Human leptin activates PI3K and MAPK pathways in human peripheral blood mononuclear cells: possible role of Sam68.
Leptin, the adipocyte-secreted hormone that centrally regulates weight control, is known to function as an immunomodulatory regulator. Thus, we have recently found that human leptin promotes stimulation and proliferation of human peripheral blood mononuclear cells. In the present work, we sought to study the mechanisms underlying these effects. First, we have assessed the presence of the long isoform of the human leptin receptor by RT-PCR. Next, we have studied tyrosine phosphorylation of cell proteins in response to leptin stimulation. We have found that leptin receptor, IRS-1 and the RNA-binding protein Sam68 are tyrosine phosphorylated upon leptin challenge in a dose-dependent manner. Moreover, tyrosine phosphorylation of IRS-1 and Sam68 promotes their association with p85, the regulatory subunit of PI3K, and this association leads to the stimulation of PI3K activity. On the other hand, the leptin-stimulated tyrosine phosphorylation of Sam68 mediates the dissociation from RNA as assessed by Sepharose-conjugated poly(U) binding. Finally, leptin receptor activation also triggers MAPK signaling pathway. Thus, leptin dose-dependently stimulates tyrosine and threonine phosphorylation of MAPK in mononuclear cells. In summary, the present work demonstrates the presence of the long isoform of the human leptin receptor in peripheral blood mononuclear cells and the activation of two signaling pathways, PI3K and MAPK. The effects on Sam68 phosphorylation may modulate its binding to RNA, although the physiological implications remain to be studied. These signal transduction pathways may mediate the described effects of human leptin on human peripheral blood mononuclear cells.
Martín-Romero
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Department of Clinical Biochemistry, Virgen Macarena University Hospital, University of Seville, 41071-Seville, Spain.
Sánchez-Margalet
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D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D002352:Carrier Proteins / Q000187:drug effects* / Q000502:physiology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D020738:Leptin / Q000494:pharmacology* / Q000502:physiology; D007963:Leukocytes, Mononuclear / Q000187:drug effects* / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000187:drug effects*; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D010768:Phosphoserine / Q000378:metabolism; D019000:Phosphotyrosine / Q000378:metabolism; D020033:Protein Isoforms / Q000187:drug effects / Q000502:physiology; D011499:Protein Processing, Post-Translational / Q000187:drug effects*; D021122:Protein Subunits; D012313:RNA / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000502:physiology*; D011956:Receptors, Cell Surface; D054411:Receptors, Leptin; D011994:Recombinant Proteins / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D020738:Leptin; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D010768:Phosphoserine; D019000:Phosphotyrosine; D020033:Protein Isoforms; D021122:Protein Subunits; D012313:RNA; D016601:RNA-Binding Proteins; D011956:Receptors, Cell Surface; D054411:Receptors, Leptin; D011994:Recombinant Proteins; C489559:KHDRBS1 protein, human; C515448:LEPR protein, human