Inhibiting proteasomal degradation of microsomal triglyceride transfer protein prevents CCl4-induced steatosis.
Carbon tetrachloride (CCl(4)) interferes with triglyceride secretion and causes steatosis, fibrosis, and necrosis. In mice, CCl(4) decreased plasma triglyceride-rich lipoproteins, increased cellular lipids, and reduced microsomal triglyceride transfer protein (MTP) without diminishing mRNA levels. Similarly, CCl(4) decreased apoB-lipoprotein production and MTP activity but had no effect on mRNA levels in primary enterocytes and colon carcinoma and hepatoma cells. CCl(4) did not affect MTP synthesis but induced post-translational degradation involving ubiquitinylation and proteasomes in McA-RH7777 cells. By contrast, MTP inhibitor increased cellular lipids without affecting MTP protein. MTP was covalently modified when cells were incubated with (14)CCl(4). This modification was prevented by the inhibition of P450 oxygenases, indicating that CCl(3)(.) generated by these enzymes targets MTP for degradation. To determine whether inhibition of proteolysis could prevent CCl(4) toxicity, mice were fed with CCl(4) with or without lactacystin. Lactacystin increased ubiquitinylated MTP and prevented lipid accumulation in tissues. Thus, CCl(4) induces post-translational degradation without affecting lipid transfer activity, whereas MTP antagonist inhibits lipid transfer activity without causing its destruction. These studies identify MTP as a major target of CCl(4) and its degradation as a novel mechanism involved in the onset of steatosis, suggesting that inhibition of proteolysis may prevent some forms of steatosis.
Pan
Xiaoyue
X
Department of Anatomy, SUNY Downstate Medical Center, Brooklyn, New York 11203, USA.
Hussain
Farah N
FN
Iqbal
Jahangir
J
Feuerman
Miriam H
MH
Hussain
M Mahmood
MM
D000818:Animals; D001055:Apolipoproteins B / Q000097:blood; D001483:Base Sequence; D002251:Carbon Tetrachloride / Q000633:toxicity*; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002460:Cell Line; D017931:DNA Primers; D005234:Fatty Liver / Q000139:chemically induced*; D020869:Gene Expression Profiling; D006868:Hydrolysis; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D061988:Proteasome Inhibitors
D001055:Apolipoproteins B; D002352:Carrier Proteins; D017931:DNA Primers; D061988:Proteasome Inhibitors; C106092:microsomal triglyceride transfer protein; D002251:Carbon Tetrachloride; D046988:Proteasome Endopeptidase Complex