Abstract We0030: The Large Ribosomal Subunit Protein 17 (RpL17) - Early Growth Response 1 (EGR1) - Breast Cancer Gene 1 (BRCA1) Feedback Loop Regulates Angiogenesis

Background: Angiogenesis, the formation of new blood vessels, plays a key role in development and cancer. In mice, we identified a critical role for Large Ribosomal Subunit Protein 17 (RpL17) in suppressing endothelial cell (EC) proliferation and migration, integral processes for angiogenesis. RNA sequencing of RpL17 knockdown (RpL17-KD) ECs revealed Early Growth Response 1 (EGR1), a known proangiogenic factor, as a key gene following RpL17-KD. In-silico , Breast Cancer gene 1 (BRCA1) was predicted as an RpL17 regulator, suggesting a novel signaling pathway. Hypothesis: BRCA1 regulates RpL17, which elevates EGR1 to increase angiogenesis in ECs. Goal: Investigate the role of the BRCA1-RpL17-EGR1 axis in angiogenesis. Methods: We performed RNA-sequencing on mouse ECs, whole retina staining, and tube formation assays in ECs to examine the effects of RpL17, BRCA1, and EGR1 knockdowns. Mouse models, bioinformatics, RT-qPCR, immunoblotting, and immunofluorescence were used to validate findings and assess gene/protein expression. Results: RpL17 deficient (RpL17+/-) mouse retinas showed increased angiogenesis (n=10, p<0.009), and RpL17 KD human ECs formed more junctions than controls (p<0.0002). RNA-seq revealed a significant increase in EGR1 expression in RpL17 KD ECs (q<2.5x10 -8 ), confirmed by RT-qPCR (p<0.002), and further enhanced by 50 nM VEGF (p<0.0001). A binding probability of 98.9% between RpL17 and BRCA1 was generated in-silico . BRCA1 expression was elevated in carotid sections from RpL17+/- mice (n=10, p<0.0403). RpL14 KD significantly decreased BRCA1, but BRCA1 KD did not decrease RpL17 in-vitro , indicating a positive feedback loop. MDM2 and p53, factors downstream of the BRCA1 pathway, were significantly increased in RpL17 +/- mice. RpL17 KD ECs lost the angiogenic phenotype in tube formation assay following EGR1 inhibition and BRCA1 overexpression. Conclusion: These findings uncover a novel feedback loop between RpL17, EGR1, and BRCA1, offering new insights for therapeutic strategies to regulate angiogenesis.