CircRSU1 Activates the hnRNPA1/HIF-1α/CD24 Signaling Axis, Promoting Stemness Features of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a highly lethal malignancy with obvious heterogeneity features. This study aims to identify circRNAs with key roles in promoting HCC malignancy and stemness properties. Through circRNA profiling comparison in HCC patients and functional screening in HCC cells, circRSU1 emerges as the top candidate. It exhibits a significant higher expression level in tumor tissues compared to adjacent non-tumor liver tissues from HCC patients. Functionally, circRSU1 promotes a spectrum of HCC malignant phenotypes both in vitro and in vivo, including an enrichment of CD24^positive cancer stem cell population. Mechanistically, circRSU1 interacts with heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) via two RNA motifs on circRSU1 and two RNA-binding domains on hnRNPA1. This interaction increases hnRNPA1 protein level via reducing its proteasomal degradation. Furthermore, hnRNPA1 enhances HIF-1α protein translation via binding to its internal ribosome entry site (IRES), which subsequently increases the CD24^positive cell population. Additionally, circRSU1 further enhances this process not only through increasing the hnRNPA1 protein level, but also through enhancing the interaction of hnRNPA1 with HIF1A IRES, consequently augmenting the CD24^positive cell population and the associated malignancy/stemness features of HCC cells. Together, circRSU1 activates the hnRNPA1/HIF-1α/CD24 signaling axis, leading to the increased HCC malignancy and stemness features.