An RNA interference-based screen identifies MAP4K4/NIK as a negative regulator of PPAR[gamma], adipogenesis, and insulin-responsive hexose transport

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), I[kappa]B 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/EBP[alpha], C/EBP[beta], and PPAR[gamma], 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 PPAR[gamma] 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 PPAR3,-responsive gene expression, adipogenesis, and insulin-stimulated glucose transport. GLUT4 function | adipocyte differentiation | protein kinase screening