Remodeling of HDL by CETP in vivo and by CETP and hepatic lipase in vitro results in enhanced uptake of HDL CE by cells expressing scavenger receptor B-I.

The transport of HDL cholesteryl esters (CE) from plasma to the liver involves a direct uptake pathway, mediated by hepatic scavenger receptor B-I (SR-BI), and an indirect pathway, involving the exchange of HDL CE for triglycerides (TG) of TG-rich lipoproteins by cholesteryl ester transfer protein (CETP). We carried out HDL CE turnover studies in mice expressing human CETP and/or human lecithin:cholesterol acyltransferase (LCAT) transgenes on a background of human apoA-I expression. The fractional clearance of HDL CE by the liver was delayed by LCAT transgene, while the CETP transgene increased it. However, there was no incremental transfer of HDL CE radioactivity to the TG-rich lipoprotein fraction in mice expressing CETP, suggesting increased direct removal of HDL CE in the liver. To evaluate the possibility that this might be mediated by SR-BI, HDL isolated from plasma of the different groups of transgenic mice was incubated with SR-BI transfected or control CHO cells. HDL isolated from mice expressing CETP showed a 2- to 4-fold increase in SR-BI-mediated HDL CE uptake, compared to HDL from mice lacking CETP. The addition of pure CETP to HDL in cell culture did not lead to increased selective uptake of HDL CE by cells. However, when human HDL was enriched with TG by incubation with TG-rich lipoproteins in the presence of CETP, then treated with hepatic lipase, there was a significant enhancement of HDL CE uptake. Thus, the remodeling of human HDL by CETP, involving CE;-TG interchange, followed by the action of hepatic lipase (HL), leads to the enhanced uptake of HDL CE by cellular SR-BI.These observations suggest that in animals such as humans in which both the selective uptake and CETP pathways are active, the two pathways could operate in a synergistic fashion to enhance reverse cholesterol transport.     

Cholesteryl ester transfer protein expression partially attenuates the adverse effects of SR-BI receptor deficiency on cholesterol metabolism and atherosclerosis

Scavenger receptor SR-BI significantly contributes to HDL cholesterol metabolism and atherogenesis in mice. However, the role of SR-BI may not be as pronounced in humans due to cholesteryl ester transfer protein (CETP) activity. To address the impact of CETP expression on the adverse effects associated with SR-BI deficiency, we cross-bred our SR-BI conditional knock-out mouse model with CETP transgenic mice. CETP almost completely restored the abnormal HDL-C distribution in SR-BI-deficient mice. However, it did not normalize the elevated plasma free to total cholesterol ratio characteristic of hepatic SR-BI deficiency. Red blood cell and platelet count abnormalities observed in mice liver deficient for SR-BI were partially restored by CETP, but the elevated erythrocyte cholesterol to phospholipid ratio remained unchanged. Complete deletion of SR-BI was associated with diminished adrenal cholesterol stores, whereas hepatic SR-BI deficiency resulted in a significant increase in adrenal gland cholesterol content. In both mouse models, CETP had no impact on adrenal cholesterol metabolism. In diet-induced atherosclerosis studies, hepatic SR-BI deficiency accelerated aortic lipid lesion formation in both CETP-expressing (4-fold) and non-CETP-expressing (8-fold) mice when compared with controls. Impaired macrophage to feces reverse cholesterol transport in mice deficient for SR-BI in liver, which was not corrected by CETP, most likely contributed by such an increase in atherosclerosis susceptibility. Finally, comparison of the atherosclerosis burden in SR-BI liver-deficient and fully deficient mice demonstrated that SR-BI exerted an atheroprotective activity in extra-hepatic tissues whether CETP was present or not. These findings support the contention that the SR-BI pathway contributes in unique ways to cholesterol metabolism and atherosclerosis susceptibility even in the presence of CETP.     

Identification of small PDZK1-associated protein,DD96/MAP17, as a regulator of PDZK1 and plasma high density lipoprotein levels

Scavenger receptor class B, type I (SR-BI) is the high density lipoprotein (HDL) receptor essential for hepatic uptake of HDL cholesterol. SR-BI was shown to impact plasma HDL levels and be anti-atherogenic. Thus, the ability to regulate hepatic SR-BI may allow for the modulation of plasma HDL cholesterol and progression of atherosclerosis. However, regulation of SR-BI in liver is not well understood. Recently, the PDZ domain containing protein PDZK1 was shown to interact with SR-BI and may serve an essential role in SR-BI cell surface expression. Here we identify an in vivo PDZK1-interacting protein that we named small PDZK1-associated protein (SPAP; also known as DD96/MAP17). Unexpectedly, we found that hepatic overexpression of SPAP in mice resulted in liver deficiency of PDZK1. The absence of PDZK1 in SPAP transgenic mice resulted in a deficiency of SR-BI in liver and markedly increased plasma HDL. Metabolic labeling experiments showed that the proteasome plays a role in the turnover of newly synthesized PDZK1, but that SPAP overexpression in liver increased PDZK1 turnover in an alternate, proteasome-independent pathway. Thus, SPAP may be an endogenous regulator of cellular PDZK1 levels by regulating PDZK1 turnover.     

Cholesteryl ester transfer protein and phospholipid transfer protein have nonoverlapping functions in vivo

Plasma phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) are homologous molecules that mediate neutral lipid and phospholipid exchange between plasma lipoproteins. Biochemical experiments suggest that only CETP can transfer neutral lipids but that there could be overlap in the ability of PLTP and CETP to transfer or exchange phospholipids. Recently developed PLTP gene knock-out (PLTP0) mice have complete deficiency of plasma phospholipid transfer activity and markedly reduced high density lipoprotein (HDL) levels. To see whether CETP can compensate for PLTP deficiency in vivo, we bred the CETP transgene (CETPTg) into the PLTP0 background. Using an in vivo assay to measure the transfer of [(3)H]PC from VLDL into HDL or an in vitro assay that determined [(3)H]PC transfer from vesicles into HDL, we could detect no phospholipid transfer activity in either PLTP0 or CETPTg/PLTP0 mice. On a chow diet, HDL-PL, HDL-CE, and HDL-apolipoprotein AI in CETPTg/PLTP0 mice were significantly lower than in PLTP0 mice (45 +/- 7 versus 79 +/- 9 mg/dl; 9 +/- 2 versus 16 +/- 5 mg/dl; and 51 +/- 6 versus 100 +/- 9, arbitrary units, respectively). Similar results were obtained on a high fat, high cholesterol diet. These results indicate 1) that there is no redundancy in function of PLTP and CETP in vivo and 2) that the combination of the CETP transgene with PLTP deficiency results in an additive lowering of HDL levels, suggesting that the phenotype of a human PLTP deficiency state would include reduced HDL levels.     

High density lipoprotein metabolism in low density lipoprotein receptor-deficient mice

The LDL receptor (LDLR) and scavenger receptor class B type I (SR-BI) play physiological roles in LDL and HDL metabolism in vivo. In this study, we explored HDL metabolism in LDLR-deficient mice in comparison with WT littermates. Murine HDL was radiolabeled in the protein (¹²⁵I) and in the cholesteryl ester (CE) moiety ([³H]). The metabolism of ¹²⁵I-/[³H]HDL was investigated in plasma and in tissues of mice and in murine hepatocytes. In WT mice, liver and adrenals selectively take up HDL-associated CE ([³H]). In contrast, in LDLR^−/− mice, selective HDL CE uptake is significantly reduced in liver and adrenals. In hepatocytes isolated from LDLR^−/− mice, selective HDL CE uptake is substantially diminished compared with WT liver cells. Hepatic and adrenal protein expression of lipoprotein receptors SR-BI, cluster of differentiation 36 (CD36), and LDL receptor-related protein 1 (LRP1) was analyzed by immunoblots. The respective protein levels were identical both in hepatic and adrenal membranes prepared from WT or from LDLR^−/− mice. In summary, an LDLR deficiency substantially decreases selective HDL CE uptake by liver and adrenals. This decrease is independent from regulation of receptor proteins like SR-BI, CD36, and LRP1. Thus, LDLR expression has a substantial impact on both HDL and LDL metabolism in mice.     

Cholesteryl ester transfer protein corrects dysfunctional high density lipoproteins and reduces aortic atherosclerosis in lecithin cholesterol acyltransferase transgenic mice

Expression of human lecithin cholesterol acyltransferase (LCAT) in mice (LCAT-Tg) leads to increased high density lipoprotein (HDL) cholesterol levels but paradoxically, enhanced atherosclerosis. We have hypothesized that the absence of cholesteryl ester transfer protein (CETP) in LCAT-Tg mice facilitates the accumulation of dysfunctional HDL leading to impaired reverse cholesterol transport and the development of a pro-atherogenic state. To test this hypothesis we cross-bred LCAT-Tg with CETP-Tg mice. On both regular chow and high fat, high cholesterol diets, expression of CETP in LCAT-Tg mice reduced total cholesterol (-39% and -13%, respectively; p < 0.05), reflecting a decrease in HDL cholesterol levels. CETP normalized both the plasma clearance of [(3)H]cholesteryl esters ([(3)H]CE) from HDL (fractional catabolic rate in days(-1): LCAT-Tg = 3.7 +/- 0.34, LCATxCETP-Tg = 6.1 +/- 0.16, and controls = 6.4 +/- 0.16) as well as the liver uptake of [(3)H]CE from HDL (LCAT-Tg = 36%, LCATxCETP-Tg = 65%, and controls = 63%) in LCAT-Tg mice. On the pro-atherogenic diet the mean aortic lesion area was reduced by 41% in LCATxCETP-Tg (21.2 +/- 2.0 micrometer(2) x 10(3)) compared with LCAT-Tg mice (35.7 +/- 2.0 micrometer(2) x 10(3); p < 0.001). Adenovirus-mediated expression of scavenger receptor class B (SR-BI) failed to normalize the plasma clearance and liver uptake of [(3)H]CE from LCAT-Tg HDL. Thus, the ability of SR-BI to facilitate the selective uptake of CE from LCAT-Tg HDL is impaired, indicating a potential mechanism leading to impaired reverse cholesterol transport and atherosclerosis in these animals. We conclude that CETP expression reduces atherosclerosis in LCAT-Tg mice by restoring the functional properties of LCAT-Tg mouse HDL and promoting the hepatic uptake of HDL-CE. These findings provide definitive in vivo evidence supporting the proposed anti-atherogenic role of CETP in facilitating HDL-mediated reverse cholesterol transport and demonstrate that CETP expression is beneficial in pro-atherogenic states that result from impaired reverse cholesterol transport.     

Role of cholesteryl ester transfer protein in selective uptake of high density lipoprotein cholesteryl esters by adipocytes

Previous reports attributed cholesteryl ester transfer protein (CETP)-mediated HDL cholesteryl ester (CE) selective uptake to the CETP-mediated transfer of CE from HDL to newly secreted apolipoprotein B-containing lipoproteins, which are then internalized by the LDL receptor (LDL-R). CETP has also been implicated in the remodeling of HDL, which renders it a better substrate for selective uptake by scavenger receptor class B type I (SR-BI). However, CETP-mediated selective uptake of HDL3-derived CE was not diminished in LDL-R null adipocytes, SR-BI null adipocytes, or in the presence of the receptor-associated protein. We found that monensin treatment or energy depletion of the SW872 liposarcoma cells with 2-deoxyglucose and NaN3 had no effect on CETP-mediated selective uptake, demonstrating that endocytosis is not required. This is supported by data indicating that CETP transfers CE into a compartment from which it can be extracted by unlabeled HDL. CETP could also mediate the selective uptake of HDL3-derived triacylglycerol (TG) and phospholipid (PL). The CETP-specific kinetics for TG and CE uptake were similar, and both reached saturation at approximately 5 microg/ml HDL. In contrast, CETP-specific PL uptake did not attain saturation at 5 microg/ml HDL and was approximately 6-fold greater than the uptake of CE. We propose two possible mechanisms to account for the role of CETP in selective uptake.     

A carboxyl-terminal PDZ-interacting domain of scavenger receptor B,type I is essential for cell surface expression in liver

Scavenger receptor B, type I (SR-BI) was recently shown to interact with a PDZ domain-containing protein, PDZK1 (CLAMP/Diphor-1/CAP70/NaPi-Cap1), but the importance of this interaction in vivo in terms of SR-BI function has not been determined. In an effort to elucidate the role of this interaction in vivo, the PDZK1-interacting domain of SR-BI was identified and mutated and expressed liver-specifically in mice. The PDZKI-interacting domain on SR-BI was identified as the last three carboxyl-terminal amino acids, Arg-Lys-Leu. A mutant SR-BI (SR-BIdel509) that lacked only the leucine in the PDZ-interacting domain failed to interact with PDZK1 in vitro, while showing normal selective uptake function in nonpolarized cells. Transgenic mice with liver overexpression of SR-BIdel509 showed marked accumulation of SR-BI mRNA with only a moderate increase in SR-BI protein in liver, with no reduction in plasma cholesterol levels. Measurement of cell surface SR-BI levels and HDL cholesteryl ester-selective uptake in primary hepatocytes from transgenic mice revealed that SR-BIdel509 was not expressed at the plasma membrane correlating with normal levels of selective uptake compared with hepatocytes from nontransgenic littermates. This study indicates that the PDZK1-interacting domain of SR-BI is essential for cell surface expression of SR-BI in liver and suggests that PDZK1 or other PDZ domain proteins may play an important role in regulating SR-BI cell surface expression and hence reverse cholesterol transport.     

Hepatic lipid accumulation in apolipoprotein C-I-deficient mice is potentiated by cholesteryl ester transfer protein

The impact of apolipoprotein C-I (apoC-I) deficiency on hepatic lipid metabolism was addressed in mice in the presence or the absence of cholesteryl ester transfer protein (CETP). In addition to the expected moderate reduction in plasma cholesterol levels, apoCIKO mice showed significant increases in the hepatic content of cholesteryl esters (+58%) and triglycerides (+118%) and in biliary cholesterol concentration (+35%) as compared with wild-type mice. In the presence of CETP, hepatic alterations resulting from apoC-I deficiency were enforced, with up to 58% and 302% increases in hepatic levels of cholesteryl esters and triglycerides in CETPTg/apoCIKO mice versus CETPTg mice, respectively. Biliary levels of cholesterol, phospholipids, and bile acids were increased by 88, 77, and 20%, respectively, whereas total cholesterol, HDL cholesterol, and triglyceride concentrations in plasma were further reduced in CETPTg/apoCIKO mice versus CETPTg mice. Finally, apoC-I deficiency was not associated with altered VLDL production rate. In line with the previously recognized inhibition of lipoprotein clearance by apoC-I, apoC-I deficiency led to decreased plasma lipid concentration, hepatic lipid accumulation, and increased biliary excretion of cholesterol. The effect was even greater when the alternate reverse cholesterol transport pathway via VLDL/LDL was boosted in the presence of CETP.     

Fenofibrate induces a novel degradation pathway for scavenger receptor B-I independent of PDZK1

Fibrate drugs improve cardiovascular health by lowering plasma triglycerides, normalize low density lipoprotein levels, and raise high density lipoprotein (HDL) levels in patients with dyslipidemias. The HDL-raising effect of fibrates has been shown to be due in part to an increase in human apolipoprotein AI gene expression. However, it has recently been shown that fibrates can affect HDL metabolism in mouse by significantly decreasing hepatic levels of the HDL receptor scavenger receptor B-I (SR-BI) and the PDZ domain containing protein PDZK1. PDZK1 is essential for maintaining hepatic SR-BI levels. Therefore, decreased SR-BI might be secondary to decreased PDZK1, but the mechanism by which fibrates lower SR-BI has not been elucidated. Here we show that feeding PDZK1-deficient mice fenofibrate resulted in the near absence of SR-BI in liver, definitively demonstrating that the effect of fenofibrate on SR-BI is PDZK1-independent. Metabolic labeling experiments in primary hepatocytes from fenofibrate-fed mice demonstrated that fenofibrate enhanced the degradation of SR-BI in a post-endoplasmic reticulum compartment. Moreover, fenofibrate-induced degradation of SR-BI was independent of the proteasome, calpain protease, or the lysosome, and antioxidants did not inhibit fenofibrate-induced degradation of SR-BI. Using metabolic labeling coupled with cell surface biotinylation assays, fenofibrate did not inhibit SR-BI trafficking to the plasma membrane. Together, the data support a model in which fenofibrate enhances the degradation of SR-BI in a post-ER, post-plasma membrane compartment. The further elucidation of this novel degradation pathway may provide new insights into the physiological and pathophysiological regulation of hepatic SR-BI.     

Scavenger receptor class B type I-mediated uptake of serum cholesterol is essential for optimal adrenal glucocorticoid production

Impaired scavenger receptor class B type I (SR-BI)-mediated uptake of HDL-cholesterol esters (HDL-CE) induces adrenal insufficiency in mice. Humans contain an alternative route of HDL-CE clearance, namely through the transfer by cholesteryl ester transfer protein (CETP) to apolipoprotein B lipoproteins for subsequent uptake via the LDL receptor. In this study, we determined whether CETP can compensate for loss of adrenal SR-BI. Transgenic expression of human CETP (CETP Tg) in SR-BI knockout (KO) mice increased adrenal HDL-CE clearance from 33–58% of the control value. SR-BI KO/CETP Tg and SR-BI KO mice displayed adrenal hypertrophy due to equally high plasma adrenocorticotropic hormone levels. Adrenal cholesterol levels and plasma corticosterone levels were 38–52% decreased in SR-BI KO mice with and without CETP expression. SR-BI KO/CETP Tg mice also failed to increase their corticosterone level after lipopolysaccharide challenge, leading to an identical >4-fold increased tumor necrosis factor-α response compared with controls. These data indicate that uptake of CE via other routes than SR-BI is not sufficient to generate the cholesterol pool needed for optimal adrenal steroidogenesis. In conclusion, we have shown that CETP-mediated transfer of HDL-CE is not able to reverse adrenal insufficiency in SR-BI knockout mice. Thus, SR-BI-mediated uptake of serum cholesterol is essential for optimal adrenal function.     

Influence of PDZK1 on lipoprotein metabolism and atherosclerosis

PDZK1 is a scaffold protein containing four PDZ protein interaction domains, which bind to the carboxy termini of a number of membrane transporter proteins, including ion channels (e.g., CFTR) and cell surface receptors. One of these, the HDL receptor, scavenger receptor class B type I (SR-BI), exhibits a striking, tissue-specific dependence on PDZK1 for its expression and activity. In PDZK1 knockout (KO) mice there is a marked reduction of SR-BI protein expression (approximately 95%) in the liver, but not in steroidogenic tissues or, as we show in this report, in bone marrow- or spleen-derived macrophages, or lung-derived endothelial cells. Because of hepatic SR-BI deficiency, PDZK1 KO mice exhibit dyslipidemia characterized by elevated plasma cholesterol carried in abnormally large HDL particles. Here, we show that inactivation of the PDZK1 gene promotes the development of aortic root atherosclerosis in apolipoprotein E (apoE) KO mice fed with a high fat/high cholesterol diet. However, unlike complete SR-BI-deficiency in SR-BI/apoE double KO mice, PDZK1 deficiency in PDZK1/apoE double knockout mice did not result in development of occlusive coronary artery disease or myocardial infarction, presumably because of their residual expression of SR-BI. These findings demonstrate that deficiency of an adaptor protein essential for normal expression of a lipoprotein receptor promotes atherosclerosis in a murine model. They also define PDZK1 as a member of the family of proteins that is instrumental in preventing cardiovascular disease by maintaining normal lipoprotein metabolism.     

Human plasma cholesteryl ester transfer protein enhances the transfer of cholesteryl ester from high density lipoproteins into cultured HepG2 cells

The role of human plasma cholesteryl ester transfer protein (CETP) in the cellular uptake of high density lipoprotein (HDL) cholesteryl ester (CE) was studied in a liver tumor cell line (HepG2). When HepG2 cells were incubated with [3H]cholesteryl ester-labeled HDL3 in the presence of increasing concentrations of CETP there was a progressive increase in cell-associated radioactivity to levels that were 2.8 times control. The CETP-dependent uptake of HDL-CE was found to be saturated by increasing concentrations of both CETP and HDL. The CETP-dependent uptake of CE radioactivity increased continuously during an 18-h incubation. In contrast to the effect on cholesteryl ester, CETP failed to enhance HDL protein cell association or degradation. Enhanced uptake of HDL cholesteryl ester was shown for the d greater than 1.21 g/ml fraction of human plasma, partially purified CETP, and CETP purified to homogeneity, but not for the d greater than 1.21 g/ml fraction of rat plasma which lacks cholesteryl ester transfer activity. HDL cholesteryl ester entering the cell under the influence of CETP was largely degraded to free cholesterol by a process inhibitable by chloroquine. CETP enhanced uptake of HDL [3H]CE in cultured smooth muscle cells and to a lesser extent in fibroblasts but did not significantly influence uptake in endothelial cells or J774 macrophages. These experiments show that, in addition to its known role in enhancing the exchange of CE between lipoproteins, plasma CETP can facilitate the in vitro selective transfer of CE from HDL into certain cells.     

Dual effects on HDL metabolism by cholesteryl ester transfer protein inhibition in HepG2 cells

Cholesteryl ester transfer protein (CETP) promotes reverse cholesterol transport via exchange of cholesteryl ester and triglyceride among lipoproteins. Here, we focused on HDL metabolism during inhibition of CETP expression by using CETP antisense oligodeoxynucleotides (ODNs) in HepG2 cells. CETP secretion was decreased by 70% in mRNA levels and by 52% in mass 20 h after ODNs against CETP were delivered to HepG2 cells. Furthermore, as a consequence of the downregulation of CETP, the expression of scavenger receptor class B type I (SR-BI), an HDL receptor, was also reduced by approximately 50% in mRNA and protein levels, whereas the apolipoprotein A-I (apoA-I) expression and secretion were increased by 30 and 92%, respectively. In a functional study, the selective uptake of (125)I-[(14)C]cholesteryl oleate-labeled HDL(3) was decreased. Cholesterol efflux to apoA-I and HDL(3) was significantly increased by 88 and 37%, respectively. Moreover, the CE levels in cells after antisense treatment were elevated by 20%, which was related to the about twofold increase of cholesterol esterification and increased acyl-CoA:cholesterol acyltransferase 1 mRNA levels. Taken together, these findings suggest that although acute suppression of CETP expression leads to an elevation in cellular cholesterol stores, apoA-I secretion, and cellular cholesterol efflux to apoA-I, the return of HDL-CE to hepatocytes via an SR-BI pathway was inhibited in vitro. Thus antisense inhibition of hepatic CETP expression manifests dual effects: namely, increased formation of HDL and suppression of catabolism of HDL-CE, probably via the SR-BI pathway.     

Triglyceride-rich lipoprotein metabolism in unique VLDL receptor, LDL receptor, and LRP triple-deficient mice

The very low density lipoprotein receptor (VLDLR), low density lipoprotein receptor (LDLR), and low density lipoprotein receptor-related protein (LRP) are the three main apolipoprotein E-recognizing endocytic receptors involved in the clearance of triglyceride (TG)-rich lipoproteins from plasma. Whereas LDLR deficiency in mice results in the accumulation of plasma LDL-sized lipoproteins, VLDLR or LRP deficiency alone only minimally affects plasma lipoproteins. To investigate the combined effect of the absence of these receptors on TG-rich lipoprotein levels, we have generated unique VLDLR, LDLR, and LRP triple-deficient mice. Compared with wild-type mice, these mice markedly accumulated plasma lipids and lipases. These mice did not show aggravated hyperlipidemia compared with LDLR and LRP double-deficient mice, but plasma TG was increased after high-fat diet feeding. In addition, these mice showed a severely decreased postprandial TG clearance typical of VLDLR-deficient (VLDLR-/-) mice. Collectively, although VLDLR deficiency in LRP- and LDLR-/- mice does not aggravate hyperlipidemia, these triple-deficient mice represent a unique model of markedly delayed TG clearance on a hyperlipidemic background.     

Analysis of expression of genes involved in apolipoprotein E-based lipoprotein metabolism in pregnant mice deficient in the receptor-associated protein, the low density lipoprotein receptor, or apolipoprotein E.

Mice deficient in receptor-associated protein (RAP) were phenotypically normal, but in contrast to results previously reported in RAP(-/-) mice, nearly 50% of the offspring died at or shortly after birth. To attempt to determine the reason for this, we analyzed the regulation of expression of genes involved in apolipoprotein E (apoE)-based mechanisms in RAP-deficient mice and compared this to results in mice deficient in low density lipoprotein receptor (LDLR) or apoE. The major finding concerned a large increase in hepatic lipoprotein receptor-related protein (LRP) mRNA and LDLR mRNA levels in pregnant RAP knockout mice. This is in contrast to the down-regulation of LRP mRNA and LDLR mRNA, which is normally seen in wild-type mice. Also in LDLR knockout mice, a significant up-regulation in expression of LRP mRNA was demonstrated. In apoE knockout mice, hepatic LRP mRNA did not change significantly, while hepatic LDLR mRNA expression was increased. In placenta and uterus, the deficiency of RAP did not markedly affect the expression of LRP and LDLR. Lipoprotein lipase mRNA and apoE mRNA increased during pregnancy in all mice, independent of their genetic status. The current study does not directly explain the increased mortality of RAP(-/-) pups. The data demonstrate, however, important relative changes in expression of the genes analyzed, an indication that LRP and LDLR play an important role in lipid metabolism during pregnancy.