SIRTUIN 1 PLAYS A CRUCIAL ROLE IN MODULATING THE PULMONARY AND SYSTEMIC HYPOXIC RESPONSE IN MICE: NOVEL ROLE FOR HYPOXIA INDUCIBLE FACTOR-3ALPHA?

Abstract

BACKGROUND: Pulmonary Hypertension (PH) is caused by occlusive remodelling of pulmonary arterioles leading to increased pulmonary vascular resistance, right ventricle hypertrophy and eventually failure. Sirtuin (Sirt)-1 is an NAD+-dependent deacetylase that has been strongly implicated in maintaining endothelium homeostasis in systemic vessels, but little is known about its role in hypoxia sensing and the lung vasculature. Thus, the goal of this study was to understand the mechanisms by which Sirt1 regulates the pulmonary vascular and systemic response to chronic hypoxia (CH)-induced PH. METHODS AND RESULTS: Mice lacking Sirt1 catalytic activity (sirt1Y/Y, H355Y) and their wild type (WT) littermates were exposed to chronic hypoxia (10% O2) for 1, 7, 14 or 21 days. Sirt1Y/Y exhibited an exaggerated increase in right ventricle systolic pressure (RVSP), apparent within the first week of hypoxic exposure (Day 7: 33 2 sirt1Y/Y vs. 27 2 WT; n1⁄48-10, p< 0.05) which progressively increased over the 3 week CH exposure period (41.5 1.8 sirt1Y/Y vs. 29.7 0.8 WT; n1⁄427 both, p< 0.001). Right ventricular (RV) hypertrophy, assessed by the RV/LV+S weight ratio was also increased (Day 7: 0.52 0.01 sirt1Y/Y vs. 0.40 0.02 WT, n1⁄48-10; p< 0.0001). Hemtaocrit levels were similar in Sirt1Y/Y and WT mice at baseline; however, there was a delayed increase after three weeks of CH in Sirt1Y/Y mice relative to WT mice (Day 7: 55 2% sirt1Y/Y vs. 52 2% WT, n1⁄48-9) (Day 21: 71 2% sirt1Y/Y vs. 63 1% WT, n1⁄417 both; p<0.001). Plasma levels of erythropoietin (EPO), a hypoxic responsive protein responsible for hematopoiesis, were assessed by ELISA at each time point. EPO was markedly increased at Day 7 in Sirt1Y/Y vs. WT mice (492 240 vs. 81 16pg/mL, respectively; n1⁄47-9, p< 0.05) but normalized at later time points (Day 21: 75 9 sirt1Y/Y vs. 96 12pg/mL WT; n1⁄44 both). Lung expression of the hypoxia inducible factors (HIF) 1a and 2a subunits was not increased by western blot analysis. Moreover, HIF1a and HIF2a responsive target genes showed no increase in transcript levels. However, there was a dramatic increase in HIF3a expression in Sirt1 mutant vs. WT lungs, which has been implicated both in repression of other HIFs transcription, activity as well as a direct mediator hypoxic signaling. CONCLUSION: Our data supports a role for Sirt1 in modulating the response to hypoxia, with loss of deacetylation activity leading to an exaggerated pulmonary and systemic induction of the hypoxic response genes.This appears to bemediatedbyHIF3a as a potential novel downstream target for Sirt1 activity in hypoxia. CIHR