The effect of canavanine on protein synthesis and protein degradation in IMR-90 fibroblasts

The Mechanism of l-Canavanine Cytotoxicity: Arginyl tRNA Synthetase as a Novel Target for Anticancer Drug Discovery

There is a clear need for agents with novel mechanisms of action to provide new therapeutic approaches for the treatment of pancreatic cancer. Owing to its structural similarity to L-arginine, L-canavanine, the beta-oxa-analog of L-arginine, is a substrate for arginyl tRNA synthetase and is incorporated into nascent proteins in place of L-arginine. Although L-arginine and L-canavanine are structurally similar, the oxyguanidino group of L-canavanine is significantly less basic than the guanidino group of L-arginine. Consequently, L-canavanyl proteins lack the capacity to form crucial ionic interactions, resulting in altered protein structure and function, which leads to cellular death. Since L-canavanine is selectively sequestered by the pancreas, it may be especially useful as an adjuvant therapy in the treatment of pancreatic cancer. This novel mechanism of cytotoxicity forms the basis for the anticancer activity of L-canavanine and thus, arginyl tRNA synthetase may represent a novel target for the development of such therapeutic agents.

Growth inhibition of A549 human lung adenocarcinoma cells by L-canavanine is associated with p21/WAF1 induction

L-Canavanine (CAV) is a higher plant nonprotein amino acid and a potent L-arginine antimetabolite. CAV can inhibit the proliferation of tumor cells in vitro and in vivo, but little is known regarding the molecular mechanisms mediating these effects. We demonstrated that the treatment of human lung adenocarcinoma A549 cells with CAV caused growth inhibition; G1 phase arrest is accompanied by accumulation of an incompletely phosphorylated form of the retinoblastoma protein, whose phosphorylation is necessary for cell cycle progression from G1 to S phase. In addition, CAV induces the expression of p53 and subsequent expression of a cyclin-dependent kinase inhibitor, p21/WAF1. The p53-dependent induction of p21/WAF1 and the following dephosphorylation of the retinoblastoma protein by CAV could account for the observed CAV-mediated G1 phase arrest.

Turnover of canavanine-containing proteins in Saccharomyces cerevisiae

Saccharomyces cerevisiae grown for 2 h in the presence of 0.5 mmol/L canavanine in a synthetic medium with ethanol as the sole carbon source (OEC) exhibited a slowing down of protein synthesis for 3-4 h after a shift to fresh ethanol-based medium containing 1.0 mmol/L arginine (OEA) in comparison with untreated cells grown on OEA. The change of carbon source from ethanol to glucose (OGA) after growth in the OEC medium resulted in an even deeper decline of protein synthesis. The degradation of canavanine-containing proteins in cells pregrown and labelled in an OEC medium after transfer to OEA was more rapid than in the OGA medium. The initial rate of protein degradation during the first hour in the OGA medium was less than 1%/h whereas in the OEA medium it reached almost 10%/h. The fraction of proteins with high turnover (half-life 0.46 h) constituted 8.3% on OEA, while during subsequent growth on OGA it was only 0.75% with a half-life of 0.12 h.

L-Canavanine and protein synthesis in the tobacco hornworm Manduca sexta

L-Canavanine, a nonprotein amino acid of certain leguminous plants, manifests potent insecticidal properties in a canavanine-sensitive insect such as the tobacco hornworm Manduca sexta (L.) (Sphingidae). This arginine analog is activated and aminoacylated by arginyl-tRNA synthetase and incorporated into nascent polypeptide chains to create structurally aberrant, canavanine-containing proteins. Analysis of incorporation of [3H]leucine into protein in M. sexta larvae that had been injected with canavanine revealed that this arginine analog stimulates protein synthesis. During the first 3 hr after injection of canavanine, canavanine-mediated net stimulation of protein formation was readily discerned. Thereafter, the stimulation of protein synthesis appeared to be offset by the preferential degradation of anomalous proteins. Double-label protein-turnover experiments with larvae injected with [14C]canavanine- and [3H]arginine-containing hemolymph proteins showed that canavanine-containing proteins were degraded preferentially.

Effects of canavanine on the secretion of plasma proteins by Hep G2 cells

Many secretory proteins contain an amino-terminal propeptide extension which is removed prior to secretion. The point of cleavage is usually marked by a basic pair of amino acids containing arginine. Canavanine, an analogue of arginine, is incorporated into protein and has been shown to inhibit the proteolytic processing of several of these prosecretory proteins. The addition of 3 mM canavanine to Hep G2 cells incubated with L-[35S]methionine inhibited the secretion of 11 plasma proteins studied. Of the secretory proteins studied only albumin is thought to contain a propeptide, which is marked by a pair of arginine residues at its point of proteolytic processing. Canavanine had varying effects on the secretion of plasma proteins; ranging from a 43-53% inhibition of secretion of alpha 1 antitrypsin and alpha 1 anti-chrymotrypsin to nearly abolishing (93% inhibition) secretion of transferrin. Canavanine also caused most of the proteins studied to migrate slower on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Two of the canavanine-treated proteins (albumin and transferrin) which underwent marked changes in electrophoretic mobility were more sensitive than untreated proteins to proteolysis by Staphylococcus Aureus V8 proteinase. The slower electrophoretic migration and the greater sensitivity to proteolysis of these proteins may be attributed to marked structural changes caused by the incorporation of canavanine. This suggests that the inhibition of plasma protein secretion by canavanine is not only due to an inhibition of the processing of proteins but may be caused by structural distortions of the secretory proteins.