Organ-specific fuel rewiring in acute and chronic hypoxia redistributes glucose and fatty acid metabolism

Abstract

Oxygen deprivation can be detrimental. However, chronic hypoxia is also associated with decreased incidence of metabolic syndrome and cardiovascular disease in high-altitude populations. Previously, hypoxic fuel rewiring has primarily been studied in immortalized cells. Here, we describe how systemic hypoxia rewires fuel metabolism to optimize whole-body adaptation. Acclimatization to hypoxia coincided with dramatically lower blood glucose and adiposity. Using in vivo fuel uptake and flux measurements, we found that organs partitioned fuels differently during hypoxia adaption. Acutely, most organs increased glucose uptake and suppressed aerobic glucose oxidation, consistent with previous in vitro investigations. In contrast, brown adipose tissue and skeletal muscle became "glucose savers," suppressing glucose uptake by 3-5-fold. Interestingly, chronic hypoxia produced distinct patterns: the heart relied increasingly on glucose oxidation, and unexpectedly, the brain, kidney, and liver increased fatty acid uptake and oxidation. Hypoxia-induced metabolic plasticity carries therapeutic implications for chronic metabolic diseases and acute hypoxic injuries.

Keywords: PET scan; TCA cycle; fatty acid metabolism; fuel rewiring; fuel uptake; glucose metabolism; hypoxia; isotope tracing; organ-specific metabolism.

Figure 1.. Acute hypoxia causes perturbations in spontaneous movement and motor coordination that attenuate over time.

(A) Representative movement traces of mice housed at 21% or 8% F_iO₂ and transferred to openfield chambers of matching F_iO₂. (B-D) (B) Total distance covered, (C) average speed, and (D) total resting time of mice placed in openfield chambers over 10 minutes. Mice were housed at 21% (grey), 11% (red), 8% (blue) F_iO₂ for 3 hours, 24 hours, 1 week, or 3 weeks. (E-F) Latency to descend (in seconds) down a pole after being placed at the top facing down (E) or facing up (F). Statistics were calculated using two-way ANOVA and post-hoc Tukey correction. N = 8 biological replicates. *p<0.5, **p<.01, ***p<.001, ****p<.0001.

Figure 2.. Physiological adaptation to hypoxia includes systemic metabolic rewiring.

(A-B) (A) Blood hemoglobin concentrations and (B) Total CO₂ measured from tail vein blood samples of mice housed in 21% (grey), 11% (red), or 8% F_iO₂ (blue) for 3 hours, 24 hours, 1 week, or 3 weeks. Reduced TCO₂ levels indicate hyperventilation. (C) Highest detectable body temperatures measured with an infrared camera daily for one week and every 2–4 days for the following 2 weeks. Mean ± SEM are shown. For 11% F_iO₂ mice, body temperatures were significantly different from normoxic mice on days 7 and 13. For 8% F_iO₂ mice, body temperatures were significantly different from normoxic mice from 3 hours to 4 days and on days 6, 7, and 13. (D) Percent change in body weight from baseline. Mean ± SEM are shown. For 11% F_iO₂ mice, differences in body weight change were statistically significant from day 1 to 4 and from day 7 to 20 when compared to normoxic mice. For 8% F_iO₂ mice, differences in body weight change were statistically significant from day 1 to 20 when compared to normoxic mice. (E) Food consumption per mouse per day. N = 2 cages. Mean ± SEM are shown. For 11% F_iO₂ mice, food consumption was significantly different from normoxic mice on days 1 and 2. For 8% F_iO₂ mice, food consumption was significantly different from normoxic mice from day 1 to 5. (F) Unfasted plasma levels of leptin and ghrelin from tail blood samples collected from mice housed at 21% F_iO₂ or from mice housed at 8% F_iO₂ for 3 hours, 24 hours, 1 week, or 3 weeks. (G) Blood urea nitrogen (BUN) concentrations measured from tail blood samples. Elevated BUN provides evidence of increased protein degradation. (H) Organ-level partial pressures of oxygen measured by a Clark-type microsensor. When compared to normoxia (grey), all organs exhibited a decreased oxygen tension when exposed to acute hypoxia (light blue), and some experienced a recovery in chronic hypoxia (dark blue). N = 5–9 biological replicates. Mean ± SEM are shown. For (A)-(F), statistics were calculated using two-way ANOVA and post-hoc Tukey correction. For (G), statistics were calculated using two-way ANOVA and post-hoc Dunnett’s test for multiple comparisons to a control group. N = 8 biological replicates for all panels except (E) and (G). *p<0.5, **p<.01, ***p<.001, ****p<.0001.

Figure 3.. Chronic hypoxia causes hypoglycemia and alters organ-specific glucose uptake.

(A) Fed and fasted blood glucose measurements after 3 hours, 24 hours, 1 week, and 3 weeks of hypoxia (8% F_iO₂) treatment (blue). Controls (grey) were housed in 21% F_iO₂. (B) Unfasted plasma levels of insulin and glucagon from tail blood samples collected from mice housed at 21% F_iO₂ or from mice housed at 8% F_iO₂ for 3 hours, 24 hours, 1 week, or 3 weeks. (C) Representative coronal and sagittal images of mouse PET scans conducted 30 minutes after tail vein injection of the glucose analogue 2-Deoxy-2-[¹⁸F]fluoro-D-glucose (FDG). (D) Representative PET scan images of organs extracted from mice 60 minutes after tail vein injection of FDG. Maximum values (% ID/cc) vary per organ: Heart: 40, Brain: 20, Lung: 10, Kidney: 5, Muscle: 7, Liver: 7, White Fat: 2, Brown Fat: 10. (E) Radioactive signal from each organ after extraction as measured by a gamma counter. Values were decay-corrected based on the time of FDG injection and the time of measurement. N = 6–9 biological replicates. Statistics were calculated using one-way ANOVA and post-hoc Tukey correction. *p<0.5, **p<.01, ***p<.001, ****p<.0001. eWAT: epidydimal white adipose tissue, BAT: brown adipose tissue.

Figure 4.. Chronic hypoxia reduces lipid accumulation and alters organ-specific free fatty acid uptake.

(A) Fat mass measured by DEXA scan and weights of eWAT depots in mice housed at 21% (grey), 11% (red), and 8% (blue) F_iO₂. (B) Representative coronal and sagittal images of mouse PET scans conducted 30 minutes after tail vein injection of the palmitate analogue 18-[18F]fluoro-4-thia-palmitate (FTP). (C) Representative PET scan images of organs extracted from mice 60 minutes after tail vein injection of FTP. Maximum values (% ID/cc) vary per organ: Heart: 20, Brain: 20, Lung: 20, Kidney: 50, Muscle: 7, Liver: 65, White Fat: 2, Brown Fat: 15. (D) Radioactive signal from each organ after extraction as measured by a gamma counter. Values were decay-corrected based on the time of FDG injection and the time of measurement. N = 4–8 biological replicates. Statistics were calculated using one-way ANOVA and post-hoc Tukey correction. *p<0.5, **p<.01, ***p<.001, ****p<.0001. eWAT: epidydimal white adipose tissue, BAT: brown adipose tissue.

Figure 5.. Hypoxia rewires metabolic flux of major fuel sources into the TCA cycle

(A) Schematic showing entry of carbons from glucose and palmitate to the TCA cycle. Red circles represent ¹³C-labeled carbons. Labeling patterns correspond to ¹³C-Acetyl CoA passing through one round of the TCA cycle. (B) Heatmap showing the median log fold change in normalized relative enrichment of the isotope label compared to the 21% F_iO₂ condition. Normalized relative enrichment was calculated by determining the fraction of carbons in TCA metabolites that were ¹³C-labeled and subsequently normalizing to the ¹³C enrichment of the fuel (glucose or palmitate) in plasma. Greater normalized relative enrichment indicates increased production of the target metabolite from the injected tracer compared to other sources. Only statistically significant changes are shown. (C-D) Normalized relative enrichment of malate 20 minutes after injection with (C) ¹³C-glucose or (D) ¹³C-palmitate. Mean ± SEM are shown. Statistics were calculated using one-way ANOVA and post-hoc Dunnett’s test for multiple comparisons to a control group. N=4–5 biological replicates. *p<0.5, **p<.01, ***p<.001, ****p<.0001.

Figure 6.. Summary of organ-specific fuel rewiring in physiological adaptation to hypoxia.

Classification of organs that exhibit significant alterations in uptake and mitochondrial oxidation of the circulating fuel sources glucose and palmitate in acute and chronic hypoxia.