Mitochondrial STAT3 contributes to transformation of Barrett's epithelial cells that express oncogenic Ras in a p53-independent fashion

Oncogenic dependency on STAT3 serine phosphorylation in KRAS mutant lung cancer

The oncogenic potential of the latent transcription factor signal transducer and activator of transcription (STAT)3 in many human cancers, including lung cancer, has been largely attributed to its nuclear activity as a tyrosine-phosphorylated (pY₇₀₅ site) transcription factor. By contrast, an alternate mitochondrial pool of serine phosphorylated (pS₇₂₇ site) STAT3 has been shown to promote tumourigenesis by regulating metabolic processes, although this has been reported in only a restricted number of mutant RAS-addicted neoplasms. Therefore, the involvement of STAT3 serine phosphorylation in the pathogenesis of most cancer types, including mutant KRAS lung adenocarcinoma (LAC), is unknown. Here, we demonstrate that LAC is suppressed in oncogenic Kras^G12D-driven mouse models engineered for pS₇₂₇-STAT3 deficiency. The proliferative potential of the transformed Kras^G12D lung epithelium, and mutant KRAS human LAC cells, was significantly reduced upon pS₇₂₇-STAT3 deficiency. Notably, we uncover the multifaceted capacity of constitutive pS₇₂₇-STAT3 to metabolically reprogramme LAC cells towards a hyper-proliferative state by regulating nuclear and mitochondrial (mt) gene transcription, the latter via the mtDNA transcription factor, TFAM. Collectively, our findings reveal an obligate requirement for the transcriptional activity of pS₇₂₇-STAT3 in mutant KRAS-driven LAC with potential to guide future therapeutic targeting approaches.

STAT3 as a mediator of oncogenic cellular metabolism: Pathogenic and therapeutic implications

The oncogenic transcription factor signal transducer and activator of transcription 3 (STAT3) is activated constitutively in a wide array of human cancers. It is an appealing molecular target for novel therapy as it directly regulates expression of genes involved in cell proliferation, survival, angiogenesis, chemoresistance and immune responsiveness. In addition to these well-established oncogenic roles, STAT3 has also been found to mediate a wide array of functions in modulating cellular behavior. The transcriptional function of STAT3 is canonically regulated through tyrosine phosphorylation. However, STAT3 phosphorylated at a single serine residue can allow incorporation of this protein into the inner mitochondrial membrane to support oxidative phosphorylation (OXPHOS) and maximize the utility of glucose sources. Conflictingly, its canonical transcriptional activity suppresses OXPHOS and favors aerobic glycolysis to promote oncogenic behavior. Apart from mediating the energy metabolism and controversial effects on ATP production, STAT3 signaling modulates lipid metabolism of cancer cells. By mediating fatty acid synthesis and beta oxidation, STAT3 promotes employment of available resources and supports survival in the conditions of metabolic stress. Thus, the functions of STAT3 extend beyond regulation of oncogenic genes expression to pleiotropic effects on a spectrum of essential cellular processes. In this review, we dissect the current knowledge on activity and mechanisms of STAT3 involvement in transcriptional regulation, mitochondrial function, energy production and lipid metabolism of malignant cells, and its implications to cancer pathogenesis and therapy.

Mitochondria: The metabolic switch of cellular oncogenic transformation

Mitochondria, well recognized as the "powerhouse" of cells, are maternally inherited organelles with bacterial ancestry that play essential roles in a myriad of cellular functions. It has become profoundly evident that mitochondria regulate a wide array of cellular and metabolic functions, including biosynthetic metabolism, cell signaling, redox homeostasis, and cell survival. Correspondingly, defects in normal mitochondrial functioning have been implicated in various human malignancies. Cancer development involves the activation of oncogenes, inactivation of tumor suppressor genes, and impairment of apoptotic programs in cells. Mitochondria have been recognized as the site of key metabolic switches for normal cells to acquire a malignant phenotype. This review outlines the role of mitochondria in human malignancies and highlights potential aspects of mitochondrial metabolism that could be targeted for therapeutic development.

Leptin activates Akt in oesophageal cancer cells via multiple atorvastatin-sensitive small GTPases

Obesity is a risk factor for Barrett’s oesophagus and oesophageal adenocarcinoma. Adipose tissue secretes the hormone leptin. Leptin is a growth factor for several cell types, including Barrett’s cells and oesophageal adenocarcinoma cells. Statins are associated with reduced rates of Barrett’s oesophagus and oesophageal cancer and exhibit anti-cancer effects in vitro. The mechanisms of these effects are not fully established. We have examined the effects of leptin and the lipid-soluble statin, atorvastatin, on signalling via monomeric GTP-binding proteins and Akt. Proliferation and apoptosis were assessed in OE33 cells. Akt activity was quantified by cell-based ELISA and in vitro kinase assay. Specific small-molecule inhibitors and a dominant-negative construct were used to reduce Akt activity. Small GTPases were inhibited using transfection of dominant-negative plasmids, prenylation inhibitors and pretreatment with atorvastatin. Leptin stimulated Akt activity and cell proliferation and inhibited camptothecin-induced apoptosis in an Akt-sensitive manner. Leptin induced phosphorylation of Bad and FOXO1 in an Akt-sensitive manner. Leptin activated Ras, Rac, RhoA and cdc42. Transfection of dominant-negative plasmids confirmed that leptin-induced Akt activation required Ras, RhoA cdc42 but not Rac. Atorvastatin inhibited leptin-induced activation of Ras, RhoA, cdc42 and Akt. Co-treatment with mevalonate prevented these effects of atorvastatin. The protein kinase Akt is essential to the growth-promoting and anti-apoptotic effects of leptin in oesophageal adenocarcinoma cells. Akt is activated via Ras-, Rho- and cdc42-dependant pathways. Atorvastatin reduces leptin-induced Akt activation by inhibiting prenylation of small GTPases. This may explain the reduced incidence of oesophageal adenocarcinoma in statin-users.

Perspectives Regarding the Intersections between STAT3 and Oxidative Metabolism in Cancer

Signal transducer and activator of transcription 3 (STAT3) functions as a major molecular switch that plays an important role in the communication between cytokines and kinases. In this role, it regulates the transcription of genes involved in various biochemical processes, such as proliferation, migration, and metabolism of cancer cells. STAT3 undergoes diverse post-translational modifications, such as the oxidation of cysteine by oxidative stress, the acetylation of lysine, or the phosphorylation of serine/threonine. In particular, the redox modulation of critical cysteine residues present in the DNA-binding domain of STAT3 inhibits its DNA-binding activity, resulting in the inactivation of STAT3-mediated gene expression. Accumulating evidence supports that STAT3 is a key protein that acts as a mediator of metabolism and mitochondrial activity. In this review, we focus on the post-translational modifications of STAT3 by oxidative stress and how the modification of STAT3 regulates cell metabolism, particularly in the metabolic pathways in cancer cells.

Helicobacter pylori infection induces STAT3 phosphorylation on Ser727 and autophagy in human gastric epithelial cells and mouse stomach

Helicobacter pylori (H. pylori) infection is considered as one of the principal risk factors of gastric cancer. Constitutive activation of the signal transducer and activator of transcription 3 (STAT3) plays an important role in inflammation-associated gastric carcinogenesis. In the canonical STAT3 pathway, phosphorylation of STAT3 on Tyr705 is a major event of STAT3 activation. However, recent studies have demonstrated that STAT3 phosphorylated on Ser727 has an independent function in mitochondria. In the present study, we found that human gastric epithelial AGS cells infected with H. pylori resulted in localization of STAT3 phosphorylated on Ser727 (P-STAT3^Ser727), predominantly in the mitochondria. Notably, H. pylori-infected AGS cells exhibited the loss of mitochondrial integrity and increased expression of the microtubule-associated protein light chain 3 (LC3), the autophagosomal membrane-associated protein. Treatment of AGS cells with a mitophagy inducer, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), resulted in accumulation of P-STAT3^Ser727 in mitochondria. In addition, the elevated expression and mitochondrial localization of LC3 induced by H. pylori infection were attenuated in AGS cells harboring STAT3 mutation defective in Ser727 phosphorylation (S727A). We also observed that both P-STAT3^Ser727 expression and LC3 accumulation were increased in the mitochondria of H. pylori-inoculated mouse stomach.

Role of Mitochondrial Markers in Improved Detection and Risk-Stratification in Barrett's Esophagus Patients

Barrett's esophagus (BE) is the only known precursor of esophageal adenocarcinoma (EAC) and is amenable to treatment. However, more than 90 percent of EAC patients are never diagnosed with antecedent BE. Identification of molecular markers for BE is needed to improve detection of BE through efficient non-endoscopic methods that are cost-effective, sensitive and can be used to cater to a larger group of the population at risk. Alterations in mitochondria and mitochondrial DNA have been shown to be associated with various cancers, including esophageal cancer. Mitochondrial response to oxidative stress, alterations in mitochondrial metabolism, changes in mitochondrial membrane potential and mitochondrial genetic mutations have been found to be associated with BE pathogenesis. This mini-review focuses on the role of mitochondria in the pathogenesis of BE and EAC and the prospects of using that knowledge to develop effective strategies for the improved detection and risk-stratification in BE patients.

Targeting STAT3 and oxidative phosphorylation in oncogene-addicted tumors

Drug resistance invariably limits the response of oncogene-addicted cancer cells to targeted therapy. The upregulation of signal transducer and activator of transcription 3 (STAT3) has been implicated as a mechanism of drug resistance in a range of oncogene-addicted cancers. However, the development of inhibitors against STAT3 has been fraught with challenges such as poor delivery or lack of specificity. Clinical experience with small molecule STAT3 inhibitors has seen efficacy signals, but this success has been tempered by drug limiting toxicities from off-target adverse events.

It has emerged in recent years that, contrary to the Warburg theory, certain tumor types undergo metabolic reprogramming towards oxidative phosphorylation (OXPHOS) to satisfy their energy production. In particular, certain drug-resistant oncogene-addicted tumors have been found to rely on OXPHOS as a mechanism of survival. Multiple cellular signaling pathways converge on STAT3, hence the localization of STAT3 to the mitochondria may provide the link between oncogene-induced signaling pathways and cancer cell metabolism.

In this article, we review the role of STAT3 and OXPHOS as targets of novel therapeutic strategies aimed at restoring drug sensitivity in treatment-resistant oncogene-addicted tumor types. Apart from drugs which have been re-purposed as OXPHOS inhibitors for-anti-cancer therapy (e.g., metformin and phenformin), several novel compounds in the drug-development pipeline have demonstrated promising pre-clinical and clinical activity. However, the clinical development of OXPHOS inhibitors remains in its infancy. The further identification of compounds with acceptable toxicity profiles, alongside the discovery of robust companion biomarkers of OXPHOS inhibition, would represent tangible early steps in transforming the therapeutic landscape of cancer cell metabolism.

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Metabolic reprogramming of cancer cells is one of the hallmarks of cancer.

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STAT3 and OXPHOS upregulation are resistance mechanisms in oncogene-addicted tumors.

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mSTAT3 has a role in the direct, non-transcriptional regulation of OXPHOS.

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Combining OXPHOS inhibitors with TKIs reverses resistance to targeted therapy.

Mitochondria-Targeted Honokiol Confers a Striking Inhibitory Effect on Lung Cancer via Inhibiting Complex I Activity

We synthesized a mitochondria-targeted honokiol (Mito-HNK) that facilitates its mitochondrial accumulation; this dramatically increases its potency and efficacy against highly metastatic lung cancer lines in vitro, and in orthotopic lung tumor xenografts and brain metastases in vivo. Mito-HNK is >100-fold more potent than HNK in inhibiting cell proliferation, inhibiting mitochondrial complex ?, stimulating reactive oxygen species generation, oxidizing mitochondrial peroxiredoxin-3, and suppressing the phosphorylation of mitoSTAT3. Within lung cancer brain metastases in mice, Mito-HNK induced the mediators of cell death and decreased the pathways that support invasion and proliferation. In contrast, in the non-malignant stroma, Mito-HNK suppressed pathways that support metastatic lesions, including those involved in inflammation and angiogenesis. Mito-HNK showed no toxicity and targets the metabolic vulnerabilities of primary and metastatic lung cancers. Its pronounced anti-invasive and anti-metastatic effects in the brain are particularly intriguing given the paucity of treatment options for such patients either alone or in combination with standard chemotherapeutics.

RAS-mediated oncogenic signaling pathways in human malignancies

Abnormally activated RAS proteins are the main oncogenic driver that governs the functioning of major signaling pathways involved in the initiation and development of human malignancies. Mutations in RAS genes and or its regulators, most frequent in human cancers, are the main force for incessant RAS activation and associated pathological conditions including cancer. In general, RAS is the main upstream regulator of the highly conserved signaling mechanisms associated with a plethora of important cellular activities vital for normal homeostasis. Mutated or the oncogenic RAS aberrantly activates a web of interconnected signaling pathways including RAF-MEK (mitogen-activated protein kinase kinase)-ERK (extracellular signal-regulated kinase), phosphoinositide-3 kinase (PI3K)/AKT (protein kinase B), protein kinase C (PKC) and ral guanine nucleotide dissociation stimulator (RALGDS), etc., leading to uncontrolled transcriptional expression and reprogramming in the functioning of a range of nuclear and cytosolic effectors critically associated with the hallmarks of carcinogenesis. This review highlights the recent literature on how oncogenic RAS negatively use its signaling web in deregulating the expression and functioning of various effector molecules in the pathogenesis of human malignancies.

Role of STAT3 in Genesis and Progression of Human Malignant Gliomas

Signal transducer and activator of transcription 3 (STAT3) is aberrantly activated in glioblastoma and has been identified as a relevant therapeutic target in this disease and many other human cancers. After two decades of intensive research, there is not yet any approved STAT3-based glioma therapy. In addition to the canonical activation by tyrosine 705 phosphorylation, concordant reports described a potential therapeutic relevance of other post-translational modifications including mainly serine 727 phosphorylation. Such reports reinforce the need to refine the strategy of targeting STAT3 in each concerned disease. This review focuses on the role of serine 727 and tyrosine 705 phosphorylation of STAT3 in glioma. It explores their contribution to glial cell transformation and to the mechanisms that make glioma escape to both immune control and standard treatment.

Mitochondrial STAT3 is negatively regulated by SOCS3 and upregulated after spinal cord injury

Suppressor of cytokine signaling-3 (SOCS3) expression is induced by the Janus kinase (JAK)-signal transducer and activator of transcription 3 (STAT3) signaling pathway. SOCS3 then acts as a feedback inhibitor of JAK-STAT signaling. Previous studies have shown that knocking down SOCS3 in spinal cord neurons with Lentiviral delivery of SOCS3-targeting shRNA (shSOCS3) increased spinal cord injury (SCI)-induced tyrosine phosphorylation of STAT3 (P-STAT3 Tyr), which in part contributed to decreased neuronal death and demyelination as well as enhanced dendritic regeneration and protection of neuronal morphology after SCI. However, the role of serine phosphorylation of STAT3 (P-STAT3 Ser) is in large part undetermined. Our purposes of this study were to evaluate the expression patterns of P-STAT3 Ser and to explore the possible role of SOCS3 in the regulation of P-STAT3 Ser expression. Immunoblot analyses demonstrated that Oncostatin M (OSM), a member of the interleukin-6 (IL-6) cytokine family, induced both P-STAT3 Tyr and P-STAT3 Ser in SH-SY5Y cells. Subcellular fractionation further revealed that P-STAT3 Ser was localized in mitochondria. Overexpression of SOCS3 with a Lentivirus-mediated approach in SH-SY5Y cells inhibited OSM-induced P-STAT3 Ser in both cytosol and mitochondria fractions. In contrast, OSM-induced P-STAT3 Ser was further upregulated in both cytosol and mitochondria when SOCS3 was knocked down by Lentivirus-delivered shSOCS3. Using a rat T8 spinal cord complete transection model, we found that SCI induced upregulation of P-STAT3 Ser in the mitochondria of macrophages/microglia and neurons both rostral and caudal to the injury site of spinal cord. Collectively, these results suggest that SOCS3 regulation of STAT3 signaling plays critical roles in stress conditions.

NOD-Like Receptor Protein 3 Inflammasome Priming and Activation in Barrett's Epithelial Cells

BACKGROUND & AIMS

Microbial molecular products incite intestinal inflammation by activating Toll-like receptors (TLRs) and inflammasomes of the innate immune system. This system's contribution to esophageal inflammation is not known. Gram-negative bacteria, which dominate the esophageal microbiome in reflux esophagitis, produce lipopolysaccharide (LPS), a TLR4 ligand. TLR4 signaling produces pro-interleukin (IL)1β, pro-IL18, and NOD-like receptor protein 3 (NLRP3), which prime the NLRP3 inflammasome. Subsequent NLRP3 inflammasome activation cleaves caspase-1, inducing secretion of proinflammatory cytokines and pyroptosis (inflammatory cell death). We explored LPS effects on NLRP3 inflammasome priming and activation in esophageal cells.

METHODS

We exposed esophageal squamous and Barrett's epithelial cells to LPS and measured the following: (1) TLR4, pro-IL1β, pro-IL18, and NLRP3 expression; (2) caspase-1 activity; (3) tumor necrosis factor-α, IL8, IL1β, and IL18 secretion; (4) lactate dehydrogenase (LDH) release (a pyroptosis marker); and (5) mitochondrial reactive oxygen species (ROS). As inhibitors, we used acetyl-Tyr-Val-Ala-Asp-CHO for caspase-1, small interfering RNA for NLRP3, and (2-(2,2,6,6,-Tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride for mitochondrial ROS.

RESULTS

Squamous and Barrett's cells expressed similar levels of TLR4, but LPS induced TLR4 signaling that increased tumor necrosis factor-α and IL8 secretion only in Barrett's cells. Barrett's cells treated with LPS showed increased expression of pro-IL18, pro-IL1β, and NLRP3, and increased mitochondrial ROS levels, caspase-1 activity, IL1β and IL18 secretion, and LDH release. Acetyl-Tyr-Val-Ala-Asp-CHO, NLRP3 small interfering RNA, and Mito-TEMPO all blocked LPS-induced IL1β and IL18 secretion and LDH release.

CONCLUSIONS

In Barrett's cells, LPS both primes and activates the NLRP3 inflammasome, causing secretion of proinflammatory cytokines and pyroptosis. By triggering molecular events promoting inflammation, the esophageal microbiome might contribute to inflammation-mediated carcinogenesis in Barrett's esophagus.

Mitochondrial Stat3, the Need for Design Thinking

Stat3 has been studied extensively as a transcription factor, however the finding that Stat3 also localizes to mitochondria has opened a new area to discover non-classical functions. Here we review the current knowledge of mitochondrial Stat3 as a regulator of the electron transport chain (ETC) and its impact on mitochondrial production of ATP and ROS. We also describe recent findings identifying Stat3 as a regulator of mitochondrial Ca(2+) homeostasis through its effect on the ETC. It is becoming evident that these non-classical functions of Stat3 can have a major impact on cancer progression, cardiovascular diseases, and inflammatory diseases. Therefore, mitochondrial Stat3 functions challenge the current design of therapies that solely target Stat3 as a transcription factor and suggest the need for "design thinking," which leads to the development of novel strategies, to intervene the Stat3 pathway.

Pivotal Importance of STAT3 in Protecting the Heart from Acute and Chronic Stress: New Advancement and Unresolved Issues

The transcription factor, signal transducer and activator of transcription 3 (STAT3), has been implicated in protecting the heart from acute ischemic injury under both basal conditions and as a crucial component of pre- and post-conditioning protocols. A number of anti-oxidant and antiapoptotic genes are upregulated by STAT3 via canonical means involving phosphorylation on Y705 and S727, although other incompletely defined posttranslational modifications are involved. In addition, STAT3 is now known to be present in cardiac mitochondria and to exert actions that regulate the electron transport chain, reactive oxygen species production, and mitochondrial permeability transition pore opening. These non-canonical actions of STAT3 are enhanced by S727 phosphorylation. The molecular basis for the mitochondrial actions of STAT3 is poorly understood, but STAT3 is known to interact with a critical subunit of complex I and to regulate complex I function. Dysfunctional complex I has been implicated in ischemic injury, heart failure, and the aging process. Evidence also indicates that STAT3 is protective to the heart under chronic stress conditions, including hypertension, pregnancy, and advanced age. Paradoxically, the accumulation of unphosphorylated STAT3 (U-STAT3) in the nucleus has been suggested to drive pathological cardiac hypertrophy and inflammation via non-canonical gene expression, perhaps involving a distinct acetylation profile. U-STAT3 may also regulate chromatin stability. Our understanding of how the non-canonical genomic and mitochondrial actions of STAT3 in the heart are regulated and coordinated with the canonical actions of STAT3 is rudimentary. Here, we present an overview of what is currently known about the pleotropic actions of STAT3 in the heart in order to highlight controversies and unresolved issues.