Down-regulation of liver JAK2-STAT5b signaling by the female plasma pattern of continuous growth hormone stimulation

Abstract

The suppression of male-specific, GH pulse-induced, liver transcription in adult female rats has been linked to the down-regulation of STAT5b activation by the female plasma pattern of near-continuous GH exposure. The mechanism underlying this down-regulation was studied in the rat liver cell line CWSV-1, where continuous GH suppressed the level of activated (tyrosine- phosphorylated) STAT5b to approximately 10-20% of the maximal GH pulse- induced STAT5b signal within 3 h. In contrast to the robust JAK2 kinase- dependent STAT5b activation loop that is established by a GH pulse, JAK2 kinase signaling to individual STAT5b molecules was found to be short lived in cells treated with GH continuously. Moreover, maintenance of the low- level STAT5b signal required ongoing protein synthesis and persisted for at least 7 days provided that GH was present in the culture continuously. Increased STAT5b DNAbinding activity was observed in cells treated with the proteasome inhibitor MG132, suggesting that at least one component of the GH receptor (GHR)JAK2-STAT5b signaling pathway becomes labile in response to continuous GH treatment. The phosphotyrosine phosphatase inhibitor pervanadate fully reversed the down-regulation of STAT5b DNAbinding activity in continuous GH-treated cells by a mechanism that involves both increased STATSb activation and decreased STAT5b dephosphorylation. Moreover, the requirement for ongoing GH stimulation and active protein synthesis to maintain STAT5b activity in continuous GH-treated cells were both eliminated by pervanadate treatment, suggesting that phosphotyrosine dephosphorylation may be an obligatory first step in the internalization/degradation pathway for the GHR-JAK2 complex. Finally, the sustaining effect of the serine kinase inhibitor H7 on GH pulse-induced JAK2 signaling to STAT5b was not observed in continuous GH-treated cells. These findings suggest a model where continuous GH exposure of liver cells downregulates the STAT5b pathway by a mechanism that involves enhanced dephosphorylation of both STAT5b and GHR-JAK2, with the latter step leading to increased internalization/degradation of the receptor-kinase complex.