[Molecular mechanisms controlling the cell cycle: fundamental aspects and implications for oncology].
BACKGROUND
Comprehension of cell cycle regulation mechanisms has progressed very quickly these past few years and regulators of the cell cycle have gained widespread importance in cancer. This review first summarizes major advances in the understanding of the control of cell cycle mechanisms. Examples of how this control is altered in tumoral cells are then described.


BACKGROUND
The typical mammalian cell cycle consists of four distinct phases occurring in a well-defined order, each of which should be completed successfully before the next begins. Progression of eukaryotic cells through major cell cycle transitions is mediated by sequential assembly and activation of a family of serine-threonine protein kinases, the cyclin dependent kinases (CDK). The timing of their activation is determined by their post-translational modifications (phosphorylations/dephosphorylations), and by the association of a protein called cyclin, which is the regulatory subunit of the kinase complex. The cyclin family is divided into two main classes. The 'G1 cyclins' include cyclins C, D1-3, and E, and their accumulation is rate-limiting for progression from the G1 to S phase. The 'mitotic or G2 cyclins', which include cyclin A and cyclin B, are involved in the control of G2/M transition and mitosis. The cyclins bind to and activate the CDK, which leads to phosphorylation (and then inhibition) of the tumor suppressor protein, pRb. pRb controls commitment to progress from the G1 to S phase, at least in part by repressing the activity of the E2F transcription factors known to promote cell proliferation. Both the D-type cyclins and their partner kinases CDK4/6 have proto-oncogenic properties, and their activity is carefully regulated at multiple levels including negative control by two families of CDK inhibitors. While members of the INK4 family (p16INK4A, p15INK4B, p18INK4C, p19INK4D) interact specifically with CDK4 and CDK6, the CIP/KIP inhibitors p21CIP1/WAF1, p27KIP1 and p57KIP2 inhibit a broader spectrum of CDK. The interplay between p16INK4A, cyclin D/CDK, and pRb/E2F together constitute a functional unit collectively known as the 'pRb pathway'. Each of the major components of this mechanism may become deregulated in cancer, and accumulating evidence points to the 'pRb pathway' as a candidate obligatory target in multistep oncogenesis of possibly all human tumor types.


CONCLUSIONS
Major advances in the understanding of cell cycle regulation mechanisms provided a better knowledge of the molecular interactions involved in human cancer. This progress has led to the promotion of new therapeutic agents presently in clinical trials or under development. Moreover, the components of the cell cycle are probably involved in other non-cancerous diseases and their role must be defined.
Viallard
J F
JF
Service de médecine interne et maladies infectieuses, centre François-Magendie, hôpital du Haut-Lévêque, 5, avenue Magellan, 33604 Pessac, France. jean-françois.viallard@chu-bordeaux.fr
Lacombe
F
F
Belloc
F
F
Pellegrin
J L
JL
Reiffers
J
J
D000818:Animals; D064007:Ataxia Telangiectasia Mutated Proteins; D002352:Carrier Proteins; D002453:Cell Cycle / Q000235:genetics / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000235:genetics / Q000502:physiology*; D002471:Cell Transformation, Neoplastic / Q000235:genetics; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000502:physiology; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology; D016213:Cyclins / Q000235:genetics / Q000502:physiology; D004249:DNA Damage; D004261:DNA Replication; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D005057:Eukaryotic Cells / Q000166:cytology; D005786:Gene Expression Regulation; D016161:Genes, Retinoblastoma; D019942:Genes, p16; D016158:Genes, p53; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D008869:Microtubule-Associated Proteins / Q000172:deficiency / Q000235:genetics / Q000502:physiology; D009363:Neoplasm Proteins / Q000235:genetics / Q000502:physiology; D014411:Neoplastic Stem Cells / Q000166:cytology; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000502:physiology; D016160:Retinoblastoma Protein / Q000502:physiology; D056465:Retinoblastoma-Binding Protein 1; D012441:Saccharomyces cerevisiae / Q000166:cytology; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology; D016159:Tumor Suppressor Protein p53 / Q000502:physiology; D025521:Tumor Suppressor Proteins
C533431:Arid4a protein, mouse; C493956:CDKN1A protein, human; D002352:Carrier Proteins; C493957:Cdkn1a protein, mouse; C493961:Cdkn1b protein, mouse; D018797:Cell Cycle Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D008869:Microtubule-Associated Proteins; D009363:Neoplasm Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; C576325:Atm protein, mouse; D017346:Protein Serine-Threonine Kinases; D018844:Cyclin-Dependent Kinases