Background: Cyanosis and myocardial hypertrophy frequently occur in combination. Hypoxia or cyanosis can be potent inducers of angiogenesis, regulating the expression of hypoxia-inducible factors (HIF), vascular endothelial growth factors (VEGF), and VEGF receptors (VEGFR-1 and 2); in contrast, pressure overload hypertrophy is often associated with impaired pro-angiogenic signaling and decreased myocardial capillary density. We hypothesized that the physiological pro-angiogenic response to cyanosis in the hypertrophied myocardium is blunted through differential HIF and VEGF-associated signaling.
Methods and results: Newborn rabbits underwent aortic banding and, together with sham-operated littermates, were transferred into a hypoxic chamber (FiO(2) = 0.12) at 3 weeks of age. Control banded or sham-operated rabbits were housed in normoxia. Systemic cyanosis was confirmed (hematocrit, arterial oxygen saturation, and serum erythropoietin). Myocardial tissue was assayed for low oxygen concentrations using a pimonidazole adduct. At 4 weeks of age, HIF-1alpha and HIF-2alpha protein levels, HIF-1alpha DNA-binding activity, and expression of VEGFR-1, VEGFR-2, and VEGF were determined in hypoxic and normoxic rabbits. At 6 weeks of age, left-ventricular capillary density was assessed by immunohistochemistry. Under normoxia, capillary density was decreased in the banded rabbits compared to non-banded littermates. As expected, non-hypertrophied hearts responded to hypoxia with increased capillary density; however, banded hypoxic rabbits demonstrated no increase in angiogenesis. This blunted pro-angiogenic response to hypoxia in the hypertrophied myocardium was associated with lower HIF-2alpha and VEGFR-2 levels and increased HIF-1alpha activity and VEGFR-1 expression. In contrast, non-hypertrophied hearts responded to hypoxia with increased HIF-2alpha and VEGFR-2 expression with lower VEGFR-1 expression.
Conclusion: The participation of HIF-2alpha and VEGFR-2 appear to be required for hypoxia-stimulated myocardial angiogenesis. In infant rabbit hearts with pressure overload hypertrophy, this pro-angiogenic response to hypoxia is effectively uncoupled, apparently in part due to altered HIF-mediated signaling and VEGFR subtype expression.