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

Whole body proteome response to a dietary lysine imbalance in zebrafish Danio rerio

Lysine (Lys) is an indispensable amino acid (AA) and is generally the first limiting AA in most vegetable proteins used in fish feeds. Lys availability may thus limit protein synthesis and accretion, and growth of fish. Metabolic effects of dietary Lys imbalance were examined by 2D-proteomics using zebrafish as model. The Control diet (Lys: 2.47 g kg(-1)) was based on zebrafish carcass AA profiles previously obtained. Two other experimental diets were deficient in Lys [Lys(-); 1.34 g kg(-1)] and Lys added in excess [Lys(+); 4.63 g kg(-1)]. Fish growth was monitored from 33 to 49 days post-fertilization and the whole body proteome screened by means of two-dimension gel electrophoresis and mass spectrometry. Growth rate was negatively affected in group Lys(-). Comparative proteomic analysis showed 45 spots differentially expressed among groups. Twenty-nine of these proteins were identified revealing proteins involved in muscle growth, energy and lipid metabolism, eye lens differentiation, chaperone activity and apoptosis. Lys deficiency is accompanied by a down-regulation of muscle proteins and up-regulation of proteins affected by fasting, energy deficit, growth arrest and apoptosis. Excess Lys was accompanied by an up-regulation of proteins related to glycolysis, steroidogenesis and sexual maturation.

Effect of retinoic acid on cell proliferation and differentiation as well as on lipid synthesis, lipoprotein secretion, and apolipoprotein biogenesis

Dietary vitamin A and its active metabolites are essential nutrients for many functions as well as potent regulators of gene transcription and growth. Although the epithelium of the small intestine is characterized by rapid and constant renewal and enterocytes play a central role in the absorption and metabolism of alimentary retinol, very little is known about the function of retinoids in the human gastrointestinal epithelium and mechanisms by which programs engage the cell cycle are poorly understood. We have assessed the effects of 10 microM 9- and 13-cis-retinoic acid (RA) on proliferation and differentiation processes, lipid esterification, apolipoprotein (apo) biogenesis and lipoprotein secretion along with nuclear factor gene transcription. Treatment of Caco-2 cells with RA at different concentrations and incubation periods revealed the reduction of thymidine incorporation in 60% preconfluent or 100% confluent cells. Concomitantly, RA 1) modulated D-type cyclins by reducing the mitogen-sensitive cyclin D1 and upregulating cyclin D3 expressions and 2) caused a trend of increase in p38 MAPK, which triggers CDX2, a central protein in cell differentiation. RA remained without effect on lipoprotein output and apo synthesis, even for apo A-I that possesses RARE in its promoter. RA, in combination with 22-hydroxycholesterol, could induce apo A-I gene expression without any impact on apo A-I mass. Only the gene expression of peroxisome proliferator-activated receptor (PPAR)beta, retinoic receptor (RAR)beta, and RARgamma was augmented and no alteration was noted in PPARalpha, PPARgamma, liver X receptor (LXR)alpha, LXRbeta, and retinoid X receptors. Taken together, these data highlight RA-induced cell differentiation via specific signaling without a significant impact on apo A-I synthesis.

Thyroid hormone regulates endogenous amyloid-beta precursor protein gene expression and processing in both in vitro and in vivo models

Thyroid hormone negatively regulates the amyloid-beta precursor protein (APP) gene in thyroid hormone receptor (TR)-transfected neuroblastoma cells. A negative thyroid hormone response element (nTRE) that mediates this regulation has been identified in the first exon of the APP gene. We demonstrate in an in vivo system that expression of APP mRNA, APP protein, and APP secretase cleavage products in mouse brain is influenced by thyroid status. Adult female mice were made hyperthyroid or hypothyroid for 3 weeks and compared to euthyroid mice. APP gene product expression was increased in hypothyroid mouse brain and reduced in hyperthyroid mouse brain, when compared to euthyroid controls. We observed similar effects of thyroid hormone on endogenous APP gene expression in human neuroblastoma cells. The incidence of hypothyroidism increases with age, and localized hypothyroidism of central nervous system has been reported in some patients with Alzheimer's disease (AD). Reduced action of thyroid hormone on the APP gene may contribute to AD pathology by increasing APP expression and the levels of processed APP products. These findings may be an underlying mechanism contributing to the association of hypothyroidism with AD in the elderly, as well as identifying a potential therapeutic target. Pharmacologic supplementation of thyroid hormone, or its analogs, may reduce APP gene expression and beta amyloid peptide accumulation.

Proteomic sensitivity to dietary manipulations in rainbow trout

Changes in dietary protein sources due to substitution of fish meal by other protein sources can have metabolic consequences in farmed fish. A proteomics approach was used to study the protein profiles of livers of rainbow trout that have been fed two diets containing different proportions of plant ingredients. Both diets control (C) and soy (S) contained fish meal and plant ingredients and synthetic amino acids, but diet S had a greater proportion of soybean meal. A feeding trial was performed for 12 weeks at the end of which, growth and protein metabolism parameters were measured. Protein growth rates were not different in fish fed different diets; however, protein consumption and protein synthesis rates were higher in the fish fed the diet S. Fish fed diet S had lower efficiency of retention of synthesised protein. Ammonia excretion was increased as well as the activities of hepatic glutamate dehydrogenase and aspartate amino transferase (ASAT). No differences were found in free amino acid pools in either liver or muscle between diets. Protein extraction followed by high-resolution two-dimensional electrophoresis, coupled with gel image analysis, allowed identification and expression of hundreds of protein. Individual proteins of interest were then subjected to further analysis leading to protein identification by trypsin digest fingerprinting. During this study, approximately 800 liver proteins were analysed for expression pattern, of which 33 were found to be differentially expressed between diets C and S. Seventeen proteins were positively identified after database searching. Proteins were identified from diverse metabolic pathways, demonstrating the complex nature of gene expression responses to dietary manipulation revealed by proteomic characterisation.

Transcriptional induction of hepatic NADPH: cytochrome P450 oxidoreductase by thyroid hormone

Studies were carried out to elucidate the mechanism whereby thyroid hormone (T3) induces NADPH:cytochrome P450 oxidoreductase (P450R) mRNA in rat liver in vivo. Northern blot analysis revealed that T3 treatment increases unspliced liver nuclear P450R RNA 4-fold within 8 h and that this induction precedes the induction of mature, cytoplasmic P450R RNA. Unspliced nuclear P450R RNA was suppressed below basal levels 24 h after T3 treatment, despite the continued presence of elevated circulating T3 levels. To determine whether the T3-stimulated increase in nuclear P450R RNA reflects an increase in P450R transcription initiation, nuclear run-on transcription assays were carried out. T3 induced a 6- to 8-fold increase in P450R transcription rate within 12 h, sufficient to account for the observed increase in nuclear P450R precursor RNA, followed by a decrease back to basal transcription levels at 24 h, consistent with the nuclear RNA profile. Similar transcriptional increases were observed in nuclear run-on transcription studies using hybridization probes corresponding to nine different fragments of the P450R gene, spanning exon 2 to exon 16. Thus, P450R transcription initiation, not transcription elongation, is the T3-regulated event. Similar results were obtained during short (5 min) compared with long (45 min) nuclear run-on transcription assays, suggesting that changes in nuclear RNA processing or regulated degradation do not contribute to the overall RNA induction. This finding was confirmed by the ability of the RNA polymerase inhibitor actinomycin D, administered in vivo, to block T3 induction of P450R transcriptional activity. We conclude that P450R transcription, rather than nuclear RNA processing or mRNA stabilization, is the primary mechanism whereby T3 induces hepatic P450R mRNA.

Activation of apolipoprotein AI gene expression by protein kinase A and kinase C through transcription factor, Sp1

Our previous finding that insulin induces apolipoprotein AI (apoAI) transcription points to the participation of intracellular signaling. This finding prompted us to ask whether two classical G-protein-coupled signaling pathways requiring activated protein kinase A (PKA) or kinase C (PKC) may also regulate apoAI. Therefore, human hepatoma, Hep G2 cells stably transfected with pAI.474-CAT, a reporter construct spanning -474 to -7 of apoAI DNA fused to chloramphenicol acetyltransferase (CAT) were treated with 10 microm forskolin (FSK) or 50 nm phorbol dibutyrate (PDBu) to activate PKA and PKC, respectively. Results showed that the apoAI promoter activity increased 4-5-fold following 24 h of treatment with either FSK or PDBu. Induction by either agent was blocked with actinomycin D but not the protein synthesis inhibitor, cycloheximide. The PKA inhibitor, PKI 14-22 amide, abrogated induction by FSK, 100 microm 8-bromo-cAMP, or 100 ng/ml cholera toxin, but it had no effect on activation via PKC. Similarly, PDBu induction was attenuated by 2 microm of the PKC inhibitor, GF109203X, but it did not affect FSK activity. Next we used deletional constructs to show that the actions of FSK and PDBu required the insulin-responsive core element (IRCE). This motif matched the consensus binding site for the transcription factor, Sp1. The binding of Sp1 to the IRCE was confirmed by gel-retardation and supershift analysis. Site-directed mutagenesis of the IRCE eliminated Sp1 action and induction by FSK or PDBu. Whereas overexpression of Sp1 enhanced basal and FSK or PDBu induced promoter activity, transfection of an antisense oligomer against Sp1 mRNA attenuated both parameters. In summary, activation of PKA or PKC increases apoAI promoter activity. The activity of both signaling pathways is mediated by the IRCE, a motif that binds the transcription factor, Sp1.

High density lipoprotein, apolipoprotein A-I, and coronary artery disease

High density lipoproteins (HDL), one of the main lipoprotein particles circulating in plasma, is involved in the reverse cholesterol transport. Several lines of evidence suggest that elevated levels of HDL is protective against coronary heart disease. The role of HDL in the removal of body cholesterol and in the regression of atherosclerosis add to the importance of understanding the molecular-cellular processes that determine plasma levels of HDL. Factors modulating plasma levels of HDL may have influence on the predisposition of an individual to premature coronary artery disease. Apolipoprotein (apo) A-I is the main apolipoprotein component of HDL and, to a large extent, sets the plasma levels of HDL. Thus, understanding the regulation of apoA-I gene expression may provide clues to raise plasma levels of HDL. This review discusses the various pathways that alter plasma levels of HDL. Since apoA-I is the main protein component of HDL and determines the plasma levels of HDL, this review also covers the regulation of apoA-I gene expression.

Cloning, characterisation and expression of the apolipoprotein A-I gene in the sea bream (Sparus aurata)

A full length cDNA clone representing apolipoprotein A-I was isolated from a sea bream (Sparus aurata) liver library. The clone encodes a 261 amino acid protein which shows highest amino acid identity (38%) with salmon apolipoprotein A-I. Northern blot analysis showed strong expression of a 1.4 kb transcript in liver with lower expression in intestine. Expression of apolipoprotein A-I in intestine was markedly reduced by treatment with triiodothyronine (T3).

Effects of L-triiodothyronine and the thyromimetic L-94901 on serum lipoprotein levels and hepatic low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and apo A-I gene expression

The mechanisms by which thyroid hormone (triiodothyronine (T3)) and a thyromimetic, 2-amino-3-(3,5-dibromo-4-[4-hydroxy-3-(6-oxo-1,6-dihydro-pyridazin -3-ylmethyl)-phenoxyl]-phenyl)propionic acid (L-94901), lower plasma low density lipoprotein (LDL) cholesterol and raise plasma high density lipoprotein (HDL) cholesterol levels was investigated in thyroidectomized and sham-operated rats. Thyroidectomy resulted in a 77% increase in plasma LDL cholesterol, a 60% decrease in plasma triglycerides, and a modest reduction in HDL cholesterol. Daily oral dosing with T3 (10-170 nmol/kg) or L94901 (100-1000 nmol/kg) for 7 days decreased plasma LDL cholesterol in thyroidectomized rats by 60-80%, respectively. This reduction in LDL cholesterol was accompanied by a dose-dependent increase in HDL cholesterol levels of up to 60%. Thus, the ratio of LDL to HDL was decreased from 1.01 to 0.12 after treatment with L-94901 and to 0.25 after dosing with T3. In sham-operated animals, T3 and L-94901 lowered LDL cholesterol by 61 and 46%, respectively, and increased HDL cholesterol by 25 and 53%, respectively. Immunoblotting analysis of liver membranes prepared from thyroidectomized or sham-operated rats demonstrated that LDL receptor protein levels were increased by up to eight-fold. Northern blotting analysis revealed similar large increases in hepatic LDL receptor mRNA levels that accounted for the increases in LDL receptor protein levels. Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA, protein, and activity were increased 2- to 3-fold. The T3- and L-94901-mediated increases in serum HDL levels were associated with 2- to 3-fold increases in apo A-I mRNA levels. In contrast with most other hypocholesterolemic agents, T3 and L-94901 significantly increase HDL cholesterol levels in addition to decreasing LDL cholesterol levels due to induction of hepatic apo A-I and LDL receptor gene expression.

Mechanisms of thyroid hormone action. Implications for the clinical manifestation of thyrotoxicosis

Serum thyroid hormone concentrations alone do not explain the variability and severity of the range of symptoms observed in thyrotoxic patients. Despite gaps in our understanding of the links between the clinical manifestations of thyrotoxicosis and the underlying mechanisms, much has been learned. A limited number of markers directly reflect T3 action. The future elucidation of T3 targets that mediate these effects should ultimately lead to additional clinical markers of tissue-specific T3 action. The availability of such tests should allow for more specific treatment of individual patients.