Y-Box Binding Protein-1 as a Potential Target to Improve Radiotherapy Outcomes

Abstract

KRAS is frequently mutated in almost 30% of all cancers and these mutations are known to be strongly associated with chemoradiotherapy resistance (CRT). Y-box bind ing protein-1 (YB-1) is an oncoprotein that is overexpressed across different cancer entities and plays a significant role in cancer progression by being involved in almost all cancer hallmarks, in particular in resistance to radiotherapy-induced cell death. Phos phorylation of YB-1 at S102 is associated with poor prognosis and this phosphorylation and activation is mainly regulated by the MAPK/ p90 ribosomal S6 kinase (RSK) sig naling pathway in KRAS mutated cells. YB-1 has been shown to facilitate the repair of IR-induced double-strand breaks (DSBs) in breast cancer cells. However, due to the lack of kinase activity in YB-1 itself, designing a direct inhibitor is challenging. Therefore, one possible approach is to target YB-1 via upstream kinases that regulate YB-1. Data from our laboratory has shown that while inhibition of RSK leads to activation of the PI3K/AKT pathway, potentially diminishing the benefits of RSK targeting, dual targeting both RSK and AKT has proven to be the most effective strategy in breast cancer cells. Fisetin, a natural flavonoid, has been known to interfere with the RSK-mediated YB-1 phosphorylation at S102 in melanoma. In our research, we demonstrated that fisetin, regardless of KRAS mutation status, blocks YB-1 phosphorylation at S102 in triple negative breast cancer (TNBC) and colorectal cancer (CRC) cells, as well as in CRC tissue samples, ex vivo. Given to the role of YB-1 in stimulating the repair of ionizing irradiation (IR)-induced DSBs, fisetin was found to inhibit DSB repair partially through its inhibitory effect on YB-1. Phosphoproteomics analysis also revealed that fisetin targets multiple DNA damage response (DDR)-related phosphosites in irradiated cells, particularly those involved in DNA repair, replication, and chromatin binding, leading to inhibition of DSB repair predominantly via homologous recombination (HR) and classical non-homologous end joining pathways (C-NHEJ). These effects were associated with the radiosensitization effect of fisetin in TNBC and CRC cells, while having no effect on human skin fibroblast, suggesting the potential benefit in combination with radiotherapy. Since RSK-mediated YB-1 phosphorylation and activity play a key role in stimulating the repair of IR-induced DSB in TNBC and CRC cells, the combination of fisetin with radiation therapy could significantly enhance radiation response, regardless of KRAS mutation status.