Carotenoid transporter CD36 expression depends on HIF-1α under normal physiological conditions in soleus muscles of mice

Background and objectives: Skeletal muscles play critical roles in physical activities, such as postural maintenance and locomotion, as well as regulating energy metabolism throughout the body. Dietary β-carotene induces muscular hypertrophy and prevents atrophy in red slow-twitch soleus muscles, but not in white fast-twitch extensor digitorum longus (EDL) and gastrocnemius muscles.1,2 However, it remains unclear why these beneficial effects of β-carotene are elicited in the soleus muscles. In this study, we focused on the expression levels of carotenoid transporters in red and white muscles to elucidate the mechanism underlying the beneficial effects of β-carotene on soleus muscles. Methods: C57BL/6J mice were orally administered micellar β-carotene, and small interfering RNA (siRNA) targeting Cd 36 was transfected into their hind limb muscles. To evaluate hypoxic levels in the muscles, the hypoxia marker pimonidazole was injected into the tail veins of mice. CD36 expression in murine C2C12 myotubes was evaluated. Results: Cd 36 mRNA levels were higher in the red muscle than in the white muscle of mice. The Scarb1 and Npc1l1 mRNA levels were not significantly different between the soleus and EDL muscles. siRNA-mediated knockdown of CD36 decreased β-carotene uptake in C2C12 myotubes. In soleus muscles, CD36 knockdown inhibited the β-carotene-induced increase in soleus muscle mass. Intravenous injection of pimonidazole produced more pimonidazole-bound proteins in soleus muscles than in EDL muscles. Hypoxia-inducible factor-1α (HIF-1α) protein levels were higher in soleus muscles than in EDL muscles. In C2C12 myotubes, hypoxia increased the CD36 expression at both the mRNA and protein levels. Consequently, HIF-1α knockdown reduced Cd36 mRNA levels in C2C12 myotubes as well as soleus muscles. Conclusions: CD36 is predominantly involved in the β-carotene-induced increase in soleus muscle mass in mice. CD36 expression depends on HIF-1α in the soleus muscles of mice even under normal physiological conditions.