An RNA interference-based screen identifies MAP4K4/NIK as a negative regulator of PPARgamma, adipogenesis, and insulin-responsive hexose transport
Program in Molecular Medicine
3T3-L1 Cells; Adipocytes; Adipogenesis; Animals; Biological Transport; CCAAT-Enhancer-Binding Protein-alpha; CCAAT-Enhancer-Binding Protein-beta; Down-Regulation; *Gene Expression Regulation; Glucose; Glucose Transporter Type 4; Insulin; Intracellular Signaling Peptides and Proteins; Mice; Oncogene Protein v-akt; PPAR gamma; Protein-Serine-Threonine Kinases; *RNA Interference; Suppression, Genetic; Tumor Necrosis Factor-alpha
Life Sciences | Medicine and Health Sciences
The insulin-regulated glucose transporter GLUT4 is a key modulator of whole body glucose homeostasis, and its selective loss in adipose tissue or skeletal muscle causes insulin resistance and diabetes. Here we report an RNA interference-based screen of protein kinases expressed in adipocytes and identify four negative regulators of insulin-responsive glucose transport: the protein kinases PCTAIRE-1 (PCTK1), PFTAIRE-1 (PFTK1), IkappaB kinase alpha, and MAP4K4/NIK. Integrin-linked protein kinase was identified as a positive regulator of this process. We characterized one of these hits, MAP4K4/NIK, and found that it is unique among mitogen-activated protein (MAP) kinases expressed in cultured adipocytes in attenuating hexose transport. Remarkably, MAP4K4/NIK suppresses expression of the adipogenic transcription factors C/EBPalpha, C/EBPbeta, and PPARgamma and of GLUT4 itself in these cells. RNA interference-mediated depletion of MAP4K4/NIK early in differentiation enhances adipogenesis and triglyceride deposition, and even in fully differentiated adipocytes its loss up-regulates GLUT4. Conversely, conditions that inhibit adipogenesis such as TNF-alpha treatment or depletion of PPARgamma markedly up-regulate MAP4K4/NIK expression in cultured adipocytes. Furthermore, TNF-alpha signaling to down-regulate GLUT4 is impaired in the absence of MAP4K4/NIK, indicating that MAP4K4 expression is required for optimal TNF-alpha action. These results reveal a MAP4K4/NIK-dependent signaling pathway that potently inhibits PPARgamma-responsive gene expression, adipogenesis, and insulin-stimulated glucose transport.
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Citation: Proc Natl Acad Sci U S A. 2006 Feb 14;103(7):2087-92. Epub 2006 Feb 3. Link to article on publisher's site