ANTIPROLIFERATIVE EFFECTS OF LICHEN-DERIVED INHIBITORS OF 5-LIPOXYGENASE ON MALIGNANT CELL-LINES AND MITOGEN-STIMULATED LYMPHOCYTES

Several lichen species have been used traditionally as medicinal plant s. It has previously been shown that two low-molecular-weight lichen m etabolites, lobaric acid isolated from Stereocaulon alpinum Laur. and protolichesterinic acid isolated from Cetraria islandica L. (Ach.), ha ve in-vitro inhibitory effects on arachidonate 5-lipoxygenase. We have studied the effects of these compounds on cultured cells from man, in cluding three malignant cell-lines (T-47D and ZR-75-1 from breast carc inomas and K-562 from erythro-leukaemia), as well as normal skin fibro blasts and peripheral blood lymphocytes. Both test substances caused a significant reduction in DNA synthesis, as measured by thymidine upta ke, in all three malignant cell-lines; the close inducing 50% of maxim um inhibition (ED50) was between 1.1 and 24.6 mu g mL(-1) for protolic hesterinic acid and between 14.5 and 44.7 mu g mL(-1) for lobaric acid . The breast-cancer cell-lines were more sensitive than K-562. The pro liferative response of mitogen-stimulated lymphocytes was inhibited wi th a mean ED50 of 8.4 mu g mL(-1) and 24.5 mu g mL(-1) for protoliches terinic acid and lobaric acid, respectively. These concentrations are of the same order of magnitude as the IC50 values in the 5-lipoxygenas e assay. Significant cell death (assessed by the MTS xymethoxyphenyl)- 2-(4-sulfophenyl)-2H-tetrazolium) assay and trypan blue exclusion) occ urred in the three malignant cell-lines at protolichesterinic acid and lobaric acid concentrations above 20 and 30 mu g mL(-1), respectively . In K-562 morphological changes consistent with apoptosis were detect ed. Up to 38% cell death was observed at 20 mu g mL(-1) for protoliche sterinic acid and 15 mu g mL(-1) for lobaric acid in mitogen-stimulate d lymphocytes but unstimulated lymphocytes were clearly less sensitive . In contrast, the DNA synthesis, proliferation and survival of normal skin fibroblasts were not affected at doses up to 20 mu g mL(-1) for protolichesterinic acid and 30 mu g mL(-1) for lobaric acid. We conclu de that the anti-proliferative and cytotoxic effects observed might be related to the 5-lipoxygenase inhibitory activity of protolichesterin ic acid and lobaric acid. These results open up the opportunity for fu ture studies of these lichen metabolites with regard to their anti-tum our and anti-inflammatory properties.