A Synthetic Lethal Interaction between Glutathione Synthesis and Mitochondrial Reactive Oxygen Species Provides a Tumor-Specific Vulnerability Dependent on STAT3

Abstract

e Increased production of mitochondrion-derived reactive oxygen species (ROS) is characteristic of a metabolic shift observed during malignant transformation. While the exact sources and roles of ROS in tumorigenesis remain to be defined, it has become clear that maintaining redox balance is critical for cancer cell proliferation and survival and, as such, may represent a vulnerability that can be exploited therapeutically. STAT3, a latent cytosolic transcription factor activated by diverse cytokines and growth factors, has been shown to exhibit an additional, nontranscriptional function in mitochondria, including modulation of electron transport chain activity. In particular, malignant transformation by Ras oncogenes exploits mitochondrial STAT3 functions. We used mass spectrometry-based metabolomics profiling to explore the biochemical basis for the STAT3 dependence of Ras transformation. We identified the gamma-glutamyl cycle, the production of glutathione, and the regulation of ROS as a mitochondrion-STAT3-dependent pathway in Ras-transformed cells. Experimental inhibition of key enzymes in the glutathione cycle resulted in the depletion of glutathione, accumulation of ROS, oxidative DNA damage, and cell death in an oncogenic Rasand mitochondrial STAT3-dependent manner. These data uncover a synthetic lethal interaction involving glutathione production and mitochondrial ROS regulation in Ras-transformed cells that is governed by mitochondrial STAT3 and might be exploited therapeutically. STAT3 is a latent cytosolic transcription factor activated by phosphorylation on tyrosine 705 in response to many growth factors and cytokines. In normal tissues, STAT3 target genes regulate proliferation, survival, angiogenesis, immune responses, inflammation, and self-renewal (1). STAT3 is also implicated in malignancy (2). Constitutively active STAT3 mutants facilitate experimental transformation (3), and STAT3 is aberrantly phosphorylated or overexpressed in many human tumors. Typically, enhanced STAT3 activation is due to the mutation of upstream tyrosine kinases or receptor tyrosine kinases (e.g., JAK2 in myeloproliferative disease, ALK in some lymphomas, or epidermal growth factor [EGFR] in head and neck cancer) or heightened secretion of cytokines (e.g., interleukin-6 [IL-6] in multiple myeloma or IL-11 in gastric cancer), and STAT3 often has been found to be essential for tumors driven by these stimuli (4). Intriguingly, STAT3 is also required for transformation driven by oncogenic mutations in the Ras-GTPase family in the absence of STAT3 Y705 phosphorylation (5-7). In the context of oncogenic Ras mutations, a mitochondrial pool of STAT3 regulates the activity of the electron transport chain, which is necessary for tumor formation (8). These varied involvements of STAT3 in human cancer have made it an attractive therapeutic candidate (9), although this promise has yet to be fulfilled (10).Metabolic reprogramming is considered a hallmark of cancer. As cells adopt a transformed phenotype, they switch the major source...

Mitochondrial STAT3: Powering up a potent factor

“…2). This hypothesis is supported by data showing that pharmacological inhibition of GSH synthesis results in accumulated ROS, DNA damage and death in Ras-transformed, STAT3 expressing cell lines [53].…”

“…Mitochondrial accumulation of STAT3 correlates with altered concentrations of Reactive Oxygen Species (ROS) [34,45,[52][53][54]. A mass-spectrometry based screen for mitochondrial-STAT3 dependent metabolites revealed that mitochondrial-STAT3 alters the glutathione (GSH) synthesis machinery [53].…”

STAT3 Undergoes Acetylation-dependent Mitochondrial Translocation to Regulate Pyruvate Metabolism

“…Constitutively acetylated STAT3 translocates into mitochondria, where it sustains altered TCA-ETC function for glycolytic and oxidative phosphorylation reactions. The STAT3-dependent metabolic regulation of pyruvate conversion in mitochondria supports Ras-dependent malignant transformation59.…”

STAT3 signaling in immunity

“…In 2009, the intriguing discovery was made that STAT3 associates with mitochondria and therein regulates electron transport chain function as well as glycolytic and oxidative phosphorylation [13, 48]. Mitochondrial STAT3 is also implicated in control of the gamma-glutamyl cycle, production of glutathione and regulation of reactive oxygen species (ROS) [49]. Furthermore, in embryonic stem (ES) cells, STAT3 controls mitochondrial gene expression and respiration, mechanisms that optimize ES cell proliferation and maintenance [50].…”