FOXO3a and the MAPK p38 are activated by cetuximab to induce cell death and inhibit cell proliferation and their expression predicts cetuximab efficacy in colorectal cancer.

BACKGROUND: Cetuximab, a monoclonal antibody against EGFR used for the treatment of colorectal cancer (CRC), is ineffective in many patients. The aim of this study was to identify the signalling pathways activated by cetuximab in CRC cells and define new biomarker of response. METHODS: We used in vitro, in vivo models and clinical CRC samples to assess the role of p38 and FOXO3a in cetuximab mechanism of action. RESULTS: We show that cetuximab activates the MAPK p38. Specifically, p38 inhibition reduced cetuximab efficacy on cell growth and cell death. At the molecular level, cetuximab activates the transcription factor FOXO3a and promotes its nuclear translocation via p38-mediated phosphorylation, leading to the upregulation of its target genes p27 and BIM and the subsequent induction of apoptosis and inhibition of cell proliferation. Finally, we found that high FOXO3a and p38 expression levels are associated with better response rate and improved outcome in cetuximab-treated patients with CRC harbouring WT KRAS. CONCLUSIONS: We identify FOXO3a as a key mediator of cetuximab mechanism of action in CRC cells and define p38 as its activator in this context. Moreover, high FOXO3a and p38 expression could predict the response to cetuximab in patients with CRC harbouring WT KRAS.

Complete switch from Mdm2 to human papillomavirus E6-mediated degradation of p53 in cervical cancer cells.

The E6 oncoprotein of human papillomaviruses (HPVs) that are associated with cervical cancer utilizes the cellular ubiquitin-protein ligase E6-AP to target the tumor suppressor p53 for degradation. In normal cells (i.e., in the absence of E6), p53 is also a target of the ubiquitin-proteasome pathway. Under these conditions, however, p53 degradation is mediated by Mdm2 rather than by E6-AP. Here we show in a mutational analysis that, surprisingly, the structural requirements of p53 to serve as a proteolytic substrate differ between E6 proteins derived from different HPV types and, as expected, between Mdm2 and E6 proteins in vitro and in vivo. Stable expression of such mutants in HPV-negative and HPV-positive cell lines demonstrates that in HPV-positive cancer cells, the E6-dependent pathway of p53 degradation is not only active but, moreover, is required for degradation of p53, whereas the Mdm2-dependent pathway is inactive. Because the p53 pathway was reported to be functional in HPV-positive cancer cells, this finding indicates clearly that the ability of the E6 oncoprotein to target p53 for degradation is required for the growth of HPV-positive cancer cells.