19 Cell Cycle Regulation

The previous chapter, Cell Division and the Cell Cycle, described the main phases of the cell cycle and the series of events in eukaryotic cells that lead one parent cell to produce genetically identical daughter cells (summarized in Figure 19.1). Cell division is essential for the overall growth of an organism, for repair of damaged cells, and replacement of worn-out cells. Each step must be carried out correctly, with the cell progressing to the next step only when the cell has completed the current step. Errors in these processes due to impaired regulation could lead to daughter cells with errors in the DNA, missing or extra DNA, or other problems that could affect the viability of cells. If uncontrolled cell division occurs due to major changes in the genetic information that impair regulation of the cell cycle, an organism could develop cancer. To ensure reliable and correct cell division, two opposing regulatory mechanisms are at work in cells. One set of proteins, when active, promotes progress through cell cycle by causing the specific steps of each cell cycle phase to occur. Another set of proteins detects steps that are incomplete or senses specific problems that need to be fixed, and will prevent progress through the cell cycle until the problems are addressed. Dysfunction of both mechanisms is typically required for cancer to arise, and better understanding of the proteins that regulate the cell cycle has led to vastly improved detection and treatment of many cancers.

Chapter Overview

Section 19.1 Promoting the Cell Cycle

Section 19.2 Cell Cycle Checkpoints

Section 19.3 Proto-oncogenes, Oncogenes, and Tumor Suppressors

Section 19.1 Promoting the Cell Cycle

As scientists began to study the process of the cell cycle, they noticed that some proteins were newly synthesized at precise points in the cell cycle and degraded at later points. These proteins, helpfully named cyclins, seemed to be associated with the timing of key steps in cell division. As shown in Figure 19.2, Cyclin D is the first cyclin to be synthesized, in G₁, and this cyclin protein remains present throughout the cell cycle, only decreasing at the end of M phase. Another cyclin, Cyclin E, is synthesized at the end of G₁, peaks in early S phase, and is degraded shortly thereafter. The expression of Cyclin A and Cyclin B are also uniquely timed with later phases of the cell cycle.  With this knowledge, one could imagine using the levels of specific cyclin proteins to determine when in the cell cycle a cell might be.

Importantly, cyclins don’t carry out chemical reactions. Rather, they bind to and activate other proteins called cyclin-dependent kinases (CDKs), whose levels are consistent throughout the cell cycle. As shown in Figure 19.3, a CDK in the absence of cyclin is inactive (Figure 19.3a). However, binding to a cyclin activates the CDK protein by changing its shape (19.3b). Now, the cyclin-CDK complex can carry out its kinase function, using phosphate from ATP to phosphorylate protein targets (Figure 19.3c). The exact protein targets and the effect of phosphorylation depend on the specific cyclin-CDK complex, and each complex has specific functions in the cell cycle that correspond to the phase of the cell cycle.

Figure 19.4 shows the approximate time that each complex is active and carrying out critical functions in the cell cycle, we will discuss each cyclin-CDK complex in more detail shortly. However, please note that the overall effect of the phosphorylation reactions carried out by every cyclin-CDK is the same: progress through the cell cycle.

Cell Cycle Entry and Progress through G₁

One important feature of eukaryotic cells, especially in multicellular organisms, is that cells only divide when cued to do so by a growth factor or mitogen signal. In Chapter 17 on Cell Communication, a mitogen signal activated receptor kinases, leading to Ras and Map Kinase signal transduction, culminating in gene expression. Figure 19.5a shows this pathway again, only now one of the genes affected by growth factor signaling is indicated: Cyclin D. The presence of growth factor mitogen causes expression of Cyclin D, which activates CDK to cause phosphorylation of proteins and signal a non-dividing cell to start a cell division cycle. Cyclin D-CDK also signals for progress through G₁, including causing cell growth and preparation for S phase. As shown in Figure 19.2, all Cyclin D is degraded during M phase, which means that a new signal and new Cyclin D expression are required for another round of cell division.

S phase

One of the outcomes of active Cyclin D-CDK is the expression of Cyclin E and later, Cyclin A. Cyclin E-CDK complexes are essential for preparing the cell for the main event of S phase, DNA replication. The mechanics and proteins needed for DNA replication are the subject matter for Chapter 20, but you can use your prior knowledge to anticipate some of the materials that will be needed to build DNA–enzymes, deoxyribonucleotides, and structural proteins to organize the DNA, to name a few key examples. Both active cyclin D-CDK and Cyclin E-CDK are needed to transition from G₁ to S phase (Figure 19.6). Then, Cyclin A-CDK phosphorylates proteins to initiate DNA replication. Importantly, Cyclin A also prevents the accidental re-replication of DNA, which helps ensure the eventual production of genetically identical daughter cells.

G₂ Phase and Entry to M phase

During G₂, several different types of activities are occurring. Any DNA replication yet to occur is completed during G₂. Cells are growing larger and synthesizing new proteins. Finally, the cell is preparing for mitosis by starting to create the mitotic spindle and by expressing Cyclin B. When the cell is ready to proceed, fully active Cyclin B-CDK complexes will initiate M phase. For example, phosphorylation of certain histone proteins that help organize the DNA cause the chromatin to condense into chromosomes. Additionally, phosphorylation of proteins in the nuclear envelope causes the breakdown of this structure, which will allow the spindle fibers to capture chromosomes by binding to kinetochores at centromeres.

Section 19.2 Cell Cycle Checkpoints

While cyclin-CDK complexes are promoting cell cycle events by phosphorylation, other proteins are sensing whether these events are occurring correctly. If problems are detected, these proteins will inhibit the function of cyclin-CDK complexes to allow the problem to be resolved, and then the cell cycle will continue. However, problems that cannot be fixed lead to a different outcome–programmed cell death, also known as apoptosis. We will focus on three major transition points, or “checkpoints” in the cell cycle that are tightly regulated. If a problem is detected at a checkpoint, we might say the checkpoint is “activated.” Each checkpoint needs to be “cleared” for the cell to move to the next part of cell cycle.

G₁ Checkpoint

The first checkpoint is at end of G₁, and is often referred to as the DNA damage checkpoint. As we will discuss in a later chapter, DNA damage that is not repaired before replication can lead to mutations, or permanent changes to the genetic information, which are rarely beneficial. Thus, checking and repairing the DNA before replication prevents mutations.

When the cell senses damaged DNA, a protein called p53 is activated. p53 is a transcription factor that activates expression of a gene that encodes a CDK inhibitor—a protein that binds to the Cyclin D-CDK and the Cyclin E-CDK complexes, preventing their interactions with ATP and the protein targets (Figure 19.8a). Without the phosphorylation reactions to cause progression, the transition to S phase is blocked and the cell remains in G₁ phase (Figure 19.8b). This pause provides time for the DNA damage to be repaired, if possible. Once the damage is repaired, p53 activation and subsequent effects are reversed, and the cell cycle resumes.

G₂ Checkpoint

The second major checkpoint occurs near the end of G₂. Recall that M phase is when the DNA is evenly divided among daughter cells by mitosis and cytokinesis (for review, see Section 18.3). However, if DNA replication is not finished and the cell proceeds into M phase, the daughter cells will not be genetically identical. To prevent premature entry into M phase, incomplete DNA replication blocks the Cyclin B-CDK complex activity, forcing the cell to pause in G₂ (Figure 19.9).

Spindle Checkpoint (Metaphase-Anaphase Transition)

A critical step ensuring the accurate division of DNA is the attachment of chromosomes to spindle fibers. Recall that in early M phase, spindle fibers from each spindle pole extend and retract to “capture” chromosomes by binding to kinetochores at the centromere of each chromatid (Figure 19.10a).  Once attached, spindle fibers push and pull chromosomes to the metaphase plate of the cell for their metaphase arrangement. At this point, sister chromatids are linked by cohesin proteins that form rings around the chromosome (Figure 19.10b). Anaphase, or the pulling apart of chromatids to opposite poles, can only occur when the cohesin rings are degraded by the protein Separase, which is activated by the anaphase-promoting complex (APC). Any unattached kinetochores release a signal that inhibits APC, and even one unattached kinetochore is sufficient to prevent APC from activating Separase.

In this way, cells ensure that anaphase only occurs when spindle fibers are appropriately positioned to accurately separate the DNA into daughter cells (Figure 19.11).

Section 19.3 Proto-oncogenes, Oncogenes, and Tumor Suppressors

Many insights into the regulation of the cell cycle were gained by studying cells with dysfunctional regulation of cell division, namely cancer cells, and some of the vocabulary from the study of cancer cells is also helpful for describing normal regulation of cell division. Early in the study of cancer, specific genes discovered in cancer cells, called oncogenes, encoded proteins that caused abnormal and uncontrolled cell division. Shockingly, similar versions of these genes were found in healthy, noncancerous cells, where they encoded proteins that promoted typical, regulated cell division. These healthy versions of oncogenes were termed proto-oncogenes. Every protein in the cell cycle that, when active, works to promote progress through the cell cycle is a proto-oncogene. Thus, cyclins, CDKs, target proteins, growth factors, activators in the receptor kinase pathway–these are all proto-oncogenes and are essential for promoting healthy cell division. However, when gene mutations change the functions of these proteins to promote cell division more often than they should, or under the wrong circumstances, these now become oncogenes.

Opposing the proteins that promote the cell cycle are proteins that, when active, slow or stop cell cycle processes. These proteins are termed tumor suppressors for their ability to slow cell division. Proteins that assist with signal termination of a growth factor pathway, along with every protein that is a part of a checkpoint, that stops the cell cycle when problems are detected, are tumor suppressors. Because checkpoints are meant to block progress so that repairs can be made or processes can occur correctly, dysfunctional checkpoints mean that cells proceed with cell division even under circumstances when they should not, inevitably leading to further disruption in genetic information. Thus, gene mutations that decrease or disrupt the functions of tumor suppressors are also associated with cancer.