Repeated transient sulforaphane stimulation in astrocytes leads to prolonged Nrf2-mediated gene expression and protection from superoxide-induced damage

Oxidative stress is a major contributor to slowly developing diseases like Parkinson's disease, Alzheimer's disease and cancer and one of the main causes of tissue damage following ischemic insults in the brain. Nrf2 is a transcription factor responsible for much of the inducible cellular defense against oxidative stress. Nrf2 can also be activated by xenobiotics like sulforaphane, a component highly enriched in cruciferous vegetables such as broccoli. Ingestion of broccoli or sulforaphane results in long-term protection against radical damage, although absorbed sulforaphane is cleared from the body within a few hours. Here we have examined whether the prolonged protection induced by sulforaphane is explained by a slow down regulation of the Nrf2 response. Furthermore, to simulate daily ingestion of sulforaphane, we examined the hypothesis that repeated transient sulforaphane stimulation results in an accumulation of Nrf2-mediated gene expression and an increased protection against oxidative damage. The kinetics of sulforaphane-induced Nrf2 response was studied in astrocytes, a cell type known to be highly involved in the defense against oxidative stress in the brain. Sulforaphane stimulation for 4 h induced an Nrf2-dependent increase of Nqo1 and Hmox1 mRNA that remained elevated for 24 h, and the corresponding proteins remained elevated for over 48 h. In addition, peroxide-clearing activity and the levels of glutathione were elevated for more than 20 h after stimulation for 4 h with sulforaphane, resulting in an increased resistance to superoxide-induced cell damage. Repeated sulforaphane stimulation resulted in an accumulation of mRNA and protein levels of Nqo1 and a persistent cell protection against oxidative damage. These findings indicate that brief stimulation of the Nrf2 pathway by sulforaphane results in long-lasting elevation of endogenous antioxidants in astrocytes. The findings also demonstrate that part of this response can be built up by repeated transient stimulation, possibly explaining how intermittent intake of sulforaphane can result in long-term protection from radical-induced disease.