Structural Biology of STAT3 and Its Implications for Anticancer Therapies Development

Abstract

Transcription factors are proteins able to bind DNA and induce the transcription of specific genes. Consequently, they play a pivotal role in multiple cellular pathways and are frequently over-expressed or dysregulated in cancer. Here, we will focus on a specific “signal transducer and activator of transcription” (STAT3) factor that is involved in several pathologies, including cancer. For long time, the mechanism by which STAT3 exerts its cellular functions has been summarized by a three steps process: (1) Protein phosphorylation by specific kinases, (2) dimerization promoted by phosphorylation, (3) activation of gene expression by the phosphorylated dimer. Consequently, most of the inhibitors reported in literature aimed at blocking phosphorylation and dimerization. However, recent observations reopened the debate and the entire functional mechanism has been revisited stimulating the scientific community to pursue new inhibition strategies. In particular, the dimerization of the unphosphorylated species has been experimentally demonstrated and specific roles proposed also for these dimers. Despite difficulties in the expression and purification of the full length STAT3, structural biology investigations allowed the determination of atomistic structures of STAT3 dimers and several protein domains. Starting from this information, computational methods have been used both to improve the understanding of the STAT3 functional mechanism and to design new inhibitors to be used as anticancer drugs. In this review, we will focus on the contribution of structural biology to understand the roles of STAT3, to design new inhibitors and to suggest new strategies of pharmacological intervention.

Can asymmetric post‐translational modifications regulate the behavior of STAT3 homodimers?

“…For example, it is still relatively easy to find articles and reviews where STAT3 is described to homodimerize only upon phosphorylation of both molecules at Y705, 27 and ourselves worked under this same assumption until very recently. For example, it is still relatively easy to find articles and reviews where STAT3 is described to homodimerize only upon phosphorylation of both molecules at Y705, 27 and ourselves worked under this same assumption until very recently.…”

“…Like us, and to the best of our knowledge, the existing scientific literature on STAT3 implicitly assumes that STAT3 homodimers are formed by two identical molecules in all aspects, including PTMs. For example, it is still relatively easy to find articles and reviews where STAT3 is described to homodimerize only upon phosphorylation of both molecules at Y705, 27 and ourselves worked under this same assumption until very recently. 28 Here, we made use of the unique properties of our BiFC system to determine the relative contribution of each residue to the dimerization and intracellular distribution of unstimulated STAT3 dimers, in an experimental paradigm similar to the one we used previously for mutant huntingtin.…”

Anticancer activity of dietary xanthone α-mangostin against hepatocellular carcinoma by inhibition of STAT3 signaling via stabilization of SHP1

“…Phosphorylation at Tyr705 in turn drives the dimerization of STAT3 through interaction with its SH2 domain 21 . We therefore transiently co-transfected green fluorescent protein (GFP)-tagged STAT3 and FLAGtagged STAT3 plasmids into HepG2 cells, and determined the interactions of GFP-STAT3 and FLAG-STAT3 by an immunoprecipitation (IP) assay.…”

“…4a, α-MGT at dose of 20 μM markedly decreased IL-6induced interactions of GFP-STAT3 and FLAG-STAT3 monomers. Dimerizated STAT3 is then translocated to the nucleus, where it binds the specific DNA-response elements in the promoters of target genes to mediate transcription 21 . The results of immunofluorescence assay indicated that α-MGT effectively blocked IL-6-induced nuclear translocation of STAT3 in HepG2 cells (Fig.…”

Targeting STAT-3 signaling pathway in cancer for development of novel drugs: Advancements and challenges

“…N-terminal domain is pivotal for the interaction of dimers of STAT-3 for the formation of tetramer, interaction with other regulators and the attachment of STAT-3 dimers to the sites of DNA, and protein interaction (Figure 3) (Sgrignani et al, 2018). The N-terminal domain has a leading role in dimer formation, so it is a captivating approach to inhibit the function of STAT-3.…”