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D-type cyclins repress transcriptional activation by the v-Myb but not the c-Myb DNA-binding domain
B Ganter1, S l Fu, J S Lipsick
1Department of Pathology, Stanford University School of Medicine, 300 Pasteur Drive, Stanford, CA 94305-5324, USA.
Abstract:
The v-Myb DNA-binding domain differs from that of c-Myb mainly by deletion of the first of three repeats. This truncation correlates with efficient oncogenic transformation and a decrease in DNA-binding activity. Here we demonstrate that the D-type cyclins, cyclin D1 and D2 in particular, specifically inhibit transcription when activated through the v-Myb DNA-binding domain, but not the c-Myb DNA-binding domain. Analysis of a cyclin D1 mutant and a dominant-negative CDK4 mutant implied that this repression is independent of complex formation with a CDK partner. Association of cyclin D1 and D2 with the Myb DNA-binding domain could be demonstrated. Increased levels of cyclin D1 and D2 resulted in a stabilization of the Myb proteins, but not in an alteration in binding of the Myb proteins to DNA. These results highlight an unexpected role for cyclin D as a CDK-independent repressor of transcriptional activation by v-Myb but not c-Myb. This differential effect of D-type cyclins on v-Myb and c-Myb might help to explain the mechanism underlying the oncogenic activity of v-Myb, which appears to be a stronger transcriptional activator following the TPA-induced differentiation of transformed monoblasts when cyclin D1 and D2 are down-regulated.
Insights
D-type cyclins, specifically cyclin D1 and D2, unexpectedly repress transcription activated by the v-Myb DNA-binding domain but not c-Myb. This CDK-independent inhibition may explain v-Myb
Area of Science:
- Molecular Biology
- Oncology
- Cell Cycle Regulation
Background:
- The viral Myb (v-Myb) oncoprotein, derived from the c-Myb proto-oncogene, possesses a DNA-binding domain truncated in its first repeat.
- This truncation in v-Myb correlates with enhanced oncogenic transformation and reduced DNA-binding activity compared to c-Myb.
Purpose of the Study:
- To investigate the regulatory role of D-type cyclins (cyclin D1 and D2) on the transcriptional activity of v-Myb and c-Myb DNA-binding domains.
- To elucidate the mechanism by which D-type cyclins modulate Myb protein function and its implications in oncogenesis.
Main Methods:
- Comparative analysis of transcriptional inhibition by cyclin D1 and D2 on v-Myb and c-Myb DNA-binding domains.
- Assessment of cyclin D1-mediated repression using cyclin D1 and CDK4 mutants to determine CDK-dependency.
- Demonstration of physical association between cyclin D1/D2 and the Myb DNA-binding domain.
- Evaluation of the impact of increased cyclin D1/D2 levels on Myb protein stability and DNA binding.
Main Results:
- D-type cyclins (cyclin D1 and D2) specifically inhibit transcription mediated by the v-Myb DNA-binding domain, but not the c-Myb DNA-binding domain.
- This repression by cyclin D1 is independent of its interaction with CDK partners, as indicated by mutant analyses.
- Cyclin D1 and D2 directly associate with the Myb DNA-binding domain, stabilizing the Myb proteins without altering their DNA-binding affinity.
- Down-regulation of cyclin D1 and D2 during TPA-induced monoblast differentiation correlates with enhanced v-Myb transcriptional activity.
Conclusions:
- Cyclin D1 and D2 function as novel, CDK-independent repressors of v-Myb transcriptional activation.
- The differential regulation of v-Myb and c-Myb by D-type cyclins provides insight into the potent oncogenic activity of v-Myb.
- Understanding this interaction may reveal therapeutic targets for v-Myb-driven malignancies.