Chronic Ethanol Administration Decreases the Ligand Binding Properties and the Cellular Content of the Mannose 6-Phosphatelinsulin-Like Growth Gactor II Receptor in Rat Hepatocytes

We have shown previously that chronic ethanol administration impairs the maturation of lysosomal enzymes in rat hepatocytes. The mannose 6-phosphate/insulin-like growth factor I1 receptor (M6PLGF-IIR) is a protein that facilitates the transport of lysosomal enzymes into the lysosome. Therefore, we examined whether ethanol consumption altered the ligand binding properties and the cellular content of M6P/IGF-IIR. Rats were pair-fed liquid diets containing either ethanol (36% of calories) or isocaloric maltose-dextrin for either 1 week or 5-7 weeks. Hepatocytes prepared from these animals were examined for receptor-ligand binding and receptor content. One week of ethanol feeding had no significant effect on ligand [radioiodinated pentamannose phosphate conjugated to bovine serum albumin (125I - PMP-BSA)] binding to hepatocytes, but cells from rats fed ethanol for 5-7 weeks bound less 125I-PMP-BSA than pair-fed controls. Scatchard plot analysis revealed that the number of 125I -PMP-BSA binding sites in hepatocytes from ethanol-fed rats was 49% lower than that of controls. 125I-PMP-BSA binding by perivenular (PV) and periportal (PP) hepatocytes from ethanol-fed rats was, respectively, 40 and 48% lower than their controls, but there was no significant difference between these two types of hepatocytes. Ligand blot analysis using 125I -insulin-like growth factor II (125I –IGF-II) also showed that the receptor in lysates of hepatocytes from ethanol-fed rats bound 26-27% less ligand than controls. Similarly, immunoblot analysis of cell lysates from ethanol-fed rats revealed 62% lower levels of immunoreactive M6P/IGF-IIR than controls. Feeding rats a low carbohydrate-ethanol diet did not exacerbate the reduction in M6P/IGF-IIR-ligand binding nor did it reduce the levels of immunoreactive receptor. Our findings indicate that chronic ethanol consumption lowers M6P/IGF-IIR activity and content in hepatocytes. This reduction may account, in part, for the impaired processing and delivery of acid hydrolases to lysosomes previously observed in ethanol-fed rats