Michael Karin

Bio: Michael Karin is an academic researcher from University of California, San Diego. The author has contributed to research in topics: IκB kinase & Signal transduction. The author has an hindex of 236, co-authored 704 publications receiving 226485 citations. Previous affiliations of Michael Karin include Sanford-Burnham Institute for Medical Research & University of California, Los Angeles.

Functional in vivo interactions between JNK1 and JNK2 isoforms in obesity and insulin resistance

Abstract: The c-Jun N-terminal kinases (JNKs) are key regulators of inflammation and interfere with insulin action in cultured cells and whole animals. Obesity increases total JNK activity, and JNK1, but not JNK2, deficiency results in reduced adiposity and improved insulin sensitivity. Interestingly, a higher-than-normal level of JNK activation is observed in Jnk2−/− mice, particularly in the liver, indicating an interaction between the isoforms that might have masked the metabolic activity of JNK2 in isolated mutant mice. To address the role of the JNK2 isoform in metabolic homeostasis, we intercrossed Jnk1−/− and Jnk2−/− mice and examined body weight and glucose metabolism in the resulting mutant allele combinations. Among all of the viable genotypes examined, we observed only reduced body weight and increased insulin sensitivity in Jnk1−/− and Jnk1+/−Jnk2−/− mice. These two groups of mice also exhibited reduced total JNK activity and cytokine expression in liver tissue compared with all other genotypes examined. These data indicate that the JNK2 isoform is also involved in metabolic regulation, but its function is not obvious when JNK1 is fully expressed because of regulatory crosstalk between the two isoforms.

Lasting N-Terminal Phosphorylation of c-Jun and Activation of c-Jun N-Terminal Kinases after Neuronal Injury

Abstract: Transcription factor c-Jun is proposed to control neuronal cell death and survival, but its activation by N-terminal phosphorylation and the underlying activity of the c-Jun N-terminal kinases (JNKs) remain to be elucidated in the adult mammalian brain. We generated a polyclonal antiserum that specifically recognizes c-Jun phosphorylated at its serine 73 (S73) residue after UV irradiation of 3T3 cells. Disruption of the c-jun locus in 3T3 cells abolished this reaction, and retransfection of the human c-jun at the c-jun-/- background restored it. The phospho-c-Jun antiserum was used to visualize N-terminally phosphorylated c-Jun in the adult rat brain with cellular resolution. Prolonged c-Jun S73 phosphorylation was detected in affected neurons up to 5 d after transient occlusion of medial cerebral artery or up to 50 d after transection of central nerve fiber tracts. After cerebral ischemia-reperfusion, phosphorylation of c-Jun was linked with induced expression of Fas-ligand (APO-1, CD95-ligand), whose gene is a putative c-Jun/AP-1 target, and with terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) reactivity, a marker for apoptosis. After nerve fiber transection, however, lasting c-Jun phosphorylation occurred in axotomized neurons negative for Fas-ligand or TUNEL and regardless of degeneration or survival. In contrast to these lasting phosphorylation patterns, transient seizure activity by pentylenetetrazole provoked only a brief c-Jun phosphorylation and JNK activation. In extracts from ischemic or axotomized brain compartments, c-Jun phosphorylation correlated with enhanced long-term JNK activity, and in-gel kinase assays visualized proteins with sizes corresponding to JNK isoforms as the only c-Jun N-terminally phosphorylating enzymes. These results demonstrate that lasting c-Jun S73 phosphorylation and JNK activity are part of neuronal stress response after neurodegenerative disorders in the adult mammalian brain with Fas-ligand as a putative apoptotic effector.

Multiple cis- and trans-acting elements mediate the transcriptional response to phorbol esters.

Abstract: Protein kinase C is important in the transduction of signals generated at the plasma membrane. The physiological activators of protein kinase C are diacylglycerols1, and the tumour-promoting phorbol esters, such as 12-O-tetradecanoyI-phorbol-14 acetate (TPA), constitute another group of specific activators1,2. Many cellular substrates for phosphorylation by protein kinase C have been described1,3, but proteins that directly control transcription in response to protein kinase C activation are yet to be identified. TPA treatment leads to induction of various proto-oncogenes4–7, growth factor genes7,8, and genes encoding secreted proteases9. In addition. TPA increases the activity of viral enhancer elements10–13. To identify trans-acting factors that mediate the transcriptional response to TPA we chose the simian virus 40 (SV40) enhancer14 as a model, because it is known to be composed of several discrete cis-acting elements15–18 which are recognized by multiple trans-acting factors19–21. We report here that the SV40 enhancer contains at least four different TPA responsive elements whose activity is dependent on cell-type. The induction response is likely to involve at least two distinct post-translational steps which modulate the activity of the proteins that recognize these elements.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

REACTIVE OXYGEN SPECIES: Metabolism, Oxidative Stress, and Signal Transduction

Abstract: Several reactive oxygen species (ROS) are continuously produced in plants as byproducts of aerobic metabolism. Depending on the nature of the ROS species, some are highly toxic and rapidly detoxified by various cellular enzymatic and nonenzymatic mechanisms. Whereas plants are surfeited with mechanisms to combat increased ROS levels during abiotic stress conditions, in other circumstances plants appear to purposefully generate ROS as signaling molecules to control various processes including pathogen defense, programmed cell death, and stomatal behavior. This review describes the mechanisms of ROS generation and removal in plants during development and under biotic and abiotic stress conditions. New insights into the complexity and roles that ROS play in plants have come from genetic analyses of ROS detoxifying and signaling mutants. Considering recent ROS-induced genome-wide expression analyses, the possible functions and mechanisms for ROS sensing and signaling in plants are compared with those in animals and yeast.