Published online before print October 14, 2004, doi: 10.1161/01.ATV.0000147896.69299.85
© 2004 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Transcription Factor Sterol Regulatory Element Binding Protein 2 Regulates Scavenger Receptor Cla-1 Gene Expression
From the National Institute for Health and Medical Research (INSERM), Dyslipoproteinemia and Atherosclerosis Research Unit, Hôpital de la Pitié, Paris Cedex 13, France.
Correspondence to Thierry Huby, INSERM, Unit 551, Pavillon Benjamin Delessert, Hôpital de la Pitié, 83 Boulevard de la Pitié, 75651 Paris Cedex 13. E-mail thuby{at}infobiogen.fr
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Abstract |
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Objective— The human scavenger receptor class B type I (Cla-1) plays a key role in cellular cholesterol movement in facilitating transport of cholesterol between cells and lipoproteins. Indirect evidence has suggested that Cla-1 gene expression is under the feedback control of cellular cholesterol content. To define the molecular mechanisms underlying such putative regulation, we evaluated whether Cla-1 is a target gene of the sterol regulatory element binding protein (SREBP) transcription factor family.
Methods and Results— Transient transfections demonstrated that SREBP factors induce Cla-1 promoter activity and that SREBP-2 is a more potent inducer than the SREBP-1a isoform. The 5'-deletion analysis of 3 kb of the 5'-flanking sequence of the Cla-1 gene, combined with site-directed mutagenesis and electrophoretic mobility shift assay, allowed identification of a unique sterol responsive element. SREBP-mediated Cla-1 regulation was confirmed in stably transfected human embryonic kidney 293 cells expressing the active form of SREBP-2 at incremental levels. In these cell lines, Cla-1 mRNA and protein levels were increased in direct proportion to the level of SREBP-2 expression.
Conclusions— These findings provide evidence that SREBP-2, a key regulator of cellular cholesterol uptake through modulation of the expression of the low-density lipoprotein receptor gene, may influence cellular cholesterol homeostasis via regulation of Cla-1 gene expression.
The putative regulation of the expression of the human scavenger receptor Cla-1 gene by intracellular cholesterol content and SREBP transcription factors was evaluated. Transient transfection, site-directed mutation, and electrophoretic mobility shift assay allowed identification of a functional SRE in the Cla-1 promoter. SREBP-stably transfected cells provided strong evidence of the implication of SREBP-2 in Cla-1 gene expression.
Key Words: SR-BI • SREBP • cholesterol homeostasis • regulation of gene expression • transcription factor
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Introduction |
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The scavenger receptor class B type I (SR-BI) has attracted considerable interest since the demonstration in vitro that it interacts with low-density lipoprotein (LDL) and high-density lipoprotein (HDL) to facilitate the bidirectional movement of cholesterol between these lipoprotein particles and SR-BI–expressing cells. On one hand, SR-BI mediates the selective uptake of cholesteryl esters (CEs) from HDL or LDL to cells; on the other, SR-BI can promote cellular efflux of free cholesterol to lipoprotein acceptor particles.1 A major role of SR-BI–mediated cellular cholesterol uptake in cholesterol metabolism is strongly supported by SR-BI gene manipulation in mice. Mice totally deficient for SR-BI display elevated plasma levels of HDL-cholesterol and decreased cholesterol content in adrenal glands.2 Conversely, mice that overexpress SR-BI in the liver exhibit a significant reduction in plasma HDL-cholesterol levels, concomitant with an increase in hepatic selective cholesterol uptake and cholesterol secretion in bile.3
The human homolog of SR-BI, termed Cla-1, exhibits similar tissue distribution, binding properties for a wide spectrum of plasma lipoproteins, and identical cholesterol transfer capacities as those of murine SR-BI. Whereas SR-BI appears as a physiologically relevant HDL receptor intimately involved in HDL metabolism in rodents, the role of Cla-1 in LDL- and HDL-mediated cholesterol homeostasis in man remains incompletely understood. However, in a human hepatic cell model, Cla-1 is responsible for the major part of the selective uptake of CE from HDL and LDL particles.4 This finding suggests that Cla-1 may play a pivotal role in metabolism of the cholesterol component of both lipoprotein classes in humans. Indeed, recent epidemiological studies have identified single-nucleotide polymorphisms (SNPs) in the Cla-1 gene that are associated with plasma lipid levels and lipoprotein composition.5,6 Interestingly, these associations between SNPs or combinations of SNPs and HDL-cholesterol or LDL-cholesterol levels appear to be influenced by gender.
Cellular cholesterol and fatty acid metabolism is tightly regulated in animal cells and involves a family of transcription factors designated as sterol regulatory element binding proteins (SREBPs). SREBP-1a and SREBP-1c are synthesized from a single gene through the use of alternate promoters and exons, whereas SREBP-2 is synthesized from a separate gene. All 3 are produced as precursors inserted into the membrane of the endoplasmic reticulum (ER). Proteolytic activation of the precursor form and subsequent release of the active Nter domain of the SREBP (nSREBP) is tightly regulated and occurs when the ER membrane becomes depleted in cholesterol. The soluble active nSREBP then enters the nucleus and activates transcription by binding either to 10-bp repeats known as sterol response elements (SREs), or to E-box sequences located in the promoter region of target genes. Use of transgenic mice overexpressing either of the active nuclear SREBP isoforms has demonstrated that the SREBP-1 isoforms are more relevant to fatty acid metabolism, whereas SREPB-2 is more selective in regulating cholesterol metabolism.7,8
Expression of the LDL-receptor, a key component of the cellular pathway for cholesterol uptake, is modulated as a function of cellular cholesterol content through SREBP-mediated transcriptional regulation. Indirect evidence suggests that SR-BI, which may modulate membrane cholesterol content and cellular cholesterol pool size through a pathway distinct from that of the LDL-receptor, may also be a target gene for SREBP factors. Indeed, SREBP-1a can induce the transcriptional activity of the rat SR-BI promoter through 2 SRE-binding sites in vitro.9 Feedback control of SR-BI expression dependent on cellular cholesterol status has also been suggested in vitro in human keratinocytes10 and in vivo in mouse adrenal glands11 and rat ovaries.12 However, in contrast, microarray analyses of RNA from livers of transgenic mice overexpressing SREBP isoforms 1a and 2 did not reveal changes in SR-BI mRNA levels.13
In this study, we evaluated the question of whether expression of the human SR-BI/Cla-1 gene is regulated via SREBP factors. Analysis of the 5'-proximal region of the Cla-1 gene allowed identification of a functional SRE-binding site through which SREBP-2 modulates Cla-1 promoter activity. Moreover, stable overexpression of nSREBP-2 in a human cell line is associated with significant upregulation of both levels of Cla-1 endogenous mRNA and protein. These findings strongly suggest that Cla-1 is a target gene for SREBP-mediated transcriptional regulation and imply that intracellular cholesterol status is implicated in regulation of its expression.
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Methods |
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Plasmid Constructs and Site-Directed Mutagenesis
The Cla-1 promoter luciferase reporter plasmid p-2913 cloned into pGL3 basic vector (Promega), the corresponding 5'-deletion constructs p-258 and p-58 and the luciferase reporter plasmid for the mouse SR-BI promoter were described previously.14 The expression vectors pCMV5-SREBP-1a and pCMV5-SREBP-2 were provided by K. Schoonjans (IGBMC, Illkirch, France). To generate the luciferase reporter plasmid for the LDL-receptor promoter, the proximal region (–171 to +57) of the gene was amplified by polymerase chain reaction (PCR) using human genomic DNA as template and cloned into pGL3 basic vector. The sequence was then verified in the final construct by DNA sequencing.
Site-directed mutants were prepared from construct p-258 using the GeneEditor in vitro Site-Directed Mutagenesis System (Promega). The synthetic complementary oligonucleotides SRE1 (5'-CAG CGG CAG CAA CCC GGG GCT TGT C-3'), SRE2 (5'-CTG CCC GTC CGT AGT GCG CCC CGC CCC GTC-3'), SRE3 (5'-CCC GCC CCG TGG CCG CCC CGG GCC CGC-3'), and the E-box sequence (5'-ACC ACT GGC CTG CTG CCG GGC TGC T-3') containing mismatched bases (underlined) were used to mutate the respective putative binding sites in the Cla-1 promoter sequence. The presence of the mutation was confirmed by sequencing.
Stable Transfection of Human Embryonic Kidney 293 Cells With nSREBP-2
The cDNA encoding for nSREBP-2 (amino acids 1–481) was subcloned into the pcDNA3.1 expression vector (Invitrogen) containing the neomycin resistance gene. Human embryonic kidney 293 (HEK293) cells were grown for 24 hours in serum-free medium and then incubated with 4 µg plasmid DNA and 5 µg Lipofectamine (Invitrogen). After 6 hours of incubation, cells were trypsinized, and a serial dilution in complete medium was performed. Selective medium (complete medium supplemented with 0.5 g/L G418-sulfate) was applied 2 days later. After 2 weeks, wells containing 1 surviving colony were selected, and HEK–nSREBP-2 clones were expanded in selective medium.
Transient Transfections, Electrophoretic Mobility Shift Assay, Immunoblot Analysis, RNA Preparation, and Real-Time Quantitative RT-PCR
Please see the expanded Methods section, available online at http://atvb.ahajournals.org.
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Results |
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Statin and SREBP-Mediated Induction of Cla-1 Promoter Activity
Because an earlier report demonstrated that rat SR-BI promoter activity was upregulated in vitro by statin treatment and the SREBP-1a transcription factor,9 we investigated whether the Cla-1 gene was regulated in a similar manner. The possibility that Cla-1 promoter activity could be modulated by intracellular sterol content was initially examined by transient transfection studies performed in hepatocellular carcinoma-derived HepG2 cells cultured in the absence or presence of cerivastatin, a potent 3-hydroxy-3-methylglutaryl–coenzyme A reductase inhibitor. Figure 1 shows that addition of cerivastatin for 24 hours resulted in differential activation of Cla-1 promoter/reporter constructs carrying
3 kb (p-2913), 0.4 kb (p-258), or 0.2 kb (p-58) of 5'-flanking sequence of the Cla-1 gene. Whereas p-2913 and p-258 promoter activities were similarly and significantly increased 1.8-fold and 1.7-fold, respectively, by statin treatment, the activity of the p-58 construct remained unchanged. To ascertain whether the increase in Cla-1 promoter activity of p-2913 and p-258 induced by cerivastatin treatment was the result of SREBP-dependent promoter activation after cellular cholesterol depletion and not the result of a cholesterol-independent effect of statin, we performed cotransfection studies of Cla-1 promoter constructs and expression vectors coding for either nSREBP-1a or nSREBP-2 (Figure 1). As seen for the cerivastatin experiments, a statistically significant upregulation of promoter activity was observed with the p-2913 and p-258 constructs with either of the SREBP isoforms, whereas no activation occurred with the shortest p-58 construct. These data indicated that the Cla-1 promoter is a target for SREBP-dependent transactivation.
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Differential Activation of mSR-BI and Cla-1 Promoter Activities by SREBP-1a and SREBP-2
It has been reported that SREBP-1a, SREBP-1c, and SREBP-2 may exhibit distinct efficacy in promoting transcriptional induction of target promoters.15–17 Comparison of the level of transcriptional activation of p-2913 and p-258 constructs by 20 ng of pCMV5-SREBP expression vectors revealed a greater induction with the SREBP-2 encoding plasmid than with the SREBP-1a form (statistical difference P=0.04 and P=0.02 for p-2913 and p-258, respectively; Figure 1). Indeed, cotransfection of p-258 with increasing amounts of either pCMV5–SREBP-1a or pCMV5–SREBP-2 in HepG2 cells demonstrated a similar dose-dependent induction with both nSREBP forms using 1 and 5 ng of expression vectors (Figure 2A). However, whereas a 2.5-fold increase in Cla-1 promoter activity was obtained with 20 ng of the SREBP-2 plasmid (dose-dependent promoter induction was maintained up to 100 ng of SREBP-2 vector; data not shown), no further increase was observed when similar amounts of SREBP-1a vector were used (plateau induction at 1.6-fold; Figure 2A). Similar results, with an identical fold increase, were obtained using the full-length p-2913 construct in place of p-258 (data not shown). To validate the capacity of SREBP-1a and SREBP-2 to transactivate target promoters in our luciferase assay, we evaluated the SREBP-mediated induction of the activity of a control reporter construct containing the proximal promoter (–171 to +57) of the human LDL-receptor gene. Under identical conditions of transfection as those used for the Cla-1 promoter plasmids, the promoter activity of the LDL-receptor reporter construct responded in a dose-dependent and similar manner when stimulated either by SREBP-1a or SREBP-2 over the range of the concentrations tested (1 to 20 ng of plasmid per well; Figure 2B). Furthermore, we evaluated the capacity of SREBP-1a and SREBP-2 to induce the activity of the mouse SR-BI promoter (2.1 kb of 5'-flanking sequence). Surprisingly, a marked increase in promoter activity was observed with both SREBP forms; however, as seen for the human promoter constructs, there was a clear differential activation of transcriptional activity of the mouse SR-BI promoter by cotransfection of either pCMV5–SREBP-1a or SREBP–2 (3.2-fold and 17.6-fold increase with SREBP-1a and SREBP-2, respectively; Figure 2C). Together, these data demonstrate that under the experimental conditions tested, SREBP-2 activates human and mouse SR-BI promoters with greater efficiency than SREBP-1a.
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Identification and Characterization of a Functional SREBP-2 Binding Site in the Promoter of the Cla-1 Gene
Data in Figure 1 revealed that p-2913 and p-258 responded in a similar manner to cerivastatin and to SREBP-mediated increase of luciferase activity. These data suggested that the 5'-deleted region between positions –2913 and –258 was not involved in the SREBP-specific response. Additionally, the lack of response of the p-58 plasmid to statin or SREBPs indicated that the DNA motif(s) required for the SREBP-mediated response lies in the region between positions –258 and –58. Analysis of this region revealed 3 putative SREs (SRE1, SRE2, and SRE3), identified according to their homology with the consensus SRE sequence (ATC ACC CCA C). The functionality of these DNA sequences as an SRE was evaluated by site-directed mutagenesis. Whereas the induction of p-258 promoter activity by SREBP-2 was not affected by mutation of the putative SRE1 and SRE3, mutation of the SRE2 sequence resulted in the complete loss of SREBP-2 transcriptional induction of Cla-1 promoter activity (Figure I, available online at http://atvb.ahajournals.org).
SREBP-2 affinity for the SRE2 sequence was evaluated by electrophoretic mobility shift assay (EMSA) analysis (Figure 3) using either in vitro–translated nSREBP-2 or nuclear extracts prepared from HEK293 cells overexpressing nSREBP-2 (see below). A retarded protein–DNA complex was detected in the presence of recombinant nSREBP-2 and HEK293 nuclear extracts (complex I, lanes 1 and 5). This complex appeared to be specific because the addition of an excess of unlabeled SRE2 probe prevented its formation (lanes 2 and 6), whereas an unlabeled mutated SRE2 oligonucleotide did not (lanes 3 and 7). The consensus SRE sequence identified in the promoter of the LDL-receptor gene was used as a control. The addition of an excess of unlabeled LDL-receptor SRE specifically competed with the SRE2–protein complex I (lanes 4 and 8). We equally observed that incubation of the SRE2 probe with nuclear extracts from HEK293 cells overexpressing nSREBP-2 produced another DNA–protein complex (complex II, lane 5). Because the SRE2 oligoprobe sequence is GC-rich, we tested the possibility that the protein involved in this second complex could be Sp1, a member of the SP transcription factor family that binds to such DNA motifs. Addition of a molar excess of an unlabeled oligomer containing an Sp1 binding sequence specifically prevented formation of complex II but not that of complex I (lane 9). Complex II was also competed out by the addition of an excess of unlabeled wild-type (lane 6) or mutated (lane 7) SRE2 oligonucleotides but not by that of cold LDL-receptor SRE (lane 8). These results suggest that Sp1 binds to a site adjacent to that of SREBP-2 on the SRE2 probe and may act as an SREBP cofactor. Further confirmation of the implication of Sp1 in this Cla-1 SRE2/protein complex II was provided by the disappearance of this complex and the formation of a supershifted band after addition of anti-Sp1 antibody (lane 10).
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Cla-1 Gene Expression Is Activated as a Function of nSREBP-2 Levels in Stably Transfected HEK293 Cells
To further evaluate whether Cla-1 gene expression is regulated by SREBP-2, we established stably transfected HEK293 cells expressing the human nuclear active form nSREBP-2. Stable HEK293 clones were assayed for the level of recombinant nSREBP-2 (rec.nSREBP-2) mRNA expression by quantitative RT-PCR using primer pairs that discriminate rec.nSREBP-2 cDNA from endogenous SREBP-2 cDNA (Table I, available online at http://atvb.ahajournals.org). Figure 4A shows the results of 4 clones that express very low (clone 1), moderate (clone 2), or high levels (clones 3 and 4) of rec.nSREBP-2. As reported previously in transgenic SREBP mouse models7 or in in vitro cell systems,18 graded overexpression of rec.nSREBP-2 was associated with a concomitant increase in the mRNA level of target genes of SREBPs, including the LDL-receptor (Figure II, available online at http://atvb.ahajournals.org). Cla-1 mRNA levels were equally increased incrementally as a function of the level of expression of rec.nSREBP-2 in the cell clone (Figure 4B, 1.5-fold, 3.4-fold, and 4.1-fold changes for clone 2, 3, and 4, respectively). Changes in protein levels were assessed by densitometric analysis of Western blots (Figure 4C). Although HEK293 cells normally express Cla-1 protein at low levels, comparison of Cla-1 protein expression between HEK–nSREBP-2 high expressor clones (clones 3 and 4) and 2 clones expressing nSREBP-2 at very low levels (clones 1 and 5) revealed greater Cla-1 protein levels in both high expressor clones (1.5-fold and 1.9-fold increase for clones 3 and 4, respectively; P<0.0002; Figure 4C). It is noteworthy that the fold changes measured at the mRNA level were 2-fold greater than those observed at the protein level, suggesting a potential control of Cla-1 expression at a post-transcriptional level. In this context, it is relevant that PDZK1, a PDZ domain containing protein essential for SR-BI protein expression,19 could not be detected by quantitative RT-PCR in HEK293 cells; therefore, PDZK1 does not appear to be implicated in this potential post-transcriptional effect.
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As a control for the Western blots, the use of an anti–LDL-receptor antibody revealed, as expected, a greater expression of LDL-receptor in high expressor clones than in very low expressor clones.
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Discussion |
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We established that the promoter regions of the mouse SR-BI gene and of its human homolog Cla-1 are transactivated by SREBP factors and preferentially by SREBP-2 compared with SREBP-1a. The significance of such regulation was highlighted by the analysis of HEK293 cells expressing the nuclear active nSREBP-2 at incremental levels, in which Cla-1 gene expression was increased as a function of that of nSREBP-2. The demonstration that SREBP-2 regulates Cla-1 gene expression, a receptor that mediates the bidirectional movement of cholesterol between cells and lipoproteins, is physiologically relevant with regard to the central role of SREBP-2 in the regulation of cellular cholesterol homeostasis. Indeed, SREBP-2 activates genes involved not only in cholesterol biosynthesis but also in the control of cellular cholesterol uptake via the LDL-receptor.
Promoter studies of mouse, rat, and human SR-BI genes have shown that they share important cis-regulatory elements and are modulated in a similar manner, as demonstrated for steriodogenic factor-1 (SF-1)–mediated9,20 and liver receptor homolog-1 (LRH-1)–mediated14 regulation of SR-BI expression. In the present work, mouse and human SR-BI promoters were shown to respond to SREBP-1a and SREBP-2. These data corroborate those of Lopez et al9 on the induction of rat SR-BI promoter activity by SREBP-1a. Whereas 2 SREs have been identified in the rat promoter,9 analysis of 3 kb of 5'-flanking sequence of the Cla-1 gene revealed the presence of a unique functional SRE (at least 2 SREBP responsive regions could be detected in the mouse SR-BI promoter; data not shown). Based on site-directed mutation and EMSA analyses, this SRE element is most likely 5'-ATCAGCGCCC-3', thereby resembling the human LDL-receptor SRE-1 (5'-ATCACCCCAC-3') and the SRE identified in the rat SR-BI proximal promoter9 (5'-AGCACCGCCC-3'). Preferential activation of different target promoters by either of the SREBP isoforms has been reported;15–17 indeed, such a mechanism has been proposed to partially account for the differential in vivo regulation of genes involved in fatty acid metabolism and cholesterol biosynthesis by SREBP-1 and SREBP-2, respectively. To date, no specific base change in SREBP recognition sites has been firmly identified that provides greater specificity in binding to individual SREBPs. However, it has been postulated that SREBP-2 could possibly display higher affinity for DNA motifs typified by the LDL-receptor SRE-1.15 Conversely, it has also been suggested that differences in the first 2 5' nucleotides of the LDL-receptor SRE-1 could explain the exclusive binding of SREBP-2 to the human squalene synthase SRE (8/10) 5'-TCCACCCCAC-3'.17,21 The Cla-1 SRE2 site, as presently identified shares an identical 5' end but diverges in its 3' end from that of the LDL-receptor SRE-1, and on the basis of the aforementioned hypotheses, would not therefore correspond to a preferential target of the SREBP-2 form. Greater promoter activation by SREBP-2 than by SREBP-1a, as observed in the present study for human and mouse SR-BI promoters, has rarely been reported previously. Therefore, the Cla-1 SRE2 site may constitute an interesting sequence to analyze, along with the SREBP-responsive element(s) of the mouse SR-BI promoter; such analysis may lead to identification of key nucleotides in SRE motifs for preferential binding specificity/affinity of SREBP-2 versus SREBP-1a.
The general transcription factors NF-Y and Sp1 are coregulatory transcription factors that are critical to high levels of SREBP-mediated activation of promoters.22,23 In this study, we observed that the GC-box 3' of the Cla-1 SRE2 sequence interacts with the Sp1 transcription factor. The presence of this functional Sp1-binding site adjacent to the Cla-1 SRE2 site further supports the notion that Cla-1 is a member of the gene family activated by SREBP factors. It is relevant that an 11-bp deletion in the Cla-1 gene, which corresponds to this Sp1-binding sequence, has been identified recently in a Taiwanese Chinese population.24 It would be of interest to evaluate whether this deletion, which was shown to be associated with reduced basal Cla-1 promoter activity in vitro but also with increased HDL-cholesterol levels in the aforementioned population, may affect SREBP-2–mediated regulation of Cla-1 gene expression.
A recent analysis of microarrays hybridized with RNA from livers of SREBP-1a and SREBP-2 transgenic mice failed to identify SR-BI as a target gene for SREBP.13 One explanation for this apparent discrepancy with the current study resides in the fact that changes in SR-BI gene expression in the livers of these SREBP transgenic mice may be modest and therefore could not meet the criteria (>2.5-fold change) required to qualify SR-BI as an SREBP target. Absence or moderate transcriptional activation of SR-BI expression may also underpin the complex regulation of this gene in hepatic tissue. Indeed, whereas SR-BI expression has been reported to be reduced in liver parenchymal cells of rats fed a high-cholesterol diet,25 thereby suggesting cholesterol-mediated feedback control of SR-BI expression, concomitant increased expression was reported in liver Kupffer cells.25 Alternatively, minor changes in the transcription levels of the cholesterol 7
-hydroxylase gene, as reported in SREBP-2 transgenic mice,13 may increase bile acid biosynthesis in the hepatocyte. On binding to bile acids, Farnesol X receptor (FXR) activates transcription of the Shp gene, a repressor of the orphan nuclear receptor LRH-1, which we identified recently as a positively acting transcription factor of SR-BI/Cla-1 expression in mouse liver.14 Whether such regulation of the SR-BI gene is relevant when SREBP action is maintained constant as in SREBP transgenic mouse models remains indeterminate.
Long-term treatment of hypercholesterolemic patients with statins has proven efficacious in reducing plasma LDL-cholesterol levels (up to 35%), whereas baseline values of HDL-cholesterol generally remain unchanged or may be slightly increased (
10%). However, early and transitory reduction of HDL-cholesterol levels after high-dose atorvastatin has been reported in patients with heterozygous26 or homozygous familial hypercholesterolemia (HFH; up to 21% reduction after 4 weeks treatment).27 Such an effect could be partially related to an early response to statin-induced cellular cholesterol deprivation, which may result in SREBP-mediated upregulation of hepatic Cla-1 expression and subsequent increase in selective uptake of HDL-cholesterol with concomitant reduction in plasma HDL-cholesterol levels. In HFH patients, the transient effect of atorvastatin on HDL-cholesterol levels was 2-fold greater in patients with total LDL-receptor deficiency compared with receptor-defective HFH subjects, thereby suggesting a direct link to the degree of LDL-receptor deficiency.27 This observation emphasizes the fact that metabolic processes influencing HDL-cholesterol levels are responsive to statin treatment and that these processes may be dependent on the presence of functional LDL-receptors. Indeed, atorvastatin therapy has been shown to decrease the mass and activity of the CE transfer protein28,29 and also to decrease hepatic lipase activity;30 both effects favor elevation of HDL-cholesterol and may be directly influenced by plasma concentrations of VLDL and LDL particles. Thus, in HFH patients treated with high-dose atorvastatin, intravascular remodeling of HDL particles could be partially altered because of LDL-receptor deficiency, revealing a statin-induced upregulation of Cla-1 expression with a subsequent lowering impact on HDL-cholesterol levels. This mechanistic hypothesis requires further studies to confirm it but suggests that 1 additional beneficial action of statins may involve minor changes in the hepatic expression of Cla-1, a low-affinity but high-capacity receptor that may positively influence the dynamic flux of the reverse cholesterol transport pathway.
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Acknowledgments |
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This study was supported by National Institute for Health and Medical Research (INSERM) and by a research grant from Bayer HealthCare (Wuppertal, Germany). M.T. was the recipient of a young investigator award of the XIII International Symposium on Atherosclerosis (Kyoto, Japan, October 2003) and of a doctoral fellowship from the French government. It is a pleasure to acknowledge stimulating discussion with Dr K.D. Bremm.
Received June 8, 2004; accepted September 8, 2004.
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