No abstract

Cytotoxic ribosome-inactivating proteins (RIPs) of type II such as ricin were investigated as anti-cancer agents, but also pose a threat as biological weapons. The molecular mechanism leading to their toxic effects is, however, not yet clear. The current paradigm, which states that the irreversible depurination of 28S rRNA results in a general translational arrest eventually leading to cell death, has been questioned. Using micro-array, qRT-PCR and Western blot, we identified the unfolded protein response (UPR), a cellular mechanism activated in response to endoplasmic reticulum stress, that is induced in HCT116 and MDA-MB-231 cells exposed to the plant type II RIPs ricin, riproximin and volkensin. Apoptosis was induced by concentrations at which translation of UPR-related genes still occurred, despite concomitant ribosomal depurination. We conclude that UPR induction represents a model that better describes the cellular effects of RIP exposure at concentrations at which selected proteins are translated despite ribosomal depurination.

The aim of this study was to identify and characterize the active component(s) of Ximenia americana plant material used to treat cancer in African traditional medicine. By a combination of preextraction, extraction, ion exchange and affinity chromatography, a mixture of two cytotoxic proteins was isolated. Using degenerated primers designed on the de novo sequence of two tryptic peptides from one of these proteins, a DNA fragment was amplified and the sequence obtained was used to determine the complete cDNA sequence by the RACE method. Sequence analysis and molecular modeling showed that the new protein, riproximin, belongs to the family of type II ribosome inactivating proteins. These results are in good agreement with the ability of riproximin to inhibit protein synthesis in a cell-free system, as well as with the cytotoxicity of riproximin, as demonstrated by its IC50 value of 0.5 pM in MCF7, 1.1 pM in HELA and 0.6 pM in CC531-lacZ cells. To assess the antineoplastic efficacy of the purified riproximin in vivo, the CC531-lacZ colorectal cancer rat metastasis model was used. Significant anticancer activity was found after administration of total dosages of 100 (perorally) and 10 (intraperitoneally) pmol riproximin/kg. These results suggest that riproximin has distinct potential for cancer treatment.