Correlation of forkhead box transcription factor O1 and myosin heavy chain isoforms in porcine skeletal muscle.

We examined the expression of myosin heavy chain (MyHC) isoforms and forkhead box transcription factor O1 (FoxO1) in porcine soleus and extensor digitorum longus (EDL) muscles to clarify the correlation of FoxO1 and the relative abundance of transcripts of MyHC isoforms. Soleus muscle was found to be redder than EDL muscles in pigs, and immunohistochemical fast MyHC staining showed more oxidative type I fibers compared to EDL. qRT-PCR quantification of MyHC isoforms I, IIa, IIx, and IIb showed that expression of MyHC I and MyHC IIa mRNAs was much higher, whereas expression of MyHC IIx and MyHC IIb mRNAs was much lower in porcine soleus muscle compared to EDL muscle. Expression of FoxO1 mRNA and p-FoxO1 protein was significantly more abundant in porcine soleus muscle compared to EDL muscle. The expression of phosphorylated FoxO1 (p-FoxO1) was positively correlated with the expression of MyHC I (R = 0.9747, P < 0.01) and negatively correlated with the expression of MyHC IIx (R = -0.9963, P < 0.01) and MyHC IIb (R = -0.9834, P < 0.01). Taken together, these results suggested that FoxO1 may play a pivotal role in the determination of muscle fiber type.