Gebert C A, Park S H, Waxman D J
Department of Biology, Boston University, Massachusetts 02215, USA.
Mol Endocrinol. 1999 Jan;13(1):38-56. doi: 10.1210/mend.13.1.0235.
STAT5b (signal transducer and activator of transcription 5b) is a key mediator of the effects of plasma GH pulses on male-specific liver gene expression. STAT5b is activated in liver cells in vivo by physiological pulses of GH and then is rapidly deactivated. Investigation of the cellular events involved in this activation/deactivation cycle using the rat liver cell line CWSV-1 established that a brief exposure to GH and the associated activation of JAK2 (Janus kinase 2) tyrosine kinase activity are both necessary and sufficient to initiate all of the downstream steps associated with STAT5b activation by tyrosine phosphorylation and the subsequent deactivation of both JAK2 kinase and STAT5b. JAK2 signaling to STAT5b at the conclusion of a GH pulse could be sustained by the protein synthesis inhibitor cycloheximide or by the proteasome inhibitor MG132, indicating that termination of this JAK2-catalyzed STAT activation loop requires synthesis of a labile or GH-inducible protein factor and is facilitated by the proteasome pathway. This factor may be a phosphotyrosine phosphatase, since the phosphatase inhibitor pervanadate both sustained GH pulse-induced JAK2 signaling to STAT5b and blocked the rapid deactivation of phosphorylated STAT5b (t(1/2) = 8.8 +/- 0.9 min) seen in its absence. Finally, the serine kinase inhibitor H7 blocked down-regulation of JAK2 signaling to STAT5b in a manner that enabled cells to respond to a subsequent GH pulse without the need for the approximately 3-h interpulse interval normally required for full recovery of GH pulse responsiveness. Termination of GH pulse-induced STAT5b signaling is thus a complex process that involves multiple biochemical events. These are proposed to include the down-regulation of JAK2 signaling to STAT5b via a cycloheximide- and H7-sensitive step, proteasome-dependent degradation of a key component or regulatory factor, and dephosphorylation leading to deactivation of the receptor-kinase signaling complex and its STAT5b substrate via the action of a phosphotyrosine phosphatase.
信号转导及转录激活因子5b（STAT5b）是血浆生长激素（GH）脉冲对雄性特异性肝脏基因表达产生影响的关键介质。在体内，STAT5b会被GH的生理脉冲激活，随后迅速失活。利用大鼠肝细胞系CWSV-1对这一激活/失活循环所涉及的细胞事件进行研究发现，短暂暴露于GH以及相关的Janus激酶2（JAK2）酪氨酸激酶活性激活，对于启动所有与STAT5b酪氨酸磷酸化激活以及随后JAK2激酶和STAT5b失活相关的下游步骤而言，既是必要的也是充分的。在GH脉冲结束时，蛋白质合成抑制剂放线菌酮或蛋白酶体抑制剂MG132可维持JAK2向STAT5b的信号传导，这表明该JAK2催化的STAT激活环的终止需要合成一种不稳定的或GH诱导的蛋白质因子，并且蛋白酶体途径对此过程有促进作用。该因子可能是一种磷酸酪氨酸磷酸酶，因为磷酸酶抑制剂过钒酸盐既能维持GH脉冲诱导的JAK2向STAT5b的信号传导，又能阻止在不存在该抑制剂时所观察到的磷酸化STAT5b的快速失活（半衰期t(1/2) = 8.8 ± 0.9分钟）。最后，丝氨酸激酶抑制剂H7以一种使细胞能够对后续GH脉冲作出反应的方式，阻断了JAK2向STAT5b的信号下调，而无需通常为完全恢复GH脉冲反应性所需的约3小时脉冲间隔。因此，GH脉冲诱导的STAT5b信号传导的终止是一个复杂的过程，涉及多个生化事件。这些事件被认为包括通过对放线菌酮和H7敏感的步骤下调JAK2向STAT5b的信号传导、蛋白酶体依赖性降解关键成分或调节因子，以及通过磷酸酪氨酸磷酸酶的作用导致受体激酶信号复合物及其STAT5b底物去磷酸化从而失活。
