Down-regulation of liver JAK2-STAT5b signaling by the female plasma pattern of continuous growth hormone stimulation.
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 DNA-binding 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 DNA-binding activity in continuous GH-treated cells by a mechanism that involves both increased STAT5b 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 down-regulates 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 re-ceptor-kinase complex.
Gebert
C A
CA
Department of Biology, Boston Universitiy, Massachusetts 02215, USA.
Park
S H
SH
Waxman
D J
DJ
D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D003513:Cycloheximide / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000502:physiology*; D015536:Down-Regulation; D004586:Electrophoresis; D005260:Female; D019833:Genistein / Q000378:metabolism; D013006:Growth Hormone / Q000502:physiology*; D006131:Growth Inhibitors / Q000378:metabolism; D053614:Janus Kinase 2; D007976:Leupeptins / Q000378:metabolism; D008099:Liver / Q000502:physiology; D008297:Male; D008894:Milk Proteins; D011500:Protein Synthesis Inhibitors / Q000378:metabolism; D017027:Protein Tyrosine Phosphatases / Q000037:antagonists & inhibitors; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D011518:Proto-Oncogene Proteins; D051381:Rats; D050799:STAT5 Transcription Factor; D015398:Signal Transduction / Q000502:physiology*; D019311:Staurosporine / Q000378:metabolism; D015534:Trans-Activators / Q000502:physiology*; D014638:Vanadates / Q000378:metabolism