Cell-cycle arrest and inhibition of Cdk4 activity by small peptides based on the carboxy-terminal domain of p21WAF1.
BACKGROUND
A common event in the development of human neoplasia is the inactivation of a damage-inducible cell-cycle checkpoint pathway regulated by p53. One approach to the restoration of this pathway is to mimic the activity of key downstream effectors. The cyclin-dependent kinase (Cdk) inhibitor p21(WAF1) is one such molecule, as it is a major mediator of the p53-dependent growth-arrest pathway, and can, by itself, mediate growth suppression. The primary function of the p21(WAF1) protein appears to be the inhibition of G1 cyclin-Cdk complexes. Thus, if we can identify the region(s) of p21(WAF1) that contain its inhibitor activity they may provide a template from which to develop novel anti-proliferative drugs for use in tumours with a defective p53 pathway.


RESULTS
We report on the discovery of small synthetic peptides based on the sequence of p21(WAF1) that bind to and inhibit cyclin D1-Cdk4. The peptides and the full-length protein are inhibitory at similar concentrations. A 20 amino-acid peptide based on the carboxy-terminal domain of p21(WAF1) inhibits Cdk4 activity with a concentration for half-maximal inhibition (l0.5) of 46 nM, and it is only four-fold less active than the full-length protein. The length of the peptide has been minimized and key hydrophobic residues forming the inhibitory domain have been defined. When introduced into cells, both a 20 amino-acid and truncated eight amino-acid peptide blocked phosphorylation of the retinoblastoma protein (pRb) and induced a potent G1/S growth arrest. These data support a physiological role for the carboxyl terminus of p21(WAF1) in the inhibition of Cdk4 activity.


CONCLUSIONS
We have discovered that a small peptide is sufficient to mimic p21(WAF1) function and inhibit the activity of a critical G1 cyclin-Cdk complex, preventing pRb phosphorylation and producing a G1 cell-cycle arrest in tissue culture cell systems. This makes cyclin D1-Cdk4 a realistic and exciting target for the design of novel synthetic compounds that can act as anti-proliferative agents in human cells.
Ball
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KL
CRC Cell Transformation Research Group, Medical Sciences Institute, University of Dundee, Dundee DD1 4HN, UK. K.L.Ball@dundee.ac.uk
Lain
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Fâhraeus
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Smythe
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D000595:Amino Acid Sequence; D002453:Cell Cycle / Q000187:drug effects*; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors*; D016213:Cyclins / Q000235:genetics* / Q000494:pharmacology; D006801:Humans; D008969:Molecular Sequence Data
C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D018844:Cyclin-Dependent Kinases