GROWTH-HORMONE STIMULATES PHOSPHORYLATION AND ACTIVATION OF ELK-1 ANDEXPRESSION OF C-FOS, EGR-1, AND JUNB THROUGH ACTIVATION OF EXTRACELLULAR SIGNAL-REGULATED KINASE-1 AND KINASE-2

Growth hormone (GR), a major regulator of normal body growth and metab olism, regulates cellular gene expression. The transcription factors E lk-1 and Serum Response Factor are necessary for GH-stimulated transcr iption of c-fos through the Serum Response Element (SRE), GH stimulate s the serine phosphorylation of Elk-1, thereby enabling Elk-1 to media te transcriptional activation. The contribution of the Ras/mitogen-act ivated protein kinase kinase (MEK)/extracellular signal-regulated kina se (ERK) pathway to Elk-1-mediated transcriptional activation of the c -fos SRE in response to GH: was examined. The MEK inhibitor PD098059 a ttenuated GH-induced expression of the endogenous SRE-regulated genes c-fos, egr-1, and junB as well as transcriptional activation mediated by the c-fos promoter. The MEK inhibitor blocked GH-stimulated activat ion of MEK, phosphorylation of ERK1/ERK2, and MAP kinase activity in 3 T3-F442A cells. Blocking MEK activation prevented GH-induced phosphory lation of Elk-1, as well as the ability of Elk-1 to mediate transcript ional activation in response to GH, Overexpression of dominant-negativ e Ras or the ERK-specific phosphatase, mitogen-activated protein kinas e phosphatase-1, blocked the Ras/MEK/ERK. pathway and abrogated GH-ind uced phosphorylation of Elk-1, GH failed to stimulate phosphorylation or activation of Jun N-terminal kinase under the conditions used. GH s lightly increased p38-mediated mitogen-activated protein kinase-activa ted protein (MAP-KAP) kinase-2 activity, but the p38 inhibitor SB20358 0 did not attenuate GH-promoted Elk-1 phosphorylation. Wortmannin, whi ch inhibited GH-induced ERK phosphorylation, also attenuated transcrip tional activation of c-fos by GH, Taken together, these data suggest t hat GH-dependent activation of the Ras/MEK/ERK pathway and subsequent serine phosphorylation of Elk-1 contribute to GH-stimulated c-fos expr ession through the SRE.