Modulating Survivin as a Radioresistant Factor, Caspase-3, and Apoptosis by Omega-3 Docosahexaenoic Acid Sensitizes Mutant-p53 Colorectal Cancer Cells to γ-Irradiation

Mutant-p53 colorectal cancer (CRC) cells are often resistant to radiotherapy. Loss of suppressor activity of wt-p53 on survivin is responsible for the enhanced expression of survivin as a radioresistant factor in tumor cells. Yet, no survivin-modulating drug has been approved for clinical application in CRC. Thus, the search for safe compounds that modulate survivin expression and induce apoptosis irrespective of p53 status may potentiate the efficacy of radiotherapy in mutant-p53 CRC cells. Omega-3 docosahexaenoic acid (DHA) induces apoptosis in malignant cells without cytotoxicity in normal cells. However, little is known whether in vitro concentrations of DHA equal to the human plasma levels are able to modulate expression of survivin and sensitize mutant-p53 CRC cells to gamma-irradiation. Radioresistant mutant-p53 HT-29 cells were pretreated with 50- and 100-mu M DHA for 48-h before 2-, 4-, 6-, 8-, and 10-Gy of gamma-irradiation. Thereafter, proliferation rates were measured after 6 d. HT-29 cells were also pretreated with 50- and 100-mu M DHA for 4-h before 2- and 10-Gy of gamma-irradiation after which, cell number, survivin expression, caspase-3 activation, apoptosis, and ED50 (gamma-irradiation dose causing 50% growth inhibition) were evaluated. Pretreatment of HT-29 cells with 50- and 100-mu M DHA for 48-h followed by 2- to 10-Gy of gamma-irradiation induced a dose-dependent additive decrease in cell proliferation rate and ED50 values were decreased by 88%, 44%, 41%, and 27% for 500-, 1500-, 2500-, and 5000 cells per well pretreated with 100-mu M DHA respectively. Pretreatment of 5 x 10(5) HT-29 cells per well with 100-mu M DHA for 4-h followed by 2- or 10-Gy of irradiation resulted in 53% and 86% decreases in cell numbers, 2- and 5.1-fold activation in caspase-3 followed by 66% and 60% decreases in survivin mRNA levels respectively. DHA in combination with radiation increased total apoptotic rate 48-h post-treatment. DHA decreases survivin expression and induces caspase-3 activation irrespective of p53 status. Significant decreases in ED50 values at concentrations of DHA equal to human plasma levels, suggesting that DHA could be used as an attractive radiosensitizer agent in CRC patients with mutant-p53.