Sterol transport by the human breast cancer resistance protein (ABCG2) expressed in Lactococcus lactis

The human breast cancer resistance protein (BCRP, also know as ABCG2, MXR, or ABCP) is one of the more recently discovered ATP-binding cassette (ABC) transporters that confer resistance on cancer cells by mediating multidrug efflux. In the present study, we have obtained functional expression of human BCRP in the Gram-positive bacterium Lactococcus lactis. BCRP expression conferred multidrug resistance on the lactococcal cells, which was based on ATP-dependent drug extrusion. BCRP-mediated ATPase and drug transport activities were inhibited by the BCRP-specific modulator fumitremorgin C. To our knowledge these data represent the first example of the functional expression of a mammalian ABC half-transporter in bacteria. Although members of the ABCG subfamily (such as ABCG1 and ABCG5/8) have been implicated in the transport of sterols, such a role has not yet been established for BCRP. Interestingly, the BCRP-associated ATPase activity in L. lactis was significantly stimulated by (i) sterols including cholesterol and estradiol, (ii) natural steroids such as progesterone and testosterone, and (iii) the anti-estrogen anticancer drug tamoxifen. In addition, BCRP mediated the efflux of [3H]estradiol from lactococcal cells. Our findings suggest that BCRP may play a role in the transport of sterols in human, in addition to its ability to transport multiple drugs and toxins.     

Genetic variability in the species Rhizopus stolonifer, assessed by random amplified polymorphic DNA analysis

Rhizopus stolonifer is an important post-harvest pathogenic fungus. Recent taxonomic findings based on morphological and growth characteristics led to a dramatic reduction in the number of accepted species within the genus. The aim of this study was to examine this situation with molecular markers. Twenty-nine R. stolonifer strains isolated from various locations and substrates were characterized by random amplified polymorphic DNA (RAPD) analysis. The numerical analysis of the RAPD data revealed four main clusters with extremely high dissimilarity values, but only low or moderate variability was observed within these groups. These results suggest a high genetic heterogeneity in the case of R. stolonifer: isolates of R. stolonifer var. stolonifer, R. stolonifer var. reflexus and R. niveus displayed species-level genetic distances, which gives rise to considerations that they might be separate species.     

Reversible transport by the ATP-binding cassette multidrug export pump LmrA: ATP synthesis at the expense of downhill ethidium uptake

The ATP dependence of ATP-binding cassette (ABC) transporters has led to the widespread acceptance that these systems are unidirectional. Interestingly, in the presence of an inwardly directed ethidium concentration gradient in ATP-depleted cells of Lactococcus lactis, the ABC multidrug transporter LmrA mediated the reverse transport (or uptake) of ethidium with an apparent K(t) of 2.0 microm. This uptake reaction was competitively inhibited by the LmrA substrate vinblastine and was significantly reduced by an E314A substitution in the membrane domain of the transporter. Similar to efflux, LmrA-mediated ethidium uptake was inhibited by the E512Q replacement in the Walker B region of the nucleotide-binding domain of the protein, which strongly reduced its drug-stimulated ATPase activity, consistent with published observations for other ABC transporters. The notion that ethidium uptake is coupled to the catalytic cycle in LmrA was further corroborated by studies in LmrA-containing cells and proteoliposomes in which reverse transport of ethidium was associated with the net synthesis of ATP. Taken together, these data demonstrate that the conformational changes required for drug transport by LmrA are (i) not too far from equilibrium under ATP-depleted conditions to be reversed by appropriate changes in ligand concentrations and (ii) not necessarily coupled to ATP hydrolysis, but associated with a reversible catalytic cycle. These findings and their thermodynamic implications shed new light on the mechanism of energy coupling in ABC transporters and have implications for the development of new modulators that could enable reverse transport-associated drug delivery in cells through their ability to uncouple ATP binding/hydrolysis from multidrug efflux.     

Effects of staurosporine,U-73122, wortmannin, 4-hydroxynonenal and sodium azide upon the release of secreted β-amyloid precursor protein from human platelets in response to thrombin stimulation

In the present study, the release of secreted -amyloid precursor protein (APPs) in response to thrombin stimulation in platelets has been investigated. Incubation of platelets with thrombin produced a concentration-dependent release of APPs with a concomitant reduction in the APP remaining in the lysates. The response to thrombin was not affected by pretreatment for 15 min with the phospholipase C inhibitor U-73122, with the protein kinase C inhibitor staurosporine, or with hydrogen peroxide (which at the concentrations used affects the phosphoinositide signalling system in human platelets). In contrast, pretreatment with wortmannin and sodium azide reduced the responses to thrombin. These data would suggest that thrombin may cause the release of APPs from human platelets via an activation of a phospholipase C-independent pathway. Thrombin-stimulated APPs release was also reduced by 4-hydroxynonenal. This finding, if it is a phenomenon also found for CNS cells, could be of relevance to the pathogenesis of Alzheimer's disease, given that an accumulation of 4-hydroxynonenal is found in this disease.     

Evaluation of a series of 2-napthamide derivatives as inhibitors of the drug efflux pump AcrB for the reversal of antimicrobial resistance

Drug efflux pumps confer multidrug resistance to dangerous pathogens which makes these pumps important drug targets. We have synthesised a novel series of compounds based on a 2-naphthamide pharmacore aimed at inhibiting the efflux pumps from Gram-negative bacteria. The archeatypical transporter AcrB from Escherichia coli was used as model efflux pump as AcrB is widely conserved throughout Gram-negative organisms. The compounds were tested for their antibacterial action, ability to potentiate the action of antibiotics and for their ability to inhibit Nile Red efflux by AcrB. None of the compounds were antimicrobial against E. coli wild type cells. Most of the compounds were able to inhibit Nile Red efflux indicating that they are substrates of the AcrB efflux pump. Three compounds were able to synergise with antibiotics and reverse resistance in the resistant phenotype. Compound A3, 4-(isopentyloxy)-2-naphthamide, reduced the MICs of erythromycin and chloramphenicol to the MIC levels of the drug sensitive strain that lacks an efflux pump. A3 had no effect on the MIC of the non-substrate rifampicin indicating that this compound acts specifically through the AcrB efflux pump. A3 also does not act through non-specific mechanisms such as outer membrane or inner membrane permeabilisation and is not cytotoxic against mammalian cell lines. Therefore, we have designed and synthesised a novel chemical compound with great potential to further optimisation as inhibitor of drug efflux pumps.     

Substitution of terminal amide with 1H-1,2,3-triazole: Identification of unexpected class of potent antibacterial agents

3-Methoxybenzamide (3-MBA) derivatives have been identified as novel class of potent antibacterial agents targeting the bacterial cell division protein FtsZ. As one of isosteres for the amide group, 1,2,3-triazole can mimic the topological and electronic features of the amide, which has gained increasing attention in drug discovery. Based on these considerations, we prepared a series of 1H-1,2,3-triazole-containing 3-MBA analogues via isosteric replacement of the terminal amide with triazole, which had increased antibacterial activity. This study demonstrated the possibility of developing the 1H-1,2,3-triazole group as a terminal amide-mimetic element which was capable of both keeping and modulating amide-related bioactivity. Surprisingly, a different action mode of these new 1H-1,2,3-triazole-containing analogues was observed, which could open new opportunities for the development of antibacterial agents.     

Zebularine Induces Long-Term Survival of Pancreatic Islet Allotransplants in Streptozotocin Treated Diabetic Rats

Background

Coping with the immune rejection of allotransplants or autologous cells in patients with an active sensitization towards their autoantigens and autoimmunity presently necessitates life-long immune suppressive therapy acting on the immune system as a whole, which makes the patients vulnerable to infections and increases their risk of developing cancer. New technologies to induce antigen selective long-lasting immunosuppression or immune tolerance are therefore much needed.

Methodology/Principal Findings

The DNA demethylating agent Zebularine, previously demonstrated to induce expression of the genes for the immunosuppressive enzymes indolamine-2,3-deoxygenase-1 (IDO1) and kynureninase of the kynurenine pathway, is tested for capacity to suppress rejection of allotransplants. Allogeneic pancreatic islets from Lewis rats were transplanted under the kidney capsule of Fischer rats previously made diabetic by a streptozotocin injection (40 mg/kg). One group was treated with Zebularine (225 mg/kg) daily for 14 days from day 6 or 8 after transplantation, and a control group received no further treatment. Survival of the transplants was monitored by blood sugar measurements. Rats, normoglycemic for 90 days after allografting, were subjected to transplant removal by nephrectomy to confirm whether normoglycemia was indeed due to a surviving insulin producing transplant, or alternatively was a result of recovery of pancreatic insulin production in some toxin-treated rats. Of 9 Zebularine treated rats, 4 were still normoglycemic after 90 days and became hyperglycemic after nephrectomy. The mean length of normoglycemia in the Zebularine group was 67±8 days as compared to 14±3 days in 9 controls. Seven rats (2 controls and 5 Zebularine treated) were normoglycemic at 90 days due to pancreatic recovery as demonstrated by failure of nephrectomy to induce hyperglycemia.

Conclusions/Significance

Zebularine treatment in vivo induces a long-lasting suppression of the immune destruction of allogeneic pancreatic islets resulting in protection of allograft function for more than 10 weeks after end of treatment.