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

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.