Exposure to the common food additive carrageenan leads to glucose intolerance, insulin resistance and inhibition of insulin signalling in HepG2 cells and C57BL/6J mice

Abstract

Aims/hypothesis The aim of this study was to determine the impact of the common food additive carrageenan (E407) on glucose tolerance, insulin sensitivity and insulin signalling in a mouse model and human hepatic cells, since carrageenan is known to cause inflammation through interaction with toll-like receptor (TLR)4, which is associated with inflammation in diabetes. Methods Male C57BL/6J mice were given carrageenan (10 mg/l) in their drinking water, and underwent a glucose tolerance test (GTT), an insulin tolerance test (ITT) and an ante-mortem intraperitoneal insulin injection. HepG2 cells were exposed to carrageenan (1 mg/l×24 h) and insulin. Levels of phospho(Ser473)-protein kinase B (Akt), phospho (Ser307)-IRS1, phosphoinositide 3-kinase (PI3K) activity and phospho(Ser32)-inhibitor of κB (IκBα) were determined by western blotting and ELISA. Results Glucose tolerance was significantly impaired in carrageenan-treated 12-week-old mice compared with untreated controls at all time points (n=12; p<0.0001). Baseline insulin and insulin levels at 30 min after taking glucose during the GTT were significantly higher following carrageenan treatment. During the ITT, glucose levels declined by more than 80% in controls, but not in carrageenan-treated mice. Carrageenan exposure completely inhibited insulininduced increases in phospho-(Ser473)-Akt and PI3K activity in vivo in mouse liver and in human HepG2 cells. Carrageenan increased phospho(Ser307)-IRS1 levels, and this was blocked when carrageenan-induced inflammation was inhibited. Conclusion This is the first report of the impact of carrageenan on glucose tolerance and indicates that carrageenan impairs glucose tolerance, increases insulin resistance and inhibits insulin signalling in vivo in mouse liver and human HepG2 cells. These effects may result from carrageenan-induced inflammation. The results demonstrate extra-colonic manifestations of ingested carrageenan and suggest that carrageenan in the human diet may contribute to the development of diabetes.

Carrageenan Inhibits Insulin Signaling through GRB10-mediated Decrease in Tyr(P)-IRS1 and through Inflammation-induced Increase in Ser(P)307-IRS1

“…Increase of phosphoinhibitor of nuclear factor B (IB) kinase ␤ caused by stimulation of the inflammatory cascades leads to increased phosphorylation of Ser 307 (in rodent; Ser 312 in human)-insulin receptor substrate 1 (IRS1), a negative mediator of downstream insulin signaling (1, 6 -8). Inhibition of the carrageenan-induced inflammation, by the combination of reactive oxygen species inhibitor Tempol and TLR4 or BCL10 siRNA, reversed the effects of carrageenan on Ser(P) 307 -IRS1, as previously reported (1). Subsequent experiments, presented in this report, have shown that the carrageenan-induced decline in the Ser(P) 473 -AKT response to insulin was not completely reversed by inhibition of the carrageenaninduced inflammatory cascades.…”

“…Carrageenan is widely used as a food additive in processed foods to thicken and solubilize ingredients and improve texture. The amount of carrageenan consumed in the typical American diet is greater than the exposure in the experiments reported previously and in the experiments in this report (1). The daily consumption of carrageenan in the experimental mice (50 g/day/25 g mouse ϭ 2 g/g ϭ 2 mg/kg) was less than the amount anticipated to be ingested daily by adults consuming the typical Western diet (ϳ250 mg/day/60 kg person ϭ ϳ4.2 mg/kg).…”

“…Some cell preparations were treated with Tempol (1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine; Axxora Life Sciences, San Diego, CA), a superoxide dismutase mimetic, which acts as a free radical scavenger and inhibits reactive oxygen species. HepG2 cells were treated with Tempol 100 nmol/liter in combination with -carrageenan (1 mg/liter) for 24 h, as previously described (1).…”

“…In this report, we present this second mechanism by which carrageenan inhibits insulin signaling in human hepatic cells (HepG2) and C57BL/6J mouse liver. The findings indicate how the carrageenan-induced decline in the insulin-initiated increase in tyrosine phosphorylation of IRS1 and the increase in serine 307 phosphorylation of IRS1 combine 1 To whom correspondence should be addressed: Dept. of Medicine, University of Illinois at Chicago, CSN 440, M/C 718, Chicago, IL 60612.…”

“…Exposure for a short duration to a low concentration of the common food additive carrageenan impaired insulin signaling in HepG2 cells and in C57BL/6J mice and produced insulin resistance and glucose intolerance in the mice (1). The mechanism of the insulin resistance was attributed to carrageenaninduced inflammation, which proceeds by activation of both innate immune-and reactive oxygen species-mediated pathways of inflammation (2)(3)(4)(5).…”