Mixed-lineage protein kinase 3 (MLK3) is a member of the mitogen-activated protein (MAP) kinase kinase kinase group that has been implicated in multiple signaling cascades, including the NF-B pathway and the extracellular signal-regulated kinase, c-Jun NH 2 -terminal kinase (JNK), and p38 MAP kinase pathways. Here, we examined the effect of targeted disruption of the murine Mlk3 gene. Mlk3 ؊/؊ mice were found to be viable and healthy. Primary embryonic fibroblasts prepared from these mice exhibited no major signaling defects. However, we did find that MLK3 deficiency caused a selective reduction in tumor necrosis factor (TNF)-stimulated JNK activation. Together, these data demonstrate that MLK3 contributes to the TNF signaling pathway that activates JNK.The mechanism of c-Jun NH 2 -terminal kinase (JNK) activation caused by tumor necrosis factor (TNF) is incompletely understood. It is established that JNK is activated by dual phosphorylation on the T-loop within the motif Thr-Pro-Tyr (7). This phosphorylation is mediated by the actions of two different mitogen-activated protein (MAP) kinase kinases: MKK4 and MKK7 (35). These MAP kinase kinases can be activated by MAP kinase kinase kinases (MAP3K), but the identity of the relevant TNF-stimulated MAP3K is unclear.Three MAP3K have been implicated in the activation of JNK caused by TNF. First, apoptosis signal-regulating kinase 1 is thought to be involved in the late phase of JNK activation in response to TNF, most likely as a result of the generation of reactive oxygen species (34). The immediate activation of JNK caused by TNF may be mediated by TAK1 and/or by one or more members of the mixed-lineage protein kinase (MLK) family. MLKs may be selectively involved in TNF-stimulated JNK activation (25), while TAK1 is implicated as a common TNF-stimulated activator of JNK, p38 MAPK,29). The relative roles of TAK1 and MLKs in TNF signaling are unclear. In this study, we have examined the possible contribution of a MLK to TNF signaling.There are three subgroups of MLKs (10). The MLK group (MLK1, MLK2, MLK3, and MLK4) shares similar structural domains, including an SH3 domain and a Crib motif that binds Cdc42 and Rac1. The DLK group (DLK and LZK) is structurally distinct and lacks the SH3 and Crib sequences. The third group of protein kinases consists of a single member (ZAK) that is distinctive because of the presence of a SAM domain. Many of these protein kinases are expressed in only a limited number of tissues; for example, MLK1 is expressed in epithelial cells and DLK is expressed in neurons (10). However, one member of this gene family is ubiquitously expressed, consistent with a possible role as a mediator of TNF signaling in many tissues-MLK3. This possibility is consistent with previous studies showing that TNF activates MLK3 (25), that MLK-family protein kinases (like TNF) (35) can selectively activate MKK7 (14, 18), and that a small-molecule MLK inhibitor can block TNF-stimulated JNK activation (25). However, recent RNA interference (RNAi)-based studies have su...

The c-Jun NH2-terminal kinase isoform (JNK) 1 is implicated in type 2 diabetes. However, a potential role for the JNK2 protein kinase in diabetes has not been established. Here, we demonstrate that JNK2 may play an important role in type 1 (insulin-dependent) diabetes that is caused by autoimmune destruction of ␤ cells. Studies of nonobese diabetic mice demonstrated that disruption of the Mapk9 gene (which encodes the JNK2 protein kinase) decreased destructive insulitis and reduced disease progression to diabetes. CD4 ؉ T cells from JNK2-deficient nonobese diabetic mice produced less IFN-␥ but significantly increased amounts of IL-4 and IL-5, indicating polarization toward the Th2 phenotype. This role of JNK2 to control the Th1͞Th2 balance of the immune response represents a mechanism of protection against autoimmune diabetes. We conclude that JNK protein kinases may have important roles in diabetes, including functions of JNK1 in type 2 diabetes and JNK2 in type 1 diabetes.insulitis ͉ T cell differentiation ͉ nonobese diabetic mouse ͉ stress-activated protein kinase