Identification of Entinostat as a Novel Modifier of STAT3 Pre-mRNA Alternative Splicing

Signal transducer and activator of transcription 3 (STAT3) is a pleiotropic factor involved in multiple vital biological processes and a key mediator of gene transcription in response to cytokines, growth factors and aberrant activation of oncogenic signaling. STAT3 has two splicing isoforms, STAT3α and STAT3β, derived from alternative splicing of exon 23 within pre-mRNA. STAT3β differs from STAT3α by replacement of 55 amino-acid residues in the C-terminal transactivation domain with 7 specific amino acids. Thus, a shorter STAT3β was originally regarded as a dominant negative isoform of STAT3α. Recently accumulating evidence from independent studies have shown STAT3 splicing isoforms confer distinct and overlapping functions in many fundamental cellular regulatory steps such as cell differentiation, inflammatory responses, and cancer progression. However, relatively little is known about the mechanisms of STAT3 pre-mRNA splicing, and it remains undiscovered which chemical compounds or bioactive substances can induce the STAT3β expression. In this study, we generated a potent reporter for detection of alternative splicing of STAT3 pre-mRNA optimized for the screening of function-known chemical library, and successfully identified entinostat, a histone deacetylase inhibitor, as a novel inducer of STAT3β through modulating mRNA splicing. Our findings demonstrate that alternative splicing of STAT3 can be regulated by a compound, providing an important clue for understanding the regulation mechanisms of the expression balance of STAT3 isoforms in a chemical biology approach. Entinostat is likely to be a promising seed compound for elucidating how the higher ratio of STAT3β expression impacts on biological responses associated with Janus kinase (JAK)/STAT3 signaling pathway.