Upregulation of E3 ligase UBR2 in acetaldehyde-treated C2C12 myotubes and its potential involvement in fast-twitch muscle atrophy in alcohol-fed rats

Abstract

Background

Chronic alcohol intake induces atrophy of type II (anaerobic, fast-twitch) muscle fibers in preference to type I (aerobic, slow-twitch) muscle fibers. However, the molecular mechanism underlying the preferential atrophy of type II muscle fibers remains unclear. The ubiquitin-proteasome system (UPS) mediates the degradation of myofibrillar proteins in skeletal muscle loss. This study investigated whether E3 ubiquitin ligases, including UBR2 and MuRF1, are involved in ethanol-induced type II muscle fiber-specific atrophy.

Methods

In a chronic alcohol intake rat model, 4- to 5-week-old male Wistar rats were fed the Lieber-DeCarli liquid diet for 8 weeks. Muscle specimens were collected from the extensor digitorum longus (EDL) and soleus muscles of both legs. In an in vitro model, myotubes differentiated from murine C2C12 myoblasts were treated with culture medium containing 100 mM ethanol or 1 mM acetaldehyde for 6 h.

Results

The muscle weight of the EDL seemed to be lesser in the ethanol group compared to the control group, and there was no difference in the muscle weight of the soleus between these groups. Cross-sectional area (CSA) of type IIA, IIB, and IIX muscle fibers in the EDL of ethanol-fed rats was smaller compared to the control rats, and CSA of type I muscle fibers in the soleus was larger in ethanol-fed rats. UBR2 protein levels seem to increase in the EDL but not in the soleus, whereas MuRF1 protein level remained unchanged in both muscles. C2C12 myotubes expressing MHC IIb, characteristics of type IIB muscle fibers, showed reduced myotube diameter after ethanol or acetaldehyde treatment. Acetaldehyde treatment increased UBR2 expression, but not MuRF1 expression, and enhanced ubiquitination. Knockdown of UBR2 using siRNA prevented acetaldehyde-induced myotube atrophy.

Conclusion

This study demonstrated that the E3 ligase UBR2 may play a role in alcohol-induced type II muscle-preferential atrophy.