Cardiomyocyte differentiation induced in cardiac progenitor cells by cardiac fibroblast-conditioned medium

Our previous study showed that after being treated with 5-azacytidine, Nkx2.5(+) human cardiac progenitor cells (CPCs) derived from embryonic heart tubes could differentiate into cardiomyocytes. Although 5-azacytidine is a classical agent that induces myogenic differentiation in various types of cells, the drug is toxic and unspecific for myogenic differentiation. To investigate the possibility of inducing CPCs to differentiate into cardiomyocytes by a specific and non-toxic method, CPCs of passage 15 and mesenchymal stem cells (MSCs) were treated with cardiac ventricular fibroblast-conditioned medium (CVF-conditioned medium). Following this treatment, the Nkx2.5(+) CPCs underwent cardiomyogenic differentiation. Phase-contrast microscopy showed that the morphology of the treated CPCs gradually changed. Ultrastructural observation confirmed that the cells contained typical sarcomeres. The expression of cardiomyocyte-associated genes, such as alpha-cardiac actin, cardiac troponin T, and beta-myosin heavy chain (MHC), was increased in the CPCs that had undergone cardiomyogenic differentiation compared with untreated cells. In contrast, the MSCs did not exhibit changes in morphology or molecular expression after being treated with CVF-conditioned medium. The results indicated that Nkx2.5(+) CPCs treated with CVF-conditioned medium were capable of differentiating into a cardiac phenotype, whereas treated MSCs did not appear to undergo cardiomyogenic differentiation. Subsequently, following the addition of Dkk1 and the blocking of Wnt signaling pathway, CVF-conditioned medium-induced morphological changes and expression of cardiomyocyte-associated genes of Nkx2.5(+) CPCs were inhibited, which indicates that CVF-conditioned medium-induced cardiomyogenic differentiation of Nkx2.5(+) CPCs is associated with Wnt signaling pathway. In addition, we also found that the activation of Wnt signaling pathway was accompanied by higher expression of GATA-4 and the blocking of the pathway inhibited the expression of GATA-4 in CVF-conditioned medium-incubated Nkx2.5(+) CPCs. This finding suggests that Wnt signaling pathway may alter GATA-4 expression and activate the cardiogenic program in the regulation of differentiation. In conclusion, Nkx2.5(+) CPCs have enormous potential for cardiomyogenic differentiation and the CVF-conditioned medium specifically induces CPCs to differentiate into a cardiac phenotype. Wnt signaling pathway is involved in CVF-conditioned medium-induced cardiomyogenic differentiation of Nkx2.5(+) CPCs.