Identification of ASF/SF2 as a Critical, Allele-Specific Effector of the Cyclin D1b Oncogene

Abstract

The cyclin D1b oncogene arises from alternative splicing of the CCND1 transcript, and harbors markedly enhanced oncogenic functions not shared by full-length cyclin D1 (cyclin D1a). Recent studies showed that cyclin D1b is selectively induced in a subset of tissues as a function of tumorigenesis; however, the underlying mechanism(s) that control tumor-specific cyclin D1b induction remain unsolved. Here, we identify the RNA-binding protein ASF/SF2 as a critical, allele-specific, disease-relevant effector of cyclin D1b production. Initially, it was observed that SF2 associates with cyclin D1b mRNA (transcript-b) in minigene analyses and with endogenous transcript in prostate cancer (PCa) cells. SF2 association was altered by the CCND1 G/A870 polymorphism, which resides in the splice donor site controlling transcript-b production. This finding was significant, as the A870 allele promotes cyclin D1b in benign prostate tissue, but in primary PCa, cyclin D1b production is independent of A870 status. Data herein provide a basis for this disparity, as tumor-associated induction of SF2 predominantly results in binding to and accumulation of G870-derived transcript-b. Finally, the relevance of SF2 function was established, as SF2 strongly correlated with cyclin D1b (but not cyclin D1a) in human PCa. Together, these studies identify a novel mechanism by which cyclin D1b is induced in cancer, and reveal significant evidence of a factor that cooperates with a risk-associated polymorphism to alter cyclin D1 isoform production. Identification of SF2 as a disease-relevant effector of cyclin D1b provides a basis for future studies designed to suppress the oncogenic alternative splicing event. Cancer Res; 70(10); 3975-84.

The Role of SF2 in Prostate Cancer Progression

“…While preliminary, this observation suggests a novel regulatory loop between D-cyclins and SF2. Moreover, robust and transient SF2 over-expression verified altered D-cyclin splicing [consistent with recently published data (8)] and the transcription of known SF2-associated genes (e.g., CLK1 and C1qBP) while having minimal impact on genes not known to be associated with SF2 (e.g., KHDRBS3). These data will provide valuable information related to dissecting signaling pathways as part of the future studies.…”

“…Published data have suggested that SF2 and cyclin D1 isoforms have oncogenic functions (6,7) suggesting a potential link between splicing and proliferation. Based on preliminary data (now published (8)), the current proposal is geared towards determining the consequence of SF2 function and splicing in the context of PCa.…”

Alternative pre-mRNA splicing regulation in cancer: pathways and programs unhinged

“…While the G870A polymorphism appears to affect the production of D1b in normal tissues, in cancer cells production of D1b occurs regardless of the identity of the final E4 nucleotide (Olshavsky et al 2010). Consistent with this, RBPs that influence the choice between D1a and D1b have also been identified.…”

“…A strong correlation between Sam68 expression levels and D1b production was also observed, underscoring the biological significance of these findings. The SR protein SRSF1 (formerly ASF/SF2) has also been shown to bind preferentially to the D1b transcript and promote the production of D1b, and SRSF1 levels were also found to correlate with D1b production (Olshavsky et al 2010). The effects of SRSF1 on D1b production were enhanced with the 870G allele, potentially providing an explanation for the fact that, in cancer, both alleles result in similar D1b production.…”

Elevated SRPK1 lessens apoptosis in breast cancer cells through RBM4-regulated splicing events

“…The alternatively spliced transcripts of Bcl-2-like 1 (BCL2L1) and myeloid cell leukemia 1 (MCL-1) genes encode either the anti-apoptotic or pro-apoptotic isoforms that manipulate the therapeutic response of cancer cells (Moore et al 2010;Gautrey and Tyson-Capper 2012). The inclusion of cyclin D intron 4 leads to the production of the cyclin D1b isoform which acts as an oncogene in prostate cancer cells (Burd et al 2006;Olshavsky et al 2010). An increase in the short S6K1 variant facilitates the transformation of breast epithelial cells, whereas the presence of the long S6K1 isoform mediates the opposite effect by inhibiting Ras-induced transformation and tumor growth (Ben-Hur et al 2013).…”