Different in vitro toxicities of structurally similar type I ribosome-inactivating proteins (RIPs)

This study was aimed at investigating and comparing the cytotoxicities of two structurally similar type I RIPs, namely trichosanthin (TCS) and free ricin A chain (RTA). A type II RIP, namely Ricinus communis agglutinin (RCA), was also included for comparison. The three RIPs were added separately to cultures of NIH 3T3 cells. The effective doses and time courses were analyzed using cell counts. Polyclonal antibodies against TCS and RTA were produced in rabbits and purified by a protein A-Sepharose CL-4B column. The mechanisms of cell death were determined by TUNEL, immunohistochemical staining, flow cytometry, and Western blotting. The effective doses for TCS, RTA and RCA were found to be 800, 50, and 50 nM, respectively. All three RIPs induced apoptosis. In all cases, activation of caspase-3 and caspase-8, but not caspase-9, was detected. Additionally, RTA caused in vivo tissue necrosis in rabbits after intradermal administration. Hence the mechanism of cell death due to RTA intoxication may vary depending on the experimental conditions, being necrosis in vivo and apoptosis in vitro. The present findings may shed light on the apoptotic pathway induced by RIPs. RTA may be useful for studying the shift in cell death.