Transcriptional regulation and functional differences driven by STAT3 isoforms, STAT3α and STAT3β

© 2018 Dr. Vincent TanoThe Signal transducer and activator of transcription 3 (STAT3) protein, a member of the STAT family of transcription factors, plays important roles in the regulation of critical biological processes. STAT3 is best known for its role in the JAK-STAT pathway, mediating the transcriptional regulation of target gene expression following its activation by extracellular signal, and continues to be the subject of intense research efforts due to its essential roles in crucial physiological processes, such as embryonic development and inflammation, as well as its involvement in numerous types of cancer. While the genome-wide DNA-binding and transcriptional regulation activities of the full-length STAT3α have been widely studied, those of a naturally occurring shorter STAT3β spliceform are less understood. Despite having identical DNA-binding domains, the STAT3 spliceforms display significant differences in their transcriptional regulation activities. Furthermore, STAT3α and STAT3β can drive opposing oncogenic and tumour suppression outcomes, respectively. Thus, the major focus of this study is to explore STAT3α- and STAT3β-specific genome-wide DNA-binding, transcriptional regulation of non-coding micro-RNAs (miRNAs), and functional outcomes in cancer cell biology. By employing ChIP-seq and miRNA expression profiling assays, this study presents results revealing that STAT3α and STAT3β display clear differences in genome-wide DNA-binding and regulation of miRNA expression. Bioinformatic analyses of transcription factor binding sites also uncovered the possible roles of co-transcription factors in the distinct STAT3 spliceform-specific transcriptional regulation activities. In addition, a Morpholino-directed splicing modulation approach to drive STAT3 knockdown and STAT3α-to-β splicing switch in cancer cells showed that the STAT3 spliceforms can differentially alter the tumourigenic properties of cancer cells, with STAT3β expression being associated with tumour suppression outcomes by driving the suppression of cell proliferation, survival and migration. Taken together, this study highlights the distinct properties of the STAT3 spliceforms in genome-wide DNA-binding which possibly underlie their different gene transcriptional regulation activities in both protein-coding and non-coding genes, and further provides evidence that STAT3β can drive distinct tumour suppression outcomes