The principles of CDKs/cyclins in cell-cycle progression
Deregulation of the cell cycle is a common feature of human cancer. The mammalian cell cycle is controlled by a subfamily of cyclin-dependent kinases (CDKs), the activity of which is modulated by several activators (cyclins) and inhibitors (INK4, and WAF1/KIP inhibitors). The activity of cell cycle CDKs is deregulated in cancer cells owing to genetic or epigenetic changes in CDKs, their regulators, or upstream mitogenic pathways. Hence, tumor-associated cell cycle defects are often mediated by alterations in CDK activity which leads to unscheduled proliferation, as well as genomic and chromosomal instability, said Marcos Malumbres from the Centro Nacional de Investigaciones Oncológicas.
Malumbres made his observations at the 2018 European Society for Medical Oncology (ESMO) Symposium on Signalling Pathways in Cancer in Barcelona in late March, which focused on modulation of the cell cycle via CDK inhibition and was held in partnership with the European Association for Cancer Research (EACR).
Malumbres added that there are up to 20 CDKs and 29 cyclins in humans, but only a certain subset of CDK-cyclin complexes is directly involved in driving the cell cycle: a classic example is the almost universal deregulation of the CDK-cyclin-retinoblastoma (RB) pathway in cell-cycle entry during malignant transformation. Genetic alterations usually affect CDK4 and CDK6, their positive (mainly cyclin D1) and negative (INK4A and INK4B) regulators and their substrates (mainly RB protein). RB protein is sequentially phosphorylated by CDK4/6-cyclin-D and CDK2-cyclin-E complexes. This phosphorylation inactivates the growth-suppression properties of RB and stimulates progression through G1 and into S phase. RB inactivation allows E2F family proteins to promote transcription of genes implicated in cell cycle progression. As it has been shown that different stimuli converge on the CDK4/6 pathway to promote cell cycle entry, a gene-signature of E2F (RBsig) can be used as a tool to measure the downstream effect of multiple signaling pathways converging on CDK4/6, with potential implications as a biomarker.
Unsurprisingly, said Malumbres, deregulation of the CDK family of proteins is a hallmark of several cancers, and drug-targeted inhibition of specific members has generated very encouraging results in clinical trials. Furthermore, different signaling pathways such as PI3K/AKT, RAS/MAPK, lead to increased Cyclin D levels, activating CDK4 and CDK6 and leading to G1-S phase transition and cell proliferation; therefore, inhibition of components involved in those pathways are also a matter of study when developing new drugs and co-treatment studies together with CDK4/6 inhibitors.
The CDK4/CDK6/RB pathway
In another presentation at the ESMO-EACR symposium, Tobias Otto (University Hospital RWTH Aachen) noted that proliferation depends on progression through four distinct phases of the cell cycle (G0/G1, S, G2 and M), which is regulated by several CDKs that act in complex with their cyclin partners. CDK4 and 6 are highly homologous and both play a major role driving cell cycle progression from G0/G1 to S phase. The activity of CDK4 and CDK6 is positively controlled by association with D-type cyclins (D1, D2 and D3) and negatively by binding to CDK inhibitors of the INK4 family (p15, p16, p18 and p19). Active cyclin D-CDK4/6 complexes phosphorylate target pocket proteins like RB, p130, p107, preventing their binding and inhibition of E2F transcription factors. Hence, CDK4/6 enable E2F transcription factors to activate transcription of a plethora of genes involved in cell cycle progression from G1 into S phase, DNA replication, chromatin structure, chromosome segregation and mitotic spindle assembly checkpoint.
Among the E2F transcriptional targets, Otto said, are cyclins E1 and E2, which bind and activate CDK2. Cyclin E-CDK2 complexes further phosphorylate RB, thereby initiating a positive feedback loop. Components of the CDK4/6-RB pathway are commonly mutated in human cancers; deletion of the RB gene occurs frequently in many tumor types allowing proliferation independently of cyclin D-CDK4/6 activity. These analyses revealed that individual D-type cyclins, CDK4 and CDK6 are required for tumor initiation, and that their continued expression is critical for tumor maintenance; therefore, D-type cyclins and/or CDK4/6 are hyperactive in tumor cells compared with normal cells, where inhibition of CDK4 and CDK6 catalytic activity had no major effects. Collectively, these facts illustrate that tumors are frequently dependent on individual cyclins and CDKs and susceptible to their targeted inhibition.
Also at the ESMO-EACR symposium, researchers examined the work conducted by Pfizer, Novartis and others on the inhibition of CDK4/6. To read the author’s full report, click here.