Faculty Bibliography

The transcription factor E2F-1 interacts stably with cyclin A via a small domain near its amino terminus and is negatively regulated by the cyclin A-dependent kinases. Thus, the activities of E2F, a family of transcription factors involved in cell proliferation, are regulated by at least two types of cell growth regulators: the retinoblastoma protein family and the cyclin-dependent kinase family. To investigate further the regulation of E2F by cyclin-dependent kinases, we have extended our studies to include additional cyclins and E2F family members. Using purified components in an in vitro system, we show that the E2F-1-DP-1 heterodimer, the functionally active form of the E2F activity, is not a substrate for the active cyclin D-dependent kinases but is efficiently phosphorylated by the cyclin B-dependent kinases, which do not form stable complexes with the E2F-1-DP-1 heterodimer. Phosphorylation of the E2F-1-DP-1 heterodimer by cyclin B-dependent kinases, however, did not result in down-regulation of its DNA-binding activity, as is readily seen after phosphorylation by cyclin A-dependent kinases, suggesting that phosphorylation per se is not sufficient to regulate E2F DNA-binding activity. Furthermore, heterodimers containing E2F-4, a family member lacking the cyclin A binding domain found in E2F-1, are not efficiently phosphorylated or functionally down-regulated by cyclin A-dependent kinases. However, addition of the E2F-1 cyclin A binding domain to E2F-4 conferred cyclin A-dependent kinase-mediated down-regulation of the E2F-4-DP-1 heterodimer. Thus, both enzymatic phosphorylation and stable physical interaction are necessary for the specific regulation of E2F family members by cyclin-dependent kinases

The pRB-related proteins p107 and p130 are thought to suppress growth in part through their associations with two important cell cycle kinases, cyclin A-cdk2 and cyclin E-cdk2, and transcription factor E2F. Although each protein plays a critical role in cell proliferation, the functional consequences of the association among growth suppressor, cyclin-dependent kinase, and transcription factor have remained elusive. In an attempt to understand the biochemical properties of such complexes, we reconstituted each of the p130-cyclin-cdk2 and p107-cyclin-cdk2 complexes found in vivo with purified, recombinant proteins. Strikingly, stoichiometric association of p107 or p130 with either cyclin E-cdk2 or cyclin A-cdk2 negated the activities of these kinases. The results of our experiments suggest that inhibition does not result from substrate competition or loss of cdk2 activation. Kinase inhibitory activity was dependent upon an amino-terminal region of p107 that is highly conserved with p130. Further, a role for this amino-terminal region in growth suppression was uncovered by using p107 mutants unable to bind E2F. To determine whether cellular complexes might display similar regulatory properties, we purified p130-cyclin A-cdk2 complexes from human cells and found that such complexes exist in two forms, one that contains E2F-4-DP-1 and one that lacks the heterodimer. These endogenous complexes behaved like the in vitro-reconstituted complexes, exhibiting low levels of associated kinase activity that could be significantly augmented by dissociation of p130. The results of these experiments suggest a mechanism whereby p130 and p107 suppress growth by inhibiting important cell cycle kinases

The cyclin-dependent kinase (Cdk) inhibitor p21 is induced by the tumor suppressor p53 and is required for the G1-S block in cells with DNA damage. We report that there are two copies of a cyclin-binding motif in p21, Cy1 and Cy2, which interact with the cyclins independently of Cdk2. The cyclin-binding motifs of p21 are required for optimum inhibition of cyclin-Cdk kinases in vitro and for growth suppression in vivo. Peptides containing only the Cy1 or Cy2 motif partially inhibit cyclin-Cdk kinase activity in vitro and DNA replication in Xenopus egg extracts. A monoclonal antibody which recognizes the Cy1 site of p21 specifically disrupts the association of p21 with cyclin E-Cdk2 and with cyclin D1-Cdk4 in cell extracts. Taken together, these observations suggest that the cyclin-binding motif of p21 is important for kinase inhibition and for formation of p21-cyclin-Cdk complexes in the cell. Finally, we show that the cyclin-Cdk complex is partially active if associated with only the cyclin-binding motif of p21, providing an explanation for how p21 is found associated with active cyclin-Cdk complexes in vivo. The Cy sequences may be general motifs used by Cdk inhibitors or substrates to interact with the cyclin in a cyclin-Cdk complex

Although a significant amount of evidence has demonstrated that there are intimate connections between transcriptional controls and cell cycle regulation, the precise mechanisms underlying these connections remain largely obscure. A number of recent advances have helped to define how critical cell cycle regulators, such as the retinoblastoma family of tumor suppressor proteins and the cyclin-dependent kinases, might function on a biochemical level and how such mechanisms of action have been conserved not only in the regulation of transcription by all three RNA polymerases but also across species lines. In addition, the use of in vivo techniques has begun to explain how the activity of the E2F transcription factor family is tied to the cell cycle dependent expression of target genes

Nonsteroidal anti-inflammatory drugs lower the incidence of and mortality from colon cancer. Sulindac reduces the number and size of polyps in patients with familial adenomatous polyposis. We have shown that sulindac and sulindac sulfide reversibly reduce the proliferation rate of HT-29 colon cancer cells, alter their morphology, induce them to accumulate in the G0/G1 phase of the cell cycle, and sulindac sulfide induces cell death by apoptosis. In this study we confirmed that sulindac and sulindac sulfide prevent HT-29 cells from progressing from the G0/G1 into the S phase. This block in cell cycle progression is associated with an initial rise, then an abrupt decrease in the levels of p34cdc2 protein. Sulindac and sulindac sulfide decrease the levels of mitotic cyclins, induce the levels of p21WAF-1/cip1, and reduce the total levels of pRB, with a relative increase in the amount of the underphosphorylated form of pRB in a time- and concentration-dependent manner. In addition, these compounds reduce the levels of mutant p53. These responses are not associated with intestinal cell differentiation and occur independent of the ability of these compounds to induce apoptosis. We conclude that sulindac and sulindac sulfide reduce the levels of major components of the molecular cell cycle machinery and alter the levels of several tumor suppressor proteins in a manner consistent with cell cycle quiescence. These mechanisms may be operative in vivo to account, in part, for the anti-neoplastic effects of these compounds

The kinase activities of the cyclin/cdk complexes can be regulated in a number of ways. The most recently discovered mechanism of regulation is the association of cdk inhibitors (CKIs), such as p21, p27, and p57, with these complexes. In this report we demonstrate that the pRB-related protein p107, like the p21 family of cdk inhibitors, can inhibit the phosphorylation of target substrates by cyclin A/cdk2 and cyclin E/cdk2 complexes, and the associations of p107 and p21 with cyclin/cdk2 rely on a structurally and functionally related interaction domain. Furthermore, interactions between p107 or p21 with cyclin/cdk2 complexes are mutually exclusive. In cells treated with DNA-damaging agents elevated levels of p21 cause a dissociation of p107/cyclin/cdk2 complexes to yield p21/cyclin/cdk2 complexes. Finally, the consequences of cyclin/cdk2 interactions with p107 have been examined. The activation of the p107-bound cyclin/cdk kinases leads to dissociation of p107 from the transcription factor E2F. Together, these results suggest that cyclin/cdk complexes can be regulated by protein molecules from different families in a mutually exclusive manner in response to certain signals and that these inhibitory proteins may have a potential role in regulating macromolecular assembly

The cyclin-dependent kinase inhibitor p16 is a candidate tumour-suppressor protein that maps to a genomic locus strongly associated with familial melanoma and other tumour types. Screening of primary tumours and linkage analysis of familial melanoma pedigrees have identified many potential mutations in p16, but the functional significance of these sequence variants has remained unclear. We report here that p16 can act as a potent and specific inhibitor of progression through the G1 phase of the cell cycle, and we demonstrate that several tumour-derived alleles of p16 encode functionally compromised proteins. The ability of p16 to arrest cell-cycle progression generally correlates with inhibition of cyclin D1/Cdk4 kinase activity in vitro, with two exceptions among the alleles tested. In vivo, the presence of functional retinoblastoma protein appears to be necessary but may not be sufficient to confer full sensitivity to p16-mediated growth arrest. Our results provide support for the notion that p16 is an important cell-cycle regulator whose inactivation contributes to the outgrowth of human tumours

p21Cip1 is a cyclin-dependent kinase (Cdk) inhibitor that is transcriptionally activated by p53 in response to DNA damage. We have explored the interaction of p21 with the currently known Cdks. p21 effectively inhibits Cdk2, Cdk3, Cdk4, and Cdk6 kinases (Ki 0.5-15 nM) but is much less effective toward Cdc2/cyclin B (Ki approximately 400 nM) and Cdk5/p35 (Ki > 2 microM), and does not associate with Cdk7/cyclin H. Overexpression of P21 arrests cells in G1. Thus, p21 is not a universal inhibitor of Cdks but displays selectivity for G1/S Cdk/cyclin complexes. Association of p21 with Cdks is greatly enhanced by cyclin binding. This property is shared by the structurally related inhibitor p27, suggesting a common biochemical mechanism for inhibition. With respect to Cdk2 and Cdk4 complexes, p27 shares the inhibitory potency of p21 but has slightly different kinase specificities. In normal diploid fibroblasts, the vast majority of active Cdk2 is associated with p21, but this active kinase can be fully inhibited by addition of exogenous p21. Reconstruction experiments using purified components indicate that multiple molecules of p21 can associate with Cdk/cyclin complexes and inactive complexes contain more than one molecule of p21. Together, these data suggest a model whereby p21 functions as an inhibitory buffer whose levels determine the threshold kinase activity required for cell cycle progression

Transcription factor IIH (TFIIH) contains a kinase capable of phosphorylating the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II (RNAPII). Here we report the identification of the Cdk-activating kinase (Cak) complex (Cdk7 and cyclin H) as a component of TFIIH after extensive purification of TFIIH by chromatography. We find that affinity-purified antibodies directed against cyclin H inhibit TFIIH-dependent transcription and that both cyclin H and Cdk7 antibodies inhibit phosphorylation of the CTD of the largest subunit of the RNAPII in the preinitiation complex. Cak is present in at least two distinct complexes, TFIIH and a smaller complex that is unable to phosphorylate RNAPII in the preinitiation complex. Both Cak complexes, as well as recombinant Cak, phosphorylate a CTD peptide. Finally, TFIIH was shown to phosphorylate both Cdc2 and Cdk2, suggesting that there could be a link between transcription and the cell cycle machinery