Type 1 Deiodinase Regulates ApoA-I Gene Expression and ApoA-I Synthesis Independent of Thyroid Hormone Signaling

Endoplasmic reticulum-resident selenoproteins and their roles in glucose and lipid metabolic disorders

Glucose and lipid metabolic disorders (GLMDs), such as diabetes, dyslipidemia, metabolic syndrome, nonalcoholic fatty liver disease, and obesity, are significant public health issues that negatively impact human health. The endoplasmic reticulum (ER) plays a crucial role at the cellular level for lipid and sterol biosynthesis, intracellular calcium storage, and protein post-translational modifications. Imbalance and dysfunction of the ER can affect glucose and lipid metabolism. As an essential trace element, selenium contributes to various human physiological functions mainly through 25 types of selenoproteins (SELENOs). At least 10 SELENOs, with experimental and/or computational evidence, are predominantly found on the ER membrane or within its lumen. Two iodothyronine deiodinases (DIOs), DIO1 and DIO2, regulate the thyroid hormone deiodination in the thyroid and some external thyroid tissues, influencing glucose and lipid metabolism. Most of the other eight members maintain redox homeostasis in the ER. Especially, SELENOF, SELENOM, and SELENOS are involved in unfolded protein responses; SELENOI catalyzes phosphatidylethanolamine synthesis; SELENOK, SELENON, and SELENOT participate in calcium homeostasis regulation; and the biological significance of thioredoxin reductase 3 in the ER remains unexplored despite its established function in the thioredoxin system. This review examines recent research advances regarding ER SELENOs in GLMDs and aims to provide insights on ER-related pathology through SELENOs regulation.

Loss of Selenov predisposes mice to extra fat accumulation and attenuated energy expenditure

Selenoprotein V (SELENOV) is a new and the least conserved member of the selenoprotein family. Herein we generated Selenov knockout (KO) mice to determine its in vivo function. The KO led to 16-19% increases (P < 0.05) in body weight that were largely due to 54% higher (P < 0.05) fat mass accumulation, compared with the wild-type (WT) controls. The extra fat accumulation in the KO mice was mediated by up-regulations of genes and proteins involved in lipogenesis (Acc, Fas, Dgat, and Lpl; up by 40%-1.1-fold) and down-regulations of lipolysis (Atgl, Hsl, Ces1d, and Cpt1a; down by 36-89%) in the adipose tissues. The KO also decreased (P < 0.05) VO₂ consumption (14-21%), VCO₂ production (14-16%), and energy expenditure (14-23%), compared with the WT controls. SELENOV and O-GlcNAc transferase (OGT) exhibited a novel protein-protein interaction that explained the KO-induced decreases (P < 0.05) of OGT protein (15-29%), activity (33%), and function (O-GlcNAcylation, 10-21%) in the adipose tissues. A potential cascade of SELENOV-OGT-AMP-activated protein kinase might serve as a central mechanism to link the biochemical and molecular responses to the KO. Overall, our data revealed a novel in vivo function and mechanism of SELENOV as a new inhibitor of body fat accumulation, activator of energy expenditure, regulator of O-GlcNAcylation, and therapeutic target of such related disorders.

Early induction of hepatic deiodinase type 1 inhibits hepatosteatosis during NAFLD progression

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) comprises a spectrum ranging from hepatosteatosis to progressive nonalcoholic steatohepatitis that can lead to cirrhosis. Humans with low levels of prohormone thyroxine (T₄) have a higher incidence of NAFLD, and thyroid hormone treatment is very promising in all patients with NAFLD. Deiodinase type 1 (Dio1) is a hepatic enzyme that converts T₄ to the bioactive T₃ and therefore regulates thyroid hormone availability within hepatocytes. We investigated the role of this intrahepatic regulation during the progression of NAFLD.

METHODS

We investigated hepatic thyroid hormone metabolism in two NAFLD models: wild-type mice fed a Western diet with fructose and Lepr^db mice fed a methionine- and choline-deficient diet. AAV8-mediated liver-specific Dio1 knockdown was employed to investigate the role of Dio1 during the progression of NAFLD. Intrahepatic thyroid hormone levels, deiodinase activity, and metabolic parameters were measured.

RESULTS

Dio1 expression and activity were increased in the early stages of NAFLD and were associated with an increased T₃/T₄ ratio. Prevention of this increase by AAV8-mediated liver-specific Dio1 knockdown increased hepatic triglycerides and cholesterol and decreased the pACC/ACC ratio and acylcarnitine levels, suggesting there was lower β-oxidation. Dio1 siRNA KD in hepatic cells treated with fatty acids showed increased lipid accumulation and decreased oxidative phosphorylation.

CONCLUSION

Hepatic Dio1 gene expression was modulated by dietary conditions, was increased during hepatosteatosis and early NASH, and regulated hepatic triglyceride content. These early adaptations likely represent compensatory mechanisms that reduce hepatosteatosis and prevent NASH progression.

Effects of thyroid hormones on thermogenesis and energy partitioning

Thyroid hormones (TH) are of central importance for thermogenesis, energy homeostasis and metabolism. Here, we will discuss these aspects by focussing on the physiological aspects of TH-dependent regulation in response to cold exposure and fasting, which will be compared to alterations in primary hyperthyroidism and hypothyroidism. In particular, we will summarise current knowledge on regional thyroid hormone status in the central nervous system (CNS) and in peripheral cells. In contrast to hyperthyroidism and hypothyroidism, where parallel changes are observed, local alterations in the CNS differ to peripheral compartments when induced by cold exposure or fasting. Cold exposure is associated with low hypothalamic TH concentrations but increased TH levels in the periphery. Fasting results in a reversed TH pattern. Primary hypothyroidism and hyperthyroidism disrupt these fine-tuned adaptive mechanisms and both, the hypothalamus and the periphery, will have the same TH status. These important mechanisms need to be considered when discussing thyroid hormone replacement and other therapeutical interventions to modulate TH status.

Differences in the Plasma Proteome of Patients with Hypothyroidism before and after Thyroid Hormone Replacement: A Proteomic Analysis

Thyroid hormone is a potent stimulator of metabolism, playing a critical role in regulating energy expenditure and in key physiological mechanisms, such as growth and development. Although administration of thyroid hormone in the form of levo thyroxine (l-thyroxine) has been used to treat hypothyroidism for many years, the precise molecular basis of its physiological actions remains uncertain. Our objective was to define the changes in circulating protein levels that characterize alterations in thyroid hormone status. To do this, an integrated untargeted proteomic approach with network analysis was used. This study included 10 age-matched subjects with newly diagnosed overt hypothyroidism. Blood was collected from subjects at baseline and at intervals post-treatment with l-thyroxine until they reached to euthyroid levels. Plasma protein levels were compared by two-dimensional difference in gel electrophoresis (2D-DIGE) pre- and post-treatment. Twenty differentially expressed protein spots were detected. Thirteen were identified, and were found to be unique protein sequences by MALDI-TOF mass spectrometry. Ten proteins were more abundant in the hypothyroid vs. euthyroid state: complement C2, serotransferrin, complement C3, Ig κ chain C region, α-1-antichymotrypsin, complement C4-A, haptoglobin, fibrinogen α chain, apolipoprotein A-I, and Ig α-1 chain C region. Three proteins were decreased in abundance in the hypothyroid vs. euthyroid state: complement factor H, paraneoplastic antigen-like protein 6A, and α-2-macroglobulin. The differentially abundant proteins were investigated by Ingenuity Pathway Analysis (IPA) to reveal their associations with known biological functions. Their connectivity map included interleukin-6 (IL-6) and tumour necrosis factor α (TNF-α) as central nodes and the pathway identified with the highest score was involved in neurological disease, psychological disorders, and cellular movement. The comparison of the plasma proteome between the hypothyroid vs euthyroid states revealed differences in the abundance of proteins involved in regulating the acute phase response.

β3-Adrenergic receptor regulates hepatic apolipoprotein A-I gene expression

BACKGROUND

β₃-adrenergic receptor (β₃-AR) was shown to upregulate hepatic apolipoprotein A-I (apoA-I) expression and reverse atherosclerotic plaques in vivo experiments. However, the effect of β₃-AR on apoA-I expression in vitro is unknown. The specific mechanism underlying β₃-AR prevention of atherosclerosis is unclear.

OBJECTIVE

The present study was designed to investigate the molecular mechanism of β₃-AR-mediated regulation of hepatic apoA-I gene expression.

METHODS

HepG2 cells were preincubated with/without a selective protein kinase A inhibitor (H-89) and then treated with a selective β₃-AR agonist (BRL37344) or antagonist (SR59230A). The hepatic apoA-I expression was detected by reverse transcription real-time quantitative polymerase chain reaction and Western blot analysis. Enzyme-linked immunosorbent assay was used to evaluate the secretion of apoA-I. A recombinant plasmid containing the apoA-I promoter was constructed and transiently transfected into HepG2 cells, and dual-luciferase reporter assays were used to examine the activity of the apoA-I promoter. A chromatin immunoprecipitation polymerase chain reaction assay was used to evaluate binding activities of hepatocyte nuclear factor-4 (HNF-4), HNF-3, and early growth response protein-1.

RESULTS

β₃-AR activation significantly upregulated apoA-I expression, promoted apoA-I secretion, and enhanced the activities of the apoA-I promoter, HNF-4, and HNF-3 in hepatocytes, whereas early growth response protein-1 was not affected. Moreover, protein kinase A inhibition partially suppressed the activation of the apoA-I promoter, HNF-4, and HNF-3 and almost completely blocked the upregulation of apoA-I expression induced by β₃-AR.

CONCLUSION

β₃-AR activation increased the activities of the apoA-I promoter, HNF-4, and HNF-3, which might account for the mechanism of β₃-AR-mediated upregulation of hepatic apoA-I expression. β₃-AR might exert an anti-atherosclerotic effect by upregulating hepatic apoA-I expression and promoting the cholesterol reverse transport process.

Treatment of Dyslipidemias to Prevent Cardiovascular Disease in Patients with Type 2 Diabetes

PURPOSE OF REVIEW

Current preventive and treatment guidelines for type 2 diabetes have failed to decrease the incidence of comorbidities, such as dyslipidemia and ultimately heart disease. The goal of this review is to describe the physiological and metabolic lipid alterations that develop in patients with type 2 diabetes mellitus. Questions addressed include the differences in lipid and lipoprotein metabolism that characterize the dyslipidemia of insulin resistance and type 2 diabetes mellitus. We also examine the relevance of the new AHA/ADA treatment guidelines to dyslipidemic individuals.

RECENT FINDINGS

In this review, we provide an update on the pathophysiology of diabetic dyslipidemia, including the role of several apolipoproteins such as apoC-III. We also point to new studies and new agents for the treatment of individuals with type 2 diabetes mellitus who need lipid therapies. Type 2 diabetes mellitus causes cardiovascular disease via several pathways, including dyslipidemia characterized by increased plasma levels of apoB-lipoproteins and triglycerides, and low plasma concentrations of HDL cholesterol. Treatments to normalize the dyslipidemia and reduce the risk for cardiovascular events include the following: lifestyle and medication, particularly statins, and if necessary, ezetimibe, to significantly lower LDL cholesterol. Other treatments, more focused on triglycerides and HDL cholesterol, are less well supported by randomized clinical trials and should be used on an individual basis. Newer agents, particularly the PCSK9 inhibitors, show a great promise for even greater lowering of LDL cholesterol, but we await the results of ongoing clinical trials.