Defects within the insulin signaling pathways may lead to insulin resistance in peripheral tissues. The prevalence of obesity generally increases with age and is associated with insulin resistance characterized by an elevation in hepatic glucose production, impaired glucose tolerance and defects in the ability of the insulin receptor to transduce insulin signals. Cross-sectional studies have associated obesity and other components of the so-called metabolic syndrome with low-grade inflammation. Hence, better understanding of the complex interplay between proinflammatory cytokines and insulin signal transduction could lead to the development of effective strategies to reduce or delay the progression of complications associated with insulin resistance states, such as Type 2 diabetes. A strong positive correlation has been reported between adipose tissue levels of adipocytokines (e.g., interleukin (IL)-6, tumor necrosis factor (TNF)) and the increased secretion of acute phase proteins by the liver of obese subjects. IL-6 activates the Janus kinase/signal transducer(s) and activator(s) of transcription (JAK/STAT) pathway to elicit the synthesis and secretion of several hepatic inflammatory proteins. In response to IL-6, JAK induces tyrosine phosphorylation of STAT3 (pY-STAT3) at a single residue, followed by its nuclear translocation for the inducible expression of genes through binding to specific DNA responsive elements. In addition to tyrosine phosphorylation, STAT3 is serine phosphorylated at a single residue in response to IL-6, which enhances its transcriptional activation. We have recently completed a study showing that the antioxidant pyrolidine dithiocarbamate (PDTC) was protective against IL-6?induced JAK/STAT3 signaling in liver-derived human HepG2 cells and a primary culture of rat hepatocytes. We found that PDTC inhibited IL-6?stimulated phosphorylation and nuclear translocation of STAT3 by interfering with the inducible association of STAT3 with an adaptor protein. This reduction in STAT3 activity led to a sharp decrease in acute phase protein gene expression in response to IL-6 as determined by microarray analysis and Northern blot. Of significance, pretreatment of rat hepatocytes with PDTC selectively reduced the adverse effects of IL-6 on insulin-stimulated Akt phosphorylation. It is established that activation of Akt plays a major role in various biological processes triggered by insulin. It remains unknown whether PDTC can inhibit the development of insulin resistance in obesity and in aging, two chronic in vivo models of low grade inflammation. In a second project, we investigated the role of actin-binding proteins in the promotion of STAT3 signaling in response to IL-6. The IL-6-mediated increase in JAK/STAT activity is initiated at the level of raft plasma membrane microdomains of diverse cell types. Despite the fact that STAT3 has been shown to localize to the actin cytoskeleton, little is known about the involvement of actin-binding proteins in regulating the JAK/STAT signaling pathway. Therefore, we hypothesized that filamin A may be a component in STAT3 activation by IL-6. Human melanoma M2 cells that lack filamin A and filamin A-expressing M2A7 cells were used in a number of experiments to assess the kinetics of STAT3 phosphorylation, its nuclear translocation and transcriptional activity following IL-6 stimulation. Because Ser727 of STAT3 is located in a conserved Pro-Xaa-Ser/Thr-Pro site, which is recognized as a phosphorylation site for members of the mitogen-activated protein (MAP) kinase family, we have also defined which subgroup(s) of the MAP kinases is involved in regulating serine phosphorylation of STAT3. Our results show a functional role of filamin A in the modulation of IL-6-induced responses. While these cell lines are suitable for initial in vitro studies, they are surrogates for more physiologically relevant models. Toward this goal, functional studies in vivo will be carried out to address the potential relevance of filamin in the proinflammatory actions of IL-6.
|Fiori, Jennifer L; Zhu, Tie-Nian; O'Connell, Michael P et al. (2009) Filamin A modulates kinase activation and intracellular trafficking of epidermal growth factor receptors in human melanoma cells. Endocrinology 150:2551-60|
|He, Hua-Jun; Zong, Yaping; Bernier, Michel et al. (2009) Sensing the insulin signaling pathway with an antibody array. Proteomics Clin Appl 3:1440-50|
|Zhu, Tie-Nian; He, Hua-Jun; Kole, Sutapa et al. (2007) Filamin A-mediated down-regulation of the exchange factor Ras-GRF1 correlates with decreased matrix metalloproteinase-9 expression in human melanoma cells. J Biol Chem 282:14816-26|
|He, Hua-Jun; Zhu, Tie-Nian; Xie, Yi et al. (2006) Pyrrolidine dithiocarbamate inhibits interleukin-6 signaling through impaired STAT3 activation and association with transcriptional coactivators in hepatocytes. J Biol Chem 281:31369-79|