Natural Polyphenols in Cancer Chemoresistance.

Resistance to chemotherapy remains a major impediment to the management of most types of cancer. Both intrinsic and acquired drug resistance are mediated by several cellular and molecular mechanisms, including alternative growth-signaling pathways unaffected by specific therapies, alterations in the tumor microenvironment (e.g., hypoxia and angiogenesis), and active transport of drugs out of the cell. Epidemiological studies have validated an inverse correlation between the consumption of dietary polyphenols and the risk of cancer, which has been attributed to polyphenol antioxidant capacity and their potential to inhibit activation of procarcinogens, cancer cell proliferation, metastasis, and angiogenesis, and inhibition or downregulation of active drug efflux transporters. Moreover, polyphenols can induce apoptosis in cancer cells and modulate immune responses and inflammatory cascades. Augmentation of the efficacy of chemotherapy and prevention of multidrug resistance are other important effects of dietary polyphenols that deserve further research, especially after the discovery of tight "crosstalk" between aberrant growth signaling and metabolic dysfunction in cancer cells. In this review, we cover what is currently known about the role of natural polyphenolic compounds in overcoming cancer drug resistance mediated by diverse primary and secondary resistance mechanisms.