Tumor-related splicing variant RAC1b in colorectal cancer: regulation and role in tumorigenesis

Purpose: Alternative splicing of the small GTPase RAC1 generates RAC1b, a hyperactivated variant that is overexpressed in a subtype of colorectal tumors. The objective of our studies is to understand the molecular regulation of this alternative splicing event and how it contributes to tumorigenesis. Experimental description: The regulation of the RAC1b splicing event in human colon cell lines was dissected using a transfected RAC1 minigene and the role of upstream regulating protein kinases through an RNA interference approach. The functional properties of the RAC1b protein were characterized by experimental modulation of Rac1b levels in colon cell lines. Results: The RAC1b protein results from an in-frame inclusion of an additional alternative exon encoding 19 amino acids that change the regulation and signaling properties of the protein. RAC1b is a hyperactive variant that exists predominantly in the GTP-bound active conformation in vivo and promotes cell cycle progression and cell survival through activation of the transcription factor NF-κB. RAC1b overexpression functionally cooperates with the oncogenic mutation in BRAF-V600E to sustain colorectal tumor cell survival. The splicing factor SRSF1 was identified to bind an exonic splice enhancer element in the alternative exon and acts as a prime regulator of Rac1b alternative splicing in colorectal cells. SRSF1 is controlled by upstream protein kinase SRPK1, the inhibition or depletion of which led to reduced SRSF1 phosphorylation and nuclear translocation with a concomitant reduction in RAC1b levels. As further SRSF1-regulating pathways we discovered kinase GSK3 and a cyclooxygenase independent effect of the non-steroidal anti-inflammatory drug ibuprofen. Conclusions: Expression of tumor-related RAC1b in colorectal cancer depends critically on SRSF1 for the observed deregulation of alternative splicing during tumorigenesis and is controlled by upstream protein kinases that can be pharmacologically targeted.