The interest in brown adipose tissue (BAT) as a target to combat metabolic disease has recently been renewed with the discovery of functional BAT in humans. In rodents, BAT can be activated by bile acids, which activate type 2 iodothyronine deiodinase (D2) in BAT via the G-coupled protein receptor TGR5, resulting in increased oxygen consumption and energy expenditure. Here we examined the effects of oral supplementation of the bile acid chenodeoxycholic acid (CDCA) on human BAT activity. Treatment of 12 healthy female subjects with CDCA for 2 days resulted in increased BAT activity. Whole-body energy expenditure was also increased upon CDCA treatment. In vitro treatment of primary human brown adipocytes derived with CDCA or specific TGR5 agonists increased mitochondrial uncoupling and D2 expression, an effect that was absent in human primary white adipocytes. These findings identify bile acids as a target to activate BAT in humans.

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The endosomal compartment of hepatic parenchymal cells contains an acidic endopeptidase, endosomal acidic insulinase, which hydrolyzes internalized insulin and generates the major primary end product A 1-21 -B 1-24 insulin resulting from a major cleavage at residues Phe B24 -Phe B25 . This study addresses the nature of the relevant endopeptidase activity in rat liver that is responsible for most receptor-mediated insulin degradation in vivo. The endosomal activity was shown to be aspartic acid protease cathepsin D (CD), based on biochemical similarities to purified CD in 1) the rate and site of substrate cleavage, 2) pH optimum, 3) sensitivity to pepstatin A, and 4) binding to pepstatin A-agarose. The identity of the protease was immunologically confirmed by removal of greater than 90% of the insulindegrading activity associated with an endosomal lysate using polyclonal antibodies to CD. Moreover, the elution profile of the endosomal acidic insulinase activity on a gel-filtration TSK-GEL G3000 SW XL high performance liquid chromatography column corresponded exactly with the elution profile of the immunoreactive 45-kDa mature form of endosomal CD. Using nondenaturating immunoprecipitation and immunoblotting procedures, other endosomal aspartic acid proteases such as cathepsin E and ␤-site amyloid precursor protein-cleaving enzyme (BACE) were ruled out as candidate enzymes for the endosomal degradation of internalized insulin. Immunofluorescence studies showed a largely vesicular staining pattern for internalized insulin in rat hepatocytes that colocalized partially with CD. In vivo pepstatin A treatment was without any observable effect on the insulin receptor content of endosomes but augmented the phosphotyrosine content of the endosomal insulin receptor after insulin injection. These results suggest that CD is the endosomal acidic insulinase activity which catalyzes the rate-limiting step of the in vivo cleavage at the Phe B24 -Phe B25 bond, generating the inactive A 1-21 -B 1-24 insulin intermediate.Proteins entering the endocytic pathway encounter an increasingly hydrolytic environment imposed by a progressive decrease in pH and an increase in protease concentrations (reviewed in Ref. 1). Ultimately, most are degraded in lysosomes to small peptides and free amino acids. For some, degradation takes place early in the endocytic pathway. This is the case for polypeptide hormones such as insulin (2-4), glucagon (5), and parathyroid hormone (6) and growth factors such as the epidermal growth factor (7) and insulin-like growth factor-I (IGF-I) 1 (8) as well as endocytosed protein antigens for major histocompatibility class II presentation (9) and plant toxins (10).In liver parenchyma the endosomal degradation of internalized insulin is thought to occur after acidification of the endosomal lumen by a soluble enzyme termed endosomal acidic insulinase (EAI) (2, 3, 11), although this protease has yet to be identified. EAI, which was easily extracted by hypotonic shock from hepatic endosomes, displayed an acidic pH opti...

In obese patients, BA alterations are related to the metabolic phenotype associated with NASH, especially IR, but not liver necroinflammation.

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