FGF2 and atrial fibrillation: These results imply that the productivity of paracrine factors secreted from AF-MSCs was stimulated by bFGF and selenium and dramatically boosted by their combination, which was proven by the elevated mRNA levels of TGF-β, VEGF, and IL-6, regarded as key paracrine factors for tissue repair (Fig. 5a) [56], and the high levels of the corresponding proteins in the AF-MSC-CMs (Fig. 5b).