The tumor suppressor protein p53 is ubiquitously expressed as a major isoform of 53 kD, but several forms of lower molecular weight have been observed. Here, we describe a new isoform, DN-p53, produced by internal initiation of translation at codon 40 and lacking the N-terminal first transactivation domain. This isoform has impaired transcriptional activation capacity, and does not complex with the p53 regulatory protein Mdm2. Furthermore, DN-p53 oligomerizes with full-length p53 (FL-p53) and negatively regulates its transcriptional and growth-suppressive activities. Consistent with the lack of Mdm2 binding, DN-p53 does not accumulate in response to DNA-damage, suggesting that this isoform is not involved in the response to genotoxic stress. However, in serum-starved cells expressing wild-type p53, DN-p53 becomes the predominant p53 form during the synchronous progression into S phase after serum stimulation. These results suggest that DN-p53 may play a role as a transient, negative regulator of p53 during cell cycle progression.

BackgroundEpidermal growth factor (EGF) receptors contribute to the development of malignant glioma. Here we considered the possible implication of the EGFR ligand epiregulin (EREG) in glioma development in relation to the activity of the unfolded protein response (UPR) sensor IRE1α. We also examined EREG status in several glioblastoma cell lines and in malignant glioma.MethodsExpression and biological properties of EREG were analyzed in human glioma cells in vitro and in human tumor xenografts with regard to the presence of ErbB proteins and to the blockade of IRE1α. Inactivation of IRE1α was achieved by using either the dominant-negative strategy or siRNA-mediated knockdown.ResultsEREG was secreted in high amounts by U87 cells, which also expressed its cognate EGF receptor (ErbB1). A stimulatory autocrine loop mediated by EREG was evidenced by the decrease in cell proliferation using specific blocking antibodies directed against either ErbB1 (cetuximab) or EREG itself. In comparison, anti-ErbB2 antibodies (trastuzumab) had no significant effect. Inhibition of IRE1α dramatically reduced EREG expression both in cell culture and in human xenograft tumor models. The high-expression rate of EREG in U87 cells was therefore linked to IRE1α, although being modestly affected by chemical inducers of the endoplasmic reticulum stress. In addition, IRE1-mediated production of EREG did not depend on IRE1 RNase domain, as neither the selective dominant-negative invalidation of the RNase activity (IRE1 kinase active) nor the siRNA-mediated knockdown of XBP1 had significant effect on EREG expression. Finally, chemical inhibition of c-Jun N-terminal kinases (JNK) using the SP600125 compound reduced the ability of cells to express EREG, demonstrating a link between the growth factor production and JNK activation under the dependence of IRE1α.ConclusionEREG may contribute to glioma progression under the control of IRE1α, as exemplified here by the autocrine proliferation loop mediated in U87 cells by the growth factor through ErbB1.