Dual Mechanisms of ABCA1 Regulation by Geranylgeranyl Pyrophosphate

Abstract

ATP-binding cassette transporter A1 (ABCA1) mediates an active efflux of cholesterol and phospholipids and is mutated in patients with Tangier disease. Expression of ABCA1 may be increased by certain oxysterols such as 22(R)-hydroxycholesterol via activation of the nuclear hormone receptor liver X receptor (LXR). In searching for potential modulators of ABCA1 expression, we have studied the effects of various mevalonate metabolites on the expression of ABCA1 in two human cell lines, THP-1 and Caco-2 cells. Most of the tested metabolites, including mevalonate, geranyl pyrophosphate, farnesyl pyrophosphate, and ubiquinone, failed to significantly change the expression levels of ABCA1. However, treatment with geranylgeranyl pyrophosphate resulted in a dose-and time-dependent reduction of ABCA1 expression. Geranylgeranyl pyrophosphate appears to reduce ABCA1 expression via two different mechanisms. One of these mechanisms is by acting directly as an antagonist of LXR since it reduces the interaction between LXR␣ or -␤ with nuclear coactivator SRC-1. Another mechanism appears to involve activation of the Rho GTP-binding proteins since treatment of Caco-2 cells with inhibitors of geranylgeranyl transferase or the Rho proteins significantly increased the expression and promoter activity of ABCA1. Further studies showed that mutations in the DR4 element of the ABCA1 promoter completely eliminate the inducible activities of these inhibitors. These data indicate that activation of the Rho proteins may change the activation status of LXR.Plasma concentration of high density lipoprotein cholesterol is inversely related to the incidence of coronary heart disease (1, 2). Our understanding of the mechanisms that regulate HDL 1 cholesterol has received a major advance with the elucidation of the cause of Tangier disease (TD). Patients with TD are characterized by near or complete absence of circulating HDL and by the accumulation of cholesteryl esters in many peripheral tissues (3, 4). Recently three groups independently reported identification of the ATP-binding cassette transporter A1 (ABCA1) as the defective gene responsible for TD (5-7). ABCA1 is a member of the ATP-binding cassette superfamily. These proteins couple the energy provided by ATP hydrolysis to the transport of a wide variety of molecules across membranes (8 -11). ABCA1 is thought to mediate the active efflux of cholesterol and phospholipids to apolipoprotein (apo) acceptors, most importantly apoA-I, the major apo of HDL (12, 13). Due to mutations, however, the function of ABCA1 in patients with TD is impaired. Therefore, cellular cholesterol efflux in TD patients is defective, which leads to accumulation of excess cellular cholesterol and defective formation of HDL (14 -16). ABCA1 is widely expressed and is particularly abundant in monocytes and macrophages (17). Studies in macrophages haveshown that the expression of ABCA1 is sterol-dependent (17). Expression of ABCA1 is up-regulated by modified low density lipoprotein and down-regulated by HDL (17, 18). The ...

Disruption of cholesterol homeostasis by plant sterols

“…This finding is consistent with a previous observation that the phytosterol-derived compound YT-32 is an LXR agonist (33). Alternatively, the accumulation of plant sterols may promote the generation of another agonist that results in increased expression of ABCA1, for example by stimulating adenosine 3′, 5′-cyclic monophosphate (cAMP) (34) or by inhibiting geranylgeranyl transferase (35).…”

Nuclear localization of liver X receptor α and β is differentially regulated

“…Such binding to p300 is associated with the retention in the nucleus of another transcription factor, p53 [Kawai et al, 2001]. If such co-activator binding to LXR a and LXR b serves as retention factor, accelerated export after antagonist binding can be explained by inhibition of co-activator binding to LXR [Gan et al, 2001]. Antagonist binding likely causes co-repressor dissociation.…”

“…Whereas these ligands as well as the RXR ligand 9-cis retinoic acid activate transcription [Willy et al, 1995], geranyl geranyl pyrophosphate is an antagonist that inhibits transcription. This antagonistic effect is partially due to inhibition of coactivator binding to LXR [Gan et al, 2001] and/or inhibition of binding of LXR-RXR heterodimers to DNA [Forman et al, 1997]. Knockout studies in mice showed that LXR a plays a greater role than LXR b in sensing cholesterol levels and for lipogenesis.…”

Cholesterol 24S-Hydroxylase Overexpression Inhibits the Liver X Receptor (LXR) Pathway by Activating Small Guanosine Triphosphate-Binding Proteins (sGTPases) in Neuronal Cells

“…LXR ligands have been previously shown to have differential effects on the activation of Rho family members, namely by inhibiting RhoA and activating Cdc42 [50,51]. Moreover, RhoA has been reported to inhibit LXR [45][46][47][48][49], suggesting integration between Rho-dependent functions and LXR activation. Our experiments on the effect of CYP46A1 overexpression on the LXR pathway show that, in contrast to the effect of added 24OHC, increased levels of CYP46A1 reduced the expression of LXR target genes ABCA1 and APOE, which are critical for cholesterol efflux in neurons [52][53][54][55][56].…”

“…Since activation of Rho GTPases has been associated with an inhibition of the LXR pathway [45][46][47][48][49] and we have observed that CYP46A1 overexpression and 24OHC treatment differentially affect RhoA membrane levels, we hypothesize that CYP46A1-dependent decrease in LXR target genes was dependent on the increase in RhoA prenylation. To test our hypothesis, we incubated cells transfected with pFLAGhCYP46A1 in the presence or absence of GGTi-2133 and observed that inhibition of prenylation abolished the CYP46A1-dependent decrease in ABCA1 and APOE mRNA levels (Fig.…”