Altered VEGF Splicing Isoform Balance in Tumor Endothelium Involves Activation of Splicing Factors Srpk1 and Srsf1 by the Wilms’ Tumor Suppressor Wt1

Abstract

Angiogenesis is one hallmark of cancer. Vascular endothelial growth factor (VEGF) is a known inducer of angiogenesis. Many patients benefit from antiangiogenic therapies, which however have limitations. Although VEGF is overexpressed in most tumors, different VEGF isoforms with distinct angiogenic properties are produced through alternative splicing. In podocytes, the Wilms’ tumor suppressor 1 (WT1) suppresses the Serine/arginine-rich protein-specific splicing factor kinase (SRPK1), and indirectly Serine/arginine-rich splicing factor 1 (Srsf1) activity, and alters VEGF splicing. We analyzed VEGF isoforms, Wt1, Srpk1, and Srsf1 in normal and tumor endothelium. Wt1, Srpk1, Srsf1, and the angiogenic VEGF164a isoform were highly expressed in tumor endothelium compared to normal lung endothelium. Nuclear expression of Srsf1 was detectable in the endothelium of various tumor types, but not in healthy tissues. Inducible conditional vessel-specific knockout of Wt1 reduced Wt1, Srpk1, and Srsf1 expression in endothelial cells and induced a shift towards the antiangiogenic VEGF120 isoform. Wt1(−KTS) directly binds and activates both the promoters of Srpk1 and Srsf1 in endothelial cells. In conclusion, Wt1 activates Srpk1 and Srsf1 and induces expression of angiogenic VEGF isoforms in tumor endothelium.

Serine-Arginine Protein Kinase 1 (SRPK1) as a Prognostic Factor and Potential Therapeutic Target in Cancer: Current Evidence and Future Perspectives

“…Second, targeting SRPK1 could simultaneously affect diverse oncogenic processes-proliferation, migration, invasion, metastasis, angiogenesis, apoptosis, acquisition of CTC phenotype, and sensitivity to chemotherapy-through various mechanisms depending on the cancer type (Table 2, Figure 1). For instance, deregulation of alternative splicing was reported to impact angiogenesis in lung [59], prostate [26], colorectal cancer [62] and melanoma [72], also apoptosis in breast [35,36], colorectal cancer [36], and leukemia [65,66] (Figure 2). Of interest, apart from cancer cells, SRPK1 inhibition influences tumor microenvironment and more specifically the tumor-associated endothelial cells, suppressing angiogenesis in vivo [59].…”

“…The SRPK1/SRSF1 axis interacts with WT-1 and regulates VEGF alternative splicing (proangiogenic vs. antiangiogenic isoforms) [59] Breast ↑SRPK1 was found in BLBC cell lines ↓SRPK1 suppressed the migration of BLBC cells in vitro and breast cancer metastasis to the lungs in vivo SRPK1 interacts with the NF-κB pathway [33] ↑SRPK1 was found in hormone-positive and triple-negative breast cancer cell lines and promoted the localization of RBM4 in the cytoplasm; this enhanced the expression of the antiapoptotic IR-A and MCL-1L isoforms in hormone-positive breast cancer cell lines ↓SRPK1 in hormone-positive breast cancer cells promoted apoptosis by reducing the phosphorylation of RBM4 and restoring its localization in the nucleus; this enhanced the expression of the proapoptotic IR-B and MCL-1s isoforms SRPK1 regulates the alternative splicing of IR and MCL-1 by modulating the localization of RBM4 inside the cell (cytoplasm vs. nucleus) [35] ↑SRPK1 was found in hormone-positive and triple-negative breast cancer cell lines ↓SRPK1 induced apoptosis and sensitivity to chemotherapy in hormone-positive breast cancer cells SRPK1 interacts with the AKT and MAPK pathways and modulates MAP2K2 alternative splicing [36] ↑SRPK1 was found in breast cancer cell lines [32] Prostate ↑SRPK1 was found in prostate cancer cell lines ↓SRPK1 suppressed angiogenesis (as shown by the reduced MVD) and thus tumor growth in vivo by inducing the expression of the antiangiogenic VEGF-165b…”

“…Except for NSCLC cells, SRPK1 acts on the endothelial cells of the tumor microenvironment. WT-1 (Wilms tumor-1) interacts with the SRPK1/SRFS1 axis and regulates the alternative splicing of VEGF (vascular endothelial growth factor) [59]. VEGF alternative splicing can result either in proangiogenic or antiangiogenic isoforms, regulating angiogenesis in cancer [19].…”

“…VEGF alternative splicing can result either in proangiogenic or antiangiogenic isoforms, regulating angiogenesis in cancer [19]. Wagner et al noted that, whereas SRPK1 overexpression induced the expression of proangiogenic VEGF164a isoform, its reduced expression resulted in a shift towards the expression of the antiangiogenic VEGF120, suppressing angiogenesis in vivo [59]. To summarize, SRPK1 downregulation affects both tumor epithelial cells and endothelial cells of the microenvironment, whilst it suppresses the acquisition of CSC phenotype, migration, invasion, metastasis, and angiogenesis and promotes apoptosis in preclinical NSCLC models.…”

“…Apart from melanoma tumor cells, VEGF alternative splicing is critical in regulating the oncogenic role of tumor-associated endothelial cells, just like in NSCLC. Low expression of SRPK1 resulted in a shift towards the expression of the antiangiogenic VEGF120, suppressing angiogenesis in vivo [59].…”