PI3K/AKT信号通路在多种癌症中上调。在I类PI3Ks（PI3Kδ/β/γ亚型）中，PI3Kδ已被认为与血液肿瘤和实体肿瘤相关。在真核细胞中，可变剪接是一种后转录过程，用于获得蛋白质多样性。新出现的证据已经强调了异常mRNA剪接在癌症的发展/进展中的参与。我们之前的研究揭示了PIK3CD-S是一个致癌剪接变体，促进前列腺癌（PCa）的肿瘤侵袭性和耐药性。为了进一步评估利用PI3Kδ-S（由PIK3CD-S编码）作为癌症生物标志物和/或药物靶点的潜力，我们对一系列内分泌/实体肿瘤患者样本和细胞系进行了全面的分析。具体地，免疫组化、免疫荧光、免疫印迹和RT-PCR实验结果表明，PI3Kδ亚型在内分泌/实体肿瘤患者标本和细胞系中高表达。在一系列内分泌/实体肿瘤细胞中发现了差异性的PIK3CD-S/PIK3CD-L表达谱。通过siRNA敲降PIK3CD-L或PIK3CD-S，我们发现它们分别不同程度地抑制了PCa、乳腺癌、结肠癌和肺癌细胞系的AKT/mTOR信号通路。此外，PTEN的siRNA敲降增加了PI3Kδ的水平并激活了AKT/mTOR信号通路，而PTEN的过表达降低了PI3Kδ的水平并抑制了AKT/mTOR信号通路。有趣的是，PI3Kδ-S的水平在siRNA敲降或PTEN过表达下保持不变。综上所述，我们的研究表明PTEN负调控PI3Kδ-L及其下游的AKT/mTOR信号通路，而PI3Kδ-S则在PTEN的调控下促进AKT/mTOR信号通路。最后，我们使用PI3Kδ抑制剂Idelalisib和SRPK1/2抑制剂SRPIN340评估了它们对抑制PI3Kδ表达的内分泌/实体肿瘤的疗效。我们的结果显示，Idelalisib有效抑制了PI3Kδ-L介导的AKT/mTOR信号通路，而SRPIN340逆转了异常的mRNA剪接，从而抑制了AKT/mTOR信号通路。体外功能分析进一步证明，Idelalisib和SRPIN340的联合使用在抑制高级肿瘤细胞方面产生了协同的药物效应（显著降低了肿瘤球的细胞生存率/生长）。总之，我们的研究揭示了在晚期内分泌癌中利用PI3Kδ-S（一种致癌亚型，具有耐药性且不受PTEN调控）作为预后生物标志物和药物靶点的潜力。版权所有 © 2023 Ha, Gujrati and Wang。

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.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.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.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.Copyright © 2023 Ha, Gujrati and Wang.