Requirement of JIP1-Mediated c-Jun N-Terminal Kinase Activation for Obesity-Induced Insulin Resistance

The c-Jun NH₂-terminal kinase (JNK) interacting protein 1 (JIP1) has been proposed to act as a scaffold protein that mediates JNK activation. However, recent studies have implicated JIP1 in multiple biochemical processes. Physiological roles of JIP1 that are related to the JNK scaffold function of JIP1 are therefore unclear. To test the role of JIP1 in JNK activation, we created mice with a germ line point mutation in the Jip1 gene (Thr¹⁰³ replaced with Ala) that selectively blocks JIP1-mediated JNK activation. These mutant mice exhibit a severe defect in JNK activation caused by feeding of a high-fat diet. The loss of JIP1-mediated JNK activation protected the mutant mice against obesity-induced insulin resistance. We conclude that JIP1-mediated JNK activation plays a critical role in metabolic stress regulation of the JNK signaling pathway.Diet-induced obesity causes insulin resistance and metabolic syndrome, which can lead to β-cell dysfunction and type 2 diabetes (15). It is established that feeding mice a high-fat diet (HFD) causes activation of c-Jun NH₂-terminal kinase 1 (JNK1) (10). Moreover, Jnk1^−/− mice are protected against the effects of HFD-induced insulin resistance (10). Together, these observations indicate that JNK1 plays a critical role in the metabolic stress response. However, the mechanism that accounts for HFD-induced JNK1 activation is unclear. Recent studies have implicated the JIP1 scaffold protein in JNK1 activation caused by metabolic stress (23, 39).JIP1 can assemble a functional JNK activation module composed of a mitogen-activated protein kinase (MAPK) kinase kinase (a member of the mixed-lineage protein kinase MLK] group), the MAPK kinase MKK7, and JNK (40, 42). This complex may be relevant to JNK activation caused by metabolic stress (23, 39). Indeed, MLK-deficient mice (14) and JIP1-deficient mice (13) exhibit defects in HFD-induced JNK activation and insulin resistance.The protection of Jip1^−/− mice against the effects of being fed an HFD may be mediated by loss of the JNK scaffold function of JIP1. However, JIP1 has also been reported to mediate other biochemical processes that would also be disrupted in Jip1^−/− mice. For example, JIP1 interacts with AKT and has been implicated in the mechanism of AKT activation (8, 17, 18, 34). Moreover, JIP1 interacts with members of the Src and Abl tyrosine kinase families (4, 16, 24), the lipid phosphatase SHIP2 (44), the MAPK phosphatase MKP7 (43), β-amyloid precursor protein (20, 31), the small GTPase regulatory proteins Ras-GRF1, p190-RhoGEF, RalGDS, and Tiam1 (2, 8, 21), ankyrin G (35), molecular chaperones (35), and the low-density-lipoprotein-related receptors LRP1, LRP2, and LRP8 (7, 37). JIP1 also interacts with other scaffold proteins, including the insulin receptor substrate proteins IRS1 and IRS2 (35). Finally, JIP1 may act as an adapter protein for kinesin-mediated (11, 12, 16, 38, 42) and dynein-mediated (35) trafficking on microtubules. The JNK scaffold properties of JIP1 therefore represent only one of the possible biochemical functions of JIP1 that are disrupted in Jip1^−/− mice.The purpose of this study was to test the role of JIP1 as a JNK scaffold protein in the response of mice to being fed an HFD. Our approach was to examine the effect of a point mutation that selectively prevents JIP1-induced JNK activation. It is established that phosphorylation of JIP1 on Thr¹⁰³ is required for JIP1-mediated JNK activation by the MLK pathway (25). Consequently, the phosphorylation-defective Thr103Ala JIP1 protein does not activate JNK (25). Here we describe the analysis of mice with a point mutation in the Jip1 gene that replaces the JIP1 phosphorylation site Thr¹⁰³ with Ala. We show that this mutation suppresses HFD-induced JNK activation and insulin resistance. These data demonstrate that JNK activation mediated by the JIP1 scaffold complex contributes to the response of mice to an HFD.