BDNF Contributes to Cisplatin Resistance Figure 7. Immunohistochemistry staining of mouse tumor tissue originated from OSC-19P and OSC-19CR. A. Immunohistochemistry staining of OSC-19P (left) and OSC-19CR (right) with Bim. B. IHC staining of OSC-19P (left) and OSC-19CR (right) with Ki-67. The pictures were taken at 1006 magnification, the inlet at 506 magnification. (C) IHC staining of OSC-19P (left) and OSC-19CR (right) with XiAP. doi:10.1371/journal.pone.0030246.g007 in response to chemotherapy. In current study, we observed changes in XiAP and Bim expression in the resistant cell lines, compared to the sensitive parental cell lines. Moreover, these changes were induced with exogenous BDNF treatment in the parental cell lines. This evidence indicates that the modulation of both XiAP and Bim expression is due, in part, to up-regulation of BDNF/TrkB system in the CR system. As an inhibitor of apoptosis, XiAP over-expression is a well-described phenomenon in various types of cancer and appears to protect cells from various insults [19]. This occurs through the inhibition of caspase enzymes, which initiate and execute apoptosis [19]. As a pro- apoptotic molecule, Bim has been demonstrated to induce apoptosis by inhibiting anti-apoptotic molecules such as bcl-2, bcl-xL, or bcl-w [20]. In the current study, we showed that Bim expression was reduced in CDDP-resistant HNSCC and exoge- nous administration of BDNF resulted in reduced Bim expression, a finding that is consistent with effects seen in various disease models including neuroblastoma model [18,21]. The increase in BDNF can cause in part of Bim degradation through a MAPK- dependent mechanism [18,21], although this phenomenon has not been evaluated in our system. These findings suggest that down- regulation of Bim by BDNF might be a potential mechanism underlying CDDP-resistance development in the HNSCC. In contrast to our findings, however, Li et al. (2007) reported that reduction in Bim expression through BDNF failed to block the CDDP-induced cell death in a neuroblastoma model [18]. This indicated that perhaps Bim does not play a central role in drug resistance in neuroblastoma, or alternatively, the acquired resistance in our model mechanistically is driven by a direct BDNF-Bim pathway. Further studies will be necessary to elucidate PLoS ONE | www.plosone.org 9 January 2012 | Volume 7 | Issue 1 | e30246