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Antisense to the Early Growth Response‐1 Gene (Egr‐1) Inhibits Prostate Tumor Development in TRAMP Mice

Abstract: Egr‐1 is a transcription factor induced by stress or injury, mitogens, and differentiation factors. Egr‐1 regulates the expression of genes involved in growth control or survival. Expression of Egr‐1 results in either promotion or regression of cell proliferation, depending on cell type and environment. Egr‐1 acts as a tumor suppressor in many cell types and loss of Egr‐1 has been proposed to contribute to cancer progression. There is strong new evidence however suggesting that Egr‐1 overexpression is involved in prostate cancer progression. For example, Egr‐1 expression levels are elevated in human prostate carcinomas in proportion to grade and stage. Furthermore, prostate cancer progression was significantly delayed in two models of prostate cancer mice lacking Egr‐1. Our objective in the present study is to test whether inhibition of Egr‐1 function would block cell proliferation and inhibit the transformed phenotype of prostate cancer cells in vitro and in vivo. We describe the development of high affinity and high specificity antisense oligonucleotides that efficiently inhibit Egr‐1 expression. We show that inhibition of Egr‐1 expression in mouse or human prostate cancer cells decreased proliferation and reduced the capacity of these cells to form colonies and to grow in soft agar. Conversely, stable expression of Egr‐1 in normal human prostate epithelial 267B1 cells promoted transformation. In TRAMP mice, treatment with Egr‐1 antisense oligonucleotides delayed the occurrence of prostate tumors. Importantly, Egr‐1 antisense showed little or no toxicity when injected into animals. Finally, we identified a few genes such as cyclin D2, p19ink4d, and Fas that are directly regulated by Egr‐1 in prostate cancer cells and that control cell cycle and survival.