PSC2002/L10 ABC Transporters & Multidrug Resistance

Question 1

What are the 2 main transporter superfamilies?

Answer 1

ATP-binding cassettes (ABC)
Solute carrier (SLC)

Question 2

Give the 4 kinds of solute carrier (SLC) proteins.

Answer 2

OAT organic ion transporter
OATP organic anion transporting peptide
OCT organic cation transporter
MATE multidrug and toxin extrusion protein

Question 3

What is the function of ATP-binding cassette transporters?

Answer 3

Couple substrate transport to ATP hydrolysis

Question 4

Give an example of exporters or efflux transporters.

Answer 4

ABCB1
MDR1
P-gp

Question 5

Where are importers/influx transporters usually found?

Answer 5

In prokaryotic species

Question 6

Give 2 importers/influx transporters in E. coli.

Answer 6

Maltose uptake transporter
Methionine uptake transporter

Question 7

Give 2 transporters with no transport function that are associated with ion channels.

Answer 7

CFTR
SUR1

Question 8

What are ABC transporters primarily involved with?

Answer 8

Xenobiotic metabolism
Normal physiological role - bile transport in liver or insulin release regulation

Question 9

Which kind of cells are ABC transporter proteins usually expressed in?

Answer 9

Cells with excretory or barrier (protective functions)
Polarised cells so expressed on apical OR basolateral side

Question 10

Describe the structure of ABC transporters. (2)

Answer 10

4 domains: 2 nucleotide-binding domains (bind & hydrolyse ATP)
2 transmembrane domains (bind & transport substrates)

Question 11

How many human transporter genes and subfamilies of ABC transporters are there?

Answer 11

48 human ABC transporter genes
7 subfamilies A-G

Question 12

How conserved are:
NBDs
TMDs?

Answer 12

NBDs highly conserved
TMDs less conserved

Question 13

How are drugs circulated from the intestine? How do ABC transporters interfere with this?

Answer 13

Enter enterocytes through brush border membrane into hepatic portal vein
ABC transporters pump compounds back into lumen

Question 14

Name 3 enterocyte influx transporters.

Answer 14

ASBT
PEPT1
MCT1

Question 15

Name an enterocyte efflux transporter.

Answer 15

OST

Question 16

What is the role of transporter proteins in the liver?

Answer 16

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)

Question 17

What is the role of transporters at the blood brain barrier? (2)

Answer 17

May prevent potentially toxic compounds reaching brain
KO mice studies show drug accumulation in brain and toxicity

Question 18

Explain how multidrug resistance occurs in tumours. (3)

Answer 18

Overexpression of ABC transporters (e.g., P-gp)
Actively pump chemotherapy drugs out of cancer cells
Reducing intracellular drug concentration to sublethal levels

Question 19

Give 2 examples of multidrug resistance (MDR) mechanisms.

Answer 19

Decreased uptake by cell
Increased metabolism of drug in cell
Alteration in cell target
Enhanced drug efflux

Question 20

What is enhanced drug efflux in MDR due to?

Answer 20

Mainly high expression of ABC transporters
MDR1, MRP1, ABCG2

Question 21

What are the 2 kinds of multidrug resistance?

Answer 21

Intrinsic
Acquired

Question 22

What was the outcome of the Victor Ling experiment in 1976?

Answer 22

Resistant cells have high levels of P-glycoprotein on cell surface
ABCB1 gene on chromosome 7 codes for P-gp

Question 23

Describe the proposed model of P-gp mediated transport. (6)

Answer 23

Substrate binds to binding site
ATP binds NBD -> hydrolysis
Conformational change -> drug efflux
ADP release -> ATP hydrolysis
Conformational reset
Process repeats

Question 24

What is the main role of P-gp?

Answer 24

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

Question 25

Where are MDR I located?

Answer 25

In cells with excretory or barrier (protective) functions Liver Intestine Kidney BBB Blood Placenta Barrier Blood testis barrier

Question 26

Give 3 effects of P-gp in tissues.

Answer 26

Limited drug absorption Active drug elimination Limited drug distribution into tissues

Question 27

Describe P-gp KO studies in mice. (3)

Answer 27

Viable, fertile, phenotypically normal Suffer toxicity with some compounds due to entry into brain Show increased absorption & decreased excretion of some drugs

Question 28

How is P-gp induced?

Answer 28

Through PXR receptor

Question 29

Describe St John's Wort. (3)

Answer 29

Herbal remedy for low mood and mild anxiety Increases P-gp expression and efflux activity Induces CYP3A4

Question 30

Describe P-gp mediated drug efflux.

Answer 30

Increased transporter gene expression Increased transporter levels Increased compound efflux

Question 31

How does low P-gp expression affect cell viability?

Answer 31

Lower P-gp expression = lower cell viability

Question 32

Describe the role of P53 and P-gp in drug resistance. (4)

Answer 32

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

Question 33

How does P-gp mediate drug resistance via lysosomal sequestration? (4)

Answer 33

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)

Question 34

Describe drug extrusion by brain endothelial cells via lysosomal drug trapping and disposal by neutrophils. (3)

Answer 34

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

Question 35

What is the role of BSEP/ABCB11? (2)

Answer 35

Transports bile salts across canalicular membrane of hepatocyte Reported to transport some drugs e.g., vinblastine

Question 36

What is PFIC2? (2)

Answer 36

Rare genetic disease Progressive familial intrahepatic cholestasis Inherited defect in BSEP

Question 37

What is ABCB4? (3)

Answer 37

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

Question 38

What do genetic mutations of ABCB4 cause? (3)

Answer 38

Progressive familal intrahepatic cholestasis type 3 (PFIC3) Gallstones Intrahepatic cholestasis of pregnancy (ICP)

Question 39

What can MDR3/ABCB4 be treated with?

Answer 39

Anthracyclines Vinca alkaloids Taxanes