Dual mechanisms of green tea extract (EGCG)-induced cell survival in human epidermal keratinocytes

Antioxidants regulate normal human keratinocyte differentiation

Cancer begins with a normal cell that, due to persistent environmental insult, is transformed, via a series of progressively more insidious steps, into a cancer cell. A major goal of chemopreventive therapy is to alter the normal cell response to the environmental agent with the goal of inhibiting disease progression. (-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive green tea antioxidant that possesses remarkable cancer chemopreventive properties. We have recently explored the hypothesis that EGCG prevents cancer by promoting keratinocyte differentiation. Based on our findings, we argue that EGCG acts to enhance the differentiation of normal keratinocytes. This is a potentially important finding, as it represents a novel mechanism of disease inhibition by EGCG--cancer preventive "differentiation therapy". However, not all antioxidant chemopreventive agents work by this mechanism. Curcumin, for example, inhibits the differentiation-promoting activity of EGCG. This report discusses the mechanism of EGCG and curcumin action in regulating expression of involucrin, a marker of keratinocyte differentiation.

Neurological mechanisms of green tea polyphenols in Alzheimer's and Parkinson's diseases

Tea consumption is varying its status from a mere ancient beverage and a lifestyle habit, to a nutrient endowed with possible prospective neurobiological-pharmacological actions beneficial to human health. Accumulating evidence suggest that oxidative stress resulting in reactive oxygen species generation and inflammation play a pivotal role in neurodegenerative diseases, supporting the implementation of radical scavengers, transition metal (e.g., iron and copper) chelators, and nonvitamin natural antioxidant polyphenols in the clinic. These observations are in line with the current view that polyphenolic dietary supplementation may have an impact on cognitive deficits in individuals of advanced age. As a consequence, green tea polyphenols are now being considered as therapeutic agents in well controlled epidemiological studies, aimed to alter brain aging processes and to serve as possible neuroprotective agents in progressive neurodegenerative disorders such as Parkinson's and Alzheimer's diseases. In particular, literature on the putative novel neuroprotective mechanism of the major green tea polyphenol, (-)-epigallocatechin-3-gallate, are examined and discussed in this review.

Catechin polyphenols: neurodegeneration and neuroprotection in neurodegenerative diseases

Neurodegeneration in Parkinson's, Alzheimer's, and other neurodegenerative diseases seems to be multifactorial, in that a complex set of toxic reactions including inflammation, glutamatergic neurotoxicity, increases in iron and nitric oxide, depletion of endogenous antioxidants, reduced expression of trophic factors, dysfunction of the ubiquitin-proteasome system, and expression of proapoptotic proteins leads to the demise of neurons. Thus, the fundamental objective in neurodegeneration and neuroprotection research is to determine which of these factors constitutes the primary event, the sequence in which these events occur, and whether they act in concurrence in the pathogenic process. This has led to the current notion that drugs directed against a single target will be ineffective and rather a single drug or cocktail of drugs with pluripharmacological properties may be more suitable. Green tea catechin polyphenols, formerly thought to be simple radical scavengers, are now considered to invoke a spectrum of cellular mechanisms of action related to their neuroprotective activity. These include pharmacological activities like iron chelation, scavenging of radicals, activation of survival genes and cell signaling pathways, and regulation of mitochondrial function and possibly of the ubiquitin-proteasome system. As a consequence these compounds are receiving significant attention as therapeutic cytoprotective agents for the treatment of neurodegenerative and other diseases.

Essential role of caspases in epigallocatechin-3-gallate-mediated inhibition of nuclear factor kappa B and induction of apoptosis

Green tea constituent (-) epigallocatechin-3-gallate (EGCG) has shown remarkable cancer-preventive and some cancer-therapeutic effects. This is partially because of its ability to induce apoptosis in cancer cells without affecting normal cells. Previous studies from our laboratory have shown the involvement of NF-kappa B pathway in EGCG-mediated cell-cycle deregulation and apoptosis of human epidermoid carcinoma A431 cells. Here we show the essential role of caspases in EGCG-mediated inhibition of NF-kappa B and its subsequent apoptosis. Treatment of A431 cells with EGCG (10-40 microg/ml) resulted in dose-dependent inhibition of NF-kappa B/p65, induction of DNA breaks, cleavage of poly(ADP-ribose) polymerase (PARP) and morphological changes consistent with apoptosis. EGCG treatment of cells also resulted in significant activation of caspases, as shown by the dose- and time-dependent increase in DEVDase activity, and protein expression of caspase-3, -8 and -9. EGCG-mediated caspase activation induces proteolytic cleavage of NF-kappa B/p65 subunit, leading to the loss of transactivation domains, and driving the cells towards apoptosis. EGCG-mediated induction of apoptosis was significantly blocked by the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone (Z-VAD-FMK), and moderately blocked by the specific caspase-3 inhibitor Z-DEVD-FMK. Further, pretreatment of cells with Z-VAD-FMK was found to suppress the cleavage of NF-kappa B/p65 subunit, thereby increasing nuclear translocation, DNA binding and transcriptional activity, thus protecting the cells from EGCG-induced apoptosis. Taken together, these studies for the first time demonstrate that EGCG-mediated activation of caspases is critical, at least in part, for inhibition of NF-kappa B and subsequent apoptosis.

Cell signaling pathways in the neuroprotective actions of the green tea polyphenol (-)-epigallocatechin-3-gallate: implications for neurodegenerative diseases

Accumulating evidence supports the hypothesis that brain iron misregulation and oxidative stress (OS), resulting in reactive oxygen species (ROS) generation from H2O2 and inflammatory processes, trigger a cascade of events leading to apoptotic/necrotic cell death in neurodegenerative disorders, such as Parkinson's (PD), Alzheimer's (AD) and Huntington's diseases, and amyotrophic lateral sclerosis (ALS). Thus, novel therapeutic approaches aimed at neutralization of OS-induced neurotoxicity, support the application of ROS scavengers, transition metals (e.g. iron and copper) chelators and non-vitamin natural antioxidant polyphenols, in monotherapy, or as part of antioxidant cocktail formulation for these diseases. Both experimental and epidemiological evidence demonstrate that flavonoid polyphenols, particularly from green tea and blueberries, improve age-related cognitive decline and are neuroprotective in models of PD, AD and cerebral ischemia/reperfusion injuries. However, recent studies indicate that the radical scavenger property of green tea polyphenols is unlikely to be the sole explanation for their neuroprotective capacity and in fact, a wide spectrum of cellular signaling events may well account for their biological actions. In this article, the currently established mechanisms involved in the beneficial health action and emerging studies concerning the putative novel molecular neuroprotective activity of green tea and its major polyphenol (-)-epigallocatechin-3-gallate (EGCG), will be reviewed and discussed.

Polyphenol (−)-epigallocatechin gallate inhibits apoptosis induced by irradiation in human HaCaT keratinocytes

Green tea is a rich source of polyphenols, and (-)-epigallocatechin-3-gallate (EGCG) is a major constituent of green tea polyphenols. In the present study, we investigated the effect of EGCG on apoptosis induced by irradiation in the human keratinocytic cell line HaCaT. Irradiation by gamma-ray induced apoptosis with concomitant cleavage of caspase-3 and its in vivo substrate poly(ADP-ribose) polymerase. Treatment of cells with EGCG inhibited irradiation-induced apoptosis as detected by Hoechst staining and internucleosomal cleavage of DNA, and prevented the cleavage of these proteins by irradiation. We also found that the treatment of cells with EGCG alone suppressed cell growth and induced apoptosis in these cells. Our results suggest that EGCG inhibits irradiation-induced apoptosis by inactivating the caspase pathway in HaCaT cells. Our study also indicates that EGCG has a dual effect on the survival of these keratinocytes.