Translocation-arrested apolipoprotein B evades proteasome degradation via a sterol-sensitive block in ubiquitin conjugation

In this study, we explored how sterol metabolism altered by the expression of cholesterol-7-alpha-hydroxylase NADPH:oxygen oxidoreductase (7 alpha-hyd roxylase) affects the ubiquitin-dependent proteasome degradation of translo cation-arrested apoB53 in Chinese hamster ovary cells. Stable expression of two different plasmids that encode either rat or human 7 alpha-hydroxylase inhibited the ubiquitin conjugation of apoB and its subsequent degradation by the proteasome. Oxysterols (25-hydroxycholesterol and 7-ketocholesterol ) reversed the inhibition of apoB degradation caused by 7 alpha-hydroxylase , The combined results suggest that the normally rapid proteasome degradati on of translocation-arrested apoB can be regulated by a sterol-sensitive po lyubiquitin conjugation step in the endoplasmic reticulum. flocked ubiquiti n-dependent proteasome degradation caused translocation-arrested apoB to be come sequestered in segregated membrane domains. Our results described for the first time a novel mechanism through which the "quality control" protea some endoplasmic reticulum degradative pathway of translocation-arrested ap oB is linked to sterol metabolism. Sterol-sensitive blocked ubiquitin conju gation appears to selectively inhibit the proteasome degradation of apoB, b ut not 7 alpha-hydroxylase protein, with no impairment of cell vitality or function. Our findings may help to explain why the hepatic production of li poproteins is increased when familial hypertriglyceridemic patients are tre ated with drugs that activate 7 alpha-hydroxylase (e.g, bile acid-binding r esins).