Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1alpha

Abstract

Chronic hypoxia induces polycythemia, pulmonary hypertension, right ventricular hypertrophy, and weight loss. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate adaptive responses to hypoxia, including erythropoietin, vascular endothelial growth factor, and glycolytic enzymes. Expression of the HIF-1alpha subunit increases exponentially as O2 concentration is decreased. Hif1a-/- mouse embryos with complete deficiency of HIF-1alpha due to homozygosity for a null allele at the Hif1a locus die at midgestation, with multiple cardiovascular malformations and mesenchymal cell death. Hif1a+/- heterozygotes develop normally and are indistinguishable from Hif1a+/+ wild-type littermates when maintained under normoxic conditions. In this study, the physiological responses of Hif1a+/- and Hif1a+/+ mice exposed to 10% O2 for one to six weeks were analyzed. Hif1a+/- mice demonstrated significantly delayed development of polycythemia, right ventricular hypertrophy, pulmonary hypertension, and pulmonary vascular remodeling and significantly greater weight loss compared with wild-type littermates. These results indicate that partial HIF-1alpha deficiency has significant effects on multiple systemic responses to chronic hypoxia.

Figure 1

Development of polycythemia in mice subjected to chronic hypoxia. Hematocrits of Hif1a^+/+ (open bars) and Hif1a^+/– (closed bars) mice exposed to room air or 10% O₂ for 1–6 weeks were determined. Results are expressed as mean ± SE (n = 8–10 mice for 0–5 weeks; n = 5–7 mice for 6 weeks). ANOVA with a post hoc Dunnet's test revealed a significant difference between genotypes (P = 0.025).

Figure 2

Development of right ventricular hypertrophy in response to chronic hypoxia. The mass ratio of the right ventricle (RV) to left ventricle and septum (LV+S) was determined for the same Hif1a^+/+ (open bars) and Hif1a^+/– (closed bars) mice analyzed in Fig. 1. Results are expressed as mean ± SE. ANOVA with a post hoc Dunnet's test revealed a significant difference between genotypes (P = 0.0001). *P < 0.01 ; **P < 0.001 (Student's t test).

Figure 3

Measurement of right ventricular pressures. Shown are representative polygraph tracings obtained from Hif1a^+/+ and Hif1a^+/– mice exposed to room air (Normoxic) or 10% O₂ (Hypoxic) for 3 weeks.

Figure 4

Right ventricular pressures of normoxic, hypoxic, and reoxygenated mice. Mean right ventricular (RV) pressure (± SE) was determined for Hif1a^+/+ (open bars) and Hif1a^+/– (closed bars) mice exposed to 21% (n = 6) or 10% (n = 11–15) _O for 3 weeks, or exposed to 10% _O for 3 weeks followed by 21% _O for 3 h (n = 5). *P = 0.003; ^#P = NS (Student's t test). NS, not significant.

Figure 5

Pulmonary histology. Lungs from Hif1a^+/+ (a and c) and Hif1a^+/+ (b and d) mice exposed to 21% (a and b) or 10% (c and d) _O for 3 weeks were formalin-fixed, paraffin-embedded, sectioned, and stained with hematoxylin and eosin for videomicroscopy. Each field shows a representative pulmonary arteriole. ×400.

Figure 6

Morphometric analysis of pulmonary vasculature in chronically hypoxic mice. (a) Neomuscularization of pulmonary arterioles. Lung sections from Hif1a^+/+ (open bars) and Hif1a^+/– (closed bars) mice exposed to 10% _O for 3 weeks were scored for nonmuscularized (N), partially muscularized (P), or completely muscularized (C) arterioles with an external diameter of ≤100 μm. For each genotype, >400 vessels were analyzed in lung sections from three to four mice to generate the mean data shown. χ² analysis revealed a significant difference between genotypes (P = 0.00001). (b) Quantitative analysis of medial thickening. Percent wall thickness (% WT) was calculated for completely muscularized arterioles, based on analysis of area or diameter, according to the following formulae: % WT = ([area_ext – area_int] / area_ext) × 100; and % WT = ([diameter_ext – diameter_int] / diameter_ext) × 100. Dimensions were demarcated by the external (ext) and internal (int) elastic laminae. For each genotype, >100 vessels were analyzed in multiple lung sections from three to four mice. Mean values are shown (SD ≤ 0.6% for each). *P < 0.001 (Student's t test).

Figure 7

Analysis of weight gain under normoxic and hypoxic conditions. Percent body weight gain (% BW gain) was determined for Hif1a^+/+ (open bars) and Hif1a^+/– (closed bars) mice exposed to 21% _O for 6 weeks (n = 10) or 10% _O for 1–6 weeks (n = 54–57), using the following formula: % BW gain = ([BW_final – BW_initial] / BW_initial) × 100. *P = 0.02; ^#P = NS (Student's t test).