Colorectal cancer: chemopreventive role of curcumin and resveratrol

The pharmacology of curcumin: is it the degradation products?

The natural product curcumin has gained considerable attention in recent years for its multiple pharmacological activities, but more efforts are needed to understand how curcumin can have these pharmacological effects considering its low bioavailability. In addition, it is unclear how curcumin exerts inhibitory effects against numerous enzymes, especially those that cannot accommodate curcumin within recognized binding pockets. By analyzing the similarities between the biological activities of curcumin and its degradation products against diseases such as Alzheimer's disease and cancer, as well as the preferential inhibition of some enzymes by degradation products, it appears that the bioactive degradation products may contribute to the pharmacological effects of curcumin. This possibility should be given full attention when elucidating the pharmacology of this promising natural product for various diseases.

Clinical trials of antioxidants as cancer prevention agents: past, present, and future

The purpose of this review is to summarize the most important human clinical trials of antioxidants as cancer prevention agents conducted to date, provide an overview of currently ongoing studies, and discuss future steps needed to advance research in this field. To date there have been several large (at least 7000 participants) trials testing the efficacy of antioxidant supplements in preventing cancer. The specific agents (diet-derived direct antioxidants and essential components of antioxidant enzymes) tested in those trials included β-carotene, vitamin E, vitamin C, selenium, retinol, zinc, riboflavin, and molybdenum. None of the completed trials produced convincing evidence to justify the use of traditional antioxidant-related vitamins or minerals for cancer prevention. Our search of ongoing trials identified six projects at various stages of completion. Five of those six trials use selenium as the intervention of interest delivered either alone or in combination with other agents. The lack of success to date can be explained by a variety of factors that need to be considered in the next generation research. These factors include lack of good biological rationale for selecting specific agents of interest; limited number of agents tested to date; use of pharmacological, rather than dietary, doses; and insufficient duration of intervention and follow-up. The latter consideration underscores the need for alternative endpoints that are associated with increased risk of neoplasia (i.e., biomarkers of risk), but are detectable prior to tumor occurrence. Although dietary antioxidants are a large and diverse group of compounds, only a small proportion of candidate agents have been tested. In summary, the strategy of focusing on large high-budget studies using cancer incidence as the endpoint and testing a relatively limited number of antioxidant agents has been largely unsuccessful. This lack of success in previous trials should not preclude us from seeking novel ways of preventing cancer by modulating oxidative balance. On the contrary, the well demonstrated mechanistic link between excessive oxidative stress and carcinogenesis underscores the need for new studies. It appears that future large-scale projects should be preceded by smaller, shorter, less expensive biomarker-based studies that can serve as a link from mechanistic and observational research to human cancer prevention trials. These relatively inexpensive studies would provide human experimental evidence for the likely efficacy, optimum dose, and long-term safety of the intervention of interest that would then guide the design of safe, more definitive large-scale trials.

Resveratrol and cellular mechanisms of cancer prevention

The use of novel and improved chemopreventive and chemotherapeutic agents for the prevention and treatment of cancer is on the rise. Natural products have always afforded a rich source of such agents. Epidemiological evidence suggests that a higher flavonoid intake is associated with low cancer risk. Accumulating data clearly indicate that the induction of apoptosis is an important component in the chemoprevention of cancer by naturally occurring dietary agents. Resveratrol, a naturally occurring polyphenol, demonstrates pleiotropic health benefits, including antioxidant, anti-inflammatory, antiaging, cardioprotective, and neuroprotective activities. Because of these properties and their wide distribution throughout the plant kingdom, resveratrol is envisioned as a potential chemopreventive/curative agent. Currently, a number of preclinical findings from our lab and elsewhere suggest resveratrol to be a promising natural weapon in the war against cancer. Remarkable progress in elucidating the molecular mechanisms underlying the anticancer properties of resveratrol has been achieved. Here, we focus on some of the myriad pathways that resveratrol targets to exert its chemopreventive role and advocate that resveratrol holds tremendous potential as an efficient anticancer drug of the future.

Tumor acidity as evolutionary spite

Most cancer cells shift their metabolic pathway from a metabolism reflecting the Pasteur-effect into one reflecting the Warburg-effect. This shift creates an acidic microenvironment around the tumor and becomes the driving force for a positive carcinogenesis feedback loop. As a consequence of tumor acidity, the tumor microenvironment encourages a selection of certain cell phenotypes that are able to survive in this caustic environment to the detriment of other cell types. This selection can be described by a process which can be modeled upon spite: the tumor cells reduce their own fitness by making an acidic environment, but this reduces the fitness of their competitors to an even greater extent. Moreover, the environment is an important dimension that further drives this spite process. Thus, diminishing the selective environment most probably interferes with the spite process. Such interference has been recently utilized in cancer treatment.