SRSF5 Regulates the Expression of BQ323636.1 to Modulate Tamoxifen Resistance in ER-Positive Breast Cancer

SIMPLE SUMMARY: Overexpression of BQ can confer tamoxifen resistance in ER +ve breast cancer. BQ is a variant of NCOR2 spliced through the exclusion of exon 11. SRSF5 is found to be involved in regulating the splicing of NCOR2. A low expression of SRSF5 enhances the chance of excluding exon 11 from mRNA, leading to the production of mRNA translating for BQ. Therefore, a low expression of SRSF5 can enhance BQ expression and, thus, tamoxifen resistance. SRPK1 can alter the activity of SRSF5 for BQ expression. Inhibition of SRPK1 can reduce TAM resistance by suppressing BQ expression in ER +ve breast cancer. Therefore, targeting the upstream pathway of BQ will be a possible strategy for reversing TAM resistance. ABSTRACT: About 70% of breast cancer patients are oestrogen receptor-positive (ER +ve). Adjuvant endocrine therapy using tamoxifen (TAM) is an effective approach for preventing local recurrence and metastasis. However, around half of the patients will eventually develop resistance. Overexpression of BQ323636.1 (BQ) is one of the mechanisms that confer TAM resistance. BQ is an alternative splice variant of NCOR2. The inclusion of exon 11 generates mRNA for NCOR2, while the exclusion of exon 11 produces mRNA for BQ. The expression of SRSF5 is low in TAM-resistant breast cancer cells. Modulation of SRSF5 can affect the alternative splicing of NCOR2 to produce BQ. In vitro and in vivo studies confirmed that the knockdown of SRSF5 enhanced BQ expression, and conferred TAM resistance; in contrast, SRSF5 overexpression reduced BQ expression and, thus, reversed TAM resistance. Clinical investigation using a tissue microarray confirmed the inverse correlation of SRSF5 and BQ. Low SRSF5 expression was associated with TAM resistance, local recurrence and metastasis. Survival analyses showed that low SRSF5 expression was associated with poorer prognosis. We showed that SRPK1 can interact with SRSF5 to phosphorylate it. Inhibition of SRPK1 by a small inhibitor, SRPKIN-1, suppressed the phosphorylation of SRSF5. This enhanced the proportion of SRSF5 interacting with exon 11 of NCOR2, reducing the production of BQ mRNA. As expected, SRPKIN-1 reduced TAM resistance. Our study confirms that SRSF5 is essential for BQ expression. Modulating the activity of SRSF5 in ER +ve breast cancer will be a potential approach to combating TAM resistance.