Diverse activity of sc-RIP saporin 6 on primary and metastatic melanoma cells in vitro

The inhibitory effect of saporin 6, a single-chain ribosome-inactivating protein (sc-RIP) purified from the seeds of Saponaria officinalis, on the proliferation of human primary (MeWo, WM 164, SK MEL 28, MEM), cloned (MEM A9, A12, A13) and metastatic (M14) melanoma cells has been tested by protein synthesis and colony formation assays in vitro. Results indicate a marked difference in the sensitivity of primary and metastatic cells to the action of saporin 6, the latter being significantly more affected, both in treated and in pretreated cultures, with a high and specific response evident after 24 h of treatment and progressively increasing up to 72 h of culture with the drug (IC50 = 0.82 microgram/ml). This effect, which was dose-dependent in exponentially growing cells, was partially reversed upon removal of the inhibitor from the culture medium. No inhibitory effect was evident in the MeWo primary cells at the highest saporin 6 concentration used: the p170 glycoprotein-mediated mechanism is not involved in such a resistance pathway.