Use of phosphoprotein profiling with high-density fluorescence protein microarray to reveal a response predictor to a multikinase inhibitor sorafenib.
e15080 Background: The efficacy of sorafenib in patients with unresectable hepatocellular carcinoma (HCC) has been revealed in the recent phase III SHARP (Sorafenib HCC Assessment Randomized Protocol) clinical trial, but only a small proportion of patients achieved the anticipated therapeutic outcomes. It is essential to identify biomarkers that will discriminate such patients. Methods: We newly developed high-density fluorescence reverse-phase protein arrays and used them to profile the phosphorylation status of 180 signaling molecules in the 120 pathways registered on the NCI-Nature curated database in 23 HCC cell lines with different sensitivities to sorafenib. Results: Among the 180 signaling nodes, we found that the level of ribosomal protein S6 phosphorylated at serine residues 235/236 (p-rpS6 S235/236) was the most significantly correlated with the sensitivity of HCC cells to sorafenib. The high expression of p-rpS6 S235/236 was confirmed immunohistochemically in biopsy samples obtained from HCC patients who responded poorly to sorafenib. Sorafenib-resistant HCC cells showed constitutive activation of the mammalian target of rapamycin (mTOR) and/or the mitogen-activated protein kinase (MAPK) pathways downstream of c-RAF. Conclusions: The present results indicate that p-rpS6 S235/236 is a potential predictor of unresponsiveness to sorafenib. Cross-talk between the mTOR and MAPK signaling pathways in sorafenib-resistant HCC cell lines was also revealed. A combination of mTOR and MAPK inhibitors may improve the efficacy of molecular targeting therapy against HCC.