Carcinogenesis and Cancer Treatment Resistance - Literature review Example

? Identification and analysis of DNA repair mechanisms that contribute to resistance against nucleoside analogues Contents Contents 2 0 Carcinogenesis and Cancer Treatment Resistance 3 2.0 Mechanisms in Cancer Cells 6 3.0 Nucleoside Analogues 7 3.1 Purine nucleobases and purine nucleoside analogues 10 3.2 Deoxyadenosine derivatives 11 3.3 Deoxycytidine analogues 11 4.0 Topoisomerases 12 5.0 The MRN complex 13 6.0 References 14 1.0 Carcinogenesis and Cancer Treatment Resistance Cellular DNA was once considered to be an inert substance, however, current molecular knowledge and research has developed considerably. It is now recognised that DNA is constantly being damaged and repaired, and thus it is much more active than was previously considered. Damage to DNA occurs frequently from sources that are inside and outside the cell. This includes mutagenic environmental agents, such as chemicals which can be obtained both from the by-products of human industry and naturally from food product, as well as many other sources of mutation. Consequently, DNA is damaged frequently, and is repaired through pathways of repair and proofreading functions by DNA polymerase. Multiple mechanisms of repair within the cell act to balance these levels of mutation. However, the large number of mutations that occur means that every so often, the repair process misses these mutations and they become part of the DNA. This process can be the first step in carcinogenesis . There are currently 346 different genes that have been associated with the development of cancer, although this number continues to grow. In some cases these genes are mutated in cancer cells and not in normal cells. In other cases, genes may show substantial levels of deregulation. Both of these processes have the potential to significantly decrease the effectiveness of certain genes, leading to altered pathways within the cancer cell and the body itself . This suggests that mutations are a crucial part of the formation of cancer. Indeed, human cancer appears to have thousands of different mutations by the time that it is first detected. Furthermore, information that cancer can be inherited from one generation to the next suggests that some of these mutations can be passed along in genes . Cancer is characterised by high levels of clonal expansion of somatic cells. These cells are not subject to the normal growth regulation components of the cell cycle. Thus, they are able to proliferate beyond the normal constraints of tissue. Likewise, the controls over apoptosis are also circumvented . Most tumours that develop in the human body are substantially heterogeneous. This suggests that multiple mutations occur within human cancers driving the creation of tumours and the change in function between normal cells and cancerous cells . Thus, cancer can be considered to be a disease, or collection of diseases, that occurs due to genetic abnormalities accumulating within cells . In each generation of cancer cells, significant mutation occurs within different cells, and those that develop mutations that facilitate cancer growth are selected for, resulting in an increase in the prevalence of these cells. Mutations may be subtle, such as changes in the sequence of nucleotides, or more substantial, involving changes in the chromosomes themselves . One significant factor in cancer treatment resistance is that therapeutic killing cancer cells, while causing cancer to go into remission, can also act to select for resistance to treatment. This can result in a progression of tumour growth, with cells that cannot be targeted by the same treatment . Another form of resistance that can occur is based on the cells location in the division cycle. Cancer therapy tends to be most effective on cells that are rapidly replicating. Furthermore, resistance to treatment can often be stronger in some stages of the cell cycle than others . Cancer cells contain a wide range of mutations, and these mutations may act to drive the formation and progression of tumours. Research suggests that cancer cells are highly unstable, and that this instability results in selection for aspects of cells that bypass traditional forms of control, especially over replication and apoptosis. This variation within individual cells results in the creation of highly heterogeneous tumours. Furthermore, cancer cells appear to be genetically unstable, and prone to mutation. This heterogeneity and instability plays another crucial role. It helps the cancer cells to become resistant to treatment. This means that any form of treatment may not be effective on all cancerous cells within the tumour. Thus, a proportion of the cells that remain are resistant to the treatment. This leads to selection for resistant elements of cancer cells. The genetic instability means that if cells do not have mutations for resistance but survive treatment due to chance are able to develop this resistance before the next treatment occurs . All forms of cancer treatment have the risk of selecting for resistance among cancer cells. This suggests that the most effective method of combating cancer would be to simultaneously target all aberrant cells at once and to attack lesions that are specific to tumours at the individual level. However, this method is beyond our current level of knowledge and sophistication . Therefore, research into cancer needs to continue examining molecular mechanisms that occur within cancer cells, with the aim of finding treatments that do not create resistance. An almost ironic observation is that carcinogenesis has a significant role both in cancer and in cancer therapy. Cancer can be caused by a cytotoxic agent, and then the same agent is sometimes used to treat the cancer. This is driven by the fact that genetic instability is a factor that acts to accelerate tumour progression and to predispose an individual to cancer. Genetic instability can be caused by a number of factors, most importantly the influence of carcinogenic agents, or errors in DNA repair. Yet, inhibiting DNA repair is also known as a method of treating cancer. Likewise, errors in DNA replication cause the mutations that drive carcinogenesis, yet, once cancer is formed, high fidelity DNA replication allows the mutant cells to continue to grow and divide efficiently. This is a factor that needs to be extensively considered when examining resistance to cancer treatment, as it shows that the most logical processes are not always the correct ones. 2.0 Mechanisms in Cancer Cells Determining the way that cancer occurs at the molecular level has been promoted as a method of developing more sophisticated and effective cancer therapies. It is often believed that cancers are highly diverse and that cancer is not a disease itself, but rather a series of different cancers. Under this approach, different cancers require different treatments, and this has lead to the development of a wide range of different treatments for cancer . Normal cells within the body rely on mitogenic signals to control their proliferations. Additionally, they can only proliferate towards the end of the G1 component of the cell cycle. Furthermore, mitogenic signalling can only be sustainably achieved under some circumstances . Therefore, the limitations on cell proliferation are significant under normal conditions. Within a normal cell, apoptosis, the controlled death of the cell is signalled through alternate two pathways. The first of these is the extrinsic pathway, which occurs via death receptors that are present on the surface of the cell. The second pathway is the intrinsic pathway, which is controlled by the mitochondria within the cell . In cancer cells, the pathways that traditionally constrain cell proliferation are abnormal. One such difference is the math that requires cells to receive mitogenic signals in order to proliferate. In cancer cells, this pathway is bypassed. Furthermore, cancer cells also bypass the mechanisms for apoptosis . This is achieved through the expression of proteins that limit apoptosis (anti-apoptosis proteins) or either mutation or downreglation of the proteins that facilitate apoptosis . High amounts of cell birth through proliferation and low levels of cell death via constrained apoptosis results in the rapid cell growth that is associated with cancer. Research suggests that the control of cell growth, proliferation and apoptosis are strongly linked to one another. Furthermore, apoptosis plays a crucial role in cancer development and maintenance. Cells that have high levels of replication, but no errors in apoptosis, do not become excessive within the body because cell death is triggered, and thus many of these cells die . Because these mechanisms are prevalent across cancer cells, they make the ideal targets for cancer therapy. Thus, many cancer treatments target the cell cycle itself. Research has shown that arresting the cell cycle is an effective method of inducing apoptosis and this plays a significant role in cancer treatment . 3.0 Nucleoside Analogues Most current drugs that are used for treating cancer do so by targeting the mitotic cycle, however, this is not a precise form of targeting. Instead, cancer treatments act by directly interfering with the machinery involved in cell division and DNA replication . One class of drugs that is becoming increasingly useful in the treatment of cancer are nucleoside analogues (NAs). Nucleoside analogues have the potential to inhibit the repair and replication of DNA within the cancer cells . Nucleoside analogues are a class of antimetabolites that have recently been used in the treatment of cancer, including solid tumours. This family of molecules includes analogues for both purine and pyrimidine DNA particles. These molecules are actively transported into cells through membrane transporters . Once inside the cell they interfere with a number of cellular processes such as cytotoxity . Many different NAs are used for treatment depending on the type of cancer. For example, fluorophyrimidine 5-fluorouracil (5-FU) is widely prescribed for a range of different cancer types . Nucleoside analogues were some of the first methods of therapy to be introduced for the treatment of cancer. Currently their use is prevalent throughout treatment for many forms of cancer. Understanding the way these molecules affect cellular processes is crucial for increasing specificity and efficiency of cancer treatment . Two NAs that have been shown to be particularly important in cancer treatment are fludarabine and gemcitabine. Both of these molecules have been shown to play a significant role in enhancing the response of tumours to radiography. This effect is thought to be driven by the NAs preventing the cell from repairing itself following damage from radiation. Thus, fludarabine and gemcitabine are able to be used as means of priming cells for radiation treatment, increasing the effectiveness of radiation on solid tumours . Thus, NAs are generally not used independently as a treatment for cancer; rather they are used in conjunction with other treatment methods, most often radiotherapy. There are three general mechanisms that have been identified that result in the cell being resistant to the NAs. The first of these is where there is a low level of concentration of NA triphosphates. This can be caused by a range of different effects, such as high levels of NA degradation, a reduction in the amount of enzyme activity or increases in the size of dNTP pools. The second mechanism is when apoptosis of cells is induced defectively, preventing NAs from being taken up efficiently. Finally, resistance may be due to NAs being unable to create the desired alterations within the strands of DNAs themselves or the pools of dNTP . However, another issue that is significantly important for many NAs is the presence of DNA repair mechanisms and their efficiency. The mechanism that NAs varies depending on the individual NA that is being used. Some NAs act by preventing the synthesis of nucleotides, while others depend on being integrated into the growing chain during DNA synthesis. When NAs are integrated into DNA they halt the replication machinery, by preventing any further bases from being added. This is achieved through the phosphorylation of the compound once it enters the cell, transferring it to an active state. For a cell to be able to continue replication, it must first remove the NA from the DNA chain. This involves the use of a repair mechanism that is able to recognise the NA as being an aberrant component of the chain and then efficiently remove it . Thus, for NAs that operate in this manner, a significant cause of treatment resistance is the removal of the NA from the growing DNA chain, and therefore the continuing of replication despite treatment. The use of NAs as a form of treatment is effective because cells in the human body are able to obtain pre-made pyrimidines and purines from their environment and incorporate this into their DNA. Approximately 20% of all drugs that are used to treat cancer are NAs of either purines or pyrimidines. Development of NAs as drugs for cancer treatment continues, with researchers looking for new and effective ways of inhibiting the ability of cancer cells to replicate . Nucleoside analogues are generally based off the structures or either pyrimidines or purines, and often mimic the structure of one of the bases in DNA. This is important, because the replication machinery must recognise the compound as component for DNA in order for it to be incorporated (Periguad, Gosselin & Imbach, 1992). 3.1 Purine nucleobases and purine nucleoside analogues The use of purine NAs are crucial in cancer treatment. Although this type of drug has been used in cancer treatment for more than 50 years, there continues to be significant developments, and the creation of new purine NAs. These may potentially create a more efficient method of treating cancer, and there remains significant potential for the development of new anticancer drugs within this type of compound .There are many types of purine NAs that are either being used or developed for therapeutic use. Some examples of such drugs are tiasofurin, heplanocin A and 3-deazaguanosine. Purine NAs vary significantly in their methods of action, the requirements that they have for activation and their biological activity . This method of treatment mostly affects cells that are actively proliferating, although it can also have some effect on cells that are not . 3.2 Deoxyadenosine derivatives Purine NAs are developed to have a chemical structure that resembles either adenosine or deoxyadenosine. The derivatives often involve the addition of a chemical group or a substitution. For example, cladribine involves a substitution of the hydrogen present in deoxyadenosine with a chlorine. Regardless of the exact chemical structure, deoxyadenosine derivatives share many aspects in common, such as the method of transport into the cell, and dephysphorylation by a 5’-nucleotidase . Deoxyadenosine derivatives are common in the treatment of cancer, and many currently used drugs are of this form . 3.3 Deoxycytidine analogues A deoxycytidine analogue that is used in the treatment of cancer is Gemcitabine. This drug is commonly used to treat patients with non-small cell lung cancer. It has a low level of toxicity, and consequently is able to be used in a wide range of patients, such as the elderly who have weakened systems . In addition, the low level of toxicity makes the drug effective in use with other forms of cancer treatment . This drug is a deoxycytidine analogue that is substituted with fluorine. When it is activated within the cell, the metabolite can be incorporated into the DNA chain, which affects multiple DNA poloymerase. In turn, this results in prevention of the DNA chain elongation, DNA repair and DNA synthesis. Nevertheless, patients are able to develop a resistance to this drug . Consequently, research needs to examine ways of increasing the efficiency of NAs and decreasing the resistance that cancer cells develop. 4.0 Topoisomerases Topoisomerases are enzymes that are involved in the regulation of DNA winding. The structure of DNA means that as replication or transcription occurs, the DNA in front of the replication site becomes overwound. This would potentially replication or transcription unable to continue. Therefore, topoisomerases create small cuts in the DNA backbone, allowing it to unwind. This cut is then sealed afterwards. Without the presence of topoisomerases, DNA replication would not be able to occur, and consequently, cells could not proliferate. This makes topoisomerases an important target for cancer treatment, as interfering with the function of these enzymes could prevent further proliferation of the cells. Furthermore, inhibiting cell replication in this manner eventually leads to apoptosis of the cell. Consequently, many anticancer drugs target these enzymes . An example of this is the drug doxorubicin, which is used in the treatment of breast cancer. This drug specifically targets DNA topoisomerase II-alpha. Studies have shown that this drug is most effective where there is expression of topoisomerases II-alpha is high, and resistance to the drug occurs when expression is low. This helps to introduce specificity to the drug’s use, as topoisomerases II-alpha is present during cell replication, thus, is highly prevalent in the rapidly replicating cancer cells . DNA topoisomerase II is the target for a wide range of cancer drugs . The reason for targeting topoisomerase II specifically is that this particular topoisomerase enzyme has been shown to have a crucial role in DNA replication. The enzyme itself is a homodimer and has two isoforms known as topoisomerase II-alpha and topoisomerase II-beta. The function of topoisomerase II-beta is not clearly defined, and consequently most therapies focus on the alpha isoform. Many drugs that target this enzyme do so by trapping it in a complex, which restricts it from performing its function . Consequently, topoisomerase plays a highly important role in cancer therapy, particularly topoisomerase II-alpha. 5.0 The MRN complex The MRN complex is a protein complex that consists of three components, Mre11, Rad50 and Nbs1. This complex plays a crucial role in DNA repair in humans as well as other mammalian species. Research has shown that MRN is one of the first compounds to respond to double-strand breaks within the cell. This complex has the potential to recognise the DNA damage caused by cancer treatment and to repair this, effectively rendering the treatment useless. Furthermore, it has a role in cell-cycle signalling cascades , which may also influence cancer treatments, as many of these rely on arresting the cell in a particular state . When a double-stranded break in DNA occurs, the MRN complex has been shown to act as a sensor for the break, recruiting specific molecules to repair the damage in the DNA . Thus, MRN involvement is crucial for DNA repair to occur. If functional MRN is not present, the ATM protein kinase is not activated, and consequently the repair does not occur . While this mechanism is important for the repair of normal cells, it has the potential decrease the effectiveness of cancer treatments significantly. As discussed above, the mechanism for some cancer treatments involves the inclusion of a NA into the growing DNA chain in order to halt DNA replication. Thus, the presence of a high fidelity DNA repair mechanism would undermine this process, and allow the cell to remove the NA from the DNA chain. This would reduce the effectiveness of the treatment and increase the resistance that the cell had. Thus, it is important to examine the role that MRN and DNA repair mechanisms as a whole on the effectiveness of cancer treatments. 6.0 References Blagosklonny, M. V. (2005). Carcinogenesis, cancer therapy and chemoprevention. Cell Death and Differenciation, 12, 592-602. Bunn Jr, P. A. (1999). Triplet chemotherapy with gemcitabine, a platinum, and a third agent in the treatment of advanced non-small cell lung cancer. Seminars in Oncology, 26, 25. 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Mre11-Rad50-Nbs1 is a keystone complex connecting DNA repair machinery, double-strand break signaling, and the chromatin template This paper is one of a selection of papers published in this Special Issue, entitled 28th International West Coast Chromatin and Chromosome Conference, and has undergone the Journal's usual peer review process. Biochemistry and Cell Biology, 85, 509-520. Zhu, C., Johansson, M., Permert, J. & Karlsson, A. (1998). Enhanced cytotoxicity of nucleoside analogs by overexpression of mitochondrial deoxyguanosine kinase in cancer cell lines. Journal of Biological Chemistry, 273, 14707-14711.  Read More