Purpose: Flavopiridol, a small molecule pan-cyclin inhibitor, has been shown to enhance the radiation response of tumor cells both in vitro and in vivo. The clinical utility of flavopiridol, however, is limited by toxicity, previously attributed to pleiotropic inhibitory effects on several targets affecting multiple signal transduction pathways. Here we utilized zebrafish embryos to investigate radiosensitizing effects of flavopiridol in normal tissues.
Methods and Materials: Zebrafish embryos at the 1-4 cell stage were treated with 500 nM flavopiridol or injected with 0.5 pmol antisense hydroxylprolyl-phosphono nucleic acid oligomers to reduce cyclin D1 expression, then subjected to ionizing radiation (IR) or no radiation.
Results: Flavopiridol-treated embryos demonstrated a 2-fold increase in mortality following exposure to 40 Gy by 96 hours post fertilization (hpf) and developed distinct radiation-induced defects in midline development (curly-up phenotype) at higher rates when compared to embryos receiving IR only. Cyclin D1-deficient embryos had virtually identical IR sensitivity profiles when compared to embryos treated with flavopiridol. This was particularly evident for the IR-induced curly-up phenotype, which was greatly exacerbated by both flavopriridol and cyclin D1 downregulation.
Conclusions: Treatment of zebrafish embryos with flavopiridol enhanced radiation sensitivity of zebrafish embryos to a degree that was very similar to that associated with downregulation of cyclin D1 expression. These results are consistent with the hypothesis that inhibition of cyclin D1 is sufficient to account for the radiosensitizing action of flavopiridol in the zebrafish embryo vertebrate model
