561 Pulmonary Arterial Hypertension in S100A1 KO Mice Encompasses Endothelial Cell Dysfunction, Impaired Nitric Oxide Production and Apoptosis

generated mice with SMC-specific Nampt deletion, using Namptflox/flox mice that express Cre under the control of the smooth muscle myosin heavy chain promoter. The progeny were born in normal Mendelian ratios and there was no evidence for vascular malformations or rupture in either neonates or adults. The muscular wall of the urinary bladder was thinned but the thickness of the aortic media in mice with SMC Nampt deletion was similar to that of control mice. However, a decrease in nuclear granularity was noted in the SMCs of the aortas of Namptflox/flox Cre mice. To determine the response to oxidizing stress, we infused Angiotensin II (1.44 mg/kg/day) by osmotic minipump for 28 days. Assessment of pressure-fixed abdominal and thoracic aortas revealed medial hemorrhage in 5 of 8 Namptflox/flox-Cre mice, compared to 0 of 5 SMC Nampt-hemizygous mice and 0 of 7 wild-type mice. As well, there was less collagen elaborated in response to Ang II in the aortic media of mutant mice. To further evaluate the role Nampt in stress resistance, we generated Namptflox/flox;CreERT2 mice from which embryonic fibroblasts were harvested and subjected to 4-OH-tamoxifen to conditionally delete Nampt. Nampt-knockout MEFs exposed to x-irradiation (10 Gy) accumulated 10-fold more -H2A.X foci, a marker of DNA strand break, compared to MEFs not subjected to Nampt deletion. This was followed by significantly more cell death in Nampt-knockout MEFs. We further determined that whereas PARP1 expression was unchanged by Nampt gene deletion, PARP1 activity, determined by the accumulation of poly-ADP-ribose chains, was markedly suppressed. CONCLUSION: Loss of Nampt in vascular SMCs predisposes to medial dissection of the aorta and increased DNA damage. Nampt-driven NAD generation thus constitutes a novel means of maintaining vascular stability during stress.