Improved Insulin Resistance and Lipid Metabolism by Cinnamon Extract through Activation of Peroxisome Proliferator-Activated Receptors

Abstract

Peroxisome proliferator-activated receptors (PPARs) are transcriptional factors involved in the regulation of insulin resistance and adipogenesis. Cinnamon, a widely used spice in food preparation and traditional antidiabetic remedy, is found to activate PPARgamma and alpha, resulting in improved insulin resistance, reduced fasted glucose, FFA, LDL-c, and AST levels in high-caloric diet-induced obesity (DIO) and db/db mice in its water extract form. In vitro studies demonstrate that cinnamon increases the expression of peroxisome proliferator-activated receptors gamma and alpha (PPARgamma/alpha) and their target genes such as LPL, CD36, GLUT4, and ACO in 3T3-L1 adipocyte. The transactivities of both full length and ligand-binding domain (LBD) of PPARgamma and PPARalpha are activated by cinnamon as evidenced by reporter gene assays. These data suggest that cinnamon in its water extract form can act as a dual activator of PPARgamma and alpha, and may be an alternative to PPARgamma activator in managing obesity-related diabetes and hyperlipidemia.

Figure 1

CE reduced the fasted blood glucose level and improved glucose tolerance in DIO and db/db mice. Mice were administered vehicle alone, or CE (equivalent to 400 mg cinnamon powder kg • day •) for 3 weeks (DIO) or 2 weeks (db/db). The blood glucose levels were measured in tail vein and the basal glucose levels were shown at 0 minute. (a) DIO C57BL/6J mice were fasted overnight and glucose levels were measured at 9:00 am at the end treatment. NF: normal food fed mice; HF: high-fat food-induced mice; CE: cinnamon water extract; (b) fasted DIO mice were intraperitoneally injected with 2 g glucose • kg⁻ body weight and glucose tolerance determined at the time indicated; (c) db/db Mice were fasted for 6 hours and the fasted blood glucose was measured. CONT: vehicle; (d) glucose tolerance was determined following 2 g glucose • kg⁻¹ body weight intraperitoneal injection in db/db mice. The data are presented as mean ± SE, n = 5 for each group. (b) *P < .05 versus HF group or (d) vehicle control (cont).

Figure 2

CE promoted 3T3-L1 adipocyte differentiation. 3T3-L1 cells were stained with oil red O at day 5. (a) Undifferentiated control cells; (b) DM (insulin 10 μg/mL, dexam: 1 μM, IBMX: 0.5 mM)-induced differentiated cells; (c) DM-induced differentiated cells + CE 0.2 mg/mL; (d) DM-induced differentiated cells + CE 0.6 mg/mL; (e) HE staining of WAT from high-fat diet control (HFC) mice; (f) HE staining of WAT from HF + CE-treated mice.

Figure 3

CE increased expression of PPARs and their target genes in differentiated 3T3-L1 cells. 3T3-L1 cells were differentiated in 24-well plate and CE 0.6 mg/mL was added at the same time. On day 5, cells were collected and total RNA was extracted and reversely transcribed into the first strand cDNA with random hexamer primers using cDNA synthesis kit. The gene expression levels were analyzed by quantitative real-time RT-PCR. (a) PPARγ and its target genes; (b) PPARα and its target gene; (c) Western blot of CE-treated 3T3-L1 differentiated cells from day 5. 1: Control; 2: DM; 3: Rosiglitazone 1 μM; 4: DM + CE 0.2 mg/mL; 5: DM + CE 0.6 mg/mL. For real-time PCR, the results were repeated in at least 3 independent experiments, and β-actin mRNA was used as an internal control. Data are presented as mean ± SE. *P < .001.

Figure 4

CE activated transactivities of PPARγ and PPARα. Full-length PPARγ or PPARα was cotransfected with PPRE-J3-TK-Luc reporter construct to 293T cell and treated with CE (0.2 mg/mL, 0.6 mg/mL), 1 μM of troglitazone or WY-14643 for 24 hours as positive controls. Rellina luc was used as a transfection efficiency control and the relative luciferase activities were measured against renilla luciferase activities. For LBD activity assay, pMCX-GAL4-LBD of PPARγ or α expression constructs were cotransfected with USA_G × 4-TK-Luc into 293T using the same protocol as described above. The empty vectors were used as control. (a) Full-length PPARγ. (b) Full-length PPARα. (c) LBD PPARγ. (d) LBD PPARα. The results represent three independent experiments. Data are presented as mean ± SE. *P < .05, **P < .01. Tro: troglitazone; WY: WY14643.

Figure 5

CE promoted gene expression of PPARγ, PPARα and their target genes in WAT and liver from DIO mice: (a) real-time PCR of PPARγ and its target gene expression in white fat tissue; (b) real-time PCR of PPARα and its target gene expression in liver. HF: high-fat diet; n = 5; data are presented as mean ± SE. *P < .05 compared to control.