Gene expression of the liver in response to chronic hypoxia

Abstract

Hypoxia is an important ecological, evolutionary, and biomedical stressor. While physiological acclimatization of mammals to hypoxia is well studied, the variation in gene expression that underlies acclimatization is not well studied. We acclimatized inbred mice for 32 days to hypoxic conditions that simulated altitudes of 1400, 3000, and 4500 m. We used oligonucleotide microarrays to measure changes in steady-state abundance of mRNA in the livers of these mice. Mice exposed to more severe hypoxia (simulated altitude of 4500 m) were smaller in mass and had higher hematocrit than mice exposed to less severe hypoxia. ANOVA and false discovery rate tests indicated that 580 genes were significantly differentially expressed in response to chronic hypoxia. Few of these 580 genes have previously been reported to respond to hypoxia. In contrast, many of these 580 genes belonged to same functional groups typically respond to acute hypoxia. That is, both chronic and acute hypoxia elicit changes in transcript abundance for genes involved in angiogenesis, glycolysis, lipid metabolism, carbohydrate metabolism, and protein amino acid phosphorylation, but the particular genes affected by the two types of hypoxia were mostly different. Numerous genes affecting the immune system were differentially expressed in response to chronic hypoxia, which supports recently proposed hypotheses that link immune function and hypoxia. Furthermore, our results discovered novel elevated mRNA abundance of genes involved in hematopoiesis and oxygen transport not reported previously, but consistent with extreme hematocrits found in hypoxic mice.

Keywords: Mus musculus; hematopoiesis; immune system; leptin receptor; microarray.

Fig. 1.

Principal component analysis of the 8 Affymetrix arrays. Note that 99.5% of the experimental variation is associated with the first principal component, and it indicates the separation between the highest altitude group and the other 2 groups.

Fig. 2.

Mean body mass of mice over 30 days of residence in hypoxia chambers simulating 3 altitudes. By day 9, 4,500 m mice were significantly smaller (20.38 ± 1.20 g) than 3,000 m mice (22.37 ± 1.35 g), and both 4,500 m and 3,000 m mice were significantly smaller than 1,400 m mice (24.24 ± 1.35 g) (P < 0.001). This pattern remained until the end of the experiment, but the differences between the 3,000 and 4,500 m experiments were no longer significant. After 30 days, mice in the 4,500 m (23.9 ± 0.91 g) and 3,000 m (24.6 ± 1.40 g) chambers were significantly smaller than mice in the 1,400 m chamber (26.6 ± 1.34 g) (P < 0.001).

Fig. 3.

Mean hematocrits (Hct) of mice after 32 days in hypoxia chambers. All Hct are significantly different (P < 0.001) with the 4,500 m mice having a mean hematocrit of 80.9% RBC (± 3.4%), the 3,000 m group having a mean Hct of 60.6% RBC (± 1.9%), and the 1,400 m group having a Hct of 56.1% (± 1.4%). RBC, red blood cell.

Fig. 4.

Heat map of 580 differentially expressed probe sets. Values are normalized across the 8 arrays and clustered via a simple hierarchical clustering procedure.

Fig. 5.

Confirmatory real-time qRT-PCR of 6 transcripts. Comparison between the expression values from the Affymetrix GeneChip Mouse Expression Set 430 and expression values from quantitative real-time PCR. LepR, leptin receptor; FABP, fatty acid binding protein; RGP5, regulator of G protein signaling 5; P450, cytochrome P450 family 4, subfamily a, protein 14; HbbA, hemoglobin-α; APO, apolipoprotein.