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Dioxin induces expression of c-fos and c-jun proto-oncogenes and a large increase in transcription factor AP-1
A Puga1, D W Nebert, F Carrier
1Laboratory of Developmental Pharmacology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892.
Abstract:
Among environmental pollutants, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is one of the most potent tumor promoters and teratogens known. The molecular mechanisms responsible for the biological activity of TCDD, however, remain largely unknown. In this report, we show that the first observable effects of TCDD in cultured murine hepatoma cells are a rapid, transient increase in Ca2+ influx and a minor but significant elevation of activated, membrane-bound protein kinase C. These changes are then followed by induction of the immediate early proto-oncogenes c-fos, jun-B, c-jun, and jun-D, and by large increases in AP-1 transcription factor activity. Induction of these changes by TCDD is delayed compared with that by phorbol esters, although the magnitude of the effects caused by both treatments is similar, and both induction processes can be blocked by staurosporine, a protein kinase C inhibitor. In cultured cells, proto-oncogene induction by TCDD appears to be independent of the presence of a functional aryl hydrocarbon (Ah) receptor or nuclear translocation protein. These results reveal early events that may lead to the elucidation of the molecular basis of TCDD-induced tumor promotion.
Insights
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) rapidly increases calcium influx and protein kinase C activity in liver cells. This leads to proto-oncogene induction, revealing early molecular events in TCDD's tumor promotion.
Area of Science:
- Environmental Toxicology
- Molecular Biology
- Oncology
Background:
- 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent environmental pollutant known for tumor promotion and teratogenicity.
- The precise molecular mechanisms underlying TCDD's biological effects are not fully understood.
Purpose of the Study:
- To investigate the early molecular events triggered by TCDD in cultured murine hepatoma cells.
- To elucidate the signaling pathways involved in TCDD-induced proto-oncogene expression.
Main Methods:
- Treatment of cultured murine hepatoma cells with TCDD.
- Measurement of intracellular calcium (Ca2+) influx.
- Assay of activated protein kinase C (PKC) activity.
- Quantification of immediate early proto-oncogene mRNA levels (c-fos, jun-B, c-jun, jun-D).
- Assessment of AP-1 transcription factor activity.
- Inhibition studies using staurosporine (PKC inhibitor).
Main Results:
- TCDD induced a rapid, transient increase in Ca2+ influx and a minor elevation of activated PKC.
- These initial changes were followed by significant induction of c-fos, jun-B, c-jun, and jun-D proto-oncogenes.
- AP-1 transcription factor activity was substantially increased following TCDD treatment.
- TCDD-induced proto-oncogene induction was delayed compared to phorbol esters but similar in magnitude.
- Both TCDD and phorbol ester effects were inhibited by staurosporine.
- Proto-oncogene induction by TCDD in cultured cells was independent of the aryl hydrocarbon (Ah) receptor.
Conclusions:
- Early cellular events following TCDD exposure include altered calcium homeostasis and protein kinase C activation.
- These early events are critical for the subsequent induction of immediate early proto-oncogenes and AP-1 activity.
- The findings provide insights into the molecular basis of TCDD-induced tumor promotion, potentially independent of the Ah receptor pathway in certain cellular contexts.