Review into the transcriptional regulation of pgp and mrp

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Background: The point of this review was to evaluate current research on the transcriptional regulation of Pgp and MRP1, as well as look into how these can be exploited to reduce multidrug resistance particularly in cancer cells.

Methods: Electronic databases were the sources used for this review, namely ISI web of knowledge and Science Direct. Search terms relating to the review were used and the general inclusion/exclusion criteria were limiting publications to the last 5 years and on the relevance of the abstract to the review.

Results: The search terms from the two databases were shown in flow charts detailing how each search term was narrowed down and a flow chart summary of included studies showing how the potentially appropriate studies for the review (81) were narrowed to the 21 actually used for the review is shown. Also, a table detailing which factors modulated transcription in Pgp and / or MRP1 is shown and explained.

Discussion/Conclusion: Some studies showed direct evidence of transcriptional control like Human AP-endonuclease (APE1) and some drugs were also very promising e.g. Glycocholic acid reducing mRNA levels of MRP1 and effectively increasing the cytotoxic effects of the anticancer drug epirubicin. This direction of transcription regulation is showing more promising results than previous attempts to just inhibit the efflux pumps and could be the route via which multidrug resistance is eliminated.

ABC transporters are carrier proteins which have two ATP- binding "cassettes". Binding of ATP to the ATPase domains leads to their dimerization, whilst the hydrolysis of ATP leads to their dissociation. As it is a transmembrane protein i.e. exposed to both sides of the membrane, it is able to use this to transport small molecules in and out of the bilayer as it has substrate binding sites exposed to either side of the membrane. This is particularly true for bacteria, however in eukaryotic cells it seems they are more specialised for transporting materials out of the cytosol.

The first eukaryotic ABC transporters were discovered because of their ability to remove hydrophobic drugs from the cytosol. As they make up the largest family of membrane transport proteins, over expression of ABC transporters capable of pumping out drugs from the cytosol would be of very important significance to the medical and pharmaceutical industry. These ABC transporters which are capable of pumping out drugs are commonly referred to as drug efflux pumps. One such example is the multidrug resistance protein (MRP). The upregulation of this protein in human cancer cells make them resistant to a wide range of chemically non-similar cytotoxic drugs used in chemotherapy and a result of this, exposure to such drugs would do little to eliminate the cancer cells, leading to increased survival and growth of the cells which have over expressed the MRP protein. This is a major problem in the treatment of cancer cells by chemotherapy as some studies have shown that up to 40% of human cancers develop multidrug resistance .

Whilst being a primary concern in cancer cells, it is also important to note that members of the ABC transporter proteins contribute to or are responsible for the increased resistance to the antimalarial drug chloroquine and in cystic fibrosis where the particular protein seems to act as Cl- channel rather than a transporter .

The human genome sequencing project shows that there are 49 ABC transporters in human and they have been classified into seven groups based on their primary amino acid sequences which define their chemical structure. The 7 groups are ABC1 (ABCA, 12 members), MDR (ABCB, 11 members), MRP (ABCC, 13 members), ALD (ABCD, 4 members), OABP (ABCE, 1 member), GCN20 (ABCF, 3 members), White (ABCG, 5 members) .

The three most notable multidrug resistance transporters are P-glycoprotein (Pgp) / multidrug transporter 1 (MDR1), multidrug resistance protein 1 (MRP1) and breast cancer-resistance protein (BCRP). In this review, the focus will be on Pgp and MRP1.

Pgp was first identified in 1976 as a result of its over expression in multidrug-resistant tumour cells . MDR1/Pgp1 (ABCB1) and MDR2/Pgp2 (ABCB2) both belong to the ABCB subfamily of transporters. They each have 2 membrane spanning domains (MSD1 and MSD2) which contain six transmembrane domains (TMD). The first ABC is located between MSD1 and MSD2, the second is positioned at the C-terminus as shown in Fig 1 below.

Figure 1. Diagram showing the structure of some ABC transporters.

MDR1, 2 & MRP 4,5,8,9 each have 12 TMDs in two MSDs, whilst MRP1, 2,3,6,7 has 17 TMDs in three MSDs. Also, ABCG2 has 6 TMDs in one MSD. The location of the various ABCs can also be noted .

Pgp1 is a phosphorylated glycoprotein with a molecular weight of 170 kDa . It is expressed in the blood brain barrier tissue, normal barrier and excretory tissues, in the kidney, in the liver, in the colon, the adrenal gland as well as the small intestine . Pgp and its various isoforms have been found in other species including amphibians, birds, fish, insects, mammals and reptiles. As an ABC drug efflux pump, Pgp exports a wide range of substrates ranging in mass from 0.3 kDa - 2 kDa . The substances transported are neutral or positively charged hydrophobic substrates and they include Ca2+ channel blockers, opioids, immunosuppressive drugs, HIV protease inhibitors, antineoplastic agents, antibiotics and a whole host of other chemotherapeutic substances . These substances are believed to enter the cells via passive diffusion as a result of the concentration gradient across the membrane and Pgp exports them out of the cell by the hydrolysis of ATP at the ABC or nucleotide-binding domain (NBD).

The exact molecular mechanism by which this export takes place is largely unknown, however crystallographic information from a bacterial multidrug transporter MsbA and studies using biochemical approaches , shows that substrate binding is the first step in Pgp drug export. Followed by conformational changes that bring the two NBDs close enough for ATP binding and then the hydrolysis of the ATP produces enough energy to release the substrate outwards via the TMDs .

MRP1 is a member of the ABCC1 subfamily and was the first member to be discovered. It consists of about 1531 amino acids including the deletion of 13 amino acid residues in NBD1/the ABC between MSD1 and MSD2 (Fig 1) that are found in the NBD2 of other ABCC members as well as in both NBDs of most eukaryotic ABC transporters .

In terms of configuration, MRP1 is relatively similar to Pgp (Fig 1) in that it has MSD1 and MSD2 with each containing 6 TMDs making a total of twelve. However, MRP1 does have an additional membrane spanning domain MSD0 at the N terminus . Deleting MSD0 does not seem to affect its transport activity, but studies have shown that it may not be functionless as mutations of some Cysteine amino acid residues on MSD0 significantly reduce substrate-transport activities .

MRP1 was originally identified in 1992 on a drug resistant cancer cell line that did not over express Pgp . Excess MRP1 expression gives the cell resistance to drugs which are similar to drugs Pgp has resistance for. These include anticancer drugs like doxorubicin, vincristine, gramicidine D, idarubicin and arsenite, arsenate which are heavy metals .

MRP1 efflux requires cofactors such as glutathione (GSH), glucuronic acid etc to aid in the transport of the anticancer drugs .

Necessity of This Review

The advent of drug resistance is one of the biggest problems facing effective chemotherapy when it comes to cancer. Over expression of drug efflux pumps is one of the major causes of this resistance and two major proteins responsible for this are MRP1 and Pgp which are members of the ABC transporter family.

A lot of research has gone into inhibiting these proteins with very limited success which is not surprising considering each of these proteins are capable of binding to and exporting a wide range of chemically unrelated drugs.

A new direction has been to focus on the transcriptional modulation of these proteins and how these can be altered to reduce the resistance to chemotherapy.

This review aims to assess current research on the transcriptional regulation of Pgp and MRP1, what the mechanisms of control are and how they can be exploited by drugs.

OBJECTIVES

To undertake a systemic analysis of the published literature on transcriptional regulation of the Pgp and MRP1 drug efflux pumps.

The primary objective was to find out the mechanisms that control the upregulation and down regulation of these pumps.

The secondary objective was to find out if there have been ways in which these mechanisms have been altered by drugs to affect the expression levels.

METHODS

The search method used for this review was limited to electronic sources. The databases covered include ISI Web of knowledge (Web of science, MEDLINE & Journal Citation Reports are under this) & Science Direct. For each of the sources mentioned above, the language was not constrained to English only and the search terms varied with each database, as well as the inclusion and exclusion criteria. Each database and the search done on it will be explained below.

ISI Web of Knowledge (Includes Web of Science, MEDLINE and Journal Citation Reports)

The search terms:

"Transcriptional regulation of ABC drug efflux pumps".

"Transcriptional regulation of p-glycoprotein" OR "Transcriptional regulation of Pgp".

"Transcriptional regulation of multidrug resistance protein 1".

"Transcriptional regulation of mrp1" OR "Transcriptional regulation of abcc1".

"Transcriptional modulation of MRP1" OR "Transcriptional modulation of Pgp".

"Transcriptional modulation of MRP1" AND "Transcriptional modulation of Pgp".

Searches in this database were carried out across all years till present and also there were no limits in terms of type of articles except for search terms 2 and 3.

Search term 2 as a result of the hits given was limited to the last 5 years (2006 - 2011) of publication and also limited to articles or reviews in terms of document type. The exact same criteria was applied to search term 3, except that the latest year of publication for this search was 2010 and hence the publication year limit was between 2005 - 2010 instead.

Science Direct

The search terms:

"Transcriptional regulation of mrp1" OR "Transcriptional modulation of mrp1".

"Transcriptional regulation of abcb1" OR "Transcriptional modulation of abcb1".

"Transcriptional regulation of abcc1" OR "Transcriptional modulation of abcc1".

"Transcriptional regulation of Pgp" OR "Transcriptional modulation of Pgp".

In this database, the searches were limited to journal only and to publications printed in the last 5 years (2006 - 2011).

Search term 2, was further limited to topics containing or tagged with the following terms: Pgp, ABC transporter, mdr1 gene, drug resistance, Pgp expression, mdr1 mRNA, p-glycoprotein, Pgp substrate.

Experimental Biology and Medicine (The royal society of medicine press)

This is not exactly a database but it did have some useful information. Also, journal articles from this source were used in this review and hence it has been included.

The search terms:

"Modulation of p-glycoprotein transport function at the blood-brain barrier"

"Modulation of p-glycoprotein transport function"

These were the only two terms that seemed to provide viable results. There were no means to use inclusion or exclusion criteria to reduce the number of hits. However, the order in which the results of the searches were presented was in the order of the most relevant articles.

Inclusion and Exclusion criteria

In general, where the number of hits given per search term was rather high, the main exclusion criteria were:

Limiting the time line to the last 5 years of publication

Limiting the document type to journals, articles and reviews

Also, going through the abstracts of the search results, articles which had content not pertaining to the point of this review were discarded, as well as articles primarily talking about inhibition of the drug efflux pumps and/or post transcriptional work were also disregarded. The main inclusion criteria used was to include articles published in peer reviewed journals and also reviews of current work on this topic.

RESULTS

The first part of this section will focus on the number of hits per search term and how these were reduced with the application of inclusion and exclusion criteria. This information will be presented in a flow diagram form and will be divided into three parts based on the database source.

ISI Web of Knowledge

"Transcriptional regulation of ABC drug efflux pumps". (Results = 18)

Results excluded based on abstract content = 15

Figure 2. Search term 1

Results potentially appropriate to the review = (3)

"Transcriptional regulation of p-glycoprotein" OR "Transcriptional regulation of Pgp" (Results =371)

(Results = 253)

Limit to publication year (2001-2011)

(Results = 199)

Limit to document type (articles)

Limit to publication year (2006-2011)

(Results = 106)

Results excluded based on abstract content = 89

Results potentially appropriate to the review = (19)

Figure 3. Search term 2

"Transcriptional regulation of mrp1" OR "Transcriptional regulation of abcc1" (Results = 39)

Results excluded based on abstract content = 32

Results potentially appropriate to the review = (7) 3)

Figure 4. Search term 4

"Transcriptional regulation of multidrug resistance protein 1"

(Results = 328)

Limit to document type (articles or reviews)

(Results = 304)

(Results = 152)

Limit to publication year (2005 - 2010)

Results excluded based on abstract content = 141

Results potentially appropriate to the review = (11) 4)

5)

Figure 6. Search term 5

"Transcriptional modulation of MRP1" OR "Transcriptional modulation of Pgp" (Results = 11)

(Results = 39)

Results excluded based on abstract content = 8

Results potentially appropriate to the review = (3)

Figure 5. Search term 3

"Transcriptional regulation of mrp1" AND "Transcriptional regulation of abcc1" (Results = 1)

Results potentially appropriate to the review = (1)

6)

Figure 7. Search term 6

Science Direct

"Transcriptional regulation of mrp1" OR "Transcriptional modulation of mrp1" (Results = 162)

(Results = 112)

Results potentially appropriate to the review = (10)

Limit to publication year (2006 - 2011)

Results excluded based on abstract content = 102

1)

Figure 8. Search term 1

"Transcriptional regulation of abcb1" OR "Transcriptional modulation of abcb1" (Results = 293)2)

Limit to publication year (2006 - 2011)

Limit to Topic = Pgp, ABC transporter, mdr1 gene, drug resistance, Pgp expression, mdr1 mRNA, P-glycoprotein, Pgp substrate.

(Results = 212)

(Results = 26)

Results potentially appropriate to the review = (10)

Results excluded based on abstract = 16

Figure 9. Search term 2

"Transcriptional regulation of abcc1" OR "Transcriptional modulation of abcc1" (Results = 162)3)

Limit to publication year (2006 - 2011)

Results excluded based on abstract content = 104

(Results = 112)

Figure 10. Search term 3

Results potentially appropriate to the review = (8)

"Transcriptional regulation of Pgp" OR "Transcriptional modulation of Pgp" (Results = 463)

(Results = 39) 4)

(Results = 167)

Results excluded based on abstract content = 160

Results potentially appropriate to the review = (7)

Limit to publication year (2006 - 2011)

Figure 11. Search term 4

Experimental Biology and Medicine (The royal society of medicine press)

As mentioned earlier, this is not a database but 2 "searches" were done and yielded 1 result each. This will be shown as a table instead.

Table 1. Search term results

Search term

Number of results

Number of results used in the review

Modulation of P-glycoprotein transport function at the blood-brain barrier

1955

Table 1 shows search terms used on the above journal's website and the results. The high number of results per search stayed that way as there was no way of applying inclusion / exclusion criteria1

Modulation of P-glycoprotein transport function

1933

1

For this source, there was no discernible means of using inclusion and/or exclusion criteria to narrow down the results. However, other than the first source on the lists for both searches, the other results had little or nothing to do with the topic of this review.

Flow Chart of Included Studies

Potentially appropriate studies (81)

Duplicates removed = (7)

Full text studies left for screening (74)

Figure 12. Summary of included studies

Studies excluded based on content and relevance = (45)

Full text studies left for the review = (29)

Studies included in the review = (21)

Table 2 on the next page shows a list of factors which have been shown via direct or indirect evidence to up regulate or down regulate Pgp and / or MRP1 at the transcription stage. The spaces with dashed lines are either still under investigation or none of the papers reviewed explicitly indicated or suggested that there was evidence of transcriptional regulation by the particular factor on the specific drug efflux pump.

Table 2. Factors involved in transcriptional control of Pgp and MRP1

Factors involved in transcriptional regulation

Pgp

MRP1

Source(s)

1) Carcinogens

Y

Y

2) Inflammation

Y

-

3) Epileptic seizures

Y

Y

4) Hypoxia

Y

-

5) PXR / SXR

Y

N

6) Brain Ischemia

Y

-

7) HIV Tat Protein

Y

-

8) APE1

Y

-

9) Heat shock

Y

-

10) p53

Y

Y

11) Fos / Jun (AP-1)

-

Y

12) Ras / WT-1

Y

-

13) APC

Y

-

14) MED-1 / iMED

Y

-

15) MEF-1

Y

-

16) N-myc

-

Y

17) Sp1

Y

Y

Table 3, showing some of the factors responsible for transcriptional control and whether or not it applies to Pgp and/or MRP1. Y = yes, N = No & the dashed line (-) = unknown..

DISCUSSION

P-glycoprotein / MDR1 (ABCB1)

Pgp being the first ABC transporter to be identified as a drug efflux pump has had the most research done in terms of understanding the mechanisms that control its expression.

The MDR1 promoter lacks a TATA box but instead contains numerous transcription factor - binding sites e.g. CCAAT-, CAGGAACA- etc; which react with transcription factors like NF-Y, ets-1. These transcription factors are part of the basal transcription regulation. However, some stressful conditions can induce MDR1 expression as a result of redox imbalance. Such conditions include Inflammation, Carcinogens, Heat shock, Hypoxia and Enhancesome .

Inflammation

The body reacts to inflammation caused by a wide variety of stimuli (e.g. infection, cell trauma etc.) by the acute response which is series of complex reactions via which pro-inflammatory cytokines (e.g. TNF) are released by macrophages and also a change in gene expression of the involved tissues. To induce inflammation experimentally, lipopolysaccharide (LPS) is injected into tissue.

According to , in liver and intestine tissues, inflammation reduces Pgp expression (mRNA and protein concentrations). Also, in hepatocytes, the increased presence of pro-inflammatory mediators reduces MDR1 expression. The exact mechanism is not quite understood but is thought that cytokines suppress the activation for MDR1 by nuclear receptors . Though, in it was found that a glucocorticoid response element (GRE) may be responsible for the repression as it a site that overlaps the binding site for CEBPβ (CAAT enhancer-binding protein). CEBPβ is IL-6 induced and hence during inflammation can activate MDR1.

Carcinogens

Early studies on the effect of carcinogens were focused on murine mrd1a and mdr1b (human MDR1 paralogues). The carcinogen 2-acetylaminofluorene (2-AAF) induces upregulation of mdr1b in rats. The mechanism involves an NF-κB binding site on the mdr1b promoter being required for the activation of the mdr1b gene by 2-AAF . shows that an increase in 2-AAF led to the increase of intracellular ROS (reactive oxygen species), which causes activation of IκB kinase (IKK), degradation of IκBβ and increase in NF-κB DNA binding activity. Excess GSH blocked the IKK kinase complex and hence NF-κB DNA binding. 2-AAF also has a similar effect in human hepatica cells but it does this via a phosphoinositide 3-kinase (PI3K), Rac1 and NADPH (ROS source) route .

Enhancesome

MDR1gene expression can be induced by UV and ionising radiation, differentiation agents, certain inhibitors and some chemotherapeutic agents. Research from shows that these stimuli all meet at a region of the MDR1 promoter referred to as the MDR1 Enhancesome. This region contains binding sites for the NF-Y and Sp family of transcription factors. Together, these transcription factors bring the histone acetyltransferase P/CAF to the MDR1 promoter which results in the promoter histones being acetylated and consequently transcriptional activation of the MDR1 gene that is most likely mediated by chromatin remodelling .

At the blood brain barrier (BBB), Pgp serves as one of the major gateways and quite a few conditions cause upregulation of Pgp.

Epilepsy and Brain Ischemia

noted that epileptic seizures tend to act as a very strong trigger for an increase in BBB Pgp expression rates, sometimes even doubling as was noted in rats right shortly after a seizure as well as in rats with chronic epilepsy. The mechanism behind this was proved to do with excess glutamate released after a seizure accumulating in brain interstitial fluid. showed that exposing isolated rat or mouse brain capillaries to glutamate increased Pgp expression and transport activity. The results from their finds show that the increased extracellular glutamate after a seizure signals through an N-methyl-d-aspartate (NMDA) receptor and cyclooxygenase-2 (COX-2) in brain capillaries to increase BBB Pgp expression. Inhibition of NMDA and COX-2 saw no increases in Pgp expression when glutamate levels where high, indicating COX-2 and NMDA in this mechanism .

Similar results in terms of upregulation of Pgp expression rates were also noticed in brain ischemia tissues especially in brain areas where cells were damaged severely. In one model, expression rates reached maximum after 24 hours and then returned to normal levels, whilst in another model Pgp expression seem to have reduced and then returned to normal . The inconsistency is possibly due to the different time points of investigation in both studies.

HIV Tat protein

The HIV tat protein is quite well accepted to be a good gene expression inducer. In they show that treating brain micro vascular endothelial cells with the HIV Tat protein induced MDR1 expression both at the mRNA and protein levels.

Other than the above conditions listed, there are whole host of other factors involved in the transcription of Pgp and some will be discussed below.

Pregane X Receptor (PXR) / Steroid Activated Receptor (SXR)

SXR is a nuclear receptor / transcription factor and part of a family of ligand activated transcription factors. SXR is the human version and PXR is for rats. PXR is activated by a whole host of ligands some of which are Pgp substrates. showed that exposing rat brain capillaries to 2 PXR ligands, pregnenolone 16α-carbonitrile (PCN) and dexamethasone increased Pgp expression both in mRNA and protein levels.

Human AP-endonuclease (APE1/Ref-1)

APE1 is an enzyme involved in the repair of broken DNA strands and oxidative base damage. However, it also functions as a transcriptional regulator. APE1 is often over expressed in tumour cells resistant to anticancer drugs. In , they showed that the acetylated form of APE1 stably interacts with the Y-box-binding protein 1 (YB-1) and considerably enhances its binding to the Y-box element, consequently leading to the activation of the MDR1 gene. The acetylated APE1 interaction with YB-1 was much more stable than the non-acetylated form and also, down regulation of APE1 sensitised the tumour cells over expressing Pgp to certain drugs e.g. Doxorubicin.

MRP1 (ABCC1)

The mechanisms which regulate MRP1 expression are still relatively unknown compared to Pgp. As MRP1 is a GS-X pump, noted that cytotoxic agents like heavy metals, carcinogens, anti tumour agents etc that induced expression of γ-GCSh also induced MRP1 expression. Though they did note that the magnitude and time course of the induced expression for both were not the same. Some other factors agents will be discussed below.

N-myc

N-myc is an oncoprotein transcription factor that belongs to the MYC family of transcription factors. Results from suggest that N-myc regulates MRP1 expression. Human MRP1 promoters contain multiple E-box sequences that are recognised by N-myc. It was observed that enforced expression of N-myc increased MRP1 levels and hence increased drug resistance. It was also noted that in cases of child neuroblastoma that increased MRP1 expression correlated with over expression of N-myc .

Epilepsy

Just like with Pgp, epileptic seizures also act as a very strong trigger for an increase in MRP1 expression in brain capillaries of epileptic tissues . Experimental studies in a variety of rodent epilepsy models showed that the increased induction of MRP1 (and Pgp) by seizure activity was done in a transient and localised manner .

P53

P53 is a tumour suppressor protein. According to , wild type p53 has been shown to repress transcription of the MPR1 promoter and loss of p53 expression is correlated with increased MRP1 expression in colorectal cancer. The exact mechanism which mediates this repression of MRP1 by p53 is still unknown, it is thought that deactivation of promoter bound Sp1 is involved .

Fos / Jun (AP-1)

AP-1 is a term given to transcription factor complexes consisting of Fos and Jun oncogene family members. The MRP1 promoter has a supposed AP-1 binding site that interacts with c-jun and junD . The role of this in regulation of MRP1 in response to inducers is not entirely understood, but in Pgp promoters, transfecting cells with c-Jun caused those cells to express higher levels of MDR1 RNA and Pgp protein .

So far, it does not seem a lot of the mechanisms behind the transcriptional control of MRP1 is entirely understood, however further research is being carried out in that effect and with time more detail should be revealed.

Modulation of Transcriptional Control by Drugs

Understanding the mechanisms that control the gene expression of these proteins is one thing, matching drugs which can exploit these mechanisms is another. Some drugs to be discussed below have been proven to effectively modulate transcription control of Pgp and MRP1 expression.

A combination of glycocholic acid (GC) and epirubicin was used to inhibit both Pgp and MRP1 as well as induce apoptosis in human colon adenocarcinoma Caco-2 cells . It was shown that GC significantly reduced mRNA expression levels of human intestinal MDR1 and MRP1, and it was observed that there was an increase in the intracellular accumulation of epirubicin in Caco-2 cells and intensified epirubicin induced apoptosis .

Ginsenoside Rd was shown to inhibit MDR1 protein expression without cytotoxicity, and without changing mRNA levels or nuclear levels of key transcriptional factors for MDR1 gene expression . It achieved this by the increased ubiquitination of MDR1 which depends on ubiquitin-dependent protein degradation for its protein stability. This inhibition reversed the doxorubicin resistance in MCF-7/ADR cells, which belong to the doxorubicin-resistant human breast cancer cell line .

showed that drug resistance in the drug-resistant colon cancer HT29-dx cells could be bypassed by designing a new liposomal doxorubicin, conjugated with a recombinant low-density lipoprotein receptor (LDLR)-binding peptide from human apoB100: this LDL-masked doxorubicin ("apo-Lipodox") was efficiently internalized by an LDLR-driven endocytosis and induced cytotoxic effects in HT29-dx cells, reversing their drug resistance .

CONCLUSION

The complexity of the mechanisms modulating the transcription of Pgp and MRP1 has been researched and with Pgp in particular a lot of it has been understood. However, considering the number of unknowns and educated guesses, there is still a lot of work to be done in fully understanding Pgp and MRP1 transcription. Regardless, current studies suggest that the transcription regulation of these drug efflux pumps would be good targets in to prevent and possibly eliminate multidrug resistance. Some drugs like Ginsenoside rd have shown that this is possible and as time and research continue, it is expected that more drugs would be able to target the transcriptional regulation of drug efflux pumps and ultimately eradicate multidrug resistance for good.

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