A potential role for sterol carrier protein-2 in cholesterol transfer to mitochondria

Mitochondrial cholesterol oxidation rapidly depletes cholesterol from the r elatively cholesterol-poor mitochondrial membranes. However, almost nothing is known regarding potential mechanism(s) whereby the mitochondrial choles terol pool is restored. Since most exogenous cholesterol enters the cell vi a the lysosomal pathway, this could be a source of mitochondrial cholestero l. In the present study, an in vitro fluorescent sterol transfer assay was used to examine whether the lysosomal membrane could be a putative choleste rol donor to mitochondria. First, it was shown that spontaneous sterol tran sfer from lysosomal to mitochondrial membranes was very slow (initial rate, 0.316 +/- 0.032 pmol/min). This was due, in part, to the fact that 90% of the lysosomal membrane sterol was not exchangeable, while the remaining 10% also had a relatively long half-time of exchange t(1/2) = 202 +/- 19 min. Second, the intracellular sterol carrier protein-2 (SCP-2) and its precurso r (pro-SCP-2) increased the initial rate of sterol transfer from the lysoso mal to mitochondrial membrane by 5.2- and 2.0-fold, respectively, but not i n the reverse direction. The enhanced sterol transfer was due to a 3.5-fold increase in exchangeable sterol pool size and to induction of a very rapid ly (t(1/2) = 4.1 +/- 0.6 thin) exchangeable sterol pool. Confocal fluoresce nce imaging and indirect immunocytochemistry colocalized significant amount s of SCP-2 with the mitochondrial marker enzyme cytochrome oxidase in trans fected L-cells overexpressing SCP-2. In summary, SCP-2 and pro-SCP-2 both s timulated molecular sterol transfer from lysosomal to mitochondrial membran es, suggesting a potential mechanism for replenishing mitochondrial cholest erol pools depleted by cholesterol oxidation. (C) 2000 Elsevier Science Ire land Ltd. All rights reserved.