Inhibiting p90 ribosomal S6 kinase modulates Y-box binding factor-1 transcription factor ability. Inhibiting p90 ribosomal S6 kinase (RSK) modulates the ability of Y-box binding factor-1 (YB-1) to act as a transcription factor for epidermal growth factor receptor (EGFR). (a) EGFR promoter activity in SUM149 cells was reduced by 80% following knockdown of YB-1 or treatment with PD098059 (***P < 0.001) and by 30% (*P = 0.02) following treatment with SL0101 (50 μM). (b) Binding of P-YB-1S102 to the EGFR promoter is reduced in the SUM149 cells following treatment with PD098059 (lane 4 compared with lane 3 (vehicle)) or SL0101 (50 μM) (lane 11 compared with lane 10 (vehicle)). IgG immunoprecipitation acts as a negative control. Input samples show amplification of the region in the cross-linked cells prior to immunoprecipitation (n = 2). DMSO, dimethylsulfoxide. (c) Transfection with RSK2 siRNA for 72 hours led to a decrease in EGFR expression in SUM149 cells. (d) Treatment of immortalized breast mammary epithelial cells (HTRY) (10 hours) or MDA-MB-231 cancer cells (12 hours) with SL0101 results in loss of P-YB-1S102 and a concomitant reduction in EGFR. (e) Model demonstrating the positive feedback loop generated on the activation of YB-1 by EGFR. Ligand binding to the receptor activates signaling pathways such as MAP kinase, resulting in the phosphorylation of RSK. Once the kinase is fully activated, it phosphorylates YB-1 at S102 – subsequently allowing YB-1 to play a role in promoting translation and to enter the nucleus as a transcription factor. AKT and PKCα can also activate YB-1 following growth factor stimulation. On binding to inverse CAAT boxes, YB-1 promotes the transcription of genes such as EGFR – resulting in increased surface expression of the receptor. Ctrl, control.