COL4A1 and atrial fibrillation: Furthermore, co-culturing with miR-21 inhibitor-treated AF-induced C28/I2 and C20A4 cells exhibited induced expression levels of chondrogenesis markers, namely GDF-5, SOX5, TGF-β1, COL4A1, and ACAN proteins in comparison with the miR-21 mimic and NC control cells (Figure 2D).