Involvement of hnRNP A1 in the matrix metalloprotease‐3‐dependent regulation of Rac1 pre‐mRNA splicing

Abstract

Rac1b is an alternatively spliced isoform of the small GTPase Rac1 that includes the 57-nucleotide exon 3b. Rac1b was originally identified through its over-expression in breast and colorectal cancer cells, and has subsequently been implicated as a key player in a number of different oncogenic signalling pathways, including tumorigenic transformation of mammary epithelial cells exposed to matrix metalloproteinase-3 (MMP-3). Although many of the cellular consequences of Rac1b activity have been recently described, the molecular mechanism by which MMP-3 treatment leads to Rac1b induction has not been defined. Here we use proteomic methods to identify heterogeneous nuclear ribonucleoprotein (hnRNP) A1 as a factor involved in Rac1 splicing regulation. We find that hnRNP A1 binds to Rac1 exon 3b in mouse mammary epithelial cells, repressing its inclusion into mature mRNA. We also find that exposure of cells to MMP-3 leads to release of hnRNP A1 from exon 3b and the consequent generation of Rac1b. Finally, we analyze normal breast tissue and breast cancer biopsies, and identify an inverse correlation between expression of hnRNP A1 and Rac1b, suggesting the existence of this regulatory axis in vivo. These results provide new insights on how extracellular signals regulate alternative splicing, contributing to cellular transformation and development of breast cancer.

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

“…Various other tumor-related splicing events were identified as targets for SRSF1 and its overexpression has oncogenic effect on cells (Karni et al 2007). Interestingly, in mouse mammary epithelial cells, Rac1b expression was reported to be primarily regulated through repression by hnRNP A1 (Pelisch et al 2012), which apparently fails to modulate Rac1b splicing in colorectal cells (Gonçalves et al 2009). It remains to be determined whether these differences reflect tissue-specific regulation patterns or differences in the nucleotide sequences between both species.…”

Tumor Cell–Derived MMP3 Orchestrates Rac1b and Tissue Alterations That Promote Pancreatic Adenocarcinoma

“…While direct targeting of MMPs has not been successful clinically (37), our results suggest that targeting MMP3-induced Rac1b could be an effective strategy. One possible approach might be to modulate the splicing factors involved in Rac1b expression (38,39), although specific targeting of splicing factors has been challenging (40). …”

Matrix metalloproteinases as drivers and therapeutic targets in breast cancer

“…MMP-3 stimulates spontaneous tumor formation in mouse mammary glands (108–110), and dissection of this process revealed that exposure of cultured mouse mammary epithelial cells to MMP-3 directly activates EMT (111, 112). MMP-3 mediates these effects by stimulating increased expression of Rac1b (113, 114), a splice variant of Rac1 with activated characteristics (115), which in turn stimulates EMT by increasing levels of cellular reactive oxygen species (113, 116, 117), through a process that depends upon cell-ECM interactions (118–120). It may be that many studies in which MMPs have been seen to stimulate cancer cell motility and invasion, although not directly investigating these phenomena in the context of EMT, have in fact been observing the cellular consequences of an incomplete or dysregulated activation of the EMT program.…”