p21 Disrupts the interaction between cdk2 and the E2F-p130 complex

Induction of p21/WAF1 and G1 cell-cycle arrest by the chemopreventive agent apigenin

Apigenin is a plant flavonoid that has been shown to significantly inhibit ultraviolet-induced mouse skin tumorigenesis when applied topically and may be an alternative sunscreen agent for humans. A long-term goal of our laboratory is to elucidate the molecular mechanism or mechanism by which apigenin inhibits skin tumorigenesis. In a previous publication, we characterized the mechanism by which apigenin induced G2/M arrest in keratinocytes. More recent studies in our laboratory have provided evidence that apigenin can induce G1 arrest in addition to arresting cells at G2/M. Here we describe the mechanism of the apigenin-induced G1 arrest in human diploid fibroblasts (HDF). Treatment of asynchronous HDF for 24 h with 10-50 microM apigenin resulted in dose-dependent cell-cycle arrest at both the G0/G1 and G2/M phases as measured by flow cytometry. The G0/G1 arrest was more clearly defined by using HDF that were synchronized in G0 and then released from quiescence by replating at subconfluent densities in medium containing 10-70 microM apigenin. The cells were analyzed for cell-cycle progression or cyclin D1 expression 24 h later. A dose of apigenin as low as 10 microM reduced the percentage of cells in S phase by 20% compared with control cultures treated with solvent alone. Western blot analysis of apigenin-treated HDF indicated that cyclin D1 was expressed at higher levels than in untreated cells, which signifies that they were arrested in G1 phase rather than in a G0 quiescent state. The G1 arrest was further studied by cyclin-dependent kinase 2 (cdk2) immune complex-kinase assays of apigenin-treated asynchronous HDF, which demonstrated a dose-dependent inhibition of cdk2 by apigenin. Inhibition of cdk2 kinase activity in apigenin-treated cells was associated with the accumulation of the hypophosphorylated form of the retinoblastoma (Rb) protein as measured by western blot analysis. The cdk inhibitor p21/WAF1 was also induced in a dose-dependent manner, with a 22-fold induction of p21/WAF1 in 70 microM apigenin-treated cells. In conclusion, apigenin treatment produced a G1 cell-cycle arrest by inhibiting cdk2 kinase activity and the phosphorylation of Rb and inducing the cdk inhibitor p21/WAF1, all of which may mediate its chemopreventive activities in vivo. To our knowledge this is the first report of a chemopreventive agent inducing p21/WAF1, a known downstream effector of the p53 tumor suppressor protein.

Accumulation of human papillomavirus type 16 E7 protein bypasses G1 arrest induced by serum deprivation and by the cell cycle inhibitor p21

The E7 oncoproteins encoded by the high-risk type of human papillomaviruses (HPVs) interact with the Rb family proteins Rb, p107, and p130. The Rb family proteins associate with the factors of the E2F family to form transcription repressor complexes, which control expression of several genes essential for S-phase entry and DNA replication. The E7 oncoproteins, by interacting with the Rb family proteins, dissociate the repressor complexes involving the factors of the E2F and Rb families, leading to a release of the E2F factors in their activator forms. In this study, we have addressed the mechanism by which the HPV type 16 (HPV16) E7 stimulates the cell cycle. Using a cell line that inducibly expresses the HPV16 E7 protein, we show that an accumulation of E7 induces quiescent cells to enter S phase and that this function of E7 depends on retention of the motif involved in binding to the Rb family proteins. To study the effects of E7 on normal human cells, we generated a recombinant adenovirus that expresses the HPV16 E7 protein. Infection of normal human fibroblasts, which were arrested in G1 phase by serum deprivation, with the E7-expressing virus induced the cells to enter S phase. The E7-induced S phase entry was accompanied by an increase in the activator form of E2F, but no increase in the cyclin-dependent kinase (cdk) activity was detected. Infection of serum-stimulated fibroblasts with a recombinant adenovirus expressing the cdk inhibitor p21 inhibited progression into S phase. Coinfection with the E7-expressing virus abrogated the p21 inhibition of progression into S phase without increasing the cdk activity. These results are consistent with the notion that E7 stimulates entry into S phase through targets downstream of the cdks such as the proteins of the E2F and Rb families.

Induction of c-myc transcription by the v-Abl tyrosine kinase requires Ras, Raf1, and cyclin-dependent kinases

v-Abl is an oncogenic form of the c-Abl nonreceptor tyrosine kinase. v-Abl induces transcription of c-myc, and c-Myc function is a necessary but not sufficient component of the v-Abl transformation program. Previously we showed that the E2F site in the c-myc promoter is a v-Abl response element and that v-Abl appears to induce c-myc by initiating a phosphorylation cascade that ultimately activates E2F-binding proteins. In this work we have investigated the signaling pathway between the v-Abl tyrosine kinase and activated E2F proteins. We show that the Ras GTPase and Raf1 serine/threonine kinase are required in this pathway. However, in contrast to other aspects of v-Abl signaling, induction of c-myc transcription is independent of the Rac GTPase. Our results also establish a requirement for activated cyclin-dependent kinases (cdks), as v-Abl-dependent induction of c-myc transcription is blocked by cdk inhibitor p21 and induction of c-myc is accompanied by activation of cdk2 and cdk4. Finally, we show that v-Abl-dependent induction of c-myc is accompanied by hyperphosphorylation of pRb, p107, and p130. On the basis of these data, we propose a model for the signaling path from v-Abl to c-myc.

p27KIP1 blocks cyclin E-dependent transactivation of cyclin A gene expression

Cyclin E is necessary and rate limiting for the passage of mammalian cells through the G1 phase of the cell cycle. Control of cell cycle progression by cyclin E involves cdk2 kinase, which requires cyclin E for catalytic activity. Expression of cyclin E/cdk2 leads to an activation of cyclin A gene expression, as monitored by reporter gene constructs derived from the human cyclin A promoter. Promoter activation by cyclin E/cdk2 requires an E2F binding site in the cyclin A promoter. We show here that cyclin E/cdk2 kinase can directly bind to E2F/p107 complexes formed on the cyclin A promoter-derived E2F binding site, and this association is controlled by p27KIP1, most likely through direct protein-protein interaction. These observation suggest that cyclin E/cdk2 associates with E2F/p107 complexes in late G1 phase, once p27KIP1 has decreased below a critical threshold level. Since a kinase-negative mutant of cdk2 prevents promoter activation, it appears that transcriptional activation of the cyclin A gene requires an active cdk2 kinase tethered to its promoter region.

The accumulation of an E2F-p130 transcriptional repressor distinguishes a G0 cell state from a G1 cell state

Previous studies have demonstrated cell cycle-dependent specificities in the interactions of E2F proteins with Rb family members. We now show that the formation of an E2F-p130 complex is unique to cells in a quiescent, G0 state. The E2F-p130 complex does not reform when cells reenter a proliferative state and cycle through G1. The presence of an E2F-p130 complex in quiescent cells coincides with the E2F-mediated repression of transcription of the E2F1 gene, and we show that the E2F sites in the E2F1 promoter are important as cells enter quiescence but play no apparent role in cycling cells. In addition, the decay of the E2F-p130 complex as cells reenter the cell cycle requires the action of G1 cyclin-dependent kinase activity. We conclude that the accumulation of the E2F-p130 complex in quiescent cells provides a negative control of certain key target genes and defines a functional distinction between these G0 cells and cells that exist transiently in G1.

Identification of a cyclin-cdk2 recognition motif present in substrates and p21-like cyclin-dependent kinase inhibitors

Understanding how cyclin-cdk complexes recognize their substrates is a central problem in cell cycle biology. We identified an E2F1-derived eight-residue peptide which blocked the binding of cyclin A and E-cdk2 complexes to E2F1 and p21. Short peptides spanning similar sequences in p107, p130, and p21-like cdk inhibitors likewise bound to cyclin A-cdk2 and cyclin E-cdk2. In addition, these peptides promoted formation of stable cyclin A-cdk2 complexes in vitro but inhibited the phosphorylation of the retinoblastoma protein by cyclin A- but not cyclin B-associated kinases. Mutation of the cyclin-cdk2 binding motifs in p107 and E2F1 likewise prevented their phosphorylation by cyclin A-associated kinases in vitro. The cdk inhibitor p21 was found to contain two functional copies of this recognition motif, as determined by in vitro kinase binding/inhibition assays and in vivo growth suppression assays. Thus, these studies have identified a cyclin A- and E-cdk2 substrate recognition motif. Furthermore, these data suggest that p21-like cdk inhibitors function, at least in part, by blocking the interaction of substrates with cyclin-cdk2 complexes.

Anchorage-dependent transcription of the cyclin A gene

NIH 3T3 cells cultured in suspension fail to express cyclin A and hence cannot enter S phase and divide. We show that loss of cell adhesion to substratum abrogates cyclin A gene expression by blocking its promoter activity through the E2F site that mediates its cell cycle regulation in adherent cells. In suspended cells, G0-specific E2F complexes remain bound to the cyclin A promoter. Overexpression of cyclin D1 restores cyclin A transcription in suspended cells and rescues them from cell cycle arrest. In suspended cells, cyclin D1 and cyclin E accumulate normally upon serum stimulation, but their associated kinases remain inactive; their substrates, pRb and p107, are not hyperphosphorylated. Concomitantly, the cyclin-dependent kinase inhibitor, p27KIP1, is stabilized. Ectopic expression of p27KIP1 blocks cyclin A promoter activity through its EN binding site. These data suggest that the block to cyclin A transcription in nonadherent NIH 3T3 cells results from stabilization of p27KIP1 and subsequent inactivation of the specific E2F moiety required for its induction.

Purification and crystallization of cyclin-dependent kinase inhibitor p21

p21, a universal inhibitor of mammalian cyclin-dependent kinases (CDK), regulates cell cycle progression by forming various distinct protein complexes with cyclins, CDKs, and the proliferating cell nuclear antigen. We have overexpressed recombinant human p21 in E. coli and purified active p21 to near homogeneity on a large scale. Crystals of recombinant p21 have been grown in the space group P2(1) a = 157.4, b = 152.7, c = 90.6 A, and beta = 92.7 degrees. The diffraction data of the recombinant p21 have been collected to 2.5 and 3.5 A resolution for the native crystal and two heavy atom derivatives of mercury and iridium.