Cyclin B is a member of the cyclin family. Cyclin B is a

mitotic In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...

cyclin. The amount of cyclin B (which binds to

Cdk1 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 t ...

) and the activity of the cyclin B-Cdk complex rise through the

cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...

until mitosis, where they fall abruptly due to degradation of cyclin B (Cdk1 is constitutively present). The complex of Cdk and cyclin B is called

maturation promoting factor 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 ...

or mitosis promoting factor (MPF).

Function

Cyclin B is necessary for the progression of the cells into and out of M phase of the cell cycle. At the end of S phase the phosphatase cdc25c dephosphorylates tyrosine15 and this activates the cyclin B/CDK1 complex. Upon activation the complex is shuttled to the nucleus where it serves to trigger for entry into mitosis. However, if DNA damage is detected alternative proteins are activated which results in the inhibitory phosphorylation of cdc25c and therefore cyclinB/CDK1 is not activated. In order for the cell to progress out of mitosis, the degradation of cyclin B is necessary. The cyclin B/CDK1 complex also interacts with a variety of other key proteins and pathways which regulate cell growth and progression of mitosis. Cross-talk between many of these pathways links cyclin B levels indirectly to induction of apoptosis. The cyclin B/CDK1 complex plays a critical role in the expression of the survival signal

survivin Survivin, also called baculoviral inhibitor of apoptosis repeat-containing 5 or BIRC5, is a protein that, in humans, is encoded by the ''BIRC5'' gene. Survivin is a member of the inhibitor of apoptosis (IAP) family. The survivin protein functions ...

. Survivin is necessary for proper creation of the

mitotic spindle In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a pr ...

which strongly affects cell viability, therefore when cyclin B levels are disrupted cells experience difficulty polarizing. A decrease in survivin levels and the associated mitotic disarray triggers apoptosis via caspase 3 mediated pathway.

Role in Cancer

Cyclin B plays in integral role in many types of cancer.

Hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of organic tissue that results from cell proliferati ...

(uncontrolled cell growth) is one of the hallmarks of cancer. Because cyclin B is necessary for cells to enter

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

and therefore necessary for cell division, cyclin B levels are often de-regulated in tumors. When cyclin B levels are elevated, cells can enter

M phase The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...

prematurely and strict control over cell division is lost, which is a favorable condition for cancer development. On the other hand, if cyclin B levels are depleted the cyclin B/CDK1 complex cannot form, cells cannot enter M phase and cell division slows down. Some anti-cancer therapies have been designed to prevent cyclin B/CDK1 complex formation in cancer cells to slow or prevent cell division. Most of these methods have targeted the CDK1 subunit, but there is an emerging interest in the oncology field to target cyclin B as well.

As a Biomarker

Cyclin levels can easily be determined through immunohistological analysis of tumor

biopsies A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a diseas ...

. The fact that cyclin B is often disregulated in cancer cells makes cyclin B an attractive

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...

. Many studies have been performed to examine cyclin levels in tumors, and it has been shown that levels of cyclin B is a strong indicator of prognosis in many types of cancer. Generally, elevated levels of cyclin B are indicative of more aggressive cancers and a poor prognosis. Immunohistologically assessed levels of cyclin B could determine if women with stage 1, node negative, hormone receptor positive breast cancer were likely to benefit from adjuvant therapy. In general women with this cancer have a very good prognosis, with mortality in 10 years of only 5%. Therefore, it is rare for oncologists to recommend adjuvant chemotherapy in these cases. However, in a small subset of patient this type of cancer is unexpectedly aggressive. These rare patients can be identified through their elevated cyclin B levels. In addition high levels of cyclin B also indicate poor prognosis and lymph node metastasis in

gastric cancer Stomach cancer, also known as gastric cancer, is a cancer that develops from the lining of the stomach. Most cases of stomach cancers are gastric carcinomas, which can be divided into a number of subtypes, including gastric adenocarcinomas. Lymph ...

s. However, not all cancers which overexpress cyclin B are more aggressive. A study in 2009 found that cyclin B overexpression in ovarian cancer indicates that the cancer is unlikely to be malignant while more aggressive ovarian cancers of epithelial cell origin do not show elevated cyclin B.

Cyclin B and p53

There is strong cross-talk between the pathways regulating cyclin B and the tumor suppressor gene

p53 p53, also known as Tumor protein P53, cellular tumor antigen p53 (UniProt name), or transformation-related protein 53 (TRP53) is a regulatory protein that is often mutated in human cancers. The p53 proteins (originally thought to be, and often s ...

. In general levels of p53 and cyclin B are negatively correlated. When p53 build-up triggers cell cycle arrest the levels of downstream proteins p21 and

WAF1 p21Cip1 (alternatively p21Waf1), also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1, is a cyclin-dependent kinase inhibitor (CKI) that is capable of inhibiting all cyclin/CDK complexes, though is primarily associated ...

are increased which prevents cyclinB/CDK1 complex activation and therefore progression through the cell cycle. It has also been observed that decreasing cyclin B levels in cells increases the levels of functional p53. Therefore,

siRNA Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, is a class of double-stranded RNA at first non-coding RNA molecules, typically 20-24 (normally 21) base pairs in length, similar to miRNA, and operating wi ...

s for cyclin B may be an effective treatment against cancers where p53 function is inhibited but the gene has not been deleted. In such cases lowering cyclin B levels restores the tumor suppressing function of p53 and also prevents cancer cells from dividing as a consequence of low cyclin B.

References

External links