PubMed was used to search for relevant publications in the past 20 years with bias to more recent publications. The search terms used were: “cyclin D”, “cyclin D1”, “cyclin D2”, “cyclin D3”, “cyclin E”, “cyclin E1”, “cyclin E2”, “retinoblastoma”, “pRB”, “cdk2”, “cdk4”, “cdk6”, “CKI”, “cdk inhibitor”, “p16ink4”, “p21cip1”, “p27kip1”, “p57kip2”, “familial melanoma”, “flavopiridol”, “UCN-01”, “staurosporine”, “E7070”, “proteosome inhibitor”, “PS-341”, “histone deacetylase inhibitor”, and
ReviewCell-cycle targeted therapies
Section snippets
Cell-cycle controversies and the continuum model
Although majority opinion accepts the restriction point as an important model on which to base the interpretation of current cell-cycle data, challenges have been raised to the concepts of restriction point, G0 phase, and cellular checkpoints, based on criticisms of experimental methods used to synchronise cell cultures. Rather than proposing that cells arrest at the restriction point on withdrawal of growth factor, the continuum model predicts that, although cells are arrested with a G1 phase
Cell-cycle regulators and restriction-point control
Disruption of restriction-point control is a common biological feature in human cancer. Cell-cycle progression is regulated by two protein classes, the cyclins and their kinase partners, the cyclin-dependent kinases (cdks). Restrictionpoint passsage is coordinated by two families of cyclins, the cyclin D family (D1, D2, and D3) and the cyclin E family (E1 and E2). The D-type cyclins bind to and activate cdks 4 and 6, and cyclins E1 and E2 interact with and activate cdk2. The activities of both
Regulation of cdk
The cdks are regulated predominantly at the posttranslational level, because protein concentrations remain constant throughout the cell cycle. Inhibitory phosphorylation on N-terminal threonine and tyrosine residues of cdk maintains kinase complexes in an inactive state. Positive regulation of cdk activity occurs in two steps: dephosphorylation of the threonine and tyrosine residues by the cdc25 phosphatase family (cdc25 A, B, and C) and phosphorylation of a central threonine residue by
The p16ink4a family
There are four members of the p16 family named according to their molecular weights, p16ink4a and p15ink4b, which share a single genomic locus (9p21), p18ink4c, and p19ink4d. p16 interacts with and inhibits cdk4 and cdk6, forming binary complexes with cdk4 in vitro (hence its name, Ink4a, inhibitor of cdk4). Addition of extracts containing proteins of the Ink4 family to active cyclin-D/cdk4 and cyclin-D/cdk6 complexes inhibits their ability to phosphorylate Rb substrate. In keeping with their
p16 mutations
The 9p21 locus is commonly disrupted in human tumours, resulting in aminoacid mutations or premature terminations that affect each of the three encoded proteins alone or in combination (homozygous deletion of this locus is common in both glioblastoma and melanoma affecting both p16 and p14ARF). The table on page 29 documents the primary cancers in which mutations or deletions at the p16 locus are found in over 15% of tumours studied. Tumourspecific alterations in p16 can affect protein function
p21cip1 family
The p21 family consists of three members, p21cip1, p27kip1, and p57kip2. Unlike the p16 proteins, which interact with and inhibit cdk4 or cdk6, the p21 family can interact with both the cyclin and cdk subunits. p21 was identified as a cdk2-interacting protein; subsequently Waf1, the same protein, was found to be expressed in diverse cell types depending on p53 expression. Gamma or ultraviolet irradiation leads to a p53-dependent G1 arrest with increased p21 protein and inhibition of
Cyclin-binding motifs
The p21/p27 interaction with the cyclin is important for cdk inhibition. Deletion of a cyclin-binding motif in p21 results in loss of binding to cyclin E and abolishes the ability of p21 to inhibit cyclin-D1/cdk4 activity. There are at least three classes of protein with similar cyclin-binding motifs: those that inhibit cyclin/cdk activity (p21 family); those that increase cdk activation (cdc25A); and the substrates themselves (E2F). The viral cyclins encoded by gammaherpesviruses including
p21/p27 localisation and degradation
Protein turnover through the ubiquitin/ proteosome is an essential means of p27 regulation. Phosphorylation of p27 on threonine 187 by cyclin-E/cdk2 leads directly to a decline in p27 protein concentrations.15 Recognition of the phosphorylated form of p27 by the ubiquitin ligase SCF(Skp2) targets p27 for degradation. Colorectal tumours with low or absent expression of p27 protein show increased proteolytic activity specific for p27.16 Patients with colon cancer and low or absent p27 protein had
G1-phase restriction point and Rb gene product
The Rb tumour suppressor is one of the mammalian regulators of passage through the restriction point. Rb deletions are frequent in retinoblastomas and osteosarcomas, and gene mutations have been found in a diverse subset of tumours, including many sarcomas, small-cell lung carcinomas, and bladder carcinomas. Changes in regulators of the Rb pathway occur in 90% of human cancers.24
Rb exists in various states of phosphorylation, progressing from an unphosphorylated transcriptionally repressive
Cyclin D family
Growth-factor and mitogenic signals converge on the cell cycle to stimulate expression of the cyclin D family (figure 6). Cyclin D is the prime integrator of these cellular signals to initiate progression through the early phase of the G1 period. The cyclin D family acts in two ways, first by cdk4/6-dependent phosphorylation and partial inactivation of the repressive activity of Rb (figure 4), and second by titrating the p27 molecule from inactive cyclin-E/cdk2 complexes.
The signalling pathways
Cyclin E family
Cyclin-D/cdk complexes regulate the activity of cyclin-E/cdk2 in two ways. First, by partly inactivating the repressive activity of Rb, the E2F transcription factor can initiate transcription of cyclin E. Second, cyclin-D/cdk complexes sequester the cdk2 inhibitor p27Kip1 from previously inactive cyclin-E/cdk2/p27Kip1x complexes.
Cyclin E, like cyclin D, is also targeted for degradation by phosphorylation. The active cyclin-E/cdk2 complex itself phosphorylates the C-terminal threonine 380. The
Therapeutic strategies
Interventions to mimic physiological cdk inhibitors have targeted the ATP-binding site of the cdk molecule. Clinical trials of one such agent, flavopiridol, have begun. The cellcycle modulator UCN-01 has a complex mode of action and can both promote and inhibit cdk activity. Pharmacological agents targeting the proteosome or histone deacetylases interfere with the degradation and expression of molecules that police the restriction point. Promising phase I and II trial data have emerged for
Kinase inhibition with flavopiridol
Flavopiridol interacts with the ATP-binding pocket of cdk2. The drug inhibits all cdks including cdk7, leads to a reduction in cyclin D1 mRNA transcription, and arrests cells in G1 or at the G2/M transition. Blockade of cdk7 can interfere with activation of cdks mediated by cdk-activating kinase, providing an alternative route for cdk inhibition.
In the earliest dose-escalation study at the US National Cancer Institute with 76 patients receiving a 72 h infusion of flavopiridol every 2 weeks,
Kinase inhibition with UCN-01
UCN-01 was first isolated from the culture broth of a Streptomyces and found to be an inhibitor of the calciumdependent protein kinase C isoenzymes. The compound was subsequently discovered to have many direct and indirect effects on cell-cycle targets. UCN-01 induces arrest with a G1-phase amount of DNA in normal cells dependent on functional Rb, attributable to a decline in cdk2 activity through increased p27/cdk2 interaction.51 In non-small-cell lung cancer cell lines expressing Rb, UCN-01
Proteosome inhibition with bortezomib
Manipulation of ubiquitination and subsequent proteosome-mediated degradation of specific cell-cycle proteins could prove advantageous in cancer therapy. Proteosome inhibition is particularly compelling, because it might exploit the genetic aberrations found in tumour cells similar to a situation when an increase in p27 protein, despite constant c-myc concentrations after proteasome inhibition, is seen to accompany apoptosis of human leukaemic HL60 cells.59 Proteosome inhibition arrests normal
Unclassified cell-cycle inhibitors
E7070 (ER-35744) was synthesised during the search for sulphonamide-related compounds with cell-cycle inhibitory activity. E7070 inhibits the activation of cyclin E/cdk2 and has cytotoxic activity against human HCT116 colon carcinoma and LX-1 non-small-cell lung carcinoma xenografts.63, 64 In the HCT116 xenograft, E7070 treatment improved activity over fluorouracil, mitomycin c, and irinotecan.64 The exact mechanism of cyclin-E/cdk2 inactivation is unclear.
Four different treatment schedules
Histone deactetylase inhibition
Histone acetylation and deacetylation is central to the control of gene transcription via modification of chromatin structure. The repression of E2F-dependent genes occurs partly by the Rb-mediated recruitment of a histone deacetylase to E2F promoter sites; removal of the acetyl group from lysine residues restores the positive charge, limiting chromatin transcriptional activity.
Paradoxically, inhibition of histone deacetylase leads to a gene expression profile associated with differentiation
Conclusion
There are many unanswered questions about cell-cycletargeted therapies. The challenges of how to assess treatment response to cytostatic agents and their sequence of administration when used in combination with conventional cytotoxic agents will have to be confronted in more advanced clinical studies. Nevertheless, with improvement in our understanding of both the mechanism of action of these agents and the intricate regulation of the cell cycle, the opportunity to offer treatment directed
Search strategy and selection criteria
References (74)
- et al.
Tumour suppression at the mouse INK4a locus mediated by the alternative reading frame product of p19ARF
Cell
(1997) - et al.
cdk phosphorylation triggers sequential intramolecular interactions that progressively block Rb functions as cells move through G1
Cell
(1999) - et al.
The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01
J Biol Chem
(2000) - et al.
Structural basis for Chk1 inhibition by UCN-01
J Biol Chem
(2002) - et al.
E7070, a novel sulphonamide agent with potent antitumour activity in vitro and in vivo
Eur J Cancer
(2001) - et al.
Phase I and pharmacokinetic study of E7070, a novel sulfonamide, given at a daily times 5 schedule in patients with solid tumours: a study by the EORTC-Early Clinical Studies Group (ECSG)
Ann Oncol
(2001) - et al.
Cell cycle control and cancer
Science
(1994) A restricition point for control of normal animal cell proliferation
Proc Natl Acad Sci USA
(1974)Reappraisal of serum starvation, the restriction point, G0 and G1 phase arrest points
FASEB J
(2003)- et al.
Alternative reading frames of the INK4a tumor suppressor gene encode two unrelated proteins capable of inducing cell cycle arrest
Cell
(1995)
Loss of p16Ink4a confers susceptibility to metastatic melanoma in mice
Nature
Loss of p16Ink4a with retention of p19Arf predisposes mice to tumourigenesis
Nature
Retinoblastoma-proteindependent cell-cycle inhibition by the tumour suppressor p16
Nature
A p16INK4a-insensitive CDK4 mutant targeted by cytolytic T lymphocytes in a human melanoma
Science
p16ink4a/CDKN2A tumour suppressor and its relatives
BBA Rev Cancer
New functional activities for the p21 family of cdk inhibitors
Genes Dev
Crystal structure of a gammaherpesvirus cyclin-cdk complex
EMBO J
Viral cyclin/cdk6 complexes evade inhibition by cdk inhibitory proteins
Nature
Selective killing of transformed cells by cyclin/cyclin-dependent kinase 2 antagonists
Proc Natl Acad Sci USA
Cdk2 is a regulator of p27kip1
Genes Dev
Expression of cell-cycle regulators p27Kip1 and cyclin E, alone and in combination, correlate with survival in young breast cancer patients
Nat Med
Increased proteosome-dependent degradation of the cyclin-dependent kinase inhibitor p27 in aggressive colorectal carcinomas
Nat Med
Decreased levels of the cell-cycle inhibitor p27kip1 protein: prognostic implications in primary breast cancer
Nat Med
PKB/Akt phosphorylates p27, impairs nuclear import of p27 and opposes p27-mediated G1 arrest
Nat Med
PKB/Akt mediates cell-cycle progression by phosphorylation of p27kip1 at threonine 157 and modulation of its cellular localization
Nat Med
Cytoplasmic relocalization and inhibition of the cyclin-dependent kinase inhibitor p27(Kip1) by PKB/Akt-mediated phosphorylation in breast cancer
Nat Med
Cytoplasmic localization of p21Cip1/WAF1 by Akt-induced phosphorylation in HER-2/neuoverexpressing cells
Nat Cell Biol
Are p27 and p21 cytoplasmic oncoproteins?
Cell Cycle
To cycle or not to cycle: a critical decision in cancer
Nat Rev Cancer
Functional inactivation of the retinoblastoma protein requires sequential modification by at least two distinct cyclin-cdk complexes
Mol Cell Biol
Differential regulation of retinoblastoma tumor suppressor protein by G1 cyclindependent kinase complexes in vivo
Mol Cell Biol
Beta-catenin regulates expression of cyclin D1 in colon carcinoma cells
Nature
Glycogen synthase kinase-3 beta regulates cyclin D1 proteolysis and subcellular localisation
Genes Dev
SSeCKS, a major protein kinase C substrate with tumour suppressor activity, regulates G1-S progression by controlling the expression and cellular compartmentalization of cyclin D
Mol Cell Biol
The p21(Cip1) and p27(Kip1) CDK ‘inhibitors’ are essential activators of cyclin D-dependent kinases in murine fibroblasts
EMBO J
Activation of cyclin D1-kinase in murine fibroblasts lacking both p21(Cip1) and p27(Kip1)
Oncogene
Regulation of CDK4 activity by a novel CDK4-binding protein, p34SEI-1
Genes Dev
Cited by (229)
Pancreatic cancer stem cell-derived exosomal miR-210 mediates macrophage M2 polarization and promotes gemcitabine resistance by targeting FGFRL1
2024, International ImmunopharmacologyEngineering prodrug nanomicelles as pyroptosis inducer for codelivery of PI3K/mTOR and CDK inhibitors to enhance antitumor immunity
2022, Acta Pharmaceutica Sinica BThe application of nanoparticles-based ferroptosis, pyroptosis and autophagy in cancer immunotherapy
2024, Journal of Nanobiotechnology
CS is a clinical scientist and specialist registrar at the Royal Marsden Hospital Breast Unit, London, UK.