Pharmacological inhibition of p90 ribosomal S6 kinase decreases Y-box binding factor-1 phosphorylation. (a) Inhibition of p90 ribosomal S6 kinases RSK1/RSK2 with SL0101 (50 μM) in SUM149 cells resulted in decreased growth-factor induced P-YB-1S102 over a time course of 6 to 16 hours. Immunoblot with densitometric analysis below. All changes are statistically significant (P < 0.01). (b) After 6 hours of treatment with SL0101 (100 μM), P-YB-1S102 was decreased in HCC1937 and MDA-MB-231 cells while Y-box binding factor-1 (YB-1) remained constant. (c) Treatment of HTRY cells with SL0101 (100 μM) decreased P-YB-1S102 in a dose-dependent manner. (d) Treatment of SUM149 cells with SL0101 (50 μM) reverses the phosphorylation of YB-1 induced by stimulation with growth factors. SL0101 has no effect on total YB-1. Vinculin was used as a loading control (n = 3). SS, stimulation of SUM149 cells; EGF, epidermal growth factor; PMA, phorbal 12-myristate 13-acetate.