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The c-Mos proto-oncogene product stimulates c-Jun transcriptional activity by a MAP kinase-dependent mechanism
Abstract:
The AP-1 transcription factor family is subject to sophisticated regulation in response to cell growth and stress stimuli. We show here that the transcriptional activity of c-Jun, a key AP-1 component, is stimulated by overexpression of the c-Mos proto-oncogene product in mammalian cells. This stimulation requires serines 63 and 73 of c-Jun, indicating that it is likely to be mediated by proline-directed kinase(s). Co-transfection of MKP-1, a specific MAP kinase antagonist, blocks the stimulation of c-Jun by c-Mos, while co-transfection of a dominant negative form of c-Raf-1 does not. Conditioned medium from c-Mos transfected cells fails to activate c-Jun in recipient cells, arguing against the involvement of a diffusible mitogen. These data suggest that c-Mos exerts its effect on c-Jun directly through a MAP kinase, acting downstream of c-Raf-1.
Insights
The proto-oncogene product c-Mos directly stimulates the transcription factor c-Jun activity via a MAP kinase pathway. This regulation is crucial for cell growth and stress responses.
Area of Science:
- Molecular Biology
- Cell Signaling
- Oncogenesis
Background:
- The AP-1 transcription factor family regulates crucial cellular processes like growth and stress response.
- c-Jun is a pivotal component of the AP-1 complex, and its activity is tightly controlled.
- The proto-oncogene product c-Mos is implicated in cell cycle regulation and signaling pathways.
Purpose of the Study:
- To investigate the mechanism by which the c-Mos proto-oncogene product influences the transcriptional activity of c-Jun.
- To identify the specific signaling pathway and key residues involved in c-Mos-mediated c-Jun activation.
Main Methods:
- Mammalian cell co-transfection assays were employed to study gene expression.
- Site-directed mutagenesis was used to examine the role of specific c-Jun serine residues (S63 and S73).
- Involvement of MAP kinase pathways was assessed using antagonists like MKP-1 and dominant-negative c-Raf-1.
Main Results:
- Overexpression of c-Mos significantly stimulates the transcriptional activity of c-Jun in mammalian cells.
- Stimulation of c-Jun by c-Mos requires specific serine residues (S63 and S73), suggesting proline-directed kinase involvement.
- Co-transfection with MKP-1 blocked c-Mos-induced c-Jun activation, while dominant-negative c-Raf-1 did not.
- Conditioned medium from c-Mos-transfected cells did not activate c-Jun in recipient cells, ruling out secreted factors.
Conclusions:
- c-Mos directly activates c-Jun transcriptional activity through a MAP kinase pathway.
- The MAP kinase acts downstream of c-Raf-1 in the signaling cascade initiated by c-Mos.
- These findings elucidate a novel regulatory mechanism of c-Jun by the c-Mos proto-oncogene product.