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Oncogene N-ras mediates selective inhibition of c-fos induction by nerve growth factor and basic fibroblast growth
T M Thomson1, S H Green, R J Trotta
1Department of Pathology, New York University Medical Center, New York 10016.
Abstract:
A cell line was generated from U7 cells (a subline of PC12 rat pheochromocytoma cells) that contains a stably integrated transforming mouse N-ras (Lys-61) gene under the control of the long terminal repeat from mouse mammary tumor virus. Such cells, designated UR61, undergo neuronal differentiation upon exposure to nanomolar concentrations of dexamethasone, as a consequence of expression of the activated N-ras gene (I. Guerrero, A. Pellicer, and D.E. Burstein, Biochem, Biophys. Res. Commun. 150:1185-1192, 1988). Exposure of UR61 cells to either nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) results in a marked induction of c-fos RNA, with kinetics paralleling those of NGF- or bFGF-induced expression of c-fos RNA in PC12 cells. Dexamethasone-induced expression of activated N-ras p21 results in blocking of c-fos RNA induction by NGF or bFGF in a time-dependent manner. Activated N-ras p21-mediated inhibition of c-fos RNA induction in UR61 cells is selective for NGF and bFGF and is not due to selective degradation of c-fos RNA. Normal and transforming N-ras can trans activate the chloramphenicol acetyltransferase gene linked to mouse c-fos regulatory sequences when transient expression assays are performed. Our observations suggest that N-ras p21 selectively interacts with pathways involved in induction of c-fos expression which initiate at the receptors for NGF and bFGF.
Insights
Activated N-ras p21 blocks nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) induced c-fos RNA. This N-ras effect is specific and does not involve c-fos RNA degradation, suggesting pathway interaction.
Area of Science:
- Molecular Biology
- Cell Biology
- Neuroscience
Background:
- PC12 rat pheochromocytoma cells are a model for neuronal differentiation.
- Nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) induce c-fos RNA expression in PC12 cells.
- Activated N-ras (Lys-61) expression can induce neuronal differentiation.
Purpose of the Study:
- To investigate the effect of activated N-ras on growth factor-induced c-fos expression.
- To determine if activated N-ras interacts with signaling pathways initiated by NGF and bFGF.
Main Methods:
- Generation of UR61 cell line expressing activated N-ras (Lys-61).
- Exposure of UR61 cells to dexamethasone, NGF, or bFGF.
- Measurement of c-fos RNA induction and N-ras p21 expression.
- Transient expression assays using chloramphenicol acetyltransferase (CAT) reporter gene linked to c-fos regulatory sequences.
Main Results:
- Dexamethasone induced neuronal differentiation in UR61 cells via activated N-ras expression.
- NGF and bFGF induced c-fos RNA in UR61 cells.
- Activated N-ras p21 blocked NGF- and bFGF-induced c-fos RNA induction in a time-dependent manner.
- This inhibition was selective for NGF and bFGF and not due to RNA degradation.
- Both normal and transforming N-ras could transactivate c-fos regulatory sequences.
Conclusions:
- Activated N-ras p21 selectively inhibits c-fos RNA induction pathways initiated by NGF and bFGF receptors.
- N-ras p21 appears to interact with specific signaling components downstream of NGF and bFGF receptors.
- These findings provide insights into the cross-talk between ras signaling and growth factor-induced gene expression.