Antineoplastic resistance, often used interchangeably with chemotherapy resistance, is the resistance of neoplastic (cancerous) cells, or the ability of cancer cells to survive and grow despite anti-cancer therapies. [1] In some cases, cancers can evolve resistance to multiple drugs, called multiple drug resistance.
There are two general causes of antineoplastic therapy failure: Inherent genetic characteristics, giving cancer cells their resistance and acquired resistance after drug exposure, which is rooted in the concept of cancer cell heterogeneity. [1] Characteristics of resistant cells include altered membrane transport, enhanced DNA repair, apoptotic pathway defects, alteration of target molecules, protein and pathway mechanisms, such as enzymatic deactivation. [1] Since cancer is a genetic disease, two genomic events underlie acquired drug resistance: Genome alterations (e.g. gene amplification and deletion) and epigenetic modifications. Cancer cells are constantly using a variety of tools, involving genes, proteins, and altered pathways, to ensure their survival against antineoplastic drugs.
Antineoplastic resistance, synonymous with chemotherapy resistance, is the ability of cancer cells to survive and grow despite different anti-cancer therapies, i.e. their multiple drug resistance. There are two general causes of antineoplastic therapy failure: [2] Inherent resistance, such as genetic characteristics, giving cancer cells their resistance from the beginning, which is rooted in the concept of cancer cell heterogeneity and acquired resistance after drug exposure. [2]
Cancer cell heterogeneity, or tumour heterogeneity, is the idea that tumours are made up of different populations of cancer cells that are morphologically, phenotypically and functionally different. [3]
Cancer cell heterogeneity can cause disease progression when molecularly targeted therapy, fails to kill those tumor cells which do not express the marker, then divide and mutate further, creating a new heterogeneous tumour. In breast cancer models of the mouse the immune microenvironment affects susceptibility to neoadjuvant chemotherapy. In breast cancer, particularly in the triple-negative subtype, immune checkpoint blockade has been used successfully in metastatic cases and neoadjuvant therapy. [4]
Since cancer is a genetic disease, [5] two genomic events underlie these mechanisms of acquired drug resistance: Genome alterations (e.g. gene amplification and deletion) and epigenetic modifications.
Chromosomal rearrangement due to genome instability can cause gene amplification and deletion. Gene amplification is the increase in copy number of a region of a chromosome. [6] which occur frequently in solid tumors, and can contribute to tumor evolution through altered gene expression. [6]
Hamster cell research in 1993 showed that amplifications in the DHFR gene involved in DNA synthesis began with chromosome break in below the gene, and subsequent cycles of bridge-breakage-fusion formations result in large intrachromosomal repeats. [7] The over amplification of oncogenes can occur in response to chemotherapy, thought to be the underlying mechanism in several classes of resistance. [6] For example, DHFR amplification occurs in response to methotrexate, [8] TYMS (involved in DNA synthesis) amplification occurs in response to 5-fluorouracil, [9] and BCR-ABL amplification occurs in response to imatinib mesylate. [10] Determining areas of gene amplification in cells from cancer patients has huge clinical implications. Gene deletion is the opposite of gene amplification, where a region of a chromosome is lost and drug resistance occurs by losing tumor suppressor genes such as TP53. [2]
Genomic instability can occur when the replication fork is disturbed or stalled in its migration. This can occur with replication fork barriers, proteins such as PTIP, CHD4 and PARP1, which are normally cleared by the cell's DNA damage sensors, surveyors, and responders BRCA1 and BRCA2. [11]
Epigenetic modifications in antineoplastic drug resistance play a major role in cancer development and drug resistance as they contribute to the regulation of gene expression. [12] Two main types of epigenetic control are DNA methylation and histone methylation/acetylation. DNA methylation is the process of adding methyl groups to DNA, usually in the upstream promoter regions, which stops DNA transcription at the region and effectively silences individual genes. Histone modifications, such as deacetylation, alters chromatin formation and silence large chromosomal regions. In cancer cells, where normal regulation of gene expression breaks down, the oncogenes are activated via hypomethylation and tumor suppressors are silenced via hypermethylation. Similarly, in drug resistance development, it has been suggested that epigenetic modifications can result in the activation and overexpression of pro-drug resistance genes. [12]
Studies on cancer cell lines have shown that hypomethylation (loss of methylation) of the MDR1 gene promoter caused overexpression and the multidrug resistance. [13]
In a methotrexate resistant breast cancer cell lines without drug uptake and folate carrier expression, giving DAC, a DNA methylation inhibitor, improved drug uptake and folate carrier expression. [14]
Acquired resistance to the alkylating drug fotemustine in melanoma cell showed high MGMT activity related to the hypermethylation of the MGMT gene exons. [15]
In Imatinib resistant cell lines, silencing of the SOCS-3 gene via methylation has been shown to cause STAT3 protein activation, which caused uncontrolled proliferation. [16]
Cancer cells can become resistant to multiple drugs by altered membrane transport, enhanced DNA repair, apoptotic pathway defects, alteration of target molecules, protein and pathway mechanisms, such as enzymatic deactivation. [12]
Many classes of antineoplastic drugs act on intracellular components and pathways, like DNA, nuclear components, meaning that they need to enter the cancer cells. The p-glycoprotein (P-gp), or the multiple drug resistance protein, is a phosphorylated and glycosylated membrane transporter that can shuttle drugs out of the cell, thereby decreasing or ablating drug efficacy. This transporter protein is encoded by the MDR1 gene and is also called the ATP-binding cassette (ABC) protein. MDR1 has promiscuous substrate specificity, allowing it to transport many structurally diverse compounds across the cell membrane, mainly hydrophobic compounds. Studies have found that the MDR1 gene can be activated and overexpressed in response to pharmaceutical drugs, thus forming the basis for resistance to many drugs. [2]
For example, the antibiotic rifampicin has been found to induce MDR1 expression. Experiments in different drug resistant cell lines and patient DNA revealed gene rearrangements which had initiated the activation or overexpression of MDR1. [17] A C3435T polymorphism in exon 226 of MDR1 has also been strongly correlated with p-glycoprotein activities. [18]
MDR1 is activated through NF-κB, a protein complex which acts as a transcription factor. [19] [20] [21] [22] In the rat, an NF-κB binding site is adjacent to the mdr1b gene, [23] NF-κB can be active in tumour cells because its mutated NF-κB gene or its inhibitory IκB gene mutated under chemotherapy. In colorectal cancer cells, inhibition of NF-κB or MDR1 caused increased apoptosis in response to a chemotherapeutic agent. [19]
Enhanced DNA repair plays an important role in the ability for cancer cells to overcome drug-induced DNA damages.
Platinum-based chemotherapies, such as cisplatin, target tumour cells by cross-linking their DNA strands, causing mutation and damage. [2] Such damage will trigger programmed cell death (e.g. apoptosis) in cancer cells. Cisplatin resistance occurs when cancer cells develop an enhanced ability to reverse such damage by removing the cisplatin from DNA and repairing any damage done. [2] [12] The cisplatin-resistant cells upregulate expression of the excision repair cross-complementing (ERCC1) gene and protein. [2]
Some chemotherapies are alkylating agents meaning they attach an alkyl group to DNA to stop it from being read. O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme which removes alkyl groups from DNA. MGMT expression is upregulated in many cancer cells, which protects them from alkylating agents. [12] Increased MGMT expression has been found in colon cancer, lung cancer, non-Hodgkin's lymphoma, breast cancer, gliomas, myeloma and pancreatic cancer. [24]
TP53 is a tumor suppressor gene encoding the p53 protein, which responds to DNA damage either by DNA repair, cell cycle arrest, or apoptosis. Losing TP53 via gene deletion can allow cells to continuously replicate despite DNA damage. The tolerance of DNA damage can grant cancer cells a method of resistance to those drugs which normally induce apoptosis through DNA damage. [2] [12]
Other genes involved in the apoptotic pathway related drug resistance include h-ras and bcl-2/bax. [25] Oncogenic h-ras has been found to increase expression of ERCC1, resulting in enhanced DNA repair (see above). [26] Inhibition of h-ras was found to increase cisplatin sensitivity in glioblastoma cells. [27] Upregulated expression of Bcl-2 in leukemic cells (non-Hodgkin's lymphoma) resulted in decreased levels of apoptosis in response to chemotherapeutic agents, as Bcl-2 is a pro-survival oncogene. [28]
During targeted therapy, oftentimes the target has modified itself and decreased its expression to the point that therapy is no longer effective. One example of this is the loss of estrogen receptor (ER) and progesterone receptor (PR) upon anti-estrogen treatment of breast cancer. [29] Tumors with loss of ER and PR no longer respond to tamoxifen or other anti-estrogen treatments, and while cancer cells remain somewhat responsive to estrogen synthesis inhibitors, they eventually become unresponsive to endocrine manipulation and no longer dependent on estrogen for growth. [29]
Another line of therapeutics used for treating breast cancer is targeting of kinases like human epidermal growth factor receptor 2 (HER2) from the EGFR family. Mutations often occur in the HER2 gene upon treatment with an inhibitor, with about 50% of patients with lung cancer found to have an EGFR-T790M gatekeeper mutation. [12]
Treatment of chronic myeloid leukemia (CML) involves a tyrosine kinase inhibitor that targets the BCR/ABL fusion gene called imatinib. In some people resistant to Imatinib, the BCR/ABL gene is reactivated or amplified, or a single point mutation has occurred on the gene. These point mutations enhance autophosphorylation of the BCR-ABL protein, resulting in the stabilization of the ATP-binding site into its active form, which cannot be bound by imatinib for proper drug activation. [30]
Topoisomerase is a lucrative target for cancer therapy due to its critical role as an enzyme in DNA replication, and many topoisomerase inhibitors have been made. [31] Resistance can occur when topoisomerase levels are decreased, or when different isoforms of topoisomerase are differentially distributed within the cell. Mutant enzymes have also been reported in patient leukemic cells, as well as mutations in other cancers that confer resistance to topoisomerase inhibitors. [31]
One of the mechanisms of antineoplastic resistance is over-expression of drug-metabolizing enzymes or carrier molecules. [2] By increasing expression of metabolic enzymes, drugs are more rapidly converted to drug conjugates or inactive forms that can then be excreted. For example, increased expression of glutathione promotes drug resistance, as the electrophilic properties of glutathione allow it to react with cytotoxic agents, inactivating them. [32] In some cases, decreased expression or loss of expression of drug-metabolising enzymes confers resistance, as the enzymes are needed to process a drug from an inactive form to an active form. Arabinoside, a commonly used chemotherapy for leukemia and lymphomas, is converted into cytosine arabinoside triphosphate by deoxycytidine kinase. Mutation of deoxycytidine kinase or loss of expression results in resistance to arabinoside. [2]
Growth factor expression levels can also promote resistance to antineoplastic therapies. [2] In breast cancer, drug resistant cells were found to express high levels of IL-6, while sensitive cells did not express significant levels of the growth factor. IL-6 activates the CCAAT enhancer-binding protein transcription factors which activate MDR1 gene expression (see Alteration of Membrane Transport). [33]
Pharmacogenetics play an increasingly important role in antineoplastic treatment. [34] Rapid sequencing technologies can identify genetic markers for treatment sensitivity and potential resistance. Certain markers are more representative and more likely to be used clinically. [34]
When BRCA1 and BRCA2 are missing, as in 5 percent to 10 percent of all breast cancers, a stalled fork remains destabilized and its newly synthesized DNA is degraded. This genomic instability means the cancer cell is actually more sensitive to DNA-damaging chemotherapy drugs. [35]
Marker | Drug | Major Conditions | Clinical Implications |
---|---|---|---|
TYMS | 5-Fluorouracil | Colorectal, stomach, pancreatic cancer | High TYMS may show poor response & less toxicity |
DPYD | 5-Fluorouracil | Colorectal, stomach, pancreatic cancer | DPD deficiency associated with higher risk of toxicity |
UGT1A1 | Irinotecan | Colorectal cancer | Decreased UGT1A1 activity may increase risk of toxicity |
CYP2D6 | Tamoxifen | Breast cancer | Patients with deficient CYP2D6 activity are at greater risks of relapse |
EGFR | Anti-EGFR therapy | Colorectal, lung cancer | Activation of EGFR pathways enhances tumor growth, progression, & resistance to therapy |
KRAS | Anti-EGFR therapy | Colorectal, lung cancer | KRAS mutation is associated with resistance to anti-EGFR therapy |
FCGR3A | Rituximab | Non-Hodgkin's lymphoma | FCRG3A 158Val/Val genotype may be associated with better response |
BRCA1/BRCA2 | Platinum | Breast, ovarian cancer | BRCA1/2-mutated cancers are more sensitive to DNA damage. Secondary intragenic mutations confer acquired resistance |
MDR proteins are known to be drug-resistance genes, and are highly expressed in various cancers. Inhibition of the MDR genes could result in sensitization of cells to therapeutics and a decrease in antineoplastic resistance. Reversin 121 (R121) is a high-affinity peptide for MDR, and use of R121 as a treatment for pancreatic cancer cells results in increased chemosensitivity and decreased proliferation. [36]
Aberrant NF-κB expression is found in many cancers, and NF-κB has been found to be involved in resistance to platinum-based chemotherapies, such as cisplatin. NF-κB inhibition by genistein in various cancer cell lines (prostate, breast, lung and pancreas) showed increased growth inhibition and an increase in chemosensitivity, seen as an increase in apoptosis induced by therapeutic agents. [37] However, targeting the NF-κB pathway can be difficult, as there can be many off-target and non-specific effects.
Expression of mutated TP53 causes defects in the apoptotic pathway, allowing cancerous cells to avoid death. Re-expression of the wild-type gene in cancer cells in vitro has been shown to inhibit cell proliferation, induce cell cycle arrest and apoptosis. [38]
In ovarian cancer, the ATP7B gene encodes for a copper efflux transporter, found to be upregulated in cisplatin-resistant cell lines and tumors. Development of antisense deoxynucleotides against ATP7B mRNA and treatment of an ovarian cancer cell line shows that inhibition of ATP7B increases sensitivity of the cells to cisplatin. [39]
Chemotherapy is the type of cancer treatment that uses one or more anti-cancer drugs in a standard regimen. Chemotherapy may be given with a curative intent, or it may aim only to prolong life or to reduce symptoms. Chemotherapy is one of the major categories of the medical discipline specifically devoted to pharmacotherapy for cancer, which is called medical oncology.
Tumor necrosis factor is a cytokine and member of the TNF superfamily, which consists of various transmembrane proteins with a homologous TNF domain. It is the first cytokine to be described as an adipokine as secreted by adipose tissue.
The Philadelphia chromosome or Philadelphia translocation (Ph) is a specific genetic abnormality in chromosome 22 of leukemia cancer cells. This chromosome is defective and unusually short because of reciprocal translocation, t(9;22)(q34;q11), of genetic material between chromosome 9 and chromosome 22, and contains a fusion gene called BCR-ABL1. This gene is the ABL1 gene of chromosome 9 juxtaposed onto the breakpoint cluster region BCR gene of chromosome 22, coding for a hybrid protein: a tyrosine kinase signaling protein that is "always on", causing the cell to divide uncontrollably by interrupting the stability of the genome and impairing various signaling pathways governing the cell cycle.
Cisplatin is a chemical compound with formula cis-[Pt(NH3)2Cl2]. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, brain tumors and neuroblastoma. It is given by injection into a vein.
P-glycoprotein 1 also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1) or cluster of differentiation 243 (CD243) is an important protein of the cell membrane that pumps many foreign substances out of cells. More formally, it is an ATP-dependent efflux pump with broad substrate specificity. It exists in animals, fungi, and bacteria, and it likely evolved as a defense mechanism against harmful substances.
Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a family of transcription factor protein complexes that controls transcription of DNA, cytokine production and cell survival. NF-κB is found in almost all animal cell types and is involved in cellular responses to stimuli such as stress, cytokines, free radicals, heavy metals, ultraviolet irradiation, oxidized LDL, and bacterial or viral antigens. NF-κB plays a key role in regulating the immune response to infection. Incorrect regulation of NF-κB has been linked to cancer, inflammatory and autoimmune diseases, septic shock, viral infection, and improper immune development. NF-κB has also been implicated in processes of synaptic plasticity and memory.
Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Rossmann fold for binding S-Adenosyl methionine (SAM). Class II methyltransferases contain a SET domain, which are exemplified by SET domain histone methyltransferases, and class III methyltransferases, which are membrane associated. Methyltransferases can also be grouped as different types utilizing different substrates in methyl transfer reactions. These types include protein methyltransferases, DNA/RNA methyltransferases, natural product methyltransferases, and non-SAM dependent methyltransferases. SAM is the classical methyl donor for methyltransferases, however, examples of other methyl donors are seen in nature. The general mechanism for methyl transfer is a SN2-like nucleophilic attack where the methionine sulfur serves as the leaving group and the methyl group attached to it acts as the electrophile that transfers the methyl group to the enzyme substrate. SAM is converted to S-Adenosyl homocysteine (SAH) during this process. The breaking of the SAM-methyl bond and the formation of the substrate-methyl bond happen nearly simultaneously. These enzymatic reactions are found in many pathways and are implicated in genetic diseases, cancer, and metabolic diseases. Another type of methyl transfer is the radical S-Adenosyl methionine (SAM) which is the methylation of unactivated carbon atoms in primary metabolites, proteins, lipids, and RNA.
Zbtb7, whose protein product is also known as Pokemon, is a gene that functions as a regulator of cellular growth and a proto oncogene.
Survivin, also called baculoviral inhibitor of apoptosis repeat-containing 5 or BIRC5, is a protein that, in humans, is encoded by the BIRC5 gene.
Annexin A1, also known as lipocortin I, is a protein that is encoded by the ANXA1 gene in humans.
Satraplatin is a platinum-based antineoplastic agent that was under investigation as a treatment of patients with advanced prostate cancer who have failed previous chemotherapy. It has not yet received approval from the U.S. Food and Drug Administration. First mentioned in the medical literature in 1993, satraplatin is the first orally active platinum-based chemotherapeutic drug; other available platinum analogues—cisplatin, carboplatin, and oxaliplatin—must be given intravenously.
Nuclear factor NF-kappa-B p105 subunit is a protein that in humans is encoded by the NFKB1 gene.
Transcription factor p65 also known as nuclear factor NF-kappa-B p65 subunit is a protein that in humans is encoded by the RELA gene.
Probable ATP-dependent RNA helicase DDX20, also known as DEAD-box helicase 20 and gem-associated protein 3 (GEMIN3), is an enzyme that in humans is encoded by the DDX20 gene.
Growth arrest and DNA-damage-inducible protein GADD45 gamma is a protein that in humans is encoded by the GADD45G gene on chromosome 9. GADD45G is also known as CR6, DDIT2, GRP17, OIG37, and GADD45gamma. GADD45G is involved in several different processes, including sexual development, human-specific brain development, tumor suppression, and the cellular stress response. GADD45G interacts with several other proteins that are involved in DNA repair, cell cycle control, apoptosis, and senescence. Low expression of GADD45G has been associated with many types of cancer.
A metastasis suppressor is a protein that acts to slow or prevent metastases from spreading in the body of an organism with cancer. Metastasis is one of the most lethal cancer processes. This process is responsible for about ninety percent of human cancer deaths. Proteins that act to slow or prevent metastases are different from those that act to suppress tumor growth. Genes for about a dozen such proteins are known in humans and other animals.
Gene expression profiling has revealed that diffuse large B-cell lymphoma (DLBCL) is composed of at least 3 different sub-groups, each having distinct oncogenic mechanisms that respond to therapies in different ways. Germinal Center B-Cell like (GCB) DLBCLs appear to arise from normal germinal center B cells, while Activated B-cell like (ABC) DLBCLs are thought to arise from postgerminal center B cells that are arrested during plasmacytic differentiation. The differences in gene expression between GCB DLBCL and ABC DLBCL are as vast as the differences between distinct types of leukemia, but these conditions have historically been grouped together and treated as the same disease.
Anticancer genes exhibit a preferential ability to kill cancer cells while leaving healthy cells unharmed. This phenomenon is achieved through various processes such as apoptosis following a mitotic catastrophe, necrosis, and autophagy. In the late 1990s, extensive research in the field of cancer cells led to the discovery of anticancer genes. Mutations in these genes due to base substitutions leading to insertions, deletions, or alterations in missense amino acids can cause frameshifts, thereby altering the protein. A change in gene copy number or rearrangements is also essential for deregulating these genes. The loss or alteration of these anticancer genes due to mutations or rearrangements may lead to the development of cancer.
Pioneer factors are transcription factors that can directly bind condensed chromatin. They can have positive and negative effects on transcription and are important in recruiting other transcription factors and histone modification enzymes as well as controlling DNA methylation. They were first discovered in 2002 as factors capable of binding to target sites on nucleosomal DNA in compacted chromatin and endowing competency for gene activity during hepatogenesis. Pioneer factors are involved in initiating cell differentiation and activation of cell-specific genes. This property is observed in histone fold-domain containing transcription factors and other transcription factors that use zinc finger(s) for DNA binding.
Ionizing radiation can cause biological effects which are passed on to offspring through the epigenome. The effects of radiation on cells has been found to be dependent on the dosage of the radiation, the location of the cell in regards to tissue, and whether the cell is a somatic or germ line cell. Generally, ionizing radiation appears to reduce methylation of DNA in cells.