Cyclins are first discovered as proteins that varied in abundance during the cell cycle (Hunt 2004). Cyclin A is one of the first members of the cyclin family to be cloned. Classical cyclins are activating subunits for cyclin-dependent kinases (CDKs) and are essential components of the cell cycle engine (Morgan 2007). Some members of the cyclin family are also known to perform functions unrelated to cell cycle control (Lim and Kaldis 2013).
Two cyclin A are present in vertebrates. Cyclin A1 is the “embryonic” form, expressing mainly in early zygotes and testis. Cyclin A2 is the “somatic” form and is widely expressed in most growing cells. In the literature, “cyclin A” generally refers to cyclin A2. Notable exceptions are papers on Xenopus embryonic cells, in which the unspecified “cyclin A” generally refers to cyclin A1.
Cyclin A is expressed periodically during the cell cycle, accumulating from early S phase and disappearing during mitosis. It is generally accepted that cyclin A functions in both S phase and mitosis. Two CDKs, CDK1 (also called CDC2) and CDK2, are catalytically activated by cyclin A. Cyclin A is also believed to play important roles in targeting CDKs to their specific substrates.
The major physiological function of cyclin A1 appears to be in the control of spermatogenesis (Fung and Poon 2006). Disruption of cyclin A1 in mice leads to a block of the first meiotic division in male mice. These mice have significantly smaller testes than the wild-type littermates and are sterile because of an arrest of spermatogenesis in the latter stages of meiotic prophase. Spermatocytes also undergo apoptosis in the absence of cyclin A1, in part through p53-dependent mechanisms.
Cyclin A1 is implicated in the tumorigenesis of several cancers, including myeloid leukemia, testicular cancer, breast cancer, and prostate cancer (Yam et al. 2002). As the expression of cyclin A1 is normally suppressed in most somatic cells, the reactivation of cyclin A1 is believed to dysregulate cell proliferation. In this connection, cyclin A1 has been implicated in cellular functions including DNA damage repair and apoptosis.
Cyclin A2 is ubiquitously expressed in proliferating somatic cells and plays essential functions during the cell cycle. Disruption of cyclin A2 in mice causes early embryonic lethality. Cyclin A2-null embryos develop normally until about day 5.5, possibly because of the persistence of a maternal pool of cyclin A2 or compensation by cyclin A1 during early embryo development. Experiments with conditional ablation of cyclin A2 indicated that while cyclin A2 is redundant in fibroblasts, it is essential for cell cycle progression in hematopoietic and embryonic stem cells (Kalaszczynska et al. 2009).
Dysregulated expression of cyclin A2 is believed to contribute to tumorigenesis. However, although increased expression of cyclin A2 can be detected in many types of cancer, it is uncertain if this merely reflects the highly proliferative nature of cancers (Yam et al. 2002).
Functions of Cyclin A
Unique among the vertebrate cyclins, cyclin A functions in both S phase and mitosis. Furthermore, multiple CDKs, including CDK1 and CDK2, can be activated by binding to cyclin A.
Activation of CDKs
Functions of cyclin A, including the G1–S transition, DNA replication, and mitosis described, below all involve the activation of CDKs. CDK1 and CDK2 are expressed at a constant level during the cell cycle. Their activity is determined predominantly by the fluctuation of the cyclin subunit. Activation of cyclin A–CDK complexes also requires the phosphorylation of a residue on the T-loop of the kinase subunit (Thr161 in CDK1; Thr160 in CDK2). Nevertheless, the activity of the enzyme responsible for this phosphorylation (CDK-activating kinase, CAK) does not appear to be regulated during the cell cycle (Morgan 2007).
Under several checkpoint conditions, such as after DNA damage and replication block, the activity of cyclin A–CDK is suppressed (Wohlbold and Fisher 2009). This is carried out by the phosphorylation of Thr14 and Tyr15 of the CDK subunit by WEE1 and MYT1. While WEE1 specifically phosphorylates Tyr15, MYT1 displays a stronger preference for Thr14. During unperturbed cell cycle, cyclin A2–CDK1, but not cyclin A2–CDK2, is regulated by Thr14/Tyr15 phosphorylation. After checkpoint activation, however, both CDK1 and CDK2 complexes are inactivated by Thr14/Tyr15 phosphorylation (Chow et al. 2003). Cyclin A–CDK2 complexes can also be inhibited by binding to members of the p21 CIP1/WAF1 family of CDK inhibitors (p21 CIP1/WAF1 , p27 KIP1 , and p57 KIP2 ).
Whether a cell stays in the cell cycle or exit to quiescence (G0) depends on the integration of extracellular growth-stimulating and inhibiting stimuli. This decision is made at a transition at the end of G1 called the restriction point (R). After passing the restriction point, a cell is committed to another round of cell cycle and becomes independent of external stimuli. Mechanistically, the restriction point involves phosphorylation of the retinoblastoma gene product (pRb) by G1 cyclin–CDK complexes, including cyclin A–CDK (Henley and Dick 2012). One of the key functions of pRb is to inhibit E2F, a transcription factor for many genes important for entry into S phase. By mechanisms including blocking the transactivation domain as well as recruiting the chromatin remodeling enzyme histone deacetylase (HDAC), pRb represses the transcription of E2F promoters. Cyclin D, of which transcription is strongly dependent on extracellular mitogenic cues, activates CDK4 and CDK6 to phosphorylate pRb (and the related p107 and p130). Hyperphosphorylation of pRb releases it from E2F (removing HDAC at the same time), thereby liberating E2F to activate transcription. Hyperphosphorylation of pRb is initiated by cyclin D–CDK4/6 and is then maintained by cyclin E–CDK2 and cyclin A–CDK2. But unlike cyclin D, the expression of cyclin E and cyclin A is independent of extracellular signals. In fact, cyclin E and cyclin A are among the genes that are transcriptionally activated by E2F, completing a positive feedback control for the G1–S transition.
Cyclin A is predominantly localized to the nucleus, with a portion specifically located at sites of DNA replication. In fact, cyclin A shuttles between the nucleus and the cytoplasm; but nuclear export is slower than the import, resulting in the nuclear localization of the protein.
DNA replication in eukaryotes involves first the formation of prereplicative complexes (components include ORC proteins, CDC6, CDT1, and MCM proteins) on the origins of replication. Components of the prereplicative complexes are then phosphorylated by protein kinase complexes including cyclin A–CDK2 and DBF4–CDC7, allowing the loading of CDC45 onto the origins. The prereplicative complexes are disassembled after origin firing, thereby preventing the refiring of the same origin during the same cell cycle.
Cyclin A is implicated in two steps during DNA replication (Woo and Poon 2003). The first step involves the initiation of DNA replication. CDK2 and CDC7 phosphorylate components of the prereplicative complexes including the MCM2–7 complex, stimulating the recruitment of two helicase coactivators, CDC45 and GINS. The MCM2–7 helicase is then activated and unwinds the origin. Finally, the unwound single-stranded DNA is stabilized by binding to replication protein A (RPA), facilitating the recruitment of DNA polymerases and other components of the DNA synthesis machinery to initiate DNA synthesis. The second step involves the inhibition of assembly of the prereplicative complex to ensure that origins are fired only once per cell cycle. Several components of the prereplicative complex are phosphorylated by cyclin A–CDK2, preventing them from forming the prereplicative complex through various mechanisms. For example, cyclin A–CDK-dependent phosphorylation excludes MCM2–7 from the nucleus, targets CDT1 and CDC6 for degradation, and dissociates ORC from the chromatin.
Although cyclin A is clearly required for mitosis, its precise role is not fully understood. One hypothesis is that cyclin A itself is a component of M phase-promoting factor (MPF). An alternative hypothesis is that cyclin A is part of the machinery that triggers the activation of MPF, mainly composed of cyclin B–CDK1 (Lindqvist et al. 2009).
Other functions attributed to cyclin A include the regulation of several transcription factors, DNA double-strand break repair, the p53-response pathway, and centrosome duplication (Poon and Fung 2007).
Regulation of Cyclin A during the Cell Cycle
The expression of cyclin A is highly regulated during the cell cycle. The protein of cyclin A starts to accumulate at early S phase, continues through S phase and G2 phase, and disappears during mitosis. In relation to other cyclins, cyclin A is synthesized and destroyed after cyclin E, but is slightly earlier than cyclin B. The periodic expression of cyclin A is regulated both at the levels of transcription and proteolysis.
Cyclin A transcription is regulated during development, ensuring the differential expression of cyclin A1 and cyclin A2. Furthermore, the promoter of cyclin A is repressed during G1 phase but is activated during S phase and G2/M during the cell cycle (Fung and Poon 2005).
Cyclin A1 promoter contains GC boxes, which bind to members of the SP1 family. Transactivation of the cyclin A1 promoter by B-MYB also depends on the SP1 binding sites. B-MYB can be phosphorylated and activated by cyclin A1–CDK2, suggesting the possibility of a positive-feedback regulation. The cyclin A1 promoter can also be transactivated by C-MYB, which interacts with the MYB-binding sites in the promoter.
One of the mechanisms that controls developmental-specific expression of cyclin A1 may involve methylation. The CpG islands of cyclin A1 promoter are highly methylated in certain tissues and somatic cell lines. However, although methylation of cyclin A1 promoter correlates with gene silencing in somatic cell lines, it has little correlation with the tissue-specific repression of the promoter. In early embryos, cyclin A1 expression is also regulated at the level of mRNA stability. The maternal cyclin A1 mRNA is destabilized once transcription is initiated in the zygote. The destabilization of cyclin A1 mRNA is due to deadenylation and relies on the 3′-untranslated region of the maternal transcript. In addition, deadenylation also causes direct translational repression of the cyclin A1 mRNA.
Several regulatory elements in cyclin A2 promoter (CCAAT boxes, NF-Y and B-MYB binding, and CHR) are also found in promoters of other genes involved in G2/M control, including cyclin B, CDK1, and CDC25C. The promoter of cyclin A2 contains CCAAT boxes, which bind the transcriptional activator NF-Y. Expression of cyclin A2 is also stimulated by B-MYB, a transcription factor controlled by E2F during G1/S transition. Expression of B-MYB is further enhanced during S phase through phosphorylation by cyclin A2–CDK2.
During G1 phase, cyclin A2 promoter is repressed by the occupation of a repressor element containing an E2F-binding site (called the cell cycle responsive element (CCRE) or the cell cycle dependent element (CDE)). E2F associates with pRb and two closely related proteins p107 and p130 during G1. These pRb-related proteins repress E2F-responsive promoters by recruiting HDAC and the chromatin remodeling complex SWI/SNF to the promoter. The activation of G1 cyclin–CDK complexes leads to hyperphosphorylation of pRb, releasing E2F and allowing the activation of the cyclin A2 promoter. Cyclin A2–CDK further phosphorylates pRb in a positive feedback loop.
In addition to transcriptional control, the expression of cyclin A2 is also regulated at the level of mRNA stability. For example, the Wilms tumor 1-associating protein (WTAP) interacts with the 3′-untranslated region of cyclin A2 mRNA and stabilizes the mRNA.
Degradation of cyclin A2 starts during prometaphase and is completed at metaphase. This differs from that of cyclin B, which is degraded before the onset of anaphase. The timely destruction of cyclin A2 is in part conferred by a short sequence at the N-terminal region known as the destruction box (D-box). D-box-containing proteins are targeted for ubiquitination by a ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C) (Skaar and Pagano 2009). Degradation of cyclin A2 during mitosis involves the APC/C targeting subunit CDC20 as well as the CDK-interacting protein CKS.
Unattached kinetochores or the absence of tension between the paired kinetochores activates the spindle-assembly checkpoint (SAC), which then prevents the activation of APC/C. Unlike canonical substrates of APC/C such as cyclin B and securin, cyclin A2 is degraded even when APC/C is inhibited by the SAC. One reason is that cyclin A2 can bind directly to CDC20 (through a domain termed the ABBA motif in cyclin A2) and outcompete the SAC components (Di Fiore and Pines 2010; Di Fiore et al. 2015).
Cyclin A is an essential regulator of the cell cycle. Cyclin A binds and activates the cyclin-dependent kinases CDK1 and CDK2. The complexes are important for the regulation of DNA replication as well as mitosis. During S phase, cyclin A–CDK complexes are critical for loading replication factors during the initiation of replication. Cyclin A–CDK complexes have a further role in S phase in preventing the refiring of the same origins. The functions of cyclin A in mitosis is less understood, but it may be involved in kick-starting the main mitotic engine. Cyclin A starts to accumulate at early S phase and disappears during mitosis. The periodic regulation of cyclin A during the cell cycle is achieved through both transcriptional control and proteolysis. Transcriptional control of cyclin A promoter involves multiple transcriptional factors that display cell cycle–dependent activities. Destruction of cyclin A during mitosis is carried out by the APC/C.
More works are required to provide a comprehensive picture of the functions of cyclin A. Presumably, many substrates of cyclin A–CDK remained to be discovered. Understanding a more complete repertoire of cyclin A–CDK’s targets will be the key to unravel the mysteries surrounding this cyclin. Also very little is known about the epigenetic control of the promoters of cyclin A1 and cyclin A2. As both cyclin A1 and cyclin A2 are frequently dysregulated in cancers, it is vital to understand how they are controlled during tumorigenesis.
- Morgan DO. The cell cycle: principles of control. Oxford: Oxford University Press; 2007.Google Scholar