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Constitutively activated neu oncoprotein tyrosine kinase interferes with growth factor-induced signals for gene
L Lehtola1, L Sistonen, P Koskinen
1Laboratory of Cancer Biology, Department of Pathology, Helsinki, Finland.
Abstract:
The neu receptor oncoprotein tyrosine kinase, capable of transforming cultured fibroblasts and causing mammary carcinomas in transgenic mice, carries a point mutation in its transmembrane domain and shows a constitutive tyrosine kinase activity. We analyzed the neu tyrosine kinase and its substrates in transfected NIH 3T3 fibroblasts by phosphotyrosine immunoblotting. Tyrosine phosphorylated proteins were similar but not identical in epidermal growth factor (EGF)-stimulated cells expressing the human EGF receptor (EGFR) or a chimeric EGFR/neu receptor but differed from phosphotyrosyl proteins constitutively expressed in neu oncogene-transformed cells. The neu oncoprotein in the latter cells was phosphorylated in tyrosine in a ligand-independent manner and had a shortened half-life in comparison with the normal neu protein. Tumor promoter pretreatment inhibited ligand-induced receptor tyrosine phosphorylation and decreased tyrosine phosphorylated neu oncoprotein. Prolonged pretreatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) also prevented the induction of immediate early growth factor-regulated genes in response to neu activation. Expression of the neu oncogene but not the protooncogene in NIH 3T3 cells was associated with enhanced levels of the jun and fos oncoproteins and loss of serum growth factor induction of immediate early mRNA responses. The constitutively activated neu oncoprotein tyrosine kinase thus deregulates cellular genomic responses to growth factors.
Insights
The neu oncoprotein tyrosine kinase, mutated and constitutively active, transforms cells and drives cancer. It deregulates growth factor signaling, impacting gene expression and promoting tumor development.
Area of Science:
- Molecular Biology
- Oncology
- Cell Signaling
Background:
- The neu receptor (also known as ErbB2) is an oncoprotein tyrosine kinase implicated in cancer development.
- Mutations in the neu receptor can lead to constitutive tyrosine kinase activity, driving cellular transformation.
Purpose of the Study:
- To investigate the activity and substrate phosphorylation of the neu tyrosine kinase in NIH 3T3 fibroblasts.
- To compare signaling pathways activated by the neu oncoprotein versus the epidermal growth factor receptor (EGFR).
- To examine the effects of the neu oncogene on gene expression and cellular responses to growth factors.
Main Methods:
- Transfection of NIH 3T3 fibroblasts with neu oncogene or chimeric EGFR/neu constructs.
- Analysis of tyrosine phosphorylated proteins using phosphotyrosine immunoblotting.
- Assessment of jun and fos oncoprotein levels and immediate early gene mRNA responses.
Main Results:
- Constitutively active neu oncoprotein showed ligand-independent tyrosine phosphorylation and a shortened half-life.
- Phosphotyrosine profiles differed between EGF-stimulated cells and neu-transformed cells.
- Tumor promoter pretreatment (TPA) inhibited neu-mediated signaling and gene induction.
- Neu oncogene expression enhanced jun and fos levels and disrupted growth factor-induced gene responses.
Conclusions:
- The constitutively activated neu oncoprotein tyrosine kinase deregulates cellular genomic responses to growth factors.
- Neu signaling pathways differ from those activated by EGFR.
- The neu oncogene plays a significant role in aberrant cell growth and transformation.