Hyperphosphorylation of the asialoglycoprotein receptor in isolated rat hepatocytes following ethanol administration

Ethanol administration leads to altered function and impaired receptor-mediated endocytosis of the hepatocyte asialoglycoprotein receptor (ASGP-R). The purpose of the present study was to examine the effects of ethanol on the phosphorylation of the ASGP-R to determine whether this post-translational modification could contribute mechanistically to the observed ethanol-induced alterations. The methodological approach of this work involved the measurement of the phosphorylation state of the receptor obtained from isolated rat hepatocytes, using a combination of experimental designs from the biosynthetic incorporation of phosphate to the determination of steady-state phosphotyrosine levels. We report here that both short-term (1- to 2-week) and chronic (5- to 7-week) periods of ethanol administration resulted in a significant increase in the steady-state phosphotyrosine protein in the ASGP-R. In addition, in vitro incorporation of [gamma-(32)P]ATP using a permeabilized cell assay system similarly showed an increase in tyrosine-phosphorylated receptors. Furthermore, metabolic radiolabeling of hepatocytes with [(32)P]orthophosphate demonstrated hyperphosphorylation of the ASGP-R in cells obtained from chronically ethanol-fed animals. Finally, our results revealed that dephosphorylation of the ASGP-R was unaffected by ethanol administration, indicating that kinase activity rather than impaired phosphatase action contributes to the increased phosphorylation state of the receptor. Overall, the results presented in this study demonstrated that the extent of tyrosine phosphorylation of the receptor is significantly higher in hepatocytes obtained from ethanol-fed animals. We conclude that hyperphosphorylation of the ASGP-R may be a contributing factor to the impaired function of the receptor elicited by ethanol administration.