Role of the cytokine-inducible SH2 protein CIS in desensitization of STAT5b signaling by continuous growth hormone

Growth hormone (GH)-inducible suppressors of cytokine signaling (SOCS/CIS p roteins) inhibit GH receptor (GHR) signaling to STAT5b via phosphotyrosine- dependent binding interactions with the tyrosine kinase JAK2 (SOCS-1) and/o r the cytoplasmic tail of GHR (CIS and SOCS-3). Presently, we investigate t he mechanism of CLS inhibition and CIS's role in down-regulating GHR JAK2 s ignaling to STAT5b in cells exposed to GH continuously, CLS is shown to inh ibit GHR-JAK2 signaling by two distinct mechanisms: by a partial inhibition that is decreased at elevated STAT5b levels and may involve competition be tween CLS and STAT5b for common GHR cytoplasmic tail phosphotyrosine-bindin g sites; and by a time-dependent inhibition, not seen with SOCS-1 or SOCS-3 , that involves proteasome action. Investigation of the latter mechanism re vealed that GH stimulates degradation of CIS, but not SOCS-3, The proteasom e inhibitor MG132 blocked this protein degradation and also blocked the inh ibitory action of CIS, but not that of SOCS-1 or SOCS-3, on STAT5b signalin g. Proteasome-dependent degradation of CIS, most likely in the form of a (G HR-JAK2)-CIS complex, is therefore proposed to be an important step in the time-dependent CIS inhibition mechanism. Finally, the down-regulation of GH R-JAK2 signaling to STAT5b seen in continuous GH-treated cells could be pre vented by treatment of cells with the proteasome inhibitor MG132 or by expr ession of CIS-R107K, a selective, dominant-negative inhibitor of CIS activi ty. These findings lead us to propose that the cytokine signaling inhibitor CIS is a key mediator of the STAT5b desensitization response seen in cells and tissues exposed to GH chronically, such as adult female rat liver.