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Cytotoxic potential of ribonuclease and ribonuclease hybrid proteins
S M Rybak1, S K Saxena, E J Ackerman
1Biochemistry Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20896.
Abstract:
Pancreatic RNase injected into Xenopus oocytes abolishes protein synthesis at concentrations comparable to the toxin ricin yet has no effect on oocyte protein synthesis when added to the extracellular medium. Therefore RNase behaves like a potent toxin when directed into a cell. To explore the cytotoxic potential of RNase toward mammalian cells, bovine pancreatic ribonuclease A was coupled via a disulfide bond to human transferrin or antibodies to the transferrin receptor. The RNase hybrid proteins were cytotoxic to K562 human erythroleukemia cells in vitro with an IC50 around 10(-7) M whereas greater than 10(-5) M native RNase was required to inhibit protein synthesis. Cytotoxicity requires both components of the conjugate since excess transferrin or ribonuclease inhibitors added to the medium protected the cells from the transferrin-RNase toxicity. Compounds that interfere with transferrin receptor cycling and compartmentalization such as ammonium chloride decreased the cytotoxicity of transferrin-RNase. After a dose-dependent lag period inactivation of protein synthesis by transferrin-RNase followed a first-order decay constant. In a clonogenic assay that measures the extent of cell death 1 x 10(-6) M transferrin-RNase killed at least 4 logs or 99.99% of the cells whereas 70 x 10(-6) M RNase was nontoxic. These results show that RNase coupled to a ligand can be cytotoxic. Human ribonucleases coupled to antibodies also may exhibit receptor-mediated toxicities providing a new approach to selective cell killing possibly with less systemic toxicity and importantly less immunogenicity than the currently employed ligand-toxin conjugates.
Insights
Ribonuclease (RNase) coupled to cell-targeting ligands becomes a potent toxin, enabling selective cancer cell killing. This approach offers a promising alternative to traditional ligand-toxin conjugates for targeted therapies.
Area of Science:
- Biochemistry
- Molecular Biology
- Oncology
Background:
- Pancreatic ribonuclease (RNase) is not toxic extracellularly but acts as a potent toxin when internalized by cells.
- Targeted delivery of cytotoxic agents is crucial for effective cancer therapy.
- Ligand-toxin conjugates are used for selective cell killing, but can have systemic toxicity and immunogenicity.
Purpose of the Study:
- To explore the cytotoxic potential of RNase toward mammalian cells by conjugating it to targeting ligands.
- To evaluate the efficacy of RNase-ligand conjugates in vitro for potential cancer treatment.
Main Methods:
- Bovine pancreatic ribonuclease A was chemically coupled to human transferrin or antibodies targeting the transferrin receptor.
- Cytotoxicity assays were performed on K562 human erythroleukemia cells.
- Inhibitors and compounds affecting receptor cycling were used to assess conjugate requirements.
- Clonogenic assays measured cell death after treatment.
Main Results:
- RNase-transferrin conjugates demonstrated significant cytotoxicity against K562 cells (IC50 ~10(-7) M), unlike native RNase.
- Cytotoxicity was dependent on both RNase and transferrin components, and sensitive to transferrin receptor pathway modulation.
- Clonogenic assays showed 10(-6) M transferrin-RNase killed >99.99% of cells, while high concentrations of native RNase were ineffective.
Conclusions:
- RNase can be rendered cytotoxic to mammalian cells when conjugated to a specific ligand.
- Receptor-mediated delivery of RNase via antibody conjugates presents a novel strategy for selective cancer cell killing.
- This approach may reduce systemic toxicity and immunogenicity compared to current ligand-toxin conjugates.