Hodge C, Liao J, Stofega M, Guan K, Carter-Su C, Schwartz J
Program in Cellular and Molecular Biology, University of Michigan Medical School, Ann Arbor, Michigan 48109-0622, USA.
J Biol Chem. 1998 Nov 20;273(47):31327-36. doi: 10.1074/jbc.273.47.31327.
Growth hormone (GH), 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 (MAPKAP) 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.
生长激素（GH）是正常身体生长和代谢的主要调节因子，可调节细胞基因表达。转录因子Elk-1和血清反应因子是GH通过血清反应元件（SRE）刺激c-fos转录所必需的。GH刺激Elk-1的丝氨酸磷酸化，从而使Elk-1介导转录激活。研究了Ras/丝裂原活化蛋白激酶激酶（MEK）/细胞外信号调节激酶（ERK）途径对GH刺激下Elk-1介导的c-fos SRE转录激活的作用。MEK抑制剂PD098059减弱了GH诱导的内源性SRE调节基因c-fos、egr-1和junB的表达以及c-fos启动子介导的转录激活。MEK抑制剂阻断了GH刺激的3T3-F442A细胞中MEK的激活、ERK1/ERK2的磷酸化和丝裂原活化蛋白激酶活性。阻断MEK激活可防止GH诱导的Elk-1磷酸化，以及Elk-1响应GH介导转录激活的能力。显性负性Ras或ERK特异性磷酸酶丝裂原活化蛋白激酶磷酸酶-1的过表达阻断了Ras/MEK/ERK途径，并消除了GH诱导的Elk-1磷酸化。在所使用的条件下，GH未能刺激Jun N末端激酶的磷酸化或激活。GH略微增加了p38介导的丝裂原活化蛋白激酶激活蛋白（MAPKAP）激酶-2的活性，但p38抑制剂SB203580并未减弱GH促进的Elk-1磷酸化。抑制GH诱导的ERK磷酸化的渥曼青霉素也减弱了GH对c-fos的转录激活。综上所述，这些数据表明，Ras/MEK/ERK途径的GH依赖性激活以及随后Elk-1的丝氨酸磷酸化通过SRE促进了GH刺激的c-fos表达。
