Growth hormone stimulates phosphorylation and activation of Elk-1 and expression of c-fos, egr-1, and junB through activation of extracellular signal-regulated kinases 1 and 2

Growth hormone (GR), a major regulator of normal body growth and metabolism, regulates cellular gene expression. The transcription factors Elk-1 and Serum Response Factor are necessary for GH-stimulated transcription of c-fos through the Serum Response Element (SRE), GH stimulates the serine phosphorylation of Elk-1, thereby enabling Elk-1 to mediate transcriptional activation. The contribution of the Ras/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway to Elk-1-mediated transcriptional activation of the c-fos SRE in response to GH: was examined. The MEK inhibitor PD098059 attenuated 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 activation of MEK, phosphorylation of ERK1/ERK2, and MAP kinase activity in 3T3-F442A cells. Blocking MEK activation prevented GH-induced phosphorylation of Elk-1, as well as the ability of Elk-1 to mediate transcriptional activation in response to GH, Overexpression of dominant-negative Ras or the ERK-specific phosphatase, mitogen-activated protein kinase phosphatase-1, blocked the Ras/MEK/ERK. pathway and abrogated GH-induced phosphorylation of Elk-1, GH failed to stimulate phosphorylation or activation of Jun N-terminal kinase under the conditions used. GH slightly increased p38-mediated mitogen-activated protein kinase-activated protein (MAP-KAP) kinase-2 activity, but the p38 inhibitor SB203580 did not attenuate GH-promoted Elk-1 phosphorylation. Wortmannin, which inhibited GH-induced ERK phosphorylation, also attenuated transcriptional activation of c-fos by GH, Taken together, these data suggest that GH-dependent activation of the Ras/MEK/ERK pathway and subsequent serine phosphorylation of Elk-1 contribute to GH-stimulated c-fos expression through the SRE.