Mitochondrial dysfunction induced by a SH2 domain-targeting STAT3 inhibitor leads to metabolic synthetic lethality in cancer cells

Abstract

In addition to its canonical role in nuclear transcription, signal transducer and activator of transcription 3 (STAT3) is emerging as an important regulator of mitochondrial function. Here, we demonstrate that a novel inhibitor that binds with high affinity to the STAT3 SH2 domain triggers a complex cascade of events initiated by interference with mitochondrial STAT3 (mSTAT3). The mSTAT3-drug interaction leads to mitochondrial dysfunction, accumulation of proteotoxic STAT3 aggregates, and cell death. The cytotoxic effects depend directly on the drug's ability to interfere with mSTAT3 and mitochondrial function, as demonstrated by site-directed mutagenesis and use of STAT3 knockout and mitochondria-depleted cells. Importantly, the lethal consequences of mSTAT3 inhibition are enhanced by glucose starvation and by increased reliance of cancer cells and tumor-initiating cells on mitochondria, resulting in potent activity in cell cultures and tumor xenografts in mice. These findings can be exploited for eliciting synthetic lethality in metabolically stressed cancer cells using highaffinity STAT3 inhibitors. Thus, this study provides insights on the role of mSTAT3 in cancer cells and a conceptual framework for developing more effective cancer therapies.S ignal transducer and activator of transcription 3 (STAT3) is a key element in multiple signaling pathways and is aberrantly activated in many human cancers (1, 2). STAT3 promotes cell proliferation, survival, angiogenesis, and immune-evasion (1-3). Phosphorylation at Tyr705 (pTyr705), catalyzed by Janus kinases (JAK) and other tyrosine kinases, induces STAT3 dimerization through the interaction of the SH2 domain (SH2D), nuclear accumulation, and target gene transcription (1, 3, 4). Emerging evidence indicates that STAT3 also localizes to mitochondria and controls mitochondrial functions (2, 5-7). Mitochondrial localized STAT3 (mSTAT3) is critical for survival of RAStransformed mouse embryo fibroblasts (MEF) under glucosestarvation, reflecting a specific dependency of cancer cells on mitochondria in certain conditions (6). Interestingly, mSTAT3 is prevalently phosphorylated at Ser727 (pSer727), which enhances its mitochondrial functions (5, 6). Furthermore, constitutive pSer727 is found in many human cancers and is apparently sufficient to drive tumorigenesis in various model systems (8-10).STAT3 is an attractive cancer therapeutic target because of its central role in multiple oncogenic processes and great effort has been devoted in recent years to discover STAT3 inhibitors (STAT3i) (11,12). To date, small-molecule STAT3i have shown relevant activity in preclinical models and few of them are currently investigated in clinical trials (11,(13)(14)(15)(16)(17). However, an important gap persists in our knowledge of the biological mechanisms of antitumor activity, the critical cellular processes affected, and the factors determining sensitivity of cancer cells to STAT3i, hindering further clinical development of these highly promising anticancer drugs. Indeed, great atten...

Circumventing senescence is associated with stem cell properties and metformin sensitivity

“…LPS can potentially change the SASP by activating STAT3 signaling (Lin et al, ), which can synergize with NF‐κB, thereby changing gene expression patterns and promoting tumorigenesis (Grivennikov & Karin, ). In addition, STAT3 can promote mitochondrial functions required for cancer cell stemness and metabolism (Genini et al, ). IL‐6 activates STAT3 and promotes the development of PDAC from PanIN (Lesina et al, ).…”

Translational and clinical advances in JAK-STAT biology: The present and future of jakinibs

“…STAT activity can also be suppressed by repurposing drugs that are not necessarily pathway specific, including agents such as pimozide, resveratrol, curcumin, and platinum‐based drugs, although it is unclear whether their inhibitory effects are due to direct interaction with STATs or indirect mechanisms. A number of these strategies have progressed to clinical trials including C188‐9 (NCT03195699), AZD9150 (NCT01563302 NCT01839604), a decoy STAT3 (NCT00696176), OPB‐31121 (NCT00955812), OPB‐ 11107 (NCT03063944), and OPB‐51602 (NCT02058017) . Napabucasin (BBI608) is a STAT inhibitor that progressed to a phase 3 study in gastrointestinal cancer but did not appear to be efficacious for this disorder.…”

“…A number of these strategies have progressed to clinical trials including C188-9 (NCT03195699), AZD9150 (NCT01563302 NCT01839604), a decoy STAT3 (NCT00696176), OPB-31121 (NCT00955812), OPB-11107 (NCT03063944), and OPB-51602 (NCT02058017). [188][189][190][191][192][193][194] Napabucasin (BBI608) is a STAT inhibitor that progressed to a phase 3 study in gastrointestinal cancer but did not appear to be efficacious for this disorder. Moreover, it is important to bear in mind that there are multiple endogenous inhibitors including the suppressor of cytokine signaling family of proteins (SOCS), which in principle could be capitalized upon therapeutically.…”

A First-in-Human Phase I Study of OPB-111077, a Small-Molecule STAT3 and Oxidative Phosphorylation Inhibitor, in Patients with Advanced Cancers

“…Previous studies have shown that STAT3 promotes tumorigenesis by at least two mechanisms: first, by acting as a nuclear transcription factor that alters expression of pro‐tumorigenic gene‐regulatory networks ; and second, by acting as a regulator of oxidative phosphorylation (OXPHOS) via interaction with complexes I and II of the mitochondrial electron transport chain . Given this broad involvement in tumorigenesis, significant effort has been devoted to the discovery of small‐molecule STAT3 inhibitors .…”

“…Research continues to identify drivers of OPB-111077 clinical activity and synergistic combinations. Name NC/NA 1 2 3 4 5 All Grades n Nausea 40 50 32 5 0 0 87 127 Fatigue 51 30 42 4 0 0 76 127 Vomiting 66 44 13 4 0 0 61 127 Constipation 80 27 17 3 0 0 47 127 Dizziness 87 31 8 1 0 0 40 127 Anorexia 97 16 14 0 0 0 30 127 Diarrhea 102 17 7 1 0 0 25 127 Hypothyroidism 103 11 13 0 0 0 24 127 Dehydration 104 4 19 0 0 0 23 Previous studies have shown that STAT3 promotes tumorigenesis by at least two mechanisms: first, by acting as a nuclear transcription factor that alters expression of pro-tumorigenic gene-regulatory networks [1][2][3][4][5][6][7][8]; and second, by acting as a regulator of oxidative phosphorylation (OXPHOS) via interaction with complexes I and II of the mitochondrial electron transport chain [9][10][11][12]…”