Multi Drug Resistance Mdr In Cancer Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Despite advances in the treatment of malignant tumours, multi-drug resistance (MDR) is a main form of resistance against a variety of antineoplastic agents and a major obstacle to improving the survival of cancer patients.1 2 MDR involves the extrusion of cytotoxic molecules 3 by transporter membrane proteins, thus preventing intracellular drug accumulation below a cell-killing threshold 4. In addition, these transporters are involved in the sequestration of drugs away from their targets, acting on sub-cellular compartments. 5

The first discovery, that certain tumours show an acquired resistance pattern whilst others develop (or increase) resistance during the course of treatment, was founded by clinical oncologists over 30 years ago.6 7 8 The MDR pattern found in the phenotype is due to the expression of plasma membrane 'pumps'.9 This phenomenon has been clearly established for decades. Multi-drug resistance in cancer is therefore a result of the up-regulation of pump expression and selection of resistant cancer cells.10

There are two classes of transporter proteins at the cellular surface which are responsible for MDR in cancer. One of these is the solute carrier transporter superfamily which mediates the cellular uptake of anticancer drugs. More predominantly major MDR proteins (found in microorganisms from bacteria to humans) are part of the adenosine triphosphate (ATP) binding cassette (ABC) superfamily.4 11 In healthy tissue, these proteins play an important physiological role in the protection of the body against xenobiotics occurring in the environment, by the active efflux of toxic agents.6 11 Otherwise these transporters have been found to hinder the effective therapy of many widespread diseases such as malaria, tuberculosis, AIDS and cancer.12 Inherited diseases have also been linked to mutation in these transporter genes.13

Major MDR proteins

ABC transporters are abundant membrane-bound proteins.14 These transporters comprise of pumps, most of which use energy released by ATP hydrolysis to move substrates against their electrochemical gradient, into or out of cells, or into cellular vesicles, and others use specific membrane channels.15 16 ABC proteins transport a number of endogenous substrates (e.g. metal ions, inorganic ions, peptides, amino acids and sugars), hydrophobic compounds and metabolites across the plasma and intracellular membranes. 17

Proteins of the ABC family are characterised by the presence of a cytoplasmic ATP binding domain with a specific structure, also known as a nucleotide-binding domain (NBD).18 The highly conserved primary structure of the NBD is between a phosphate-binding loop (Walker A) and a magnesium binding site (Walker B). Other conserved motifs which make up the rest of the nucleotide binding site consists of: the 'switch region' which contains a histidine loop and partakes in the hydrolysis of ATP, the 'signature conserved motif' which is specific to the ABC transporter and the 'Q-motif' located between Walker A and the signature motif. Another membrane-spanning component of ABC transporters are trans-membrane domains (TMDs) which offer binding sites for substrates or chemotherapeutic drugs for translocation from the cytoplasm to the cell membrane.4 6 19 It is generally accepted that the minimum functional unit requirement for an ABC transporter is the presence of two TMD's and two NBD's. These may be present within one polypeptide chain, 'full transporters', or within a membrane-bound homo- or heterodimer of 'half transporters'.20 21

There are 49 human proteins of the ABC superfamily which are divided into seven subfamilies (class A to G), based on the number and combination of TMDs and NBDs (Table 1).11 13 14 22 Three decades ago, P-glycoprotein (P-gp; MDR1/ABCB1) was the first ABC transporter known to be associated with MDR to chemotherapeutic agents. The later realisation that P-gp alone could not account for all the MDR in many independently established MDR cells, led to the discoveries of other drug transporters, more notably MDR associated protein (MRP; ABCC1) and breast cancer resistance protein (BCRP; ABCG2). More than 80% of drugs currently used in cancer chemotherapy are transported by these major MDR proteins.11 13 23

Figure 1 presents a plausible membrane topology models for the major MDR proteins.23 24 As shown in Figure 1, Pgp-MDR1 (ABCB1) is a full transporter with six trans-membrane (TM) helices in both TMDs of the protein; MRP1 (ABCC1) is also a full transporter with five TM helices (TMD0) and ABCG2 (BCRP/MXR) is a half transporter with six TM helices. The structures of the three major proteins are further explained in Table 2.20 21 25 26

Gene

Chromosome Location

Exons

Amino Acids

Function

ABC1, ABCA

ABCA1

9q31.1

36

2261

Cholesterol efflux onto HDL

ABCA2

9q34

27

2436

Drug resistance

ABCA3

16p13.3

26

1704

Multidrug resistance

ABCA4

1p22

38

2273

N-retinylidene-phosphatidylethanolamine (PE) efflux

ABCA5

17q24.3

31

1642

Urinary diagnostic marker for prostatic intraepithelial neoplasia (PIN)

ABCA6

17q24.3

35

1617

Multidrug resistance

ABCA7

19p13.3

31

2146

Cholesterol efflux

ABCA8

17q24

31

1581

Transports certain lipophilic drugs

ABCA9

17q24.2

31

1624

Might play a role in monocyte differentiation and macrophage lipid homeostasis

ABCA10

17q24

27

1543

Cholesterol-responsive gene

ABCA12

2q34

37

2595

Has implications for prenatal diagnosis

ABCA13

7p12.3

36

5058

Inherited disorder affecting the pancreas

MDR, ABCB

ABCB1

7q21.1

20

1280

Multidrug resistance

ABCB2

6p21.3

11

808

Peptide transport

ABCB3

6p21.3

11

703

Peptide transport

ABCB4

7q21.1

25

1279

Phosphatidylcholine (PC) transport

ABCB5

7p15.3

17

812

Melanogenesis

ABCB6

2q36

19

842

Iron transport

ABCB7

Xq12-q13

14

753

Fe/S cluster transport

ABCB8

7q36

15

718

Intracellular peptide trafficking across membranes

ABCB9

12q24

12

766

Located in lysosomes

ABCB10

1q42.13

13

738

Export of peptides derived from proteolysis of inner-membrane proteins

ABCB11

2q24

26

1321

Bile salt transport

MRP, ABCC

ABCC1

16p13.1

31

1531

Drug resistance

ABCC2

10q24

26

1545

Organic anion efflux

ABCC3

17q22

19

1527

Drug resistance

ABCC4

13q32

19

1325

Nucleoside transport

ABCC5

3q27

25

1437

Nucleoside transport

ABCC6

16p13.1

28

1503

Expressed primarily in liver and kidney

ABCC7 (CFTR)

7q31.2

23

1480

Chloride ion channel (same as CFTR gene in cystic fibrosis)

ABCC8

11p15.1

30

1581

Sulfonylurea receptor

ABCC9

12p12.1

32

1549

Encodes the regulatory SUR2A subunit of the cardiac K+(ATP) channel

ABCC10

6p21.1

19

1464

Multidrug resistance

ABCC11

16q12.1

25

1382

Drug resistance in breast cancer

ABCC12

16q12.1

25

1359

Multidrug resistance

ABCC13

21q11.2

6

325

Encodes a polypeptide of unknown function

ALD, ABCD

ABCD1

Xq28

9

745

Very-long-chain fatty acid (VLCFA) transport

ABCD2

12q11-q12

10

740

Major modifier locus for clinical diversity in X-linked ALD (X-ALD)

ABCD3

1p22-p21

16

659

Involved in import of fatty acids and/or fatty acyl-coenzyme As into the peroxisome

ABCD4

14q24

19

606

May modify the ALD phenotype

OABP, ABCE

ABCE1

4q31

14

599

Oligoadenylate-binding protein

GCN20, ABCF

ABCF1

6p21.33

19

845

Susceptibility to autoimmune pancreatitis

ABCF2

7q36

14

634

Tumour suppression at metastatic sites and in endocrine pathway for breast cancer/drug resistance

ABCF3

3q27.1

21

709

Also present in promastigotes (one of five forms in the life cycle of trypanosomes)

White, ABCG

ABCG1

21q22.3

13

678

Cholesterol transport

ABCG2 (BCRP)

4q22

16

655

Toxicant efflux, drug resistance

ABCG4

11q23.3

15

646

Found in macrophage, eye, brain and spleen

ABCG5

2p21

11

651

Sterol transport

ABCG8

2p21

10

673

Sterol transport

Table 1. Human ABC transporter superfamily This table outlines all of the human ABC genes according to subfamily, chromosome location, exon number and function.22

Figure 1. Membrane topology models for the key MDR-related ABC transporters The green bars represent predicted transmembrane helices, the purple circles represent the ABC domains, and the gold trees are glycosylation sites at the extracellular surface.24

Protein

Amino Acids (AA)

Full/Half transporter

Important structures

P-gp

1280 AA in membrane bound glycoprotein

Full transporter

No communication between ATP sites of NBD's when L0 is unavailable so no drug transportation

MRP1

Encodes membrane bound glycoprotein of 1531 AA

Full transporter

L0 linker region is essential for drug transport. TMD0 is not required for transport. Also contains an extra N terminal segment.

BCRP

Encodes plasma membrane glycoprotein of 655 AA

Half transporter

Two ABCG2 form a functioning homodimer by a disulphide bridge for drug transport. Also consists of a unique domain arrangement (ABC is in the N-terminus).

Table 2. Structure of the three major MDR proteins This table shows the structure of the three major MDR proteins in more depth considering the number of amino acids and whether the protein is a full or half transporter (which is determinant to its respective mechanism). Other important structures relating to drug transportation of the proteins are outlined.20 21 25 26MDR mechanisms in cancer chemotherapy

Response to anti-cancer chemotherapy is affected by several factors including cell kinetic, pharmacokinetic and cellular drug resistance mechanisms (Table 2). The phenomenon that more than one mechanism of MDR is simultaneously active in cancer cells has been called multi-factorial MDR.5

Mechanism

Individual process

Cell kinetic resistance

Tumour growth

Pharmacokinetic resistance

Poor absorption

Excessive metabolism

Poor penetration to certain sites

Blood supply of the tumour

Drug diffusion

Cellular drug resistance

Increased drug efflux

Decreased drug uptake

Sequestration of drugs

Alterations in drug targets

Activation of detoxifying systems

Increased repair of drug-induced DNA damage

Blocked apoptosis

Disruption in signalling pathways

Alterations of factors involved in cell cycle regulation

Table 3. Mechanisms of drug resistance This tables shows the individual processes within cell kinetic resistance, pharmacokinetic resistance and cellular drug resistance mechanisms. 5The mechanism for drug transport in P-gp is coupled by two hydrolyses reactions of ATP. The first reaction converts ATP to adenosine diphosphate (ADP). In this stage the nucleoside diphosphate trapping is essential to form a transition state intermediate of the P-gp complex. The second ATP hydrolysis, the drug is extruded from P-gp and the ADP dissociates from the complex. An additional molecule of ATP is hydrolysed binding to the alternate ATP site on P-gp, whereby the dissociation of ADP allows conformation of P-gp to be restored to its original state. This step initiates the next cycle of drug transport. 13 27

The transportation mechanism of chemotherapeutic drugs for MRP1 is different in that of P-gp, even though MRP1 also requires two ATP's as the energy source. The functions of the two NBD's in P-gp are 'equal' and the two NBD binding sites operate alternately. However in MRP1, NBD1 has a higher affinity than NBD2 for ATP. A substrate then binds to the TMD's in MRP1 which induces ATP binding at NBD1, this is achieved by the conformational change of MRP1. Further conformational change of MRP1 enhances ATP binding at NBD2. The bound substrate is then transported out of the cell when both NBD1 and NBD2 are occupied by two ATP molecules. After the substrate is extruded the ATP bound at NBD2 is hydrolysed first, releasing an ADP and an inorganic phosphate. This partially brings the protein back to its original confirmation and further facilitates the dissociation of ATP bound at NBD1. The MRP1 protein returns fully to its original confirmation following the subsequent release of ADP and inorganic phosphate from NBD1. It should also be noted that MRP1 cannot transport unmodified cancer drugs without the presence of glutathione (GSH) thus implies that GSH binds to the protein to enhance transport of hydrophobic drugs across biological membranes. 13 25

The ATP cleavage cycle for BCRP, as for MRP1 and P-gp, has not yet been investigated in much detail but is speculated to be very similar in its basic steps. 21 23

The major MDR proteins are highly promiscuous compounds, sharing the ability of recognising and translocating an array of structurally diverse compounds, with overlapping substrate specifity and ability to handle unique compounds (Figure 2).5 13 24 P-gp is a transporter for large hydrophobic, either uncharged or slightly cationic compounds whilst MRP's primarily transport hydrophobic anionic conjugates and extrude hydrophobic uncharged drugs. Transported substrates for BCRP (MXR) have yet to be explored in further detail.13 23

MDR1/Pgp-mediated transport can be competitively inhibited by MDR-reversal agents or Pgp blockers. In polarised cells, Pgp-MDR1 and MXR are localized in the apical (luminal) membrane surface, whereas MRP1 expression is restricted to the basolateral membrane (Table 4).13 17

Circumvention of MDR in cancer chemotherapy

Preventing MDR is the ultimate way to drastically improve survival in cancer patients. Developing anti-cancer agents that do not interact with MDR transporters would be ideal, yet it is impossible, as cytotoxic drugs must penetrate the cell membrane and MDR have a wide recognition pattern.3 Decades ago, pharmacological modulators (figure 3) were presented to increase cytotoxic action of MDR-related drugs by preventing the extrusion of anti-cancer drugs from target cells. The first generation of modulators included calcium channel blockers (Verapamil Diltiazem, Azidopine), quinine derivatives, calmodulin inhibitors (Trifluoroperazine, Chlorpromazine) and the immunosuppressive agent, Cyclosporin A.3 7 The second generation modulators consisted of derivatives of the first generation compounds,3 which had a less pronounced effect on their original target, yet retained their modulatory effects.10 The differences in the substrate-specificities and inhibitor-sensitivities of different proteins expressed in different tumour cells, proper therapeutic intervention required an advanced diagnosis and targeted modulator agents.10 28 Finally, third generation of MDR modulators are molecules specifically devised to interact with specific MDR transporters; these have yet to prove their clinical efficiency. 3 7 10

Figure 2. MDR substrates This Venn-diagram illustrates common chemotherapeutic drugs which are transported by the major MDR proteins which also happen to be MDR substrates.24

ABC Transporter

Resistance Spectrum

Reversal Agents

Important Endogenous substrates

Major tissue expression

MDR1 (ABCB1)

Anthracyclines, Vinca, Etoposide, Colchicine

Verpamil, Cyclosporin A, GF120918

Phospholipid

Blood-brain barrier, choroid plexus, adrenal gland, liver

MRP1 (ABCC1)

Anthracyclines, Vinca, Etoposide, Topotecan

V-104

LTC4

Ubiquitous

Table 4. Properties of major MDR transporter proteins This table outlines a non-exhaustive list of the resistance spectrum, reversal agents, important endogenous substrates (in healthy tissue), and major expression in tissue. It is important to note that the MRP is ubiquitously expressed in tissue.13 23BCRP (ABCG2/MXP)

Dox, Mitoxantrone, Anthracyclines, ST1571, Topotecan

GF120918, Grfitnib

Porphyrin, heme

Placenta, various stem cells, kidney, liver

New treatment strategies have been developed by understanding the clinically active mechanisms of drug resistance (Table 5).29-40 It has been recently discovered that the expression of a major glycosaminoglycan in the extracellular matrix, hyaluronan (HA), and its receptor, CD44, a cell surface marker for both normal and cancer stem cells, are tightly linked to MDR and tumour progression. Anti-CD44 antibody blocks HA-CD44 binding and inhibits ABCB1-mediated efflux activity. Thus, anti-CD44 antibody may be used in combination with chemotherapy to enhance chemosensitivity.29 In addition, a gene therapy treatment involves an MDR gene inserted into bone marrow stem cells are then placed back into the patient. The increased expression of MDR in the bone marrow cells renders them less susceptible to harmful effects of chemotherapy drugs and allows the patient to tolerate higher doses. It is hoped that the increased levels of the drugs will more effectively eliminate the cancer.30

New Treatment Strategies

New cytotoxic drugs

Resistance modifiers

Gene therapy

Antisense oligonucleotides

Monoclonal antibodies

Signal transduction inhibitors

Immunotherapy

Hammerhead ribozymes

Small interference RNAs

Table 5. New treatment strategies This table lists the current development of treatment strategies to circumvent MDR in cancer chemotherapy. 29-40 Modulation of ubiquitination

Conclusion

In conclusion, the three major MDR transporters have been recognised for decades and their mechanisms comprehensively studied yet, there is no definitive conclusion with regard to the impact of the expression of drug resistance factors in most malignant diseases. Thus prospective studies with large numbers of patients and the association between the various drug resistance genes and response to chemotherapy will need to be assessed in more detail. As multidrug resistance in malignant disease is multifactorial, there is a large distribution of resistance mechanisms that are needed to be studied to determine the relative contribution of drug resistance markers to treatment failure. These studies may lead to novel strategies for improving chemotherapeutic efficacies through targeted interventions of ABC transporters.

The extensive data currently available on the clinical role of drug resistance mechanisms is the basis for future studies in the search for potential MDR transporter inhibitors and the clinical implementation of drug resistance factors. Another significant strategy for cancer treatment is to ascertain the subtle differences between the tumour and normal stem cells so that approaches can be developed to eliminate tumour cells without excessive toxicity to normal stem cells.

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.