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Cdk6-cyclin D3 complex evades inhibition by inhibitor proteins and uniquely controls cell's proliferation competence
1Department of Biochemistry and Molecular Biology, Graduate School of Medicine, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Abstract:
Mammalian cells require a cyclin D-dependent kinase for the cell cycle start, yet many mesenchymal cells express three seemingly redundant D cyclins and similarly, seemingly redundant Cdk4 and Cdk6 as their kinase partners. We have found that the Cdk6-cyclin D3 complex is unique among the D cyclin and kinase combinations in the ability to promote the cell cycle start. In an anchorage-minus G(1)-arrested rat fibroblast, only Cdk6-D3 retains kinase activity due mainly to its ability to evade inhibition by p27(KIP1) and p21(CIP1) with a resemblance to viral cyclin-bound Cdk6. Rodent fibroblasts engineered to overexpress both Cdk6 and cyclin D3 highly resist serum starvation- or cell-cell contact-imposed G(1)-arrest. In BALB/c 3T3 cells, D3 is constitutively expressed, but Cdk6 is markedly induced with concomitant activation upon stimulation with a growth-promoting factor. These results suggest a role for the Cdk6-D3 complex in regulating cell's proliferation ability in response to external stimuli.
Insights
The Cdk6-cyclin D3 complex uniquely drives cell cycle start in mammalian cells. This complex overcomes inhibitors, promoting cell proliferation even under restrictive conditions.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Mammalian cells utilize cyclin D-dependent kinases (CDKs) for cell cycle progression.
- Mesenchymal cells express multiple D cyclins and CDK partners (Cdk4, Cdk6), suggesting functional redundancy.
- Understanding the specific roles of these complexes in cell cycle regulation is crucial.
Purpose of the Study:
- To identify the specific cyclin D-CDK complex responsible for initiating the cell cycle.
- To investigate the unique properties of the Cdk6-cyclin D3 complex.
- To determine the role of the Cdk6-D3 complex in cellular response to growth stimuli and arrest conditions.
Main Methods:
- Assessed kinase activity of different cyclin D-CDK combinations in anchorage-minus G(1)-arrested rat fibroblasts.
- Evaluated the susceptibility of Cdk6-D3 complex to inhibitors p27(KIP1) and p21(CIP1).
- Engineered rodent fibroblasts to overexpress Cdk6 and cyclin D3 to study resistance to G(1)-arrest.
- Analyzed Cdk6 and cyclin D3 expression and activation in BALB/c 3T3 cells upon growth factor stimulation.
Main Results:
- The Cdk6-cyclin D3 complex demonstrated unique ability to promote cell cycle start among D cyclin-kinase combinations.
- Cdk6-D3 retained kinase activity by evading inhibition from p27(KIP1) and p21(CIP1), similar to viral cyclin-bound Cdk6.
- Overexpression of Cdk6 and cyclin D3 conferred resistance to serum starvation and cell-cell contact-induced G(1)-arrest.
- Cyclin D3 was constitutively expressed, while Cdk6 was induced and activated upon growth factor stimulation in BALB/c 3T3 cells.
Conclusions:
- The Cdk6-cyclin D3 complex plays a critical, non-redundant role in initiating the cell cycle.
- This complex's resistance to CDK inhibitors contributes to its unique function in cell cycle progression.
- The Cdk6-D3 complex is a key regulator of cellular proliferation in response to external growth signals.