The human amniotic fluid stem cell secretome triggers intracellular Ca2+ oscillations, NF-κB nuclear translocation and tube formation in human endothelial colony-forming cells

Second trimester foetal human amniotic fluid-derived stem cells (hAFS) have been shown to possess remarkable cardioprotective paracrine potential in different preclinical models of myocardial injury and drug-induced cardiotoxicity. The hAFS secretome, namely the total soluble factors released by cells in their conditioned medium (hAFS-CM), can also strongly sustain in vivo angiogenesis in a murine model of acute myocardial infarction (MI) and stimulates human endothelial colony-forming cells (ECFCs), the only truly recognized endothelial progenitor, to form capillary-like structures in vitro. Preliminary work demonstrated that the hypoxic hAFS secretome (hAFS-CMHypo) triggers intracellular Ca2+ oscillations in human ECFCs, but the underlying mechanisms and the downstream Ca2+-dependent effectors remain elusive. Herein, we found that the secretome obtained by hAFS undergoing hypoxic preconditioning induced intracellular Ca2+ oscillations by promoting extracellular Ca2+ entry through Transient Receptor Potential Vanilloid 4 (TRPV4). TRPV4-mediated Ca2+ entry, in turn, promoted the concerted interplay between inositol-1,4,5-trisphosphate- and nicotinic acid adenine dinucleotide phosphate-induced endogenous Ca2+ release and store-operated Ca2+ entry (SOCE). hAFS-CMHypo-induced intracellular Ca2+ oscillations resulted in the nuclear translocation of the Ca2+-sensitive transcription factor p65 NF-kappa B. Finally, inhibition of either intracellular Ca2+ oscillations or NF-kappa B activity prevented hAFS-CMHypo-induced ECFC tube formation. These data shed novel light on the molecular mechanisms whereby hAFS-CMHypo induces angiogenesis, thus providing useful insights for future therapeutic strategies against ischaemic-related myocardial injury.