The ribosomal protein S6 in renal cell carcinoma: functional relevance and potential as biomarker
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Maximilian Knoll1,2, Stephan Macher-Goeppinger1,2, Jürgen Kopitz1, Stefan Duensing3, Sascha Pahernik3, Markus Hohenfellner3, Peter Schirmacher1 and Wilfried Roth1,2,4
1 Institute of Pathology, University of Heidelberg, Im Neuenheimer Feld, Heidelberg, Germany
2 Molecular Tumor Pathology, German Cancer Research Center, Im Neuenheimer Feld, Heidelberg, Germany
3 Department of Urology, University of Heidelberg, Im Neuenheimer Feld, Heidelberg, Germany
4 Institute of Pathology, University Medical Center Mainz, Langenbeckstrasse, Mainz, Germany
Wilfried Roth, email:
Keywords: mTOR, ribosomal protein S6, everolimus, renal cell cancer, biomarkers
Received: June 25, 2015 Accepted: October 08, 2015 Published: October 25, 2015
Inhibitors of the mTOR pathway, such as everolimus, are promising compounds to treat patients with renal cell carcinomas (RCCs). However, the precise mechanisms of action are far from clear, and biomarkers predicting the response to mTOR inhibitors are still missing. Here, we provide evidence that in RCCs the rpS6 protein is the major mediator of anti-tumoral effects exerted by everolimus. Inhibition of mTOR signaling results in substantially decreased clonogenicity and proliferation of RCC cells, but did not significantly induce apoptosis. Everolimus effectively blocked protein biosynthesis both in vitro and in a novel ex vivo tissue slice model using fresh vital human RCC tissue. Compared to other components of the mTOR pathway, phosphorylation of rpS6 was most effectively downregulated by everolimus. Importantly, siRNA-mediated downregulation of rpS6, but not of 4ebp1 or p27, abolished the inhibitory effects of everolimus on proliferation and protein synthesis. Moreover, we analyzed the tissue expression of phosphorylated rpS6 (p-rpS6) and non-phosphorylated rpS6 in a large collection of patients with RCCs (n=598 and n=548, respectively). Expression of both proteins qualified as independent negative prognostic markers with a substantially shorter survival of patients with RCCs exhibiting high levels of rpS6 and p-rpS6. Taken together, our functional studies identified rpS6 as a main mediator of the anti-tumoral activity of Everolimus. Therefore, further (pre-)clinical evaluations of rpS6 as a predictive marker for everolimus-based treatment for RCC patients are warranted. Finally, the combined detection of phosphorylated and non-phosphorylated rpS6 could represent a robust prognostic marker to identify patients with high risk RCCs.
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