Rapid purification and activity of apolipoprotein C1 on the proliferation of bovine vascular endothelial cells in vitro

Upregulation of APOC1 Promotes Colorectal Cancer Progression and Serves as a Potential Therapeutic Target Based on Bioinformatics Analysis

Background

Approximately 10% of cancer patients worldwide have colorectal cancer (CRC), a prevalent gastrointestinal malignancy with substantial mortality and morbidity. The purpose of this work was to investigate the APOC1 gene's expression patterns in the CRC tumor microenvironment and, using the findings from bioinformatics, to assess the biological function of APOC1 in the development of CRC.

Methods

The TCGA portal was employed in this investigation to find APOC1 expression in CRC. Its correlation with other genes and clinicopathological data was examined using the UALCAN database. To validate APOC1's cellular location, the Human Protein was employed. In order to forecast the relationship between APOC1 expression and prognosis in CRC patients, the Kaplan-Meier plotter database was used. TISIDB was also employed to evaluate the connection between immune responses and APOC1 expression in CRC. The interactions of APOC1 with other proteins were predicted using STRING. In order to understand the factors that contribute to liver metastasis from CRC, single-cell RNA sequencing (scRNA-seq) was done on one patient who had the disease. This procedure included sampling preoperative blood and the main colorectal cancer tissues, surrounding colorectal cancer normal tissues, liver metastatic cancer tissues, and normal liver tissues. Finally, an in vitro knockdown method was used to assess how APOC1 expression in tumor-associated macrophages (TAMs) affected CRC cancer cell growth and migration.

Results

When compared to paracancerous tissues, APOC1 expression was considerably higher in CRC tissues. The clinicopathological stage and the prognosis of CRC patients had a positive correlation with APOC1 upregulation and a negative correlation, respectively. APOC1 proteins are mostly found in cell cytosols where they may interact with APOE, RAB42, and TREM2. APOC1 was also discovered to have a substantial relationship with immunoinhibitors (CD274, IDO1, and IL10) and immunostimulators (PVR, CD86, and ICOS). According to the results of scRNA-seq, we found that TAMs of CRC tissues had considerably more APOC1 than other cell groups. The proliferation and migration of CRC cells were impeded in vitro by APOC1 knockdown in TAMs.

Conclusion

Based on scRNA-seq research, the current study shows that APOC1 was overexpressed in TAMs from CRC tissues. By inhibiting APOC1 in TAMs, CRC progression was reduced in vitro, offering a new tactic and giving CRC patients fresh hope.

Apolipoprotein C1: Its Pleiotropic Effects in Lipid Metabolism and Beyond

Apolipoprotein C1 (apoC1), the smallest of all apolipoproteins, participates in lipid transport and metabolism. In humans, APOC1 gene is in linkage disequilibrium with APOE gene on chromosome 19, a proximity that spurred its investigation. Apolipoprotein C1 associates with triglyceride-rich lipoproteins and HDL and exchanges between lipoprotein classes. These interactions occur via amphipathic helix motifs, as demonstrated by biophysical studies on the wild-type polypeptide and representative mutants. Apolipoprotein C1 acts on lipoprotein receptors by inhibiting binding mediated by apolipoprotein E, and modulating the activities of several enzymes. Thus, apoC1 downregulates lipoprotein lipase, hepatic lipase, phospholipase A2, cholesterylester transfer protein, and activates lecithin-cholesterol acyl transferase. By controlling the plasma levels of lipids, apoC1 relates directly to cardiovascular physiology, but its activity extends beyond, to inflammation and immunity, sepsis, diabetes, cancer, viral infectivity, and-not last-to cognition. Such correlations were established based on studies using transgenic mice, associated in the recent years with GWAS, transcriptomic and proteomic analyses. The presence of a duplicate gene, pseudogene APOC1P, stimulated evolutionary studies and more recently, the regulatory properties of the corresponding non-coding RNA are steadily emerging. Nonetheless, this prototypical apolipoprotein is still underexplored and deserves further research for understanding its physiology and exploiting its therapeutic potential.

Glycation of apolipoprotein C1 impairs its CETP inhibitory property: pathophysiological relevance in patients with type 1 and type 2 diabetes

OBJECTIVE Apolipoprotein (apo)C1 is a potent physiological inhibitor of cholesteryl ester transfer protein (CETP). ApoC1 operates through its ability to modify the electrostatic charge at the lipoprotein surface. We aimed to determine whether the inhibitory ability of apoC1 is still effective in vivo in patients with diabetes and whether in vitro glycation of apoC1 influences its electrostatic charge and its CETP inhibitory effect. RESEARCH DESIGN AND METHODS ApoC1 concentrations and CETP activity were measured in 70 type 1 diabetic (T1D) patients, 113 patients with type 2 diabetes, and 83 control subjects. The consequences of in vitro glycation by methylglyoxal on the electrostatic properties of apoC1 and on its inhibitory effect on CETP activity were studied. An isoelectric analysis of apoC1 was performed in patients with T1D and in normolipidemic-normoglycemic subjects. RESULTS An independent negative correlation was found between CETP activity and apoC1 in control subjects but not in patients with diabetes. HbA1c was independently associated with CETP activity in T1D patients. In vitro glycation of apoC1 modified its electrostatic charge and abrogated its ability to inhibit CETP activity in a concentration-dependent manner. The isoelectric point of apoC1 in T1D patients was significantly lower than that in control subjects. CONCLUSIONS The ability of apoC1 to inhibit CETP activity is impaired in patients with diabetes. Glycation of apoC1 leads to a change in its electrostatic properties that might account, at least in part, for a loss of constitutive CETP inhibition and an increase in plasma CETP activity in patients with diabetes.

Apolipoprotein CI is a physiological regulator of cholesteryl ester transfer protein activity in human plasma but not in rabbit plasma

Plasma cholesteryl ester transfer protein (CETP) activity is high in rabbits, intermediate in humans, and nondetectable in rodents. Human apolipoprotein CI (apoCI) was found to be a potent inhibitor of CETP. The aim of this study was to compare the ability of rabbit and human apoCI to modulate the interaction of CETP with HDLs and to evaluate to which extent apoCI contributes to plasma cholesteryl ester transfer rate in normolipidemic humans and rabbits. Rabbit apoCI gene was cloned and sequenced, rabbit and human apoCI were purified to homogeneity, and their ability to modify the surface charge properties and the CETP inhibitory potential of HDL were compared. It is demonstrated that unlike human apoCI, rabbit apoCI does not modulate cholesteryl ester transfer rate in total plasma. Whereas both human and rabbit apoCI readily associate with HDL, only human apoCI was found to modify the electrostatic charge of HDL. In humans, both CETP and apoCI at normal, physiological levels contribute significantly to the plasma cholesteryl ester transfer rate. In contrast, CETP is the sole major determinant of cholesteryl ester transfer in normolipidemic rabbit plasma as a result of the inability of rabbit apoCI to change HDL electronegativity.

Molecular mechanisms responsible for the antiinflammatory and protective effect of HDL on the endothelium

In addition to their role in reverse cholesterol transport, high-density lipoproteins (HDL) exert several beneficial effects, including the prevention and correction of endothelial dysfunction. HDL promote endothelium proliferation and diminish endothelial apoptosis; they play a key role in vasorelaxation by increasing the release of nitric oxide and prostacyclin through the induction of the expression and the activity of endothelial nitric oxide synthase and the coupling of cyclooxygenase 2 and prostacyclin synthase. In addition, HDL affect coagulation, fibrynolisis, platelet adhesion, adhesion molecules, and protease expression, and they exert antioxidant activity. These effects are achieved at the gene expression level and are dependent on the activation of several intracellular signaling pathways, including PI3K/Akt, ERK1/2, PKC, and p38MAPK. The complexity of the signaling pathways modulated by HDL reflects the different effects of the components of this class of lipoproteins such as apolipoproteins or lipids on endothelial cell gene expression and the subsequent modulation of endothelial function observed. The in vivo relevance of these findings to endothelial recovery during physiological or pathological conditions remains to be addressed; nevertheless, the results of clinical studies with synthetic HDL, ApoA-I mimetics, and drugs that are becoming available that selectively affect HDL plasma levels and biological functions support the importance of the correction of endothelial function by HDL.

Molecular Mechanism of the Blockade of Plasma Cholesteryl Ester Transfer Protein by Its Physiological Inhibitor Apolipoprotein CI

Genetically engineered mice demonstrated that apolipoprotein (apo) CI is a potent, physiological inhibitor of plasma cholesteryl ester transfer protein (CETP) activity. The goal of this study was to determine the molecular mechanism of the apoCI-mediated blockade of CETP activity. Kinetic analyses revealed that the inhibitory property of apoCI is independent of the amount of active CETP, but it is tightly dependent on the amount of high density lipoproteins (HDL) in the incubation mixtures. The electrostatic charge of HDL, i.e. the main carrier of apoCI in human plasma, is gradually modified with increasing amounts of apoCI, and the neutralization of apoCI lysine residues by acetylation produces a marked reduction in its inhibitory potential. The inhibitory property of full-length apoCI is shared by its C-terminal alpha-helix with significant electrostratic properties, whereas its N-terminal alpha-helix with no CETP inhibitory property has no effect on HDL electronegativity. Finally, binding experiments demonstrated that apoCI and to a lower extent its C-terminal alpha-helix are able to disrupt CETP-lipoprotein complexes in a concentration-dependent manner. It was concluded that the inhibition of CETP activity by apoCI is in direct link with its specific electrostatic properties, and the apoCI-mediated reduction in the binding properties of lipoproteins results in weaker CETP-HDL interactions and fewer cholesteryl ester transfers.

Apolipoprotein CI deficiency markedly augments plasma lipoprotein changes mediated by human cholesteryl ester transfer protein (CETP) in CETP transgenic/ApoCI-knocked out mice

Transgenic mice expressing human cholesteryl ester transfer protein (HuCETPTg mice) were crossed with apolipoprotein CI-knocked out (apoCI-KO) mice. Although total cholesterol levels tended to be reduced as the result of CETP expression in HuCETPTg heterozygotes compared with C57BL6 control mice (-13%, not significant), a more pronounced decrease (-28%, p < 0.05) was observed when human CETP was expressed in an apoCI-deficient background (HuCETPTg/apoCI-KO mice). Gel permeation chromatography analysis revealed a significant, 6.1-fold rise (p < 0.05) in the cholesteryl ester content of very low density lipoproteins in HuCETPTg/apoCI-KO mice compared with control mice, whereas the 2.7-fold increase in HuCETPTg mice did not reach the significance level in these experiments. Approximately 50% decreases in the cholesteryl ester content and cholesteryl ester to triglyceride ratio of high density lipoproteins (HDL) were observed in HuCETPTg/apoCI-KO mice compared with controls (p < 0.05 in both cases), with intermediate -20% changes in HuCETPTg mice. The cholesteryl ester depletion of HDL was accompanied with a significant reduction in their mean apparent diameter (8.68 +/- 0.04 nm in HuCETPTg/apoCI-KO mice versus 8.83 +/- 0.02 nm in control mice; p < 0.05), again with intermediate values in HuCETPTg mice (8.77 +/- 0.04 nm). In vitro purified apoCI was able to inhibit cholesteryl ester exchange when added to either total plasma or reconstituted HDL-free mixtures, and coincidently, the specific activity of CETP was significantly increased in the apoCI-deficient state (173 +/- 75 pmol/microg/h in HuCETPTg/apoCI-KO mice versus 72 +/- 19 pmol/microg/h in HuCETPTg, p < 0.05). Finally, HDL from apoCI-KO mice were shown to interact more readily with purified CETP than control HDL that differ only by their apoCI content. Overall, the present observations provide direct support for a potent specific inhibition of CETP by plasma apoCI in vivo.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.

Human apolipoprotein C-I accounts for the ability of plasma high density lipoproteins to inhibit the cholesteryl ester transfer protein activity

The aim of the present study was to identify the protein that accounts for the cholesteryl ester transfer protein (CETP)-inhibitory activity that is specifically associated with human plasma high density lipoproteins (HDL). To this end, human HDL apolipoproteins were fractionated by preparative polyacrylamide gradient gel electrophoresis, and 30 distinct protein fractions with molecular masses ranging from 80 down to 2 kDa were tested for their ability to inhibit CETP activity. One single apolipoprotein fraction was able to completely inhibit CETP activity. The N-terminal sequence of the 6-kDa protein inhibitor matched the N-terminal sequence of human apoC-I, the inhibition was completely blocked by specific anti-apolipoprotein C-I antibodies, and mass spectrometry analysis confirmed the identity of the isolated inhibitor with full-length human apoC-I. Pure apoC-I was able to abolish CETP activity in a concentration-dependent manner and with a high efficiency (IC(50) = 100 nmol/liter). The inhibitory potency of total delipidated HDL apolipoproteins completely disappeared after a treatment with anti-apolipoprotein C-I antibodies, and the apoC-I deprivation of native plasma HDL by immunoaffinity chromatography produced a mean 43% rise in cholesteryl ester transfer rates. The main localization of apoC-I in HDL and not in low density lipoprotein in normolipidemic plasma provides further support for the specific property of HDL in inhibiting CETP activity.

Proliferative effect of high density lipoprotein (HDL) and HDL fractions (HDL1,2, HDL3) on virus transformed lymphoblastoid cells

The growth-promoting activities of plasma lipoproteins (LDL, HDL, HDL1,2, HDL3) and total HDL apolipoproteins on a virus transformed lymphoblastoid cell line in vitro, has been compared. When maintained in lipoprotein-deficient serum-supplemented medium, these cells do not proliferate optimally. The addition of either HDL, HDL1,2 or HDL3 induced optimal cell proliferation as compared to the result observed in fetal calf serum-supplemented medium. The HDL1,2 subfraction was found to be more potent than the HDL3 subfraction in supporting cell growth. Total HDL apolipoproteins were able to support significant cell proliferation. In contrast, LDL did not promote cell growth. In serum-free conditions and in the presence of transferrin, only HDL and HDL subfractions induced cell proliferation. These results suggest that HDL and HDL subfractions could initiate B lymphoblastoid cell growth and that total HDL apolipoproteins could support a part of cell proliferation.

Effects of low- and high-density lipoproteins on the proliferation of human breast cancer cells in vitro: differences between hormone-dependent and hormone-independent cell lines

The influence of low- and high-density lipoproteins on the proliferation of human breast cancer cells in culture was studied. We compared total cell number after incubation for 48 hr in culture medium containing or lacking plasma lipoproteins. Marked differences were found between hormone-dependent (MCF-7, T-47-D, ZR-75) and hormone-independent (MDA-MB-231, HBL-100) mammary tumor cell lines. The cells also reacted differently on the different lipoproteins offered in the medium. Human low-density lipoproteins (LDL) exhibited a marked stimulation of the growth of hormone-independent cell lines but no or only toxic effects upon the hormone-sensitive lines. Human high-density lipoproteins (HDL) stimulated the proliferation of all cell lines in a dose-dependent manner but hormone-independent cells showed a higher response. These findings point towards different utilizations of nutrients in hormone-dependent and hormone-independent cells.

The role of lipoproteins in EBV early antigen induction in Raji cells

Synthesis of the Epstein-Barr virus (EBV) associated early antigen (EA) can be induced by a variety of agents in Raji cells, a latently EBV-infected Burkitt lymphoma line. We investigated the role of lipoproteins in this EA induction system. Cell growth was not affected by lipoprotein-deficiency, but EA induction by most combinations of the inducers TPA (tetradecanoyl-phorbol-acetate), IdU (iododeoxyuridine), n-BA (n-butyric acid), anti-IgM and EA inducing factor (EIF), was greatly reduced. Only the inducer combination TPA/n-BA was completely independent of the presence of lipoproteins, indicating a different induction pathway. Removing the lipid moieties of the culturing serum did not result in reduced EA induction. Thus, the lowered EA inducibility in lipoprotein-deficiency is due to the absence of the protein moiety (apolipoprotein). Addition of HDL or VLDL partially reconstituted the original EA inducibility, whereas LDL had no effect. Lipoproteins were particularly important during the first 4 hours of induction, the phase where inducers may act on cell membrane structures (e.g., receptors). But lipoproteins were also required throughout the incubation period, even in a late and inducer independent phase.

Role of lipoproteins in growth of human adult arterial endothelial and smooth muscle cells in low lipoprotein-deficient serum

Recently improved culture conditions for human adult arterial endothelial and smooth muscle cells from a wide variety of donors have been used to study the effects of lipoproteins on proliferation of both cell types in low serum culture medium. Optimal growth of endothelial and smooth muscle cells in an optimal nutrient medium (MCDB 107) containing epidermal growth factor, a partially purified fraction from bovine brain, and 1% (v/v) lipoprotein-deficient serum was dependent on either high- or low-density lipoprotein. High- and low-density lipoprotein stimulated cell growth by three- and five-fold, respectively, over a 6-day period. Optimal stimulation of both endothelial and smooth muscle cell growth occurred between 20 and 60 micrograms/ml of high- and low-density lipoproteins, respectively. No correlation between the activation of 3-hydroxyl-3-methylglutaryl coenzyme. A reductase activity and lipoprotein-stimulated cell proliferation was observed. Lipid-free total apolipoproteins or apolipoprotein C peptides from high-density lipoprotein were partially effective and together with oleic acid effectively replaced native high-density lipoprotein for the support of endothelial cell growth. In contrast, apolipoproteins or apolipoprotein C peptides from high-density lipoprotein alone or with oleic acid had no effect on smooth muscle cell proliferation. The results suggest a functional role of high- and low-density lipoproteins and apolipoproteins in the proliferation of human adult endothelial and smooth muscle cells.

Purified HDL-apolipoproteins, A-I and C-III, substitute for HDL in promoting the growth of SV40-transformed REF52 cells in serum-free medium

The lipid-free apolipoproteins of human high density lipoprotein (HDL) have been assayed for their ability to substitute for native HDL in promoting the growth of a SV40-transformed REF52 cell line in serum-free medium. Total HDL-apolipoproteins (apoHDL) were found to mimic almost exactly the growth promoting effects of whole HDL. The apoHDL-associated growth promoting activity eluted from a Sephacryl S-200 column in two separate fractions coinciding with the protein peaks of apolipoprotein A-I and the C group of apolipoproteins. These two fractions, designated S-II and S-IV, respectively, acted additively in promoting WT1A cell growth when tested at saturating concentrations. The active component in the S-II fraction maximally stimulated WT1A cell growth at 40-60 micrograms/ml and was identified as apolipoprotein A-1 by NaDodSO4 polyacrylamide gel electrophoresis and affinity chromatography on anti-(apoA-I). The active component in the S-IV fraction was maximally active at 1-2 micrograms/ml and was identified as apolipoprotein C-III by DEAE ion exchange high pressure liquid chromatography and polyacrylamide gel electrophoresis (at pH 8.3) in 6 M urea. These results indicate that the growth promoting effect of HDL on WT1A cells is mediated via the HDL-apolipoproteins, A-I and C-III, and that the mechanism responsible does not necessarily involve their participation in the uptake (or utilization) of HDL-associated lipids.