Kazuki Nakashima

We investigated the effects of a low‐protein diet and feed restriction on the mRNA expression of cationic amino acid transporters (CATs) in the longissimus dorsi (LD), rhomboideus (RH), and biceps femoris (BF) muscles of pigs. Eighteen piglets were divided into three groups: a control (CP21%), low‐protein diet (LP, CP16%), and feed‐restricted diet (FR, CP21%, 76% feed intake of control pigs) groups. The expression levels of CAT‐1 in the LD and BF muscles of LP pigs were higher than that of control pigs, whereas that of FR pigs showed no difference. The CAT‐2A expression levels in the RH muscle of FR pigs were higher than that of control pigs. The free lysine concentrations in all muscles of LP and FR pigs were lower than that of control pigs. To examine the factors that affect CATs mRNA expression, we evaluated the effects of lysine, arginine, insulin‐like growth factor‐I, and dexamethasone on the expression of CATs in C2C12 myotubes. CAT‐1 expression levels increased in lysine and/or arginine deprivation. We show that CAT‐1 and CAT‐2A expression levels in skeletal muscles differ in response to dietary treatments and CAT‐1 expression in skeletal muscles appears to increase in response to low free lysine concentrations.

β 2 ‐Adrenoceptor (β 2 ‐AR) signaling decreases the transcriptional activity of forkhead box O (FoxO), but the underlying mechanisms remain incompletely understood. Here, we investigated how β 2 ‐AR signaling regulates the protein abundance of FoxO and its transcriptional activity in skeletal muscle. We observed that stimulation of β 2 ‐AR with its selective agonist, clenbuterol, rapidly decreased FoxO1 mRNA expression, and this was accompanied by a decrease in either FoxO1 protein level or FoxO transcriptional activity. We subsequently observed that miR‐374b‐5p and miR‐7a‐1‐3p were rapidly upregulated in response to β 2 ‐AR stimulation. Transfection with mimics of either miRNA successfully decreased FoxO1 mRNA. Moreover, because β 2 ‐AR stimulation increased cAMP concentration, pretreatment with an adenylyl cyclase inhibitor canceled out these effects of β 2 ‐AR stimulation. These results suggest that β 2 ‐AR stimulation results in rapid upregulation of miR‐374b‐5p and miR‐7a‐1‐3p in myotubes, which in turn results in a decrease in FoxO1 mRNA expression via the β 2 ‐AR–cAMP signaling pathway.