Role of thyroid hormone in the expression of apolipoprotein A-IV and C-III genes in rat liver

ApoCIII: A multifaceted protein in cardiometabolic disease

ApoCIII has a well-recognized role in triglyceride-rich lipoproteins metabolism. A considerable amount of data has clearly highlighted that high levels of ApoCIII lead to hypertriglyceridemia and, thereby, may influence the risk of cardiovascular disease. However, recent findings indicate that ApoCIII might also act beyond lipid metabolism. Indeed, ApoCIII has been implicated in other physiological processes such as glucose homeostasis, monocyte adhesion, activation of inflammatory pathways, and modulation of the coagulation cascade. As the inhibition of ApoCIII is emerging as a new promising therapeutic strategy, the complete understanding of multifaceted pathophysiological role of this apoprotein may be relevant. Therefore, the purpose of this work is to review available evidences not only related to genetics and biochemistry of ApoCIII, but also highlighting the role of this apoprotein in triglyceride and glucose metabolism, in the inflammatory process and coagulation cascade as well as in cardiovascular disease.

Stable isotope labeling with amino acids in cell culture (SILAC)-based quantitative proteomics study of a thyroid hormone-regulated secretome in human hepatoma cells

The thyroid hormone, 3, 3',5-triiodo-l-thyronine (T(3)), regulates cell growth, development, differentiation, and metabolism via interactions with thyroid hormone receptors (TRs). However, the secreted proteins that are regulated by T(3) are yet to be characterized. In this study, we used the quantitative proteomic approach of stable isotope labeling with amino acids in cell culture coupled with nano-liquid chromatography-tandem MS performed on a LTQ-Orbitrap instrument to identify and characterize the T(3)-regulated proteins secreted in human hepatocellular carcinoma cell lines overexpressing TRα1 (HepG2-TRα1). In total, 1742 and 1714 proteins were identified and quantified, respectively, in three independent experiments. Among these, 61 up-regulated twofold and 11 down-regulated twofold proteins were identified. Eight proteins displaying increased expression and one with decreased expression in conditioned media were validated using Western blotting. Real-time quantitative RT-PCR further disclosed induction of plasminogen activator inhibitor-1 (PAI-1), a T(3) target, in a time-course and dose-dependent manner. Serial deletions of the PAI-1 promoter region and subsequent chromatin immunoprecipitation assays revealed that the thyroid hormone response element on the promoter is localized at positions -327/-312. PAI-1 overexpression enhanced tumor growth and migration in a manner similar to what was seen when T(3) induced PAI-1 expression in J7-TRα1 cells, both in vitro and in vivo. An in vitro neutralizing assay further supported a crucial role of secreted PAI-1 in T(3)/TR-regulated cell migration. To our knowledge, these results demonstrate for the first time that proteins involved in the urokinase plasminogen activator system, including PAI-1, uPAR, and BSSP4, are augmented in the extra- and intracellular space of T(3)-treated HepG2-TRα1 cells. The T(3)-regulated secretome generated in the current study may provide an opportunity to establish the mechanisms underlying T(3)-associated tumor progression and prognosis.

Apo A-IV: an update on regulation and physiologic functions

Apolipoprotein (apo) A-IV, first identified 28 years ago as a plasma lipoprotein moiety, is now known to participate in the regulation of various metabolic pathways. It is synthesized primarily in the enterocytes of the small intestine during fat absorption. After entry into the bloodstream, the 46-kDa glycoprotein apo A-IV appears associated with chylomicrons, high-density lipoproteins, and in the lipoprotein-free fraction. It has a role in lipid absorption, transport and metabolism, and may act as a post-prandial satiety signal, an anti-oxidant and a major factor in the prevention of atherosclerosis. After summarizing and discussing these functions for reader's comprehension, the current review focuses on the regulation of apo A-IV by nutrients, biliary components, drugs, hormones and gastrointestinal peptides. The understanding of the involved mechanisms that underline apo A-IV regulation may in the long run allow us to switch on its gene, which may confer multiple beneficial effects, including the protection from atherosclerosis.

Thyroid hormone regulates hepatic triglyceride mobilization and apolipoprotein B messenger ribonucleic Acid editing in a murine model of congenital hypothyroidism

Thyroid hormone modulates the expression of numerous genes that in turn regulate lipoprotein metabolism in vivo. We have examined the thyroid hormone-dependent regulation of apolipoprotein B (apoB) RNA editing in a strain of congenitally hypothyroid mice (Pax8(-/-)) that lacks thyroid follicular cells. Neonatal Pax8(-/-) mice demonstrate an approximately 10-fold increase in hepatic triglyceride content associated with a decrease in hepatic apoB RNA editing. Thyroid hormone administration resulted in hepatic triglyceride mobilization in conjunction with an increase in hepatic, but not intestinal, apoB RNA editing and without changing total apoB RNA abundance. ApoB RNA editing is mediated by a multicomponent enzyme complex whose catalytic core contains two proteins, apobec-1 and apobec-1 complementation factor (ACF). Hepatic ACF mRNA and protein abundance decreased in Pax8(-/-) mice, with restoration after thyroid hormone administration, whereas apobec-1 mRNA and protein abundance were unchanged. Immunohistochemical analysis revealed increased staining intensity of ACF within hepatocyte nuclei of treated mice, findings confirmed by Western analysis of isolated nuclei. In vitro RNA editing assays demonstrated that supplementation with recombinant ACF alone restored enzymatic activity of S100 extracts from hypothyroid, Pax8(-/-) mice. These data demonstrate that thyroid hormone modulates murine hepatic lipoprotein metabolism in association with tissue-specific effects on apoB RNA editing mediated through alterations in ACF gene expression.

Identification of Rev-erbalpha as a physiological repressor of apoC-III gene transcription

Elevated serum levels of triglyceride-rich remnant lipoproteins (TRL) are a major risk factor predisposing a subject to atherosclerosis. Apolipoprotein C-III (apoC-III) is a major constituent of TRL that impedes triglyceride hydrolysis and remnant clearance and, as such, may exert pro-atherogenic activities. In the present study, transient cotransfection experiments in rat hepatocytes in primary culture and rabbit kidney RK13 cells demonstrated that overexpression of Rev-erbalpha specifically decreases basal and HNF-4 stimulated human apoC-III promoter activity. A Rev-erbalpha response element was mapped by promoter deletion, mutation analysis, and gel-shift experiments to a AGGTCA half-site located at position -23/-18 (downstream of the TATA box) in the apoC-III promoter. Finally, Rev-erbalpha-deficient mice displayed elevated serum and liver mRNA levels of apoC-III together with increased serum VLDL triglycerides. Taken together, our data identify Rev-erbalpha as a regulator of apoC-III gene expression, providing a novel, physiological role for this nuclear receptor in the regulation of lipid metabolism.

Transcriptional regulation of apolipoprotein C-III gene expression by the orphan nuclear receptor RORalpha

Triglyceride-rich remnant lipoproteins are considered as major risk factors contributing to the pathogenesis of atherosclerosis. Because apolipoprotein (apo) C-III is a major determinant of plasma triglyceride and remnant lipoprotein metabolism, it is important to understand how the expression of this gene is regulated. In the present study, we identified the orphan nuclear receptor RORalpha1 as a regulator of human and mouse apo C-III gene expression. Plasma triglyceride and apo C-III protein concentrations in staggerer (sg/sg) mice, homozygous for a deletion in the RORalpha gene, were significantly lower than in wild type littermates. The lowered plasma apo C-III levels were associated with reduced apo C-III mRNA levels in liver and intestine of sg/sg mice. Transient transfection experiments in human hepatoma HepG2, human colonic CaCO2, and rabbit kidney RK13 cells demonstrated that overexpression of the human RORalpha1 isoform specifically increases human apo C-III promoter activity, indicating that RORalpha1 enhances human apo C-III gene transcription. RORalpha1 response elements were mapped by promoter deletion analysis and gel shift experiments to two AGGTCA half-sites located at positions -83/-78 (within the C3P site) and -23/-18 (downstream of the TATA box) in the human apo C-III promoter, with the -23/-18 site exhibiting the highest binding affinity. Transfection of site-directed mutated constructs in HepG2 cells indicated that the RORalpha1 effect is predominantly mediated by the -23/-18 site. This site is conserved in the mouse apo C-III gene promoter. Moreover, RORalpha binds to the equivalent mouse site and activates constructs containing three copies of the mouse site cloned in front of an heterologous promoter. Taken together, our data identify RORalpha as a transcriptional regulator of apo C-III gene expression, providing a novel, physiological role for RORalpha1 in the regulation of genes controlling triglyceride metabolism.

The genetic background modifies the effects of the obesity mutation, 'fatty', on apolipoprotein gene regulation in rat liver

BACKGROUND

Obesity is associated with disorders of plasma lipid transport in many, but not in all obese subjects. The effects of obesity on the regulation of genes involved in plasma lipid transport may depend on specific mutations causing or contributing to obesity and/or on interactions of a specific obesity mutation with the genetic background. The 'fatty' (Glu269Pro) leptin receptor mutation causes severe obesity associated with hypertriglyceridaemia and altered hepatic apolipoprotein gene regulation in Zucker fatty rats.

OBJECTIVE

To determine whether the effects of the obesity mutation 'fatty' on apolipoprotein gene regulation in rat liver depend on the genetic background.

METHODS

We studied hepatic apolipoprotein (apo) A-IV, A-I, and C-III gene expression in obese rats carrying the 'fatty' mutation on the background of the Zucker or Wistar strain.

RESULTS

Basal apoA-IV gene expression was increased in fatty rats of both strains, whereas apoA-I and apoC-III gene expression differed between Wistar and Zucker fatty rats: apoA-I gene transcription was reduced to half and apoC-III mRNA was increased two-fold in Wistar fatty, but not in Zucker fatty rats vs lean controls. A fish oil diet suppressed apoA-IV, but not apoA-I gene transcription in Wistar fatty rats, whereas in Zucker fatty rats apoA-IV transcription was unaffected, but apoA-I transcription was suppressed.

CONCLUSIONS

Interactions of the 'fatty' leptin receptor mutation with the genetic background significantly affect the basal and diet-induced regulation of the apoA-IV, C-III and A-I genes in rat liver. The genetic background may therefore be a major determinant of the consequences of a specific obesity mutation for plasma lipid transport.

A thyroid hormone binding motif is evolutionarily conserved in apolipoproteins

High-density lipoproteins (HDL) are the major lipoprotein (Lp) plasma carriers of thyroid hormones, binding mediated by a specific interaction with their apolipoproteins (A-I, A-II, A-IV, C-I, C-II, C-III and E). The single binding site of these apolipoproteins is encoded by exon 3 (exon 2 for apoA-IV) of the respective gene and has amino acid sequence homology with regions of the three major thyroid hormone plasma transport proteins (TBG, TTR, and albumin) known to contain the corresponding hormone binding site(s). Within the hormone domain, we identified a 5-residue hydrophobic motif "Y, L/I/M, X, X, V/L/I" that is extremely well conserved in apolipoproteins. The exon-3 coded region of human apo E contains a hydrophobic pocket which is formed by Trp 34 and a number of neighboring leucines (residues no. 28, 30, 37 or 43). The location of this pocket overlaps strikingly that of the region (aa 26-40) where the said homology is maximal and where the motif YLRVW, (aa 36-40) lies. This hydrophobic pocket should represent the thyroid hormone site of apo E and, because of the said homology, should exist in the other HDL apolipoproteins. Because TBG and/or TTR are not present in all animal species, but Lp are, and because fish HDL bind thyroid hormones, I postulated that thyroid hormone binding to HDL apolipoproteins is conserved through the phylum. To this end, I evaluated the conservation of the 5-residue motif in all the apolipoprotein sequences known (PIR data bank no. 42) and tested the thyroid hormone binding properties of two animal apolipoproteins that were available (bovine apo A-I and rabbit apo E). I found that the conservation does exist and that the binding properties of the two animal apolipoproteins match those of the respective human counterpart. In addition, I found that the 5-residue motif is spared by naturally occurring mutations, which is not the case for other domains. I therefore conclude that the interaction of thyroid hormones with Lp represents the first plasma transport for these hormones that appeared in the animal world and that preservation of the structure of the hormone domain appears to be more important than preservation of other domains.

Regulation of human apolipoprotein A-I gene expression by gramoxone

To induce oxidative stress, HepG2 cells were exposed to a compound known as gramoxone. This compound undergoes a one-electron reduction to form a stable free radical which is capable of generating reactive oxygen species. We demonstrated that exposure of HepG2 cells to gramoxone (0.1 microM) resulted in a 2-fold decrease in apoA-I mRNA with no significant change in apoB and apoE mRNA levels. To examine if increased rates of mRNA degradation were responsible for the reduction in apoA-I mRNA levels, mRNA half-lives were measured in the presence of actinomycin D with and without gramoxone treatment. These studies revealed a 4-fold increase in the rate of apoA-I mRNA degradation in cells exposed to gramoxone. In similarly treated cells, nuclear run-off assays indicated that the transcription rate of the apoA-I gene was also increased 2-fold. Consistent with nuclear run-off assays, transient transfection experiments using a series of pGL2-derived luciferase reporter plasmids containing the human apoAI proximal promoter demonstrated that gramoxone treatment increased apoA-I promoter activity 2-fold. We have identified a potential "antioxidant response element" (ARE) in the apoA-I promoter region that may be responsible for the increase in apoA-I transcriptional activity by gramoxone. Gel mobility shift assays with an ARE oligonucleotide revealed increased levels of a specific protein-DNA complex that formed with nuclear extracts from gramoxone-treated cells. UV cross-linking experiments with the ARE and nuclear extracts from either untreated or gramoxone-treated cells detected proteins of approximately 100 and 115 kDa. When a single copy of the ARE was inserted upstream of the SV40 promoter in a luciferase reporter plasmid, a significant 2-fold induction in luciferase activity was observed in HepG2 cells in the presence of gramoxone. In contrast, a plasmid containing a mutant apoAI-ARE did not confer responsiveness to gramoxone. Furthermore, pGL2 (apoAI-250 mutant ARE), in which point mutations eliminated the ARE in the apoAI promoter, showed no increase in luciferase activity in response to gramoxone. Taken together, the data suggest that gramoxone affects apoA-I mRNA levels by both transcriptional and post-transcriptional mechanisms.

Control of synthesis and secretion of intestinal apolipoprotein A-IV by lipid

Apolipoprotein (apo) A-IV, a component of intestinally secreted, triacylglycerol-rich lipoproteins, has recently been proposed as a physiological controller of gastric function and food intake. Thus, it is important to understand the mechanisms involved in the control of expression, synthesis and secretion of apo A-IV. Apo A-IV is a member of a closely linked, multigene cluster which includes apolipoproteins A-I and C-III. Expression and synthesis of apo A-IV display marked variability with regard to species, tissue, stage of development and response to hormones, but intestinal apo A-IV is consistently stimulated by dietary lipid. The precise molecular mechanisms underlying the response of apo A-IV to lipid have not been clearly defined. Most evidence supports the hypothesis that some aspect of lipid transport is necessary for the apo A-IV response, but only part of this response may be due to a direct effect of intestinal lipid: recent findings suggest a connection between intestinal production of apo A-IV and hormonal and/or neural factors associated with operation of the "ileal brake." Thus, apo A-IV may play an integrative role in the modulation of both upper gastrointestinal function and ingestive behavior.

Thyroid hormone modulates apolipoprotein-AI gene expression at the post-transcriptional level in Hep G2 cells

Hyperthyroidism is associated with elevated plasma levels of apolipoprotein AI (apo AI). We have examined the effects of 3,3',-5-triiodothyronine on apo AI mRNA, transcription run-on activity, apo AI mRNA half-life, and the rate of protein synthesis in Hep G2 cells, to understand the molecular mechanism by which thyroid hormone regulates apo AI gene expression. Incubation with thyroid hormone increased the apo AI and apo AII mRNA concentrations twofold. Cycloheximide alone caused a significant increase in apo AI mRNA. Nuclear run-on assays indicate that thyroid hormone did not change the rate of the apo AI gene transcription at 6, 12 or 24 h, showing that thyroid hormone did not modulate apo AI gene transcription. Kinetic studies performed in the presence of actinomycin D showed that the half-life of apo AI mRNA was increased 2-3-fold by thyroid hormone over control cells. Thyroid hormone did not change the incorporation of [35S]methionine into immunoprecipitable apo AI. Pulse-chase experiments demonstrated that there was no change in the secretion and degradation rates of labeled apo AI in response to T3. This suggests that thyroid hormone does not affect the catabolism of apo AI (degradation or/and uptake) and that translation control strongly influences the regulation of apo AI gene expression. The stabilization of apo AI mRNA by thyroid hormone and its role in translation remain to be elucidated.

Thyroid hormone influences the maturation of apolipoprotein A-I messenger RNA in rat liver

Chronic administration of thyroid hormone (T3) increases apolipoprotein (apo) A-I gene expression in rat liver. That transcriptional activity of the apoA-I gene is reduced to 50% of control, whereas abundance levels of nuclear and total cellular apoA-I mRNA are increased 3-fold, implies more effective apoA-I mRNA maturation. To study hormonal effects on apoA-I RNA processing, we quantified mRNA precursors in control and T3-treated rats (50 micrograms/100 g body weight for 7 days). Northern blotting, amplification of reverse-transcribed RNA, and ribonuclease protection assays showed that the splicing pathway is branched, in that either intron 1 or intron 2 is removed first from the primary transcript, whereas intron 3 is removed last. In T3-treated rats, abundance levels of the primary transcript, the intron 1-containing precursor devoid of intron 2, the intron 2-containing precursor devoid of intron 1, the intron 3-containing precursor lacking both introns 1 and 2, and nuclear mRNA were 65, 183, 78, 195, and 268% of controls. Compared with control rats, the half-life of the intron 1-containing precursor, measured after injection of actinomycin D, was increased 2-fold in T3-treated rats. In contrast, half-lives of the primary transcript and the intron 2-containing precursor were similar in control and T3-treated rats. Ribonuclease protection assays revealed an RNA species extending from the transcription start site close to the 3' end of intron 1. The abundance of this RNA fragment, probably representing a degradation product, was 2.5-fold higher in control than in T3-treated animals (p < 0.001). Sequences of apoA-I mRNA precursors were identical in control and T3-treated rats which excluded hormonal effects on splice-site selection or post-transcriptional editing of apoA-I transcripts. Compartmental modeling of apoA-I mRNA processing suggested that chronic thyroid hormone administration enhances apoA-I mRNA maturation more than 7-fold by protecting the intron 1-containing precursor devoid of intron 2 from degradation and by facilitating the splicing of intron 1 from this precursor.