PSC2002/L10 ABC Transporters & Multidrug Resistance Flashcards
What are the 2 main transporter superfamilies?
ATP-binding cassettes (ABC)
Solute carrier (SLC)
Give the 4 kinds of solute carrier (SLC) proteins.
OAT organic ion transporter
OATP organic anion transporting peptide
OCT organic cation transporter
MATE multidrug and toxin extrusion protein
What is the function of ATP-binding cassette transporters?
Couple substrate transport to ATP hydrolysis
Give an example of exporters or efflux transporters.
ABCB1
MDR1
P-gp
Where are importers/influx transporters usually found?
In prokaryotic species
Give 2 importers/influx transporters in E. coli.
Maltose uptake transporter
Methionine uptake transporter
Give 2 transporters with no transport function that are associated with ion channels.
CFTR
SUR1
What are ABC transporters primarily involved with?
Xenobiotic metabolism
Normal physiological role - bile transport in liver or insulin release regulation
Which kind of cells are ABC transporter proteins usually expressed in?
Cells with excretory or barrier (protective functions)
Polarised cells so expressed on apical OR basolateral side
Describe the structure of ABC transporters. (2)
4 domains: 2 nucleotide-binding domains (bind & hydrolyse ATP)
2 transmembrane domains (bind & transport substrates)
How many human transporter genes and subfamilies of ABC transporters are there?
48 human ABC transporter genes
7 subfamilies A-G
How conserved are:
NBDs
TMDs?
NBDs highly conserved
TMDs less conserved
How are drugs circulated from the intestine? How do ABC transporters interfere with this?
Enter enterocytes through brush border membrane into hepatic portal vein
ABC transporters pump compounds back into lumen
Name 3 enterocyte influx transporters.
ASBT
PEPT1
MCT1
Name an enterocyte efflux transporter.
OST
What is the role of transporter proteins in the liver?
Transport of drugs into hepatocyte from circulation across sinusoidal membrane
Drugs transported out across canalicular membrane (to bile) or sinusoidal membrane (back to blood for renal excretion)
What is the role of transporters at the blood brain barrier? (2)
May prevent potentially toxic compounds reaching brain
KO mice studies show drug accumulation in brain and toxicity
Explain how multidrug resistance occurs in tumours. (3)
Overexpression of ABC transporters (e.g., P-gp)
Actively pump chemotherapy drugs out of cancer cells
Reducing intracellular drug concentration to sublethal levels
Give 2 examples of multidrug resistance (MDR) mechanisms.
Decreased uptake by cell
Increased metabolism of drug in cell
Alteration in cell target
Enhanced drug efflux
What is enhanced drug efflux in MDR due to?
Mainly high expression of ABC transporters
MDR1, MRP1, ABCG2
What are the 2 kinds of multidrug resistance?
Intrinsic
Acquired
What was the outcome of the Victor Ling experiment in 1976?
Resistant cells have high levels of P-glycoprotein on cell surface
ABCB1 gene on chromosome 7 codes for P-gp
Describe the proposed model of P-gp mediated transport. (6)
Substrate binds to binding site
ATP binds NBD -> hydrolysis
Conformational change -> drug efflux
ADP release -> ATP hydrolysis
Conformational reset
Process repeats
What is the main role of P-gp?
Removal of xenobiotics from cells
Pumps out xenobiotics from enterocytes following initial absorption
Transports xenobiotics into bile across canalicular membrane
Prevents access of many xenobiotics to brain
Transports xenobiotics into lumen of kidney on brush-border membrane
Where are MDR I located?
In cells with excretory or barrier (protective) functions
Liver
Intestine
Kidney
BBB
Blood Placenta Barrier
Blood testis barrier
Give 3 effects of P-gp in tissues.
Limited drug absorption
Active drug elimination
Limited drug distribution into tissues
Describe P-gp KO studies in mice. (3)
Viable, fertile, phenotypically normal
Suffer toxicity with some compounds due to entry into brain
Show increased absorption & decreased excretion of some drugs
How is P-gp induced?
Through PXR receptor
Describe St John’s Wort. (3)
Herbal remedy for low mood and mild anxiety
Increases P-gp expression and efflux activity
Induces CYP3A4
Describe P-gp mediated drug efflux.
Increased transporter gene expression
Increased transporter levels
Increased compound efflux
How does low P-gp expression affect cell viability?
Lower P-gp expression = lower cell viability
Describe the role of P53 and P-gp in drug resistance. (4)
Inactivation of tumour suppressor gene in 50% of cancers
Wt P53 represses P-gp transcription via direct DNA binding
Wt P53 mediated downregulation of P-gp via miR-34a and LRPPRC
Mutant P53 cooperates with ETS-1 to upregulate ABCB1 expression
How does P-gp mediate drug resistance via lysosomal sequestration? (4)
Lysosomal accumulation of anticancer drugs as novel mechanism
As endosome matures into lysosome, it’s acidified
Drug (e.g., Doxorubicin) enters cell & lysosome
Becomes charged under acidified conditions and trapped in lysosome unable to reach nucleus (target)
Describe drug extrusion by brain endothelial cells via lysosomal drug trapping and disposal by neutrophils. (3)
Lysosomal sequestration in endothelial cells of BBB
Shedding of vesicles attached at apical side followed by phagocytosis by neutrophils
Novel mechanism of drug disposal and BBB protection
What is the role of BSEP/ABCB11? (2)
Transports bile salts across canalicular membrane of hepatocyte
Reported to transport some drugs e.g., vinblastine
What is PFIC2? (2)
Rare genetic disease
Progressive familial intrahepatic cholestasis
Inherited defect in BSEP
What is ABCB4? (3)
Specific translocase (floppase) for phosphatidylcholine
Translocates PC from inner to outer leaflet of canalicular membrane for extraction into lumen by bile salts
Form micelles with biles salts to protect hepatocyte biliary membrane
What do genetic mutations of ABCB4 cause? (3)
Progressive familal intrahepatic cholestasis type 3 (PFIC3)
Gallstones
Intrahepatic cholestasis of pregnancy (ICP)
What can MDR3/ABCB4 be treated with?
Anthracyclines
Vinca alkaloids
Taxanes
