6. Miscellaneous specialized metabolites
Cancer suppression through the use of natural plant specialized metabolites, such as flavonoids, phenolic acids, carotenoids have been shown more effective and beneficial (Russo et al., 2005). The main mode of action of flavonoids and its derivatives is to inhibit cell proliferation and angiogenesis (development of abnormal blood cells), cause cell cycle arrest, induce cell apoptosis, and reverse multidrug resistance. Some derivatives such as quercetin targets the cell cycle at G1/S and G2/M checkpoints by inducing the p21 CDK inhibitor while decreasing pRb phosphorylation, thereby blocking an important transcription factor of DNA synthesis proteins which is E2F1.
In vitro studies of Fagonia indica revealed its potential to produce important specialized metabolites such as phenolic compounds (gallic acid, caffeic acid, catechin, epigenin, myricetin) with anticancer activity by the use of plant growth regulator thidiazuron (TDZ) in callus culture of stem and leaf of this species. TZD acts as an elicitor in the callus culture of this plant (Khan et al., 2016). There is also improved production of anticancer compound naphthodianthrones fromHypericum perforatum L. when elicitor salicylic acid is incorporated in the shoot and callus cultures of this plant (Gadzovska et al., 2013). Secondary compounds, β-glucan, and galactomannan, polysaccharides extracted from lichens, are also active against several cancer cell lines (Shrestha and Clair, 2013; Watanabe et al., 1986). Another class of secondary metabolites, rosmarinic acid (RA) and salvianolic acid B (Sal. B) obtained by in vitro techniques such as callus cultures of stems and leaves of Salvia miltiorrhiza (Danshen) is yet another anticancer compound known to produce cytotoxicity against leukemia cell lines. Here, callus stem extracts proved to show more cytotoxicity towards leukemia cell lines than callus leaf extracts. Thus, callus culture techniques from Danshen for producing RA and Sal. B can be promising for anticancer therapy (Wu et al., 2016).