ABSTRACT
Polarized hepatocytes expressing hyperactive Ha-Ras adopt an invasive and metastatic phenotype in cooperation with transforming growth factor (TGF)-beta. This dramatic increase in malignancy is displayed by an epithelial to mesenchymal transition (EMT), which mimics the TGF-beta-mediated progression of human hepatocellular carcinomas. In culture, hepatocellular EMT occurs highly synchronously, facilitating the analysis of molecular events underlying the various stages of this process. Here, we show that in response to TGF-beta, phosphorylated Smads rapidly translocated into the nucleus and activated transcription of target genes such as E-cadherin repressors of the Snail superfamily, causing loss of cell adhesion. Within the TGF-beta superfamily of cytokines, TGF-beta1, -beta2 and -beta3 were specific for the induction of hepatocellular EMT. Expression profiling of EMT kinetics revealed 78 up- and 235 downregulated genes, which preferentially modulate metabolic activities, extracellular matrix composition, transcriptional activities and cell survival. Independent of the genetic background, platelet-derived growth factor (PDGF)-A ligand and both PDGF receptor subunits were highly elevated, together with autocrine secretion of bioactive PDGF. Interference with PDGF signalling by employing hepatocytes expressing the dominant-negative PDGF-alpha receptor revealed decreased TGF-beta-induced migration in vitro and efficient suppression of tumour growth in vivo. In conclusion, these results provide evidence for a crucial role of PDGF in TGF-beta-mediated tumour progression of hepatocytes and suggest PDGF as a target for therapeutic intervention in liver cancer.
Subject(s)
Carcinoma, Hepatocellular/metabolism , Hepatocytes/metabolism , Liver Neoplasms/metabolism , Platelet-Derived Growth Factor/physiology , Transforming Growth Factor beta/metabolism , Animals , Carcinoma, Hepatocellular/pathology , Carcinoma, Hepatocellular/prevention & control , Cell Nucleus/metabolism , Cyclin-Dependent Kinase Inhibitor p16 , Disease Progression , Humans , Liver Neoplasms/pathology , Liver Neoplasms/prevention & control , Mesoderm/metabolism , Mesoderm/pathology , Mice , Mice, Inbred BALB C , Mice, Knockout , Mice, SCID , Phosphorylation , Rats , Receptor, Platelet-Derived Growth Factor alpha/antagonists & inhibitors , Receptor, Platelet-Derived Growth Factor alpha/metabolism , Receptors, Platelet-Derived Growth Factor/metabolism , Smad Proteins/metabolism , Tumor Suppressor Protein p14ARF/genetics , Tumor Suppressor Protein p14ARF/physiology , beta Catenin/metabolismABSTRACT
RE repeats encoded (RERE) was identified recently as a protein with high homology to the atrophin-1 protein, which appears to be causal in the hereditary neurodegenerative disorder termed dentatorubral-pallidoluysian atrophy (DRPLA) caused by an abnormal glutamine expansion. We have independently identified RERE in a search for genes localized to the translocation breakpoint region at chromosome 1p36.2 in the neuroblastoma cell line NGP. Here we show that neuroblastoma tumor cell lines display reduced abundance of RERE transcripts. Furthermore, we detected RERE protein mainly in the nucleus, where it colocalizes with the promyelocytic leukemia protein in promyelocytic leukemia oncogenic domains (PODs). Overexpression of RERE recruits a fraction of the proapoptotic protein BAX to PODS: This observation correlates with RERE-induced apoptosis, which occurs in a caspase-dependent manner. These results identify RERE as a novel component of PODs and suggest an important role of RERE in the control of cell survival.
