Circular RNA‐circLRP6 protects cardiomyocyte from hypoxia‐induced apoptosis by facilitating hnRNPM‐mediated expression of FGF‐9

Coronary atherosclerosis‐induced myocardial ischemia leads to cardiomyocyte apoptosis. The regulatory mechanisms for cardiomyocyte apoptosis have not been fully understood. Circular RNAs are non‐coding RNAs which play important roles in heart function maintenance and progression of heart diseases by regulating gene transcription and protein translation. Here, we reported a conserved cardiac circular RNA, which is generated from the second exon of LRP6 and named circLRP62‐2. CircLRP62‐2 can protect cardiomyocyte from hypoxia‐induced apoptosis. The expression of circLRP62‐2 in cardiomyocytes was down‐regulated under hypoxia, while forced expression of circLRP62‐2 inhibited cell apoptosis. Normally, circLRP62‐2 was mainly localized in the nucleus. Under hypoxia, circLRP62‐2 is associated with heterogeneous nuclear ribonucleoprotein M (hnRNPM) to be translocated into the cytoplasm. It recruited hnRNPM to fibroblast growth factor 9 (FGF9) mRNA to enhance the expression of FGF9 protein, promoting hypoxia‐adaption and viability of cardiomyocytes. In summary, this study uncovers a new inhibitor of apoptosis and reveals a novel anti‐apoptotic pathway composed of circLRP62‐2, hnRNPM, and FGF9, which may provide therapeutic targets for coronary heart disease and ischemic myocardial injury. Here, we propose an anti‐apoptotic function of circLRP62‐2 in cardiomyocytes under hypoxic conditions. In hypoxia, FGF9 translation in cardiomyocytes was inhibited (1), and nuclear circLRP62‐2 and hnRNPM translocated into the cytoplasm (2), thereby facilitating FGF9 mRNA‐hnRNPM association and maintenance of FGF9 expression, consequently resisting hypoxic stress and apoptosis. Prolonged hypoxia, however, gradually disrupts FGF9 mRNA‐hnRNPM interaction through downregulating circLRP62‐2, leading to FGF9 deficiency. This eventually results in cardiomyocyte apoptosis (3).