A high-throughput fluorescence polarization assay for discovering inhibitors targeting the DNA-binding domain of signal transducer and activator of transcription 3 (STAT3)

Biophysical Assays for Investigating Modulators of Macromolecular Complexes: An Overview

Drug discovery is a lengthy and intricate process, and in its early stage, crucial steps are the selection of the therapeutic target and the identification of novel ligands. Most targets are dysregulated in pathogenic cells; typically, their activation or deactivation leads to the desired effect, while in other cases, interfering with the target-natural binder complex achieves the therapeutic results. Biophysical assays are a suitable strategy for finding new ligands or interferent agents, being able to evaluate ligand-protein interactions and assessing the effect of small molecules (SMols) on macromolecular complexes. This mini-review provides a detailed analysis of widely used biophysical methods, including fluorescence-based approaches, circular dichroism, isothermal titration calorimetry, microscale thermophoresis, and NMR spectroscopy. After a brief description of the methodologies, examples of interaction and competition experiments are described, together with an analysis of the advantages and disadvantages of each technique. This mini-review provides an overview of the most relevant biophysical technologies that can help in identifying SMols able not only to bind proteins but also to interfere with macromolecular complexes.

The role of the STAT3 signaling transduction pathways in radioresistance

The efficacy of radiotherapy has long known to be limited by the emergence of resistance. The four Rs of radiotherapy (DNA damage repair, reoxygenation, redistribution of the cell cycle, and repopulation) are generally accepted concepts in radiobioolgy. Recent studies have strongly linked signal transducer and activator of transcription 3 (STAT3) to the regulation of cancer stemness and radioresistance. In particular, a STAT3 pathway inhibitor napabucasin, claimed to be the first cancer stemness antagonist in clinical trials, strengthens the link. However, no reviews connect STAT3 with the four Rs of radiotherapy. Herein, the evidence-based role of STAT3 in radioresistance is discussed in relation to the four Rs of radiotherapy. The proposed mechanisms include upstream and downstream effector proteins of STAT3, including FOXM1, MELK, NEK2, AKT, EZH2, and HIF1α. Downstream transcriptional products of the mechanistically-related proteins are involved in cancer stemness, anti-apoptosis, and the four Rs of radiotherapy. Utilizing selective inhibitors of the mechanistically-related proteins has shown promising antagonism of radioresistance, suggesting that the expression levels of these proteins may be biomarkers for the prediction of radiotherapeutic outcomes, and that this molecular mechanism may provide a rational axis through which to treat radioresistance.

Role of STAT3 signaling transduction pathways in cancer stem cell-associated chemoresistance

Chemoresistance resulting from cancer stem cells (CSCs) and epithelial-mesenchymal transition (EMT) results in inconsistent chemotherapeutic efficacy. The co-existence of CSCs and the EMT allows cancer cells to interconvert between differentiated and stem-like states, a phenomenon known as cellular plasticity. Phosphorylated signal transducer and activator of transcription 3 (pSTAT3) has been increasingly identified as a major contributor to CSCs and the EMT, as evidenced from preclinical studies that reversed chemoresistance through STAT3 pathway inhibition. In this review, we discuss mechanisms that center on STAT3 and its target genes responsible for regulating the EMT. We also highlight the current status of clinical trials using STAT3 pathway inhibitors.

Optimization of a high-throughput fluorescence polarization assay for STAT5B DNA binding domain-targeting inhibitors

Signal transducer and activator of transcription 5B (STAT5B) is constitutively activated in multiple cancers as a result of hyperactivating mutations or dysregulation of upstream effectors. Therapeutic strategies have predominantly targeted the Src homology 2 (SH2) domain to inhibit STAT phosphorylation, a prerequisite for STAT5B transcriptional activation. An alternative approach for STAT5B pharmacologic inhibition involves targeting the DNA-binding domain (DBD). However, this strategy remains relatively unexplored and is further hindered by the lack of a high-throughput in vitro engagement assay. Herein, we present the development and optimization of a STAT5B DBD fluorescence polarization (FP) assay, which facilitates rapid screening of small molecules targeting the STAT5B DBD though displacement of a fluorescently labelled oligonucleotide. The assay can generate a complete DNA-binding profile in 10 min, with signal stability up to 2 h, and minimal changes under a range of conditions including 10 % (v/v) glycerol, 15 % (v/v) DMSO, 1 mM NaCl, 0.02 % (w/v) BSA, and 1 mM EDTA. This assay is compatible with both unphosphorylated and phosphorylated STAT5B and demonstrates suitability for high-throughput screening with a Z' factor of 0.68 ± 0.07 and a signal to noise ratio of 6.7 ± 0.84.

Revisiting signal transducer and activator of transcription 3 (STAT3) as an anticancer target and its inhibitor discovery: Where are we and where should we go?

As a transcription factor, STAT3 protein transduces extracellular signals to the nucleus and then activates transcription of target genes. STAT3 has been well validated as an attractive anticancer target due to its important roles in cancer initiation and progression. Identification of specific and potent STAT3 inhibitors has attracted much attention, while there has been no STAT3 targeted drug approved for clinical application. In this review, we will briefly introduce STAT3 protein and review its role in multiple aspects of cancer, and systematically summarize the recent advances in discovery of STAT3 inhibitors, especially the ones discovered in the past five years. In the last part of the review, we will discuss the possible new strategies to overcome the difficulties of developing potent and specific STAT3 inhibitors and hope to shed light on future drug design and inhibitor optimization.

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.

Modulation of nuclear receptor function: Targeting the protein-DNA interface

Nuclear receptors (NRs) are a superfamily of ligand-dependent transcription factors that modulate several biological processes. Traditionally, modulation of NRs has been focused on the development of ligands that recognize and bind to the ligand binding domain (LBD), resulting in activation or repression of transcription through the recruitment of coregulators. However, for more severe diseases, such as breast and prostate cancer, the conventional treatment addressing LBD modulation is not always successful, due to tumor resistance. To overcome these challenges and aiming to modulate NR activity by inhibiting the NR-DNA interaction, new studies focus on the development of molecules targeting alternative sites and domains on NRs. Here, we discuss two different approaches for this alternative NR modulation: one targeting the NR DNA binding domain (DBD); and the other targeting the DNA sites recognized by NRs. Our aim is to present the challenges and perspectives for developing specific inhibitors for each purpose, alongside with already reported examples.