Synthetic triterpenoids attenuate cytotoxic retinal injury: cross-talk between Nrf2 and PI3K/AKT signaling through inhibition of the lipid phosphatase PTEN.

PURPOSE: Evidence implicating oxidative stress in the pathogenesis of age-related macular degeneration suggests that antioxidant therapy could play a role in preventing its progression. The aim of this study was to determine whether derivatives of the triterpenoid (TP) 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid (CDDO; CDDO-imidazolide [-Im], CDDO-ethylamide [-EA], and CDDO-trifluoroethylamide [-TFEA]) confer cytoprotection from oxidative- and photooxidative-induced cellular damage and to explore the molecular mechanisms of this cytoprotection.
METHODS: Retinal pigment epithelial and retinal photoreceptor cell lines were treated with TP derivatives. Induction of Nrf2 signaling was measured by reporter assay. Cytoprotection was quantified by MTT assay. To determine whether TPs confer in vivo cytoprotection, BALB/c mice were pretreated with CDDO-TFEA, and retinal degeneration was induced by light exposure. To explore the association of TPs with PTEN, a biotinylated derivative of CDDO (CDDO-Bt) was used.
RESULTS: Treatment with CDDO-Im-, -TFEA-, or -EA-induced Nrf2 signaling and TP pretreatment protected retinal cell lines from oxidant-induced cell death. The antioxidant and cytoprotective potential of these compounds was then examined in vivo. Treatment of BALB/c mice with CDDO-TFEA induced the Nrf2-regulated transcripts glcl and trx1 in retinal tissue and was protective from photooxidative retinal damage. Treatment with CDDO-Im leads to phosphorylation of AKT. CDDO-Bt directly binds cysteine 124 within PTEN's active site and inhibits PTEN's lipid phosphatase activity in vitro. Thus the stimulation of AKT activity is mediated by TP inhibition of PTEN activity.
CONCLUSIONS: These studies highlight the potential of TPs in retinal cytoprotection and implicate PTEN inhibition as a target in cytoprotection.