Y-box binding protein-1 (YB-1) serine 102 is a downstream target of p90 ribosomal S6 kinase in breast cancer cells
Authors: Anna Stratford, Isabelle Berquin, Philippe Roux, Sandra Dunn
Journal: Clinical Cancer Research (2008)

AACR Centennial Conference: Translational Cancer Medicine-- July 20-23, 2008; Monterey, CA

A20 

Unlike estrogen receptor positive or HER2 over-expressing breast cancers the treatment of the aggressive basal-like breast cancer (BLBC) is currently still dependent on the use of chemotherapeutic agents. These have limited benefit in many patients and survival beyond 2 years is low. Expression of Y-box binding protein-1 (YB-1) is associated with all breast cancer subtypes and predicts poor survival and relapse. We previously reported that YB-1 appears to be enriched in the BLBC subtype where the frequency is 73%. There is currently no method of inhibiting YB-1 directly; therefore understanding its regulation may provide alternative approaches to impact the function of this protein. Phosphorylation of YB-1 allows this protein to act in an oncogenic manner by promoting transcription and translation. Of particular interest is the serine 102 (S102) residue that is required for the transcriptional activation of growth enhancing genes, such as epidermal growth factor receptor (EGFR), by YB-1. Although it is known Akt phosphorylates this site, it is not clear which other factors play a role in regulating the S102 phospho-status. Using predictive algorithms within Motif Scanner, a second kinase was identified; p90 ribosomal S6 kinase (RSK) which is a serine/threonine kinase of the same family as Akt and is over-expressed in breast cancer. This was particularly interesting to us because in the past unlike YB-1 we were not able to link Akt to BLBC or poor survival and could therefore not explain how this transcription factor might be activated in these tumors.    Herein, we show that the phosphorylation of YB-1(S102) is blocked by the inhibition of the MAP kinase pathway but further RSK1/2 indeed phosphorylates YB-1 at S102. We observed that YB-1 was phosphorylated at S102 in immortalized breast epithelial cells when RSK was active yet AKT was not. Further, inhibition of RSK1 or RSK2 through genetic and pharmacological methods decreased YB-1(S102) phosphorylation. This subsequently interfered with the ability of YB-1 to regulate gene expression; the RSK inhibitor SL0101 decreased the ability of YB-1 to bind the promoter, transactivate and ultimately reduced the expression of EGFR. Interestingly, in concordance with these results we found that expression of constitutively active RSK1 increased YB-1 phosphorylation. Furthermore, transfection of a kinase-dead RSK1 mutant was unable to phosphorylate YB-1 at this site. We therefore conclude that RSK1/2 is able to phosphorylate YB-1(S102). This provides a newly described mechanism whereby this transcription factor is activated in breast cancer. As the interest in developing small molecules against RSK has increased over the past two years, we believe this could be an important opportunity for therapeutic intervention. RSK has never before been associated with BLBC; we therefore introduce a new mechanistic understanding and potentially a therapeutic strategy for treating this aggressive disease.