If You Cannot Win Them, Join Them: Understanding New Ways to Target STAT3 by Small Molecules

Abstract

Signal transducer activator of transcription 3 (STAT3) is among the most investigated oncogenic transcription factors, as it is highly associated with cancer initiation, progression, metastasis, chemoresistance, and immune evasion. Evidences from both preclinical and clinical studies have demonstrated that STAT3 plays a critical role in several malignancies associated with poor prognosis such as glioblastoma and triple-negative breast cancer, and STAT3 inhibitors have shown efficacy in inhibiting cancer growth and metastasis. Constitutive activation of STAT3 by mutations occurs frequently in tumor cells and directly contributes to many malignant phenotypes. Unfortunately, detailed structural biology studies on STAT3 as well as target-based drug discovery efforts have been hampered by difficulties in the expression and purification of the full-length STAT3 and a lack of ligand-bound crystal structures. Considering these, molecular modeling and simulations offer an attractive strategy for the assessment of the “druggability” of STAT3 dimers and allow investigations of reported activating and inhibiting STAT3 mutants at the atomistic level of detail. In the present study, we focused on the effects exerted by reported STAT3 mutations on the protein structure, dynamics, DNA-binding, and dimerization, thus linking structure, dynamics, energetics, and the biological function. By employing atomistic molecular dynamics and umbrella-sampling simulations to a series of human STAT3 dimers, which comprised wild-type protein and four mutations, we explained the modulation of STAT3 activity by these mutations. Counter-intuitively, our results show that the D570K inhibitory mutation exerts its effect by enhancing rather than weakening STAT3–DNA interactions, which interfere with the DNA release by the protein dimer and thus inhibit STAT3 function as a transcription factor. We mapped the binding site and characterized the binding mode of a clinical candidate napabucasin/BBI-608 at STAT3, which resembles the effect of a D570K mutation. Our results contribute to understanding the activation/inhibition mechanism of STAT3, to explain the molecular mechanism of STAT3 inhibition by BBI-608. Alongside the characterization of the BBI-608 binding mode, we also discovered a novel binding site amenable to bind small-molecule ligands, which may pave the way to design novel STAT3 inhibitors and to suggest new strategies for pharmacological interventions to combat cancers associated with poor prognosis.

Hippo pathway monomerizes STAT3 to regulate prostate cancer growth

“…STAT3 dimerization is carried out by its SH2 domain, as SH2 domain from one STAT3 monomer associates with the phosphorylated Y705 located in the transactivation domain of another STAT3 monomer. 29 We demonstrate that MST2‐mediated phosphorylation at STAT3 T622 dampened the STAT3 dimerization, thereby inhibiting STAT3 activation. Further, prostate cancer cells harboring STAT3 T622A grew much faster than the WT cells, and patients with a high level of STAT3 T622 phosphorylation showed a longer average survival time.…”

“…STAT3 strongly binds to DNA when it is in the dimeric form. STAT3 dimerization is carried out by its SH2 domain, as SH2 domain from one STAT3 monomer associates with the phosphorylated Y705 located in the transactivation domain of another STAT3 monomer 29 . We demonstrate that MST2‐mediated phosphorylation at STAT3 T622 dampened the STAT3 dimerization, thereby inhibiting STAT3 activation.…”

Looking for the Holy Grail—Drug Candidates for Glioblastoma Multiforme Chemotherapy

“…BBI608 (Napabucasin) is a small molecule that can bind to the STAT3 protein, limiting its cellular activity. Zaraquiey et al [64] mapped the binding site and characterized the binding mode of NNI608 at STAT3, which resembles the effect of a D570K mutation in a linker domain known to interrupt STAT3 activity [65]. Han et al [66] reported that BBI608 treatment significantly blocked GBM cells' (U87 and LN229 cell lines) proliferation, migration, invasion, and sphere formation.…”