The Thyroid Hormone Transporter Mct8 Restricts Cathepsin-Mediated Thyroglobulin Processing in Male Mice through Thyroid Auto-Regulatory Mechanisms That Encompass Autophagy.

The thyroid gland is both a thyroid hormone (TH) generating as well as a TH responsive organ. It is hence crucial that cathepsin-mediated proteolytic cleavage of the precursor thyroglobulin is regulated and integrated with the subsequent export of TH into the blood circulation, which is enabled by TH transporters such as monocarboxylate transporters Mct8 and Mct10. Previously, we showed that cathepsin K-deficient mice exhibit the phenomenon of functional compensation through cathepsin L upregulation, which is independent of the canonical hypothalamus-pituitary-thyroid axis, thus, due to auto-regulation. Since these animals also feature enhanced Mct8 expression, we aimed to understand if TH transporters are part of the thyroid auto-regulatory mechanisms. Therefore, we analyzed phenotypic differences in thyroid function arising from combined cathepsin K and TH transporter deficiencies, i.e., in Ctsk-/-/Mct10-/-, Ctsk-/-/Mct8-/y, and Ctsk-/-/Mct8-/y/Mct10-/-. Despite the impaired TH export, thyroglobulin degradation was enhanced in the mice lacking Mct8, particularly in the triple-deficient genotype, due to increased cathepsin amounts and enhanced cysteine peptidase activities, leading to ongoing thyroglobulin proteolysis for TH liberation, eventually causing self-thyrotoxic thyroid states. The increased cathepsin amounts were a consequence of autophagy-mediated lysosomal biogenesis that is possibly triggered due to the stress accompanying intrathyroidal TH accumulation, in particular in the Ctsk-/-/Mct8-/y/Mct10-/- animals. Collectively, our data points to the notion that the absence of cathepsin K and Mct8 leads to excessive thyroglobulin degradation and TH liberation in a non-classical pathway of thyroid auto-regulation.

Visceral Adipose Tissue in Patients with Crohn's Disease Correlates with Disease Activity, Inflammatory Markers, and Outcome.

BACKGROUND: Visceral adipose tissue (VAT) could affect Crohn's disease (CD); however, no prospective clinical studies have explored the issue. METHODS: We measured VAT with magnetic resonance imaging and total fat mass (FM) with air-displacement plethysmography in 31 women with CD in remission and 19 matched control women. We assessed the VAT/FM ratio as index of VAT accumulation, measured cytokines, and monitored clinical features (duration of remission, disease behavior, and outcome) in patients with CD retrospectively and prospectively. We also tested whether ultrasound could provide a surrogate marker of VAT in patients with CD. RESULTS: Patients with CD had higher percentage of FM (37 ± 10% versus 31 ± 10%, P = 0.03), VAT (1885 ± 1403 mL versus 941 ± 988 mL, P = 0.02), and VAT/FM ratio (65 ± 24 mL/kg versus 37 ± 25 mL/kg, P = 0.004) than control women. In patients with CD, VAT/FM ratio was associated with leptin (P = 0.009) and interleukin 6 (P = 0.032) concentrations, and higher in short-term than in long-term remission (72.6 ± 27.1 mL/kg versus 54.8 ± 16.1 mL/kg, P = 0.079). Patients with CD with stricturing/fistulizing disease had a higher VAT/FM ratio than patients with nonstricturing/nonfistulizing behavior (79 ± 0.15 mL/kg versus 63 ± 28 mL/kg, P = 0.067). A higher baseline VAT/FM ratio was associated with an increased disease activity at follow-up (P = 0.029). The ultrasound-determined thickness between the abdominal wall and the aorta was strongly associated with VAT as measured by magnetic resonance imaging (P < 0.001). CONCLUSIONS: VAT accumulation could be a prospective risk factor for increased disease activity in CD.

Nutritional strategy to prevent fatty liver and insulin resistance independent of obesity by reducing glucose-dependent insulinotropic polypeptide responses in mice.

AIMS/HYPOTHESIS: High intake of carbohydrates, particularly sucrose, in western societies is associated with the development of non-alcoholic fatty liver (NAFL) and diabetes mellitus. It is unclear whether this is related primarily to the carbohydrate quantity or to the hormonal responses, particularly glucose-dependent insulinotropic polypeptide (GIP), which is released in the proximal intestine. Therefore, we investigated the role of GIP by comparing two glucose-fructose dimers, sucrose and Palatinose (isomaltulose), resorbed proximally or distally. METHODS: The glycaemic and incretin responses to sucrose and Palatinose were studied by oral gavage and meal tests. We then analysed phenotypic and metabolic diet-induced changes in C57Bl/6J mice exposed to isoenergetic diets differing in carbohydrate type. Studies were repeated in GIP receptor knockout (Gipr(-/-)) mice and their wild-type littermates. RESULTS: Compared with sucrose, Palatinose intake resulted in slower glucose absorption and reduced postprandial insulin and GIP levels. After 22 weeks, Palatinose feeding prevented hepatic steatosis (48.5%) compared with sucrose and improved glucose tolerance, without differences in body composition and food intake. Ablation of GIP signalling in Gipr(-/-) mice completely prevented the deleterious metabolic effects of sucrose feeding. Furthermore, our microarray analysis indicated that sucrose increased 2.3-fold the hepatic expression of Socs2, which is involved in the growth hormone signalling pathway and participates in the development of NAFL. CONCLUSIONS/INTERPRETATION: Our results suggest that the site of glucose absorption and the GIP response determine liver fat accumulation and insulin resistance. GIP may play a role in sucrose induced fatty liver by regulating the expression of Socs2.

Thyroid function is maintained despite increased oxidative stress in mice lacking selenoprotein biosynthesis in thyroid epithelial cells.

AIMS: We have tested the hypothesis that selenium (Se)-containing antioxidative enzymes protect thyroid epithelial cells from oxidative damage associated with enzymatic production of hydrogen peroxide required for thyroid hormone biosynthesis. Thyroid epithelial cells therefore express antioxidative enzymes, including catalase, peroxiredoxins, thioredoxin reductases, and glutathione peroxidases (GPxs). The latter two enzyme families contain highly active peroxide-degrading enzymes that carry selenocysteine (Sec) in their active centers. Since low Se status has been associated with thyroid disorders, selenoproteins are considered essential for thyroid integrity and function. We have conditionally inactivated selenoprotein biosynthesis in thyrocytes by targeting Sec tRNA. RESULTS: Constitutive and inducible Cre/loxP-mediated recombination of tRNA([Ser]Sec) drastically reduced activities of selenoenzymes GPx and type I-deiodinase in thyroid extracts. Immunohistochemical staining revealed increased 4-hydroxynonenal and 3-nitro-tyrosine levels consistent with increased oxidative stress. However, gross thyroid morphology remained intact for at least 6 months after recombination. Circulating thyroid hormone levels remained normal in mutant mice, while thyrotropin (TSH) levels were moderately elevated. Challenging mutant mice with low iodine diet increased TSH, but did not lead to destruction of selenoprotein-deficient thyroids. INNOVATION: This is the first report probing the assumed physiological roles of selenoproteins in the thyroid using a genetic loss-of-function approach. CONCLUSION: We conclude that selenoproteins protect thyrocytes from oxidative damage and modulate thyroid hormone biosynthesis, but are not essential for thyrocyte survival.

Inhibition of xanthine oxidase reduces wasting and improves outcome in a rat model of cancer cachexia.

Cachexia is a common co-morbidity in cancer occurring in up to 80% of patients depending on the type of cancer. Uric acid (UA), the end-product of the purine metabolism, is elevated in cachexia due to tissue wasting and upregulated xanthine oxidase (XO) activity. High serum UA levels indicate increased XO-dependent production of oxygen free radicals (reactive oxygen species; ROS) and correlate with metabolic illness and poor survival. We hypothesized that XO-inhibition might reduce inflammatory signals accounting for tissue wasting and improve survival in experimental cancer cachexia. Animals were inoculated intraperitoneally with AH-130 hepatoma cells and treated with two XO-inhibitors: allopurinol [Allo, low (LD) and high dose (HD) 4 and 40 mg/kg/d] and its more effective active metabolite oxypurinol (Oxy, 4 and 40 mg/kg/d) or placebo for 15 days. Weight loss and tissue wasting of both fat and lean tissue (assessed by NMR-scanning) was reduced by both LD and HD Allo and LD-Oxy, but not by HD-Oxy. A robust induction of XO-activity for generation of reactive oxygen species was seen in the placebo group (assessed by electron paramagnetic spectroscopy), which was reduced by XO-inhibition. Increased ROS induced cytokine signaling, proteolytic activity and tissue degradation were all attenuated by XO inhibition. Survival was significantly and dose dependently improved. Food intake and spontaneous locomotor activity were higher, indicating a higher quality of life. Inhibition of XO can reduce tissue wasting and improve survival in cancer cachexia and clearly clinical studies are needed.

The HPA axis modulates the CNS melanocortin control of liver triacylglyceride metabolism.

The central melanocortin system regulates lipid metabolism in peripheral tissues such as white adipose tissue. Alterations in the activity of sympathetic nerves connecting hypothalamic cells expressing melanocortin 3/4 receptors (MC3/4R) with white adipocytes have been shown to partly mediate these effects. Interestingly, hypothalamic neurons producing corticotropin-releasing hormone and thyrotropin-releasing hormone co-express MC4R. Therefore we hypothesized that regulation of hypothalamo-pituitary adrenal (HPA) and hypothalamo-pituitary thyroid (HPT) axes activity by the central melanocortin system could contribute to its control of peripheral lipid metabolism. To test this hypothesis, we chronically infused rats intracerebroventricularly (i.c.v.) either with an MC3/4R antagonist (SHU9119), an MC3/4R agonist (MTII) or saline. Rats had been previously adrenalectomized (ADX) and supplemented daily with 1mg/kg corticosterone (s.c.), thyroidectomized (TDX) and supplemented daily with 10 µg/kgL-thyroxin (s.c.), or sham operated (SO). Blockade of MC3/4R signaling with SHU9119 increased food intake and body mass, irrespective of gland surgery. The increase in body mass was accompanied by higher epididymal white adipose tissue (eWAT) weight and higher mRNA content of lipogenic enzymes in eWAT. SHU9119 infusion increased triglyceride content in the liver of SO and TDX rats, but not in those of ADX rats. Concomitantly, mRNA expression of lipogenic enzymes in liver was increased in SO and TDX, but not in ADX rats. We conclude that the HPA and HPT axes do not play an essential role in mediating central melanocortinergic effects on white adipose tissue and liver lipid metabolism. However, while basal hepatic lipid metabolism does not depend on a functional HPA axis, the induction of hepatic lipogenesis due to central melanocortin system blockade does require a functional HPA axis.

Monocarboxylate transporter 8 deficiency: altered thyroid morphology and persistent high triiodothyronine/thyroxine ratio after thyroidectomy.

CONTEXT: Thyroid hormone transport across the plasma membrane depends on transmembrane transport proteins, including monocarboxylate transporter 8 (MCT8). Mutations in MCT8 (or SLC16A2) lead to a severe form of X-linked psychomotor retardation, which is characterised by elevated plasma triiodothyronine (T(3)) and low/normal thyroxine (T(4)). MCT8 contributes to hormone release from the thyroid gland. OBJECTIVE: To characterise the potential impact of MCT8-deficiency on thyroid morphology in a patient and in Mct8-deficient mice. DESIGN: Thyroid morphology in a patient carrying the A224V mutation was followed by ultrasound imaging for over 10 years. After thyroidectomy, a histopathological analysis was carried out. The findings were compared with histological analyses of mouse thyroids from the Mct8(-/y) model. RESULTS: We show that an inactivating mutation in MCT8 leads to a unique, progressive thyroid follicular pathology in a patient. After thyroidectomy, histological analysis revealed gross morphological changes, including several hyperplastic nodules, microfollicular areas with stromal fibrosis and a small focus of microfollicular structures with nuclear features reminiscent of papillary thyroid carcinoma (PTC). These findings are supported by an Mct8-null mouse model in which we found massive papillary hyperplasia in 6- to 12-month-old mice and nuclear features consistent with PTC in almost 2-year-old animals. After complete thyroidectomy and substitution with levothyroxine (l-T(4)), the preoperative, inadequately low T(4) and free T(4) remained, while increasing the l-T(4) dosage led to T(3) serum concentrations above the normal range. CONCLUSIONS: Our results implicate peripheral deiodination in the peculiar hormonal constellation of MCT8-deficient patients. Other MCT8-deficient patients should be closely monitored for potential thyroid abnormalities.

Thyroid hormone transporters in the brain.

Thyroid hormones are essential for brain development. The active thyroid hormone, T3, binds to several products of two genes, the nuclear thyroid hormone receptors alpha and beta, and thus regulates gene expression. Mutations in a thyroid hormone transmembrane transport protein, monocarboxylate transporter 8 (MCT8), underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome. This discovery sparked great interest in the process of thyroid hormone transmembrane transport. Iodothyronines are charged amino acid derivatives and require protein facilitators to cross cellular membranes. Thyroid hormones are translocated across lipid bilayers by several members of the major facilitator superfamily, including monocarboxylate transporters, amino acid transporters, and organic anion transporting polypeptides. Although until recently few researchers considered thyroid hormone transporters an important object of study, there is now a large number of candidate transporters to be reckoned with in the brain. Moreover, to finally cross the neuronal plasma membrane, any iodothyronine molecule on its way toward a neuronal nucleus has to cross consecutively the lumenal and ablumenal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane. Moreover, microglia, oligodendrocytes, and precursor and stem cells are thyroid hormone responsive and likely express thyroid hormone transporters. Hence, the many roles played by thyroid hormones in the development, function, and regeneration of the nervous system are dependent on the spatiotemporal expression of several transmembrane transport proteins.