Amphiregulin (AR) is a secreted heparin-binding growth factor that is structurally and functionally related to epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha). GEO cells are from a human colon cancer cell line that expresses high levels of AR protein and mRNA. To assess the role of AR in colon-cancer cell proliferation and transformation, 2 different anti-sense 20-mer phosphorothioate oligodeoxynucleotides (AR AS-1 and AR AS-2 S-oligos) complementary to the 5' sequence of AR mRNA were synthesized. Both AR AS S-oligos were able to inhibit the anchorage-dependent growth (ADG) of GEO cells. The 2 AR AS S-oligos were equipotent when used in equimolar concentrations. In particular, a 40% growth inhibition was observed at a concentration of 10 microM, while a mis-sense S-oligo used as control had no effect on GEO cell growth. The AR AS-1 S-oligo used at the same concentration also inhibited by 40% the 3H-thymidine incorporation by DNA of GEO cells. The anchorage-independent growth (AIG) of GEO cells was even more significantly affected by AR AS S-oligo treatment. In fact, up to 80% inhibition of the AIG of GEO cells was observed when cells were treated with 10 microM of both AR AS S-oligos. Finally, the AR AS S-oligos were able to specifically inhibit AR protein expression in GEO cells, as assessed by immunocytochemistry. These data suggest that AR is involved in colon-cancer cell transformation, and that AR may represent a suitable target for gene therapy in human colon carcinomas.