The problem of chemoresistance is multidimensional, and several factors affect tumour drug sensitivity. However, evidence has emerged from research over recent decades that can partially explain the mechanisms of resistance (Longley et al.,2006). These processes include the reduction of active drug concentration at the target level, due to activation of transporter proteins, detoxification mechanisms within the cell, alterations affecting drug-target interactions and induction of factors that influence cellular responses that affecting tumour cell survival (Fig 1.11).
A key drug-resistance mechanism in cancer cells is the ability to decrease drug concentration at the drug target. Two possible mechanisms capable of reducing drug concentration at the target site includes reduced drug uptake or enhanced drug efflux. Drug transporters aim to remove chemotherapeutic drugs from the cell, and this mechanism is a natural defence by which means the tumour cells can evade the action of the drug (Kachalaki et al., 2013).
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Figure 1.11: Mechanisms of chemoresistance. (Adapted from Gatti and Zunino, 2005)
An illustration of the resistance mechanisms by cells. (a) represents the drug transporters P- glycoprotein 1 (P-gp), multidrug-resistant associated protein (MRP) and breast cancer resistance protein (BCRP). (b) the drug target site such as the DNA; (c) cellular response to the drug: cell cycle arrest, initiation of apoptosis or cell survival.
1.13.1 Resistance mechanisms of cancer cells
The strategies employed by cancer cells to evade the effect of anti-cancer drugs are described in this section.
1.13.2 Drug transporters
Most drugs are transported from the digestive tract into the portal blood vessel, but drug bioavailability at the site of action is dependent upon local cellular active transport processes (Zhou, 2010). Based on metabolic needs or the presence of toxic substances, membrane transporters relocate substances in and out of cells (Cucillo et al., 2016).
(a)
(b)
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Several cytotoxins found naturally and used in chemotherapy as drugs enter cells by passive diffusion, and these amphipathic drugs are hydrophobic enough to diffuse through a lipid bilayer but also hydrophilic enough to reach their target (Gatti and Zunino, 2005).
A decrease in drug influx, as well as increase of efflux from the cells, may reduce the accumulation of drug inside the cell, as most chemotherapeutic drugs move into cells via passive diffusion through the plasma membrane. Thus, changes in the cell membrane structure associated with oncogenes will also affect drug influx (Bush and Li, 2002).
The ATP-binding cassette (ABC) family of proteins is responsible for influencing the intracellular concentration of drugs and several compounds in cells and tissues (Gatti et al., 2005). Approximately 49 ABC transporter genes have been discovered in the human genome, which are expressed naturally in a diversity of tissues remove xenobiotics from the body (Kachalaki et al., 2013).
Drug transporters typically are composed of two transmembrane domains that recognise and translocate substances through the plasma membrane and two other nucleotide-binding domains, which help to generate the energy needed, by the hydrolysis of ATP (Kachalaki et
al., 2013).
1.13.3 Modifications of drug targets
Any alteration or modification in the expression levels of the drug target during patient treatment can lead to resistance. An example of such change is with the antimetabolite drug 5-Fluorouracil, which inside the cell is converted to the compound fluorodeoxyuridine monophosphate (FdUMP). FdUMP is a strong inhibitor of the enzyme thymidylate synthase (TS) which is needed for DNA synthesis (Longley et al., 2003). An increase in TS affects the quantity of 5FU required to inhibit all targets, and if insufficient, cancer cells will continue multiplying (Kachalaki et al., 2016). Research has shown that patients with elevated levels of TS were resistant to chemotherapy when compared with responsive patients (Leichman et al., 1997).
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Another example is that of decreased oestrogen receptor expression in oestrogen positive breast cancer cells which leads to the failure of tamoxifen, a major drug of choice for treatment to subdue cell growth (Miller, 2004; Campos 2004).
1.13.4 Cellular response to chemotherapeutic agents
There are several different cellular responses to chemotherapeutic agents, and the initiation of DNA repair processes is considered to be the first and most frequent response of cells to toxic damage. Cells have developed complex signalling pathways to stop cell cycle progression in the presence of DNA damage, thus facilitating DNA repair (Zhou et al., 2000; Friedberg, E. C. 2003). However, when the impact of cellular insult surpasses cellular ability to repair, cells enter into apoptosis (Cory and Adams, 2002).
1.13.5 Alterations in DNA damage repair
Cells can develop chemoresistance following recovery from DNA damage probably caused by oncosis. The excision repair cross-complementing protein group 1 (ERCC1), which is a DNA-repair protein, has been shown to be prominent in platinum-based drug resistance and the overexpression of this protein correlates with platinum drug resistance in certain cancers, such as non-small cell lung cancers, ovarian and gastric cancers (Chang et al. 2005; Olaussen
et al., 2006; Bouwman and Jonkers, 2012).
1.13.6 Problems with cell cycle regulation
The tumour suppressor protein (p53) is encoded in humans by the homologous gene TP53 and functions as a tumour suppressor gene by regulating the cell cycle, DNA repairs and inducing apoptosis (Khoury and Bourdon, 2003). p53 controls apoptosis and cell cycle arrest after DNA damage and prevents the production of new daughter cells with damaged DNA (Lowe et al., 2004; Kachalaki et al., 2016). p53 controls the G1 to S phase and also at the G2/M phases, and its activation promotes cell cycle arrest and encourages DNA repair or
triggers apoptosis. Therefore, its inactivation has dire consequences in cells, leading to the emergence of several cancers (Kapil and Fok 2007).
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Previous reports have revealed that in healthy growing cells, p53 translocates between the nucleus and cytoplasm, but when the cell is under stress such as in cases of hypoxia, UV irradiation or in the presence of chemotherapeutic drugs, p53 stabilises its location in the nucleus (Mesaeli and Philipson, 2003). In the nucleus, p53 binds to DNA stimulating the production of p21Cip1 (CDK-interacting protein 1) which interacts with cdk2, a cell division stimulating protein. The interaction between p21Cip1 and cdk2 prevents the cell from advancing to the next cell division stage with the resultant cell cycle arrest and apoptosis. So, a mutant p53 will be unable to bind to DNA effectively with the resultant consequence that p21Cip1 cannot act with cdk2 to stop the cell proliferation. Thus, cell division continues uncontrollably leading to the production of abnormal daughter cells resulting in tumours (NCBI 1998; Zukerman et
al., 2009).
Increased expression and/or mutation of the p53 gene has been linked with poor breast cancer prognosis, chemotherapy radiotherapy. Cancer is an example where the normal machinery of the cell cycle regulation is defective, with either decreased removal of abnormal cells and/or an overproliferation of cells (King and Cidlowski,1998) and, the tumour suppressor gene is a significant player (Pucci et al., 2000).
Another essential regulator of cell cycle progression is the phosphatase and tensin homolog (PTEN) protein which is also involved in tumour suppression activities, such as inhibition of cell growth, migration, invasion and focal adhesion (Kapil and Fok 2007). It induces cell cycle arrest by upregulating p27KIP1 (a member of the universal cyclin-dependent kinase inhibitor
(CDKI) family), a cell cycle inhibitor, and collaboration between p53and p27KIP1 appears critical
in cancer development (Minami et al., 2016).
1.13.7 Changes in B-cell lymphoma 2 (Bcl-2) regulation
The Bcl-2 family of proteins regulates and activates caspases, which are the essential effectors of apoptosis. The Bcl-2 family contains both anti-apoptotic and pro-apoptotic proteins, and the pro-apoptotic members initiate the activities of the caspases leading to
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apoptosis. Previously, a study has shown an association between high expressions of Bcl-2 in cancers with reduced response to anti-cancer agents (Kachalaki et al., 2016).