Aberrant PI3Kδ splice isoform as a potential biomarker and novel therapeutic target for endocrine cancers

Abstract

IntroductionPI3K/AKT signaling pathway is upregulated in a broad spectrum of cancers. Among the class I PI3Ks (PI3Kδ/β/δ isoforms), PI3Kδ has been implicated in hematologic cancers and solid tumors. Alternative splicing is a post-transcriptional process for acquiring proteomic diversity in eukaryotic cells. Emerging evidence has highlighted the involvement of aberrant mRNA splicing in cancer development/progression.MethodsOur previous studies revealed that PIK3CD-S is an oncogenic splice variant that promotes tumor aggressiveness and drug resistance in prostate cancer (PCa). To further evaluate the potential of utilizing PI3Kδ-S (encoded from PIK3CD-S) as a cancer biomarker and/or drug target, comprehensive analyses were performed in a series of patient samples and cell lines derived from endocrine/solid tumors. Specifically, IHC, immunofluorescence, western blot and RT-PCR assay results have demonstrated that PI3Kδ isoforms were highly expressed in endocrine/solid tumor patient specimens and cell lines.ResultsDifferential PIK3CD-S/PIK3CD-L expression profiles were identified in a panel of endocrine/solid tumor cells. SiRNA knockdown of PIK3CD-L or PIK3CD-S differentially inhibits AKT/mTOR signaling in PCa, breast, colon and lung cancer cell lines. Moreover, siRNA knockdown of PTEN increased PI3Kδ levels and activated AKT/mTOR signaling, while overexpression of PTEN reduced PI3Kδ levels and inhibited AKT/mTOR signaling in cancer cells. Intriguingly, PI3Kδ-S levels remained unchanged upon either siRNA knockdown or overexpression of PTEN. Taken together, these results suggested that PTEN negatively regulates PI3Kδ-L and its downstream AKT/mTOR signaling, while PI3Kδ-S promotes AKT/mTOR signaling without regulation by PTEN. Lastly, PI3Kδ inhibitor Idelalisib and SRPK1/2 inhibitor SRPIN340 were employed to assess their efficacies on inhibiting the PI3Kδ-expressing endocrine/solid tumors. Our results have shown that Idelalisib effectively inhibited PI3Kδ-L (but not PI3Kδ-S) mediated AKT/mTOR signaling. In contrast, SRPIN340 reversed the aberrant mRNA splicing, thereby inhibiting AKT/mTOR signaling. In-vitro functional assays have further demonstrated that a combination of Idelalisib and SRPIN340 achieved a synergistic drug effect (with drastically reduced cell viabilities/growths of tumor spheroids) in inhibiting the advanced tumor cells.ConclusionIn summary, our study has suggested a promising potential of utilizing PI3Kδ-S (an oncogenic isoform conferring drug resistance and exempt from PTEN regulation) as a prognostic biomarker and drug target in advanced endocrine cancers.

Organoids: An Emerging Precision Medicine Model for Prostate Cancer Research

“…Recently, our research group evaluated the efficacies of the Idelalisib, SRPIN340, and Idelalisib/SRPIN340 combination in 3D spheroids derived from PCa and other solid tumor cell lines (22Rv1, PC-3, LNCaP, MDA PCa 2b, DU-145, C4-2B, HT-29, SW620, A549, H1299, MSA MB 231, and MCF-7). Our results suggested that combined Idelalisib/SRPIN340 therapy resulted in a synergistic effect on inhibiting PCa and other solid tumor spheroids expressing PI3Kδ isoforms [142]. Additionally, we established PCa organoids from LNCaP, C4-2B, 22Rv1 and MDA PCa 2b cells for exploring the mTOR/AR signaling in the in African American (AA) PCa disparities and CRPC progression.…”

Editorial: The RNA revolution and cancer

“…This was the first systematic analysis of the expression profiles of PI3Kδ splice isoforms across different endocrine/solid tumors. In their conclusion, they suggested that the synergistic inhibitory effects of the Idelalisib/SRPIN340 combination may pave a new path for developing novel therapeutics for the Idelalisib-resistant endocrine/solid tumors (and hematologic cancers, in theory) that express PI3Kδ-S ( 12 ).…”