act as an independent response element for either thyrotropin-rele asing hormone or Ca2+. Two copies of site 1P conferred upon a heterolo gous metallothionein promoter a response to Q227L-alpha(s). This impli es that site 1P can also serve as an independent response element for alpha(s) and suggests that pit-1 may be a mediator of the cellular reg ulation by alpha(s) of the prolactin promoter.