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

by Selleckchem | Dec 15 2023

Introduction: PI3K/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.

Methods: Our 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.

Results: Differential 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.

Conclusion: In 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.

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**Title: Targeting PI3Kδ-Splice Variant as a Prognostic Biomarker and Therapeutic Strategy in Advanced Endocrine Cancers**

**Abstract:**
The dysregulation of the PI3K/AKT signaling pathway is a hallmark feature in various cancer types. Among the class I PI3K isoforms, PI3Kδ has been implicated in both hematologic and solid tumors. Alternative splicing, a crucial post-transcriptional process, contributes to proteomic diversity and is increasingly recognized in cancer development. This study delves into the role of the oncogenic splice variant, PIK3CD-S, in promoting tumor aggressiveness and drug resistance in prostate cancer (PCa). Through comprehensive analyses of patient samples and various cancer cell lines, we demonstrate high expression levels of PI3Kδ isoforms in endocrine and solid tumor specimens. Our findings reveal distinct expression profiles of PIK3CD-S and PIK3CD-L in different cancer cells. Knockdown experiments elucidate the differential regulation of AKT/mTOR signaling by PIK3CD-S and PIK3CD-L in PCa, breast, colon, and lung cancer cells. Importantly, PTEN, a known tumor suppressor, was found to modulate PI3Kδ-L but not PI3Kδ-S, suggesting a unique regulation pattern for the splice variant. Moreover, our study highlights the efficacy of Idelalisib, a PI3Kδ inhibitor, in inhibiting PI3Kδ-L-mediated AKT/mTOR signaling, while SRPIN340, a splicing modulator, effectively targets aberrant mRNA splicing, leading to AKT/mTOR inhibition. Combining Idelalisib and SRPIN340 synergistically reduced cell viability and tumor growth in advanced cancer models, providing a promising therapeutic strategy for PI3Kδ-expressing endocrine cancers. Overall, our findings propose PI3Kδ-S as both a prognostic biomarker and a therapeutic target, shedding light on novel avenues for advanced endocrine cancer treatment.