Apr 7, 2016

How do phytochemicals affect your body?

The precise biochemical mechanisms through which phytochemicals exert their anti-cancer effects are still being explored, as their actions are wide-ranging and complex but significant advances have been made of late in the understanding the mode of action.

Antioxidant properties

The most quoted cancer prevention mechanism is via their vegetablesantioxidant activity, elicited either through direct free-radical absorption or through induction of antioxidant enzymes, such as superoxide dismutase (SOD), catalase and glutathione via a variety of molecular mechanisms1,2. One of these mechanisms is activation of Nrf2, which switches on genes that code for antioxidant as well as detoxification enzymes1,2.

Phytochemicals, particularly the thiol class such as sulforaphane, have also been shown to inhibit the conver­sion of pro-carcinogens to their electrophilic, DNA-damaging chemicals2,3.

A number studies involving known common carcinogens have highlighted the antioxidant properties of phytochemicals:

  • A good example of their protective effect was an experiment involving the known house-hold carcinogen triclocarban, commonly found in detergents and cleaning agents. Healthy cells exposed to triclocarban tend to mutate into pre-malignant cells, however, the amount and rate of carcinogenesis was significantly reduced by adding curcumin to the petri dish culture feeds4.
  • In another study, volunteers who ate a diet rich in kaempferol were found, on serum and urine analysis, to have improved SOD activity and higher urinary concentration of these polyphenols5. Rats exposed to cigarette smoke given indole-3-carbinol, a phytochemical rich in cruciferous vegetables, had a lower lung cancer rate than those not given idole-3-carbinol6.
  • Subjects eating a meal of onions, which increased their serum levels of quercetin, demonstrated decreased levels of oxidative metabolites including 8-hydroxydeoxyguanosine (8-OHdG) a marker of DNA damage and repair7,8. Quercetin supplementation has also been shown to improving mitochondrial dysfunctions induced by the toxin 3-nitropropionic acid9.
  • A clinical study in Singapore gave Chinese smokers 170g of watercress a day, rich in the indole-3-carbinol, found a significant effect on urinary markers of DNA damage10.
  • Finally, marinating meat in rosemary and thyme, has been reported to reduced the serum levels of carcinogenic heterocyclic amines (HCA) by 87% compared to subjects who eat the meat unseasoned11.

Inflammatory properties

Another key anti-cancer mechanism of phytochemicals appears to be their ability to reduce inflammation.

It is now well established that inappropriate inflammation is intimately involved in the cancer process, particularly in the promotion and progression stages of cancer. Inflammation is closely associated with oxidative stress and activation of NF-kappa B family of transcription fac­tors. These factors regulate more than 150 genes involved in mechanisms of cell survival and these target genes are not just pro-inflammatory but also oncogenic. Numerous phytochemicals have been shown to inhibit NF-kappa B signalling, particularly the green tea polyphenol epigallocatechin-3-gallate (EGCG), quercetin, curcumin, caffeic acid and caffeic acid phenethyl ester and the phytochemicals within bilberries1,12.

Affecting signaling pathways in cancer

More recently, it has been reported mainly from laboratory studies that phytochemicals have an effect on several cancer processes through modulation of cellular and signalling events involved in growth, invasion and metastasis2.

Pomegranate, for example, rich in the polyphenol ellagic acid, pomegranatehas been shown to directly inhibit cell growth and induce apoptosis in androgen-sensitive and aggressive human prostate cancer cells13. Pomegranate extract has also been reported to inhibit processes involved in cancer meta
stasis in a study involving oestrogen-sensitive and -resistant breast cancer cell lines, showing increased markers of cell adhesion and migration in cancer but not normal cells14. In another study, it inhibited a chemokine that attracts breast cancer cells to the bone15.

Curcumin, a substance in turmeric, slows cancer cell growth by blocking the cell cycle, increasing the rate of apoptosis and preventing the invasion and migration of cells16,17,18,19. It has also been found to halt the growth of stem cells that give rise to breast cancer without harming normal breast stem cells20. Curcumin has been shown to modulate microRNA expression in breast cancer cells leading to a reduced expression of Bcl-221 and stabilisation of tumour suppressor gene in colorectal cancer cell lines22.

Green tea, rich in epigallocatechin gallate (EGCG), has demonstrated significant reduction of several factors that promote cancer cell proliferation by inhibiting DNA synthesis, de-differentiation and angiogenesis23,24,25. It has also been shown to block ornithine decarboxylase, an enzyme that signals cells to proliferate faster and bypass apoptosis21,26. Resveratrol has demonstrated epigenetic regulatory properties that influence cell proliferation, survival and apoptosis in prostate cancer by global modulation of gene expression through deacetylation of FOXO transcription factor17. Caffeic acid and phenethyl ester, as well as inhibiting NF-kappa B signaling, also have been shown to inhibit cell motility in vitro and inhibit metastasis of tumour models in vivo18. Luteolin, as well as inhibiting tumour growth and metastasis, inhibits epithelial mesenchymal transition, which is a basic biological process related to cancer initiation and development18.

Hormonal actions

Some polyphenols and other phytochemicals are also able to influence cancer via a hormonal mechanism.

Phytoestrogenic compounds, most notably isoflavones and lignans found in soy products, legumes and some cruciferous vegetables, weakly bind to the oestrogen receptor without stimulating proliferation of the cells, yet at the same time blocking the binding of more harmful oestrogens, including those produced endogenously10. This explains why in the previously mentioned Shanghai Breast Cancer Survival Study, women with the highest intake of isoflavone- and flavanone-rich foods had a lower risk of death27. In men, phytoestrogenic compounds have been shown to affect 5-alpha reductase-lowering endogenous testosterone levels. This may partly explain why men who eat phytoestrogenic foods such as beans and pulses have a lower risk of prostate cancer.


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