Cholesterol substrate pools and steroid hormone levels are normal in the face of mutational inactivation of NPC1 protein

Abstract

Mutational inactivation of NPC1 largely blocks the movement of LDL-derived cholesterol from the lysosome to the metabolically active, cytosolic pool of sterol that is the substrate for steroid hormone production. Such a block might, in theory, lead to deficiencies in circulating levels of testosterone, progesterone, and corticosterone. However, there are at least two other sources for cellular cholesterol, de novo synthesis and scavenger receptor class B type I-mediated uptake of HDL cholesteryl ester (CE). In this study, we measured the rates of net cholesterol acquisition by these three pathways in the adrenal, ovary, and testis. In all three organs, the majority (81-98%) of cholesterol acquisition came from the selective uptake of CE from HDL and de novo synthesis. Furthermore, in the npc1 2/2 mouse, the cytosolic storage pool of CE in a tissue such as the adrenal remained constant (z25 mg/g). As a result of these alternative pathways, the plasma concentrations of testosterone (3.5 vs. 2.5 ng/ml), progesterone (8.5 vs. 6.7 ng/ml), and corticosterone (391 vs. 134 ng/ml) were either the same or elevated in the npc1 2/2 mouse, compared with the control animal.Thus, impairment of cholesterol acquisition through the NPC1-dependent, clathrin-coated pit pathway did not limit the availability of cholesterol substrate for steroid hormone synthesis in the steroidogenic cells. Essentially all cells acquire cholesterol primarily through de novo synthesis (1-3). In addition, at least three different proteins located in the plasma membrane are recognized that can also bring about the net transfer of sterol from the extracellular environment into the metabolically active pools of cholesterol within these cells. The first of these, the LDL receptor (LDLR), can take up lipoproteins that contain one of two ligands, apolipoprotein B 100 (apoB 100 ) or apoE, and these include LDL and the remnants of both VLDL and chylomicrons (4, 5). This transport process involves binding of the ligand to the receptor, clustering of the bound lipoproteins into clathrin-coated pits, and finally, internalization of these particles into the endosomal/ lysosomal compartment of the cell (6). There, acidification leads to a change in the configuration of the binding site on the LDLR, release of the lipoprotein, and ultimately, hydrolysis of the cholesteryl ester (CE) contained in these particles by an acidic cholesteryl ester esterase, i.e., acid lipase (7-9). Thus, this LDLR-dependent endocytosis results in the uptake of the cholesterol, both esterified and unesterified, carried in particles like LDL, and deposits it as unesterified sterol in the endosomal compartment of the cell.The second plasma membrane protein, scavenger receptor class B type I (SR-BI), binds lipoproteins containing apoAI such as HDL, and then processes these particles through a very different pathway (10,11). Although the details of this internalization step are not clear, in some manner cholesteryl ester from the HDL particle is selectively translocated into the cell ...

GM2/GD2 and GM3 gangliosides have no effect on cellular cholesterol pools or turnover in normal or NPC1 mice

“…Because about 80% of this sterol is in myelin, in the whole brain, only approximately 3 mg/g of the total of 16 mg/g represents unesterified cholesterol in the plasma membranes of neurons and glial cells (40). The endocrine glands are the second exception, and in the adrenal, for example, where the concentration of plasma membrane unesterified cholesterol is about 5 mg/g (41), there is a second pool of sterol exceeding 20 mg/g, made up of cholesteryl esters stored in lipid droplets in the cytosol (Figs. 3, 7, 8).…”

Impact of Aging on Steroid Hormone Biosynthesis and Secretion

“…For example, in the rodent (rat) Leydig cells, the cholesterol that is synthesized de novo within the cell is the most important source of testosterone synthesis [36][37][38][39], but in the human testis, both endogenously synthesized cholesterol as well as LDL-derived cholesterol contribute to testosterone production [39,40]. As noted above, all steroidogenic cells, however, contain the intracellular cholesterol pool in the form of cholesterol esters (lipid droplets) which are regarded as a short-term store of substrates that enables cells to respond rapidly to trophic hormone stimulation [25,33,39].…”

Lysosomal unesterified cholesterol content correlates with liver cell death in murine Niemann-Pick type C disease

“…Most cells express low density lipoprotein receptors (LDLRs) that can bind lipoproteins carrying apolipoprotein E (apoE) with very high affinity as well as those carrying apoB-100 with lesser affinity (19)(20)(21)(22). The density of these receptors is highest in the endocrine tissues and liver, but they can also be detected in most other organs (23,24). However, the tight endothelial lining of most capillary beds limits the access of circulating lipoproteins to these LDLRs in most organs, including the central nervous system (25).…”

“…However, despite the block in the movement of unesterified cholesterol out of the late endosomal/lysosomal compartment in cells of the liver and other tissues, there is no shortage of sterol in the metabolically active pool of these cells. This is because de novo synthesis increases to supply adequate amounts of cholesterol for the turnover of membrane sterol and for the synthesis of bile acids and, in the endocrine glands, steroid hormones (4,24,30). In the mouse with this mutation, for example, total body cholesterol synthesis increases from ?120 mg/day/kg body weight (bw) in the normal animal to 182 mg/day/kg.…”