Curcumin prevents high fat diet induced insulin resistance and obesity via attenuating lipogenesis in liver and inflammatory pathway in adipocytes

Abstract

Background: Mechanisms underlying the attenuation of body weight gain and insulin resistance in response to high fat diet (HFD) by the curry compound curcumin need to be further explored. Although the attenuation of the inflammatory pathway is an accepted mechanism, a recent study suggested that curcumin stimulates Wnt signaling pathway and hence suppresses adipogenic differentiation. This is in contrast with the known repressive effect of curcumin on Wnt signaling in other cell lineages.

Methodology and principal findings: We conducted the examination on low fat diet, or HFD fed C57BL/6J mice with or without curcumin intervention for 28 weeks. Curcumin significantly attenuated the effect of HFD on glucose disposal, body weight/fat gain, as well as the development of insulin resistance. No stimulatory effect on Wnt activation was observed in the mature fat tissue. In addition, curcumin did not stimulate Wnt signaling in vitro in primary rat adipocytes. Furthermore, curcumin inhibited lipogenic gene expression in the liver and blocked the effects of HFD on macrophage infiltration and the inflammatory pathway in the adipose tissue.

Conclusions and significance: We conclude that the beneficial effect of curcumin during HFD consumption is mediated by attenuating lipogenic gene expression in the liver and the inflammatory response in the adipose tissue, in the absence of stimulation of Wnt signaling in mature adipocytes.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Long term dietary curcumin supplementation prevents HFD-induced obesity and fat mass.

(A) Comparison of the body weight change of mice fed with LFD, HFD or HFD plus curcumin for 28 weeks (n = 4 for LFD or HFD, and n = 8 for HFD/curcumin). a, LFD versus HFD; b, HFD versus HFD/curcumin. *, p<0.05. (B) Weights of the epididymal fat pad (n = 4 for LFD and HFD, n = 8 for HFD/curcumin). **, p<0.01. (C) Total body fat volume assessed by MRI at 26 weeks (n = 4 for all three groups).

Figure 2. Curcumin supplementation improves glucose disposal and insulin sensitivity.

Assessment of glucose metabolism in animals with LFD, HFD or HFD plus curcumin feeding. (A) IPGTT (n = 4, 20 weeks), (B) IPPTT (n = 4, 23 weeks), and (C) IPITT (n = 4, 26 weeks). AUC, area under the curve. *, p<0.05; **, p<0.01.

Figure 3. Curcumin improves insulin stimulated PKB phosphorylation in fat tissue and hepatocytes.

(A) and (B) The three groups of mice fed with the diets as indicated for 28 weeks were fasted overnight and injected with PBS or insulin. After 30 min, the mice were sacrificed. Samples of epididymal fat pad (A) and liver (B) were prepared and immunoblotted with PKB or Ser473 phosphorylated PKB (p-PKB) antibody. (C) HepG2 cells were pre-treated with or without curcumin at indicated concentration overnight, followed by a 6 h treatment with or without glucose oxidase (GO). The cells were then further treated with or without insulin (100 nM) for 10 min, followed by PKB and p-PKB immunoblotting. The blots shown are representative of 3 separated experiments.

Figure 4. Curcumin does not stimulate the Wnt signaling pathway in mature adipocytes.

(A) Samples from epididymal fat pad of the three groups of mice were prepared for Western blotting. β-cat and GSK-3 represent Wnt pathway effectors, while cyclin D1 and c-Myc are two known Wnt target genes. (B) Rat primary adipocytes were prepared and treated with insulin (100 nM), or curcumin (20 µM), or insulin plus curcumin for 0, 5, 30, and 60 min. Samples were collected for Western blotting, with indicated antibody. (C) Rat primary adipocytes were prepared and treated with 10 or 20 µM curcumin for 4 h, followed by Western blotting with indicated antibody. All panels show the representative blot (n = 3).

Figure 5. Curcumin increases HO-1 expression and reduces inflammatory markers in mature adipocytes.

(A) Rat primary adipocytes were prepared and treated with 10 or 20 µM curcumin for 4 h. Western blotting was performed with the indicated antibodies. (B) Immunostaining was performed for the detection of macrophage infiltration marker F4/80 in the fat tissue of the three groups of mice. Arrows show the positive staining. No macrophage infiltration was observed in LFD or HFD/Curcumin animals. (C) Samples from epididymal fat pad of the three groups of mice were prepared for Western blotting. WSL, whole cell lysates; Nuclear, nuclear extracts. Panel A and C show representative blots (n = 3).

Figure 6. Histological assessment of mouse liver.

Representative histological slides show lipid content (A, H&E staining; B, Oil Red O staining) and macrophage infiltration (C) in the three groups of mice.

Figure 7. Curcumin reduces intra-hepatic lipid content and lipogenic gene expression.

(A) Curcumin supplementation reduced liver weight in HFD fed mice (n = 4 for all 3 groups). (B) MRI shows that curcumin supplementation reduced intra-hepatic lipid content (n = 4 for all 3 groups). (C) Curcumin supplementation reduced the expression of TxNIP, ChREBP and SREBP-1c in liver of HFD fed mice (A representative blot, n = 3). (D) Curcumin supplementation reduced liver ChREBP, SREBP-1c, L-PK and TxNIP mRNA expression in HFD fed mice (n = 3). (E) The ChREBP-LUC reporter plasmid construct (3 µg) were transfected into HepG2 cells for 24 h, followed by a 20 h serum starvation and a 4 h curcumin (20 µM) treatment. The data are presented as mean ± SEM (n = 3). *; p<0.05; **, p<0.01. F. Curcumin supplementation blocked the stimulatory effect of HFD on S6K1 phosphorylation in the liver.