Malignant melanoma is notoriously difficult to treat due to lack of targeted systemic therapies and emergence of resistance to BRAF inhibitors, currently the treatment of choice for metastatic melanoma. The p90 ribosomal S6 kinase (RSK), an important effector of RAF/ERK signaling promotes melanoma growth and contributes to melanoma chemoresistance by altering response to chemotherapeutic agents. Y-box binding protein-1 (YB-1) is an oncogenic transcription/translation factor involved in multiple cellular functions including proliferation, migration, and epithelial mesenchymal transition (EMT). Recent studies show that RSK1/2 can activate YB-1 through phosphorylation at its Ser102 residue. We found YB-1 expression to be significantly upregulated in metastatic melanoma cell lines when compared to normal human melanocytes. Treatment of A375 human melanoma cells with the dietary flavonoid fisetin (20-60µM) resulted in inhibition of phosphorylation of YB-1 at Ser102, with subsequent decrease in nuclear translocation and activation. We further studied the effect of fisetin on YB-1 signaling in a 3-D melanoma skin equivalent model, comprising of A375 melanoma cells, cultured with epidermal keratinocytes and dermal fibroblasts that closely simulates the human skin. Studies in the 3-D model showed that fisetin treatment resulted in reduction in melanocytic lesions and tumor progression (p< .01) associated with decreased expression of YB-1, and its downstream targets c-Myc and MTA-1. Fisetin suppressed the expression of EMT markers N-cadherin and Vimentin, with a concomitant increase in E-cadherin in melanoma cells. In addition, a significant decrease in the protein expression of matrix-metalloproteinases -2 and -9 was observed. Next we investigated whether fisetin interfered with RSK signaling involved in YB-1 activation. Immunoblot studies revealed that fisetin inhibited the phosphorylation of p90RSK at the Thr359/Ser363 residue but not at Ser380. Notably, the MDR-1 gene product, P-glycoprotein (P-gp), was markedly decreased in fisetin-treated cells in a dose-dependent manner. Fisetin-mediated increase in the phosphorylation of ERK1/2 at Thr202/Tyr204 residues suggests that diminished P-gp expression is linked to suppression of RSK/YB-1 signaling in fisetin treated cells. Together, our results suggest that fisetin is a potent inhibitor of the p90RSK/YB-1 pathway and may be used to modulate chemosensitivity, which is one of the major obstacles to melanoma treatment.

Citation Format: Deeba N. Syed, Rahul K. Lall, Mohammad Imran Khan, Maria Shabbir, Hasan Mukhtar. Fisetin inhibits p90RSK/YB-1 signaling and downregulates chemoresistance associated P-glycoprotein in A375 melanoma cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3125. doi:10.1158/1538-7445.AM2014-3125

©2014 American Association for Cancer Research.
2014