These opposing events are thought to be mediated by phosphorylation sites around the CDK inhibitors. human cancers. However, it has only recently been appreciated that dysfunction or loss of many of these same genes can also mediate resistance versus sensitivity to currently used cancer therapies. In this review, Lanabecestat we focus on two important cell cycle regulators – the cyclin-dependent kinase (CDK) inhibitors p21 (CDKN1A) and p27 (CDKN1B) – by discussing their early discovery as mediators of cell cycle checkpoints and subsequent work showing how their loss or dysfunction prospects to carcinogenesis and, more recently, resistance to therapeutic drugs. We also briefly discuss their paradoxical effects as cell cycle and tumour promoters. Cell cycle, cyclins, CDKs and CDK inhibitors The cell cycle comprises a very carefully orchestrated set of events that can lead to cell proliferation, senescence or apoptosis. Cells progress through the various phases of the cell cycle via the interactions of different cyclins with their respective CDK subunits (Fig. 1). The name cyclin comes from the fact that these proteins were first identified as molecules that accumulated and were then degraded at unique points during the cell cycle of embryonic sea urchin eggs (Ref. 1). There are now several recognised classes or types of cyclins, active in different stages of the cell cycle. The D- and E-type cyclins are associated with the G1-S phase transition of the cell cycle (Refs 2, 3). Human cyclin D1 (CCND1; also called PRAD1 and BCL-1) and E were recognized from mRNAs able to restore cyclin function in cyclin-deficient yeast (Refs 4, 5). Cyclin D1 quickly generated interest when it was mapped to chromosomal locus 11q13 close to a region known to be a breakpoint for certain chromosomal inversions that lead to parathyroid adenomas (Ref. 6). This ultimately led to further studies that uncovered additional cyclins in human and other mammalian systems. Open in a separate window Physique 1 Cell cycle progression is regulated by cyclin-CDK complexesFollowing a mitogenic stimulus, the decision to enter the cell cycle is regulated by complexes of cyclin-B-CDK1 and cyclin-C-CDK3 complexes (not shown). Next, during G1, D- and E-type cyclins are upregulated and assemble with their respective CDK binding partners. The cyclin-D-CDK4/6 and cyclin-E-CDK2 complexes then phosphorylate the retinoblastoma protein (pRB), thus inactivating it and releasing the transcription factor E2F from its inhibition. E2F then activates a variety of growth-promoting genes, which sends the cell cycle into the synthesis (S) phase. The cyclin-B-CDK1 complex continues to inhibit pRB at the S-G2 and G2-mitosis (M) checkpoints via phosphorylation to ensure completion of the cell cycle. Finally, following dephosphorylation of pRB, cells exit the M phase and pRB once again inhibits E2F. The CDK inhibitors p21 and p27 regulate the cell cycle by inhibiting the functions of the various CDK proteins as indicated. Abbreviations: CDK, cyclin-dependent kinase; P, phosphorylation. CDKs were recognized by virtue of the fact that mutations of these proteins in yeast affected their growth rates (Refs 7, 8). Because many of the early CDKs were discovered independently before their functions were well comprehended, the nomenclature of these proteins was nonuniform and often confusing. In 1991, experts met at Cold Planting season Harbor for the Symposium on Cell Cycle to agree on the following designations: cdc2 would become.For p27, the underlying mechanism found in the majority of these studies was thought to be phosphorylation of Thr157. lost and/or have loss of function in many different human cancers. However, it has only recently been appreciated that dysfunction or loss of many of these same genes can also mediate resistance versus sensitivity to currently used cancer therapies. In this review, we focus on two important cell cycle regulators – the cyclin-dependent kinase (CDK) inhibitors p21 (CDKN1A) and p27 (CDKN1B) – by discussing their early discovery as mediators of cell cycle checkpoints and subsequent work showing how their loss or dysfunction prospects to carcinogenesis and, more recently, resistance to therapeutic drugs. We also briefly discuss their paradoxical effects as cell cycle and tumour promoters. Cell cycle, cyclins, CDKs and CDK inhibitors The cell cycle comprises a very carefully orchestrated set of events that can lead to cell proliferation, senescence or apoptosis. Cells progress through the various phases of the cell cycle via the interactions of different cyclins with their respective CDK subunits (Fig. 1). The name cyclin comes from the fact that these proteins were first identified as molecules that accumulated and were then degraded at unique points during the cell cycle of embryonic sea urchin eggs (Ref. 1). There are now several recognised classes or types of cyclins, active in different stages of the cell cycle. The D- and E-type cyclins are associated with the G1-S phase transition of the cell cycle (Refs 2, 3). Human cyclin D1 (CCND1; also called PRAD1 and BCL-1) and E were identified from mRNAs able to restore cyclin function in cyclin-deficient yeast (Refs 4, 5). Cyclin D1 quickly generated interest when it was mapped to chromosomal locus 11q13 close to a region known to be a breakpoint for certain chromosomal inversions that lead to parathyroid adenomas (Ref. 6). This ultimately led to further studies that uncovered additional cyclins in human IQGAP1 and other mammalian systems. Open in a separate window Figure 1 Cell cycle progression is regulated by cyclin-CDK complexesFollowing a mitogenic stimulus, the decision to enter the cell cycle is regulated by complexes of cyclin-B-CDK1 and cyclin-C-CDK3 complexes (not Lanabecestat shown). Next, during G1, D- and E-type cyclins are upregulated and assemble with their respective CDK binding partners. The cyclin-D-CDK4/6 and cyclin-E-CDK2 complexes then phosphorylate the retinoblastoma protein (pRB), thus inactivating it and releasing the transcription factor E2F from its inhibition. E2F then activates a variety of growth-promoting genes, which sends the cell cycle into the synthesis (S) phase. The cyclin-B-CDK1 complex continues to inhibit pRB at the S-G2 and G2-mitosis (M) checkpoints via phosphorylation to ensure completion of the cell cycle. Finally, following dephosphorylation of pRB, cells exit the M phase and pRB once again inhibits E2F. The CDK inhibitors p21 and p27 regulate the cell cycle by inhibiting the functions of the various CDK proteins as indicated. Abbreviations: CDK, cyclin-dependent kinase; P, phosphorylation. CDKs were identified by virtue Lanabecestat of the fact that mutations of these proteins in yeast affected their growth rates (Refs 7, 8). Because many of the early CDKs were discovered independently before their functions were well understood, the nomenclature of these proteins was nonuniform and often confusing. In 1991, researchers met at Cold Spring Harbor for the Symposium on Cell Cycle to agree on the following designations: cdc2 would become.
