Structural determinants of mitochondrial STAT3 targeting and function.

Signal transducer and activator of transcription (STAT) 3 has been found within mitochondria in addition to its canonical role of shuttling between cytoplasm and nucleus during cytokine signaling. Mitochondrial STAT3 has been implicated in modulation of cellular metabolism, largely through effects on the respiratory electron transport chain. However, the structural requirements underlying mitochondrial targeting and function have remained unclear. Here, we show that mitochondrial STAT3 partitions between mitochondrial compartments defined by differential detergent solubility, suggesting that mitochondrial STAT3 is membrane associated. The majority of STAT3 was found in an SDS soluble fraction copurifying with respiratory chain proteins, including numerous components of the complex I NADH dehydrogenase, while a minor component was found with proteins of the mitochondrial translation machinery. Mitochondrial targeting of STAT3 required the amino-terminal domain, and an internal linker domain motif also directed mitochondrial translocation. However, neither the phosphorylation of serine 727 nor the presence of mitochondrial DNA was required for the mitochondrial localization of STAT3. Two cysteine residues in the STAT3 SH2 domain, which have been previously suggested to be targets for protein palmitoylation, were also not required for mitochondrial translocation, but were required for its function as an enhancer of complex I activity. These structural determinants of STAT3 mitochondrial targeting and function provide potential therapeutic targets for disrupting the activity of mitochondrial STAT3 in diseases such as cancer.

Mitochondrial STAT3 regulates antioxidant gene expression through complex I-derived NAD in triple negative breast cancer.

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor with roles in inflammation and tumorigenicity. A fraction of STAT3 localizes in mitochondria, where it augments tumorigenesis via regulation of mitochondrial functions, including modulation of respiration and redox status. We show a novel mechanism for mitochondrial STAT3 regulation of redox homeostasis in triple-negative breast cancer cells. Loss of STAT3 diminished complex I dehydrogenase activity and impaired NAD+ regeneration, leading to impaired expression of glutathione biosynthetic genes and other antioxidant genes. Expressing mitochondrially restricted STAT3 or replenishment of the cellular NAD pool restored antioxidant gene expression, as did complementation of the NADH dehydrogenase activity by expression of the STAT3-independent yeast dehydrogenase, NDI1. These NAD-regulated processes contributed to malignant phenotypes by promoting clonal cell growth and migration. Proximity interaction and protein pull-down assays identified three components of complex I that associated with mitochondrial STAT3, providing a potential mechanistic basis for how mitochondrial STAT3 affects complex I activity. Our data document a novel mechanism through which mitochondrial STAT3 indirectly controls antioxidant gene regulation through a retrograde NAD+ signal that is modulated by complex I dehydrogenase activity.

STAT3 Inhibitor OPB-51602 Is Cytotoxic to Tumor Cells Through Inhibition of Complex I and ROS Induction.

STAT3 is a transcription factor involved in several cellular activities including inflammation, proliferation, and survival, but it also plays a non-transcriptional role in modulating mitochondrial metabolism. Given its diverse functions in human cancers, it is an emerging therapeutic target. Here we show that OPB-51602, a small molecule inhibitor of STAT3, is highly toxic in a STAT3-dependent manner. Specifically, drug toxicity depends on mitochondrial STAT3 as tumor cells expressing only a mitochondrially restricted form of STAT3 are sensitive to the compound, whereas STAT3-null cells are protected. OPB-51602 inhibited complex I activity and led to increased ROS production, which in turn induced mitophagy, actin rearrangements, and cell death. Cells undergoing reduced oxidative phosphorylation or expressing NDI1 NADH dehydrogenase from Saccharomyces cerevisiae, which bypasses mammalian complex I, were resistant to OPB-51602 toxicity. These results show that targeting mitochondrial STAT3 function causes synthetic lethality through complex I inhibition that could be exploited for cancer chemotherapy.

PKR Transduces MDA5-Dependent Signals for Type I IFN Induction.

Sensing invading pathogens early in infection is critical for establishing host defense. Two cytosolic RIG-like RNA helicases, RIG-I and MDA5, are key to type I interferon (IFN) induction in response to viral infection. Mounting evidence suggests that another viral RNA sensor, protein kinase R (PKR), may also be critical for IFN induction during infection, although its exact contribution and mechanism of action are not completely understood. Using PKR-deficient cells, we found that PKR was required for type I IFN induction in response to infection by vaccinia virus lacking the PKR antagonist E3L (VVΔE3L), but not by Sendai virus or influenza A virus lacking the IFN-antagonist NS1 (FluΔNS1). IFN induction required the catalytic activity of PKR, but not the phosphorylation of its principal substrate, eIF2α, or the resulting inhibition of host translation. In the absence of PKR, IRF3 nuclear translocation was impaired in response to MDA5 activators, VVΔE3L and encephalomyocarditis virus, but not during infection with a RIG-I-activating virus. Interestingly, PKR interacted with both RIG-I and MDA5; however, PKR was only required for MDA5-mediated, but not RIG-I-mediated, IFN production. Using an artificially activated form of PKR, we showed that PKR activity alone was sufficient for IFN induction. This effect required MAVS and correlated with IRF3 activation, but no longer required MDA5. Nonetheless, PKR activation during viral infection was enhanced by MDA5, as virus-stimulated catalytic activity was impaired in MDA5-null cells. Taken together, our data describe a critical and non-redundant role for PKR following MDA5, but not RIG-I, activation to mediate MAVS-dependent induction of type I IFN through a kinase-dependent mechanism.

A structural basis for the regulation of an H-NOX-associated cyclic-di-GMP synthase/phosphodiesterase enzyme by nitric oxide-bound H-NOX.

Biofilms are surface-attached communities of bacteria enclosed in a polysaccharide matrix. Bacteria in a biofilm are extremely resistant to antibiotics. Several recent reports have linked the signaling molecule nitric oxide (NO) with biofilm dispersal. We have previously reported that an H-NOX (heme-nitric oxide/oxygen binding) protein in the biofilm-dwelling bacterium Shewanella woodyi mediates NO-induced biofilm dispersal. In S. woodyi, H-NOX (SwH-NOX) is cocistronic with a gene encoding a dual-functioning diguanylate cyclase/phosphodiesterase enzyme, designated here as HaCE (H-NOX-associated cyclic-di-GMP processing enzyme). Enzymes such as these are responsible for regulating the intracellular concentrations of cyclic-di-GMP, a secondary signaling molecule essential to biofilm formation in bacteria. We have demonstrated that NO-bound SwH-NOX regulates both enzymatic activities of SwHaCE, resulting in decreased cellular cyclic-di-GMP levels and disruption of biofilm formation. Thus, H-NOX/HaCE represents a potential drug target for regulating biofilm formation. In this work, the SwH-NOX surface residues critical for the formation of a protein complex with SwHaCE are identified using nuclear magnetic resonance, fluorescence quenching, and cosedimentation. Enzyme assays confirm this protein-protein interface and its importance for H-NOX/HaCE function.