Small heterodimer partner deletion prevents hepatic steatosis and when combined with farnesoid X receptor loss protects against type 2 diabetes in mice.

Nuclear receptors farnesoid X receptor (FXR) and small heterodimer partner (SHP) are important regulators of bile acid, lipid, and glucose homeostasis. Here, we show that global Fxr -/- Shp-/- double knockout (DKO) mice are refractory to weight gain, glucose intolerance, and hepatic steatosis when challenged with high-fat diet. DKO mice display an inherently increased capacity to burn fat and suppress de novo hepatic lipid synthesis. Moreover, DKO mice were also very active and that correlated well with the observed increase in phosphoenolpyruvate carboxykinase expression, type IA fibers, and mitochondrial function in skeletal muscle. Mechanistically, we demonstrate that liver-specific Shp deletion protects against fatty liver development by suppressing expression of peroxisome proliferator-activated receptor gamma 2 and lipid-droplet protein fat-specific protein 27 beta. CONCLUSION: These data suggest that Fxr and Shp inactivation may be beneficial to combat diet-induced obesity and uncover that hepatic SHP is necessary to promote fatty liver disease. (Hepatology 2017;66:1854-1865).

Myocardial Induction of Type 3 Deiodinase in Dilated Cardiomyopathy.

BACKGROUND: The thyroid hormone-inactivating enzyme type 3 deiodinase (D3) is induced during hypertrophic and ischemic cardiomyopathy, leading to a state of local cardiac hypothyroidism. Whether D3 induction occurs in dilated cardiomyopathy is unknown. METHODS: This study characterized changes in cardiac D3 and thyroid hormone signaling in a transgenic model of progressive dilated cardiomyopathy (TG9 mice). RESULTS: Cardiac D3 was dramatically induced 15-fold during the progression of dilated cardiomyopathy in TG9 mice. This D3 induction localized to cardiomyocytes and was associated with a decrease in myocardial thyroid hormone signaling. CONCLUSIONS: Cardiac D3 is induced in a mouse model of dilated cardiomyopathy, indicating that D3 induction may be a general response to diverse forms of cardiomyopathy.

Endoplasmic reticulum-associated degradation of the human type 2 iodothyronine deiodinase (D2) is mediated via an association between mammalian UBC7 and the carboxyl region of D2.

The type 2 iodothyronine selenodeiodinase (D2) is an endoplasmic reticulum (ER)-resident selenoprotein that activates T4 to T3, playing a critical role in thyroid homeostasis. D2 has an approximately 45-min half-life due to selective ubiquitin-mediated ER-associated degradation (ERAD), a process of particular interest because it is accelerated by exposure to D2 substrates, T4 or rT3. The present in vitro binding studies indicate that glutathione-S-transferase (GST)-human D2 fusion proteins specifically associate with a mammalian homolog of the ubiquitin conjugase UBC7 (MmUBC7), with localization to amino acids 169-234 of D2. Coexpression of D2 with an inactive D2 mutant or a truncated version containing amino acids 169-234 stabilizes D2 half-life, supporting the importance of the carboxyl region of D2 for ERAD. Mammalian UBC6 (MmUBC6) does not directly associate with D2 but can associate with a complex containing UBC7 and D2. At the same time, functional studies in human embryonic kidney-293 cells indicate that D2 activity half-life and protein levels are stabilized only when inactive mutants of both UBC6 and UBC7 are overexpressed with D2, suggesting that redundancy may exist at the level of the E2 for both basal and substrate-accelerated D2 ERAD. In conclusion, D2 ERAD in human cells proceeds via an association between UBC7 and the carboxyl region of D2, a unique mechanism for the control of thyroid hormone activation.