Peptide transport by the multidrug resistance protein MRP1.

Small hydrophobic peptides were studied as possible substrates of the multidrug resistance protein (MRP)-1 (ABCC1) transmembrane transporter molecule. As observed earlier for P-glycoprotein- (Pgp; ABCB1) overexpressing cells, MRP1-overexpressing cells, including cells stably transfected with the MRP1 cDNA, showed distinct resistance to the cytotoxic peptide N-acetyl-Leu-Leu-norleucinal (ALLN). Resistance to this peptide and another toxic peptide derivative, which is based on a Thr-His-Thr-Nle-Glu-Gly backbone conjugated to butyl and benzyl groups (4A6), could be reversed by MRP1 inhibitors. The reduced toxicity of 4A6 in MRP1-overexpressing cells was found to be associated with lower accumulation of a fluorescein-labeled derivative of this peptide. Glutathione (GSH) depletion had a clear effect on resistance to ALLN but hardly affected 4A6 resistance. In a limited structure-activity study using peptides that are analogous to 4A6, MRP1-overexpressing cells were found to be resistant to these peptides as well. Remarkably, when selecting A2780 ovarian cancer cells for resistance to ALLN, even in the absence of Pgp blockers, resulting cell lines had up-regulated MRP1, rather than any of the other currently known multidrug resistance transporter molecules including Pgp, MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCCS), and the breast cancer resistance protein ABCG2. ALLN-resistant, MRP1-overexpressing cells were found to be cross-resistant to 4A6 and the classical multidrug resistance drugs doxorubicin, vincristine, and etoposide. This establishes MRP1 as a transporter for small hydrophobic peptides. More extensive structure-activity relationship studies should allow the identification of clinically useful peptide antagonists of MRP1.

Synthetic peptide vaccines: unexpected fulfillment of discarded hope?

In the early eighties it was realized that the ultimate vaccine would be a synthetic peptide. Major efforts were put into the development of a synthetic vaccine for foot-and-mouth disease virus (FMDV) for which even today no alternative exists besides the classical vaccine based on inactivated virus. Despite impressive progress, a peptide vaccine that could match the classical vaccine with respect to efficacy (i.e. full protection of all animals after a single vaccination) has not materialized. This has led to the belief that synthetic vaccines were not possible. However, in the early nineties we developed a synthetic peptide vaccine for canine parvovirus that did match the classical vaccine based on inactivated virus (i.e. protected all animals). Based on the difference of FMDV (an RNA virus) and canine parvovirus (a DNA virus), we suggested that in the case of FMDV, more than one antigenic site should be used, instead of the single one used previously. In our opinion multiple sites are necessary to prevent the development of escape mutants of FMDV. Unfortunately, the additional sites of FMDV are highly discontinuous. Until recently it was impossible to reconstruct these sites in the form of synthetic peptides. In the past few years, new methods have been developed that allow recombination of such sites into synthetic molecules. If successfully applied to FMDV, synthetic peptide vaccines and many others may become feasible in the near future. Moreover, the ability to mimic complex discontinuous sites by synthetic peptides will have a major impact on the rapidly developing area of therapeutic vaccines.

Mimotopes: realization of an unlikely concept.

Theoretically it seems highly unlikely that relatively small peptides could mimic functionally discontinuous epitopes of antigens. Nevertheless various recent reports show this to be the case. Peptide mimics of protein-, polysaccharide- and DNA-epitopes have been shown to be able to replace the native epitope. Moreover, some of them are able to induce, when used in a vaccine, antibodies with the same activity as that of the antibody used as a template. These mimics, called mimotopes, can be used in vaccines and diagnostics and can be developed more or less systematically using solely antibodies and random, semi-random and dedicated peptide arrays or libraries. Furthermore, the mimotope concept which seems to have proven itself for antibody and antigen interaction can be applied equally well to many receptor ligand interactions and thus may form a new generic approach to the development of drugs. Ltd.

The Mr 193,000 vault protein is up-regulated in multidrug-resistant cancer cell lines.

Vaults are 13 megadalton ribonucleoprotein particles composed largely of the major vault protein (MVP) and two high molecular weight proteins, p240 and p193, and a small vault RNA (vRNA). Increased levels of MVP expression, vault-associated vRNA, and vaults have been linked directly to multidrug resistance (MDR). To further define the putative role of vaults in MDR, we produced monoclonal antibodies against the Mr 193,000 vault protein and studied its expression levels in various multidrug-resistant cell lines. We find that, like MVP, p193 mRNA and protein levels are increased in various multidrug-resistant cell lines. Subcellular fractionation of vault particles revealed that vault-associated p193 levels are increased in multidrug-resistant cells as compared with the parental, drug-sensitive cells. Furthermore, protein analysis of postnuclear supernatants and co-immunoprecipitation studies show that drug-sensitive MVP-transfected tumor cells lack this up-regulation in vault-associated p193. Our observations indicate that vault formation is limited not only by the expression of the MVP but also by the expression or assembly of at least one of the other vault proteins.

In vitro inhibition of the bioactivity of follicle-stimulating hormone by antisera against a peptide representing part of the FSH-receptor.

The aim of the present work was to define an FSH receptor (FSHR) peptide that can induce antibodies that will inhibit the bioactivity of FSH. Therefore, the hFSHR sequence was aligned with that of all other known G-protein coupled receptors. An area with increased sequence homology was identified between the FSH-, LH-, TSH receptors, the C5a receptor and the IL8 receptor. The similarity consists of a richness in acidic (D and E) and hydrophobic (Y and F) residues. In hFSHR the sequence is EDNESSYSRGFDMTYTEFDYDLCNEVVD (amino acid 299-326). Research on both the C5a- and IL8-receptor has indicated that this part is responsible for hormone binding but not for signal transduction. Protamine. an antagonist for both the C5a- and IL8 receptor also inhibited the bioactivities of FSH and LH when tested in a bioassay. This suggests that in the hFSHR this region might also be involved in hormone binding. Specificity of this region towards the diverse ligands all binding to the C5a or to the IL8 receptor might be attributed to differences in the profile of alternating basic and hydrophobic residues. Therefore, the hypothesis was tested as to whether antisera raised against peptides of this FSHR-domain would inhibit FSH-bioactivity but not LH-bioactivity. Indeed antisera were found (anti-hFSHR 309-322) that inhibited the biological activity of FSH in a bioassay. These antisera proved to be specific since they did not inhibit the bioactivity of LH. These data suggest that the core sequence (hFSHR 309-322) of the aligned domain of the hFSHR, in analogy to the IL8- and C5a receptors, is involved in hormone binding and ligand specificity. This domain therefore forms a valuable tool in FSH- and FSHR research for scientific and medical purposes.

Identification of new tag sequences with differential and selective recognition properties for the anti-FLAG monoclonal antibodies M1, M2 and M5.

The FLAG peptides DYKDDDDK and MDYKDDDDK are widely used affinity tags. Here we describe new variants of the FLAG peptides which, in direct ELISA, showed selective and differential binding to the commercially available anti-FLAG monoclonal antibodies M1, M2 and M5. Variants of the FLAG peptides were synthesized on polymer-grafted plastic pins, and in an ELISA incubated with mAbs M1, M2 and M5. Among the newly identified tag sequences are those that bind only one of the anti-FLAG mAbs and those that bind only two or all three of the anti-FLAG mAbs. Examples of new tag sequences are MDFKDDDDK (which binds mAb M5 and does not bind mAbs M1 and M2) and MDYKAFDNL (which binds mAb M2 and does not bind mAbs M1 and M5). The sensitivity in direct ELISA of some variants was increased, e.g. using mAb M2 it was found that replacing DDDDK in MDYKDDDDK by AFDNL increased the sensitivity in ELISA at least 10-fold. The activity of this peptide was studied in more detail. In different direct ELISAs, in which MDYKAFDNL was synthesized on polyethylene pins, coated onto polystyrene microtiter plates or onto nitrocellulose paper, the activity of this peptide was similar, i.e. increased at least 10-fold over that of MDYKDDDDK. Remarkably, in competitive ELISA the binding activity of soluble MDYKAFDNL was decreased 10-fold over those of soluble MDYKDDDDK or DYKDDDDK. The results seem to suggest that, in solution, the conformation of MDYKAFDNL is more 'unstructured' compared to its conformation when coated or linked to a carrier. We postulate that the newly described tag sequences may be used as affinity tags to separately detect, quantify and purify multiple co-expressed proteins and/or subunits.

In vitro inhibition of the biological activity of follicle-stimulating hormone by anti-peptide antisera representing the human follicle-stimulating hormone beta subunit sequence 33-53.

There are few male contraceptive methods, and research is required to broaden the scope of available male antifertility methods. Two approaches toward hormonal contraception are currently being investigated. The first relies on elimination of testosterone while the second is based upon immunizations against FSH. However, most anti-whole FSH antisera cross-react with LH, thereby possibly inhibiting testosterone and leading to potential loss of libido. Therefore, a more effective alternative would be to define an FSH peptide that differs significantly from LH in order to prevent cross-reactivity between anti-FSH antisera and LH. Two peptides were selected from the beta subunit of FSH that were considered to be inducers of anti-FSH activity but not anti-LH activity. The first peptide (sequence beta33-53) is a linear antigenic site of human FSH found only in anti-FSH antisera that do not cross-react with LH. The second peptide (sequence beta81-95) is a part of FSH that confers receptor specificity. These peptides, in monomer and tandem form, were used to immunize rabbits. The antisera were tested for inhibition of FSH activity in a bioassay; they were also tested in a Leydig cell assay to detect anti-LH activity. It was found that antisera raised against the beta33-53 tandem could inhibit the FSH bioactivity but not that of LH. Antisera against the beta33-53 monomer or the beta81-95 monomer or tandem did not inhibit FSH. Thus, the tandem peptide beta33-53 is an attractive candidate for use as antigen in a male contraceptive vaccine. The better results obtained with tandem vaccinations might be related to the ability of the tandem peptide to direct the antibody response toward the N-terminal end of the peptide and to raise antisera with the ability to react with shorter chains of amino acids.

Screening of a small set of random peptides: a new strategy to identify synthetic peptides that mimic epitopes.

Small diversity libraries, composed of 4550 synthetic dodecapeptides and 8000 synthetic tripeptides, have been used to identify sequences homologous to small linear and non-linear parts of epitopes. Here we report that synthetic peptides identified through alignment of dodecapeptides and tripeptides derived from these small libraries have, in direct ELISA and/or competitive ELISA, activities similar to that of peptides covering the native epitope and similar to that of peptides derived from large expression libraries composed of 10(6)-10(7) random peptides. This result was obtained with the monoclonal antibodies 6A.A6 and M2. Mab 6A.A6 binds the transmissible gastroenteritis virus (TGEV) and mAb M2 binds the FLAG-peptide, an affinity tag. It was also found that the antibody binding activity of peptides, derived from small or large libraries, can strongly depend on the way in which the peptide is presented to the antibody, i.e. high antibody titers were obtained when these peptides were synthesized on pins or coated onto microtiter plates, whereas low IC50s were obtained with these peptides in solution. We postulate that small peptide libraries may be a powerful tool to quickly identify new peptides that can be used as sensitive markers for mAbs of interest.

Detection of epitopes on follicle-stimulating hormone and FSH-antiserum-induced suppression of bioactivity of follicle-stimulating hormone and luteinizing hormone.

There are currently two major approaches to hormonal male contraception. One relies on testosterone (analogs) either alone or in combination with gonadotropin releasing hormone (GnRH) (analogs or immunizations), the other on immunizations against follicle-stimulating hormone (FSH). Theoretically, the latter method will suppress spermatogenesis whilst not interfering with libido. An absolute requirement is, however, that an anti-FSH vaccine does not include anti-luteinizing hormone (LH) antibodies (LH being responsible for the induction of testosterone which is necessary to maintain libido). In this report we show that when whole FSH is used for vaccination, in most cases in addition to biological activity against FSH, anti-LH activity is also induced. By systematic analysis of the antisera raised with FSH using systematic epitope scanning (PEPSCAN) we found differences between the FSH-specific and FSH-nonspecific sera. Only the FSH-specific antiserum contained antibodies that recognized amino acid sequence 37-55 on the beta-subunit in a linear manner. Because antibodies against this epitope have not been found in the cross-reactive sera this epitope forms a prime candidate for an anti-FSH contraceptive vaccine.

Structural aspects of antibody-antigen interaction revealed through small random peptide libraries.

Two small random peptide libraries, one composed of 4550 dodecapeptides and one of 8000 tripeptides, were synthesized in newly developed credit-card format miniPEPSCAN cards (miniPEPSCAN libraries). Each peptide was synthesized in a discrete well (455 peptides/card). The two miniPEPSCAN libraries were screened with three different monoclonal antibodies (Mabs). Two other random peptide libraries, expressed on the wall of bacteria (recombinant libraries) and composed of 10(7) hexa- and octapeptides, were screened with the same three Mabs. The aim of this study was to compare the amino acid sequence of peptides selected from small and large pools of random peptides and, in this way, investigate the potential of small random peptide libraries. The screening of the two miniPEPSCAN libraries resulted in the identification of a surprisingly large number of antibody-binding peptides, while the screening of the large recombinant libraries, using the same Mabs, resulted in the identification of only a small number of peptides. The large number of peptides derived from the small random peptide libraries allowed the determination of consensus sequences. These consensus sequences could be related to small linear and nonlinear parts of the respective epitopes. The small number of peptides derived from the large random peptide libraries could only be related to linear epitopes that were previously mapped using small libraries of overlapping peptides covering the antigenic protein. Thus, with respect to the cost and speed of identifying peptides that resemble linear and nonlinear parts of epitopes, small diversity libraries based on synthetic peptides appear to be superior to large diversity libraries based on expression systems.

Possible active site of the sweet-tasting protein thaumatin.

Epitopes on thaumatin and monellin were studied using the PEPSCAN-technology. The antibodies used were raised against thaumatin. Only antibodies that, in an ELISA, both recognized thaumatin and monellin were used in the PEPSCAN-analyses. On thaumatin two major overlapping epitopes were identified. On monellin no epitopes could be identified. The identified epitope region on thaumatin shares structural features with various peptide and protein sweeteners. It contains an aspartame-like site which is formed by Asp21 and Phe80, tips of the two extruding loops KGDAALDAGGR19-29 and CKRFGRPP77-84, which are spatially positioned next to each other. Furthermore, sub-sequences of the KGDAALDAGGR19-29 loop are similar to peptide-sweeteners such as L-Asp-D-Ala-L-Ala-methyl ester and L-Asp-D-Ala-Gly-methyl ester. Since the aspartame-like Asp21-Phe80 site and the peptide-sweetener-like sequences are also not present in non-sweet thaumatin-like proteins it is postulated that the KGDAALDAGGR19-29- and CKRFGRPP77-84 loop contain important sweet-taste determinants. This region has previously not been implicated as a sweet-taste determinant of thaumatin.

Two receptor binding regions of human FSH show sense-antisense similarity to the human FSH receptor.

The sequences of two receptor binding regions of the beta-subunit of the human follicle-stimulating hormone (hFSH-beta) were compared with the DNA-derived antisense peptide sequence of the hFSH receptor. A striking sense-antisense similarity was established between these receptor binding regions and the hFSH receptor. Based on this sense-antisense similarity four putative hormone binding regions on the N-terminal extracellular region of the hFSH receptor are identified.

The bioelectrical activity of the body wall of the pulmonate freshwater snail Lymnaea stagnalis: effects of neurotransmitters and the sodium influx stimulating neuropeptides.

A method is described to dissect segments of the head skin of Lymnaea stagnalis. These skin segments were used to study the effects of neurotransmitters and of the Lymnaea sodium influx stimulating (SIS) peptides on ion transport, using the Ussing-cell technique. The electrical activity of the segments, with Lymnaea ringer solution at both sides, was low: the mean electrical potential difference (PD) across the skin was 0.7 +/- 0.6 mV (inside positive) at a resistance of 160 +/- 57 ohm.cm2 (inward short-circuit current, SCC, 4.2 +/- 3.0 microA/cm2; n = 25). Acetylcholine, adrenalin, noradrenalin, histamine, dopamine, and GABA, at 10(-5) M, did not affect skin resistance and PD. 5-Hydroxytryptamine-HCl (5-HT), however, in a dose-dependent way, increased the PD, SCC, and, to a much lesser extent, the resistance of skin segments. Extracts of the medium lip nerves, which contain the SIS peptides, had similar effects, but of much longer duration. Effects comparable to those of 5-HT and the SIS peptides could also be brought about by cAMP analogs. The inward current stimulation by 5-HT, SIS peptides, and cAMP was abolished by ouabain. The inward current induced by 5-HT and the SIS peptides was partly (10-25%) inhibited by amiloride. The presence of tetrodotoxin (10(-6) M) did not prevent the inward current stimulation by 5-HT and the SIS peptides.