Ferroptosis inhibitor improved the pathological process of MI
The LVEF% and LVFS% in the MI group were significantly lower than those in the control group (Fig. 1Aand 1B). The cardiomyocytes were closely arranged, and its morphology was natural in the sham group, but in the MI group, the injuried myocardial cells presented the coagulative necrosis (black arrow), the disappearing cardiomyocytes with new granulation tissue and gradual fibrosis (blue arrow), the inflammatory cells infiltration (orange arrow), and bleeding (green arrow) (Fig. 1C). Further the MI group had higher levels of MDA and Fe2+ in infarcted myocardial tissue (Fig. 1D). The ferroptosis inhibitor reversed the low LVEF% and LVFS% in infarcted mice (Fig. 1A and 1B), and showed the mild cells injury, and a few coagulative necrosis of cardiomyocytes (black arrow) (Fig. 1C), and decreased the levels of MDA and Fe2+ in infarcted myocardial tissue (Fig. 1D). Moreover, Fer-1 significantly improved the viability of cardiomyocytes treated with erastin (ferroptosis inducer), but ZVAD-FMK (apoptosis inhibitor) and necrosulfonamide (necrosis inhibitor) did not have similar effect(Fig. 1E).
Thus, in the established mouse MI model, we found that ferroptosis occurred in mouse MI. Further ZVAD-FMK (apoptosis inhibitor) and necrosulfonamide (necrosis inhibitor) did not improve the viability of cardiomyocytes treated with erastin (ferroptosis inducer), but Fer-1 (ferroptosis inhibitor) could improve left ventricular function and pathological changes of MI.
LRP6 was involved in the process of ferroptosis in cardiomyocytes through autophagy
The expression of LRP6 in the infarcted myocardial tissue was significantly lower in comparison to the control group (Fig. 2A). LRP6 interference fragment siRNA was transfected into cardiomyocytes, and the expression of LRP6 in cardiomyocytes was significantly down-regulated after transfection with LRP6-siR-mus-4649 compared with LRP6-siR-mus-2488 or LRP6-siR-mus-513(Fig. 2B). After induction of hypoxia and ferroptosis, compared with the control group, the activity of cardiomyocytes interfered by LRP6 was lower (Fig. 2C), and the levels of MDA and Fe2+ in cardiomyocytes were higher (Fig. 2D and 2E).
In the cardiomyocytes treated with hypoxia and erastin, the expression of autophagy-related proteins LC3-A/B(microtubule associated protein 1 light chain 3-A/B) and ATG5 after LRP6 interference was higher in comparing with the control group, but the expression of p62 (sequestosome-1) was lower (Fig. 3A). ATG5 siRNA significantly decreased the expression of ATG5 in cardiomyocytes (Fig. 3B). In cardiomyocytes treated with hypoxia and erastin, the cardiomyocytes with LRP6 siRNA and ATG5 siRNA could repair the decreasing survival rate induced by LRP6 deletion (Fig. 3C), and reverse the increase of Fe2+ and MDA induced by LRP6 deletion (Fig. 3D and 3E).
Thus, to explore whether LRP6 regulates ferroptosis through autophagy, we detected the expression of LRP6 in myocardial tissue, and found the low expression of LRP6 in MI group, interference with LRP6 could promote ferroptosis of cardiomyocytes, increase the expression of autophagy-related proteins LC3-A/B and ATG5, and decrease the expression of p62. In addition, the cell was interfered with LRP6 and inhibited autophagy simultaneously, its survival rate increased, and the level of ferroptosis decreased.
Screening and identification of circRNA1615 plays a role as a miRNA sponge by targeting LRP6
We screened the miRNA targeting LRP6 and the circRNAs, that bind to these miRNA through CLIP database to construct circRNA-miRNA-LRP6 regulatory network (Fig. 4A). Based on the analysis of LRP6 regulatory network, it was found that mmu-miR-466c-5p, mmu-miR-466o-5p, and mmu-miR-679-5p miRNA had strong binding ability, while GEO database data (GSE81636) showed that the expression of miR-152-3p in myocardial tissue of MI mice was significantly higher than that of sham-operated group at 2 days after MI, and it significantly rose at 7 day of MI. In addition, the expression of miR-152-3p was significantly increased in MI group and hypoxia-treated cardiomyocytes (Fig. 4B and 4C). The expression of five circRNAs targeting miR-152-3p was detected in mouse myocardial tissue, and the expression of circRNA1615 was significantly down-regulated in the MI group (Fig. 4D). Further the expression of circRNA1615 decreased significantly after hypoxia (Fig. 4E). Using polymeric primers and divergent primers to amplify gDNA and cDNA of mouse myocardial tissue, only divergent primers amplified circRNA1615 in cDNA. Sequencing of PCR products revealed that circRNA1615 was a circular RNA derived from Copb1 gene (Fig. 4F and 4G).
We constructed the overexpressed circRNA1615 plasmids, then transfected it to cardiomyocytes. It was found that the expression of circRNA1615 in mouse cardiomyocytes transfected with overexpressed plasmids was higher than that of control plasmids (Fig. 5A). The survival rate of overexpressed circRNA cells increased in cardiomyocytes treated with hypoxia and erastin (Fig. 5B), suggesting that circRNA1615 inhibits ferroptosis in cardiomyocytes.
Thus, to study whether the circRNAs are involved in the regulation of ferroptosis of cardiomyocytes by LRP6, we constructed a circRNA-miRNA-LRP6 regulatory network. Bioinformatics analysis combined with experimental tests showed that the expression of miR-152-3p in myocardial tissue of infarcted mice and hypoxia-treated cardiomyocytes was increased, while the matching circRNA1615 (circBase: mmu_circ_0001615) expression was down-regulated. Noticeably, the overexpression of circRNA1615 could inhibit cardiomyocyte death (Figs. 4 and 5).
In addition, we studied whether circRNA1615 has the ability to bind to miRNA. The binding sites of circRNA1615 and miR-152-3p were analyzed in database, and psicheck-2 reporter gene wild-type vector (including miR-152-3p target sequence) and mutant vector (miR-152-3p target sequence point mutation) of LRP6 were constructed and cotransfected with miR-152-3p mimics. Compared with the negative control RNA (NC), miR-152-3p decreased the luciferase activity of the wild-type vector (Fig. 6A); RIP experiment showed that circRNA1615 could be precipitated by miR-152-3p and AGO2 antibodies, indicating the sponge adsorption ability of circRNA1615 to miR-152-3p (Fig. 6B). The expression of miR-152-3p in cardiomyocytes was decreased after overexpression of circRNA1615, while the expression of miR-152-3p was up-regulated by knocking down circRNA1615 (Fig. 6C). To test whether miR-152-3p targets the binding sites of miR-152-3p and LRP6 in the database, the wild-type psicheck-2 reporter gene vector and mutant vector of LRP6 were constructed and cotransfected with miR-152-3p mimics and miR-NC. Compared with the negative control RNA (NC), miR-152-3p decreased the luciferase activity of the wild-type vector (Fig. 6D). Overexpression of miR-152-3p in cardiomyocytes inhibited the expression of LRP6, while the knocking down miR-152-3p upregulated the expression of LRP6 (Fig. 6E).
Therefore, further the overexpression or knockdown of circRNA1615 in cardiomyocytes negatively regulated the expression of miR-152-3p, the luciferase and RIP experiments verified the spongy adsorption of circRNA1615 to miR-152-3p. In addition, the overexpression or knockdown of miR-152-3p in cardiomyocytes could negatively regulate the expression of LRP6, and luciferase reporter gene assay confirmed the targeting effect of miR-152-3p on LRP6 (Fig. 6). These results suggest that circRNA1615 regulates the expression of LRP6 through sponge adsorption of miR-152-3p, and then regulates ferroptosis in cardiomyocytes.