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The permissive effect of p21(Waf1/Cip1) on DNA synthesis is dependent on cell type: effect is absent in p53-inactive
1Division of Nephrology, Department of Internal Medicine, University of California, Davis, CA 95616, USA. rhweiss@ucdavis.edu
Abstract:
The cyclin-dependent kinase inhibitors (CKI) interact with cyclin-cdk complexes to arrest mitogen-stimulated transit through the cell cycle, but we and others have recently shown that these molecules can exert permissive effects on cell cycle transit as well. The p53 protein induces transcription of the p21(Waf1/Cip1) gene, but whether p53 has any effect on the stimulatory versus inhibitory state of p21(Waf1/Cip1) toward cell growth is not known. The focus of the current study was to examine the effect of p21(Waf1/Cip1) inhibition on growth in cells which possess an inactive p53 protein. We found that there was significant and specific inhibition of p21(Waf1/Cip1) protein transcription in human squamous carcinoma A431 cells after transfection of an antisense p21(Waf1/Cip1) oligodeoxynucleotide, yet there was no significant growth inhibition in these cells after stimulation with 10% serum or with PDGF-BB, in contrast to what was observed in vascular smooth muscle (VSM) cells. Furthermore, there was no attenuation of either cyclinD/cdk4 association or of Rb hyperphosphorylation after antisense p21(Waf1/Cip1) oligodeoxynucleotide transfection, suggesting that an alternate pathway exists to allow association and phosphorylation of these cell cycle components in the absence (or with lower levels) of p21(Waf1/Cip1). Thus, the permissive effect of p21(Waf1/Cip1) toward growth is dependent on cell type, and active p53 is likely required for this effect.
Insights
Cyclin-dependent kinase inhibitors like p21(Waf1/Cip1) can promote cell growth. This study found that p21(Waf1/Cip1)
Area of Science:
- Cell biology
- Molecular biology
- Cancer research
Background:
- Cyclin-dependent kinase inhibitors (CKI) regulate cell cycle progression.
- While typically inhibitory, CKIs like p21(Waf1/Cip1) can also permit cell cycle transit.
- The role of p53 in modulating p21(Waf1/Cip1)'s effect on growth is unclear.
Purpose of the Study:
- To investigate the impact of p21(Waf1/Cip1) inhibition on cell growth in cells with inactive p53.
- To determine if p21(Waf1/Cip1)'s permissive growth effects are cell-type dependent.
- To explore the molecular mechanisms underlying p21(Waf1/Cip1)'s permissive role.
Main Methods:
- Transfection of human squamous carcinoma A431 cells with antisense p21(Waf1/Cip1) oligodeoxynucleotide.
- Assessment of p21(Waf1/Cip1) protein transcription and cell growth.
- Analysis of cyclinD/cdk4 association and Rb hyperphosphorylation.
Main Results:
- Antisense p21(Waf1/Cip1) oligodeoxynucleotide specifically inhibited p21(Waf1/Cip1) transcription in A431 cells.
- No significant growth inhibition was observed in A431 cells upon serum or PDGF-BB stimulation after p21(Waf1/Cip1) inhibition.
- CyclinD/cdk4 association and Rb hyperphosphorylation were not attenuated, indicating an alternative pathway.
- These findings contrast with observations in vascular smooth muscle (VSM) cells.
Conclusions:
- The permissive effect of p21(Waf1/Cip1) on cell growth is cell-type specific.
- Active p53 is likely essential for mediating the growth-promoting role of p21(Waf1/Cip1).
- Alternative molecular pathways can support cell cycle progression independently of p21(Waf1/Cip1) levels in certain cell types.