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Ras-independent transformation by v-Src
D T Aftab1, J Kwan, G S Martin
1Department of Molecular and Cell Biology, University of California, Berkeley 94720-3204, USA.
Abstract:
Signaling by a variety of receptor and nonreceptor tyrosine kinases is mediated by Ras, a membrane-associated GTPase. Expression of v-Src, a transforming nonreceptor tyrosine kinase, results in Ras activation, and inhibition of Ras function in NIH 3T3 cells suppresses transformation by v-Src, indicating that in these cells Ras-dependent signaling pathways are required for v-Src to exert its biological effects. However, we show here that Ras was not activated in Rat-2 fibroblasts transformed by wild-type v-Src, or in chicken embryo fibroblasts transformed by SRX5, a v-Src mutant with a linker insertion at the major site of autophosphorylation. Expression of a dominant-negative mutant of Ras completely inhibited the ability of v-Src to activate the mitogen-activated protein kinase ERK2, which is downstream of Ras. However, dominant-negative Ras did not suppress transformation by v-Src as judged by a variety of criteria. Thus, v-Src can transform at least some cell types in the absence of Ras activation or Ras-stimulated ERK2 activity, and in these cells activation of Ras-independent signaling pathways must therefore be sufficient for transformation.
Insights
v-Src transformation can occur without Ras activation. This study shows v-Src transforms cells via Ras-independent pathways, challenging previous understandings of tyrosine kinase signaling.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Receptor and nonreceptor tyrosine kinases signal through Ras, a GTPase.
- v-Src, a tyrosine kinase, typically activates Ras, mediating cellular transformation.
- Ras-dependent pathways are considered crucial for v-Src's biological effects.
Purpose of the Study:
- To investigate whether Ras activation is essential for v-Src-mediated cellular transformation.
- To determine if Ras-independent signaling pathways contribute to v-Src transformation.
- To elucidate the role of Ras in v-Src's downstream signaling, including ERK2 activation.
Main Methods:
- Expression of wild-type and mutant v-Src in Rat-2 and chicken embryo fibroblasts.
- Utilizing dominant-negative Ras mutants to inhibit Ras function.
- Assessing cellular transformation criteria and mitogen-activated protein kinase ERK2 activation.
Main Results:
- Ras was not activated in Rat-2 fibroblasts transformed by wild-type v-Src or in chicken embryo fibroblasts transformed by a v-Src mutant (SRX5).
- Dominant-negative Ras inhibited v-Src-induced ERK2 activation, a downstream target of Ras.
- Despite inhibiting ERK2, dominant-negative Ras did not suppress v-Src-mediated cellular transformation.
Conclusions:
- v-Src can transform certain cell types independently of Ras activation.
- Ras-independent signaling pathways are sufficient for v-Src-mediated transformation in specific cellular contexts.
- This finding necessitates a re-evaluation of Ras's role in tyrosine kinase signaling and cellular transformation.