Regulation of cyclins during the cell cycle pathway
This poster explores the role of the various cyclins during the progression of the cell cycle.
Eukaryotic cell division, regulated by a well-defined cell cycle, is one of the most fundamental biological processes, controlling how we develop and grow, how we heal, and playing a critical role in diseases such as cancer. The cell cycle has an interphase broken into G1, S, and G2 phases, and the regulation of cyclins during these is fundamental to ensuring that cells progress through the various phases in a controlled and orderly manner. This regulation is perhaps most significant during the G1/S and G2/M transitions, key checkpoints in the cell cycle whereby a cell commits itself to undergoing cell division, or an alternative pathway.
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Cyclins are the main regulators of the cell cycle, whose function is to specifically activate a corresponding cyclin-dependent kinase. Different cyclin family members are sequentially expressed to ensure timely activation of the relevant CDKs, which themselves then act upon downstream effectors such retinoblastoma protein (Rb) to ensure the cell cycle progresses, or arrests. 1
The G1/S transition is a critical checkpoint where the cell commits to DNA replication. This transition is regulated by the G1 cyclins (Cyclin D) and the G1/S cyclins (Cyclin E). Cyclin D binds to CDK4 and CDK6, forming active complexes that phosphorylate the Rb. Phosphorylated Rb releases E2F transcription factors, which then activate the transcription of genes required for DNA synthesis. The expression of Cyclin D and activity of Cyclin D-CDK4/6 complexes is regulated by growth factors and mitogenic signals. Cyclin D levels increase in the presence of these signals, promoting cell cycle progression.
Cyclin E binds to CDK2, forming a complex that further phosphorylates Rb and other substrates, driving the cell from the G1 phase into the S phase. Cyclin E levels peak at the G1/S transition and are tightly regulated by proteolysis. The SCF (Skp, Cullin, F-box containing complex) ubiquitin ligase targets Cyclin E for degradation, ensuring that CDK2 activity is temporary2.
The G2/M transition is another crucial checkpoint where the cell, having undergone DNA replication, now prepares for mitosis. This transition is regulated by the mitotic cyclins (Cyclin B) and CDK1 (also known as Cdc2). Cyclin B binds to CDK1, forming the maturation-promoting factor (MPF). MPF activity is vital for initiating mitosis, as it phosphorylates various substrates involved in chromosome condensation, nuclear envelope breakdown, and spindle assembly. The activity of Cyclin B-CDK1 is regulated by phosphorylation and dephosphorylation. CDK1 is kept inactive by phosphorylation at specific inhibitory sites. Activation of CDK1 requires dephosphorylation by the phosphatase Cdc25. Moreover, Cyclin B levels are controlled by the anaphase-promoting complex/cyclosome (APC/C), which targets Cyclin B for degradation, ensuring that mitosis progresses immediately.
Proper regulation of cyclins and CDKs is essential for maintaining genomic integrity and preventing uncontrolled cell proliferation. Cyclin dysregulation can contribute to numerous health issues, including cancer. For instance, overexpression or genetic amplification of cyclins, particularly Cyclin D and Cyclin E, is commonly observed in cancers, leading to hyperactivation of CDKs, and resulting in uncontrolled cell division and tumor growth. Mutations in CDK inhibitors, such as p16INK4A, can also contribute to cancer by failing to restrain CDK activity3.
Abnormal cell cycle re-entry and cyclin expression have also been implicated in neurodegenerative diseases like Alzheimer's disease4. Neurons, typically post-mitotic, may aberrantly express cyclins, leading to cell cycle re-entry and apoptosis. In addition, mutations in genes encoding cyclins or CDKs can lead to developmental disorders. For example, mutations in Cyclin D1 are linked with microcephaly, a condition characterized by reduced brain size.
Cyclin regulation during the cell cycle is a complex and tightly controlled process that ensures proper cell division and genomic integrity. Through their interaction with CDKs, cyclins drive the cell cycle transitions at the G1/S and G2/M checkpoints. Dysregulation of cyclins can lead to various diseases, including cancer, neurodegenerative diseases, and developmental disorders. Understanding the molecular mechanisms underlying cyclin regulation provides valuable insights into potential therapeutic targets for these conditions. Our pathway poster explores the regulation of cyclins, detailing the phases involved and their implications for cellular health and disease. It also includes a list of antibodies, related products, and their applications.
References
1. Ding, L. et al. The Roles of Cyclin-Dependent Kinases in Cell-Cycle Progression and Therapeutic Strategies in Human Breast Cancer. Int. J. Mol. Sci. 21, 1960 (2020).
2. Pellarin, I. et al. Cyclin-dependent protein kinases and cell cycle regulation in biology and disease. Sig. Transduct. Target Ther. 10, 11 (2025).
3. Casimiro, M. C., Crosariol, M., Loro, E., Li, Z. & Pestell, R. G. Cyclins and cell cycle control in cancer and disease. Genes Cancer 3, 649–657 (2012).
4. Nagy, Z. The dysregulation of the cell cycle and the diagnosis of Alzheimer's disease. Biochim. Biophys. Acta 1772, 402–408 (2007).