Circular RNAs (circRNAs) are generally formed by back splicing and are expressed in various cells. Vascular calcification (VC), a common complication of chronic kidney disease (CKD), is often associated with cardiovascular disease. The relationship between circRNAs and VC has not yet been studied. Inorganic phosphate (Pi) was used to treat rat vascular smooth muscle cells to induce VC. circRNAs were identified by analyzing RNA sequencing (RNA-seq) data, and their expression change during VC was validated. The selected circRNAs, including circSamd4a, circSmoc1-1, circMettl9, and circUxs1, were resistant to RNase R digestion and mostly localized in the cytoplasm. While silencing circSamd4a promoted VC, overexpressing it reduced VC in calcium assay and Alizarin red S (ARS) staining. In addition, microRNA (miRNA) microarray, luciferase reporter assay, and calcium assay suggested that circSamd4a could act as a miRNA suppressor. Our data show that circSamd4a has an anti-calcification role by functioning as a miRNA sponge. Moreover, mRNAs that can interact with miRNAs were predicted from RNA-seq and bioinformatics analysis, and the circSamd4a-miRNA-mRNA axis involved in VC was verified by luciferase reporter assay and calcium assay. Since circSamd4a is conserved in humans, it can serve as a novel therapeutic target in resolving VC.

Vascular calcification is characterized by the accumulation of hydroxyapatite crystals, which is a result of aberrant mineral metabolism. Although many clinical studies have reported its adverse effects on cardiovascular morbidity, the molecular mechanism of vascular calcification, especially the involvement of long noncoding RNAs (lncRNAs), is not yet reported. From the transcriptomic analysis, we discovered hundreds of lncRNAs differentially expressed in rat vascular smooth muscle cells (VSMCs) treated with inorganic phosphate, which mimics vascular calcification. We focused on Lrrc75a-as1 and elucidated its transcript structure and confirmed its cytoplasmic localization. Our results showed that calcium deposition was elevated after knockdown of Lrrc75a-as1, while its overexpression inhibited calcium accumulation in A10 cells. In addition, Lrrc75a-as1 attenuated VSMCs calcification by decreasing the expression of osteoblast-related factors. These findings suggest that Lrrc75a-as1 acts as a negative regulator of vascular calcification, and may serve as a possible therapeutic target in vascular calcification.

MicroRNAs (miRNAs) regulate the expression of mRNA through sequence-specific binding of the 3 0 untranslated region (UTR). The seed sequence of miRNAs is the key determinant for target site recognition. Paralogous miRNAs, which share the same seed sequences but differ in their 3 0 regions, are known to regulate largely overlapping groups of mRNAs. However, no study has analyzed functional differences between paralogous miRNAs with proper experimental methods. In this study, we compared the targets of paralogous miRNAs, miR-221 and miR-222. Using a nuclease-mediated genome engineering technique, we established knockout cell lines for these miRNAs, and precisely analyzed differences in target regulation. We found that miR-221 and miR-222 suppress the previously identified targets, CDKN1B and CDKN1C, differentially. Whereas both miRNAs suppressed CDKN1B, only miR-221 suppressed CDKN1C. From transcriptome analyses, we found that several different target mRNAs were regulated by each of miR-221 and miR-222 independently, although a large number of mRNAs responded commonly to miR-221 and miR-222. This is the first study to compare the mRNA regulations by paralogous miRNAs and illustrate that paralogous miRNAs with the same seed sequence also have difference in target regulation.

Vascular calcification (VC), or calcium deposition inside the blood vessels, is common in patients with atherosclerosis, cardiovascular disease, and chronic kidney disease. Although several treatments are available to reduce calcification, the incidence of VC continues to rise. Recently, there have been several reports describing the regulation of circular RNAs (circRNAs) in various diseases. However, the role of circRNAs in VC has not yet been fully explored. Here, we investigated the function of circSmoc1-2, one of the circRNAs generated from the Smoc1 gene, which is downregulated in response to VC. CircSmoc1-2 is localized primarily to the cytoplasm and is resistant to exonuclease digestion. Inhibition of circSmoc1-2 worsens VC, while overexpression of circSmoc1-2 reduces VC, suggesting that circSmoc1-2 can prevent calcification. We went on to investigate the mechanism of circSmoc1-2 as a microRNA sponge and noted that miR-874-3p, the predicted target of circSmoc1-2, promotes VC, while overexpression of circS-moc1-2 reduces VC by suppressing miR-874-3p. Additionally, we identified the potential mRNA target of miR-874-3p as Adam19. In conclusion, we revealed that the circSmoc1-2/ miR-874-3p/Adam19 axis regulates VC, suggesting that circS-moc1-2 may be a novel therapeutic target in the treatment of VC.