Usnic acid: anti-proliferative, apoptotic and morphological effects on human malignant cell lines

The aim was to investigate the effects of two enantiomers of usnic acid on proliferation and survival of human malignant cell lines. R- and S- usnic acid were isolated in pure form from the lichens Cladonia arbuscula (Wallr.) Flot. and Alectoria ochroleuca (Hoffm.) Massal., respectively, and solubilized in DMSO. T47-D (breast cancer) and Capan-2 (pancreatic cancer) were from ATCC and the myeloma cell lines RPMI-8226*, U266–84* and LP-1*. Anti-proliferative effects were tested by thymidine-uptake, results expressed as IC₅₀. To test for apoptosis cells were exposed to four times this concentration for 24 hours; a commercial TUNEL assay was used. The morphology of MG-G-stained cells was investigated after 2, 6 and 24 hour-exposure to usnic acid at the four times IC₅₀ concentration. Usnic acid had anti-proliferative effects against T47-D (IC50=4.2µg/mL) and Capan-2 (IC₅₀=5.3µg/mL). No difference was found between the enantiomers and only (+) usnic acid was used for further testing. None of the myeloma cell lines was significantly affected. Usnic acid did not induce apoptosis in any of the cell lines. T47-D cells, but not Capan-2 cells, showed morphological changes indicative of necrosis in a small proportion of cells after 6 hours exposure to usnic acid. In conclusion, both enantiomers of usnic acid have significant anti-proliferative effects against two human carcinoma cell lines but not against myeloma cell lines. Usnic acid did not induce apoptosis, which is in line with its non-genotoxic mode of action (1), but mild signs of necrosis were seen.

*A kind gift from Kenneth Nilsson.