Background & aims: Tumor progression is promoted by the ability of tumor cells to resist adverse environmental conditions such as hypoxia. We have shown that translational dysregulation contributes to transformed cell growth in malignant cholangiocytes. Translational regulation of gene expression can contribute to an immediate and rapid response to environmental changes such as hypoxia. Thus, our aims were to assess translational mechanisms involved in cell survival during hypoxia and to identify specific translationally regulated proteins involved in the cellular response to hypoxia.
Methods: Cell viability and apoptosis in response to hypoxia were assessed in human cholangiocarcinoma cells. Translational processes were deregulated by cycloheximide or rapamycin or by targeted deletion of eukaryotic initiation factor (eIF)-4E, a rate-limiting translational initiation factor using small interfering RNA (siRNA). A protein antibody microarray was used to screen for eIF-4E-dependent proteins expressed during hypoxia. Expression of the X-linked inhibitor of apoptosis (XIAP) was decreased using siRNA.
Results: Malignant cholangiocytes are resistant to hypoxia-induced apoptosis. Furthermore, cell survival during hypoxia required protein translation. eIF-4E was over expressed in malignant cholangiocytes. Reduction in eIF-4E expression by siRNA decreased tumor cell resistance to hypoxia, increased caspase-3 activation and apoptosis, and decreased cell survival compared with controls. XIAP was identified as a translationally regulated protein expressed during hypoxia. Modulation of XIAP expression by siRNA decreases cell death during hypoxia in vitro and in vivo.
Conclusions: Human cholangiocarcinoma cells are highly resistant to hypoxia. Translational regulation of survival proteins such as XIAP is a mechanism mediating cholangiocarcinoma survival during hypoxia.