Efflux protein

Chan LM, Lowes S, Hirst BH (2004) The ABCs of drug transport in intestine and liver efflux proteins limiting drug absorption and bioavailability. Eur J Pharm Sci 21 25—51... 

Fig. 9.1 Schematic representation of possible mechanisms of resistance in Gram-negative and Gram-positive bacteria. 1, antibiotic-inactivating enzymes 2, antibiotic efflux proteins 3, alteration or duplication of intracellular targets 4, alteration of the cell membrane reducing antibiotic uptake 5, alterations in porins or lipopolysaccharide reducing antibiotic uptake or binding.

Plasmid- or transposon-encoded tetracycline efflux proteins have been described in a number of bacteria. These efflux profeins are fhoughf to span fhe cytoplasmic membrane and are dependenf on the proton-motive force for their action, ft is thought that the efflux proteins bind tetracyclines and initiate proton transfer, although no functional domains have been identified. Eight distinct tetracycline efflux profeins have been idenfified thus far. 

Three ofher mechanisms of chloramphenicol resisfance have been described. Firsf, a fransposon-encoded chloramphenicol efflux protein has been idenfified in E. coli. Second, some bacterial sfrains have been found to possess drug-resisfanf ribosomes, and fhird, low level resisfance can arise by chromosomal mufafions which reduce fhe amounf ofporins and fherefore impair uptake. This last mechanism is essentially that described for the AG AC antibiotics. 

A gene designated msrA has been identified in Staph, aureus which confers resistance to macrolides and streptogramins but not to lincosamides. Its function is unknown but the DNA sequence is homologous to genes coding for known efflux proteins. 

B. Sjostrom et al. Correlation of gene expression of ten drug efflux proteins of the atp-binding cassette transporter family in normal human jejunum and in human intestinal epithelial caco-2 cell monolayers, /. Pharmacol. Exp. Ther. 2001, 299, 164-170... 

Measurement of absorption can be complicated by efflux mechanisms. It is clear that many compounds are actively transported back into the GIT, into the bile or into the urine by efflux proteins. In the case of those in the GIT these may have an impact on the apparent absorption of a compound. Some understanding of the substrate specificity for one of these proteins, P-glycoprotein, is becoming apparent [8, 9], but currently the understanding is limited. At the moment there are no published reliable methods either in vivo or in vitro for predicting the importance of efflux mechanisms for a particular compound in man [10-12],... 

Cheng, J., Hicks, D. B. and Krulwich, T. A. (1996). The purified Bacillus subtilis tetracycline efflux protein TetA(L) reconstitutes both tetracycline-cobalt/H+ and Na+(K+)/H+ exchange, Proc. Natl Acad. Sci. USA, 93, 14446-14451. 

Either Transwell inserts or side-by-side diffusion chambers can be used for transport studies. Bode et al. have provided an excellent review on this subject [60], Briefly, cells are incubated for 30-60 min with a buffer solution. To initiate the transport study, a transport buffer containing the drug under investigation is added to either the apical or the basal chamber depending on the transport direction of interest. At predetermined time points, the respective receiver chamber is sampled and the withdrawn volume is replaced with the same volume of fresh buffer. The permeability coefficient (Papp) is calculated and the ratio of /apP in the basolateral-to-apical direction versus that in the apical-to-basolateral direction gives the efflux ratio. These sort of transport experiments are well suited to determine if drugs/xenobiotics are substrates of the placental efflux proteins. 

Menkes disease, an X-linked recessive condition, is caused by mutations in the gene encoding a Cu efflux protein. Cells from an affected individual accumulate high concentrations of Cu " that cannot be released from the cell. The symptoms result from functional Cu deficiency inasmuch as Cu absorbed from the intestine becomes trapped in the intestinal epithelial cells and delivery to other tissues is inadequate. 

Smyth MJ, Drasovskis E, Sutton VR, Johnstone RW (1998) The drug efflux protein, P-glycoprotein, additionally protects drug-resistant tumor cells from multiple forms of caspase-dependent apoptosis. Proc Natl Acad Sd USA 95 7024-7029... 

Liver injury is clinically defined as an increase of serum alanine amino transferase (ALT) levels of more than three times the upper limit of normal and a total bilirubin level of more than twice the upper limit of normal [4]. The clinical patterns of liver injury can be characterized as hepatocellular (with a predominant initial elevation of ALT), cholestatic (with an initial elevation of alkaline phosphatase) or mixed. The mechanisms of drug-induced hepatotoxicity include excessive generation of reactive metabolites, mitochondrial dysfunction, oxidative stress and inhibition of bile salt efflux protein [5]. Better understandings of these mechanisms in the past decades led to the development of assays and models suitable for studying such toxic mechanisms and for selecting better leads in the drug discovery stage. 

Inhibition of Bile Salt Efflux Protein and Drug-Induced Cholestasis... 

Both functional assays [vincristine transport (381) and rhodamine 123 transport (383)] and biochemical assays involving immunohistochemical analysis (381,384) have confirmed the expression of P-gp in the luminal membrane of BMECs cultured on polycarbonate membranes. Additionally, immunohistochemical methods showed the expression of P-gp in BMEC to be constant and at a high level in five- to seven-day-old old primary cultures (384). Like many other barrier-forming cells, BMECs appear to express other efflux proteins, for example, RT-PCR and immunoblot analysis have shown the presence of MRP1 in rat BMECs (385,386). Functional evidence has also been presented to confirm the expression of MRP1 in BMECs (387). 

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