Fisetin induces DNA double-strand break and interferes with the repair of radiation-induced damage to radiosensitize triple negative breast cancer cells.

backgroundTriple-negative breast cancer (TNBC) is associated with aggressiveness and a poor prognosis. Besides surgery, radiotherapy serves as the major treatment modality for TNBC. However, response to radiotherapy is limited in many patients, most likely because of DNA damage response (DDR) signaling mediated radioresistance. Y-box binding protein-1 (YB-1) is a multifunctional protein that regulates the cancer hallmarks among them resisting to radiotherapy-induced cell death. Fisetin, is a plant flavonol of the flavonoid family of plant polyphenols that has anticancer properties, partially through inhibition of p90 ribosomal S6 kinase (RSK)-mediated YB-1 phosphorylation. The combination of fisetin with radiotherapy has not yet been investigated.

methodsActivation status of the RSK signaling pathway in total cell lysate and in the subcellular fractions was analyzed by Western blotting. Standard clonogenic assay was applied to test post-irradiation cell survival. γH2AX foci assay and 3 color fluorescence in situ hybridization analyses were performed to study frequency of double-strand breaks (DSB) and chromosomal aberrations, respectively. The underlying repair pathways targeted by fisetin were studied in cells expressing genomically integrated reporter constructs for the DSB repair pathways via quantifying the expression of green fluorescence protein by flow cytometry. Flow cytometric quantification of sub-G1 cells and the protein expression of LC3-II were employed to measure apoptosis and autophagy, respectively. Kinase array and phosphoproteomics were performed to study the effect of fisetin on DDR response signaling.

resultsWe showed that the effect of fisetin on YB-1 phosphorylation in TNBC cells is comparable to the effect of the RSK pharmacological inhibitors. Similar to ionizing radiation (IR), fisetin induces DSB. Additionally, fisetin impairs repair of IR-induced DSB through suppressing the classical non-homologous end-joining and homologous recombination repair pathways, leading to chromosomal aberration as tested by metaphase analysis. Effect of fisetin on DSB repair was partially dependent on YB-1 expression. Phosphoproteomic analysis revealed that fisetin inhibits DDR signaling, which leads to radiosensitization in TNBC cells, as shown in combination with single dose or fractionated doses irradiation.

conclusionFisetin acts as a DSB-inducing agent and simultaneously inhibits repair of IR-induced DSB. Thus, fisetin may serve as an effective therapeutic strategy to improve TNBC radiotherapy outcome.