G2/mitotic-specific cyclin-B1 is a protein that in humans is encoded by the CCNB1 gene. [5]
G2/mitotic-specific cyclin-B1 is a protein that in humans is encoded by the CCNB1 gene. [5]
Cyclin B1 is a regulatory protein involved in mitosis. The gene product complexes with p34 (Cdk1) to form the maturation-promoting factor (MPF). Two alternative transcripts have been found, a constitutively expressed transcript and a cell cycle-regulated transcript that is expressed predominantly during G2/M phase of the cell cycle. The different transcripts result from the use of alternate transcription initiation sites. [6]
Cyclin B1 contributes to the switch-like all or none behavior of the cell in deciding to commit to mitosis. Its activation is well-regulated, and positive feedback loops ensure that once the cyclin B1-Cdk1 complex is activated, it is not deactivated. Cyclin B1-Cdk1 is involved in the early events of mitosis, such as chromosome condensation, nuclear envelope breakdown, and spindle pole assembly.
Once activated, cyclin B1-Cdk1 promotes several of the events of early mitosis. The active complex phosphorylates and activates 13S condensin, [7] which helps to condense chromosomes.
Another important function of the cyclin B1-Cdk1 complex is to break down the nuclear envelope. The nuclear envelope is a membranous structure containing large protein complexes supported by a network of nuclear lamins. Phosphorylation of the lamins by cyclin B1-Cdk1 causes them to dissociate, [8] compromising the structural integrity of the nuclear envelope so that it breaks down. The destruction of the nuclear envelope is important because it allows the mitotic spindle to access the chromosomes.
Like all cyclins, levels of cyclin B1 oscillate over the course of the cell cycle. Just prior to mitosis, a large amount of cyclin B1 is present in the cell, but it is inactive due to phosphorylation of Cdk1 by the Wee1 kinase. The complex is activated by dephosphorylation by the phosphatase Cdc25. [9] Cdc25 is always present in the cell but must be activated by phosphorylation. A possible trigger for activation is phosphorylation by cyclin A-Cdk, which functions before cyclin B1-Cdk in the cell cycle. Active Cdk1 is also capable of phosphorylating and activating Cdc25 and thus promote its own activation, resulting in a positive feedback loop. Once cyclin B1-Cdk1 is activated, it remains stably active for the rest of mitosis.
Another mechanism by which cyclin B1-Cdk1 activity is regulated is through subcellular localization. Before mitosis almost all cyclin B1 in the cell is located in the cytoplasm, but in late prophase it relocates to the nucleus. This is regulated by the phosphorylation of cyclin B1, in contrast to phosphorylation of Cdk1 regulating the activity of the complex. Phosphorylation of cyclin B1 causes it to be imported to the nucleus, [10] and phosphorylation also prevents export from the nucleus by blocking the nuclear export signal. [11] Cyclin B1 is phosphorylated by Polo kinase and Cdk1, again setting up a positive feedback loop that commits cyclin B1-Cdk1 to its fate.
At the end of mitosis, cyclin B1 is targeted for degradation by the APC through its APC localization sequence, permitting the cell to exit mitosis.
One of the hallmarks of cancer is the lack of regulation in the cell cycle. The role of cyclin B1 is to transition the cell from G2 to M phase but becomes unregulated in cancer cells where overexpression of cyclin B1 can lead to uncontrolled cell growth by binding to its partner Cdks. Binding of Cdks can lead to phosphorylation of other substrates at inappropriate time and unregulated proliferation. [19] This is a consequence of p53, tumor suppressor protein, being inactivated. Wild-type p53 have been shown to suppress cyclin B1 expression. [20] [21]
Previous work has shown that high cyclin B1 expression levels are found in variety of cancers such as breast, cervical, gastric, colorectal, head and neck squamous cell, non-small-cell lung cancer, colon, prostate, oral and esophageal. [19] [22] [23] [24] [25] High expression levels are usually seen before the tumor cells become immortalized and aneuploid which can contribute to the chromosomal instability and the aggressive nature of certain cancers. [26] These high levels of cyclin B1 can also be associated to the extent of tumor invasion and aggressiveness therefore concentration of cyclin B1 can be used to determine the prognosis of cancer patients. [22] [27] For example, an increase in expression of cyclin B1/cdc2 is significantly higher in breast tumor tissue and shown to increase lymph node metastasis in breast cancer. [22] [28]
Cyclin B1 can reside in the nucleus or the cytoplasm which can have an effect on the malignant potential of cyclin B1 when overexpressed in each location. Nuclear-dominant expression of cyclin B1 leads to poorer prognosis due to its weak activity compared to cytoplasmic cyclin B1. [26] This trend has been observed in esophageal cancer, head and neck squamous cell cancer and breast cancer. [19] [29]
While the exact mechanism that explains how cyclin B1 becomes overexpressed is not very well understood, previous work has shown that down regulation of cyclin B1 can lead to tumor regression. A possible treatment option for tumor suppression is to deliver gene or protein to target the degradation of cyclin B1. Previous work done has shown that cyclin B1 is essential for tumor cell survival and proliferation and that a decrease in expression levels only leads to tumor-specific and not normal cell death. [30] Reduction of cyclin B1 can stop cells in the G2 phase of the cell cycle and triggers cell death by preventing the chromosomes from condensing and aligning. The specific downregulation of cyclin B1, however, did not influence other molecules that facilitated the transition from the G2 to M phase such as Cdk1, Cdc25c, Plk1 and cyclin A. Therefore the delivery of a therapeutic gene to correct these mutations is a viable treatment option for tumor suppression. [19]
In early stages of cancer when the cyclin B1 concentration is high, it is recognized by the immune system, leading to the production of antibodies and T cells. It would then be possible to take advantage of this to monitor the immune response for early cancer detection. [31] An ELISA (Enzyme-linked immunosorbent assay) can be performed to the measure the antibodies recognizing cyclin B1.
Cyclin B1 expression levels can be used as a tool to determine prognosis of patients with breast cancers. The intracellular concentration can have important implications for cancer prognosis. High levels of nuclear cyclin B1 is associated with high tumor grade, larger tumor size and higher metastasis probability, therefore a high level of cyclin B1 is a predictor of poor prognosis. [26]
Studies in non-small cell lung cancer demonstrated that high levels of cyclin B1 are associated with poorer prognosis. The study also found that this correlation between expression levels was only found in patients with squamous cell carcinoma. This finding indicates the possibility of using cyclin B1 expression as a prognostic marker for patients with early stage non-small cell lung cancer. [32]
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that causes it to divide into two daughter cells. These events include the duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process called cell division.
Cell growth refers to an increase in the total mass of a cell, including both cytoplasmic, nuclear and organelle volume. Cell growth occurs when the overall rate of cellular biosynthesis is greater than the overall rate of cellular degradation.
Cyclin-dependent kinases (CDKs) are a predominant group of serine/threonine protein kinases involved in the regulation of the cell cycle and its progression, ensuring the integrity and functionality of cellular machinery. These regulatory enzymes play a crucial role in the regulation of eukaryotic cell cycle and transcription, as well as DNA repair, metabolism, and epigenetic regulation, in response to several extracellular and intracellular signals. They are present in all known eukaryotes, and their regulatory function in the cell cycle has been evolutionarily conserved. The catalytic activities of CDKs are regulated by interactions with CDK inhibitors (CKIs) and regulatory subunits known as cyclins. Cyclins have no enzymatic activity themselves, but they become active once they bind to CDKs. Without cyclin, CDK is less active than in the cyclin-CDK heterodimer complex. CDKs phosphorylate proteins on serine (S) or threonine (T) residues. The specificity of CDKs for their substrates is defined by the S/T-P-X-K/R sequence, where S/T is the phosphorylation site, P is proline, X is any amino acid, and the sequence ends with lysine (K) or arginine (R). This motif ensures CDKs accurately target and modify proteins, crucial for regulating cell cycle and other functions. Deregulation of the CDK activity is linked to various pathologies, including cancer, neurodegenerative diseases, and stroke.
A cyclin-dependent kinase complex is a protein complex formed by the association of an inactive catalytic subunit of a protein kinase, cyclin-dependent kinase (CDK), with a regulatory subunit, cyclin. Once cyclin-dependent kinases bind to cyclin, the formed complex is in an activated state. Substrate specificity of the activated complex is mainly established by the associated cyclin within the complex. Activity of CDKCs is controlled by phosphorylation of target proteins, as well as binding of inhibitory proteins.
Maturation-promoting factor (abbreviated MPF, also called mitosis-promoting factor or M-Phase-promoting factor) is the cyclin-Cdk complex that was discovered first in frog eggs. It stimulates the mitotic and meiotic phases of the cell cycle. MPF promotes the entrance into mitosis (the M phase) from the G2 phase by phosphorylating multiple proteins needed during mitosis. MPF is activated at the end of G2 by a phosphatase, which removes an inhibitory phosphate group added earlier.
G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with the onset of prophase, the first phase of mitosis in which the cell’s chromatin condenses into chromosomes.
Cdc25 is a dual-specificity phosphatase first isolated from the yeast Schizosaccharomyces pombe as a cell cycle defective mutant. As with other cell cycle proteins or genes such as Cdc2 and Cdc4, the "cdc" in its name refers to "cell division cycle". Dual-specificity phosphatases are considered a sub-class of protein tyrosine phosphatases. By removing inhibitory phosphate residues from target cyclin-dependent kinases (Cdks), Cdc25 proteins control entry into and progression through various phases of the cell cycle, including mitosis and S ("Synthesis") phase.
Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint. Progression through these checkpoints is largely determined by the activation of cyclin-dependent kinases by regulatory protein subunits called cyclins, different forms of which are produced at each stage of the cell cycle to control the specific events that occur therein.
Cyclin B is a member of the cyclin family. Cyclin B is a mitotic cyclin. The amount of cyclin B and the activity of the cyclin B-Cdk complex rise through the cell cycle until mitosis, where they fall abruptly due to degradation of cyclin B. The complex of Cdk and cyclin B is called maturation promoting factor or mitosis promoting factor (MPF).
Cyclin A is a member of the cyclin family, a group of proteins that function in regulating progression through the cell cycle. The stages that a cell passes through that culminate in its division and replication are collectively known as the cell cycle Since the successful division and replication of a cell is essential for its survival, the cell cycle is tightly regulated by several components to ensure the efficient and error-free progression through the cell cycle. One such regulatory component is cyclin A which plays a role in the regulation of two different cell cycle stages.
Cyclin-dependent kinase 1 also known as CDK1 or cell division cycle protein 2 homolog is a highly conserved protein that functions as a serine/threonine protein kinase, and is a key player in cell cycle regulation. It has been highly studied in the budding yeast S. cerevisiae, and the fission yeast S. pombe, where it is encoded by genes cdc28 and cdc2, respectively. With its cyclin partners, Cdk1 forms complexes that phosphorylate a variety of target substrates ; phosphorylation of these proteins leads to cell cycle progression.
Checkpoint kinase 1, commonly referred to as Chk1, is a serine/threonine-specific protein kinase that, in humans, is encoded by the CHEK1 gene. Chk1 coordinates the DNA damage response (DDR) and cell cycle checkpoint response. Activation of Chk1 results in the initiation of cell cycle checkpoints, cell cycle arrest, DNA repair and cell death to prevent damaged cells from progressing through the cell cycle.
M-phase inducer phosphatase 2 is an enzyme that in humans is encoded by the CDC25B gene.
WEE1 homolog , also known as WEE1, is a protein which in humans is encoded by the WEE1 gene.
M-phase inducer phosphatase 3 is an enzyme that in humans is encoded by the CDC25C gene.
Large tumor suppressor kinase 1 (LATS1) is an enzyme that in humans is encoded by the LATS1 gene.
Membrane-associated tyrosine- and threonine-specific cdc2-inhibitory kinase also known as Myt1 kinase is an enzyme that in humans is encoded by the PKMYT1 gene.
Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast Schizosaccharomyces pombe. Wee1 has a molecular mass of 96 kDa and is a key regulator of cell cycle progression. It influences cell size by inhibiting the entry into mitosis, through inhibiting Cdk1. Wee1 has homologues in many other organisms, including mammals.
A series of biochemical switches control transitions between and within the various phases of the cell cycle. The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases. The phases include the G1 and G2 phases, DNA replication or S phase, and the actual process of cell division, mitosis or M phase. During the M phase, the chromosomes separate and cytokinesis occurs.
The G2-M DNA damage checkpoint is an important cell cycle checkpoint in eukaryotic organisms that ensures that cells don't initiate mitosis until damaged or incompletely replicated DNA is sufficiently repaired. Cells with a defective G2-M checkpoint will undergo apoptosis or death after cell division if they enter the M phase before repairing their DNA. The defining biochemical feature of this checkpoint is the activation of M-phase cyclin-CDK complexes, which phosphorylate proteins that promote spindle assembly and bring the cell to metaphase.
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