Sterol carrier protein2. Delivery of cholesterol from adrenal lipid droplets to mitochondria for pregnenolone synthesis.

The ability of sterol carrier protein2 (SCP2) to mediate transfer of unesterified cholesterol from adrenal lipid inclusion droplets to mitochondria has been tested in an in vitro model system. Unlike mitochondrial utilization of cholesterol added in acetone or dimethyl sulfoxide, the unesterified cholesterol of lipid droplets did not provide a readily available source of substrate for mitochondrial pregnenolone production, without the addition of a transport mediator. Addition of SCP2, but not albumin, stimulated mitochondrial utilization of droplet cholesterol in a concentration-dependent manner. In the absence of mitochondria, SCP2 sequestered lipid droplet cholesterol, and in the presence of mitochondria, which were unable to convert cholesterol to pregnenolone, this cholesterol was quantitatively accumulated by mitochondria. Both processes were concentration-dependent and demonstrated a molar ratio of SCP2 and cholesterol for both binding and transport of 1. SCP2 also enhanced pregnenolone formation by mitochondria which were incubated in the absence of an extramitochondrial source of cholesterol. However, SCP2 had no effect on steroid release from a crude particulate fraction. These studies suggest that the effects of SCP2 are related to delivery of cholesterol from preformed stores to and into mitochondria for initiation of steroid hormone synthesis, and may represent an important modulator of sterol metabolism in adrenal cortical cells.