Bone morphogenetic protein signaling is required for RAD51-mediated maintenance of genome integrity in vascular endothelial cells.

The integrity of blood vessels is fundamental to vascular homeostasis. Inactivating mutations in the bone morphogenetic protein (BMP) receptor type II (BMPR2) gene cause hereditary vascular disorders, including pulmonary arterial hypertension and hereditary hemorrhagic telangiectasia, suggesting that BMPR2 and its downstream signaling pathway are pivotal to the maintenance of vascular integrity through an unknown molecular mechanism. Here we report that inactivation of BMPR2 in pulmonary vascular endothelial cells results in a deficit of RAD51, an enzyme essential for DNA repair and replication. Loss of RAD51, which causes DNA damage and cell death, is also detected in animal models and human patients with pulmonary arterial hypertension. Restoration of BMPR2 or activation of the BMP signaling pathway rescues RAD51 and prevents DNA damage. This is an unexpected role of BMP signaling in preventing the accumulation of DNA damage and the concomitant loss of endothelial integrity and vascular remodeling associated with vascular disorders.

Extracellular ATP protects endothelial cells against DNA damage.

Cell damage can lead to rapid release of ATP to extracellular space resulting in dramatic change in local ATP concentration. Evolutionary, this has been considered as a danger signal leading to adaptive responses in adjacent cells. Our aim was to demonstrate that elevated extracellular ATP or inhibition of ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1/CD39) activity could be used to increase tolerance against DNA-damaging conditions. Human endothelial cells, with increased extracellular ATP concentration in cell proximity, were more resistant to irradiation or chemically induced DNA damage evaluated with the DNA damage markers γH2AX and phosphorylated p53. In our rat models of DNA damage, inhibiting CD39-driven ATP hydrolysis with POM-1 protected the heart and lung tissues against chemically induced DNA damage. Interestingly, the phenomenon could not be replicated in cancer cells. Our results show that transient increase in extracellular ATP can promote resistance to DNA damage.

Loss of PPARγ in endothelial cells leads to impaired angiogenesis.

Tie2-promoter-mediated loss of peroxisome proliferator-activated receptor gamma (PPARγ, also known as PPARG) in mice leads to osteopetrosis and pulmonary arterial hypertension. Vascular disease is associated with loss of PPARγ in pulmonary microvascular endothelial cells (PMVEC); we evaluated the role of PPARγ in PMVEC functions, such as angiogenesis and migration. The role of PPARγ in angiogenesis was evaluated in Tie2CrePPARγ(flox/flox) and wild-type mice, and in mouse and human PMVECs. RNA sequencing and bioinformatic approaches were utilized to reveal angiogenesis-associated targets for PPARγ. Tie2CrePPARγ(flox/flox) mice showed an impaired angiogenic capacity. Analysis of endothelial progenitor-like cells using bone marrow transplantation combined with evaluation of isolated PMVECs revealed that loss of PPARγ attenuates the migration and angiogenic capacity of mature PMVECs. PPARγ-deficient human PMVECs showed a similar migration defect in culture. Bioinformatic and experimental analyses newly revealed E2F1 as a target of PPARγ in the regulation of PMVEC migration. Disruption of the PPARγ-E2F1 axis was associated with a dysregulated Wnt pathway related to the GSK3B interacting protein (GSKIP). In conclusion, PPARγ plays an important role in sustaining angiogenic potential in mature PMVECs through E2F1-mediated gene regulation.