Specific reduction of G6PT may contribute to downregulation of hepatic 11ß-HSD1 in diabetic mice.

Pre-receptor activation of glucocorticoids via 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1 (HSD11B1)) has been identified as an important mediator of the metabolic syndrome. Hexose-6-phosphate dehydrogenase (H6PDH) mediates 11ß-HSD1 amplifying tissue glucocorticoid production by driving intracellular NADPH exposure to 11ß-HSD1 and requires glucose-6-phosphate transporter (G6PT (SLC37A4)) to maintain its activity. However, the potential effects of G6PT on tissue glucocorticoid production in type 2 diabetes and obesity have not yet been defined. Here, we evaluated the possible role of G6PT antisense oligonucleotides (G6PT ASO) in the pre-receptor metabolism of glucocorticoids as related to glucose homeostasis and insulin tolerance by examining the production of 11ß-HSD1 and H6PDH in both male db/+ and db/db mouse liver tissue. We observed that G6PT ASO treatment of db/db mice markedly reduced hepatic G6PT mRNA and protein levels and substantially diminished the activation of hepatic 11ß-HSD1 and H6PDH. Reduction of G6pt expression was correlated with the suppression of both hepatic gluconeogenic enzymes G6Pase and PEPCK and corresponded to the improvement of hyperglycemia and insulin resistance in db/db mice. Addition of G6PT ASO to mouse hepa1-6 cells led to a dose-dependent decrease in 11B-Hsd1 production. Knockdown of G6PT with RNA interference also impaired 11B-Hsd1 expression and showed comparable effects to H6pdh siRNA on silencing of H6pdh and 11B-Hsd1 expression in these intact cells. These findings suggest that G6PT plays an important role in the modulation of pre-receptor activation of glucocorticoids and provides new insights into the role of G6PT in the development of type 2 diabetes.

Additive effects of nicotine and high-fat diet on hepatic steatosis in male mice.

Smoking is a major risk factor for diabetes and cardiovascular disease and may contribute to nonalcoholic fatty liver disease. We hypothesize that in the presence of nicotine, high-fat diet (HFD) causes more severe hepatic steatosis in obese mice. Adult C57BL6 male mice were fed a normal chow diet or HFD and received twice daily injections of nicotine (0.75 mg/kg body weight, ip) or saline for 10 wk. Light microscopic image analysis revealed significantly higher lipid accumulation in livers from mice on HFD plus nicotine (190 ± 19 µm(2)), compared with mice on HFD alone (28 ± 1.2 µm(2)). A significant reduction in the percent volume of endoplasmic reticulum (67.8%) and glycogen (49.2%) was also noted in hepatocytes from mice on HFD plus nicotine, compared with mice on HFD alone. The additive effects of nicotine on the severity of HFD-induced hepatic steatosis was associated with significantly greater oxidative stress, increased hepatic triglyceride levels, higher incidence of hepatocellular apoptosis, inactivation (dephosphorylation) of AMP-activated protein kinase, and activation of its downstream target acetyl-coenzyme A-carboxylase. Treatment with acipimox, an inhibitor of lipolysis, significantly reduced nicotine plus HFD-induced hepatic lipid accumulation. We conclude that: 1) greater oxidative stress coupled with inactivation of AMP-activated protein kinase mediate the additive effects of nicotine and HFD on hepatic steatosis in obese mice and 2) increased lipolysis is an important contributor to hepatic steatosis. We surmise that nicotine exposure is likely to exacerbate the metabolic abnormalities induced by high-fat intake in obese patients.

Effect of nicotine on body composition in mice.

Nicotine induces weight loss in both humans and rodents consuming a regular diet; however, the effect of nicotine on body weight and fat composition in rodents consuming a high-fat diet (HFD) has not been well studied. Thus, this study examined the effect of nicotine vs saline on body weight and fat composition in mice fed with either an HFD (62% of kcal from fat) or a standard normal chow diet (NCD) for 7 weeks. Nicotine dose dependently reduced body weight gain in mice that consumed both diets, but this effect was significantly greater in mice on the HFD. Caloric intake was decreased in nicotine-treated mice. Estimates of energy intake suggested that decreased caloric intake accounted for all the reduced weight gain in mice on an NCD and 66% of the reduced weight gain on an HFD. Computed tomography analysis for fat distribution demonstrated that nicotine was effective in reducing abdominal fat in mice that consumed the HFD, with nicotine treatment leading to lower visceral fat. The effect of nicotine on weight loss in mice on an HFD was completely blocked by mecamylamine, a nonselective nicotinic acetylcholine receptor (nAChR) antagonist, but only partially blocked by the α4ß2 nAChR partial agonist/antagonist, varenicline. We conclude that nicotine is effective in preventing HFD-induced weight gain and abdominal fat accumulation.