Repression of rRNA gene transcription by endothelial SPEN deficiency normalizes tumor vasculature via nucleolar stress

Human cancers induce a chaotic, dysfunctional vasculature that promotes tumor growth and dampens most current therapies, but the underlying mechanism has been unclear. Here we show that SPEN (split end), a transcription repressor, coordinates ribosome RNA (rRNA) synthesis in endothelial cells (ECs) and is required for physiological and tumor angiogenesis. SPEN deficiency attenuated EC proliferation and blunted retinal angiogenesis, which was attributed to p53 activation. Furthermore, SPEN knockdown activated p53 by upregulating the noncoding promoter RNA (pRNA), which represses rRNA transcription and triggers p53-mediated nucleolar stress. In human cancer biopsies, low endothelial SPEN level correlated with extended overall survival. Consistently in mice, endothelial SPEN deficiency compromised rRNA expression and repressed tumor growth and metastasis by normalizing tumor vessels, which was abrogated by p53 haploinsufficiency. rRNA gene transcription is driven by RNA polymerase I (RNPI). We found that CX-5461, an RNPI inhibitor, recapitulated the effect of Spen ablation on tumor vessel normalization, and combining CX-5461 with cisplatin substantially improved the efficacy on treating tumors in mice. Together, these results demonstrate that SPEN is required for angiogenesis by repressing pRNA to enable rRNA gene transcription and ribosomal biogenesis, and that RNPI represents a target for tumor vessel normalization therapy of cancer.