Transport of cholesterol into mitochondria

The next events in steroidogenesis must obviously include the transport of cholesterol and cholesterol ester to the required organelles, in particular, cholesterol into 

The release of cholesterol from cholesterol esters occurs extra-mitochondrially by means of a cholesterol ester hydrolase in adrenals, ovaries and testicular Ley dig cells (see Ref. 6 for review). This enzyme has been studied mostly in adrenal preparations, and is known to be activated and de-activated by reversible phosphorylation [14] and that the phosphorylation was brought about by a c-AMP-dependent protein kinase [15]. Hence, ACTH stimulation of cholesterol ester activity in the adrenal occurs via the kinase and, in a similar way, trophic hormone stimulation of ovarian and testicular cholesterol ester hydrolases may occur and provide a large pool of cholesterol for steroidogenesis [16-18], 

In 1979, Simpson [24] postulated that the outer mitochondrial membrane was the site of action of a labile protein factor, necessary to facilitate the transport of cholesterol, and Privalle et al. [19] provided evidence to support the notion that transference of cholesterol from the outer to the inner membrane required an agent that is cycloheximide dependent. When rats were ether-stressed in vivo and cholesterol SCC was deliberately inhibited, cholesterol accumulated in the adrenal mitochondria, most (90%) of this being associated with the inner membrane cytochrome P-450scc. After administration of aminoglutethimide to rats to block SCC, there was a two-fold increase in inner membrane cholesterol, while cycloheximide abolished this increase. Thus, it appears that cholesterol accumulates in the inner mitochondrial membrane as a result of stress and that transference from outer to inner membrane requires a protein factor. 

Pederson and co-workers (see Ref. 25) have isolated a peptide of Mr 2200, from ACTH-stimulated rat adrenals, which contained 15% of basic aminoacid residues. The polar side-chain groups were thought to alter membrane structure so that transference of cholesterol towards the cyt P-450scc on the inner membrane would be favoured. 

Phospholipids are also thought to be involved in cholesterol transference. Increases in both the degree of unsaturation of fatty acyl groups and length of fatty acyl chains of mitochondrial phospholipids are known to increase the rate of cholesterol transfer [25]. Further, the concentrations of some phospholipids in the inner mitochondrial membrane of rat adrenals were shown to increase after ACTH stimulation and to be related to cholesterol SCC activity [25], 

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As the SCCE described above (Section 6.3.1.1) maybe part of a protein complex in the mitochondria, more effort was directed to study the possible interaction partners, especially the peripheral-t)q)e benzodiazepine receptor (PBR) (Papadopoulos et al, 1997 Koch, 2002) and the acyl-CoA-binding protein (ACBP Metzner et al, 2000). The ACBPs bind to the peripheral-t)q)e PBR present in the envelope of mitochondria (Gamier et al, 1994). This interaction stimulates the transport of cholesterol into mitochondria (Papadopoulos and Brown, 1995). The cholesterol taken up into the mitochondria is available as a substrate to the side-chain cleavage enzyme which transforms cholesterol into pregnenolone (Papadopoulos et al, 1997). Because of its interaction with PBR, ACBP is also described as diazepam-binding inhibitor or endozepine. Some isoforms of the latter were isolated and characterized from D. lanata (Metzner et al, 2000). Lindemann and Luckner (1997) speculated that cardenolide formation is regulated mainly by the availability of cholesterol and its transport into mitochondria, where the P450scc is assumed to be located. 

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