Additive effect of adenovirus-mediated E2F-1 gene transfer and topoisomerase II inhibitors on apoptosis in human osteosarcoma cells

Recently, it has been demonstrated that Etoposide, a topoisomerase II inhib itor, can induce apoptosis in MDM2- overexpressing tumor cells by inhibitio n of MDM2 synthesis. We have previously shown that E2F-1 overexpression ind uces apoptosis of MDM2- overexpressing sarcoma cells, which is related to t he inhibition of MDM2 expression. Therefore, the present study was designed to investigate the in vitro and in vivo effect of combined treatment of ad enovirus - mediated E2F - 1 and topoisomerase II inhibiters on the growth i nhibition and apoptosis in human sarcoma cells. Two human sarcoma cell line s, OsACL and U20S, were treated with topoisomerase II inhibitors (Etoposide and Adriamycin), alone or in combination with adenoviral vectors expressin g galactosidase (Ad- LacZ) or E2F-1 (Ad- E2F-1). E2F-1 expression was confi rmed by Western blot analysis. Ad- E2F-1 gene transfer at a low dose (multi plicity of infection, 2) markedly increased the sensitivity of human sarcom a cells to topoisomerase II inhibitor treatment. This cooperative effect of E2F-1 and topoisomerase II inhibitors was less marked in SAOS-2 cells (p53 and pRb null). Topoisomerase II inhibitors also cooperated with E2F-1 over expression to enhance tumor cell killing in an in vivo model using xenograf ts in nude mice. When combined with Adriamycin or Etoposide, E2F-1 adenovir us therapy resulted in approximately 95% and 85% decrease in tumor size, re spectively, compared to controls ( P<.05). These results suggest a new chem osensitization strategy that is effective in MDM2-overexpressing tumors and may have clinical utility.