Hypoxia-lnducible Factor 1: Molecular Physiology: HIF-1 Expression in the LungWe reasoned that the increase in cardiac mass and work would require an increase in vascularization to supply the myocardium with adequate perfusion. Capillary morphometry demonstrated significant increases in the mean capillar) density, minimal capillary diameter, and capillary: myocardial volume ratio and a significant decrease in intercapillary distance in the hearts of six anemic compared with six control fetuses. Mean VEGF mRNA, VEGF protein, and HIF-la protein levels were each significantly increased threefold to fivefold in response to anemia, whereas HIF-1 P levels showed a nonsignificant increase. These data suggest the following physiologic pathway: anemia results in hypoxemia, which is sensed by the carotid body. Neurohumoral signals to the heart stimulate myocardial inotropy and hypertrophy necessary for the chronically increased stroke volume and cardiac output that provide systemic compensation for the reduction in blood 02-carrying capacity. However, the basal rate of myocardial angiogenesis is not sufficient to maintain adequate oxygenation under these conditions, resulting in a local myocardial hypoxia that stimulates HIF-la expression. Increased levels of HIF-1 are associated with in creased VEGF gene transcription, increased levels of VEGF mRNA and protein, and increased angiogenesis.
HIF-1 Expression in the Lung
To study the kinetics of HIF-1 induction in vivo, we utilized a perfused and ventilated ferret lung preparation in which the pulmonary circulation was isolated in situ and the lungs were ventilated with either 16% or 0% 02. www.canadian-familypharmacy.com this Whereas HIF-1 P protein was constitutively expressed and levels did not vary as a function of inspired 02 concentration, HIF-la protein was undetectable in lungs ventilated with 16% 02 and was induced upon exposure to 0% 02 with maximal expression at 4 h of continuous hypoxia and overall kinetics remarkably similar to those observed in tissue culture cells (AY Yu, MD, CM Wiener, MD, GL Semenza, MD, et al; unpublished data; 1997). Upon reoxygenation, HIF-la protein levels decayed within 1 min, thus demonstrating remarkable posthypoxic instability. Expression of HIF-la also varied as a function of inspired 02 concentration in vivo.
Analysis of HIF-la protein, HIF-ip protein, and HIF-1 DNA-binding activity in either primary or transformed cultures of a variety of pulmonary parenchymal and vascular cell types revealed low or undetectable levels in cells exposed to 20% 02 and a marked induction in cells exposed to 1% 02 for 4 h (AY Yu, MD, CM Wiener, MD, GL Semenza, MD, et al; unpublished data; 1997). Among the cell types tested were the following: alveolar macrophage, bronchial epithelial, and type II alveolar cells; aortic, pulmonary arterial, and pulmonary microvascular endothelial cells; and aortic and pulmonary arterial smooth muscle cells. Pulmonary artery smooth muscle cells differed from all other cell types in demonstrating high levels of HIF-la protein, HIF-1 p protein, and HIF-1 DNA-binding activity when exposed to 20% 02. This is an intriguing observation that warrants further investigation.