LAUSR

Simple Summary Higher incidence and mortality rates were observed in African American vs. European American prostate cancers. Alternative mRNA splicing has been suggested as an oncogenic driver in aggressive cancers as well as cancer disparities. Herein, we show that aberrant PIK3CD-S splice variant is overexpressed in aggressive African American prostate cancer, and its resulting PI3Kδ-S isoform is resistant to PI3Kδ inhibitors, such as Idelalisib and Seletalisib. Molecular modeling and functional validations implicate that a structural change due to exon 20 skipping greatly reduces the affinity between drugs and PI3Kδ-S isoform. Further targeting alternative splicing mechanism drastically sensitizes Idelalisib/Seletalisib-resistant prostate cancers to PI3Kδ inhibitors. These results have highlighted the therapeutic potential of modulating RNA splicing mechanism to overcome drug resistance in aggressive cancers. Abstract Targeting PI3Kδ has emerged as a promising therapy for hematologic and non-hematologic malignancies. Previously, we identified an oncogenic splice variant, PIK3CD-S, conferring Idelalisib resistance in African American (AA) prostate cancer (PCa). In the current study, we employed a comprehensive analysis combining molecular biology, biochemistry, histology, in silico simulation, and in vitro functional assays to investigate the PIK3CD-S expression profiles in PCa samples and to elucidate the drug resistance mechanism mediated by PI3Kδ-S (encoded by PIK3CD-S). The immunohistochemistry, RT-PCR, and Western blot assays first confirmed that PI3Kδ-S is highly expressed in AA PCa. Compared with PCa expressing the full-length PI3Kδ-L, PCa expressing PI3Kδ-S exhibits enhanced drug resistance properties, including a higher cell viability, more antiapoptotic and invasive capacities, and constitutively activated PI3K/AKT signaling, in the presence of PI3Kδ/PI3K inhibitors (Idelalisib, Seletalisib, Wortmannin, and Dactolisib). Molecular docking, ATP-competitive assays, and PI3 kinase assays have further indicated a drastically reduced affinity of PI3Kδ inhibitors with PI3Kδ-S vs. PI3Kδ-L, attributed to the lack of core binding residues in the PI3Kδ-S catalytic domain. Additionally, SRSF2 has been identified as a critical splicing factor mediating exon 20 skipping in PIK3CD pre-mRNA. The inhibition of the SRSF2 activity by SRPIN340 successfully sensitizes AA PCa cells to PI3Kδ inhibitors, suggesting a novel therapeutic option for Idelalisib-resistant tumors.