Possible immunotherapeutic potentiation with D-Fraction in prostate cancer cells

BACKGROUND: Prostate cancer remains the most common malignancy among elderly men and the second leading cause of cancer death in the United States. Although several conventional therapies are currently available, they have a low efficacy and the more effective treatment modalities need to be established. Interferons (IFNs) are one of such options known as immunotherapy and demonstrated their antitumor effects on certain cancer types. Yet such antitumor activity should be improved or potentiated to have the satisfactory outcomes. In fact, combination therapy has been proposed as an alternative approach and is being underway in human and animal studies. Accordingly, we studied whether the combination of IFN-α and D-fraction (PDF), a bioactive mushroom extract, might potentiate anticancer activity of IFN-α in prostate cancer PC-3 cells in vitro. RESULTS: Potential effects of recombinant IFN-α(2b )(0–100,000 IU/ml), PDF (0–1,000 μg/ml), or their combinations were assessed on the growth of PC-3 cells at 72 h. Cell cycle analysis using a flow cytometer and Western blot analysis were performed to explore antiproliferative mechanism of these agents. The dose-dependent study showed that IFN-α(2b )up to 20,000 (20 K) IU/ml had no significant effects, but >60% growth reduction was attained ≤50 K IU/ml. Similarly, PDF showed no effects up to 250 μg/ml but ~65% growth reduction was seen at 1,000 μg/ml. When IFN-α(2b )and PDF were combined, a relatively low concentration (10 K IU/ml) of IFN-α(2b )and PDF (250 μg/ml) resulted in a ~65% growth reduction. This was accompanied by a G(1 )cell cycle arrest, indicated by cell cycle analysis. Western blots also revealed that the G(1)-specific cell cycle regulators, CDK2, CDK4, CDK6, cyclin D(1), and cyclin E, had been significantly (>60%) down-regulated in IFN/PDF-treated cells. CONCLUSION: The combination of IFN-α(2b )(10 K IU/ml) and PDF (250 μg/ml) is capable of inducing a ~65% reduction in PC-3 cell growth. This appears to be due to a synergistic potentiation of two agents, leading to a G(1 )cell cycle arrest. Thus, it is conceivable that PDF may potentiate IFN-α(2b )activity, improving immunotherapy for prostate cancer.