Fig. 6.

Proposed model for the cytoprotective effect of 18α-GA against MMC cytotoxicity. Apart from the canonical mechanism of action of MMC on DNA topology, MMC alters the cellular redox status, promoting ROS production and contributing to oxidative stress-mediated DNA damage. Moreover, MMC leads to decreased p-ERK levels, thereby limiting Nrf2 activation and its antioxidant effect and further contributing to oxidative stress-mediated DNA damage. 18α-GA induces ERK phosphorylation that promotes Nrf2 phosphorylation and nuclear translocation. Nuclear Nrf2 heterodimerizes with p-c-JUN (among other possible partners) upregulating the transcription of several target genes like NQO1 and -TXNRD1 (among others). The proteins encoded by those genes are part of the antioxidant cellular mechanisms leading to reduced ROS levels that normally promote DNA damage. This cascade may be inhibited through treatment with the specific MEK inhibitor PD035901 that effectively prevents ERK phosphorylation thus abolishing the protective effect of 18α-GA against MMC cytotoxicity. 18α-GA may also activate Nrf2 through other pathways as well (e.g. through the redox regulation of Keap1) while it may impact cellular ROS levels through additional direct or indirect mechanisms.