Norman J. Kennedy

The Bax subfamily of Bcl2-related proteins is essential for apoptotic signal transduction by c-Jun NH(2)-terminal kinase.

ABSTRACT Targeted gene disruption studies have established that the c-Jun NH2-terminal kinase (JNK) signaling pathway is required for stress-induced release of mitochondrial cytochrome c and apoptosis. Here we demonstrate that activated JNK is sufficient to induce rapid cytochrome c release and apoptosis. However, activated JNK fails to cause death in cells deficient of members of the Bax subfamily of proapoptotic Bcl2-related proteins. Furthermore, exposure to stress fails to activate Bax, cause cytochrome c release, and induce death in JNK-deficient cells. These data demonstrate that proapoptotic members of the Bax protein subfamily are essential for JNK-dependent apoptosis.

Regulation of innate and adaptive immune responses by MAP kinase phosphatase 5.

Mitogen-activated protein (MAP) kinases are essential regulators in immune responses, and their activities are modulated by kinases and phosphatases. MAP kinase phosphatase (MKP) is a family of dual-specificity phosphatases whose function is evolutionarily conserved. A number of mammalian MKPs have been identified so far, but their specific physiological functions in negative regulation of MAP kinases have not been genetically defined. Here we examine innate and adaptive immune responses in the absence of MKP5. JNK activity was selectively increased in Mkp5 (also known as Dusp10)-deficient mouse cells. Mkp5-deficient cells produced greatly enhanced levels of pro-inflammatory cytokines during innate immune responses and exhibited greater T-cell activation than their wild-type counterparts. However, Mkp5-deficient T cells proliferated poorly upon activation, which resulted in increased resistance to experimental autoimmune encephalomyelitis. By contrast, Mkp5-deficient CD4+ and CD8+ effector T cells produced significantly increased levels of cytokines compared with wild-type cells, which led to much more robust and rapidly fatal immune responses to secondary infection with lymphocytic choriomeningitis virus. Therefore, MKP5 has a principal function in both innate and adaptive immune responses, and represents a novel target for therapeutic intervention of immune diseases.

c-Jun NH 2 -Terminal Kinase Is Essential for the Regulation of AP-1 by Tumor Necrosis Factor

ABSTRACT The c-Jun NH2-terminal kinase (JNK) is activated by the cytokine tumor necrosis factor (TNF). This pathway is implicated in the regulation of AP-1-dependent gene expression by TNF. To examine the role of the JNK signaling pathway, we compared the effects of TNF on wild-type and Jnk1 −/− Jnk2 −/− murine embryo fibroblasts. We show that JNK is required for the normal regulation of AP-1 by TNF. The JNK-deficient cells exhibited decreased expression of c-Jun, JunD, c-Fos, Fra1, and Fra2; decreased phosphorylation of c-Jun and JunD; and decreased AP-1 DNA binding activity. The JNK-deficient cells also exhibited defects in the regulation of the AP-1-related transcription factor ATF2. These changes were associated with marked defects in TNF-regulated gene expression. The JNK signal transduction pathway is therefore essential for AP-1 transcription factor regulation in cells exposed to TNF.

c-Jun NH2-Terminal Kinase (JNK)1 and JNK2 Have Distinct Roles in CD8+ T Cell Activation

The c-Jun NH2-terminal kinase (JNK) signaling pathway is induced by cytokines and stress stimuli and is implicated in cell death and differentiation, but the specific function of this pathway depends on the cell type. Here we examined the role of JNK1 and JNK2 in CD8+ T cells. Unlike CD4+ T cells, the absence of JNK2 causes increased interleukin (IL)-2 production and proliferation of CD8+ T cells. In contrast, JNK1-deficient CD8+ T cells are unable to undergo antigen-stimulated expansion in vitro, even in the presence of exogenous IL-2. The hypoproliferation of these cells is associated with impaired IL-2 receptor α chain (CD25) gene and cell surface expression. The reduced level of nuclear activating protein 1 (AP-1) complexes in activated JNK1-deficient CD8+ T cells can account for the impaired IL-2 receptor α chain gene expression. Thus, JNK1 and JNK2 play different roles during CD8+ T cell activation and these roles differ from those in CD4+ T cells.

Sirtuin 1 (SIRT1) Protein Degradation in Response to Persistent c-Jun N-terminal Kinase 1 (JNK1) Activation Contributes to Hepatic Steatosis in Obesity

SIRT1 is involved in the pathogenesis of obesity, diabetes, and aging. However, it is not clear how SIRT1 activity is regulated by intracellular kinases in cells. In this study, we investigated SIRT1 phosphorylation and protein degradation in response to JNK1 activation in obese mice. Mouse SIRT1 is phosphorylated by JNK1 at Ser-46 (Ser-47 in human SIRT1), which is one of the four potential residues targeted by JNK1. The phosphorylation induces a brief activation of SIRT1 function and degradation of SIRT1 thereafter by the proteasome. Ubiquitination occurs in SIRT1 protein after the phosphorylation. Mutation of Ser-46 to alanine prevents the phosphorylation, ubiquitination, and degradation. In vivo, SIRT1 undergoes an extensive degradation in hepatocytes in obesity as a consequence of persistent activation of JNK1. The degradation leads to inhibition of SIRT1 function, which contributes to development of hepatic steatosis. The degradation disappears in obesity when JNK1 is inactivated in mice. JNK2 exhibits an opposite activity in the regulation of SIRT1 degradation. The JNK1-SIRT1 pathway provides a new molecular mechanism for the pathogenesis of hepatic steatosis in obesity.

Role of Muscle c-Jun NH2-Terminal Kinase 1 in Obesity-Induced Insulin Resistance

ABSTRACT Obesity caused by feeding of a high-fat diet (HFD) is associated with an increased activation of c-Jun NH2-terminal kinase 1 (JNK1). Activated JNK1 is implicated in the mechanism of obesity-induced insulin resistance and the development of metabolic syndrome and type 2 diabetes. Significantly, Jnk1−/− mice are protected against HFD-induced obesity and insulin resistance. Here we show that an ablation of the Jnk1 gene in skeletal muscle does not influence HFD-induced obesity. However, muscle-specific JNK1-deficient (MKO) mice exhibit improved insulin sensitivity compared with control wild-type (MWT) mice. Thus, insulin-stimulated AKT activation is suppressed in muscle, liver, and adipose tissue of HFD-fed MWT mice but is suppressed only in the liver and adipose tissue of MKO mice. These data demonstrate that JNK1 in muscle contributes to peripheral insulin resistance in response to diet-induced obesity.

Novel Observations From Next-Generation RNA Sequencing of Highly Purified Human Adult and Fetal Islet Cell Subsets.

Understanding distinct gene expression patterns of normal adult and developing fetal human pancreatic α- and β-cells is crucial for developing stem cell therapies, islet regeneration strategies, and therapies designed to increase β-cell function in patients with diabetes (type 1 or 2). Toward that end, we have developed methods to highly purify α-, β-, and δ-cells from human fetal and adult pancreata by intracellular staining for the cell-specific hormone content, sorting the subpopulations by flow cytometry, and, using next-generation RNA sequencing, we report the detailed transcriptomes of fetal and adult α- and β-cells. We observed that human islet composition was not influenced by age, sex, or BMI, and transcripts for inflammatory gene products were noted in fetal β-cells. In addition, within highly purified adult glucagon-expressing α-cells, we observed surprisingly high insulin mRNA expression, but not insulin protein expression. This transcriptome analysis from highly purified islet α- and β-cell subsets from fetal and adult pancreata offers clear implications for strategies that seek to increase insulin expression in type 1 and type 2 diabetes.

Requirement of c-Jun NH2-Terminal Kinase for Ras-Initiated Tumor Formation

ABSTRACT The c-Jun NH2-terminal kinase (JNK) signal transduction pathway causes increased gene expression mediated, in part, by members of the activating transcription factor protein (AP1) group. JNK is therefore implicated in the regulation of cell growth and cancer. To test the role of JNK in Ras-induced tumor formation, we examined the effect of compound ablation of the ubiquitously expressed genes Jnk1 plus Jnk2. We report that JNK is required for Ras-induced transformation of p53-deficient primary cells in vitro. Moreover, JNK is required for lung tumor development caused by mutational activation of the endogenous KRas gene in vivo. Together, these data establish that JNK plays a key role in Ras-induced tumorigenesis.

Signal Transduction Cross Talk Mediated by Jun N-Terminal Kinase-Interacting Protein and Insulin Receptor Substrate Scaffold Protein Complexes

ABSTRACT Scaffold proteins have been established as important mediators of signal transduction specificity. The insulin receptor substrate (IRS) proteins represent a critical group of scaffold proteins that are required for signal transduction by the insulin receptor, including the activation of phosphatidylinositol 3 kinase. The c-Jun NH2-terminal kinase (JNK)-interacting proteins (JIPs) represent a different group of scaffold molecules that are implicated in the regulation of the JNK. These two signaling pathways are functionally linked because JNK can phosphorylate IRS1 on the negative regulatory site Ser-307. Here we demonstrate the physical association of these signaling pathways using a proteomic approach that identified insulin-regulated complexes of JIPs together with IRS scaffold proteins. Studies using mice with JIP scaffold protein defects confirm that the JIP1 and JIP2 proteins are required for normal glucose homeostasis. Together, these observations demonstrate that JIP proteins can influence insulin-stimulated signal transduction mediated by IRS proteins.

Regulation of Adipose Tissue Inflammation and Insulin Resistance by MAPK Phosphatase 5

Background: MKP5 regulates inflammation in innate immunity. Results: MKP5 knock-out mice developed insulin resistance and glucose intolerance spontaneously, associated with increased inflammatory macrophage infiltration in visceral adipose tissue. Conclusion: MKP5 critically controls adipose tissue inflammation and insulin sensitivity. Significance: Understanding the role of MKP5 in metabolism will shed new light on the pathogenesis and treatment of metabolic disorders. Obesity and metabolic disorders such as insulin resistance and type 2 diabetes have become a major threat to public health globally. The mechanisms that lead to insulin resistance in type 2 diabetes have not been well understood. In this study, we show that mice deficient in MAPK phosphatase 5 (MKP5) develop insulin resistance spontaneously at an early stage of life and glucose intolerance at a later age. Increased macrophage infiltration in white adipose tissue of young MKP5-deficient mice correlates with the development of insulin resistance. Glucose intolerance in MKP5-deficient mice is accompanied by significantly increased visceral adipose weight, reduced AKT activation, enhanced p38 activity, and increased inflammation in visceral adipose tissue when compared with wild-type (WT) mice. Deficiency of MKP5 resulted in increased inflammatory activation in macrophages. These findings thus demonstrate that MKP5 critically controls inflammation in white adipose tissue and the development of metabolic disorders.