Molecular cloning of hamster lipid transfer inhibitor protein (apolipoprotein F) and regulation of its expression by hyperlipidemia

Unveiling the Impact of ApoF Deficiency on Liver and Lipid Metabolism: Insights from Transcriptome-Wide m6A Methylome Analysis in Mice

Lipid metabolism participates in various physiological processes and has been shown to be connected to the development and progression of multiple diseases, especially metabolic hepatopathy. Apolipoproteins (Apos) act as vectors that combine with lipids, such as cholesterol and triglycerides (TGs). Despite being involved in lipid transportation and metabolism, the critical role of Apos in the maintenance of lipid metabolism has still not been fully revealed. This study sought to clarify variations related to m6A methylome in ApoF gene knockout mice with disordered lipid metabolism based on the bioinformatics method of transcriptome-wide m6A methylome epitranscriptomics. High-throughput methylated RNA immunoprecipitation sequencing (MeRIP-seq) was conducted in both wild-type (WT) and ApoF knockout (KO) mice. As a result, the liver histopathology presented vacuolization and steatosis, and the serum biochemical assays reported abnormal lipid content in KO mice. The m6A-modified mRNAs were conformed consensus sequenced in eukaryotes, and the distribution was enriched within the coding sequences and 3' non-coding regions. In KO mice, the functional annotation terms of the differentially expressed genes (DEGs) included cholesterol, steroid and lipid metabolism, and lipid storage. In the differentially m6A-methylated mRNAs, the functional annotation terms included cholesterol, TG, and long-chain fatty acid metabolic processes; lipid transport; and liver development. The overlapping DEGs and differential m6A-modified mRNAs were also enriched in terms of lipid metabolism disorder. In conclusion, transcriptome-wide MeRIP sequencing in ApoF KO mice demonstrated the role of this crucial apolipoprotein in liver health and lipid metabolism.

A pan-cancer analysis of the oncogenic and immunological roles of apolipoprotein F (APOF) in human cancer

BACKGROUND

Apolipoprotein F (APOF) has been less studied in cancers. Thus, we aimed to perform a pan-cancer analysis of the oncogenic and immunological effects of APOF on human cancer.

METHODS

A standardized TCGA pan-cancer dataset was downloaded. Differential expression, clinical prognosis, genetic mutations, immune infiltration, epigenetic modifications, tumor stemness and heterogeneity were analyzed. We conducted all analyses through software R (version 3.6.3) and its suitable packages.

RESULTS

Overall, we found that the common cancers differentially expressed between tumor and normal samples and prognostic-associated were BRCA, PRAD, KIRP, and LIHC in terms of overall survival (OS), disease-free survival (DFS) and progression-free survival (PFS). The pan-cancer Spearman analysis showed that the mRNA expression of APOF was negatively correlated with four tumor stemness indexes (DMPss, DNAss, ENHss, and EREG-METHss) with statistical significance for PRAD and was positively correlated for LIHC. In terms of BRCA and PRAD patients, we found negative correlation of APOF with TMB, MSI, neo, HRD and LOH. The mutation frequencies of BRCA and LIHC were 0.3%. APOF expression was negatively correlated with immune infiltration and positively correlated with tumor purity for PRAD patients. The mRNA expression of APOF was negatively associated with most TILs for LIHC, B cells, CD4+ T cells, neutrophils, macrophages and dendritic cells, but was positively associated with CD8+ T cells.

CONCLUSIONS

Our pan-cancer study offered a relatively comprehensive understanding of the roles of APOF on BRCA, PRAD, KIRP, and LIHC.

Apolipoprotein F concentration, activity, and the properties of LDL controlling ApoF activation in hyperlipidemic plasma

Apolipoprotein F (ApoF) modulates lipoprotein metabolism by selectively inhibiting cholesteryl ester transfer protein activity on LDL. This ApoF activity requires that it is bound to LDL. How hyperlipidemia alters total plasma ApoF and its binding to LDL are poorly understood. In this study, total plasma ApoF and LDL-bound ApoF were quantified by ELISA (n = 200). Plasma ApoF was increased 31% in hypercholesterolemic plasma but decreased 20% in hypertriglyceridemia. However, in donors with combined hypercholesterolemia and hypertriglyceridemia, the elevated triglyceride ameliorated the rise in ApoF caused by hypercholesterolemia alone. Compared with normolipidemic LDL, hypercholesterolemic LDL contained ∼2-fold more ApoF per LDL particle, whereas ApoF bound to LDL in hypertriglyceridemia plasma was <20% of control. To understand the basis for altered association of ApoF with hyperlipidemic LDL, the physiochemical properties of LDL were modified in vitro by cholesteryl ester transfer protein ± LCAT activities. The time-dependent change in LDL lipid composition, proteome, core and surface lipid packing, LDL surface charge, and LDL size caused by these factors were compared with the ApoF binding capacity of these LDLs. Only LDL particle size correlated with ApoF binding capacity. This positive association between LDL size and ApoF content was confirmed in hyperlipidemic plasmas. Similarly, when in vitro produced and enlarged LDLs with elevated ApoF binding capacity were incubated with LPL to reduce their size, ApoF binding was reduced by 90%. Thus, plasma ApoF levels and the activation status of this ApoF are differentially altered by hypercholesterolemia and hypertriglyceridemia. LDL size is a key determinate of ApoF binding and activation.

The lipid substrate preference of CETP controls the biochemical properties of HDL in fat/cholesterol-fed hamsters

Cholesteryl ester transfer protein (CETP) modulates lipoprotein metabolism by transferring cholesteryl ester (CE) and triglyceride (TG) between lipoproteins. However, differences in the way CETP functions exist across species. Unlike human CETP, hamster CETP prefers TG over CE as a substrate, raising questions regarding how substrate preference may impact lipoprotein metabolism. To understand how altering the CE versus TG substrate specificity of CETP might impact lipoprotein metabolism in humans, we modified CETP expression in fat/cholesterol-fed hamsters, which have a human-like lipoprotein profile. Hamsters received adenoviruses expressing no CETP, hamster CETP, or human CETP. Total plasma CETP mass increased up to 70% in the hamster and human CETP groups. Hamsters expressing human CETP exhibited decreased endogenous hamster CETP, resulting in an overall CE:TG preference of plasma CETP that was similar to that in humans. Hamster CETP overexpression had little impact on lipoproteins, whereas human CETP expression reduced HDL by 60% without affecting LDL. HDLs were TG enriched and CE depleted and much smaller, causing the HDL3:HDL2 ratio to increase threefold. HDL from hamsters expressing human CETP supported higher LCAT activity and greater cholesterol efflux. The fecal excretion of HDL-associated CE in human CETP animals was unchanged. However, much of this cholesterol accumulated in the liver and was associated with a 1.8-fold increase in hepatic cholesterol mass. Overall, these data show in a human-like lipoprotein model that modification of CETP's lipid substrate preference selectively alters HDL concentration and function. This provides a powerful tool for modulating HDL metabolism and impacting sterol balance in vivo.

Apolipoprotein F: a natural inhibitor of cholesteryl ester transfer protein and a key regulator of lipoprotein metabolism

PURPOSE OF REVIEW

The aim of this study is to highlight recent studies that have advanced our understanding of apolipoprotein F (ApoF) and its role in lipid metabolism.

RECENT FINDINGS

Previous studies showed that ApoF hepatic mRNA levels are suppressed by fat-enriched diets. Recent studies show this downregulation is mediated by agonist-induced binding of liver X receptor (LXR) and PPARalpha to a regulatory element in the ApoF promoter. First-of-kind in-vivo studies show ApoF lowers low-density lipoprotein levels and enhances reverse cholesterol transport in fat-fed hamsters.

SUMMARY

Diverse studies collectively provide compelling evidence that cholesteryl ester transfer protein (CETP) plays an important role in regulating lipid metabolism. Inhibiting CETP raises HDL cholesterol. However, considering the recent failures of pharmacological inhibitors of CETP in clinical trials, it does not seem likely that global inhibition of CETP will be beneficial. ApoF is a minor apolipoprotein that functions as a natural inhibitor of CETP. However, ApoF is not a general inhibitor of CETP, but rather it preferentially inhibits CETP activity with LDL. Therefore, ApoF tailors CETP activity so that less tissue-derived cholesterol traffics from HDL into the LDL compartment. Lower LDL cholesterol levels have recognized clinical benefit for reduced cardiovascular disease.

The lipid transfer properties of CETP define the concentration and composition of plasma lipoproteins

Cholesteryl ester transfer protein (CETP) facilitates the net transfer of cholesteryl esters (CEs) and TGs between lipoproteins, impacting the metabolic fate of these lipoproteins. Previous studies have shown that a CETP antibody can alter CETP's preference for CE versus TG as transfer substrate, suggesting that CETP substrate preference can be manipulated in vivo. Hamster and human CETPs have very different preferences for CE and TG. To assess the effect of altering CETP's substrate preference on lipoproteins in vivo, here, we expressed human CETP in hamsters. Chow-fed hamsters received adenoviruses expressing no CETP, hamster CETP, or human CETP. Plasma CETP mass increased 2-fold in both the hamster and human CETP groups. Although the animals expressing human CETP still had low levels of hamster CETP, the CE versus TG preference of their plasma CETP was similar to that of the human ortholog. Hamster CETP overexpression had little impact on lipoproteins. However, expression of human CETP reduced HDL up to 50% and increased VLDL cholesterol 2.5-fold. LDL contained 20% more CE, whereas HDL CE was reduced 40%, and TG increased 6-fold. The HDL3:HDL2 ratio increased from 0.32 to 0.60. Hepatic expression of three cholesterol-related genes (LDLR, SCARB1, and CYP7A1) was reduced up to 40%. However, HDL-associated CE excretion into feces was unchanged. We conclude that expression of human CETP in hamsters humanizes their lipoprotein profile with respect to the relative concentrations of VLDL, LDL, HDL, and the HDL3:HDL2 ratio. Altering the lipid substrate preference of CETP provides a novel approach for modifying plasma lipoproteins.

Identification of a hormone response element that mediates suppression of APOF by LXR and PPARα agonists

Apolipoprotein F (ApoF) regulates cholesteryl ester transfer protein activity. We previously observed that hepatic APOF mRNA levels are decreased by high fat, cholesterol-enriched diets. Here we show in human liver C3A cells that APOF mRNA levels are reduced by agonists of LXR and PPARα nuclear receptors. This negative regulation requires co-incubation with the RXR agonist, retinoic acid. Bioinformatic analysis of the ~2 kb sequence upstream of the APOF promoter identified one potential LXR and 4 potential PPARα binding sites clustered between nucleotides -2007 and -1961. ChIP analysis confirmed agonist-dependent binding of LXRα, PPARα, and RXRα to this hormone response element complex (HREc). A luciferase reporter containing the 2 kb 5' APOF sequence was negatively regulated by LXR and PPARα ligands as seen in cells. This regulation was maintained in constructs lacking the ~1700 nucleotides between the HREc and the APOF proximal promoter. Mutations of the HREc that disrupted LXRα and PPARα binding led to the loss of reporter construct inhibition by agonists of these nuclear receptors. siRNA knockdown studies showed that APOF gene regulation by LXRα or PPARα agonists did not require an interaction between these two nuclear receptors. Thus, APOF is subject to negative regulation by agonist-activated LXR or PPARα nuclear receptors binding to a regulatory element ~1900 bases 5' to the APOF promoter. High fat, cholesterol-enriched diets likely reduce APOF gene expression via these receptors interacting at this regulatory site.

Obesity-linked suppression of membrane-bound O-acyltransferase 7 (MBOAT7) drives non-alcoholic fatty liver disease

Recent studies have identified a genetic variant rs641738 near two genes encoding membrane bound O-acyltransferase domain-containing 7 (MBOAT7) and transmembrane channel-like 4 (TMC4) that associate with increased risk of non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), alcohol-related cirrhosis, and liver fibrosis in those infected with viral hepatitis (Buch et al., 2015; Mancina et al., 2016; Luukkonen et al., 2016; Thabet et al., 2016; Viitasalo et al., 2016; Krawczyk et al., 2017; Thabet et al., 2017). Based on hepatic expression quantitative trait loci analysis, it has been suggested that MBOAT7 loss of function promotes liver disease progression (Buch et al., 2015; Mancina et al., 2016; Luukkonen et al., 2016; Thabet et al., 2016; Viitasalo et al., 2016; Krawczyk et al., 2017; Thabet et al., 2017), but this has never been formally tested. Here we show that Mboat7 loss, but not Tmc4, in mice is sufficient to promote the progression of NAFLD in the setting of high fat diet. Mboat7 loss of function is associated with accumulation of its substrate lysophosphatidylinositol (LPI) lipids, and direct administration of LPI promotes hepatic inflammatory and fibrotic transcriptional changes in an Mboat7-dependent manner. These studies reveal a novel role for MBOAT7-driven acylation of LPI lipids in suppressing the progression of NAFLD.

ApoF knockdown increases cholesteryl ester transfer to LDL and impairs cholesterol clearance in fat-fed hamsters

Cholesteryl ester transfer protein (CETP) regulates intravascular lipoprotein metabolism. In vitro studies indicate that ApoF alters CETP function by inhibiting its activity with LDL. To explore in vivo the complexities driving ApoF's effects on CETP, we developed a siRNA-based hamster model of ApoF knockdown. In both male and female hamsters on chow- or fat-fed diets, we measured lipoprotein levels and composition, determined CETP-mediated transfer of cholesteryl esters (CEs) between lipoproteins, and quantified reverse cholesterol transport (RCT). We found that apoF knockdown in chow-fed hamsters had no effect on lipoprotein levels or composition, but these ApoF-deficient lipoproteins supported 50-100% higher LDL CETP activity in vitro. ApoF knockdown in fat-fed male hamsters created a phenotype in which endogenous CETP-mediated CE transfer from HDL to LDL increased up to 2-fold, LDL cholesterol increased 40%, HDL declined 25%, LDL and HDL lipid compositions were altered, and hepatic LDLR gene expression was decreased. Diet-induced hypercholesterolemia obscured this phenotype on occasion. In fat-fed female hamsters, ApoF knockdown caused similar but smaller changes in plasma CETP activity and LDL cholesterol. Notably, ApoF knockdown impaired HDL RCT in fat-fed hamsters but increased sterol excretion in chow-fed animals. These in vivo data validate in vitro findings that ApoF regulates lipid transfer to LDL. The consequences of ApoF knockdown on lipoproteins and sterol excretion depend on the underlying lipid status. By minimizing the transfer of HDL-derived CE to LDL, ApoF helps control LDL cholesterol levels when LDL clearance mechanisms are limiting.

Influence of maternal hypercholesterolemia and phytosterol intervention during gestation and lactation on dyslipidemia and hepatic lipid metabolism in offspring of Syrian golden hamsters

SCOPE

Although there is a normal physiological rise in maternal lipids during pregnancy, excessive maternal hyperlipidemia during pregnancy increases cardiovascular disease risk for both the mother and offspring. There are limited safe lipid-lowering treatment options for use during pregnancy, therefore, we evaluated the influence of maternal phytosterol (PS) supplementation on lipid and lipoprotein metabolism in mothers and progeny.

METHODS AND RESULTS

Female Syrian golden hamsters were randomly assigned to three diets throughout prepregnancy, gestation, and lactation (n = 6/group): (i) Chow (Chow), (ii) chow with 0.5% cholesterol (CH), and (iii) chow with 0.5% CH and 2% PS (CH/PS). Compared with newly weaned pups from Chow dams, pups from dams fed the CH-enriched diet demonstrated increases (p < 0.05) in total-C, LDL-C, HDL-C, and total LDL and VLDL particle number. Pups from CH-fed mothers also exhibited higher hepatic CH concentration and differential mRNA expression pattern of CH regulatory genes. Pups from PS-supplemented dams demonstrated reductions (p < 0.05) in serum total-C, non-HDL-C, and LDL-C but also increased triglycerides compared with pups from CH-fed dams. Maternal PS supplementation reduced (p < 0.05) hepatic CH and increased the abundance of HMG-CoAr and LDLr protein in newly weaned pups compared with the CH group.

CONCLUSION

Results suggest that maternal PS supplementation is largely effective in normalizing CH in pups born to mothers with hypercholesterolemia, however, the cause and long-term influence of increased triglyceride is not known.

Transcriptional regulation of the apolipoprotein F (ApoF) gene by ETS and C/EBPα in hepatoma cells

Apolipoprotein F (ApoF) inhibits cholesteryl ester transfer protein (CETP) activity and plays an important role in lipid metabolism. In the present study, the full-length human ApoF promoter was cloned, and the molecular mechanism of the regulation of ApoF was investigated. The ApoF promoter displayed higher activities in hepatoma cell lines, and the -198 nt to +79 nt promoter region contained the maximum promoter activity. In the promoter region of -198 nt to -2 nt there were four putative binding sites for transcription factors ETS-1/ETS-2 (named EBS-1 to EBS-4) and one for C/EBP. Mutation of EBS-2, EBS4 and the C/EBP binding site abolished the promoter activity, and ETS-1/ETS-2 and C/EBPα could interact with corresponding binding sites. In addition, overexpression of ETS-1/2 or C/EBPα enhanced, while dominant-negative mutants of ETS-1/2 and knockdown of C/EBPα decreased, ApoF promoter activities. ETS-1 and C/EBPα associated physically, and acted synergistically to activate ApoF transcription. These results demonstrated combined activation of the ApoF promoter by liver-enriched and ubiquitous transcription factors. Direct interactions between C/EBPα and ETS-1 were important for high liver-specific expression of ApoF.

High-fat and fructose intake induces insulin resistance, dyslipidemia, and liver steatosis and alters in vivo macrophage-to-feces reverse cholesterol transport in hamsters

Reverse cholesterol transport (RCT) promotes the egress of cholesterol from peripheral tissues to the liver for biliary and fecal excretion. Although not demonstrated in vivo, RCT is thought to be impaired in patients with metabolic syndrome, in which liver steatosis prevalence is relatively high. Golden Syrian hamsters were fed a nonpurified (CON) diet and normal drinking water or a high-fat (HF) diet containing 27% fat, 0.5% cholesterol, and 0.25% deoxycholate as well as 10% fructose in drinking water for 4 wk. Compared to CON, the HF diet induced insulin resistance and dyslipidemia, with significantly higher plasma non-HDL-cholesterol concentrations and cholesteryl ester transfer protein activity. The HF diet induced severe liver steatosis, with significantly higher cholesterol and TG levels compared to CON. In vivo RCT was assessed by i.p. injecting ³H-cholesterol labeled macrophages. Compared to CON, HF hamsters had significantly greater ³H-tracer recoveries in plasma, but not HDL. After 72 h, ³H-tracer recovery in HF hamsters was 318% higher in liver and 75% lower in bile (P < 0.01), indicating that the HF diet impaired hepatic cholesterol fluxes. However, macrophage-derived cholesterol fecal excretion was 45% higher in HF hamsters than in CON hamsters. This effect was not related to intestinal cholesterol absorption, which was 89% higher in HF hamsters (P < 0.05), suggesting a possible upregulation of transintestinal cholesterol excretion. Our data indicate a significant increase in macrophage-derived cholesterol fecal excretion in a hamster model of metabolic syndrome, which may not compensate for the diet-induced dyslipidemia and liver steatosis.

The effects of apolipoprotein F deficiency on high density lipoprotein cholesterol metabolism in mice

Apolipoprotein F (apoF) is 29 kilodalton secreted sialoglycoprotein that resides on the HDL and LDL fractions of human plasma. Human ApoF is also known as Lipid Transfer Inhibitor protein (LTIP) based on its ability to inhibit cholesteryl ester transfer protein (CETP)-mediated transfer events between lipoproteins. In contrast to other apolipoproteins, ApoF is predicted to lack strong amphipathic alpha helices and its true physiological function remains unknown. We previously showed that overexpression of Apolipoprotein F in mice reduced HDL cholesterol levels by 20–25% by accelerating clearance from the circulation. In order to investigate the effect of physiological levels of ApoF expression on HDL cholesterol metabolism, we generated ApoF deficient mice. Unexpectedly, deletion of ApoF had no substantial impact on plasma lipid concentrations, HDL size, lipid or protein composition. Sex-specific differences were observed in hepatic cholesterol content as well as serum cholesterol efflux capacity. Female ApoF KO mice had increased liver cholesteryl ester content relative to wild type controls on a chow diet (KO: 3.4+/−0.9 mg/dl vs. WT: 1.2+/−0.3 mg/dl, p<0.05). No differences were observed in ABCG1-mediated cholesterol efflux capacity in either sex. Interestingly, ApoB-depleted serum from male KO mice was less effective at promoting ABCA1-mediated cholesterol efflux from J774 macrophages relative to WT controls.