Phosphorylation of SRSF1 by SRPK1 regulates alternative splicing of tumor-related Rac1b in colorectal cells

Abstract

The premessenger RNA of the majority of human genes can generate various transcripts through alternative splicing, and different tissues or disease states show specific patterns of splicing variants. These patterns depend on the relative concentrations of the splicing factors present in the cell nucleus, either as a consequence of their expression levels or of post-translational modifications, such as protein phosphorylation, which are determined by signal transduction pathways. Here, we analyzed the contribution of protein kinases to the regulation of alternative splicing variant Rac1b that is overexpressed in certain tumor types. In colorectal cells, we found that depletion of AKT2, AKT3, GSK3β, and SRPK1 significantly decreased endogenous Rac1b levels. Although knockdown of AKT2 and AKT3 affected only Rac1b protein levels suggesting a post-splicing effect, the depletion of GSK3β or SRPK1 decreased Rac1b alternative splicing, an effect mediated through changes in splicing factor SRSF1. In particular, the knockdown of SRPK1 or inhibition of its catalytic activity reduced phosphorylation and subsequent translocation of SRSF1 to the nucleus, limiting its availability to promote the inclusion of alternative exon 3b into the Rac1 pre-mRNA. Altogether, the data identify SRSF1 as a prime regulator of Rac1b expression in colorectal cells and provide further mechanistic insight into how the regulation of alternative splicing events by protein kinases can contribute to sustain tumor cell survival.

SRPK1 gene silencing promotes vascular smooth muscle cell proliferation and vascular remodeling via inhibition of the PI3K/Akt signaling pathway in a rat model of intracranial aneurysms

“…Its overexpression has been implicated in promoting anchorage‐independent cell development in vitro as well as tumor growth in nude mice . Furthermore, SRPK1 plays a heterogeneous role in different cancers, such as regulating angiogenesis in prostate and colorectal cancers . SRPK1 is a downstream Akt target for transducing growth signals to regulate splicing.…”

“…13 Furthermore, SRPK1 plays a heterogeneous role in different cancers, such as regulating angiogenesis in prostate and colorectal cancers. [14][15][16] SRPK1 is a downstream Akt target for transducing growth signals to regulate splicing. Its aberrant expression has been found to induce constitutive Akt activation.…”

miR-302b suppresses cell invasion and metastasis by directly targeting AKT2 in human hepatocellular carcinoma cells

“…3). AKT2, a critical oncogene in many cancers such as neuroblastoma [22], human breast cancer [23], HCC [24], colorectal cancer [25], and muscular tumors [26], is involved in regulating proliferation, apoptosis, metastasis, and the invasive potential of cancers. In a study on neuroblastoma, AKT2 was identified as a downstream target of GRP/GRP-R-regulated cell proliferation, anchorageindependent growth, metastasis, and invasion.…”

Targeting the angiogenic 'splice' switch; inhibition of SRPK1 as a novel therapeutic approach in prostate cancer

“…There are a number of known SRPK1 targets, not all of which are inhibited by treatment with SRPK1 inhibitors at concentrations that inhibit pro-angiogenic VEGF-A splicing. It appears that the VEGF-A system is exquisitely sensitive to SRPK1 activity and subsequent SRSF1 phosphorylation -other targets such as Rac1B, hnRNPA2/B1 and MKNK2 [19,20,34] , have differing sensitivities -MKNK2 and hnRNPA2B1 are both altered in highly expressing SRPK1 cell types [17] whereas Rac1B is not and neither hnRNPA2/B1 nor Rac1B are affected in other cell types such as RPE cells (unpublished data). Future assessments of the role of additional splicing proteins and interplay with other signaling pathways would lead to a comprehensive understanding of the role of the SRPK1-SRSF1 axis in prostate cancer.…”