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

Abstract

Summary 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.

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

“…Importantly, the requirement for mitochondrial STAT3 in these processes could be circumvented by modulation of NAD+ levels through supplementation with NAM or restoration of dehydrogenase activity. These results could have therapeutic implications, as it has been shown that targeting NAD+ metabolism can sensitize tumor cells to drug and radiation therapy [85,86], adding another potential rationale for targeting mitochondrial STAT3 during cancer therapy [40].…”

“…To check this, we augmented NADH dehydrogenase activity in STAT3 KO cells by ectopically expressing yeast NDI1. Yeast NDI1 is a rotenone-resistant and STAT3-independent NADH dehydrogenase that is capable of bypassing defects in complex I function when expressed in mammalian cells [40,55,56]. Interestingly, NDI1 expression restored antioxidant gene expression in STAT3 KO cells to WT levels (Fig.…”

“…Cell viability was measured in triplicate by crystal violet staining, as previously described [40]. In brief, cells seeded in 96-well plates (Corning Incorporated, Corning, NY, USA) at a density of 10 4 cells/well were allowed to attach overnight (16 h at 37°C), followed by addition of 0.375 mM hydrogen peroxide with or without supplementation with N-Acetyl cysteine (NAC) (25 mM), Trolox (250 µM), or NAM (10 mM), and then cultured for an additional 8 h at 37°C, followed by staining with 50 µL per well of crystal violet.…”

“…To determine whether STAT3 is critical for mitochondrial functions in triple-negative breast cancer (TNBC), we created STAT3 KO cell lines by CRISPR/Cas9 gene targeting in the TNBC cell line MDA-MB-231 [40] and compared them to wild-type (WT) counterparts. We also generated STAT3 KO cell lines reconstituted with either WT STAT3 or with artificially mitochondrially restricted forms of STAT3 (MTS), including a version that encoded the phosphorylation-deficient S727A mutation, previously shown to impair mitochondrial STAT3 functions [31].…”

Nujiangexanthone A Inhibits Cervical Cancer Cell Proliferation by Promoting Mitophagy

“…Therapies that target tumor cell mitochondria can effectively release the coupling between tumor cells and the “parasitic host,” cutting off the energy source of tumor cells. Mitophagy is correlated with various cancer cell characteristics, including the promotion of ROS production, disrupting the redox balance [ 22 ], accelerating the mitochondrial unfolded protein response (UPRmt), increasing the production of unfolded proteins and toxic metabolites [ 23 ], increasing mitochondrial membrane permeability, promoting cytochrome C release into the cytoplasm to induce cell apoptosis [ 24 ], adjusting intracellular energy metabolism to cause drug resistance [ 25 ], and inducing mitochondrial DNA damage [ 26 ]. Since mitophagy is generally induced in cancer cells, therapeutic strategies targeting mitophagy possess better selectivity and efficiency, which can be a direction for anticancer drug screening.…”