Mazurkiewicz-Munoz Anna M, Argetsinger Lawrence S, Kouadio Jean-Louis K, Stensballe Allan, Jensen Ole N, Cline Joel M, Carter-Su Christin
Graduate Program in Cellular and Molecular Biology, The University of Michigan Medical School, Ann Arbor, MI 48109-0622, USA.
Mol Cell Biol. 2006 Jun;26(11):4052-62. doi: 10.1128/MCB.01591-05.
The tyrosine kinase JAK2 is a key signaling protein for at least 20 receptors in the cytokine/hematopoietin receptor superfamily and is a component of signaling for multiple receptor tyrosine kinases and several G-protein-coupled receptors. In this study, phosphopeptide affinity enrichment and mass spectrometry identified serine 523 (Ser523) in JAK2 as a site of phosphorylation. A phosphoserine 523 antibody revealed that Ser523 is rapidly but transiently phosphorylated in response to growth hormone (GH). MEK1 inhibitor UO126 suppresses GH-dependent phosphorylation of Ser523, suggesting that extracellular signal-regulated kinases (ERKs) 1 and/or 2 or another kinase downstream of MEK1 phosphorylate Ser523 in response to GH. Other ERK activators, phorbol 12-myristate 13-acetate and epidermal growth factor, also stimulate phosphorylation of Ser523. When Ser523 in JAK2 was mutated, JAK2 kinase activity as well as GH-dependent tyrosyl phosphorylation of JAK2 and Stat5 was enhanced, suggesting that phosphorylation of Ser523 inhibits JAK2 kinase activity. We hypothesize that phosphorylation of Ser523 in JAK2 by ERKs 1 and/or 2 or other as-yet-unidentified kinases acts in a negative feedback manner to dampen activation of JAK2 in response to GH and provides a mechanism by which prior exposure to environmental factors that regulate Ser523 phosphorylation might modulate the cell's response to GH.
酪氨酸激酶JAK2是细胞因子/造血因子受体超家族中至少20种受体的关键信号蛋白，并且是多种受体酪氨酸激酶和几种G蛋白偶联受体信号传导的组成部分。在本研究中，磷酸肽亲和富集和质谱分析确定JAK2中的丝氨酸523（Ser523）为磷酸化位点。一种磷酸化丝氨酸523抗体显示，Ser523在生长激素（GH）刺激下迅速但短暂地被磷酸化。MEK1抑制剂UO126可抑制GH依赖的Ser523磷酸化，这表明细胞外信号调节激酶（ERK）1和/或2或MEK1下游的另一种激酶可响应GH使Ser523磷酸化。其他ERK激活剂，佛波酯12-肉豆蔻酸酯13-乙酸酯和表皮生长因子，也能刺激Ser523的磷酸化。当JAK2中的Ser523发生突变时，JAK2激酶活性以及JAK2和Stat5的GH依赖型酪氨酸磷酸化增强，这表明Ser523的磷酸化会抑制JAK2激酶活性。我们推测，ERK1和/或2或其他尚未确定的激酶使JAK2中的Ser523磷酸化，以负反馈方式抑制JAK2对GH的激活，并提供了一种机制，通过该机制，先前暴露于调节Ser523磷酸化的环境因素可能会调节细胞对GH的反应。
