Cytotoxic activity of ribonucleolytic toxin restrictocin-based chimeric toxins targeted to epidermal growth factor receptor

Abstract

Targeted toxins represent a new approach to specific cytocidal therapy. The ribonucleolytic protein toxin restrictocin is a potent protein synthesis inhibitor produced by the fungus Aspergillus restrictus. In the present study we have constructed two restrictocin based chimeric toxins where human transforming growth factor alpha (TGFα) has been used as a ligand. TGFα is a single chain polypeptide, which binds to epidermal growth factor receptor (EGFR) and causes proliferation in a large number of cancers. The ligand has been separately fused either at the amino terminus or carboxyl terminus of restrictocin, giving rise to TGFα-restrictocin and restrictocin-TGFα respectively. The fusion proteins were overexpressed in Escherichia coli and purified from inclusion bodies by a denaturation-renaturation protocol. Both the chimeric toxins actively inhibited eukaryotic protein synthesis in a cell free in vitro translation assay system. These chimeric toxins selectively killed human epidermal growth factor receptor positive target cells in culture. Among the two proteins, restrictocin-TGFα was more active than TGFα-restrictocin on all the cell lines studied.