HIF, HAPE, and Hilltop rats: a paradox unfolding

Abstract

Hilltop rats develop a pathological syndrome resembling chronic mountain sickness (CMS) in humans. Manifestations of CMS include polycythemia, pulmonary vascular remodeling, pulmonary hypertension and right ventricular hypertrophy. Paradoxically, Hilltop rats are more resistant to acute hypoxic pulmonary hypertension and high altitude pulmonary edema (HAPE) than control Madison rats. Hypoxia Inducible Factor-1α (HIF-1α) is exquisitely coupled to cellular hypoxia, enhancing expression of erythropoietin (EPO), vascular endothelial growth factor (VEGF) and inducible nitric oxide synthase (NOS II), a potent generator of nitric oxide. Therefore, it was hypothesized that differences in HIF-1α activity or HIF-1 induced proteins would explain, in part, the apparent paradox. Methods: Hilltop and Madison rats were subjected to zero, 6 or 18 hours of acute hypobaria (18,000 ft). Pulmonary edema was documented by blood free ww/dw (BFww/dw) ratios. Lung nuclear or total protein extracts were assessed by immunoblot assay for HIF-1α, HIF-1β, NOS II, endothelial NOS (NOS III), and VEGF expression. Electrophoretic mobility shift assay and enzyme linked immunosorbant assay determined HIF-1 DNA-binding activity. Results: Hilltop rats had lower BFww/dw after 18 hours of hypoxic exposure (p<0.001). There was no difference between strains in baseline HIF-1α and NOS II proteins. Neither strain increased HIF-1α after 18 hours of hypoxia, but Hilltop rats showed greater expression of HIF-1β and HIF- 1 DNA-binding activity (p<0.05 and p<0.01). Hilltop rats expressed greater NOS II and VEGF after 6 and 18 hours of hypoxia (p<0.01 and p<0.001), whereas NOS II decreased and VEGF did not change in Madison rats. Surprisingly, Hilltop rats expressed negligible NOS III except briefly at 6 hours of hypoxia, compared to Madison rats, which expressed abundant baseline NOS III, but exhibited progressive hypoxic downregulation. Conclusion: Hilltop rats may be more resistant to HAPE than Madison rats because they are molecular "high-responders" to hypoxia, having greater HIF-1 activity, inducing greater NOS II expression. They may also benefit from reduced dependence on normoxic NOS Ill-generated NO-mediated pulmonary vasodilation. The Hilltop paradox may be a manifestation of this "high-response" profile, as acutely exaggerated HIF-1-induced NOS II reduces HAPE risk, but chronically excessive HIF-1 mediated VEGF and EPO may increase risk for CMS.