Modulation of splicing factors SF3B1 and SRSF1 by polypurine reverse Hoogsteen hairpins affects the splicing pattern of estrogen receptor α in breast cancer cells.

Multiple mRNA splice variants for the human estrogen receptor α (ERα) with one or more skipped exons have been identified, and some encode isoforms with modified functions compared with the wild-type 66 kDa protein. Our aim was to evaluate the effect of Polypurine Reverse Hoogsteen Hairpins (PPRHs) targeting the splicing factors SF3B1 and SRSF1 in breast cancer cells to analyze the potential changes in ERα splicing pattern. Two PPRHs against SF3B1 and three against SRSF1 were designed, and their cytotoxic effect was evaluated in vitro. HpSF3B1 against the SF3B1 promoter and HpSRSF1 against SRSF1 exon 3 decreased the protein levels of their respective targets in MCF-7 breast cancer cells. Inhibition of either SF3B1 or SRSF1 in these cells altered the inclusion/skipping of several ERα exons and decreased the protein levels of ERα isoforms. Changes in ERα splicing variants were also observed in the ZR-75-1 cell line. Additionally, modulation on mRNA and protein levels upon transfection with both PPRHs was also detected in patient-derived xenograft cells. Finally, we showed that combining PPRHs targeting SF3B1 or SRSF1 with Tamoxifen or 4-hydroxytamoxifen reduced MCF-7 cell viability in an additive or synergistic manner, respectively. Our results indicate that SF3B1 and SRSF1 participate in the generation of different ERα splicing variants and their inhibition by PPRHs could have therapeutic potential in the treatment of estrogen receptor positive breast tumors.