An extract from the Atlantic brown algae Saccorhiza polyschides counteracts diet-induced obesity in mice via a gut related multi-factorial mechanisms

Abstract

In this study we addressed the questions whether an Atlantic brown algae extract (BAE) affects diet induced obesity in mice and which would be the primary targets and underlying key mechanisms. Male C57 BL/6 mice were fed a hypercaloric diet, referred to as high fat diet (HFD), supplemented with a freeze-dried aqueous BAE from Saccorhiza polyschides (5 %) for 8 months. Compared to the control group, dietary BAE supplementation significantly attenuated increase in body weight and fat mass. We observed apparent metabolic improvement including normalization of blood glucose, reduced plasma leptin, reduced fecal bile salt hydrolase activity with lower microbial production of toxic bile acid metabolites in the gut and increased systemic bile acid circulation in BAE-fed mice counteracting adverse effects of long term HFD feeding. Survival of mice receiving dietary BAE supplementation appeared slightly enhanced; however, median and maximal life spans as well as hepatic mTOR activation were not significantly different between BAE and control mice. We suggest that the beneficial metabolic effects of our BAE are at least partly mediated by alterations in gut microbiota associated with fermentation of indigestible polysaccharides that are major components of brown algae such as alginates and fucoidans. We moreover propose a multi-factorial mechanism that involves profound alterations in bile acid homeostasis, changes in intestinal and systemic glucose metabolism likely including increased intestinal gluconeogenesis, increased activity of the intestinally derived hormone GLP-1 contributing to promote systemic insulin sensitivity, and inhibition of α-amylase activity, which expectably limits dietary carbohydrate digestion and glucose release.

Keywords: Gerotarget; bile salt hydrolase activity; circulating bile acids; intestinal gluconeogenesis; life span; mTOR activation.

Figure 1. The brown algae extract (BAE) reduces body weight gain

A. and fat mass B. and improves the metabolic phenotype C.-F. in mice fed a high fat diet. Abdominal adiposity (C) is illustrated in two images from the micro-computed tomography of mice representative for the differences between groups. Subcutaneous fat is illustrated in light grey and visceral fat as dark grey area. Fasted plasma leptin (D) and glucose (E) concentration were significantly lower, while insulin and the HOMA index showed a downward trend in BAE mice. Data are means ± SEM (n = 5 - 8). Statistically significant differences (p < 0.05) are indicated as *.

Figure 2. Dietary supplementation of a high fat diet with a brown algae extract (BAE) alters microbial metabolism of bile acids and elevates the concentration of bile acids in systemic circulation of mice

Fecal bile salt hydrolase (BSH) activity A. as well as concentration of fecal secondary bile acids were significantly reduced in BAE mice, while primary bile acid excretion in feces was similar compared to control mice B. The concentration of total bile acids in plasma C. and urine D. was significantly higher in BAE than control mice, but hepatic bile acids were not different E. Urinary bile acid concentration was related to creatinine (Cr) concentration. In plasma, the ratio of conjugated to unconjugated bile acids was significantly modified with a relative decrease of unconjugated bile acids and an increase of glycine and taurine conjugation in the BAE group F., G. Furthermore, the ratio of cholic acid (CA) to muricholic (MCA) and chenodeoxycholic acid (CDCA) tended to be higher, whereas the secondary-to-primary bile acid-ratio was not different in BAE mice G. Data are means ± SEM (n = 6 - 7). Statistically significant differences (p < 0.05) are indicated as *.

Figure 3. Substrate oxidation, intestinal glucose metabolism and genes involved in glucose and bile acid metabolism are modulated in mice fed a high fat diet supplemented with a brown algae extract (BAE)

The respiratory exchange ratio (RER) representing the substrate used for oxidation is significantly higher in BAE mice indicative of increased glucose oxidation especially during the light phase A. The mRNA levels of G6pc encoding the catalytic unit of the glucose-6-phosphatase that mainly drives gluconeogenesis is higher in the intestine, but not affected in the liver and kidney of 12 h fasted BAE mice B. Furthermore, the BAE dose-dependently inhibits the in vitro activity of α-amylase suggesting an inhibition of carbohydrate digestion and glucose release C. The intestinal mRNA levels of Gcg and Pcsk1 were significantly higher in 4 h fasted BAE mice D., while the expression of Fxr was time dependently modulated E. Relative mRNA levels were determined by relating the expression of target to housekeeping genes. The mean of the control group was set to be 1. Data are means + SEM (n = 7 - 8, except for C, where n = 2 - 3). Statistically significant differences (p < 0.05) comparing control with BAE mice (except for C, when BAE was compared to water) are indicated as *.

Figure 4. Brown algae extract (BAE) supplementation showed no significant effect on life span and activation of the key ageing pathway mTOR, but tended to increase AMPK signaling in the liver of mice fed a high fat diet

Survival of 18 months old mice placed on the high fat diet with half of them receiving dietary BAE supplementation was assessed. The median and maximal life spans were not significantly different between BAE and control mice A. and likewise no activation of mTOR (p-mTOR/mTOR) was evident B. However, AMPK tended to be activated in BAE mice determined as the ratio of phosphorylated AMPK (p-AMPK) to total AMPK. Cropped Western blot images visualizing the respective target bands show representative animals of the groups. Total protein load per lane was used for target protein band normalization. The relative protein band intensity was densitometrically analyzed for of all animals of the group, individual ratios calculated and the means + SEM (n = 6 - 7) per group are given in C.