Endothelial cells form the inner lining of blood vessels and are integral to the maintenance of vascular homeostasis. Precursors of endothelial cells, namely endothelial progenitor cells (EPCs) contribute to this process via vasculogenesis where they act in a paracrine manner secreting pro-angiogenic factors or by differentiating into mature endothelial cells (ECs) to repair or form new vessels. Because EPCs have been shown to be regulators of both physiological and pathological vasculogenesis these cells have become the target of over 200 clinical trials aiming to control the biggest killers worldwide, cancer and cardiovascular disease. To this end, elucidation of how EPCs and, in turn, ECs are regulated is necessary.
The role of transcription factors as regulators of cellular function has been widely described; herein we aim to implicate peroxisome proliferator-activated receptor gamma (PPARγ), a transcriptional member of the nuclear receptor super family, and a specific ligand and co-activator signature in endothelial biology. Endogenous levels of PPARγ protein and mRNA are detectable in human EPCs and ECs, with significantly elevated levels detected in EPCs. Importantly, in both EPCs and ECs a proportion of PPARγ protein was shown to be localised to the nucleus, suggesting the presence of transcriptionally active PPARγ in these cells. In addition, pharmacological manipulation of PPARγ altered vascular cell phenotype and cell function in Matrigel, a tube formation assay that mimics in vivo vasculogenesis.
Herein, we reveal a previously undescribed PPARγ regulatory complex in EPCs and ECs, with data suggesting that the bioactive second messenger, sphingosine-1-phosphate (S1P) plays a role in PPARγ gene regulation by binding to and activating PPARγ. In response to this activation the co-activator, PPARγ co-activator 1 beta is recruited, binds to the complex and potentially contributes to PPARγ’s target gene specificity. Taken together, this new regulatory system controls vascular cell phenotype and function in a manner distinct from its control of metabolism. More importantly, our in vivo data suggest that PPARɣ is a new target for controlling vasculogenesis in disease.