Insulin Reverses Growth Hormone-induced Homologous Desensitization

Abstract

Growth hormone (GH) is secreted in a pulsatile pattern to promote body growth and metabolism. GH exerts its function by activating several signaling pathways, including JAK2/STAT and MEK/ERK. ERK1/2 activation by GH plays important roles in gene expression, cell proliferation, and growth. We previously reported that in rat H4IIE hepatoma cells after an initial GH exposure, a second GH exposure induces STAT5 phosphorylation but not ERK1/2 phosphorylation (Ji, S., Frank, S. J., and Messina, J. L. (2002) J. Biol. Chem. 277, 28384 -28393). In this study the mechanisms underlying GH-induced homologous desensitization were investigated. A second GH exposure activated the signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. This correlated with recovery of GH receptor levels, but was insufficient for GH-induced phosphorylation of MEK1/2 and ERK1/2. Insulin restored the ability of a second GH exposure to induce phosphorylation of MEK1/2 and ERK1/2 without altering GH receptor levels or GH-induced phosphorylation/activation of JAK2 and Raf-1. GH and insulin synergized in promoting cell proliferation. Further investigation suggested that insulin increased the amount of MEK bound to KSR (kinase suppressor of Ras) and restored GH-induced tyrosine phosphorylation of KSR. Previous GH exposure also induced desensitization of STAT1 and STAT3 phosphorylation, but this desensitization was not reversed by insulin. Thus, insulin-regulated resensitization of GH signaling may be necessary to reset the complete response to GH after a normal, physiologic pulse of GH. Binding of GH to its receptor (GHR)2 results in tyrosine phosphorylation of Janus-activating kinase (JAK) 2 and GHR (1, 2), leading to the activation of signal transducer and activator of transcription (STAT) proteins (2-4). GH also activates ERK1/2 via the Ras/Raf-1/MEK/ERK cascade (5-8). An activated GHR⅐JAK2 complex recruits and phosphorylates Shc, bringing Grb2 and Son of Sevenless (SOS) to the cell membrane, resulting in the formation of active GTP-bound Ras. Ras activation initiates the activation of Raf-1 kinase, which leads to the phosphorylation and activation of MEK1/2 and then ERK1/2. Activation of ERK1/2 plays an important role in GH-induced gene expression, cell proliferation and growth, and cross-talk between GH and other growth factor signaling pathways (9 -13).GH is secreted in a pulsatile fashion (14, 15). In young adult male rats, GH is released in ϳ1-h pulses with peak serum concentrations of 150 -400 ng/ml and interpulse intervals of 2 h or more, where serum GH concentrations are negligible (16). In female rats, GH is secreted more frequently, resulting in the continuous presence of GH in the circulation, which peaks at 50 -150 ng/ml (15-18). After stimulation by a GH pulse, an obligatory recovery period is required for a succeeding GH pulse to induce STAT5 phosphorylation (19 -21). Reactivation of STAT5b by the masculine, but not the feminine, GH secretory pattern is important in transducing the sexually dimorphic pattern of GH ...

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Insulin regulation of growth hormone receptor gene expression: involvement of both the PI-3 kinase and MEK/ERK signaling pathways

“…The confounding presence of other hormones and growth factors makes these studies difficult to perform in vivo as well. The rat H4IIE hepatoma cell line is an ideal model for the present studies, in that it is extremely sensitive to both insulin and GH, and has been used in several studies examining the effects of insulin and GH [3,[7][8][9][10][11][12][13][14][15]. The H4IIE cell line is a derivative of the minimal deviation H35 hepatoma cell line, and expresses receptors that bind insulin and GH with a similar affinity to isolated rat hepatocytes and adipocytes.…”