Chronic hypoxia reduces cold tolerance via hepatic ANGPTL3-driven mitochondrial damage and adipose thermogenesis suppression

To investigate the role and mechanism of hepatic-derived angiopoietin-like 3(ANGPTL3) in mediating reduced cold tolerance induced by chronic hypoxic exposure.

① Male Sprague-Dawley (SD) rats (7 to 8 weeks old, weighing 220 to 260 g) were randomly divided into a normoxic group (N, housed at an ambient condition of altitude 308 m and 24.0±0.5 ℃) and a hypoxic group (H, housed in a hypobaric hypoxic chamber of simulating an altitude 5800 m and 24.0±0.5 ℃), with 20 rats in each group. Twelve rats from each group were implanted intraperitoneally with temperature-sensing transponders for subsequent measurement of core body temperature (Tc). ② Male SD rats were randomly divided into (n=20 per group): a hypoxic (H) group and a hypoxic+ANGPTL3 inhibitor [Vupanorsen 2 mg/(kg·w)] intervention (H+V) group. Twelve rats from each group were implanted intraperitoneally with temperature-sensing transponders for subsequent Tc measurement. The rats in the H and H+V groups were all housed in a hypobaric hypoxic chamber (simulated altitude 5800 m, 24.0±0.5 ℃). After 6 weeks, 8 non-implanted rats per group were anesthetized for blood and liver tissue collection. Serum aspartate aminotransferase (AST) level was measured using an automatic biochemical analyzer. Blood levels of hepatic factors were detected with ELISA. Hepatic mitochondrial morphology was assessed by transmission electron microscopy (TEM). Hepatic ANGPTL3 protein level was quantified by Western blotting. The 12 implanted rats from each group were exposed to either a hypoxic cold environment (5800 m, 6 ℃ for 24 h) or a normoxic cold environment (308 m, -10 ℃ for 6 h). Tc was monitored using a physiological signal telemetry system to assess cold tolerance. Western blotting was used to detect the protein level uncoupling protein 1(UCP1) in subcutaneous white adipose tissue as an indicator of browning potential.

① Compared to normothermic hypoxia (5800 m, 24 ℃), the group H exhibited a significant Tc decrease during hypoxic cold exposure (5800 m, 6 ℃)(P < 0.05). ② During normoxic cold exposure (308 m, -10 ℃, 6 h), Tc was significantly decreased in both groups N and H, and it was obviously lower in the group H than the group N (P < 0.05). ③ Compared to the N group, serum ANGPTL3 level (t=7.3, df=10, 95%CI: 2.553 to 2.687, P < 0.0001) and hepatic ANGPTL3 protein level (t=6.532, df=10, 95%CI: 26.98 to 54.92, P < 0.0001) were notably elevated in the group H. ④ Compared to the H group, serum ANGPTL3 level (t=7.803, df=10, 95%CI: -196.8 to -109.4, P=0.0004) and hepatic ANGPTL3 protein level (t=3.514, df=10, 95%CI: -66.14 to -14.81. P=0.006) were remarkably decreased in the group H+V. ⑤ During normoxic cold exposure (308 m, -10 ℃, 6 h), Tc was significantly decreased in both groups H and H+V, with that of the group H+V significantly higher that of the group H (95%CI: -0.629 to -0.452, P=0.0004). ⑥ Under hypoxic cold exposure (5800 m, 6 ℃, 24 h), Tc was decreased significantly in both groups H and H+V, with that in the H+V group higher than that of the group H (95%CI: -0.568 to -4.431, P=0.0181). ⑦ Compared to the N group, hepatocyte mitochondrial morphology was significantly damaged and serum AST level was significantly increased (t=14.15, df=14, 95%CI: 51.61 to 70.04, P < 0.0001) in the group H. ⑧ Compared to the group H, hepatocyte mitochondrial morphology was significantly improved and the serum AST level was significantly decreased (t=4.879, df=14, 95%CI: -47.24 to -18.39, P=0.0002) in the group H+V. ⑨ Following hypoxic cold exposure (5800 m, 6 ℃, 24 h), the protein level of UCP1 in subcutaneous white adipose tissue was significantly increased in the group H+V than the group H (t=2.26, df=10, 95%CI: 0.849 to 118.1, P=0.047).

Chronic hypoxic exposure increases hepatic ANGPTL3 production and release into blood circulation, subsequently impairing cold tolerance. This effect is mediated through ANGPTL3-induced hepatic mitochondrial damage and suppression of white adipose tissue browning potential.