Background: The presence of p53 mutations and associated mutant p53 overexpression has been demonstrated in many cancer systems. Whether the overexpression of mutant p53 represents cause or effect, and whether p53 mutation contributes actively to the malignant phenotype is a matter of controversy. We examined the growth effects of oligonucleotides designed to interfere with p53 expression and/or activity in p53-mutant/overexpressing endometrial cancer cell lines.
Methods: Phosphorothioate oligonucleotides were used to target p53-related sequences in two p53-mutant/overexpressing endometrial cancer cell lines (KLE and RL95-2) and a normal fibroblast control. The ATP cell viability assay was used to measure growth effects after 6-day treatments with 27-mer and 14-mer sense (S) or antisense (AS) phosphorothioate oligodeoxyribonucleotides (oligos) targeting the promoter/ATG region of p53 and/or the p53 consensus (CON) DNA binding sequence. These sequences were designed to interfere with p53 expression and activity, respectively. Random sequences of the p53 27- and 14-mer were used as controls for nonspecific oligo effects, and a normal fibroblast cell line was used to compare oligo effects and serve as a negative p53 immunostaining control.
Results: Mean +/- SE IC50 (50% growth inhibition) of the S, AS p53, and p53 CON oligos were 4.2 +/- 1.3, 4.7 +/- 0.9, and 7.6 +/- 1.4 microM, respectively, for the two endometrial cell lines combined. The AS and S p53 oligos demonstrated dose-dependent inhibitory effects in both cell lines, while p53 CON produced variable effects alone and in combination with p53 AS. In KLE, a uniform inhibitory dose response was seen with p53 CON oligos. In RL95-2, the approximate IC50 for p53 CON was 0.5-1.0 microM, but at increasing doses above this, an inverse dose response was consistently observed. Combinations of p53 AS and p53 CON oligos produced predominantly synergistic growth inhibition. Although combinations of p53 AS and p53 CON in KLE were synergistic at low doses, antagonistic effects occurred at higher concentrations. Oligos had little effect on normal fibroblast growth, with calculated IC50 > 16 microM. Equimolar combinations of p53 S and AS were antagonistic, indicating that antiproliferative effects were sequence-specific. Random oligos demonstrated some nonspecific inhibitory effects, with >25% growth inhibition at 16 microM and beyond. Immunoperoxidase staining for mutant p53 after exposure to 16 microM concentrations of p53 AS oligos demonstrated reductions in p53 staining but persistent overexpression relative to wild-type (fibroblast) cells.
Conclusion: Phosphorothioate oligos directed against p53 sequences in two p53-mutant endometrial cancer cell lines demonstrated antiproliferative effects. Combined anti-p53 and anti-p53 binding site oligos resulted in predominantly synergistic antiproliferative effects. The activity of sense oligos, the variable responses to p53 CON, and the persistent overexpression of mutant p53 at high concentrations of growth-inhibiting anti-p53 oligos suggest that, while promising, the antineoplastic effects of these oligos occur through complex and incompletely understood mechanisms.