The health benefits of green tea are begin ever discovered and researched. Below is a scientific (complicated and full of jargon) article about a substance in green tea, polyphenols, which may prevent skin cancer in not one, but two ways; reducing inflammation from UV exposure (lessening the damage a sunburn does to your skin) and healing the DNA which changes to create skin cancer. Now there’s a cup of tea that packs a bang for your buck!

May 2009 Journal Club Article: Green Tea Prevents Skin Cancer by Two Mechanisms

Inhibition of UVB-Induced Skin Tumor Development by Drinking Green Tea Polyphenols Is Mediated Through DNA Repair and Subsequent Inhibition of Inflammation
Syed M Meeran, Suhail Akhtar and Santosh K Katiyar
Journal of Investigative Dermatology (2009) 129: 1258-1270; doi:10.1038/jid.2008.354

Green Tea Prevents Skin Cancer by Two Mechanisms
Navid Bouzari 1, Yvonne Romagosa 1 and Robert S. Kirsner 1
Journal of Investigative Dermatology (2009) 129, 1054. doi:10.1038/jid.2009.64

Skin cancer accounts for more new cases of cancer than all other cancers combined. Exposure to UV radiation is the most important risk factor for development of skin cancer (LeBlanc et al., 2008). Primary prevention has reduced the incidence of skin cancer in the geographic regions at highest risk, such as Australia (Geller et al., 2009), but researchers are actively seeking ways to augment prevention through chemoprevention. Candidate agents include commonly used medications and foods such as nonsteroidal anti-inflammatory agents, lipid-lowering agents, angiotensin-converting enzyme and receptor blockers, and green tea (Christian et al., 2008). Polyphenols from green tea (GTPs) have been shown to reduce UV-induced skin cancer in animal models (Mantena et al., 2005). The mechanisms by which this is achieved remain unclear, but it is thought that the ability of GTPs to reduce the inflammation associated with UV exposure may be an important factor in this process.

To study the mechanisms by which GTPs affect inflammation and DNA repair and by which GTPs may prevent skin cancers induced by chronic UV exposure, Meeran and co-workers (2009) fed GTPs through drinking water to IL-12 (an important mediator of inflammation) knockout (KO) and wild-type mice. Compared with control mice, chronically UV-exposed and GTP-fed mice exhibited a delay in tumor onset, as well as smaller and fewer tumors. The benefit of GTPs was observed in IL-12 KO mice, but it was markedly less dramatic.

Mediators of inflammation, such as prostaglandin E2 and cyclooxygenase-2 (Katiyar and Meeran, 2007), as well as markers of cell proliferation, such as proliferating cell nuclear antigen and cyclin D1, were also affected by GTP administration; these effects were weaker in IL-12 KO mice. Repletion of IL-12 in KO mice and anti-IL-12 administration in wild-type mice confirmed the role of IL-12. Because GTP benefits appeared to be only partly mediated through IL-12, DNA repair of cyclopyridine dimers was studied after acute UV exposure. GTP-fed mice showed greater DNA repair than control mice, suggesting a dual mechanism by which GTP might reduce skin cancer development in animal models.

Through the following questions, we examine this paper in greater detail.

QUESTIONS
1. What are the mechanisms by which UV induces skin cancer?
2. What is the rationale for studying green tea to prevent skin cancer?
3. What were the hypotheses in this study, and how were they tested?
4. What were the findings of this study?
5. What were the limitations of this study?
6. How might the article affect your recommendations to patients?

ANSWERS
1. The human skin is a highly dynamic and protective organ. It is exposed daily to UVR from sun and occupational light sources, and sometimes to phototherapy systems. UVR is part of the spectrum of electromagnetic radiation, and it is divided into three regions: UVC, 200–290 nm; UVB, 290–320 nm; and UVA, 320–400 nm. UVC radiation is completely absorbed by the Earth’s atmosphere and is therefore irrelevant to solar-induced cutaneous carcinogenesis. UVB radiation is only partly absorbed by the atmospheric ozone layer, and UVA radiation is not absorbed at all (de Laat et al., 1996). When the photons that make up UVR enter the skin, they may be absorbed by any one of several biomolecules, leading to excited energy states. Following this, the excited molecule can re-emit some of the energy (as fluorescence), lose energy as heat, or undergo photochemical reactions that form new molecules (photoproducts).

In the skin, the main absorbing biomolecules are melanin, DNA, amino acids, carotene, and urocanic acids (Anderson and Parrish, 1981). Among these, DNA is most critical. UVR, especially that in the UVB range, leads to the formation of cyclobutane–pyrimidine dimer and pyrimidine pyridone photoproducts in DNA. Other effects of UVR on DNA are the induction of cytosin photohydrates, purine photoproducts, and single-strand breaks. Erroneous repair of these products may lead to mutations, most commonly substitutions of T for C or TT for CC. If these mutations affect the function of specific oncogenes, tumor-suppressive genes, or important housekeeping genes such that cell growth and site or residence are no longer regulated, transformation is a recognized consequence (Hussein, 2005; de Laat et al., 1996). This is referred to as mutagenesis.

In addition to direct mutagenesis, UVR can indirectly affect the frequency of mutations, usually by altering the cell cycle to allow less time for DNA repair prior to subsequent rounds of replication. In this process, alterations in tumor suppressor genes (TSGs), oncogenes, and mismatch repair proteins occurs. The tumor-suppressor genes, such as p53, p16, and PTCH, ordinarily regulate normal cellular growth and differentiation. Every living cell has two copies of these TSGs; if one is lost or damaged, the other copy can maintain normal cell function. The proto-oncogenes involved in photocarcinogenesis are bcl-2, ras, and c-fos. UVR can activate these proto-oncogenes, transforming them into oncogenes.

2. Green tea is one of the most commonly consumed beverages, accounting for almost 20% of tea consumption worldwide. It is manufactured from the fresh leaves of the plant Camellia sinensis by drying and steaming them at high temperatures. Its polyphenols have antioxidant and anti-inflammatory properties, and they have been shown to have a protective role against a variety of cancers, including skin cancer in laboratory animals. Green tea polyphenols (GTPs) have protective effects against the UV-induced sunburn response, UV-induced immunosuppression, and photoaging of skin.

Several basic and epidemiologic studies have suggested that green tea prevents cancer in many organ systems, including skin (Katiyar and Mukhtar, 1996). Wang et al. (1991) were among the first to suggest that green tea might have protective effects against UV-induced skin cancers. They showed that administration of green tea to SKH-1 hairless mice led to a dose-dependent prolongation in the mean time of tumor development when the mice were exposed to UVR. Although the mechanism of photoprotection by green tea is not known, there is a growing body of evidence concerning its protective effects at the cellular, molecular, and biochemical levels (Figure 1).

Figure 1. Protective effects of green tea at the cellular, molecular, and biochemical levels.

At the cellular level, GTPs have been shown to induce apoptosis in premalignant and squamous cell carcinoma keratinocytes but have anti-apoptotic effects in normal keratinocytes. GTPs deter the movement of macrophages and neutrophils to UV-irradiated skin and, as a result, decrease the reactive oxygen radicals. On the other hand, GTPs have been shown to increase the number of Langerhans cells. These important antigen-presenting cells are thought to have a protective role in the pathogenesis of UV-induced skin cancers. Additionally, GTPs have been shown to reduce the number of sunburn cells, increase the level of IL-12, decrease IL-10, inhibit tumor angiogenesis, decrease oxidative stress, increase DNA-repair ability, and increase apoptosis of photodamaged cells (Yusuf et al., 2007). Because GTPs have been shown to reverse the immunosuppressive effects of UV radiation, there has been considerable interest in defining the effects of green tea on UV-induced modifications in cytokine production.

At the molecular level, there is considerable evidence to indicate that GTPs protect against UV-induced oxidative injury in the skin. They not only decrease the production of free oxygen radicals but also protect against depletion of glutathione, glutathione peroxidase, and catalase, which have antioxidant activity. Several studies have demonstrated that GTPs can directly prevent UV-induced DNA damage. It is thought that this protective effect is at least partly due to GTP-induced p53 increase. Other mechanisms include inhibition of NF-κB and suppression of angiogenic factors such as vascular endothelial growth factor and matrix metalloproteinases. Because GTPs have been shown to reverse the immunosuppressive effects of UV radiation, there has been considerable interest in defining the effects of green tea on UV-induced modifications in cytokine production.

3. Based on the facts that GTPs increase IL-12 and that IL-12 plays a role in the removal or repair of UVB-induced DNA damage, Meeran et al. (2009) hypothesized that treating mice with GTPs would enhance IL-12 production and decrease UVB-induced DNA damage, resulting in reduced inflammation and tumorigenesis. If this theory is correct, GTPs should not be able to show the above effects in IL-12-deficient mice. With this in mind, the authors conducted two experiments, one using wild-type (C3H/HeN) mice and the other using IL-12-KO mice on a C3H/HeN background. Each experiment included three groups of mice: mice that were not UVB exposed (control), mice that were exposed to a photocarcinogenesis protocol, and mice that were exposed to a photocarcinogenesis protocol and provided drinking water containing GTPs. The photocarcinogenesis protocol consisted of exposure to longer-term UV light, predominantly in the UVB range. Following the prescribed protocols, the authors recorded the number of tumors, their size, and the time elapsed for tumor development. They subsequently examined the level of cytokines (TNF-α, IL-1β, and IL-6) using ELISA, COX-2 expression using western blot, and level of PGE2 using enzyme immunoassay. Then they looked at the differences in the above factors between the two sets of mice (wild-type vs. IL-12-KO), as well as the differences among the groups in each set (control vs. UVB-treated vs. GTP- and UVB-treated). Markers of keratinocyte proliferation were also examined (PCNA and cyclin D1). Additionally, using a different protocol consisting of acute UV exposure, the authors examined the skin as well as tumor cells for formation of cyclobutane pyrimidine dimers (CPDs) using immunohistochemical and southwestern dot-blot analysis.

4. The investigators demonstrated that UVB induces skin cancers in wild-type mice. In this group, treatment with GTPs not only decreased tumor incidence but also increased the latency period of the tumors by 2 weeks. IL-12-KO mice were more susceptible to UVB-induced carcinogenesis compared with their wild-type counterparts. Moreover, GTPs did not protect against UVB-induced tumorigenesis in these mice. Hence, the authors concluded that prevention of photocarcinogenesis by oral GTPs is IL-12 dependent. In the same manner, Meeran et al. observed more inflammation in IL-12-KO mice compared with wild-type mice and a reduced effect of GTPs in decreasing the inflammatory mediators (COX-2 and PGE2) in these mice compared with the wild type following UVB radiation.

When the investigators evaluated two of the markers of keratinocyte proliferation, PCNA and cyclin D1, the results were similar to the findings observed for inflammatory mediators. That is, greater keratinocyte proliferation was seen in IL-12-KO than in wild-type mice. In addition, a significant decrease in these proliferation markers was observed in wild-type mice treated with GTPs; this was not observed in GTP-treated IL-12-KO mice. The investigators found the inhibitory effect of GTPs on UVB-induced proinflammatory cytokines (TNF-α, IL-1β, and IL-6) to be higher in wild-type but not in IL-12-KO mice.

Subsequently, these researchers undertook additional experiments to determine whether the inhibitory effect of GTPs on UVB-induced inflammation and tumorigenesis was attributable to a GTP-mediated enhancement of the repair or removal of UVB-induced DNA damage. They evaluated the CPDs and inflammatory mediators (COX-2 and PGE2) at several time points (30 minutes, and 24, 48, and 72 hours) after acute UV exposure and found that GTPs significantly decreased the above factors at 48 and 72 hours in wild-type mice, but not in their IL-12-KO counterparts. Injection of recombinant IL-12 in UVB-irradiated IL-12-KO mice led to exhibition of GTP-mediated anti-tumorigenesis and anti-inflammatory activity. Conversely, injection of anti-IL-12 antibodies in UVB-irradiated wild-type mice resulted in a failure in the repair or removal of CPD and in high levels of COX-2 and PGE2.

5. While the study of Meeran et al. provides valuable data on the mechanism by which GTPs exert their antitumorigenesis and anti-inflammatory activities, the study was limited in several respects. The investigators accurately showed that the chemical composition of GTPs added to the drinking water was consistent over the 3 days. But they did not measure the level of GTPs in the serum or tissue of the study animals, so we cannot exclude the possibility that the lack of GTP-mediated antitumorigenesis effects in IL-12-KO mice is attributable to a lack of GI or tissue absorption of the GTPs in this model. Furthermore, the amount of fluid (GTP-containing water and regular water) consumed by the mice was not measured (or at least not reported). One could argue that mice did (or did not) like the taste of the GTP-containing water and hence drank more (or less) fluid.

Another possible explanation for the lack of GTP-mediated antitumorigenesis effects in IL-12-KO mice might be the high incidence of the tumors in this model (80% of the irradiated mice developed skin cancer), which statistically decreases the power of GTPs in the prevention of cancer. Simply, one could argue that GTPs are effective in both wild-type and IL-12-KO mice but they lose their significance in the IL-12-KO mouse because there are more cancers in this model. In this study, the GTPs were started 2 weeks prior to the initiation of UVB irradiation and continued throughout the study. In humans, the likely scenario is that UVB exposure occurs months to years before the consumption of green tea begins. It would be interesting to examine whether GTPs could exert their effects if started after UVB irradiation. It would also be interesting to study what would happen if the GTPs were discontinued.

6. The mechanistic study by Meeran et al. (2009) supports the potential nutritional value of green tea in skin photoprotection. The authors show that GTPs are effective via oral administration in drinking water. Hence, GTPs, in combination with sunscreens, may provide an effective strategy for reducing the risk of skin cancers. In addition, the data presented in this study indicate that UVB-induced DNA damage and inflammatory responses are causally related to an increased risk of photocarcinogenesis. The ability of IL-12 to repair UV-induced DNA damage and decrease subsequent inflammation makes it a potential “cytokine sunscreen” with the ability to protect against both skin cancer and sunburn. In summary, the outcome of this study suggests that endogenous enhancement of IL-12-mediated DNA repair by GTPs may be considered an effective strategy for the prevention of inflammation-associated skin diseases, including skin cancers. Nonetheless, clinical trials are needed to validate the usefulness of GTPs, either alone or in combination with existing modes of skin cancer prevention.

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1 Department of Dermatology and Cutaneous Surgery, University of Miami Miller School of Medicine, Miami, Florida, USA