Crosstalk between Wnt Signaling and RNA Processing in Colorectal Cancer

Abstract

RNA processing involves a variety of processes affecting gene expression, including the removal of introns through RNA splicing, as well as 3' end processing (cleavage and polyadenylation). Alternative RNA processing is fundamentally important for gene regulation, and aberrant processing is associated with the initiation and progression of cancer. Deregulated Wnt signaling, which is the initiating event in the development of most cases of human colorectal cancer (CRC), has been linked to modified RNA processing, which may contribute to Wnt-mediated colonic carcinogenesis. Crosstalk between Wnt signaling and alternative RNA splicing with relevance to CRC includes effects on the expression of Rac1b, an alternatively spliced gene associated with tumorigenesis, which exhibits alternative RNA splicing that is influenced by Wnt activity. In addition, Tcf4, a crucial component of Wnt signaling, also exhibits alternative splicing, which is likely involved in colonic tumorigenesis. Modulation of 3' end formation, including of the Wnt target gene COX-2, also can influence the neoplastic process, with implications for CRC. While many human genes are dependent on introns and splicing for normal levels of gene expression, naturally intronless genes exist with a unique metabolism that allows for intron-independent gene expression. Effects of Wnt activity on the RNA metabolism of the intronless Wnt-target gene c-jun is a likely contributor to cancer development. Further, butyrate, a breakdown product of dietary fiber and a histone deacetylase inhibitor, upregulates Wnt activity in CRC cells, and also modulates RNA processing; therefore, the interplay between Wnt activity, the modulation of this activity by butyrate, and differential RNA metabolism in colonic cells can significantly influence tumorigenesis. Determining the role played by altered RNA processing in Wnt-mediated neoplasia may lead to novel interventions aimed at restoring normal RNA metabolism for therapeutic benefit. Therefore, this minireview presents a brief overview of several aspects of RNA processing of relevance to cancer, which potentially influence, or are influenced by, Wnt signaling activity.

Keywords: RNA processing; Wnt activity; butyrate; colon cancer; polyadenylation.; splicing.

Fig 1

Wnt activity influences splicing efficiency in SW620 CRC cells. Splicing constructs pTN23 (TN23) or pTN24 (TN24) were transfected into SW620 cells with pcDNA3 (Ctl) or DN-Tcf4 (DN). Luciferase and beta-galactosidase activities were assayed with the Dual Light kit (Applied Biosystems) and the relative activity of luciferase vs. beta-galactosidase, indicative of splicing efficiency, is shown.

Fig 2

Wnt activity and Rac1b expression in CRC cells. Wnt activity promotes expression of SRp20 which blocks expression of Rac1b through control of alternative splicing. In contrast, the splicing factor ASF/SF2, the expression of which is repressed by PI3-K signaling, enhances expression of Rac1b through alternative splicing. Rac1b itself not only promotes cell survival, but enhances Wnt signaling, forming a negative feedback loop that controls Rac1b levels through Wnt-mediated SRp20 expression. Histone deacetylase inhibitors (HDACis) influence these processes by affecting both Wnt activity and the expression of factors such as SRp20. Arrows represent known or potential positive interactions; blocked lines represent known or potential repressive interactions.

Fig 3

Interactions between Wnt activity and aspects of RNA metabolism. Wnt signaling can influence alternative splicing through the differential expression of RNA splicing factors. In some cases, such as Rac1b, the products of alternative processing can affect Wnt signaling. Wnt activity may also affect alternative polyadenylation. In addition, Wnt activity can influence the stability of the products of alternative polyadenylation; these effects on RNA stability may be mediated by the HuR protein. Further, our preliminary data suggest a possible role for Wnt activity in repressing general RNA splicing activity in CRC cells. Finally, Wnt activity can also promote expression of certain intronless genes, whose gene products influence Wnt activity, with possible consequences for colonic tumorigenesis. Note that the overall net effect of HDACis on CRC is believed to be inhibitory; however, at the same time, HDACis can enhance expression of potentially pro-tumorigenic factors, such as the product of the intronless c-jun gene. Arrows represent known or potential positive interactions; blocked lines represent known or potential repressive interactions. Steps involving RNA metabolism are underlined.