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

The suppression of male-specfic, GH pulse-induced, liver transcription in a dult female rats has been linked to the down-regulation of STAT5b activatio n by the female plasma pattern of near-continuous GH exposure. The mechanis m underlying this down-regulation was studied in the rat liver cell line CW SV-1, where continuous OH suppressed the level of activated (tyrosine- phos phorylated) STAT5b to approximately 10-20% of the maximal GH pulse-induced STAT5b signal within 3 h. In contrast to the robust JAK2 kinase-dependent S TAT5b activation loop that is established by a GH pulse, JAK2 kinase signal ing to individual STAT5b molecules was found to be short lived in cells tre ated with GH continuously. Moreover, maintenance of the low-level STAT5b si gnal required ongoing protein synthesis and persisted for at least 7 days p rovided that GH was present in the culture continuously. Increased STAT5b D NA-binding activity was observed in cells treated with the proteasome inhib itor MG132, suggesting that at least one component of the GH receptor (GHR) JAK2-STAT5b signaling pathway becomes labile in response to continuous GH t reatment. The phosphotyrosine phosphatase inhibitor pervanadate fully rever sed the down-regulation of STAT5b DNA-binding activity in continuous GH-tre ated cells by a mechanism that involves both increased STAT5b activation an d 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 fir st step in the internalization/degradation pathway for the GHR-JAK2 complex . Finally, the sustaining effect of the serine kinase inhibitor H7 on GH pu lse-induced JAK2 signaling to STAT5b was not observed in continuous GH-trea ted cells. These findings suggest a model where continuous GH exposure of l iver cells down-regulates the STAT5b pathway by a mechanism that involves e nhanced dephosphorylation of both STAT5b and GHR-JAK2, with the latter step leading to increased internalization/degradation of the receptor-kinase co mplex.