Exercise- and training-induced upregulation of skeletal muscle fatty acid oxidation are not solely dependent on mitochondrial machinery and biogenesis

Abstract

Regulation of skeletal muscle fatty acid oxidation (FAO) and adaptation to exercise training have long been thought to depend on delivery of fatty acids (FAs) to muscle, their diffusion into muscle, and muscle mitochondrial content and biochemical machinery. However, FA entry into muscle occurs via a regulatable, protein-mediated mechanism, involving several transport proteins. Among these CD36 is key. Muscle contraction and pharmacological agents induce CD36 to translocate to the cell surface, a response that regulates FA transport, and hence FAO. In exercising CD36 KO mice, exercise duration (-44%), and FA transport (-41%) and oxidation (-37%) are comparably impaired, while carbohydrate metabolism is augmented. In trained CD36 KO mice, training-induced upregulation of FAO is not observed, despite normal training-induced increases in mitochondrial density and enzymes. Transfecting CD36 into sedentary WT muscle (+41%), comparable to training-induced CD36 increases (+44%) in WT muscle, markedly upregulates FAO to rates observed in trained WT mice, but without any changes in mitochondrial density and enzymes. Evidently, in vivo CD36-mediated FA transport is key for muscle fuel selection and training-induced FAO upregulation, independent of mitochondrial adaptations. This CD36 molecular mechanism challenges the view that skeletal muscle FAO is solely regulated by muscle mitochondrial content and machinery.

Figure 1. Relative and absolute quantities of fuel selection during exercise by humans at selected exercise intensities

Bars show the energy expenditure at rest and at different exercise intensities (40, 55 and 75%W_max) (scale on the right). The line graph shows the relative contributions (%) of lipid and carbohydrate use at rest and during each exercise condition (scale on the left). Redrawn fromvan Loon et al. (2001).

Figure 2. Effects of electrically stimulated muscle contraction on fatty acid transport, and plasma membrane CD36

Data are means ± SEM. Fatty acid transport was examined in giant sarcolemmal vesicles obtained from rat skeletal muscles and plasma membrane CD36 was measured in these vesicles. A, the effect of muscle contraction and recovery from muscle contraction on the rate of fatty acid transport. B, the relationship between the rate of muscle contraction and the rate of fatty acid transport. C, the relative changes in the rate of fatty acid transport and plasma membrane CD36 at the end of 30 min muscle contraction and at the end of 55 min recovery (rest = 100%). D, the effect of the CD36 inhibitor (SSO) on fatty acid transport in resting and in 30 min contracted muscle. Note that control (–SSO) and the inhibition studies were conducted on vesicles from the same pools of resting and contracting muscles as shown by the similar plasma membrane CD36 in each of these two conditions. *P < 0.05, contraction vs. rest (A–D), and contraction vs. recovery (C). **P < 0.05, 40 Hz vs. 20 Hz (B). ***P < 0.05, +SSO treatment vs.–SSO treatment in resting and contracting muscle (D). From Bonen et al. (2000); © American Society for Biochemistry and Molecular Biology.

Figure 3. Effects of CD36 overexpression and CD36 ablation on muscle fatty acid oxidation

Data are means ± SEM. A, the effects of muscle contraction in isolated soleus muscle on fatty acid oxidation in WT and CD36 transgenic mice (redrawn from Ibrahimi et al. (1999)) with permission from the American Physiological Society for Biochemistry and Molecular Biology. B, the effects of AICAR stimulation on fatty acid oxidation by perfused hindquarter muscles of WT and CD36 KO mice (redrawn from Bonen et al. (2007b) with permission from the American Physiological Society). *P < 0.05, in resting muscle CD36 transgenic or CD36 KO vs. WT. **P < 0.05, contracting or AICAR-stimulated muscle vs. respective resting muscle. ***P < 0.05, contracting CD36 transgenic muscle vs. contracting WT muscle, and AICAR-stimulated CD36 KO muscle vs. AICAR-stimulated WT muscle.

Figure 4. Phenotype of CD36 KO mice compared to WT mice (100%, see dashed line)

Data are means ± SEM. *P < 0.05, CD36 KO vs. WT. From McFarlan et al. (2012). © American Society for Biochemistry and Molecular Biology.

Figure 5. Effects of acute exercise on selected parameters in WT and CD36 KO mice, including run time to fatigue and fatty acid metabolism (A) and carbohydrate metabolism (B)

Data are means ± SEM. The exercise response in WT mice was set to 100% and the data in CD36 KO mice were expressed relative to the WT mice. Fatty acid oxidation (A) and carbohydrate (CHO) utilization (B) were determined from the respiratory exchange ratio during exercise. Circulating fatty acids increased during exercise in both WT and CD36 KO mice. The relative (%) comparisons in Fig. 5A of the circulating fatty acids are those at the end of exhaustive exercise. The rate of muscle and hepatic glycogen use (Fig. 5B) was based on their absolute reduction during exercise to exhaustion divided by the exercise time. It is recognized that these depletion rates are not necessarily linear as assumed by this calculation. WT muscle and hepatic glycogen depletion were set to 100%, and CD36 KO muscle and hepatic glycogen depletion were expressed relative to the respective WT tissues. Note that exercise-induced hepatic glycogen depletion is 5- to 10-fold greater than in skeletal muscle (see McFarlan et al. 2012). *P < 0.05, CD36 KO vs. WT. From McFarlan et al. (2012). © American Society for Biochemistry and Molecular Biology.

Figure 6. Exercise training induced changes in WT and CD36 KO mice relative to their respective sedentary control groups (100%)

Data are means ± SEM. Fatty acid oxidation was examined when muscles were stimulated with caffeine (3 mm). From McFarlan et al. (2012). © American Society for Biochemistry and Molecular Biology.

Figure 7. Comparison of exercise training-induced changes and CD36-transfection in WT soleus muscles on muscle CD36, mitochondrial content and enzymes, and caffeine-stimulated (3 mm) fatty acid oxidation

Data are means ± SEM. Note that CD36 transfection increased muscle CD36 protein comparably to exercise training, and this increase in CD36 upregulated fatty acid oxidation independent of any changes in muscle mitochondrial content or enzymes. *P < 0.05, exercise-trained vs. sedentary, and CD36 transfected vs. transfection with empty vector in the same animal. **P < 0.05, exercise-trained vs. CD36 transfected. From McFarlan et al. (2012). © American Society for Biochemistry and Molecular Biology.

Figure 8. Summary of experimental treatments showing schematically the dissociation between skeletal muscle fatty acid oxidation and mitochondrial biogenesis and enzymes, and the coordinate regulation between the fatty acid transporter CD36 and skeletal muscle fatty acid oxidation

↑= increase; →= no change; –= not present