Cancer chemotherapy--ribonucleases to the rescue

Abstract

Ribonucleases, once dismissed as uninteresting digestive enzymes, have been shown to have remarkable biological activities. Onconase, from the Northern leopard frog, is currently in clinical trials as a cancer chemotherapeutic. Recent research has revealed some key factors responsible for the cytotoxicity of ribonucleases, and may lead to a new class of drugs.

Figure 1

Biochemical basis for the potential cytotoxicity of ribonucleases.

Figure 2

Putative mechanism for catalysis of RNA cleavage by RNase A. His12, Lys41 and His119 are conserved in all homologous ribonucleases (see Figure 3a).

Figure 3

(a) Amino acid sequences of RNase A and three of its cytotoxic homologs. Sequences were aligned using the PILEUP program (Genetics Computer Group—Version 10; Madison, WI) with a Gap Creation Penalty of 8 and a Gap Extension Penalty of 2. Residues are numbered according to RNase A. RNase A residues that contact porcine ribonuclease inhibitor in the pRI·RNase A complex are white on black. Conserved residues are boxed. The three residues most important for catalysis by RNase A (His12, Lys41 and His119), and the corresponding residues in the cytotoxic homologs, are blue. Cysteine residues are yellow. (b) Three-dimensional structures of RNase A, onconase, and the Rana catesbeiana ribonuclease. The secondary structural context of each half-cystine is indicated by H (helix), S (sheet), or L (loop). Ribbon diagrams were created with the programs MOLSCRIPT [76] and Raster3d [77] using coordinates derived from X-ray diffraction [12,33] or NMR spectroscopy [78].

Figure 4

Three-dimensional structure of the complex between porcine ribonuclease inhibitor (red) and RNase A (blue). Ribbon diagrams were created with the programs MOLSCRIPT [76] and Raster3d [77] using coordinates derived from X-ray diffraction [25].

Figure 5

Putative cellular routing of cytotoxic pancreatic-type ribonucleases. (I) The ribonuclease first interacts with the surface of the target cell. Onconase and the Rana catesbeiana and Rana japonica ribonucleases appear to bind receptors on the plasma membrane. (II) The ribonuclease is internalized by endocytosis and crosses a lipid bilayer to reach the cytosol. The mechanism of bilayer transversal is unknown. (III) In the cytosol, ribonucleases encounter the ribonuclease inhibitor (RI) protein. Ribonucleases that evade RI catalyze cleavage of cellular RNA, which leads to cell death.