The thiolprotease bromelain, isolated from pine apple stem, was suggested for use in adjuvant tumor therapy. This study examined the in vitro effects of crude bromelain, bromelain F9 and papain on B16F10 mouse melanoma cell lung colonization, in vitro cell proliferation, invasion through matrigel and CD44 expression. In vitro treatment of the melanoma cells with bromelain F9 and papain before i.v. injection into mice prevented lung colonization. The lung weight at day 20 was significantly reduced from 5.1% (untreated cells) to 1.6% (bromelain F9 treated cells). Papain was as effective as bromelain F9. However, there was no difference in the lung weight between bromelain F9 treated and the untreated group at day 27. Protease removal and further incubation of the B16F10 cells retained their capacity to induce lung tumor metastases. The proteases inhibited growth of the melanoma cells in a dose dependent manner. Crude bromelain was most active with a half maximal value of 7.5 mu g/ml. However, the antiproliferative effects did not correlate with the proteolytic activity. In a matrigel invasion assay, the proteases reduced the invasive capacity of the melanoma cells maximally by about 30%. Using flow cytometry, the proteases were found to reduce the CD44 density, present on the melanoma cells, to a different degree: crude bromelain was more active than bromelain F9 and papain, which had higher proteolytic activity. Crude bromelain was most active in abolishing the CD44 re-expression after protease treatment.

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