Transcriptional and post-transcriptional induction of the TGFalpha gene in transformed rat liver epithelial cells.
Although TGFalpha 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 TGFalpha mRNA in two series of clonally-derived rat liver epithelial cell lines that were transformed either by exposure to chemical carcinogen or stable transfection of activated Ha-ras. We found that steady-state levels of TGFalpha 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 TGFalpha 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 TGFalpha gene transcription, while a third site correlated with TPA-induced expression. Finally, measurement of TGFalpha mRNA decay in the presence of Actinomycin D revealed a consistent 1.5- to 3.2-fold increase in the half-life of the TGFalpha transcript in the various transformed cell lines. These results indicate that transformation-mediated induction of TGFalpha gene expression in rat liver epithelial cells occurs through both transcriptional and post-transcriptional mechanisms, but is primarily the result of TGFalpha promoter activation.
Berkowitz
E A
EA
Lineberger Comprehensive Cancer Center, School of Medicine, University of North Carolina at Chapel Hill 27599-7295, USA.
Hissong
M A
MA
Lee
D C
DC
D000818:Animals; D002460:Cell Line; D002461:Cell Line, Transformed; D003850:Deoxyribonuclease I / Q000378:metabolism; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism; D008099:Liver / Q000166:cytology / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D014158:Transcription, Genetic; D016211:Transforming Growth Factor alpha / Q000235:genetics*
D012333:RNA, Messenger; D016211:Transforming Growth Factor alpha; D003850:Deoxyribonuclease I