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BACKGROUND: The MDM2 oncogene is amplified or overexpressed in many human cancers and MDM2 levels are associated with poor prognosis. MDM2 not only serves as a negative regulator of p53 but also has p53-independent activities. This study investigates the functions of the MDM2 oncogene in colon cancer growth and the potential value of MDM2 as a drug target for cancer therapy, by inhibiting MDM2 expression with an antisense anti-human-MDM2 oligonucleotide. MATERIALS AND METHODS: The selected antisense mixed-backbone oligonucleotide was evaluated for its in vitro and in vivo antitumor activity in human colon cancer models: LS174T cell line containing wild-type p53 and DLD-1 cell line containing mutant p53. The levels of MDM2, p53 and p21 proteins were quantified by Western blot analysis. RESULTS: In vitro antitumor activity was found in both cell lines, resulting from specific inhibition of MDM2 expression. In vivo antitumor activity of the oligonucleotide occurred in a dose-dependent manner in both models and synergistically or additive therapeutic effects of MDM2 inhibition and the cancer chemotherapeutic agents 10-hydroxycamptothecin and 5-fluorouracil were also observed. CONCLUSIONS: These results suggest that MDM2 have a role in tumor growth through both p53-dependent and p53- independent mechanisms. We speculate that MDM2 inhibitors have a broad spectrum of antitumor activities in human cancers regardless of p53 status. This study should provide a basis for future development of anti-MDM2 antisense oligonucleotides as cancer therapeutic agents used alone or in combination with conventional chemotherapeutics.