Ethanol consumption alters trafficking of lysosomal enzymes and affects the processing of procathepsin L in rat liver.
In order to determine whether ethanol consumption alters the targeting of hepatic lysosomal enzymes to their organelles, we examined the sedimentation properties of lysosomal hydrolases in ethanol-fed rats and their pair-fed controls. Rats were fed a liquid diet containing either ethanol (36% of calories) or isocaloric maltose dextrin for one to five wk. Liver extracts were fractionated by Percoll density gradient centrifugation and fractions obtained were analyzed for the distribution of lysosomal marker enzymes. Heavy lysosomes were further purified from these gradients and the activity of specific hydrolases was determined. Compared with those from controls, isolated lysosomes from ethanol-fed rats showed a 20-50% reduction in the activity of lysosomal acid phosphatase and beta-galactosidase. Decreased intralysosomal hydrolase activity in ethanol-fed rats was associated with a significant redistribution of these enzymes as well as those of cathepsins B and L to lighter fractions of Percoll density gradients. This indicated an ethanol-elicited shift of these enzymes to lower density cellular compartments. In order to determine whether ethanol administration affects the synthesis and proteolytic maturation of hepatic procathepsin L, we conducted immunoblot analyses to quantify the steady-state levels of precursor and mature forms of cathepsin L in hepatic post-nuclear fractions. Ethanol administration caused a significant elevation in the steady-state level of the 39 kDa cathepsin L precursor relative to its 30 kDa intermediate and 25 kDa mature product. These results were confirmed by pulse-chase experiments using isolated hepatocytes exposed to [35S]methionine. Hepatocytes from both control and ethanol-fed rats incorporated equal levels of radioactivity into procathepsin L. However, during the chase period, the ratios of the 39 kDa procathepsin L to its 30 kDa intermediate and 25 kDa mature product in cells from ethanol-fed rats were 1.5-3-fold higher than those in controls. These results demonstrate that ethanol consumption caused a marked impairment in the processing of procathepsin L to mature enzyme, without affecting its synthesis. Taken together, our findings suggest that chronic ethanol consumption caused a deficiency in intralysosomal enzyme content by altering the trafficking and processing of these hydrolases into lysosomes.
Kharbanda
K K
KK
Liver Study Unit, Department of Veterans Affairs (VA) Medical Center, Omaha, NE, USA.
McVicker
D L
DL
Zetterman
R K
RK
Donohue
T M
TM
D000135:Acid Phosphatase / Q000378:metabolism; D000818:Animals; D002401:Cathepsin B / Q000378:metabolism; D056668:Cathepsin L; D002403:Cathepsins / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism*; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases; D010450:Endopeptidases; D004792:Enzyme Precursors / Q000737:chemistry / Q000378:metabolism*; D000431:Ethanol / Q000494:pharmacology*; D008099:Liver / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D008247:Lysosomes / Q000201:enzymology*; D008297:Male; D008970:Molecular Weight; D011499:Protein Processing, Post-Translational / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D001616:beta-Galactosidase / Q000378:metabolism
D004792:Enzyme Precursors; D000431:Ethanol; D000135:Acid Phosphatase; D001616:beta-Galactosidase; D002403:Cathepsins; D010450:Endopeptidases; D003546:Cysteine Endopeptidases; C063905:procathepsin L; D002401:Cathepsin B; D056668:Cathepsin L; C534280:Ctsl protein, rat