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Down-regulation of tropomyosin-2 expression in c-Jun-transformed rat fibroblasts involves induction of a
S Ljungdahl1, S Linder, B Franzén
1Department of Oncology-Pathology, Cancer Center Karolinska, Karolinska Institute and Hospital, Stockholm, Sweden.
Abstract:
Overexpression of the c-Jun transcription factor in rodent fibroblasts may result in cell transformation or in apoptosis. The mechanisms whereby c-Jun induces transformation are unknown. We show here that the expression of high-molecular weight tropomyosin-2 (TM-2) is down-regulated in c-jun-transformed FR3T3 rat fibroblasts. However, down-regulation did not seem to be a direct effect of c-Jun on TM-2 gene expression. Thus, TM down-regulation in c-jun-transformed cells was alleviated by inhibitors of Ras (BZA-5B) or MEK1 (PD98059). Furthermore, medium conditioned by c-jun-transformed cells induced TM-2 down-regulation in untransformed cells by a mechanism requiring MEK1. Consistent with a central role for the MEK/ERK, but not SEK/JNK, pathway for TM down-regulation, constitutively active mutants of Raf induced TM down-regulation, whereas constitutively active Rac did not. We also show that anchorage-independent growth of c-jun-transformed cells requires MEK1. These findings suggest that indirect induction of the MEK/ERK pathway is central to c-Jun-induced transformation of rat fibroblasts.
Insights
Overexpression of c-Jun transcription factor leads to cell transformation by indirectly activating the MEK/ERK pathway, which down-regulates tropomyosin-2 (TM-2) expression. This pathway is crucial for the anchorage-independent growth characteristic of transformed cells.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Overexpression of the c-Jun transcription factor in rodent fibroblasts can lead to cell transformation or apoptosis.
- The precise mechanisms by which c-Jun induces cell transformation remain largely unknown.
- High-molecular weight tropomyosin-2 (TM-2) expression is observed to be down-regulated in c-jun-transformed FR3T3 rat fibroblasts.
Purpose of the Study:
- To investigate the mechanisms underlying c-Jun-induced cell transformation.
- To determine the role of tropomyosin-2 (TM-2) down-regulation in c-Jun-mediated transformation.
- To elucidate the signaling pathways involved in c-Jun-induced fibroblast transformation.
Main Methods:
- Analyzing TM-2 expression in c-jun-transformed FR3T3 rat fibroblasts.
- Utilizing Ras (BZA-5B) and MEK1 (PD98059) inhibitors to assess TM-2 down-regulation.
- Investigating the effect of conditioned medium from transformed cells on TM-2 expression in untransformed cells.
- Employing constitutively active mutants of Raf and Rac to study pathway involvement.
- Assessing the requirement of MEK1 for anchorage-independent growth of c-jun-transformed cells.
Main Results:
- Down-regulation of TM-2 in c-jun-transformed cells was not a direct effect of c-Jun.
- TM-2 down-regulation was alleviated by Ras and MEK1 inhibitors.
- Conditioned medium from c-jun-transformed cells induced TM-2 down-regulation via MEK1.
- The MEK/ERK pathway, not the SEK/JNK pathway, was central to TM-2 down-regulation.
- Constitutively active Raf, but not Rac, induced TM-2 down-regulation.
- MEK1 is essential for the anchorage-independent growth of c-jun-transformed cells.
Conclusions:
- Indirect induction of the MEK/ERK pathway is a key mechanism in c-Jun-induced transformation of rat fibroblasts.
- TM-2 down-regulation is a consequence of MEK/ERK pathway activation during c-Jun-mediated transformation.
- The MEK/ERK pathway plays a critical role in mediating the oncogenic effects of c-Jun, including anchorage-independent growth.