Alternative splicing of human plasma cholesteryl ester transfer protein mRNA in Caco-2 cells and its modulation by oleic acid

Both full length-cholesteryl ester transfer protein and exon 9-deleted cholesteryl ester transfer protein promote triacylglycerol storage in cultured hepatocytes

We previously reported that overexpression of full-length cholesteryl ester transfer protein (FL-CETP), but not its exon 9-deleted variant (∆E9-CETP), in an adipose cell line reduces their triacylglycerol (TAG) content. This provided mechanistic insight into several in vivo studies where FL-CETP levels are inversely correlated with adiposity. However, increased FL-CETP is also associated with elevated hepatic lipids, suggesting that the effect of CETP on cellular lipid metabolism may be tissue-specific. Here, we directly investigated the role of FL-CETP and ∆E9-CETP in hepatic lipid metabolism. FL- or ∆E9-CETP was overexpressed in HepG2-C3A by adenovirus transduction. Overexpression of either FL or ∆E9-CETP in hepatocytes increased cellular TAG mass by 25% but reduced TAG secretion. This cellular TAG was contained in larger and more numerous lipid droplets. Analysis of TAG synthetic and catabolic pathways showed that this elevated TAG content was due to increased incorporation of fatty acid into TAG (24%), and higher de novo synthesis of fatty acid (50%) and TAG from acetate (40%). siRNA knockdown of CETP had the opposite effect on TAG synthesis and lipogenesis, and decreased cellular TAG. This novel increase in cellular TAG by FL-CETP overexpression was reproduced in Caco-2 intestinal epithelial cells. We conclude that, unlike that seen in adipocyte cells, overexpression of either CETP isoform in lipoprotein-secreting cells promotes the accumulation of TAG. These data suggest that the in vivo correlation between CETP levels and hepatic steatosis can be explained, in part, by a direct effect of CETP on hepatocyte cellular metabolism.

mRNA transcript diversity creates new opportunities for pharmacological intervention

Most protein coding genes generate multiple RNA transcripts through alternative splicing, variable 3' and 5'UTRs, and RNA editing. Although drug design typically targets the main transcript, alternative transcripts can have profound physiological effects, encoding proteins with distinct functions or regulatory properties. Formation of these alternative transcripts is tissue-selective and context-dependent, creating opportunities for more effective and targeted therapies with reduced adverse effects. Moreover, genetic variation can tilt the balance of alternative versus constitutive transcripts or generate aberrant transcripts that contribute to disease risk. In addition, environmental factors and drugs modulate RNA splicing, affording new opportunities for the treatment of splicing disorders. For example, therapies targeting specific mRNA transcripts with splice-site-directed oligonucleotides that correct aberrant splicing are already in clinical trials for genetic disorders such as Duchenne muscular dystrophy. High-throughput sequencing technologies facilitate discovery of novel RNA transcripts and protein isoforms, applications ranging from neuromuscular disorders to cancer. Consideration of a gene's transcript diversity should become an integral part of drug design, development, and therapy.

Cholesteryl Ester Transfer Protein (CETP) polymorphisms affect mRNA splicing, HDL levels, and sex-dependent cardiovascular risk

Polymorphisms in and around the Cholesteryl Ester Transfer Protein (CETP) gene have been associated with HDL levels, risk for coronary artery disease (CAD), and response to therapy. The mechanism of action of these polymorphisms has yet to be defined. We used mRNA allelic expression and splice isoform measurements in human liver tissues to identify the genetic variants affecting CETP levels. Allelic CETP mRNA expression ratios in 56 human livers were strongly associated with several variants 2.5-7 kb upstream of the transcription start site (e.g., rs247616 p = 6.4 × 10(-5), allele frequency 33%). In addition, a common alternatively spliced CETP isoform lacking exon 9 (Δ9), has been shown to prevent CETP secretion in a dominant-negative manner. The Δ 9 expression ranged from 10 to 48% of total CETP mRNA in 94 livers. Increased formation of this isoform was exclusively associated with an exon 9 polymorphism rs5883-C>T (p = 6.8 × 10(-10)) and intron 8 polymorphism rs9930761-T>C (5.6 × 10(-8)) (in high linkage disequilibrium with allele frequencies 6-7%). rs9930761 changes a key splicing branch point nucleotide in intron 8, while rs5883 alters an exonic splicing enhancer sequence in exon 9.The effect of these polymorphisms was evaluated in two clinical studies. In the Whitehall II study of 4745 subjects, both rs247616 and rs5883T/rs9930761C were independently associated with increased HDL-C levels in males with similar effect size (rs247616 p = 9.6 × 10(-28) and rs5883 p = 8.6 × 10(-10), adjusted for rs247616). In an independent multiethnic US cohort of hypertensive subjects with CAD (INVEST-GENE), rs5883T/rs9930761C alone were significantly associated with increased incidence of MI, stroke, and all-cause mortality in males (rs5883: OR 2.36 (CI 1.29-4.30), p = 0.005, n = 866). These variants did not reach significance in females in either study. Similar to earlier results linking low CETP activity with poor outcomes in males, our results suggest genetic, sex-dependent CETP splicing effects on cardiovascular risk by a mechanism independent of circulating HDL-C levels.

Cholesteryl Ester Transfer Protein Biosynthesis and Cellular Cholesterol Homeostasis Are Tightly Interconnected

Cholesteryl ester transfer protein (CETP) mediates triglyceride and cholesteryl ester (CE) transfer between lipoproteins, and its activity is strongly modulated by dietary cholesterol. To better understand the regulation of CETP synthesis and the relationship between CETP levels and cellular lipid metabolism, we selected the SW872 adipocytic cell line as a model. These cells secrete CETP in a time-dependent manner at levels exceeding those observed for Caco-2 or HepG2 cells. The addition of LDL, 25OH-cholesterol, oleic acid, or acetylated LDL to SW872 cells increased CETP secretion (activity and mass) up to 6-fold. In contrast, CETP production was decreased by almost 60% after treatment with lipoprotein-deficient serum or beta-cyclodextrin. These effects, which were paralleled by changes in CETP mRNA, show that CETP biosynthesis in SW872 cells directly correlates with cellular lipid status. To investigate a possible, reciprocal relationship between CETP expression and cellular lipid homeostasis, CETP biosynthesis in SW872 cells was suppressed with CETP antisense oligonucleotides. Antisense oligonucleotides reduced CETP secretion (activity and mass) by 60% compared with sense-treated cells. When CETP synthesis was suppressed for 24 h, triglyceride synthesis was unchanged, but cholesterol biosynthesis was reduced by 20%, and acetate incorporation into CE increased 31%. After 3 days of suppressed CETP synthesis, acetate incorporation into the CE pool increased 3-fold over control. This mirrored a similar increase in CE mass. The efflux of free cholesterol to HDL was the same in sense and antisense-treated cells; however, HDL-induced CE hydrolysis in antisense-treated cells was diminished 2-fold even though neutral CE hydrolase activity was unchanged. Thus, CETP-compromised SW872 cells display a phenotype characterized by inefficient mobilization of CE stores leading to CE accumulation. These results strongly suggest that CETP expression levels contribute to normal cholesterol homeostasis in adipocytic cells. Overall, these studies demonstrate that lipid homeostasis and CETP expression are tightly coupled.

Molecular biology and pathophysiological aspects of plasma cholesteryl ester transfer protein

Plasma cholesteryl ester transfer protein (CETP) facilitates the transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to apolipoprotein B-containing lipoproteins. Since CETP regulates the plasma levels of HDL cholesterol and the size of HDL particles, CETP is considered to be a key protein in reverse cholesterol transport, a protective system against atherosclerosis. CETP, as well as plasma phospholipid transfer protein, belongs to members of the lipid transfer/lipopolysaccharide-binding protein (LBP) gene family, which also includes the lipopolysaccharide-binding protein (LBP) and bactericidal/permeability-increasing protein. Although these four proteins possess different physiological functions, they share marked biochemical and structural similarities. The importance of plasma CETP in lipoprotein metabolism was demonstrated by the discovery of CETP-deficient subjects with a marked hyperalphalipoproteinemia (HALP). Two common mutations in the CETP gene, intron 14 splicing defect and exon 15 missense mutation (D442G), have been identified in Japanese HALP patients with CETP deficiency. The deficiency of CETP causes various abnormalities in the concentration, composition, and functions of both HDL and low density lipoprotein. Although the pathophysiological significance of CETP in terms of atherosclerosis has been controversial, the in vitro experiments showed that large CE-rich HDL particles in CETP deficiency are defective in cholesterol efflux. Epidemiological studies in Japanese-Americans and in the Omagari area where HALP subjects with the intron 14 splicing defect of CETP gene are markedly frequent, have shown an increased incidence of coronary atherosclerosis in CETP-deficient patients. The current review will focus on the recent findings on the molecular biology and pathophysiological aspects of plasma CETP, a key protein in reverse cholesterol transport.

Tyrosol, the major olive oil biophenol, protects against oxidized-LDL-induced injury in Caco-2 cells

Experimental and clinical evidence suggest that oxidative stress causes cellular damage, leading to functional alterations of the tissue. Free radicals may thus play an important role in the pathogenesis of a number of human diseases. Among pro-oxidant agents, oxidized LDL lead to the production of cytotoxic reactive species, e.g., lipoperoxides, causing tissue injury and various subsequent pathologies including intestinal diseases. Thus, to analyze the oxidative damage induced by oxidized LDL to intestinal mucosa, we evaluated morphological and functional changes induced in the human colon adenocarcinoma cell line, Caco-2. In addition, we examined the protective effects exerted by tyrosol, 2-(4-hydroxyphenyl)ethanol, the major phenolic compound present in olive oil. Caco-2 cell treatment (24 and/or 48 h) with oxidized LDL (0.2 g/L) resulted in cytostatic and cytotoxic effects characterized by a series of morphological and functional alterations: membrane damage, modifications of cytoskeleton network, microtubular disorganization, loss of cell-cell and cell-substrate contacts, cell detachment and cell death. The oxidized LDL-induced alterations in Caco-2 cells were almost completely prevented by tyrosol which was added 2 h before and present during the treatments. Our results suggest that some biophenols, such as those contained in olive oil, may counteract the reactive oxygen metabolite-mediated cellular damage and related diseases, by improving in vivo antioxidant defenses.