Pancreatic cancer is an extremely aggressive
neoplasm whose incidence equals its death rate. Despite intensive analysis, the genetic changes that mediate
pancreatic cancer development and effective
therapies for diminishing the morbidity associated with this disease remain unresolved. Through subtraction hybridization, we have identified a gene associated with induction of irreversible growth arrest,
cancer reversion, and terminal differentiation in human
melanoma cells,
melanoma differentiation associated gene-7 (mda-7). Ectopic expression of mda-7 when using a recombinant adenovirus, Ad.mda-7, results in growth suppression and apoptosis in a broad spectrum of human
cancers with diverse genetic defects, without exerting deleterious effects in normal human epithelial or fibroblast cells. Despite the apparently ubiquitous antitumor effects of mda-7,
pancreatic carcinoma cells are remarkably refractory to Ad.mda-7 induced growth suppression and apoptosis. In contrast, the combination of Ad.mda-7 with antisense
phosphorothioate oligonucleotides, which target the K-ras oncogene (a gene that is mutated in 85 to 95% of
pancreatic carcinomas), induces a dramatic suppression in growth and a decrease in cell viability by induction of apoptosis. In mutant K-ras
pancreatic carcinoma cells, programmed cell death correlates with expression and an increase, respectively, in MDA-7 and BAX
proteins and increases in the ratio of BAX to BCL-2
proteins. Moreover, transfection of mutant K-ras
pancreatic carcinoma cells with an antisense K-ras expression vector and
infection with Ad.mda-7 inhibits colony formation in vitro and
tumorigenesis in vivo in nude mice. These intriguing observations demonstrate that a combinatorial approach, consisting of a
cancer-specific apoptosis-inducing gene and an oncogene inactivation strategy, may provide the foundation for developing an effective
therapy for
pancreatic cancer.