Effect of Dietary Pretreatment and Obesity on (-)- Epigallocatechin-3-gallate (egcg) Mediated Hepatotoxicity and the Underlying Mechanism
Open Access
- Author:
- James, Karma Delores
- Graduate Program:
- Food Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 22, 2016
- Committee Members:
- Joshua D Lambert, Dissertation Advisor/Co-Advisor
Ryan John Elias, Committee Member
Gary H Perdew, Committee Member
Jairam Kp Vanamala, Committee Member - Keywords:
- EGCG
hepatotoxicity
bioavailability
mitochondria
antioxidant - Abstract:
- A growing body of evidence suggests the hepatotoxic potential of high doses of EGCG. In vitro several mechanisms of EGCG cytotoxicity have been examined; however potential mechanisms of in vivo toxicity remain understudied. Additionally factors that can potentiate EGCG induced hepatotoxicity, such as pre-existing physiological conditions that attenuate liver function have not been investigated. With the lack of data to support an association between dietary supplements and liver toxicity, I hypothesized that dietary pretreatment with EGCG markedly decreases EGCG bioavailability and can potentially mitigate the toxic potential of EGCG, and that ORFLD increases susceptibility to high dose EGCG-mediated hepatotoxicity. In addition to the effect of these factors on high dose EGCG hepatotoxicity, I further hypothesized that the underlying mechanism by which EGCG mediates hepatotoxicity is through induction of mitochondrial oxidative stress and increased mitochondrial dysfunction. This study investigated the effect of dietary pretreatment with EGCG in male CF-1 mice. EGCG pretreated (EP) mice were pretreated with 3.2mg/kg EGCG in their diet for 2 weeks, a dietary dose that is equivalent to 500 mg/kg body weight. Non-pretreated (NP) and negative control (NC) mice were given a basal diet. NP mice showed increased levels of ALT and phosphorylated histone 2Ax compared to control mice. Pretreatment with EGCG reduced these elevated levels. NP mice showed reduced levels of glutathione, which was partially ameliorated in EP mice. In addition to blunting the markers of oxidative stress dietary pretreatment enhanced hepatic level of antioxidant genes and reduced plasma and tissue concentrations of unconjugated EGCG indicating that EGCG impacted its own bioavailability. In summary, these results show that dietary pretreatment with EGCG reduced the bioavailability and hepatotoxic potential of subsequent acute high dose oral bolus EGCG. In another study we examined the effect of pre-existing obesity and ORFLD on EGCG-mediated hepatotoxicity in mice. EGCG treatment decreased survival rates of both lean and obese mice, but the effects were more pronounced in obese mice. EGCG caused increased liver injury as seen in elevated levels of liver ALT in all treatment groups. EGCG treatment increased DNA oxidative damage in both lean and obese mice. Markers of liver tissue damage assessed using histopathology showed that EGCG caused a dose dependent increase in tissue apoptosis, necrosis, hemorrhage, inflammation and hepatic lipidosis in both lean and obese mice. Lipid peroxidation increased following EGCG treatment, and depleted glutathione levels were observed. Obese mice exhibited significantly lower levels of glutathione compared to lean mice indicating elevated levels of oxidative stress caused by EGCG treatment. This study showed that EGCG treatment caused hepatotoxicity in lean and obese mice in a dose dependent manner, with the effects being more pronounced in obese mice. Our third study examined the effects of EGCG on hepatic markers of antioxidant response and mitochondrial biogenesis/function in lean and obese mice. EGCG significantly reduced hepatic antioxidant capacity. Lean and obese mice showed reduce gene and protein expression, and activity of antioxidant enzymes. EGCG treatment significantly reduced mitochondrial superoxide dismutase (Sod2) mRNA levels in lean mice; as well as catalase mRNA levels in both lean and obese mice. Obese mice showed lower catalase activity and protein expression following EGCG treatment. A significant decrease in glutathione peroxidase 1 (Gpx1) protein expression was also seen. mRNA expression of glutathione transferase zeta 1 (Gstz1) and peroxiredoxin were significantly decreased by EGCG. In response to oxidative stress induced by EGCG, increases in gene expression of metallothionein transcription factor 1 (MTF-1) and metallothionein I and II were observed. Reduced antioxidant activity was a result of the impairment of antioxidant regulators by EGCG treatment. EGCG reduce mRNA levels of Sirtuin 3 (Sirt3), forkhead box O3 (Foxo3a) and nuclear factor (erythroid-derived 2)-like 2 (Nrf 2). EGCG treatment reduced mitochondrial biogenesis by decreasing mRNA levels of transcription factor nuclear respiratory factor 1 (Nrf1), co-activator peroxisome proliferator-activator receptor coactivator-1a (Pgc-1α) and mtDNA copy number in both lean and obese mice. EGCG impaired mitochondrial function by reducing gene expression of mitochondrial transcription factors A, (Tfam), B1 (Tfb1m) and B2 (Tfb2m), as well as subunits of mitochondrial Complex I and mitochondrial complex III. Overall, based on our results and observations, EGCG treatment induced hepatotoxicity in lean and obese mice in a dose dependent manner and it induced oxidative stress by inhibiting antioxidant response. Mitochondrial function was impaired based on reduced biogenesis and inhibition of complexes following EGCG treatment. However, EGCG was shown to mitigate its toxic effects by reducing its own bioavailability following chronic dietary treatment.