Attenuation of leptin action and regulation of obesity by protein tyrosine phosphatase 1B.

Common obesity is primarily characterized by resistance to the actions of the hormone leptin. Mice deficient in protein tyrosine phosphatase 1B (PTP1B) are resistant to diabetes and diet-induced obesity, prompting us to further define the relationship between PTP1B and leptin in modulating obesity. Leptin-deficient (Lep(ob/ob)) mice lacking PTP1B exhibit an attenuated weight gain, a decrease in adipose tissue, and an increase in resting metabolic rate. Furthermore, PTP1B-deficient mice show an enhanced response toward leptin-mediated weight loss and suppression of feeding. Hypothalami from these mice also display markedly increased leptin-induced Stat3 phosphorylation. Finally, substrate-trapping experiments demonstrate that leptin-activated Jak2, but not Stat3 or the leptin receptor, is a substrate of PTP1B. These results suggest that PTP1B negatively regulates leptin signaling, and provide one mechanism by which it may regulate obesity.

T-cell protein tyrosine phosphatase (Tcptp) is a negative regulator of colony-stimulating factor 1 signaling and macrophage differentiation.

Mice null for the T-cell protein tyrosine phosphatase (Tcptp-/-) die shortly after birth due to complications arising from the development of a systemic inflammatory disease. It was originally reported that Tcptp-/- mice have increased numbers of macrophages in the spleen; however, the mechanism underlying the aberrant growth and differentiation of macrophages in Tcptp-/- mice is not known. We have identified Tcptp as an important regulator of colony-stimulating factor 1 (CSF-1) signaling and mononuclear phagocyte development. The number of CSF-1-dependent CFU is increased in Tcptp-/- bone marrow. Tcptp-/- mice also have increased numbers of granulocyte-macrophage precursors (GMP), and these Tcptp-/- GMP yield more macrophage colonies in response to CSF-1 relative to wild-type cells. Furthermore, we have identified the CSF-1 receptor (CSF-1R) as a physiological target of Tcptp through substrate-trapping experiments and its hyperphosphorylation in Tcptp-/- macrophages. Tcptp-/- macrophages also have increased tyrosine phosphorylation and recruitment of a Grb2/Gab2/Shp2 complex to the CSF-1R and enhanced activation of Erk after CSF-1 stimulation, which are important molecular events in CSF-1-induced differentiation. These data implicate Tcptp as a critical regulator of CSF-1 signaling and mononuclear phagocyte development in hematopoiesis.

The T cell protein tyrosine phosphatase is a negative regulator of janus family kinases 1 and 3.

BACKGROUND: The immune response is regulated through a tightly controlled cytokine network. The counteracting balance between protein tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP) activity regulates intracellular signaling in the immune system initiated by these extracellular polypeptides. Mice deficient for the T cell protein tyrosine phosphatase (TCPTP) display gross defects in the hematopoietic compartment, indicating a critical role for TCPTP in the regulation of immune homeostasis. To date, the molecular basis underlying this phenotype has not been reported. RESULTS: We have identified two members of the Janus family of tyrosine kinases (JAKs), JAK1 and JAK3, as bona fide substrates of TCPTP. Inherent substrate specificity in the TCPTP-JAK interaction is demonstrated by the inability of other closely related PTP family members to form an in vivo interaction with the JAKs in hematopoietic cells. In keeping with a negative regulatory role for TCPTP in cytokine signaling, expression of TCPTP in T cells abrogated phosphorylation of STAT5 following interleukin (IL)-2 stimulation. TCPTP-deficient lymphocytes treated with IL-2 had increased levels of tyrosine-phosphorylated STAT5, and thymocytes treated with interferon (IFN)-alpha or IFN-gamma had increased tyrosine-phosphorylated STAT1. Hyperphosphorylation of JAK1 and elevated expression of iNOS was observed in IFN-gamma-treated, TCPTP-deficient, bone marrow-derived macrophages. CONCLUSIONS: We have identified JAK1 and JAK3 as physiological substrates of TCPTP. These results indicate a negative regulatory role for TCPTP in cytokine signaling and provide insight into the molecular defect underlying the phenotype of TCPTP-deficient animals.