Nicotinamide phosphoribosyltransferase activates the mitochondrial unfolded protein response to promote pulmonary arterial endothelial cell proliferation

Vascular remodeling leading to occlusion of the pulmonary artery and increased pulmonary vascular resistance is the central feature of Pulmonary Arterial Hypertension (PAH) which leads to death due to right heart failure. The expression of Nicotinamide phosphoribosyltransferase (Nampt) is increased in the lungs and isolated PAECs of PAH patients. Inhibition of NAMPT is protective in preclinical models of pulmonary hypertension. NAMPT regulates multiple mitochondrial processes that control cell proliferation and survival. Using rodent models of PH, we demonstrated that the mitochondrial unfolded protein response (UPRmt) promotes vascular remodeling. Hence we, hypothesized that NAMPT activates the UPRmt to promote abnormal pulmonary arterial endothelial cell proliferation. Analysis of PAECs isolated from PAH patients show increased expression of the UPRmt pathway mediators, ATF-5, mtHSP70 and ClpP. Human PAEC cell lines were exposed to recombinant Nampt or transduced with lentiviral-Nampt. We observed increased phosphorylation and activation of eIF2 which permits preferential translation of the ATF-5 transcription factor. ATF-5 expression and nuclear localization was increased. Further, we observed increased expression of ATF-5 target genes, mtHSP70, HSP60, ClpP and LonP1 as well as increased PAEC proliferation. Blocking mtHSP70 function reversed the Nampt induced increase in proliferation. Neither UPRmt activation nor proliferation was increased in the presence of enzymatically inactive-Nampt. Our observations demonstrate that Nampt can promote PAEC proliferation by activating the UPRmt and uncovers novel potential targets to address vascular remodeling in PAH.