The mitogenic response of T cells to interleukin‐2 requires Raf‐1

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Abstract
The product of the c‐raf‐1 proto‐oncogene, Raf‐1, is known to encode a 74‐kDa ubiquitously expressed cytoplasmic serine/threonine kinase. Various growth factors such as epidermal growth factor, acidic fibroblast growth factor, platelet‐derived growth factor, insulin, granulocyte‐macrophage colony‐stimulating factor, interleukin (IL)‐2, IL‐3 and erythropoietin have been shown to induce phosphorylation of Raf‐1, thereby activating Raf‐1 kinase. Raf‐1 is, thus, believed to play a role in coupling growth factor receptors to proliferation. We have examined the role of Raf‐1 in the mitogenic response of human peripheral blood‐derived IL‐2 receptor expressing T cells to human recombinant IL‐2 employing c‐raf antisense (AS) oligodeoxyribonucleotide. Uptake studies of oligonucleotides indicated that incorporation of oligomers was maximal at 4 h and oligdeoxynucleotides remained stable in these cells for up to 24 h. Treatment of T cells with the AS oligodeoxyribonucleotide in intracellular duplex formation followed by efficient translation blockade of c‐raf‐1. In contrast, sense (S) and nonsense (NS) oligodeoxynucleotides failed to form intracellular duplexes and did not interfere with translation of c‐raf‐1, suggesting specific elimination of c‐raf‐1 by the AS oligomer. Proliferation of T cells ([3H]thymidine incorporation) following exposure to IL‐2 was substantially reduced when the c‐raf‐1 AS oligodeoxyribonucleotide was added to cultures, while the mitogenic response to this factor remained almost unaffected in the presence of S and NS oligodeoxy‐ribonucleotides.