If You Cannot Win Them Join Them: Understanding NewWays to Target STAT3 by Small Molecules

摘要

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 inhibitingSTAT3 mutants at the atomistic level of detail. In the present study,we focused on the effects exerted by reported STAT3 mutations on theprotein structure, dynamics, DNA-binding, and dimerization, thus linkingstructure, dynamics, energetics, and the biological function. By employingatomistic molecular dynamics and umbrella-sampling simulations toa series of human STAT3 dimers, which comprised wild-type proteinand four mutations, we explained the modulation of STAT3 activityby these mutations. Counter-intuitively, our results show that theD570K inhibitory mutation exerts its effect by enhancing rather thanweakening STAT3–DNA interactions, which interfere with theDNA release by the protein dimer and thus inhibit STAT3 function asa transcription factor. We mapped the binding site and characterizedthe binding mode of a clinical candidate napabucasin/BBI-608 at STAT3,which resembles the effect of a D570K mutation. Our results contributeto understanding the activation/inhibition mechanism of STAT3, toexplain the molecular mechanism of STAT3 inhibition by BBI-608. Alongsidethe characterization of the BBI-608 binding mode, we also discovereda novel binding site amenable to bind small-molecule ligands, whichmay pave the way to design novel STAT3 inhibitors and to suggest newstrategies for pharmacological interventions to combat cancers associatedwith poor prognosis.