Background RNA interference has the potential to be more selective than small molecule inhibitors and can be used to target proteins, such as Ras, that are currently undruggable. The purpose of our study was to determine the optimal cotargeting strategy of the commonly mutated PI3K/AKT/mTOR and Ras pathways by a selective RNA interference approach in colorectal cancer cell lines possessing coexistent PIK3CA and KRAS mutations. Methods Human colorectal cancer cell lines HCT116 and DLD-1 were treated with a panel of small interfering RNAs directed against the PI3K/AKT/mTOR and Ras pathways; proliferation, apoptosis, and protein expression were assessed. Combined treatment with small interfering RNA and 5-fluorouracil was then evaluated. Results PIK3CA and KRAS small interfering RNAs were most effective as single treatments; combined treatments with PIK3CA and KRAS small interfering RNA resulted in a more pronounced inhibition of colorectal cancer cell proliferation. Either KRAS small interfering RNA alone or combined PIK3CA and KRAS small interfering RNA treatments increased apoptosis in HCT116 cells but not in the DLD-1 cell line. Inhibition of 4E-BP1 phosphorylation correlated with increased apoptosis. In addition, small interfering RNA treatment combined with 5-fluorouracil further inhibited colorectal cancer cell proliferation. Conclusion Combined PIK3CA and KRAS small interfering RNA treatments offer an effective therapy against colorectal cancer cells with coexisting mutations in both pathways. Decreased 4E-BP1 phosphorylation correlates with increased apoptosis and may provide a biomarker indicative of treatment success. In addition, small interfering RNA directed to PIK3CA and KRAS may be used to enhance the effects of current chemotherapy.
