As far as known, 24-isopropylcholesterol is not synthesized by eumetazoans (cnidarians plus bilaterian animals).
Steroids containing a tertiary butyl group(s) [or tert-butyl unit(s)] are rather rare compounds that have been found in nature.
In animal tissues, especially in the liver, adrenals and plasma lipids (more the 70% in circulating lipoproteins), cholesterol is esterified by a variety of fatty acids and most frequently by essential fatty acids, thus forming cholesterol esters.
These sterols could be related to the use of that mushroom in thetraditional Chinese and Japanese medicine to cure several pathologies (hypertension,hypercholesterolemia, hepatitis, gastritis, diabetes, bronchitis, andcardiovascular problems) ().
The 24-alkyl group is characteristic of all phytosterols and ispreserved during subsequent steroid metabolism in both fungi and plants to givehormones that regulate growth and reproduction in a manner similar to animals.
The study of the physical properties of model membranes mimicking plant plasmamembranes suggests that phytosterols are more efficient than cholesterol inextending the temperature range in which membrane-associated biologicalprocesses can take place ().
Furthermore, cholesterol has been detected as one of the major sterolsin the surface lipids of higher plant leaves (rape) where he may amount to about72% of the total sterols in that fraction ().
The secretions of human meibomian glands (sebaceous glands at the rim of eyelids) are particularly rich in cholesterol esters (about 30% of the lipid pool) which are characterized by a saturated or monounsaturated fatty acid moiety with C18-C34 carbon chain (Butovich IA, J Lipid Res 2009, 50, 501).
In conclusion, the results of the present studydemonstrated that the TLR4 inhibitor CLI-095 was able toeffectively reduce atherosclerosis in ApoE mice bysuppressing foam cell formation. The present study also providesnovel insights into the protective effects of TLR4 inhibition onenhancing cholesterol efflux by upregulating the expression ofABCA1, and reducing CE biosynthesis by downregulating theexpression of ACAT-1, which is mediated by inhibiting theTLR4/NF-κB signaling pathway. These results suggested that TLR4 maybe considered a potential therapeutic target for the prevention ofatherosclerotic progression.
In animals, the esterification of free cholesterol within intestinal cells (by acyl CoA:cholesterol acyltransferase, ACAT) allows the cholesterol to be stored as a neutral lipid in cytosolic droplets and in the packing of cholesterol into lipoprotein particles for export via the plasma to liver cells.
Both the present and previous studies demonstratedthat TLR4 regulates cholesterol biosynthesis ;however, whether TLR4 is able to regulate lipid metabolism remains controversial. The results of the present studyrevealed that CLI-095 did not reduce increased serum cholesteroland triglyceride levels in mice receiving a high-fat diet. Inaddition, Higashimori () reported that TLR4 deficiency was notassociated with reduced levels of cholesterol and triglycerides,whereas Aspichueta () reported that endotoxic ratsexhibited increased levels of serum very low-densitylipoprotein-apoB, -triglyceride, and -cholesterol. In addition, Lu () reported thatRs-LPS, a TLR4 antagonist, decreased the serum levels ofcholesterol and triglycerides in non-diabetic mice; therefore,further research is required to determine whether and how TLR4affects lipid metabolism in animals.
Cholesteryl linoleate and cholesteryl arachidonate present in nasal fluid have been shown to contribute to the inherent antibacterial activity of that secretion ().
Foam cell formation has a critical role inatherosclerosis and its mechanisms include uptake of atherogeniclipoproteins, impaired cellular cholesterol efflux and disturbedintracellular cholesterol processing. Notably, Lox-1, ABCA1 andACAT-1 have an important role in these three mechanismsrespectively. The present data suggested that CLI-095 mayupregulate the expression of ABCA1 and downregulate that of Lox-1and ACAT-1 . These findings were consistent withthose from previous studies; both ABCA1 overexpression and Lox-1deficiency in mice have been shown to lead to decreasedatherosclerotic lesions (,–).However, previous studies have demonstrated that loss of ACAT-1expression may lead to severe atherosclerosis (,).A possible explanation for the controversy between previous studiesand the present results is that global ACAT-1 knockout causesmonocytosis in ApoE mice during the development ofatherosclerosis; however, there is no evidence to demonstrate thatthe inhibition of TLR4 signaling causes monocytosis.
The accumulation of CE in macrophages has a criticalrole in foam cell formation. The possible effects of CLI-095 onmacrophage CE accumulation were examined in thepresent study. Treatment with CLI-095 resulted in a marked decreasein the abundance of cellular CE in MPMs, as compared with thevehicle group ().Furthermore, CLI-095 resulted in a marked reduction in the ratio ofcellular CE to total cholesterol in MPMs (). During the process of foam cellformation, excess cellular free cholesterol is converted to CE bythe enzyme ACAT-1, or is removed from the cell by ABCA1-dependentcholesterol efflux (–). In addition, activation of NF-κB cansuppress ABCA1 and enhance ACAT-1 expression to promote CE-ladencell formation (,). In the present study, Ox-LDLstimulation resulted in enhanced TLR4 expression as previouslyreported (,); however, the expression of TLR4 wasnot altered in the CLI-095-treated MPMs, as compared with thevehicle-treated MPMs ().Notably, treatment with TLR4 inhibitor CLI-095 significantlyreduced Ox-LDL-induced phosphorylation of NF-κB P65 (), suggesting that CLI-095 mayinhibit TLR4 signaling by affecting its adaptor proteins butwithout downregulating its expression. Furthermore, it was observedthat CLI-095 markedly promoted ABCA1 expression and attenuatedACAT-1 expression ().These data strongly indicate that CLI-095 may exert its vascularprotective function by restricting CE synthesis and enhancingcholesterol efflux in macrophages.