MITOGEN-ACTIVATED PROTEIN-KINASE KINASE INHIBITION DECREASES GROWTH-HORMONE STIMULATED TRANSCRIPTION MEDIATED BY STAT5

We have investigated the possible involvement of the MAPK pathway in t he growth hormone(GH)-induced activation of one of the members of sign al transducers and activators of transcription, STAT5, by using the MA PK kinase (MEK) inhibitor PD98059. PD98059 treatment of Chinese hamste r ovarian cells, stably transfected with the GH receptor (CHOA cells), abolished the GH-induced MAPK activity. PD98059 decreased the amount of GH-induced STAT5 in nuclear extract with DNA-binding capacity. Furt hermore, GH dependent transcription of a STAT5 regulated reporter gene was inhibited by PD98059. The MEK inhibitor did not reduce GH-stimula ted nuclear translocation of STAT5. We also investigated if PD98059 di fferentially influences the activation of the two STAT5 homologs, STAT 5a and STAT5b, which differ mainly at the C-terminal end, one of the d ifferences being the presence of a possible MAPK phosphorylation site in STAT5a. Expression plasmids for these transcription factors were tr ansfected into CHOA cells together with a reporter gene. GH-stimulated fold induction of transcription was reduced by PD98059 in STAT5a but not in STAT5b overexpressing cells. A MAPK phosphorylation site-mutate d version of STAT5a was also transfected into CHOA cells. GH-stimulate d fold induction of cotransfected reporter gene was not reduced by PD9 8059 in cells overexpressing mutant STAT5a. The above data show that t he MAPK pathway is required for the full activation of one of the STAT 5 isoforms (STAT5a). (C) 1997 Elsevier Science Ireland Ltd.