A nonantisense sequence-selective effect of a phosphorothioate oligodeoxynucleotide directed against the epidermal growth factor receptor in A431 cells

The overexpression of epidermal growth factor receptor (EGFr) has been implicated as a causative factor and a poor prognostic marker in a number of carcinomas. Therefore, strategies that down-regulate EGFr expression may be therapeutically useful. We designed antisense ODNs complementary to the initiation codon region of the EGFr mRNA and evaluated their efficacy in several tumor-derived cells, including the A431 cell line that express amplified levels of EGFr. A 15-mer phosphorothioate (PS) antisense ODN (erbB1AS15) induced a concentration-dependent reduction in proliferation that was accompanied by a change in the morphology of A431 cells into more tightly clustered and discrete colonies. A 15-mer sense (PS) control oligodeoxynucleotide (ODN) and a phosphodiester (PO) version of erbB1AS15 had little or no effect on cell number of morphology, and erbB1AS15 (PS) did not induce these effects in control cell lines expressing lower levels of EGFr. The effects of erbB1AS15 (PS) on A431 cells were not mediated by a true antisense mechanism in that there was no reduction in the level of EGFr mRNA or protein over a 24-hr period, as determined by Northern and Western blotting, respectively. However, autophosphorylation of the receptor was significantly reduced by erbB1AS15 (PS) and not by control ODNs. The results of further studies suggested that this effect was mediated by a direct, dose-dependent inhibition of the EGFr tyrosine kinase enzyme and was not due to impairment of either ligand-binding or receptor dimerization. These data suggest that erbB1AS15 (PS) can inhibit proliferation and alter the morphology of A431 cells by a sequence-selective, but nonantisense mechanism affecting receptor tyrosine kinase activity.