The mechanism for normal cell proliferation (cell growth) has been discussed in the “Cell growth factors” section in Chapter 14 as well as in Chapter 13. When the orderly mechanism of cell growth become abnormal, some cells start to multiply abnormally, thereby making it difficult for the body to maintain homeostasis. Cancer in multicellular organisms can essentially be explained as abnormal proliferation of cells, often described as “malignant alteration of cells”. Cells that have turned cancerous (cancer cells) multiply quickly, but this does not mean that the cell cycle itself has been accelerated. The fact is, cancer cells are unable to stay at the G0 phase in the cell cycle (see Chapter 13), making growth regulation difficult. This unregulated cell growth is essentially due to the abnormal state of signal transduction in these cells.
As discussed in previous sections, cell growth is regulated by balancing positive and negative regulation, which promotes and inhibits cell growth, respectively. All of these regulation mechanisms are controlled by signal transduction, which is participated in by various components such as signal molecules and receptors. If any one of these regulatory mechanisms is disrupted, the regulation of cell growth becomes abnormal. An example is when the G protein Ras (activated by the epidermal growth factor (EGF) receptor), which regulates cell cycle progression, becomes mutated. Mutated Ras proteins that have lost their GTP hydrolytic enzyme activity remain constantly activated. As a result, cell cycle progression becomes uncontrolled, causing unlimited cell proliferation (Figure 15-6A). Ras was the first oncogene discovered to cause cancer in humans (see Column at the bottom).
Helicobacter pylori and stomach cancer
The percentage of Helicobacter pylori (H. pylori) carriers tends to be reduced in countries with good sanitary conditions; however, the total number of infected persons is relatively high with about half of the global population being infected. In Japan, the carrier percentage is low in younger populations; on the other hand, more than half of those over 60 years are estimated to be infected. When H. pylori is ingested orally, e.g., through water, it remains in the stomach until it is sterilized. It is believed that the H. pylori infection has spread among many of the Japanese adults who lived through the post-war era, in which sanitary conditions were poor.
The association of H. pylori with gastric ulcers has been suggested since decades ago. Given that the sterilization of the bacteria in patients with repeated gastric ulcers prevents further recurrence, the sterilization of the bacteria has become a standard treatment for ulcer patients tested positive for H. pylori.
Recent studies have revealed how H. pylori causes gastric cancer. H. pylori injects a protein called CagA into epithelial cells in the stomach. If CagA is phosphorylated in the epithelial cells, it disrupts endogenous signaling pathways. It is particularly known to continuously activate the MAP kinase cascade*2 (see Selection 2 of Chapter 14, Figure 14-3), a pathway involved in cell growth signals. Such activation is thought to be closely associated with the malignant alteration of cells (development of gastric cancer) (COLUMN Figure 15-2). Although much information about the mechanism still remains unknown, large epidemiological follow-up studies conducted to compare Japanese H. pylori-positive and -negative patients have shown that an overwhelmingly high percentage of H. pylori-positive patients develop gastric cancer. Compared to Europe and the U.S., the incidence of gastric cancer is relatively high in East Asia, including Japan. One of the reasons for this difference may be that most of the H. pylori found in East Asia produces CagA, whereas only 30–40% of the bacteria detected in Europe and the U.S. produces CagA.
H. pylori was discovered by Robin Warren and Barry Marshall. They received the Nobel Prize for Physiology and Medicine in 2005 for their achievements.
*2 Phosphorylated CagA activates the cascade independent of Ras.
Column Fig. 15.2 Helicobacter pylori and stomach cancers
When H. pylori injects the CagA protein into the epithelial cells of the stomach, CagA is phosphorylated in the cells. The phosphorylated CagA continuously activates the MAP kinase cascade through the activation of the SHP-2 protein, inducing the abnormal growth of cells.
Retinoblastoma (Rb) is a protein that inhibits the activation of transcription factors that are involved in the expression of proteins required by cells in the S phase (see Chapter 13). However, if Rb activity becomes abnormal due to mutations, these transcription factors become activated and promote cell division at inappropriate times (Figure 15-6B). The protein p53 is also involved in the negative regulation of cell growth, and causes cancer if its activity becomes unregulated. Thus, genes for proteins such as Rb and p53, whose loss of activity is related to malignant alteration, are called tumor suppressor genes*3 . To date, numerous oncogenes and tumor suppressor genes have been discovered, all of which are closely associated with various cell functions (Table 15-1). Cancer cells are able to continue autonomous proliferation without depending on signals from outside because both pro-proliferative and proliferation-inhibiting mechanisms, required for cells to maintain the homeostasis in the organism, become abnormal. In nature, cells have several different signaling pathways for achieving the same goal. This redundancy is thought to serve as a safety net in case of trouble in one of the pathways. A multistep carcinogenesis model for colon cancer (Figure 15-7) shows how cells become cancerous only after multiple abnormal changes occur in the complex proliferation-inhibiting mechanisms built by cells. In the model, adenomatous polyposis coli (APC) gene activity becomes abnormal in the normal colon mucosa, resulting in the development of an adenoid tumor. When combined with Ras abnormality, the adenoid tumor is encouraged to grow, and develops into colon cancer when the p53 gene expression becomes abnormal. Genetic mutations further accumulate, eventually acquiring characteristics specific to cancer cells (such as metastasis).
*3 The tumor suppressor gene Rb causes cancer when it undergoes “loss-of-function” mutation (recessive) (see Chapter 3, 5), whereas the oncogene Ras causes cancer when it undergoes a “gain-of-function” mutation (dominant).
Figure 15-7 Multistep carcinogenesis model for colon cancer
Several abnormal changes in genes are accumulated as normal tissues become cancerous. APC is a tumor suppressor gene and is related to the cell skeleton. K-ras, a type of ras, is an oncogene. Abnormal changes in these genes cause adenoma in the colon mucosa. If further abnormal changes occur in p53, which is a tumor suppressor gene involved in transcription control, cancer will develop. Accumulation of additional gene abnormalities will lead to metastasis.
When DNA extracted from human cancer cells is fragmented, transferred to normal mouse cells and implanted in the mouse DNA, normal mouse cells can be malignantly altered (COLUMN Figure 15-3). The DNA fragments implanted in cells that became cancerous include a sequence of genes called oncogenes. The first oncogene discovered in humans was the mutant of Ras, a gene that encodes a type of small G protein. Oncogenes in their pre-cancerous (i.e, unmutated, normal) state are called proto-oncogenes as opposed to oncogenes.
As long as activated Ras proteins are produced, signal-transduction pathways continue sending downstream signals even without signals from growth factors. In normal cells that have not been malignantly altered, Ras functions by following the cycle of activation-inactivation according to growth signals. Comparisons of the amino acid sequence between the cancer-causing Ras protein and normal Ras protein revealed mutation in one amino acid as a result of one mutated DNA base. The mutation was located at the GTP-binding site, constantly activating the abnormal Ras protein by constantly binding to GTP. In this way, even in the absence of growth stimuli, cell proliferation does not stop.
Similarly, other studies analyzing cancer-causing gene defects have found that most oncogenes are genes encoding signaling molecules. Mutations in oncogenes disrupt signaling pathways that result in uncontrolled cell proliferation.
Column Fig. 15-3 Identification of oncogenes
(1) DNA is extracted from human cancer cells.
(2) The DNA of (1) is transfected into murine cells and cultured.
(3) Of the cells in (2), those that have transformed divide abnormally or form clusters.
(4) The DNA of transformed cells is extracted.
(5) The DNA of (4) is fragmented and transformed into phages.
(6) Of the phages, those including sequences unique to human DNA are selected. Again, the DNA is
transfected into murine cells. If the same transformation as (3) is seen, it means that oncogenes
are included in that phage.
Cancer treatment by inhibition of tyrosine phosphorylation
Leukemia is a cancer of the blood in which white blood cells proliferate in an uncontrolled and deregulated manner. In the past, chronic myelogenous leukemia (CML), which accounts for <20% of all leukemias, would result in death within several years of diagnosis. In 1960, it was already known that the cause of this type of leukemia was an abnormal chromosome. More than 20 years ago, it was discovered that the chromosome abnormality caused a tyrosine kinase, which controls cell growth, to stay constantly activated. As a result, abnormal cells proliferate endlessly (malignant alteration). This suggested that if it was possible to control the activity of the abnormal tyrosine kinase it would become possible to treat this type of leukemia. Subsequently, new cancer treatment strategies were developed as a result of the discovery of this cancer cell growth mechanism. In the early 1990s, drugs to control the abnormal activity of the protein kinase were developed, and one with enhanced inhibitory activity and specificity for the kinase was clinically used to treat patients (approved in 2001). As a result, this drug has saved the lives of more than 80% of all CML patients. This is a good example of how cancer can be treated by controlling the abnormal signal transduction of cancer cells using molecular biology approaches. In recent years, in addition to synthetic compounds, antibodies showing similar effects are being developed for treating cancers.
Column Fig. 15-4 Abnormal chromosome in chronic myelocytic leukemia (CML)
A translocation in the chromosome (mutual interchange of a part of chromosome 9 and part of chromosome 22) is frequently seen in CML patients. This chromosome defect encourages the production of abnormal protein, which eventually causes the malignant alteration of white blood cells.