A mouse model of sitosterolemia: absence of Abcg8/sterolin-2 results in failure to secrete biliary cholesterol

BACKGROUND: Mutations in either of two genes comprising the STSL locus, ATP-binding cassette (ABC)-transporters ABCG5 (encoding sterolin-1) and ABCG8 (encoding sterolin-2), result in sitosterolemia, a rare autosomal recessive disorder of sterol trafficking characterized by increased plasma plant sterol levels. Based upon the genetics of sitosterolemia, ABCG5/sterolin-1 and ABCG8/sterolin-2 are hypothesized to function as obligate heterodimers. No phenotypic difference has yet been described in humans with complete defects in either ABCG5 or ABCG8. These proteins, based upon the defects in humans, are responsible for regulating dietary sterol entry and biliary sterol secretion. METHODS: In order to mimic the human disease, we created, by a targeted disruption, a mouse model of sitosterolemia resulting in Abcg8/sterolin-2 deficiency alone. Homozygous knockout mice are viable and exhibit sitosterolemia. RESULTS: Mice deficient in Abcg8 have significantly increased plasma and tissue plant sterol levels (sitosterol and campesterol) consistent with sitosterolemia. Interestingly, Abcg5/sterolin-1 was expressed in both liver and intestine in Abcg8/sterolin-2 deficient mice and continued to show an apical expression. Remarkably, Abcg8 deficient mice had an impaired ability to secrete cholesterol into bile, but still maintained the ability to secrete sitosterol. We also report an intermediate phenotype in the heterozygous Abcg8+/- mice that are not sitosterolemic, but have a decreased level of biliary sterol secretion relative to wild-type mice. CONCLUSION: These data indicate that Abcg8/sterolin-2 is necessary for biliary sterol secretion and that loss of Abcg8/sterolin-2 has a more profound effect upon biliary cholesterol secretion than sitosterol. Since biliary sitosterol secretion is preserved, although not elevated in the sitosterolemic mice, this observation suggests that mechanisms other than by Abcg8/sterolin-2 may be responsible for its secretion into bile

Heparan sulfate-dependent and low density lipoprotein receptor-related protein-dependent catabolic pathways for lipoprotein lipase in mouse embryonic fibroblasts

Heparan sulfate and low density lipoprotein receptor related protein (LRP) have been shown to participate in the uptake and degradation of the enzyme lipoprotein lipase (LPL). Yet, the contribution of each of these pathways to LPL metabolism and their possible dependence is unknown. In the present study we examined the metabolism of 125I-labeled LPL in untreated and heparinase-treated primary wild-type mouse embryonic fibroblasts (MEF) and in mouse fibroblasts that express single LRP allele (PEA-10) or are lacking the LRP (PEA-13). The degradation of LPL in PEA-13 cells was 30% lower than in MEF and PEA-10 cells. Heparinase treatment decreased the LPL degradation by 58%, 79% and 92%, whereas heparin reduced such degradation by 87%, 90% and 94% in MEF, PEA-10 and PEA-13 cultures, respectively. Assuming that a) heparinase treatment abolished the heparan-sulfate pathway, and that b) the degradation remaining in heparin-treated cultures represents nonspecific values, it appears that heparan sulfate contributes about 61%, 83% and 95% of total LPL degradation, whereas the LRP pathway contributes 39%, 17% and less than 5% of LPL degradation in MEF, PEA-10 and PEA-13 cells, respectively. In addition, the data indicate that LPL interaction with heparan sulfate and the LRP pathways is independent of each other. The study shows that these cells possess both a heparan sulfate-dependent pathway and an LRP-dependent pathway for LPL metabolism and that the two pathways are independent of each other