Lipid infusion and high fat feeding are established causes of systemic and adipose tissue insulin resistance. In this study, we treated 3T3-L1 adipocytes with a mixture of free fatty acids (FFAs) to investigate the molecular mechanisms underlying fat-induced insulin resistance. FFA treatment impaired insulin receptor-mediated signal transduction and decreased insulin-stimulated GLUT4 translocation and glucose transport. FFAs activated the stress/inflammatory kinases c-Jun N-terminal kinase (JNK) and IKKβ, and the suppressor of cytokine signaling protein 3, increased secretion of the inflammatory cytokine tumor necrosis factor (TNF)-α, and decreased secretion of adiponectin into the medium. RNA interference-mediated down-regulation of JNK blocked JNK activation and prevented most of the FFA-induced defects in insulin action. Blockade of TNF-α signaling with neutralizing antibodies to TNF-α or its receptors or with a dominant negative TNF-α peptide had a partial effect to inhibit FFA-induced cellular insulin resistance. We found that JNK activation by FFAs was not inhibited by blocking TNF-α signaling, whereas the FFA-induced increase in TNF-α secretion was inhibited by RNA interference-mediated JNK knockdown. Together, these results indicate that 1) JNK can be activated by FFAs through TNF-α-independent mechanisms, 2) activated JNK is a major contributor to FFA-induced cellular insulin resistance, and 3) TNF-α is an autocrine/paracrine downstream effector of activated JNK that can also mediate insulin resistance.

Insulin stimulates glucose transport by promoting translocation of GLUT4 proteins from the perinuclear compartment to the cell surface. It has been previously suggested that the microtubule-associated motor protein kinesin, which transports cargo toward the plus end of microtubules, plays a role in translocating GLUT4 vesicles to the cell surface. In this study, we investigated the role of Rab4, a small GTPase-binding protein, and the motor protein KIF3 (kinesin II in mice) in insulin-induced GLUT4 exocytosis in 3T3-L1 adipocytes. Photoaffinity labeling of Rab4 with [γ-³²P]GTP-azidoanilide showed that insulin stimulated Rab4 GTP loading and that this insulin effect was inhibited by pretreatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor LY294002 or expression of dominant-negative protein kinase C-λ (PKC-λ). Consistent with previous reports, expression of dominant-negative Rab4 (N121I) decreased insulin-induced GLUT4 translocation by 45%. Microinjection of an anti-KIF3 antibody into 3T3-L1 adipocytes decreased insulin-induced GLUT4 exocytosis by 65% but had no effect on endocytosis. Coimmunoprecipitation experiments showed that Rab4, but not Rab5, physically associated with KIF3, and this was confirmed by showing in vitro association using glutathione S-transferase-Rab4. A microtubule capture assay demonstrated that insulin stimulation increased the activity for the binding of KIF3 to microtubules and that this activation was inhibited by pretreatment with the PI3-kinase inhibitor LY294002 or expression of dominant-negative PKC-λ. Taken together, these data indicate that (i) insulin signaling stimulates Rab4 activity, the association of Rab4 with kinesin, and the interaction of KIF3 with microtubules and (ii) this process is mediated by insulin-induced PI3-kinase-dependent PKC-λ activation and participates in GLUT4 exocytosis in 3T3-L1 adipocytes.

The number of patients suffering from diabetic nephropathy resulting in end-stage kidney disease is increasing worldwide. In clinical settings, there are limited data regarding the impact of the urinary albumin-to-creatinine ratio (UACR) and reduced estimated glomerular filtration rate (eGFR) on renal and cardiovascular outcomes and all-cause mortality.