2 " cut & paste's " i though were worth throwing in here....
Alcohol Impairs Protein Synthesis and Degradation in Cultured Skeletal Muscle Cells
Ly Q. Hong-Brown 1 , Robert A. Frost 1 Charles H. Lang 1
1 Department of Cellular and Molecular Physiology, Pennsylvania State College of Medicine, Hershey, Pennsylvania.
Correspondence to Ly Q. Hong-Brown, PhD, Department of Cellular and Molecular Physiology (H166), Pennsylvania State College of Medicine, Hershey, PA 17033; Fax: 717-531-7667; E-mail:
[email protected]
Supported by Grant AA11290 from NIAAA.
KEYWORDS
Alcohol • Protein Synthesis • Proteolysis • IGF-I/Insulin Hybrid Receptors
ABSTRACT
Background: Acute and chronic alcohol intoxication decreases skeletal muscle protein synthesis under in vivo conditions. We investigated whether ethanol (EtOH) and its major metabolites, acetaldehyde and acetate, can directly modulate protein balance under in vitro conditions.
Methods: Human myocytes were incubated with different doses of EtOH for varying periods of time (i.e., 4–72 hr). Alternatively, cells were incubated with acetaldehyde, acetate, insulin, insulin-like growth factor-I (IGF-I), or with a combination of EtOH plus insulin or IGF-I. Rates of protein synthesis or degradation were determined by 35S-methionine/cysteine incorporation into or release from cellular protein.
Results:
A significant, 15% to 20%, decrease in basal protein synthesis was observed after 24 hr, but not at earlier time points, in response to 80 mM EtOH. Incubation of myocytes for 72 hr decreased synthesis in cells incubated with EtOH ranging between 60 and 120 mM.
The ability of IGF-I or insulin to stimulate protein synthesis was impaired by 30% and 60%, respectively, in cells incubated with 80 mM EtOH for 72 hr. Exposure of cells to 200 μM acetaldehyde or 5 mM Na-acetate also decreased basal protein synthesis. In contrast, neither EtOH, acetaldehyde, nor acetate altered the basal rate of protein degradation. However, EtOH completely impaired the ability of insulin and IGF-I to inhibit proteolysis. Finally, EtOH did not impair IGF-I receptor autophosphorylation, but inhibited the ability of insulin to phosphorylate its own receptor. EtOH also did not alter the number of insulin or IGF-I receptors or the formation of insulin/IGF-I hybrid receptors.
Conclusions: We have demonstrated that
alcohol can directly inhibit muscle protein synthesis under in vitro conditions. Neither EtOH nor its metabolites altered basal protein degradation, although EtOH did compromise the ability of both insulin and IGF-I to slow proteolysis. This impairment seems to be mediated by different defects in signal transduction.
Inhibition of muscle protein synthesis by alcohol is associated with modulation of eIF2B and eIF4E
Charles H. Lang, Duanqing Wu, Robert A. Frost, Leonard S. Jefferson, Scot R. Kimball, and Thomas C. Vary
Departments of Cellular and Molecular Physiology and Surgery, Pennsylvania State College of Medicine, Hershey, Pennsylvania 17033
The present study examined potential mechanisms for the inhibition of protein synthesis in skeletal muscle after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were pair fed.
Alcohol-induced myopathy was confirmed by a reduction in lean body mass as well as a decrease in the weight of the gastrocnemius and psoas muscles normalized for tibial length. No alcohol-induced decrease in total RNA content (an estimate of ribosomal RNA) was detected in any muscle examined, suggesting that alcohol reduced translational efficiency but not the capacity for protein synthesis. To identify mechanisms responsible for regulating translational efficiency, we analyzed several eukaryotic initiation factors (eIF). There was no difference in the muscle content of either total eIF2 or the amount of eIF2 in the phosphorylated form between alcohol-fed and control rats. Similarly, the relative amount of eIF2B in muscle was also not different. In contrast, alcohol decreased eIF2B activity in psoas (fast-twitch) but not in soleus or heart (slow-twitch) muscles. Alcohol feeding also dramatically influenced the distribution of eIF4E in the gastrocnemius (fast-twitch) muscle. Compared with control values, muscle from alcohol-fed rats demonstrated 1) an increased binding of the translational repressor 4E-binding protein 1 (4E-BP1) with eIF4E, 2) a decrease in the phosphorylated -form of 4E-BP1, and 3) a decrease in eIF4G associated with eIF4E. In summary,
these data suggest that chronic alcohol consumption impairs translation initiation in muscle by altering multiple regulatory sites, including eIF2B activity and eIF4E availability.