Plasma membrane steroidogenic cholesterol: The relative importance of membrane internalization rate and cholesterol extraction rate of internalized membrane

Most cholesterol destined to become steroid hormone passes through the plas ma membrane. Using MA-10 Leydig tumor cells as a model system, we determine d that plasma membrane cholesterol disappeared 2.5-fold faster in cAMP-stim ulated cells. Stimulation of MA-10 cells increased internalization so that 1.4-fold more membrane cycled through the cell per unit time. Increased int ernalization accounted for 26% of the cholesterol actually lost from cyclin g membrane. The remaining 74% was accounted for by increasing the extractio n of cholesterol from internalized membrane.