Redox characterization of usnic acid and its cytotoxic effect on human neuron-like cells (SH-SY5Y)

Usnic acid (UA) is the most common and abundant lichenic secondary metabolite with potential therapeutic application. Anti-inflammatory and antitumour properties have already been reported and UA-enriched extracts are widely used to treat several diseases in the folk medicine. First, we performed in silico evaluation of UA interactions with genes/proteins and important compounds for cellular redox balance and NO pathway. Then, we assessed UA redox properties against different reactive species (RS) generated in vitro, and evaluated its action on SH-SY5Y neuronal like cells upon hydrogen peroxide (H2O2), since no in vitro neurotoxicological data has been reported so far. Total reactive antioxidant potential index (TRAP) showed a significant antioxidant capacity of UA at the highest tested concentration; UA was also effective against hydroxyl radicals and reduced the formation of nitric oxide. In vitro, lipoperoxidation was enhanced by UA and changed the cellular viability at highest concentration of 20 μg/mL for 1 and 4 h, as well as 2 and 20 μg/mL for 24 h of treatment, according to MTT reduction assay. Moreover, UA did not display protective effects against H2O2-induced cell death in any case. Evaluation of intracellular RS production by the DCFH-based assay indicated that UA was able to induce changes in basal RS production at concentration of 20 μg/mL for 1 h and from 2 ng/mL to 20 μg/mL for 4 and 24 h. In conclusion, UA could display variable redox-active properties, according to different system conditions and/or cellular environment. Moreover, our results suggest that potential neurotoxicological effects of UA should be further studied by additional approaches; for instance, in vivo and clinical studies.

► Usnic acid is a hydroxyl and nitric oxide scavenger in vitro. ► Usnic acid acts as a general antioxidant in TRAP/TAR assays. ► Usnic acid enhances lipid peroxidation generated in vitro. ► Usnic acid was slightly neurotoxic SH-SY5Y cells in vitro. ► Usnic acid enhanced intracellular reactive species production in SH-SY5Y cells.