CD36 is indispensable for thermogenesis under conditions of fasting and cold stress

Abstract

Hypothermia can occur during fasting when thermoregulatory mechanisms, involving fatty acid (FA) utilization, are disturbed. CD36/FA translocase is a membrane protein which facilitates membrane transport of long-chain FA in the FA consuming heart, skeletal muscle (SkM) and adipose tissues. It also accelerates uptake of triglyceride-rich lipoprotein by brown adipose tissue (BAT) in a cold environment. In mice deficient for CD36 (CD36(-/-) mice), FA uptake is markedly reduced with a compensatory increase in glucose uptake in the heart and SkM, resulting in lower levels of blood glucose especially during fasting. However, the role of CD36 in thermogenic activity during fasting remains to be determined. In fasted CD36(-/-) mice, body temperature drastically decreased shortly after cold exposure. The hypothermia was accompanied by a marked reduction in blood glucose and in stores of triacylglycerols in BAT and of glycogen in glycolytic SkM. Biodistribution analysis using the FA analogue (125)I-BMIPP and the glucose analogue (18)F-FDG revealed that uptake of FA and glucose was severely impaired in BAT and glycolytic SkM in cold-exposed CD36(-/-) mice. Further, induction of the genes of thermogenesis in BAT was blunted in fasted CD36(-/-) mice after cold exposure. These findings strongly suggest that CD36(-/-) mice exhibit pronounced hypothermia after fasting due to depletion of energy storage in BAT and glycolytic SkM and to reduced supply of energy substrates to these tissues. Our study underscores the importance of CD36 for nutrient homeostasis to survive potentially life-threatening challenges, such as cold and starvation.

Keywords: Brown adipose tissue; CD36; Fatty acid; Hypothermia; Skeletal muscle.

Fig. 1

Cold intolerance in CD36^−/− mice with prior fasting. Cold tolerance was tested for a maximum of 4 h or until body temperature dropped below 25 °C as described in Methods. Body temperature was measured from the shaved mid-dorsal body surface. Fasting was for 20 h n = 5/group. *p < 0.05. RT, room temperature (22 °C); CT, cold temperature (4 °C).

Fig. 2

Blood energy substrates and uptake of glucose and NEFAs by BAT and SkM of fasted WT and CD36^−/− mice before and after a 2 h cold exposure. (A to D) Blood was collected from the retro-orbital plexus before and 2 h after cold exposure to measure serum levels of glucose (A), NEFAs (B), TG (C), and ketone bodies (D) in the fasted states. n = 5—6/group. (E to H) The mice received intravenous injections of ¹⁸F-FDG (100 kBq) and ¹²⁵I-BMIPP (5 kBq) via the lateral tail vein after a 20 h fast. The mice were maintained at room temperature or in cold rooms (4 °C) for 2 h and then sacrificed to isolate tissues and determine uptake of ¹⁸F-FDG (E and F) and ¹²⁵I-BMIPP (G and H) by BAT (E and G) and SkM (F and H) (n = 4—6/group). *p < 0.05; **p < 0.01; ***p < 0.001. RT, room temperature; CT, cold temperature.

Fig. 3

Storage of TG in BAT and glycogen in SkM in fasted WT and CD36^−/− mice after a 2 h cold exposure. Interscapular BAT and quadriceps femoris muscle were isolated before and 2 h after cold exposure (4 °C) of 20 h fasted mice. (A) The ratio of the BAT weight to the body weight (B) TG concentration in the BAT. (C) Glycogen content in the muscle was normalized by protein concentration, n = 4—5/group. *p < 0.05; **p < 0.01; ***p < 0.001. RT, room temperature; CT, cold temperature.

Fig. 4

Induction of genes associated with thermogenesis in BAT of fasted WT and CD36^−/− mice after a 2 h cold exposure. The mice were maintained at room temperature or in a cold room (4 °C) for 2 h with or without prior fasting. The total RNA from BAT was extracted for quantitative real-time PCR. n = 4—5/group. *p < 0.05; **p < 0.01; ***p < 0.001. RT, room temperature; CT, cold temperature.