22 Model of Ras and Cancer

Simply defined, cancer is the uncontrolled division of cells due to mutations that disrupt cell cycle control mechanisms. We can categorize regulators of cell cycle events into one of two groups: tumor suppressors and proto-oncogenes (for more detail, see Section 19.3). Tumor suppressor genes encode proteins that slow the cell cycle when they are active, while proto-oncogenes encode proteins that promote the cell cycle when active. Cancerous cells contain mutations that inactivate tumor suppressors and mutations that make proto-oncogenes overactive, turning them into oncogenes.

As discussed in Chapter 21, mutations arise randomly, but several factors influence the likelihood that a mutation will occur. For example, a cell that divides frequently is more likely to develop mutations, as each round of DNA replication provides an opportunity for errors or DNA damage to become permanent. Additionally, a cell with defective checkpoint or DNA repair proteins will be more likely to proceed with the cell cycle under conditions when a typical cell would not. Both increase the likelihood of cancer-causing mutations arising. Individuals with mutations in p53 have significantly higher rates of developing many types of cancer, while the inability to repair certain DNA replication errors is associated with hereditary colorectal and other cancers.

Our knowledge of cell signaling and the events of the cell cycle have informed our knowledge of how to detect and treat cancer, and vice versa. In this last chapter of Unit 3, we will synthesize ideas from multiple class days to build a model of a mutation associated with cancer and use this model to make predictions about a drug that was developed to treat the type of cancer caused by that mutation. To build our models and explain the effect of the mutation, we will use our knowledge of a specific cell signaling pathway that responds to growth factors, principles of signal transduction, and our understanding of key proteins that regulate the cell cycle.

Chapter Overview

Section 22.1 Cell Cycle Recap

Section 22.2 Ras and Cancer

Section 22.3 Model for the In-class Activity

Section 22.1 Cell Cycle Recap

As shown in Figure 22.1, a typical cell division first requires a growth factor ligand to bind to receptor kinases on a cell. After phosphorylation, the active receptor dimer causes signal transduction in the cell that eventually leads to a change in gene expression.

One of the genes that is expressed is Cyclin D, which binds to CDK and causes the cell to enter G₁ and initiate events that lead to subsequent steps of the cell cycle, including expression of other cyclins at specific times. At key points in the cell cycle, different cyclin-CDK complexes phosphorylate proteins to cause the key steps to occur. Cyclins are also degraded at specific times in the cell cycle, and no cyclins remain at the conclusion of M phase. As a result, no additional cell cycles occur until a new growth factor signal is detected.

The growth factor, the proteins activated in the receptor kinase pathway, cyclins, and CDKs are all proto-oncogenic proteins, as they are promoting events in the cell cycle. If mutations in the genes for these proteins were to create versions that were always active, we would call them oncogenes (cancer-causing genes). Conversely, the proteins involved in signal termination, such as phosphatases, proteins that cause the degradation of cyclins, and proteins associated with cell cycle checkpoints are all tumor suppressors, since these act to restrain the cell cycle. Healthy cell division involves the balance of proto-oncogene and tumor suppressor function, while disruption of this balance can lead to abnormal cell division and eventually cancer.

Section 22.2 Ras and Cancer

Since cancer is the uncontrolled division of cells, one approach to treating cancer is to inhibit the proteins and processes necessary for cell division. The earliest cancer therapies were drugs that interfered with DNA replication or the function of the mitotic spindle, or treatments that caused extensive DNA damage. However, these therapies affected all dividing cells, not just cancerous cells, leading to serious side effects.

A more modern approach is to develop drugs that target specific mutant proteins found in cancer cells. One such protein that has long been the desired target for cancer treatment is Ras. As shown in Figure 22.1a, Ras is a G protein that, when bound to GTP, activates the Map Kinase phosphorylation cascade, eventually resulting in Cyclin D expression. We previously discussed G proteins in Section 17.2. In a typical cell, Ras is only active for a short period before interactions with a GTPase activating protein cause Ras to hydrolyze GTP to GDP, inactivating itself (Figure 22.3).

However, because of the signal amplification that occurs in the Map Kinase cascade, one briefly active Ras molecule can lead to a significant cellular response to promote cell division, making Ras a potent proto-oncogene. Furthermore, a mutant form of Ras was one of the first oncogenes to be identified, and Ras mutations are found in roughly 3 out of every 10 cancers. While the exact mutations are different, the general effect is the same: mutant Ras is slow to hydrolyze its GTP. Based on the pathway in Figure 22.1, consider why these kinds of mutations convert Ras from proto-oncogene to oncogene.

Due to the central role of Ras in the growth factor-receptor kinase pathway, several decades of research have been focused on identifying drugs to block Ras activation by preventing Ras from binding to GTP. However, because Ras, like all G proteins, binds with very high affinity to GTP, this was historically considered impossible (although promising new research seems to demonstrate otherwise). As an alternative approach, scientists instead focused on the unique shape of a particular form of mutant Ras called Ras G12C. This naming means that the 12th amino acid of Ras, glycine, is mutated to cysteine, which is caused by a single change in the DNA sequence (Figure 22.4). By focusing on the structure of this Ras mutant, scientists were able to develop a molecule that binds to the mutant Ras without affecting the healthy version of Ras. Subsequently, scientists then showed that this drug blocked cell division in cancer cells with this mutation, and the drug is now an FDA-approved therapy!

Section 22.3 Model for the In-class Activity

In the in-class activity, we will explore a receptor kinase pathway like the one shown in Figure 22.1. This expanded model has a few additional proteins and gives specific protein names as opposed to the generic designations previously used. However, the underlying principles you already learned are the same. In a healthy cell, when growth factor ligand is absent, all proteins in the signal transduction pathway are present but in their inactive forms (Figure 22.5a). Each of these proteins has a different shape when turned “on” in signal transduction (Figure 22.5b).

As we go through the pathway, think about how you would describe differences between active and inactive forms of each protein, what causes those differences, and how each change is reversed in signal termination. Notice how the receptor kinases are depicted as monomers that are not interacting, no proteins are phosphorylated, and transcription of Cyclin D is not occurring.

If growth factor is present, several events are activated in sequence (Figure 22.6). The ligand brings the receptor kinases together, activating their kinase functions. Although it is not shown in this figure, remember that phosphorylation reactions are the transfer of a phosphate group from ATP to another protein (review the detailed reaction in Figure 17.6). The phosphorylated receptor can activate signal transduction by binding to and activating the adaptor, which in turn binds to and activates the Ras G protein (review the detailed reaction in Figure 22.3; a protein in the adaptor is a GEF). Ras-GTP binds to and activates Raf, which is the first kinase of a phosphorylation cascade. Raf activates Mek, which is another kinase that in turn activates Erk. Once activated, Erk moves into the nucleus to activate a positive transcription factor (pos TF). As a result, RNA polymerase is recruited to the regulatory region and initiates transcription. Signal amplification occurs each phosphorylation reaction (similar to Figure 17.7), with Raf activating several Mek proteins, each phospho-Mek activating several Erk proteins, and each phospho-Erk activating multiple transcription factors. This amplification strengthens the cellular response, which is expression of Cyclin D and other genes to initiate the cell cycle in responding cells. Now you have the active pathway for reference and further modification in class!