Triglyceride-rich lipoproteins of subjects heterozygous for apolipoprotein E2(Lys146-->Gln) are inefficiently converted to cholesterol-rich lipoproteins

From degenerative disease to malignant tumors: Insight to the function of ApoE

Apolipoprotein E (ApoE) is a multifunctional protein involved in lipid transport and lipoprotein metabolism, mediating lipid distribution/redistribution in tissues and cells. It can also regulate inflammation and immune function, maintain cytoskeleton stability, and improve neural tissue Function. Due to genetic polymorphisms of ApoE (ε2, ε3, and ε4), its three common structural isoforms (ApoE2, ApoE3, ApoE4) are also associated with the risk of many diseases, especially degenerative diseases, such as vascular degenerative diseases including atherosclerosis (AS), coronary heart disease (CHD), and neurodegenerative disease like Alzheimer's disease (AD). The frequency of the ε4 allele and APOE variants were significantly higher than that of the ε2 and ε3 alleles in the patients with CHD or AD. In recent years, ApoE has frequently appeared in tumor research and become a tumor biomarker gradually. It has been found that ApoE is highly expressed in most solid tumor tissues, such as glioblastoma, gastric cancer, pancreatic ductal cell carcinoma, etc. Studies illustrated that ApoE could regulate the polarization changes of macrophages, participate in the construction of tumor immune microenvironment, regulate tumor inflammation and immune response and play a role in tumor progression, invasion, and metastasis. Of course, many functions of ApoE and its relationship with diseases are still under research. By reviewing the structure and function of ApoE from degeneration diseases to tumor neoplasms, we hope to better understand such a biomarker and further explore the value of ApoE in later studies.

Establishing the relationship between Familial Dysbetalipoproteinemia and genetic variants in the APOE gene

Familial Dysbetalipoproteinemia (FD) is the second most common monogenic dyslipidemia and is associated with a very high cardiovascular risk due to cholesterol‐enriched remnant lipoproteins. FD is usually caused by a recessively inherited variant in the APOE gene (ε2ε2), but variants with dominant inheritance have also been described. The typical dysbetalipoproteinemia phenotype has a delayed onset and requires a metabolic hit. Therefore, the diagnosis of FD should be made by demonstrating both the genotype and dysbetalipoproteinemia phenotype. Next Generation Sequencing is becoming more widely available and can reveal variants in the APOE gene for which the relation with FD is unknown or uncertain. In this article, two approaches are presented to ascertain the relationship of a new variant in the APOE gene with FD. The comprehensive approach consists of determining the pathogenicity of the variant and its causal relationship with FD by confirming a dysbetalipoproteinemia phenotype, and performing in vitro functional tests and, optionally, in vivo postprandial clearance studies. When this is not feasible, a second, pragmatic approach within reach of clinical practice can be followed for individual patients to make decisions on treatment, follow‐up, and family counseling.

Apolipoprotein E1 Baden (Arg(180)-->Cys). A new apolipoprotein E variant associated with hypertriglyceridemia

Apolipoprotein (apo) E mediates the removal of chylomicron and very low density lipoprotein remnants from plasma. It is polymorphic in sequence and the products of the three common alleles (epsilon 2, epsilon 3, epsilon 4) differ from one another in their binding to lipoprotein receptors. ApoE2 is defective in binding and homozygosity for apoE2 is associated with type III hyperlipoproteinemia (HLP). Other rare isoforms of apoE have been found to be associated either with dominant type III HLP or with the development of hypertriglyceridemia. We identified a 42 year-old hypertriglyceridemic woman with an apoE phenotype 3/1. Restriction isotyping using AflIII/HaeII resulted in an apparent apoE genotype 3/2, suggesting that the mutation occurred in an epsilon 2 allele. DNA sequence analysis revealed a C-->T point mutation at the first position of the codon for amino acid residue 180 of the mature apoE. This predicted a change Arg(180)-->Cys. The mutation altered a recognition site for the endonuclease HaeII, which allowed us rapidly to screen for this mutation. In relatives of the proband, apoE1 Baden was consistently associated with hypertriglyceridemia. Similar to other apoE variants linked to hypertriglyceridemia, the Arg(180)-->Cys mutation is located within the lipid binding domain of apoE. We therefore suggest that apoE1 Baden may cause hypertrigylceridemia, possibly by inhibiting the hydrolysis of triglycerides associated with very low density lipoproteins.

Apolipoprotein E2 (Lys146-->Gln) causes hypertriglyceridemia due to an apolipoprotein E variant-specific inhibition of lipolysis of very low density lipoproteins-triglycerides

The apolipoprotein E2 (Lys146-->Gln) variant is associated with a dominant form of familial dysbetalipoproteinemia. Heterozygous carriers of this variant have elevated levels of plasma triglycerides, cholesterol, and apolipoprotein E (apoE). It was hypothesized that the high amounts of triglycerides in the very low density lipoprotein (VLDL) fraction are due to a disturbed lipolysis of VLDL. To test this hypothesis, apoE knockout mice were injected with an adenovirus containing the human APOE*2 (Lys146-->Gln) gene, Ad-E2(146), under the control of the cytomegalovirus promoter. ApoE knockout mice injected with an adenovirus vector encoding human apoE3 (Ad-E3) were used as controls. Five days after adenovirus injection, plasma cholesterol levels of mice injected with a high dose of Ad-E2(146) (2x10(9) plaque-forming units) were not changed compared with preinjection levels, whereas in the group who received a low dose of Ad-E2(146) (5x10(8) plaque-forming units) and in the groups injected with a low or a high dose of Ad-E3, plasma cholesterol levels were decreased 5-, 6-, and 12-fold, respectively. Plasma triglycerides were not affected in mice injected with Ad-E3. In contrast, a 7-fold increase in plasma triglycerides was observed in mice injected with the low dose of Ad-E2(146) compared with mice injected with Ad-E3. Injection with the high dose of Ad-E2(146) resulted in a dramatic increase of plasma triglycerides (50-fold compared with Ad-E3 injection). In vitro lipolysis experiments showed that the lipolysis rate of VLDLs containing normal amounts of apoE2 (Lys146-->Gln) was decreased by 54% compared with that of VLDLs containing comparable amounts of apoE3. The in vivo VLDL-triglyceride production rate of Ad-E2(146)-injected mice was not significantly different from that of Ad-E3-injected mice. These results demonstrate that expression of apoE2 (Lys146-->Gln) causes hypertriglyceridemia due to an apoE variant-specific inhibition of the hydrolysis of VLDL-triglycerides.

Hyperlipidemia of ApoE2(Arg(158)-Cys) and ApoE3-Leiden transgenic mice is modulated predominantly by LDL receptor expression

To investigate the relative roles of the LDL receptor- and non-LDL receptor-mediated pathways in the clearance of apolipoprotein E (apoE) variants in vivo, we have generated apoE2(Arg(158)-Cys) (apoE2) and apoE3-Leiden transgenic mice deficient for the endogenous mouse Apoe and Ldl receptor genes (Apoe-/-.Ldlr-/- mice). Unexpectedly, on the Apoe-/-.Ldlr-/- background, expression of neither apoE2 nor apoE3-Leiden results in a decrease of the hyperlipidemia. In contrast, serum cholesterol levels are increased by the introduction of apoE2 and apoE3-Leiden in Apoe-/-.Ldlr-/- mice (to 39.1+/-7.1 and 37.6+/-7.6 mmol/L, respectively, from 25. 9+/-6.5 mmol/L). In addition, in these transgenic mice, the serum triglyceride levels are substantially increased (to 9.6+/-7.0 and 5. 8+/-2.8 mmol/L, respectively, from 0.7+/-0.5 mmol/L), which is associated with a decreased efficiency of in vitro LPL-mediated lipolysis of circulating VLDL. The VLDL-triglyceride secretion rate is not affected by the expression of apoE2 or apoE3-Leiden on the Apoe-/-.Ldlr-/- background. These results indicate that in the absence of the LDL receptor, clearance of triglyceride-rich apoE2 and apoE3-Leiden-containing lipoproteins via alternative hepatic receptors, such as the LDL receptor-related protein (LRP) is inefficient. Although apoE2 and apoE3-Leiden are disturbed in binding to the LDL receptor in vitro, expression of 1 or 2 mouse Ldlr alleles in an apoE2.Apoe-/- or apoE3-Leiden.Apoe-/- background results in a gene dose-dependent decrease of the hyperlipidemia. Furthermore, overexpression of the LDL receptor via adenovirus-mediated gene transfer rescues the hyperlipidemia associated with apoE2 and apoE3-Leiden expression. These data indicate that in apoE2 and apoE3-Leiden transgenic mice, the LDL receptor constitutes the predominant route for clearance of VLDL remnants, carrying even poorly binding apoE variants, and that this pathway is functional despite an apoE-mediated disturbance in VLDL triglyceride lipolysis.

Dissection of the complex role of apolipoprotein E in lipoprotein metabolism and atherosclerosis using mouse models

Transgenic and knockout mice have been instrumental in delineating the role of apolipoprotein (apo) E in lipoprotein metabolism and atherosclerosis. The severe hypercholesterolemia and premature atherosclerosis of the apoE knockout mouse have been the starting point from which various physiologic processes have been identified in which apoE plays a critical role. These processes include 1) very low density lipoprotein (VLDL) triglyceride production; 2) lipoprotein lipase mediated triglyceride lipolysis; 3) VLDL remnant clearance and intracellular processing; and 4) the efflux of cellular cholesterol. In this review we will discuss the recent insight in the role of apoE in these processes, which has been obtained using a variety of in vivo and in vitro approaches to modify apoE expression and function.

Nascent very-low-density lipoprotein triacylglycerol hydrolysis by lipoprotein lipase is inhibited by apolipoprotein E in a dose-dependent manner

In the present study it was investigated whether apolipoprotein (apoE) can inhibit the lipoprotein lipase (LPL)-mediated hydrolysis of very-low-density-lipoprotein (VLDL) triacylglycerols (TAGs). Previous studies have suggested such an inhibitory role for apoE by using as a substrate for LPL either plasma VLDL or artificial TAG emulsions. To mimic the in vivo situation more fully, we decided to investigate the effect of apoE on the LPL-mediated TAG hydrolysis by using VLDL from apoE-deficient mice that had been enriched with increasing amounts of apoE. Furthermore, since plasma VLDL isolated from apoE-deficient mice was relatively poor in TAGs and strongly enriched in cholesterol as compared with VLDL from wild-type mice, we used nascent VLDL obtained by liver perfusions. Nascent VLDL (d<1. 006) isolated from the perfusate of the apoE-deficient mouse liver was rich in TAGs. Addition of increasing amounts of apoE to apoE-deficient nascent VLDL effectively decreased TAG lipolysis as compared with that of apoE-deficient nascent VLDL without the addition of apoE (63.1+/-6.3 and 20.8+/-1.8% of the control value at 2.7 microg and 29.6 microg of apoE/mg of TAG added respectively). Since, in vivo, LPL is attached to heparan sulphate proteoglycans (HSPG) at the endothelial matrix, we also performed lipolysis assays with LPL bound to HSPG in order to preserve the interaction of the lipoprotein particle with the HSPG-LPL complex. In this lipolysis system a concentration-dependent decrease in the TAG lipolysis was also observed with increasing amounts of apoE on nascent VLDL, although to a lesser extent than with LPL in solution (72.3+/-3.6% and 56.6+/-1.7% of control value at 2.7 microg and 29.6 microg of apoE/mg TAGs added respectively). In conclusion, the enrichment of the VLDL particle with apoE decreases its suitability as a substrate for LPL in a dose-dependent manner.

In the absence of endogenous mouse apolipoprotein E, apolipoprotein E*2(Arg-158 --> Cys) transgenic mice develop more severe hyperlipoproteinemia than apolipoprotein E*3-Leiden transgenic mice

Apolipoprotein E*2(Arg-158 --> Cys) (APOE*2) transgenic mice were generated and compared to the previously generated apolipoprotein E*3-Leiden (APOE*3-Leiden) transgenic mice to study the variable expression of hyperlipoproteinemia associated with these two APOE variants. In the presence of the endogenous mouse Apoe gene, the expression of the APOE*3-Leiden gene resulted in slightly elevated levels of serum cholesterol as compared with control mice (2.7 +/- 0. 5 versus 2.1 +/- 0.2 mmol/liter, respectively), whereas the expression of the APOE*2(Arg-158 --> Cys) gene did not affect serum cholesterol levels, even after high/fat cholesterol feeding. The extreme cholesterol level usually found in apoE-deficient mice (Apoe-/- mice; 23.6 +/- 5.0 mmol/liter) could be rescued by introducing the APOE*3-Leiden gene (APOE*3-Leiden.Apoe-/-; 3.6 +/- 1. 5 mmol/liter), whereas the expression of the APOE*2(Arg-158 --> Cys) gene in Apoe-/- mice minimally reduced serum cholesterol levels (APOE*2.Apoe-/-; 16.6 +/- 2.9 mmol/liter). In vivo very low density lipoprotein (VLDL) turnover studies revealed that APOE*2.Apoe-/- VLDL and APOE*3-Leiden.Apoe-/- VLDL display strongly reduced fractional catabolic rates as compared with control mouse VLDL (4.0 and 6.1 versus 22.1 pools/h). In vitro low density lipoprotein (LDL) receptor binding studies using HepG2 and J774 cells showed that APOE*2. Apoe-/- VLDL is completely defective in binding to the LDL receptor, whereas APOE*3-Leiden.Apoe-/- VLDL still displayed a considerable binding activity to the LDL receptor. After transfection of APOE*2.Apoe-/- and APOE*3-Leiden.Apoe-/- mice with adenovirus carrying the gene for the receptor-associated protein (AdCMV-RAP), serum lipid levels strongly increased (15.3 to 42.8 and 1.4 to 15.3 mmol/liter for cholesterol and 5.0 to 35.7 and 0.3 to 20. 7 mmol/liter for triglycerides, respectively). This indicates that RAP-sensitive receptors, possibly the LDL receptor-related protein (LRP), mediate the plasma clearance of both APOE*2.Apoe-/- and APOE*3-Leiden. Apoe-/- VLDL. We conclude that in vivo the APOE*2 variant is completely defective in LDL receptor binding but not in binding to LRP, whereas for the APOE*3-Leiden mutant both LRP and LDL receptor binding activity are only mildly affected. As a consequence of this difference, APOE*2.Apoe-/- develop more severe hypercholesterolemia than APOE*3-Leiden.Apoe-/- mice.

Apolipoprotein E1-Hammersmith (Lys146-->Asn;Arg147-->Trp), due to a dinucleotide substitution, is associated with early manifestation of dominant type III hyperlipoproteinaemia

Apolipoprotein E (apoE) is one of the major protein constituents of chylomicron and very low density lipoprotein (VLDL) remnants and plays a central role as a ligand in the receptor-mediated uptake of these particles by the liver. Here we describe a new variant of apoE, apoE1-Hammersmith, which is associated with dominantly expressed type III hyperlipidaemia. The propositus, aged 26, developed tubero-eruptive xanthomas at the age of 3, her daughter developed similar lesions at age 7 but her son, aged 3, shows no clinical abnormality so far. All three cases had an apoE3E1 phenotype and a broad beta band on lipoprotein electrophoresis. Cysteamine modification resulted in a shift of apoE1 to the apoE2 isoform position, indicating that the mutation leading to apoE1-Hammersmith occurred on an apoE3 background. ApoE genotyping confirmed these results. Sequence analysis of DNA of the propositus was performed for exons 3 and 4 and revealed a dinucleotide substitution causing two amino acid changes at adjacent positions (Lys146-->Asn) and (Arg147-->Trp).

Both lipolysis and hepatic uptake of VLDL are impaired in transgenic mice coexpressing human apolipoprotein E*3Leiden and human apolipoprotein C1

Transgenic mice overexpressing human APOE*3Leiden are highly susceptible to diet-induced hyperlipoproteinemia and atherosclerosis due to a defect in hepatic uptake of remnant lipoproteins. In addition to the human APOE*3Leiden gene, these mice carry the human APOC1 gene (APOE*3Leiden-C1). To investigate the possible effect of simultaneous expression of the human APOC1 gene, we examined the phenotypic expression in these APOE*3Leiden-C1 mice in relation to transgenic mice expressing the APOE*3Leiden gene without the APOC1 gene (APOE*3Leiden-HCR). APOE*3Leiden-C1 and APOE*3Leiden-HCR mice had comparable liver expression for the APOE*3Leiden transgene and high total cholesterol levels on a sucrose-based diet compared with control mice (4.3 and 4.3 versus 2.1 mmol/L). In addition, on this diet APOE*3Leiden-C1 mice displayed significantly higher serum triglyceride levels than APOE*3Leiden-HCR mice and control mice (4.4 versus 0.6 and 0.2 mmol/L). Elevated triglyceride and cholesterol levels were mainly in the VLDL-sized lipoproteins. In vivo turnover studies with endogenously triglyceride-labeled VLDL showed a reduced VLDL triglyceride fractional catabolic rate for APOE*3Leiden-C1 and APOE*3Leiden-HCR mice compared with control mice (3.5 and 11.0 versus 20.4 pools per hour). To study whether the difference in fractional catabolic rates between the two transgenic strains was due to an inhibiting effect of apoC1 on the extrahepatic lipolysis or hepatic-mediated uptake of VLDL, turnover experiments were performed in functionally hepatectomized mice. Strikingly, both APOE*3Leiden-C1 and APOE*3Leiden-HCR mice showed a decreased lipolytic rate of VLDL triglyceride in the extrahepatic circulation compared with control mice (1.5 and 1.8 versus 6.3 pools per hour). We conclude that next to an impaired hepatic uptake, overexpression of the APOE*3Leiden gene influences the extrahepatic lipolysis of VLDL triglycerides, whereas simultaneous overexpression of the APOC1 gene leads to a further decrease in hepatic clearance of VLDL.