Ethanol Consumption Decreases the Synthesis of the Mannose 6-Phosphatelinsulin-Like Growth Factor II Receptor but Does Not Decrease its Messenger RNA

Authors

• James Haorah, The Veterans Ajjt~irs( VA) Medical Center; 4101 Woolworth Ave., Omaha, NE
• Richard MacDonald, University of Nebraska College of Medicine, Omaha, NE
• Julie A. Stoner, University of Nebraska College of Medicine, Omaha, NE
• Terrence M. Donohue, Jr., The Veterans Ajjt~irs( VA) Medical Center; 4101 Woolworth Ave., Omaha, NE

Date of this Version

2003

Comments

Published in Biochemical Pharmacology 65 (2003) 637-648.

Abstract

The mannose 6-phosphatelinsulin-like growth factor I1 receptor (M6P/IGF-IIR) is a protein that facilitates the transport of acid hydrolases into the lysosome. We have shown that chronic ethanol consumption lowers the M6P/IGF-IIR content in rat hepatocytes. Here, we determined the steady-state level of mRNA encoding M6P/IGF-IIR, as well as the rate of receptor synthesis, to ascertain whether the ethanol-elicited reduction in receptor protein content is related to changes in either or both of these parameters. Rats were pair-fed the normal carbohydrate (NC) or low carbohydrate high-fat (LC) liquid diets containing either ethanol or isocaloric maltose-dextrin for 7-8 weeks. RNA was isolated from hepatocytes and from whole livers of these animals and subjected to reverse transcription-polymerase chain reaction (RT-PCR) to determine the mRNA levels encoding M6P/IGF-IIR. Hepatocytes isolated from these animals were also radiolabeled with Pro-mix L-[³⁵S] in vitvo cell labeling mix to measure incorporation into total cellular protein and the immune-precipitated M6P/IGF-IIR protein. The steady-state levels of M6P/IGF-IIR mRNA in both hepatocytes and whole livers from ethanol-fed rats were the same as those from their respective controls regardless of whether they were fed the NC or the LC diets. Hepatocytes from ethanol-fed rats showed a 36% lower rate of total protein synthesis and an even greater reduction (70%) in receptor synthesis. When the relative rate of receptor synthesis was calculated, hepatocytes from ethanol-fed rats had a 53% lower relative rate of receptor synthesis compared with controls. Autoradiographic analysis of the immune-precipitated receptor protein from ethanol-fed rats also indicated a 79% decline in the total M6P/IGF-IIR protein synthetic rate compared with pair-fed controls. We conclude that the ethanol-elicited reduction of M6P/IGF-IIR content was, in part, related to a concomitant reduction of receptor protein synthesis but not to a decline in its mRNA level. Thus, the ethanol-elicited decline in receptor protein synthesis may be due to defective M6P/IGF-IIR mRNA translation.