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. 2010 Jun 1:7:47.
doi: 10.1186/1743-7075-7-47.

Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones

Affiliations

Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones

Jie Hu et al. Nutr Metab (Lond). .

Abstract

Steroid hormones regulate diverse physiological functions such as reproduction, blood salt balance, maintenance of secondary sexual characteristics, response to stress, neuronal function and various metabolic processes. They are synthesized from cholesterol mainly in the adrenal gland and gonads in response to tissue-specific tropic hormones. These steroidogenic tissues are unique in that they require cholesterol not only for membrane biogenesis, maintenance of membrane fluidity and cell signaling, but also as the starting material for the biosynthesis of steroid hormones. It is not surprising, then, that cells of steroidogenic tissues have evolved with multiple pathways to assure the constant supply of cholesterol needed to maintain optimum steroid synthesis. The cholesterol utilized for steroidogenesis is derived from a combination of sources: 1) de novo synthesis in the endoplasmic reticulum (ER); 2) the mobilization of cholesteryl esters (CEs) stored in lipid droplets through cholesteryl ester hydrolase; 3) plasma lipoprotein-derived CEs obtained by either LDL receptor-mediated endocytic and/or SR-BI-mediated selective uptake; and 4) in some cultured cell systems from plasma membrane-associated free cholesterol. Here, we focus on recent insights into the molecules and cellular processes that mediate the uptake of plasma lipoprotein-derived cholesterol, events connected with the intracellular cholesterol processing and the role of crucial proteins that mediate cholesterol transport to mitochondria for its utilization for steroid hormone production. In particular, we discuss the structure and function of SR-BI, the importance of the selective cholesterol transport pathway in providing cholesterol substrate for steroid biosynthesis and the role of two key proteins, StAR and PBR/TSO in facilitating cholesterol delivery to inner mitochondrial membrane sites, where P450scc (CYP11A) is localized and where the conversion of cholesterol to pregnenolone (the common steroid precursor) takes place.

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Figures

Figure 1
Figure 1
Principal steps involved in the biosynthesis of various steroid hormones. Modified from Payne and Hales and website [1,305]
Figure 2
Figure 2
Potential sources of cholesterol for product formation (steroids, vitamin D and bile acids) and membrane biogenesis
Figure 3
Figure 3
Diagrammatic representation of the molecular and cellular events involved in the selective and endocytic uptake and intracellular processing of the lipoprotein-derived cholesteryl esters for steroid hormone biosynthesis by adrenal and gonadal tissues. ACATA1, acyl-coenzyme A:cholesterol acyltransferase I; CEs, cholesteryl esters; CS, cytoskeleton; CYP11A1, cytochrome P450 side-chain cleavage enzyme (P450scc); FC, free cholesterol; NPC1, Nieman-Pick type C1; NPC2, Nieman-Pick type C2; SCP2, sterol carrier protein2; SREBP, sterol-regulatory element-binding proteins; SCAP, SREBP cleavage-activating protein. StAR, steroidogenic acute regulatory protein; TGs, triglycerides; TSPO, translocator protein. Modified from Chang et al, Rone et al, and Farese and Walther [68,288,306].
Figure 4
Figure 4
Correlation between the cellular levels of SR-BI dimers and the functional efficiency of selective HDL-CE uptake. Appropriate Western blots from various cell types were scanned for SR-BI monomers and dimers and dimer/monomer ratios were plotted against the respective selective HDL-CE uptake data. The results show that dimer/monomer ratios determined for individual cell types correlate significantly with their respective SR-BI-mediated selective HDL-CE uptake.

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