Vitamin D receptor and paclitaxel in triple negative breast cancer: is there a link bethween them?

Abstract

Paclitaxel (PTX) is an antitumor agent employed in the treatment of Triple-Negative Breast Cancer (TNBC). TNBC expresses Vitamin D Receptor (VDR), a member of the nuclear receptor superfamily. The aim of this work was to investigate the involvement of VDR in the antitumor action of PTX in TNBC cells. To this end, viability assays by crystal violet staining were performed in murine 4T1 TNBC cells and in 4T1 stably expressing a shRNA against VDR (4T1 shVDR), treated with PTX (10 nM) or vehicle. Also, cell cycle was studied by flow cytometry. Cellular studies were complemented with in silico analyses including molecular docking and molecular dynamics (MD) simulations to describe the pharmacodynamic interaction between PTX and VDR. The results show that PTX reduced the viability of 4T1 wild type cells (p<0.001). These viability effects were lost in 4T1 shVDR cells which display approximately 53% of VDR levels with respect to control cells. Cell cycle analysis of 4T1 wild type and 4T1 shVDR cells treated with PTX showed that the chemotherapy causes an increase in the percentage of cells in sub G0/G1 phase compared to vehicle-treated cells. However, this PTX effect was significantly higher in wild type than in VDR-silenced cells (13.72 ± 2.37% vs 6.18 ± 1.07%, p<0.001). Docking and MD studies showed that PTX was not able to bind to the classical ligand-binding pocket of VDR. However, an exhaustive search of allosteric sites identified its stable binding to a cavity adjacent to the activating factor 2 (AF- 2) region. MD studies verified a conformational restraint on AF-2, which triggers transcriptional and antitumor effects. Furthermore, a potential cooperativity in the interaction with VDR between PTX and the natural ligand of the receptor was observed. Altogether, these results suggest that PTX could interact with VDR to display its anti- tumor effects in TNBC by its binding in an alternative site to that of the classical VDR agonists