Dysregulation of VEGF-induced proangiogenic Ca2+ oscillations in primary myelofibrosis-derived endothelial colony-forming cells

Endothelial progenitor cells could be implicated in the aberrant neoangiogenesis that occurs in bone marrow and spleen in patients with primary myelofibrosis (PMF). However, antivascular endothelial growth factor (VEGF) monotherapy had only a modest and transient effect in these individuals. Recently it was found that VEGF-induced proangiogenic intracellular Ca2+ oscillations could be impaired in endothelial progenitor cells of subjects with malignancies. Therefore, we employed Ca2+ imaging, wavelet analysis, and functional assays to assess whether and how VEGF-induced Ca2+ oscillations are altered in PMF-derived endothelial progenitor cells. We focused on endothelial colony-forming cells (ECFCs), which are the only endothelial progenitor cell subtype capable of forming neovessels both in vivo and in vitro. VEGF triggers repetitive Ca2+ spikes in both normal ECFCs (N-ECFCs) and ECFCs obtained from PMF patients (PMF-ECFCs). However, the spiking response to VEGF is significantly weaker in PMF-ECFCs. VEGF-elicited Ca2+ oscillations are patterned by the interaction between inositol-1,4,5-trisphosphate-dependent Ca2+ mobilization and store-operated Ca2+ entry. However, in most PMF-ECFCs, Ca2+ oscillations are triggered by a store-independent Ca2+ entry pathway. We found that diacylglycerol gates transient receptor potential canonical 1 channel to trigger VEGF-dependent Ca2+ spikes by recruiting the phospholipase C/inositol-1,4,5-trisphosphate signaling pathway, reflected as a decrease in endoplasmic reticulum Ca2+ content. Finally, we found that, apart from being less robust and dysregulated as compared with N-ECFCs, VEGF-induced Ca2+ oscillations modestly stimulate PMF-ECFC growth and in vitro angiogenesis. These results may explain the modest effect of anti-VEGF therapies in PMF.