Ricin is a potent plant toxin that inhibits protein synthesis of eukaryotic cells resulting eventually in cell death. The mechanism of ricin-mediated cytotoxicity on human tumor cell lines was examined in view of the recent findings that bacterial toxins like diphtheria toxin (DTX) and Pseudomonas aeruginosa exotoxin A (PEA) cause programmed cell death or apoptosis in addition to their activity of inhibition of protein synthesis. Ricin is shown to be cytolytic in a short term assay (less-than-or-equal-to 24 h) and cytostatic in a long term assay (greater-than-or-equal-to 72 h) as determined by the MTT assay. The cytostatic activity of ricin paralleled its protein synthesis inhibitory activity whereas ricin-mediated cytolytic activity did not correlate with protein synthesis inhibition. Ricin is shown to mediate programmed cell death or apoptosis against several drug sensitive and drug resistant cell lines. DNA fragmentation is initially detected following 4 h of treatment. Although cytotoxicity by ricin is energy dependent, it is not reduced by the addition of a lysosomotropic agent NH4Cl. We have recently demonstrated that the bacterial toxins DTX and PEA synergize with tumor necrosis factor-alpha (TNF-alpha) (J Immunol 147: 2609-2616, 1991 and J Immunol 149: 2089-2094, 1992). However, unlike DTX or PEA, ricin did not synergize with TNF-alpha, suggesting that ricin and DTX/PEA may mediate their cytolytic activity by different mechanisms. These findings demonstrate that ricin can mediate both protein synthesis inhibition and programmed cell death or apoptosis against a variety of drug sensitive and resistant human tumor cell lines. The significance of these findings in cancer therapy is discussed.

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