Jak family tyrosine kinases mediate signaling by cytokine receptors to regulate diverse biological processes. Although Jak2 and other Jak kinase family members are phosphorylated on numerous sites during cytokine signaling, the identity and function of most of these sites remains unknown. Using tandem mass spectroscopic analysis of activated Jak2 protein from intact cells, we identified Tyr 221 and Tyr 570 as novel sites of Jak2 phosphorylation. Phosphorylation of both sites was stimulated by cytokine treatment of cultured cells, and this stimulation required Jak2 kinase activity. While we observed no gross alteration of signaling upon mutation of Tyr 221 , Tyr 570 lies within the inhibitory JH2 domain of Jak2, and mutation of this site (Jak2 Y570F ) results in constitutive Jak2-dependent signaling in the absence of cytokine stimulation and enhances and prolongs Jak2 activation during cytokine stimulation. Mutation of Tyr 570 does not alter the ability of SOCS3 to bind or inhibit Jak2, however. Thus, the phosphorylation of Tyr 570 in vivo inhibits Jak2-dependent signaling independently of SOCS3-mediated inhibition. This Tyr 570 -dependent mechanism of Jak2 inhibition likely represents an important mechanism by which cytokine function is regulated.Type I cytokines mediate a plethora of physiologic processes, ranging from hematopoietic and immune functions (such as those mediated by erythropoietin [EPO] and the interleukins [ILs]) to growth and neuroendocrine responses (such as those mediated by growth hormone and leptin) (12,14,16,23). These actions are mediated by the activation of cytokine receptor proteins found on the surface of target cells. Cytokine receptors each contain an extracellular domain that recognizes its specific cytokine ligand, a single transmembrane domain, and an intracellular domain that, although devoid of enzymatic activity, transmits intracellular signals by means of an associated Jak family tyrosine kinase. Ligand binding activates the associated intracellular Jak kinase, resulting in the tyrosine phosphorylation of the Jak kinase and the intracellular domain of the cytokine receptor. These tyrosine phosphorylation events mediate the recruitment of downstream signaling molecules that contain phosphotyrosine-binding SH2 domains (such as STAT proteins) (12, 16); tyrosine phosphorylation may also mediate other regulatory events during cytokine signaling (8, 31).The Jak kinase family contains four members: Jak1 to Jak3 and Tyk2 (12, 16). Of these, Jak1, Jak2, and Tyk2 are ubiquitously expressed, while Jak3 is found predominantly in immune and hematopoietic tissues. Jak kinases are composed of four conserved domains. The NH 2 -terminal FERM domain is required for interaction with cytokine receptors (24, 30), while the adjacent SH2-like fold has no known function. The COOH-terminal portion of Jak kinases contains a kinase-like JH2 domain that is devoid of enzymatic activity but that inhibits the activity of the COOH-terminal JH1 tyrosine kinase domain (10,19,22,28,29).Our laboratory studies s...

The leptin receptor, LRb, and other cytokine receptors are devoid of intrinsic enzymatic activity and rely upon the activity of constitutively associated Jak family tyrosine kinases to mediate intracellular signaling. In order to clarify mechanisms by which Jak2, the cognate LRb-associated Jak kinase, is regulated and mediates downstream signaling, we employed tandem mass spectroscopic analysis to identify phosphorylation sites on Jak2. We identified Ser 523 as the first-described site of Jak2 serine phosphorylation and demonstrated that this site is phosphorylated on Jak2 from intact cells and mouse spleen. Ser 523 was highly phosphorylated in HEK293 cells independently of LRb-Jak2 activation, suggesting a potential role for the phosphorylation of Ser 523 in the regulation of LRb by other pathways. Indeed, mutation of Ser 523 sensitized and prolonged signaling by Jak2 following activation by the intracellular domain of LRb. The effect of Ser 523 on Jak2 function was independent of Tyr 570 -mediated inhibition. Thus, the phosphorylation of Jak2 on Ser 523 inhibits Jak2 activity and represents a novel mechanism for the regulation of Jak2-dependent cytokine signaling.Type I cytokines mediate a plethora of physiologic processes, ranging from hematopoietic and immune functions (such as those mediated by erythropoietin [Epo] and the interleukins) to growth and neuroendocrine responses (such as those mediated by growth hormone and leptin) (11,15,16,23,31). These actions are mediated by the activation of cytokine receptor proteins found on the surface of target cells. Cytokine receptors each contain an extracellular domain that recognizes its specific cytokine ligand, a single transmembrane domain, and an intracellular domain that, although devoid of enzymatic activity, transmits intracellular signals by means of an associated Jak family tyrosine kinase. Ligand binding activates the associated intracellular Jak kinase, resulting in Jak kinase autophosphorylation and activation and the subsequent tyrosine phosphorylation of the intracellular domain of the cytokine receptor. These tyrosine phosphorylation events mediate downstream signaling by the LRb/Jak2 complex (11,16,19,23).The Jak kinase family contains four members: Jak1 to Jak3 and Tyk2 (11,16). Of these, Jak1 and -2 and Tyk2 are ubiquitously expressed, while Jak3 is found predominantly in immune and hematopoietic tissues. Jak kinases are composed of four conserved domains. The NH 2 -terminal FERM domain is required for interaction with cytokine receptors (32, 35), while the adjacent SH2-like fold has no known function. The COOHterminal portion of Jak kinases contains a kinase-like JH2 domain that is devoid of enzymatic activity but that regulates the activity of the COOH-terminal JH1 tyrosine kinase domain (9,21,28,33,34).Our laboratory studies signaling by the long form of the leptin receptor (LRb), which regulates feeding, neuroendocrine, and immune function in response to leptin, which is in turn regulated by nutritional cues (8,10,23,31). Stimulation of LRb ...

The left two lanes should be labeled Jak2 (not pcDNA3), and the middle two lanes should be labeled S523A (not Jak2).

To determine feasibility of implementation of a weight loss program for overweight Latinos with severe mental illness. In this quasi-experimental study, a 14-week behavioral weight loss course (extended) was implemented at one clinic. A one-time nutrition class (brief) was given at a sister clinic. Implementation feasibility was assessed by consent and participation rates. Weight was followed for 6 months. Consent rates were high [77 % (49/64) extended; 68 % (39/57) brief], and 88 % (43/49) of extended subjects participated and 88 % (38/43) completed follow-up. Weight loss did not differ between groups. A behavioral weight loss course is feasible to implement for this population.