Cell Cycle

The cell cycle is the process by which a cell grows, duplicates its DNA, and divides into identical daughter cells. Cell cycle duration varies according to cell type and organism. In mammals, cell division occurs over a period of approximately twenty-four hours.

In multicellular organisms, only a subset of cells go through the cycle continuously. Those cells include the stem cells of the hematopoietic system, the basal cells of the skin, and the cells in the bottom of the colon crypts. Other cells, such as those that make up the endocrine glands, as well as liver cells, certain renal (kidney) tubular cells, and cells that belong to connective tissue, exist in a nonreplicating state but can enter the cell cycle after receiving signals from external stimuli. Finally, postmitotic cells are incapable of cell division even after maximal stimulation, and include most neurons, striated muscle cells, and heart muscle cells.

The cell cycle is functionally divided into discrete phases. During the DNA synthesis (S) phase, the cell replicates its chromosomes. During the mitosis (M) phase, the duplicated chromosomes are segregated, migrating to opposite poles of the cell. The cell then divides into two daughter cells, each having the same genetic components as the parental cell. Mammalian cells undergo two gap, or growth, phases (G₁ and G₂). G₁ occurs prior to the S phase, and G₂ occurs before the M phase.

Control of the Cycle

During the G₁ and G₂ phases, cells grow and make sure that conditions are proper for DNA replication and cell division. During the G₁ phase, cells monitor their environment and determine if conditions, including the availability of nutrients, growth factors and hormones, justify DNA replication. The decision to initiate replication is made at a specific "checkpoint" in G₁ called the "restriction point."

The processes of DNA replication and mitosis, and intervening events during the cell cycle, occur in a highly ordered and specific manner. A complex network of proteins ensures that these events occur at the proper times. Intracellular and extracellular signals block cell-cycle progression at checkpoints if certain events have not yet been completed. After the restriction point, the cell is committed to replicating its genome and dividing, completing one round of the cell cycle. If, prior to the restriction point, cells sense inadequate growth conditions or receive inhibitory signals from other cells, they enter G₀ (G-zero) phase, also called quiescence. In the G₀ phase, they are maintained for prolonged periods in a nondividing state. If cells sense such conditions after the restriction point, they complete the current round of the cell cycle and exit to G₀ during the subsequent G₁ phase. The G₂ phase is shorter than G₁, but it, too, consists of important mechanisms that control the completion and fidelity of DNA replication and that prepare the cell for entry into mitosis. Whereas some conditions cause cells to enter the G₀ phase, others trigger apoptosis. One such signal that may trigger apoptosis is if a cell's DNA has undergone significant damage.

After the restriction point, at the transition from the G₂ to the M phase, another checkpoint occurs. Mitosis is prevented if DNA damage has occurred or if genomic replication is not complete. The final key checkpoint occurs at the end of mitosis, when the cycle stops if chromosomes are not properly attached to the mitotic spindle.

Proteins That Regulate the Cycle

The mammalian cell cycle control system is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins. (The alteration of protein activity by the attachment of phosphate groups occurs frequently in cells.)

CDKs are called "cyclin-dependent" because their activity requires their association with activating subunits called cyclins. While the number of CDKs in a cell remains constant during the cell cycle, the levels of cyclins oscillate. There are G₁ cyclins, S-phase cyclins, and G₂/M cyclins, each of which interact differently with CDK subunits to regulate the various phases of the cell cycle. CDKs can also associate with inhibitory subunits called CDK inhibitors (CKIs). In response to signals that work against proliferation, such as growth factor deprivation, DNA damage, cell-cell contact inhibition and lack of cell adhesion, CKIs cause the cell cycle to halt.

By the end of 2001, many structurally related cyclins (A1, A2, B1, B2, B3, B4, B5, C, D1, D2, D3, E1, E2, F, G1, G2, H, I, L, and T) and nine CDKs (CDK1 to CDK9) were identified in mammalian cells. Complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, operate during the G₁ phase. Complexes of cyclin A and CDK2, as well as complexes of cyclin E and CDK2, act during the transition from the G₁ to the S phase. Complexes of cyclin A and CDK1, as well as cyclin B and CDK1, function during the transition from the G₂ to the M phase.

Active complexes of cyclins and CDKs exert their biological effects by phosphorylating proteins. During the G₁ phase, a major target of cyclin/CDK complexes is the retinoblastoma protein (pRb). pRb is a growth-suppressing protein whose activity is controlled by whether or not it is phosphorylated.

When pRb is in the dephosphorylated form, during the G₀ phase and early in the G₁ phase, it is active. pRb exerts its growth-suppressing effects by binding to many cellular proteins, including the transcription factors of the E2F family (Figure 1). E2F transcription factors regulate the expression of numerous genes that are expressed during G₁, or at the transition from the G₁ to the S phase, to initiate DNA replication.

pRb that is bound to an E2F transcription factor inhibits the transcription factor's activity. Following phosphorylation by cyclin/CDK complexes, pRb dissociates from E2F, allowing the transcription factor to bind DNA sequences and activate the expression of genes necessary for the cell to enter the S phase. Cyclin D1/CDK4 complexes phosphorylation of pRb during the middle of the G₁ phase. They allow for subsequent phosphorylation of pRb by additional cyclin/CDK complexes that act later in the cell cycle.

Two families of CKIs have been identified, based on their amino acid sequence similarity and the specificity of their interactions with CDKs. One of the families of CKIs, the INK family, includes four proteins (p15, p16 p18 and p20). These CKIs exclusively bind complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, to block cells that are in the G₁ phase of the cell cycle. The other family of CKIs, the Cip/Kip family, consists of three proteins (p21, p27, and p57). These inhibitors bind to all complexes of cyclins and CDKs that function during the G₁ phase and during the transition from the G₁ to the S phase. They act preferentially, however, to block the activity of complexes containing CDK2.

Deregulation and Cancer

Deregulation of cell cycle control proteins plays a key role in the development of cancer. Overactivation of proteins that favor cell cycle progression, namely cyclins and CDKs, and the inactivation of proteins that impede cell cycle progression, such as CKIs, can result in uncontrolled cell proliferation.

In human tumors, it is genes encoding the proteins that control the transition from the G₁ to the S phase that are most commonly altered. These genes include those for cyclins, CKIs, and pRb. Such mutations overcome the inhibitory effects of pRb on the cell cycle, causing cells to have a growth advantage. In some cancers, this occurs after the direct mutation of the pRb gene, resulting in the protein's loss of function. In a larger set of cancers, pRb is indirectly inactivated by the hyper-activation of CDKs. This may result from over expression of cyclins, from an activating mutation in CDK4, or from inactivation of CKIs.

There is much evidence to suggest that cyclins can act as oncogenes to induce cells to become cancerous. In particular the G₁ cyclins, cyclin D1, and cyclin E have been implicated in the development of cancer. Over-expression of the cyclin D1 protein is frequently detected in human breast cancer, and increasing evidence suggests that cyclin E overexpression plays an important role in the pathogenesis of breast cancer.

CKIs antagonize the function of cyclins, and considerable evidence suggests that these proteins function as tumor suppressors. CKI function is often altered in cancer cells. The gene encoding p16, a protein that belongs to the INK family of CKIs, is mutated, deleted, or inactivated in a large number of human malignancies and tumors. Such alterations prevent the inhibition of cyclin D/CDK4 and cyclin D/CDK6 complexes during G₁.

Decreased expression of p21 and p27, proteins that belong to the Cip/Kip family of CKIs, also has been demonstrated in numerous human tumors. In contrast to the genetic mutations observed with p16, the decrease in p27 levels in tumors is due to enhanced degradation of the p27 protein. One of the proteins required for the degradation of p27, Skp2, has oncogenic properties. Skp2 over expression is observed in several human cancers and likely contributes to the uncontrolled progression of the cell cycle by increasing the degradation of p27. Understanding of the fine details of cell cycle regulation is likely to lead to specific cancer therapies targeting one or more of these important proteins.

Joanna Bloom

and Michele Pagano

Bibliography

Goldberg, Alfred L., Stephen J. Elledge, and J. Wade Harper. "The Cellular Chamber of Doom." Scientific American 284, no. 1 (2001): 68-73.

Gutkind, J. Silvio, ed. Signaling Networks and Cell Cycle Control. Totowa, NJ: Humana Press, 2000.

Murray, Andrew, and Tim Hunt. The Cell Cycle: An Introduction. Oxford, U.K.: Oxford University Press, 1993.

Pagano, Michele, ed. Cell Cycle Control. New York: Springer-Verlag, 1998.

Weinberg, Robert A. "How Cancer Arises." Scientific American 275, no. 3 (1996): 62-70.