Adaptative response to inflammatory stressors

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Earlier work has demonstrated that the transcription factor, STAT3, is involved in the inducible expression of target genes responsible for acute-phase response in response to the interleukin-6 family of cytokines and growth factors. It has previously demonstrated that the STAT3 protein is a target for post-translational modifications such as phosphorylation, glutathionylation and acetylation, all of which adding layers of STAT3 regulation by extracellular signals. For example, we reported that STAT3 is susceptible to S-glutathionylation after cell treatment with the small thiol compound, pyrrolidine dithiocarbamate (PDTC), thus preventing the IL-6-mediated phosphorylation, dimerization and binding of STAT3 to DNA. Moreover, STAT3 is subject to reversible acetylation that occurs both in the cytosol and the nucleus by multiple enzymes, such that in response to cytokine stimulation, the histone acetyltransferase p300/CREB-binding protein associates with STAT3 and promotes its acetylation. Conversely, several histone deacetylases, including SIRT1, interact with STAT3 and negatively regulate its transcriptional activity through deacetylation of key STAT3 lysine residues. It is unclear whether other cellular functions of STAT3 involve SIRT1. In this study, Sirt1 gene knockout murine embryonic fibroblasts (MEF) cells were used to delineate the role of SIRT1 in the expression and subcellular localization of STAT3 and various markers of mitochondrial function. We demonstrate that Sirt1-null cells have significantly higher serine phosphorylated STAT3 level in mitochondria that correlated with increase in mitochondrial bioenergetics and formation of ATP. Various mitochondrial bioenergetic parameters were measured, such as the oxygen consumption rate in cell cultures, enzyme activities of the electron transport chain complexes in isolated mitochondria, and production of ATP and lactate. Two independent approaches, including ectopic expression of SIRT1 and siRNA-mediated knockdown of STAT3, led to reduction in intracellular ATP and increased lactate production in Sirt1-KO cells to levels that were approaching those of wild-type controls. We also performed extensive phospho-antibody array analysis and further establish a constitutive activation of the pro-inflammatory NF-κB pathway in Sirt1-null cells. Of significance, suppression of NF-κB activity by p65Rel gene knockdown blocked STAT3 gene induction while reducing cellular respiration in Sirt1-KO cells. These results suggest that the activation of NF-κB may play an important role in triggering a pro-inflammatory program of increased cellular respiration in Sirt1-null cells via higher expression and activation of mitochondrial STAT3. These results have implications for understanding the interplay between STAT3 and SIRT1 in pro-inflammatory conditions. The transcription factor HSF1 controls longevity through a stress-response mechanism, enabling adaptive responses to physical and inflammatory stressors. We recently reported that PDTC confers cellular protection against inflammation partly via HSF1-mediated transcriptional activation. Whether HSF1 contributes to the expression and/or activation of SIRT1 and alters the capacity of mitochondria to meet energy demands through non-genomic STAT3 functions remains to be established. This work may lead to new ways of fighting chronic inflammatory diseases and increase longevity.