P-glycoprotein 1 (permeability glycoprotein, abbreviated as P-gp or Pgp) 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.
P-gp is a
glycoprotein that in humans is encoded by the ABCB1 gene.[4] P-gp is a well-characterized
ABC-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the
MDR/
TAP subfamily.[5] The normal excretion of xenobiotics back into the gut lumen by P-gp
pharmacokinetically reduces the
efficacy of some
pharmaceutical drugs (which are said to be P-gp
substrates). In addition, some
cancer cells also express large amounts of P-gp, further amplifying that effect and rendering these cancers
multidrug resistant. Many drugs inhibit P-gp, typically incidentally rather than as their main
mechanism of action; some foods do as well.[6] Any such substance can sometimes be called a P-gp inhibitor.
A 2015 review of polymorphisms in ABCB1 found that "the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance."[7]
Protein
P-gp is a 170 kDa transmembrane
glycoprotein, which includes 10–15 kDa of N-terminal glycosylation. The N-terminal half of the protein contains six transmembrane helixes, followed by a large cytoplasmic domain with an ATP-binding site, and then a second section with six transmembrane helixes and an ATP-binding domain that shows over 65% of amino acid similarity with the first half of the polypeptide.[8] In 2009, the first structure of a mammalian P-glycoprotein was solved (3G5U).[9] The structure was derived from the mouse MDR3 gene product heterologously expressed in Pichia pastoris yeast. The structure of mouse P-gp is similar to structures of the bacterial ABC transporter MsbA (3B5W and 3B5X)[10] that adopt an inward facing conformation that is believed to be important for binding substrate along the inner leaflet of the membrane. Additional structures (3G60 and 3G61) of P-gp were also solved revealing the binding site(s) of two different cyclic peptide substrate/inhibitors. The promiscuous binding pocket of P-gp is lined with aromatic amino acid side chains.
Through Molecular Dynamic (MD) simulations, this sequence was proved to have a direct impact in the transporter's structural stability (in the nucleotide-binding domains) and defining a lower boundary for the internal drug-binding pocket.[11]
Species, tissue, and subcellular distribution
P-gp is expressed primarily in certain cell types in the liver, pancreas, kidney, colon, and
jejunum.[12] P-gp is also found in brain
capillaryendothelial cells.[13]
Function
Substrate enters P-gp either from an opening within the
inner leaflet of the membrane or from an opening at the cytoplasmic side of the protein. ATP binds at the cytoplasmic side of the protein. Following binding of each, ATP hydrolysis shifts the substrate into a position to be excreted from the cell. Release of the phosphate (from the original ATP molecule) occurs concurrently with substrate excretion. ADP is released, and a new molecule of ATP binds to the secondary ATP-binding site. Hydrolysis and release of ADP and a phosphate molecule resets the protein, so that the process can start again.
The protein belongs to the superfamily of
ATP-binding cassette (ABC) transporters. ABC proteins transport various molecules across extra- and intra-cellular membranes. ABC genes are divided into seven distinct subfamilies (ABC1, MDR/TAP, MRP, ALD, OABP, GCN20, White). This protein is a member of the MDR/TAP subfamily. Members of the MDR/TAP subfamily are involved in
multidrug resistance. P-gp is an ATP-dependent drug efflux pump for
xenobiotic compounds with broad substrate specificity. It is responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs. This protein also functions as a transporter in the
blood–brain barrier. Mutations in this gene are associated with colchicine resistance and Inflammatory bowel disease 13. Alternative splicing and the use of alternative promoters results in multiple transcript variants. [14]
P-gp transports various substrates across the cell membrane including:
Its ability to transport the above substrates accounts for the many roles of P-gp including:
Regulating the distribution and bioavailability of drugs
Increased intestinal expression of P-glycoprotein can reduce the absorption of drugs that are substrates for P-glycoprotein. Thus, there is a reduced bioavailability, and therapeutic plasma concentrations are not attained. On the other hand, supratherapeutic plasma concentrations and drug toxicity may result because of decreased P-glycoprotein
expression
Regulation of expression and function of P-gp in cancer cells
At the
transcriptional level, the expression of P-gp has been intensively studied, and numerous
transcription factors and pathways are known to play roles. A variety of transcription factors, such as
p53,[17]YB-1,[18] and
NF-κB[19] are involved in the direct regulation of P-gp by binding to the
promoter regions of the P-gp gene. Many
cell signaling pathways are also involved in transcriptional regulation of P-gp. For example, the
PI3K/Akt pathway[18] and the
Wnt/β-catenin pathway[20] were reported to positively regulate the expression of P-gp.
Mitogen-activated protein kinase (MAPK) signaling includes three pathways: the classical
MAPK/ERK pathway, the
p38 MAPK pathway, and the
c-Jun N-terminal kinase (JNK) pathway, all of which were reported to have implications in the regulation of the expression of P-gp. Studies suggested that the MAPK/ERK pathway is involved in the positive regulation of P-gp;[21] the p38 MAPK pathway negatively regulates the expression of the P-gp gene;[22] and the JNK pathway was reported to be involved in both positive regulation and negative regulation of P-gp.[23][24]
After 2008,
microRNAs (miRNAs) were identified as new players in regulating the expression of P-gp in both transcriptional and
post-transcriptional levels. Some miRNAs decrease the expression of P-gp. For example,
miR-200c down-regulates the expression of P-gp through the JNK signaling pathway[23] or
ZEB1 and
ZEB2;[25]miR-145 down-regulates the mRNA of P-gp by directly binding to the
3'-UTR of the gene of P-gp and thus suppresses the
translation of P-gp.[26] Some other miRNAs increase the expression of P-gp. For example,
miR-27a up-regulates P-gp expression by suppressing the
Raf kinase inhibitor protein (RKIP);[27] alternatively, miR-27a can also directly bind to the promoter of the P-gp gene, which works in a similar way with the mechanism of action of transcriptional factors.[28]
Altered P-gp function has also been linked to
inflammatory bowel diseases (IBD);[40] however, due to its ambivalent effects in intestinal inflammation many questions remain so far unanswered.[41] While decreased efflux activity may promote disease susceptibility and drug toxicity, increased efflux activity may confer resistance to therapeutic drugs in IBD.[41] Mice deficient in MDR1A develop chronic intestinal inflammation spontaneously, which appears to resemble human
ulcerative colitis.[42]
Cancer
P-gp efflux activity is capable of lowering intracellular concentrations of otherwise beneficial compounds, such as chemotherapeutics and other medications, to sub-therapeutic levels. Consequently, P-gp overexpression is one of the main mechanisms behind decreased intracellular drug accumulation and development of multidrug resistance in human multidrug-resistant (MDR) cancers.[43][44]
History
P-gp was first characterized in 1976. P-gp was shown to be responsible for conferring multidrug resistance upon mutant cultured cancer cells that had developed resistance to cytotoxic drugs.[5][45]
The structure of mouse P-gp, which has 87% sequence identity to human P-gp, was resolved by
x-ray crystallography in 2009.[9] The first structure of human P-gp was solved in 2018, with the protein in its ATP-bound, outward-facing conformation. [46]
Homozygous subjects, identified with the TT
genotype, are usually more able to extrude xenobiotics from the cell. A Homozygous genotype for the
allele ABCB1/MDR1 is capable of a higher absorption from the blood vessels and a lower extrusion into the
lumen.
Xenobiotics are extruded at a lower rate with heterozygous (CT) alleles compared to homozygous ones. [50]
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^Wilk JN, Bilsborough J, Viney JL (2005). "The mdr1a-/- mouse model of spontaneous colitis: a relevant and appropriate animal model to study inflammatory bowel disease". Immunologic Research. 31 (2): 151–9.
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