Subcellular compartmentation of free and esterified cholesterol in the interstitial cells of the mouse testis

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Abstract
The subcellular distribution of free and esterified cholesterol in mouse testis and the changes occurring in cholesterol content of whole testes and cell fractions after inhibition of gonadotropins with methallibure (ICI 33, 828) and restimulation with human chorionic gonadotropin (HCG) are reported in the present paper. In subcellular fractions, the bulk of free cholesterol is associated with organelles sedimented in the microsomal fraction while esterified cholesterol is mainly stored in isolated lipid droplets. The latter compartment increases in methallibure-treated mice 2.3 fold and is remarkably depleted after administration of HCG. There is a close parallelism between the changes in esterified cholesterol content and the variation in the numbers of lipid droplets found in electron micrographs of interstitial cells of mice receiving similar treatments. By contrast no significant changes were noticed in either free cholesterol concentrations of the microsomal fractions or in the fine structure of organelles associated with this fraction. The dynamic nature of steroidogenesis in the microsomal fraction requires the existence of a free cholesterol pool with a high turnover rate for use as an intermediate in androgen synthesis. On the other hand, the large content of free cholesterol in microsomes and its stability under different conditions suggest the presence of a cholesterol compartment with a slow turnover, as a constituent of the membranes.