The neuroprotective activities of natural products through the Nrf2 upregulation.

Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a basic-region leucine zipper transcription factor that regulates the expression of many Phase II detoxifying/antioxidant enzymes. Many natural products act on the Nrf2 pathway and exhibit their preventive and/or therapeutic effects by activating Nrf2 and subsequently antioxidant enzymes. In this article, we reviewed the research literature that described Nrf2 activation as a mechanism for the neuroprotective effect of natural products. Our results showed that the inhibition of Kelch-like ECH-associated protein 1 and the activation of different intracellular kinases that lead to Nrf2 phosphorylation were the main mechanisms of Nrf2 upregulation suggested for these the natural products. Although some natural compounds such as terpenoids and alkaloids were reported to protect the nervous system by Nrf2 activating, flavonoids and phenolic compounds represent the majority of the reported neuroprotective natural substances that activate Nrf2. We also evaluated the physiochemical properties of some of the same substances to predict their oral bioavailability and central nervous system penetration. The results showed that sulforaphane and berberine have good oral bioavailability and would be predicted to cross through blood-brain barrier (BBB) easily, whereas the flavonoids in general would be predicted to be less effective in BBB transition because of their relatively high polar surface area.