in: Discovery and Development of Anti-Breast Cancer Agents from Natural Products, Goutam Brahmachari, Editor, Elsevier Science, Oxford/Amsterdam , Oxford, pp.147-196, 2021
Flavonoids are plant pigments and widely distributed in plants. They are from the most common group of polyphenolic compounds in the human diets and found in fruits, vegetables, cereals, chocolate, and beverages such as tea, coffee, and wine. Flavonoids are generated from three acetate units and one phenylpropane unit and classified according to the state of oxygenation of the C3 unit. The main flavonoid subclasses are flavones, flavonols, flavanones, flavan-3-ols (flavanols), isoflavones, anthocyanidins, and proanthocyanidins. It is widely accepted that the dietary flavonoids in fruits and vegetables play a key role in the prevention of human diseases including cancer. Apoptosis and autophagy are evolutionarily conserved processes in all eukaryotic cells. Apoptosis (type I programmed cell death) occurs normally during development and as a homeostatic mechanism to maintain cell populations in tissues. Apoptosis also occurs as a defense mechanism under pathological conditions, when cells are damaged too severely to repair. Autophagy (type II programmed cell death) is another cell homeostatic mechanism by recycling a cell's own nonessential macromolecular components or damaged organelles. Some flavonoids induce cell death through apoptosis and autophagy in cancer cells via unique molecular mechanisms. In this chapter, we will discuss the general extraction and isolation techniques for natural flavonoids from fruits and vegetables, and the synthesis of some flavones, flavonols, flavanones, isoflavones in detail. In addition, we will discuss the apoptosis- and autophagy-inducing activities of certain flavonoids as well as the underlying molecular mechanisms. Flavan-3-ols and proanthocyanidins will not be discussed in this chapter.