Dephosphorylation of p-ERK1/2 in relation to tumor remission after HER-2 and Raf1 blocking therapy in a conditional mouse tumor model

Several studies have shown that HER‐2/neu (erbB‐2) blocking therapy strategies can cause tumor remission. However, the responsible molecular mechanisms are not yet known. Both ERK1/2 and Akt/PKB are critical for HER‐2‐mediated signal transduction. Therefore, we used a mouse tumor model that allows downregulation of HER‐2 in tumor tissue by administration of anhydrotetracycline (ATc). Switching‐off HER‐2 caused a rapid tumor remission by more than 95% within 7 d of ATc administration compared to the volume before switching‐off HER‐2. Interestingly, HER‐2 downregulation caused a dephosphorylation of p‐ERK1/2 by more than 80% already before tumor remission occurred. Levels of total ERK protein were not influenced. In contrast, dephosphorylation of p‐Akt occurred later, when the tumor was already in remission. These data suggest that in our HER‐2 tumor model dephosphorylation of p‐ERK1/2 may be more critical for tumor remission than dephosphorylation of p‐Akt. To test this hypothesis we used a second mouse tumor model that allows ATc controlled expression of BXB‐Raf1 because the latter constitutively signals to ERK1/2, but cannot activate Akt/PKB. As expected, downregulation of BXB‐Raf1 in tumor tissue caused a strong dephosphorylation of p‐ERK1/2, but did not decrease levels of p‐Akt. Interestingly, tumor remission after switching‐off BXB‐Raf1 was similarly efficient as the effect of HER‐2 downregulation, despite the lack of p‐Akt dephosphorylation. In conclusion, two lines of evidence strongly suggest that dephosphorylation of p‐ERK1/2 and not that of p‐Akt is critical for the rapid tumor remission after downregulation of HER‐2 or BXB‐Raf1 in our tumor model: (i) dephosphorylation of p‐ERK1/2 but not that of p‐Akt precedes tumor remission after switching‐off HER‐2 and (ii) downregulation of BXB‐Raf1 leads to a similarly efficient tumor remission as downregulation of HER‐2, although no p‐Akt dephosphorylation was observed after switching‐off BXB‐Raf1. © 2006 Wiley‐Liss, Inc.