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Transforming growth factor beta 1 selectively regulates ornithine decarboxylase gene expression in malignant H-ras
R A Hurta1, A H Greenberg, J A Wright
1Manitoba Institute of Cell Biology, University of Manitoba, Winnipeg, Canada.
Abstract:
Negative growth regulators such as the transforming growth factor beta (TGF-beta) family appear to be important inhibitors in most tissue types. However, inhibition of DNA synthesis and cell proliferation is frequently lost during malignant transformation, and in some cases, tumor cell proliferation is actually stimulated by TGF-beta. The present study demonstrates a novel link between alterations in TGF-beta regulation during malignant conversion, and the expression of ornithine decarboxylase, a key rate-limiting activity in the biosynthesis of polyamines, and an enzyme that plays an important role in cell growth and differentiation. A panel of radiation and H-ras transformed mouse 10T1/2 cell lines exhibiting increasing malignant potential was investigated for possible TGF-beta 1 mediated changes in ornithine decarboxylase gene expression. Selective induction of gene expression was observed since only H-ras transformed cell lines with malignant potential exhibited marked elevations in ornithine decarboxylase message levels. Ornithine decarboxylase gene expression in nontransformed 10T1/2 cells and cell lines capable of only benign tumor formation was unaffected by TGF-beta 1 treatment. H-ras transformed cells were transfected with a plasmid placing the TGF-beta 1 coding region under the control of a zinc sensitive metallothionein promoter. When these cells were cultured in the presence of zinc an elevation of TGF-beta 1 mRNA was observed within 30 min. This increase in TGF-beta 1 message closely coincided with an elevation in ornithine decarboxylase message, and preceded an induction of jun-B, an early response gene in cells sensitive to TGF-beta 1 stimulation. Evidence for regulation of ornithine decarboxylase gene expression by TGF-beta 1 at both transcription and posttranscription was found. Actinomycin D pretreatment of malignant cells prior to TGF-beta 1 exposure prevented the increase in ornithine decarboxylase message. Marked differences in the rates of ornithine decarboxylase message decay were observed when cells treated with TGF-beta 1 were compared to untreated controls, with the half-life of ornithine decarboxylase mRNA increasing from 2.5 h in untreated cells to 17.5 h in cells exposed to TGF-beta 1. In addition, evidence was obtained for a cycloheximide sensitive regulator of ornithine decarboxylase gene expression, since the presence of this protein synthesis inhibitor increased the levels of ornithine decarboxylase message, and this effect was synergistically augmented by exposure of cells to cycloheximide and induction of TGF-beta 1 gene expression together.(ABSTRACT TRUNCATED AT 400 WORDS)
Insights
Transforming growth factor beta (TGF-beta) can stimulate tumor cell proliferation by altering ornithine decarboxylase (ODC) gene expression. This study reveals a novel link between TGF-beta dysregulation and ODC in malignant transformation.
Area of Science:
- Cell Biology
- Molecular Oncology
- Cancer Research
Background:
- Transforming growth factor beta (TGF-beta) typically inhibits cell growth but can paradoxically stimulate proliferation in malignant cells.
- Loss of TGF-beta growth inhibition is a hallmark of cancer, with implications for tumor progression.
- Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis, crucial for cell growth and differentiation.
Purpose of the Study:
- To investigate the novel link between altered TGF-beta regulation and ornithine decarboxylase (ODC) gene expression during malignant transformation.
- To determine if TGF-beta 1 influences ODC expression in H-ras transformed mouse 10T1/2 cell lines with varying malignant potential.
Main Methods:
- Utilized radiation and H-ras transformed mouse 10T1/2 cell lines with differential malignant potential.
- Administered TGF-beta 1 and measured ODC gene expression via mRNA levels.
- Employed gene transfection with a TGF-beta 1 expression vector under a metallothionein promoter, followed by zinc induction.
- Investigated transcriptional and post-transcriptional regulation using Actinomycin D and cycloheximide.
Main Results:
- TGF-beta 1 selectively induced ODC gene expression only in H-ras transformed malignant cell lines, not in benign or non-transformed cells.
- Zinc-induced TGF-beta 1 expression in H-ras cells elevated ODC mRNA and preceded jun-B induction.
- Evidence suggests both transcriptional and post-transcriptional regulation of ODC by TGF-beta 1, including increased mRNA half-life from 2.5 to 17.5 hours.
Conclusions:
- Altered TGF-beta 1 regulation is linked to increased ornithine decarboxylase (ODC) expression in malignant cells.
- TGF-beta 1 promotes ODC expression through transcriptional and post-transcriptional mechanisms, contributing to malignant cell proliferation.
- This study uncovers a novel pathway involving TGF-beta 1 and ODC in cancer progression.