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Cyclin D-cdk6 complex is targeted by p21(WAF) in growth-arrested lymphoma cells
D Tvrdík1, R Djaborkhel, A Nagy
1Laboratory of Gene Expression, 1st Faculty of Medicine, Charles University, Albertov 4, Prague, Czech Republic.
Abstract:
Normal human B lymphocytes are sensitive to the growth-inhibitory action of transforming growth factor beta1 (TGFbeta1) whereas malignant B lymphoma cells are mostly resistant to TGFbeta1 effects. We examined the phosphorylation status of retinoblastoma protein and the activity of G(1) cyclin-dependent kinases (cdk) in TGFbeta1-sensitive malignant follicular lymphoma cells during the TGFbeta1 treatment. The kinase activity of cdk2, cdk4, and cdk6 was significantly reduced and hypophosphorylation of pRb on serine 795 (S795) and threonine 373 (T373) was observed. We examined the composition of cdk complexes and the level of cdk inhibitors to explain the inhibitory action of TGFbeta1 toward cdk activity. Both cdk4 and cdk6 were notably dissociated from cyclin D cofactors, while cyclin E-cdk2 complexes remained coupled in TGFbeta1-treated cells. TGFbeta1-induced growth arrest was associated with notably increased binding of p21(WAF1) to cdk4 and cdk6. No induction of cdk-inhibitor molecules of INK family was observed in TGFbeta1-treated DoHH2 cells. As shown, TGFbeta1-induced growth arrest of malignant B cells was associated with the activation of CIP/KIP family members of cdk inhibitors.
Insights
Transforming growth factor beta1 (TGFbeta1) inhibits malignant B cell growth by reducing cyclin-dependent kinase (cdk) activity. This involves p21(WAF1) binding to cdk4/cdk6, leading to cell cycle arrest in follicular lymphoma.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Normal B lymphocytes are sensitive to TGFbeta1 growth inhibition.
- Malignant B lymphoma cells often exhibit resistance to TGFbeta1.
- Understanding TGFbeta1's mechanism in sensitive lymphoma cells is crucial.
Purpose of the Study:
- To investigate the molecular mechanisms of TGFbeta1-induced growth arrest in malignant B cells.
- To examine the role of cyclin-dependent kinases (cdks) and retinoblastoma protein (pRb) phosphorylation.
- To identify the specific cdk inhibitors involved in TGFbeta1's action.
Main Methods:
- Treatment of TGFbeta1-sensitive malignant follicular lymphoma cells (DoHH2) with TGFbeta1.
- Analysis of retinoblastoma protein (pRb) phosphorylation status.
- Assay of G(1) cyclin-dependent kinase (cdk) activity and complex composition.
- Western blot analysis for cdk inhibitors.
Main Results:
- TGFbeta1 significantly reduced the activity of cdk2, cdk4, and cdk6.
- Hypophosphorylation of pRb at S795 and T373 was observed.
- cdk4 and cdk6 dissociated from cyclin D, while cyclin E-cdk2 complexes remained intact.
- TGFbeta1 treatment increased p21(WAF1) binding to cdk4 and cdk6, activating CIP/KIP family inhibitors.
Conclusions:
- TGFbeta1 induces growth arrest in malignant B cells via inhibition of cdk activity.
- The mechanism involves pRb hypophosphorylation and increased binding of p21(WAF1) to cdk4/cdk6.
- Activation of CIP/KIP cdk inhibitors mediates TGFbeta1's anti-proliferative effects in follicular lymphoma.