Background: Specific inhibition of target proteins by antisense oligodeoxynucleotides is an extensively studied experimental approach. This technique is currently being tested in clinical trials applying phosphorothioate-modified oligonucleotides as therapeutic agents. These polyanionic molecules, however, may also exert non-antisense-mediated effects.
Materials and methods: We examined the influence of oligonucleotides on lipopolysaccharide (LPS)-stimulated tumor necrosis factor alpha (TNF alpha) synthesis in freshly isolated human peripheral blood mononuclear cells. Oligonucleotides (18 mer) with different degrees of phosphorothioate modification were studied.
Results: The addition of phosphorothioate oligonucleotides (5 microM) caused amplification of TNF synthesis of up to 410% compared with the control with LPS alone. Without LPS stimulation, phosphorothioate oligonucleotides did not induce TNF production. We demonstrate that the enhancement of LPS-stimulated TNF production by phosphorothioate oligonucleotides does not rely on the intracellular presence of oligonucleotides and is not mediated by LPS contamination. Partially phosphorothioate-modified oligonucleotides and unmodified oligonucleotides did not increase TNF synthesis. High concentrations of the polyanion heparin reversed the oligonucleotide-induced enhancement of TNF synthesis.
Conclusions: The data suggest that amplification of TNF synthesis may be caused by binding of the polyanionic phosphorothioate oligonucleotide to cationic sites on the cell surface. Such binding sites have been proposed for polyanionic glycoaminoglycans of the extracellular matrix, which have also been described to augment LPS-stimulated TNF synthesis. The present results are relevant to all in vitro studies attempting to influence protein synthesis in monocytes by using phosphorothioate oligonucleotides. The significance of our findings for in vivo applications of phosphorothioates in situations where there is a stimulus for TNF synthesis, such as in sepsis, should be elucidated.