Mechanism of anti-tumor activity of adenovirus type 5 E1A in ovarian cancer

The adenovirus type 5 E1A gene was originally developed as a gene therapy to inhibit tumorigenicity of HER-2-overexpressing cells by transcriptional downregulation of HER-2. Our goal is to improve the overall efficacy of E1A gene therapy. To achieve this goal, we have conducted two preclinical experiments. ^ First, we hypothesized that Bcl-2 overexpressing ovarian cancer is resistant to E1A gene therapy. This hypothesis is based on that the 19 kDa protein product of the adenoviral E1B gene which is homologous to Bcl-2 inhibits E1A-induced apoptosis. Treating high Bcl-2-xpressing cells with E1A in combination with an antisense oligonucleotide to Bcl-2 (Bcl-2-ASO) resulted in a significant decrease in cell viability due to an increased rate of apoptosis relative to cells treated with E1A alone. In an ovarian cancer xenograft model, mice implanted with low HER-2, high Bcl-2 cells, treated with E1A plus Bcl-2-ASO led to prolonged survival. Bcl-2 thus may serve as a predictive molecular marker enabling us to select patients with ovarian cancer who will benefit significantly from E1A gene therapy. ^ Second, we elucidated the molecular mechanism governing the anti-tumor effect of E1A in ovarian cancer to identify a more potent tumor suppressor gene. We identified PEA-15 (phospho-protein enriched in astrocytes) upregulated in E1A transfected low HER-2-expressing OVCAR-3 ovarian cancer cell, which showed decreased cell proliferation. PEA-15 moved ERK from the nucleus to the cytoplasm and inhibited ERK-dependent transcription and proliferation. Using small interfering RNA to knock down PEA-15 expression in OVCAR-3 cells made to constitutively express E1A resulted in accumulation of phosphoERK in the nucleus, an increase in Elk-1 activity, DNA synthesis, and anchorage-independent growth. PEA-15 also independently suppressed colony formation in some breast and ovarian cancer cell lines in which E1A is known to have anti-tumor activity. We conclude that the anti-tumor activity of E1A depends on PEA-15. ^ In summary, (1) Bcl-2 may serve as a predictive molecular marker of E1A gene therapy, allowing us to select patients and improve efficacy of E1A gene therapy. (2) PEA-15 was identified as a component of the molecular mechanism governing the anti-tumor activity of E1A in ovarian cancer, (3) PEA-15 may be developed as a novel therapeutic gene. ^