Sensitivity of Leishmaniaspp.to Glibenclamide and 4-Aminopiridine: A Tool for the Study of Drug Resistance Development
Alicia Ponte-Sucre, Yelitza Campos^*, Jorge Vazquez, Heidrun Moll^**, Alexis Mendoza-Leon^*/+
Laboratorio de Fisiologia Molecular, Instituto de Medicina Experimental,
Universidad Central de Venezuela, Apartado 68256, Caracas 1062-A,
^+Corresponding author. Fax: +58-2-753.5897
Received 11 November 1996; Accepted 11 June 1997
Code Number:OC97112 Sizes of Files: Text: 21.5K Graphics: Tables (jpg) - 37.9K Line drawings and photographs (jpg) - 107.5K
We have demonstrated that Leishmania spp. grown as promastigotes, are sensitive to the K^+ channel inhibitors 4-aminopyridine and glibenclamide. Their host cells, the macrophages, are not affected by similar concentrations of the drugs. We have also initiated the molecular characterization of the mechanisms involved in the development of drug resistance to glibenclamide by the parasite. Therefore, we have selected experimentally and begun to characterize the Venezuelan Leishmania (Leishmania) strain, NR resistant to glibenclamide [NR(Gr)]. The analysis of genomic DNA evidenced the existence of a fragment which apparently is amplified in NR(Gr). The fragment recognized by the pgpA probe, related to the Leishmania P-glycoprotein family and which was originally isolated from L. tarentolae, showed a size polymorfism between the sensitive and the resistant strain. These results suggest that the development of resistance to glibenclamide in the strain NR(Gr) might be associated with the amplification of the ltpgpA or related gene(s).
Key words: Leishmania - chemotherapy - glibenclamide - 4-aminopyridine - drug resistance - P-glycoprotein
Leishmaniasis is a disease with different clinical manifestations. The toxicity of conventional drugs such as the antimonials, and the possible development of resistance by the parasite to such compounds, emphasize the importance of the search for alternative strategies, both for its prevention and its treatment (Olliaro & Bryceson 1993). The accurate and early diagnoses of the disease, and its effective control, are extremely important and must be adequately addressed.
From the chemotherapeutic point of view, the development and evaluation of new drugs is mainly based on their efficacy against parasites. Such activity is sometimes opposed either by the complex life cycle of the parasite or by the intracellular nature of the developmental stages of the organisms as happens in Leishmania (Olliaro & Bryceson 1993). In this work we have attempted to evaluate the effect of 4-aminopyridine and glibenclamide, two classical inhibitors of K^+ channels, on the viability of Leishmania promastigotes and analyze some of the molecular mechanisms involved in the development of resistance to glibenclamide.
MATERIALS AND METHODS
Drugs - The drugs used were the classic K^+ channel inhibitors 4-aminopyridine (4-AP) for voltage gated channels, and glibenclamide (GLIB) for ATP gated channels. 4-AP was dissolved in deionized water to 0.5 M and GLIB was dissolved in ethanol to 4 x 10^-3 M. Both drugs were further diluted in Schneider's Drosophila medium to the desired concentrations. As GLIB has an afinity for serum albumin greater than 96%, a correction has to be done to estimate the free drug concentration in the medium. Such estimation was done using the methodology described by Bennet et al. (1996). 4-AP does not have a high affinity for serum proteins as albumin thus, the free drug concentration in the medium is considered to be the same added.
Strains, cell lines and culture conditions - Table shows the different Leishmania strains used in this study. The Venezuelan strain NR was originally isolated in 1980 from a Venezuelan patient with tegumentary diffuse leishmaniasis and was kindly provided by Dr Angel Hernandez, Instituto de Biologia Celular, Universidad Central de Venezuela. Its identification was carried out in this study by RFLP analysis of the beta-tubulin gene region as previously described (Mendoza-Leon et al. 1995).
The leishmanicidal activity of the compounds was studied in Leishmania NR promastigotes cultured in Schneider's Drosophila medium (GIBCO) supplemented with 10% fetal calf serum. The parasites were harvested at their stationary-phase of growth, the cell suspension was adjusted to a density of 2 x 10^6 cells ml^-1 in a final volume of 0.1 ml of fresh medium. Aliquots of stationary-phase Leishmania were then exposed to various concentrations of the drug (i.e., 10^-10 - 10^-2 M) at room temperature for 2 or 4 hr. Subsequently, the number of live cells was estimated in each experimental vial and compared to the number of live cells of control vials incubated without drugs. The viability of Leishmania was determined by a modification of a previously described procedure (Jackson et al. 1985). At the end of the incubation time, the cells were washed twice in phosphate buffer (PBS), resuspended in the same buffer at the same density, stained for 2 min at 25 C with 125 ug ml^-1 fluorescein diacetate and observed with a Nikon fluorescence microscope at 495 nm. The results are expressed as the percentage of live parasites at each drug concentration.
The isolation of a resistant Leishmania strain was assayed in three isolates of New World Leishmania, i.e., the Venezuelan strains NR, 9012 and AZV, all of them from the subgenus Leishmania (Table), and one from the Old World, L. (L.) major P. The strains were serially cultured at increasing concentrations of GLIB (5 uM to 100 uM). At each concentration step, the cells remained at least three weeks. The development of resistance was monitored comparing the growth of experimental and control cells grown in parallel.
Molecular analysis - The DNA isolation from sensitive and resistant Leishmania, their restriction analysis, as well as the DNA labelling and hybridization conditions were carried out as described by Mendoza-Leon et al. (1995). The pgpA probe, a genomic fragment of 800 bp from L. tarentolae, present in the recombinant clone PM12, recognizes the P-glycoprotein (P-gp) family of Leishmania (Legare et al. 1994). Following the digestion of the plasmid pM12 with the restriction enzyme Sac I and the fragment separation, the pgpA probe was recovered from the agarose gel and purified by the Wizard DNA purification System (Promega).
We have evaluated the effect of the K^+ channel inhibitors on stationary-phase promastigote cultures of the Venezuelan strain NR. This strain was identified as a member of the Leishmania subgenus by the polymorphism of the beta-tubulin region (Fig. 1).
Previously, we have demonstrated that Leishmania strains are sensitive to K^+ channel inhibitors (Ponte-Sucre et al. 1995). A typical dose-response curve obtained for the viability of the strain NR is illustrated in Fig. 2. Both K^+ inhibitors, 4-AP and GLIB, affect the viability of the promastigotes along their stationary phase of growth. The LD50 for 4-AP was 39 uM and for GLIB was even smaller, i.e., 7 uM. In addition, we have demonstrated that although the infection of the macrophage cell line J774 and peritoneal-exudate macrophages by L. (L.) major is not significantly decreased by concentrations of the drugs 3-4 times smaller than the promastigote LD50, 464 uM 4-AP and 10 uM GLIB for this strain, the survival of intracellular parasites decreased significantly in the presence of these drugs without affecting the viability of the macrophages (data not shown). For instance, a concentration of 3 uM GLIB, affects the viability of the macrophages less than 5%.
Figure 3b: sensitivity of the Venezuelan strain Leishmania (L.) NR strain (sensitive to glibenclamide) to glibenclamide 3 uM (u) and 16 uM (t) and development of resistance by NR to GLIB 3 uM at week 2 (l) and 3 (s) of subculture (a) and number of alive Leishmania (L.) NR cells per field in a sensitive strain NR(Gs) under control conditions and number of alive NR cells in a resistant strain NR(Gr) exposed to GLIB 16 uM (b). Magnification 400 x.
The study of ion channels in parasites such as Leishmania is mainly justified by their biomedical importance (Backer-Grunwald 1992). Their complex life cycles, their flexibility and their ability to adapt to diverse environments, where the membrane plays an important role in maintaining the cellular homeostasis, also justify the study of the membrane proteins implicated on structures such as ion channels, pores and membrane transporters (Zilberstein & Shapira 1994). The drug 4-AP has been used extensively for identifying a high variety of voltage-gated K^+ channels (Castle et al. 1989) and GLIB which is a K^+ ATP channel blocker, has a widespread use to identify K^+ channels associated with insulin secretion (Schmid-Antomarchi et al. 1987, Panten et al. 1989). More strikingly, the sulphonylurea (GLIB-sensitive) receptor is a member of the ATP-binding cassette or traffic ATP-ase superfamily (Aguilar-Bryan et al. 1995) to which the Leishmania P-glycoprotein (PgpA) has been associated (Legare et al. 1994). The fact that both 4-AP and GLIB are deleterious to cultured promastigotes, have low DL50 and do not seem to impair the macrophage survival at concentrations where the survival of intracellular amastigotes decreased significantly (data not shown), reflects the importance of understanding the response and mechanism of action of these transport inhibitors. In this work we have begun to study the response of the parasite against these drugs and to acknowledge the molecular mechanisms involved in the development of GLIB resistance. For this study we have used the Venezuelan Leishmania (L.) NR strain, a member of the Leishmania subgenus. We have selected and began the characterization of Leishmania (L.) NR resistant to GLIB [NR(Gr)]. Thus, we found that NR(Gr) survived at concentrations higher than 11 uM as the fraction of free drug present in the medium. Its viability is not impaired at a GLIB concentration of 16 uM, although the viability of the sensitive strain, NR(Gs), is dramatically affected by this drug concentration. Striking was the strong signal found, independent of the restriction enzymes tested, for the pgpA in NR(Gr) in the Southern analysis. Since equal amounts of DNA was used, these results suggest an amplification of sequences related to pgpA in the GLIB resistant strain. The pgpA probe was originally characterized as the nucleotide binding site (nbs) of the gene family of P-glycoproteins in Leishmania (Ouellette et al. 1990, Legare et al. 1994). Whether the resistance found could be related to the amplification of genes associated to P-glycoproteins is an open question. Amplification of particular gene sequences in response to drug pressure has been well documented in Leishmania; however, many of these amplifications are unrelated to drug resistance (Beverly 1991). Therefore is worthy to continue to explore the biochemical and molecular mechanisms which determine the response of the parasite to this drug and the development of resistance by this strain. On the other hand, there is a size polymorphism for the Pst I-fragment recognized by pgpA both in NR(Gs) (6.2 Kb) and NR(Gr) (4.4 Kb). The comparison of a partial DNA sequence of the polymorphic fragment from both strains showed more than 95% of homology between them, and around 60% homology with multidrug resistance genes from Plasmodium falciparum's (not shown). We do not have yet an explanation for it; nevertheless, internal gene rearrangements has been postulated as one mechanism where by it could arise; to answer this question we are currently working on the restriction map and final DNA sequence of the fragment from both strains.
To Dr M Ouellette for the generous gift of the pgpA probe.
This work was partially supported by the Venezuelan grants CDCH-UCV 09.33.0018.96, PC.09.010.94, 03.10.3750.96, 09.33.3514.95, CONICIT RPIV-110034 S1-96001411 and S1-2674 and the German Bundes-ministerium fur Forschung and Technologie grant K I8906.
Copyright 1997 Fundacao Oswaldo Cruz
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