Transcriptional and post-transcriptional induction of the TGFalpha gene in transformed rat liver epithelial cells
Berkowitz, E.A.; Hissong, M.A.; Lee, D.C.
Oncogene 12(9): 1991-2002
1996
ISSN/ISBN: 0950-9232 PMID: 8649860 Document Number: 456132
Although TGF-alpha mRNA and protein are frequently elevated in neoplastic cells, neither the level at which deregulation occurs nor the mechanism(s) responsible have been well characterized. As a first step, we examined the induction of TGF-alpha mRNA in two series of clonally-derived rat liver epithelial cell fines that were transformed either by exposure to chemical carcinogen or stable transfection of activated Ha-ras. We found that steady-state levels of TGF-alpha mRNA in both series of transformed lines were induced 25- to 50-fold over those in the respective normal parental cells. This induction, which occurred without amplification of the TGF-alpha gene, was accompanied by at least a five- to 10-fold increase in transcription along the entire length of the gene with no evidence of a transcriptional attenuation or arrest mechanism in the normal cells. Analysis of the TGF-alpha promoter and flanking regions did not support a correlation between the extent of methylation and the level of expression, but did reveal several DNase I hypersensitive sites spanning from -14 to +8 kilobases. Two of these sites were differentially observed in cells displaying high and low TGF-alpha gene transcription, while a third site correlated with TPA-induced expression. Finally, measurement of TGF-alpha mRNA decay in the presence of Actinomycin D revealed a consistent 1.5- to 3.2-fold increase in the half-life of the TGF-alpha transcript in the various transformed cell lines. These results indicate that transformation-mediated induction of TGF-alpha gene expression in rat liver epithelial cells occurs through both transcriptional and post-transcriptional mechanisms, but is primarily the result of TGF-alpha promoter activation.