Biochemical mechanisms of calcium mobilisation induced by mastoparan and chimeric hormone-mastoparan constructs

Pomiferin targets SERCA, mTOR, and P-gp to induce autophagic cell death in apoptosis-resistant cancer cells, and reverses the MDR phenotype in cisplatin-resistant tumors in vivo

Drug resistance in cancer has been classified as innate resistance or acquired resistance, which were characterized by apoptotic defects and ABC transporters overexpression respectively. Therefore, to preclude or reverse these resistance mechanisms could be a promising strategy to improve chemotherapeutic outcomes. In this study, a natural product from Osage Orange, pomiferin, was identified as a novel autophagy activator that circumvents innate resistance by triggering autophagic cell death via SERCA inhibition and activation of the CaMKKβ-AMPK-mTOR signaling cascade. In addition, pomiferin also directly inhibited the P-gp (MDR1/ABCB1) efflux and reversed acquired resistance by potentiating the accumulation and efficacy of the chemotherapeutic agent, cisplatin. In vivo study demonstrated that pomiferin triggered calcium-mediated tumor suppression and exhibited an anti-metastatic effect in the LLC-1 lung cancer-bearing mouse model. Moreover, as an adjuvant, pomiferin potentiated the anti-tumor effect of the chemotherapeutic agent, cisplatin, in RM-1 drug-resistant prostate cancer-bearing mouse model by specially attenuating ABCB1-mediated drug efflux, but not ABCC5, thereby promoting the accumulation of cisplatin in tumors. Collectively, pomiferin may serve as a novel effective agent for circumventing drug resistance in clinical applications.

Selective Inhibition of Plasmodium falciparum ATPase 6 by Artemisinins and Identification of New Classes of Inhibitors after Expression in Yeast

Treatment failures with artemisinin combination therapies (ACTs) threaten global efforts to eradicate malaria. They highlight the importance of identifying drug targets and new inhibitors and of studying how existing antimalarial classes work. Here, we report the successful development of a heterologous expression-based compound-screening tool. The validated drug target Plasmodium falciparum ATPase 6 (PfATP6) and a mammalian orthologue (sarco/endoplasmic reticulum calcium ATPase 1a [SERCA1a]) were functionally expressed in Saccharomyces cerevisiae, providing a robust, sensitive, and specific screening tool. Whole-cell and in vitro assays consistently demonstrated inhibition and labeling of PfATP6 by artemisinins. Mutations in PfATP6 resulted in fitness costs that were ameliorated in the presence of artemisinin derivatives when studied in the yeast model. As previously hypothesized, PfATP6 is a target of artemisinins. Mammalian SERCA1a can be mutated to become more susceptible to artemisinins. The inexpensive, low-technology yeast screening platform has identified unrelated classes of druggable PfATP6 inhibitors. Resistance to artemisinins may depend on mechanisms that can concomitantly address multitargeting by artemisinins and fitness costs of mutations that reduce artemisinin susceptibility.

Sarco/Endoplasmic Reticulum Calcium ATPase Inhibitors: Beyond Anticancer Perspective

The sarco/endoplasmic reticulum calcium ATPase (SERCA), which plays a key role in the maintenance of Ca²⁺ ion homeostasis, is an extensively studied enzyme, the inhibition of which has a considerable impact on cell life and death decision. To date, several SERCA inhibitors have been thoroughly studied and the most notable one, a derivative of the sesquiterpene lactone thapsigargin, is gradually approaching a clinical application. Meanwhile, new compounds with SERCA-inhibiting properties of natural, synthetic, or semisynthetic origin are being discovered and/or developed; some of these might also be suitable for the development of new drugs with improved performance. This review brings an up-to-date comprehensive overview of recently discovered compounds with the potential of SERCA inhibition, discusses their mechanism of action, and highlights their potential clinical applications, such as cancer treatment.

The cationic tetradecapeptide mastoparan as a privileged structure for drug discovery: Enhanced antimicrobial properties of mitoparan analogues modified at position-14

Mastoparan (MP) peptides, distributed in insect venoms, induce a local inflammatory response post envenomation. Most endogenous MPs share common structural elements within a tetradecapeptide sequence that adopts an amphipathic helix whilst traversing biological membranes and when bound to an intracellular protein target. Rational modifications to increase cationic charge density and amphipathic helicity engineered mitoparan (MitP), a mitochondriotoxic bioportide and potent secretagogue. Following intracellular translocation, MitP is accreted by mitochondria thus indicating additional utility as an antimicrobial agent. Hence, the objectives of this study were to compare the antimicrobial activities of a structurally diverse set of cationic cell penetrating peptides, including both MP and MitP sequences, and to chemically engineer analogues of MitP for potential therapeutic applications. Herein, we confirm that, like MP, MitP is a privileged structure for the development of antimicrobial peptides active against both prokaryotic and eukaryotic pathogens. Collectively, MitP and target-selective chimeric analogues are broad spectrum antibiotics, with the Gram-negative A. baumannii demonstrating particular susceptibility. Modifications of MitP by amino acid substitution at position-14 produced peptides, Δ14MitP analogues, with unique pharmacodynamic properties. One example, [Ser¹⁴]MitP, lacks both cytotoxicity against human cell lines and mast cell secretory activity yet retains selective activity against the encapsulated yeast C. neoformans.

(Z)3,4,5,4'-trans-tetramethoxystilbene, a new analogue of resveratrol, inhibits gefitinb-resistant non-small cell lung cancer via selectively elevating intracellular calcium level

Calcium is a second messenger which is required for regulation of many cellular processes. However, excessive elevation or prolonged activation of calcium signaling would lead to cell death. As such, selectively regulating calcium signaling could be an alternative approach for anti-cancer therapy. Recently, we have identified an effective analogue of resveratrol, (Z)3,4,5,4′-trans-tetramethoxystilbene (TMS) which selectively elevated the intracellular calcium level in gefitinib-resistant (G-R) non-small-cell lung cancer (NSCLC) cells. TMS exhibited significant inhibitory effect on G-R NSCLC cells, but not other NSCLC cells and normal lung epithelial cells. The phosphorylation and activation of EGFR were inhibited by TMS in G-R cells. TMS induced caspase-independent apoptosis and autophagy by directly binding to SERCA and causing endoplasmic reticulum (ER) stress and AMPK activation. Proteomics analysis also further confirmed that mTOR pathway, which is the downstream of AMPK, was significantly suppressed by TMS. JNK, the cross-linker of ER stress and mTOR pathway was significantly activated by TMS. In addition, the inhibition of JNK activation can partially block the effect of TMS. Taken together, TMS showed promising anti-cancer activity by mediating calcium signaling pathway and inducing apoptosis as well as autophagy in G-R NSCLC cells, providing strategy in designing multi-targeting drug for treating G-R patients.

A diversity of SERCA Ca2+ pump inhibitors

The SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) is probably the most extensively studied membrane protein transporter. There is a vast array of diverse inhibitors for the Ca2+ pump, and many have proved significant in helping to elucidate both the mechanism of transport and gaining conformational structures. Some SERCA inhibitors such as thapsigargin have been used extensively as pharmacological tools to probe the roles of Ca2+ stores in Ca2+ signalling processes. Furthermore, some inhibitors have been implicated in the cause of diseases associated with endocrine disruption by environmental pollutants, whereas others are being developed as potential anticancer agents. The present review therefore aims to highlight some of the wide range of chemically diverse inhibitors that are known, their mechanisms of action and their binding location on the Ca2+ ATPase. Additionally, some ideas for the future development of more useful isoform-specific inhibitors and anticancer drugs are presented.

Alisol B, a novel inhibitor of the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase pump, induces autophagy, endoplasmic reticulum stress, and apoptosis

Emerging evidence suggests that autophagic modulators have therapeutic potential. This study aims to identify novel autophagic inducers from traditional Chinese medicinal herbs as potential antitumor agents. Using an image-based screen and bioactivity-guided purification, we identified alisol B 23-acetate, alisol A 24-acetate, and alisol B from the rhizome of Alisma orientale as novel inducers of autophagy, with alisol B being the most potent natural product. Across several cancer cell lines, we showed that alisol B-treated cells displayed an increase of autophagic flux and formation of autophagosomes, leading to cell cycle arrest at the G(1) phase and cell death. Alisol B induced calcium mobilization from internal stores, leading to autophagy through the activation of the CaMKK-AMPK-mammalian target of rapamycin pathway. Moreover, the disruption of calcium homeostasis induces endoplasmic reticulum stress and unfolded protein responses in alisol B-treated cells, leading to apoptotic cell death. Finally, by computational virtual docking analysis and biochemical assays, we showed that the molecular target of alisol B is the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase. This study provides detailed insights into the cytotoxic mechanism of a novel antitumor compound.

The widely utilized brominated flame retardant tetrabromobisphenol A (TBBPA) is a potent inhibitor of the SERCA Ca2+ pump

TBBPA (tetrabromobisphenol A) is currently the most widely used type of BFR (brominated flame retardant) employed to reduce the combustibility of a large variety of electronic and other manufactured products. Recent studies have indicated that BFRs, including TBBPA, are bio-accumulating within animal and humans. BFRs including TBBPA have also been shown to be cytotoxic and potentially endocrine-disrupting to a variety of cells in culture. Furthermore, TBBPA has specifically been shown to cause disruption of Ca2+ homoeostasis within cells, which may be the underlying cause of its cytotoxicity. In this study, we have demonstrated that TBBPA is a potent non-isoform-specific inhibitor of the SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) (apparent K(i) 0.46-2.3 microM), thus we propose that TBBPA inhibition of SERCA contributes in some degree to Ca2+ signalling disruption. TBBPA binds directly to the SERCA without the need to partition into the phospholipid bilayer. From activity results and Ca2+-induced conformational results, it appears that the major effect of TBBPA is to decrease the SERCA affinity for Ca2+ (increasing the K(d) from approx. 1 microM to 30 microM in the presence of 10 microM TBBPA). Low concentrations of TBBPA can quench the tryptophan fluorescence of the SERCA and this quenching can be reversed by BHQ [2,5-di-(t-butyl)-1,4-hydroquinone] and 4-n-nonylphenol, but not thapsigargin, indicating that TBBPA and BHQ may be binding to similar regions in the SERCA.

Myotoxic effects of mastoparan from Polybia paulista (Hymenoptera, Epiponini) wasp venom in mice skeletal muscle

In a previous study, we showed that the Polybia paulista wasp venom causes strong myonecrosis. This study was undertaken to characterize the myotoxic potency of mastoparan (Polybia-MPII) isolated from venom (0.25 microg/microl) and injected in the tibial anterior (TA) muscle (i.m.) of Balb/c mice. The time course of the changes was followed at muscle degenerative (3 and 24h) and regenerative (3, 7, and 21 days) periods (n=6) after injection and compared to matched controls by calculation of the percentage of cross-sectional area affected and determination of creatine kinase (CK) activity (n=10). The results showed that although MP was strongly myotoxic, its capacity for regeneration was maintained high. Since the extent of tissue damage was not correlated with the CK serum levels, which remained very low, we raised the hypothesis that the enzyme underwent denaturation by the peptide. Evidence suggested that MP induced the death of TA fibers by necrosis and apoptosis and had the sarcolemma as its primordial target. Given its amphiphilic polycationic nature and based on the vast spectrum of functions attributed to the peptide, we suggest that MP interaction with cell membrane impaired the phosphorylation of dystrophin essential for sarcolemma mechanical stability, and disturbed Ca2+ mobilization with obvious implications on sarcoplasmic reticulum and mitochondrial functioning.

The interaction of the brominated flame retardant: Tetrabromobisphenol A with phospholipid membranes

Tetrabromobisphenol A (TBBPA) is one of the most widely used members of the family of brominated flame retardants (BFRs). BFRs, including TBBPA have been shown to be widely distributed within the environment and there is growing evidence of their bio-accumulation within animals and man. Toxicological studies have shown that TBBPA can be harmful to cells by modulating a number of cell signalling processes. In this study, we employed fluorescence spectroscopy and differential scanning calorimetry to investigate the interaction of TBBPA with phospholipid membranes, as this is the most likely route for it to influence membrane-associated cellular processes. TBBPA readily and randomly partitions throughout all regions of the phospholipid bilayer with high efficacy [partition coefficient (Log K(p))=5.7+/-0.7]. A decrease in membrane fluidity in both liquid-crystalline and gel-phase membranes was detected at concentrations of TBBPA as low as 2.5 microM. TBBPA also decreases the phase transition temperature of dipalmitoyl phoshatidylcholine (DPPC) membranes and broadened transition peaks, in a fashion similar to that for cholesterol. TBBPA, however, also prefers to partition into membrane regions not too highly enriched with cholesterol. Our findings therefore suggests that, the toxic effects of TBBPA, may at least in part, be due to its lipid membrane binding/perturbing effects, which in turn, could influence biological processes involving cell membranes.

The effects of the phenylalanine 256 to valine mutation on the sensitivity of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) Ca2+ pump isoforms 1, 2, and 3 to thapsigargin and other inhibitors

Three isoforms of the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) are known to exist in mammalian cells. This study investigated the effects of thapsigargin and a variety of commonly used hydrophobic inhibitors on these SERCA isoforms (i.e. SERCA1b, SERCA2b, and SERCA3a), which were transiently expressed in COS-7 cells. In addition, the study assessed whether the introduction of the phenylalanine to valine mutation at position 256 (F256V), known to reduce the potency of thapsigargin inhibition in avian SERCA1, affects the other SERCA isoforms in a similar manner and whether this mutation also affects the inhibition by other inhibitors. This study has shown that the sensitivity to thapsigargin is different for the SERCA isoforms (apparent K(i) values being 0.21, 1.3, and 12 nm for SERCA1b, SERCA2b, and SERCA3a, respectively). The reduction in thapsigargin sensitivity caused by the F256V mutation was also different for the three isoforms, with SERCA2b only being modestly affected by this mutation. Although some of the other inhibitors investigated (i.e. cyclopiazonic acid and curcumin) showed some differences in their sensitivity toward the SERCA isoforms, most were little affected by the F256V mutation, indicating that they inhibit the Ca(2+)-ATPase by binding to sites on SERCA distinct from that of thapsigargin.

Localization of intracellular calcium release in cells injured by venom from the ectoparasitoid Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) and dependence of calcium mobilization on G-protein activation

Venom from the ectoparasitic wasp Nasonia vitripennis induces cellular injury that appears to involve the release of intracellular calcium stores via the activation of phospholipase C, and culminates in oncotic death. A linkage between release of intracellular Ca2+ and oncosis has not been clearly established and was the focus of this study. When BTI-TN-5B1-4 cells were treated with suramin, an uncoupler of G-proteins, venom-induced swelling and oncotic death were inhibited in a dose-dependent manner for at least 24 h. Suramin also blocked increases in free cytosolic [Ca2+], arguing that venom induces calcium mobilization through G-protein signaling pathways. Endoplasmic reticulum (ER) was predicted to be the source of intracellular calcium release, but labeling with the fluorescent probe ER-tracker revealed no indication of organelle swelling or loss of membrane integrity as would be expected if the Ca(2+)-ATPase pump was disabled by crude venom. Incubation of cell monolayers with calmodulin or nitrendipine, modulators of ER calcium release channels, neither attenuated nor augmented the effects of wasp venom. These results suggest that wasp venom stimulates calcium release from ER compartments distinct from RyRs, L-type Ca2+ channels, and the Ca(2+)-ATPase pump, or calcium is released from some other intracellular store. A reduction of mitochondrial membrane potential delta psi(m) appeared to precede a rise in cytosolic free Ca2+ as evidenced by fluorescent microscopy using the calcium-sensitive probe fluo-4 AM. This argues that the initial insult to the cell resulting from venom elicits a rapid loss of (delta psi(m)), followed by unregulated calcium efflux from mitochondria into the cytosol. Mobilization of calcium in this fashion could stimulate cAMP formation, and subsequently promote calcium release from NAADP-sensitive stores.

The expression, activity and localisation of the secretory pathway Ca2+ -ATPase (SPCA1) in different mammalian tissues

The distribution of the secretory pathway Ca2+ -ATPase (SPCA1) was investigated at both the mRNA and protein level in a variety of tissues. The mRNA and the protein for SPCA1 were relatively abundant in rat brain, testis and testicular derived cells (myoid cells, germ cells, primary Sertoli cells and TM4 cells; a mouse Sertoli cell line) and epididymal fat pads. Lower levels were found in aorta (rat and porcine), heart, liver, lung and kidney. SPCA activities from a number of tissues were measured and shown to be particularly high in brain, aorta, heart, fat pads and testis. As the proportion of SPCA activity compared to total Ca2+ ATPase activity in brain, aorta, fat pads and testis were relatively high, this suggests that SPCA1 plays a major role in Ca2+ storage within these tissues. The subcellular localisation of SPCA1 was shown to be predominantly around the Golgi in both human aortic smooth muscle cells and TM4 cells.

Charge delocalisation and the design of novel mastoparan analogues: enhanced cytotoxicity and secretory efficacy of [Lys5, Lys8, Aib10]MP

The formation of an amphipathic helix is a major determinant of the biological activity of the tetradecapeptide mastoparan (MP). To address the functional significance of lysyl residues at positions 4, 11 and 12 of MP, we synthesised five novel analogues using sequence permutation and arginine-substitution to delocalise cationic charge. Comparative bioassays determined cytotoxicity, beta-hexoseaminidase secretory efficacy and peptide-activated extracellular receptor-stimulated kinase (ERK)1/2 phosphorylation. The monosubstitution of individual lysine residues with arginine produced differential changes to the indices of cytotoxicity and secretion indicating that these conservative substitutions are compatible with membrane translocation and the selective binding and activation of intracellular proteins. More profound changes to the predicted hydrophilic face of MP, resulting from the relocation or substitution of additional lysyl residues, enhanced both the cytotoxicity and secretory efficacy of novel peptides. Significantly, the more amphipathic peptide [Lys5, Lys8, Aib10]MP was identified to be both the most cytotoxic and the most potent secretagogue of all the peptides compared here. Charge delocalisation within the hydrophilic face of MP analogues was also compatible with peptide-induced activation of ERK1/2 phosphorylation. Our data indicate that charge delocalisation is a suitable strategy to engineer more potent analogues of MP that differentially target intracellular proteins.

Mastoparan binds to glycogen phosphorylase to regulate sarcoplasmic reticular Ca2+ release in skeletal muscle

The ryanodine receptor, a Ca(2+)-releasing channel in sarcoplasmic reticulum (SR), plays an important role in the excitation-contraction coupling of skeletal muscle. In a previous study [Hirata, Nakahata and Ohizumi (2000) Mol. Pharmacol. 57, 1235-1242], we reported that mastoparan caused Ca(2+) release through ryanodine receptor from the heavy fraction of SR (HSR) isolated from rabbit skeletal muscle, and that it specifically bound to a 97 kDa protein which was distinct from Ca(2+)-pump or triadin. The present study was undertaken to identify and characterize the 97 kDa mastoparan-binding protein. The 97 kDa protein was purified from solubilized HSR by DEAE-Sepharose column chromatography and preparative SDS/PAGE. The partial amino acid sequence of the purified 97 kDa protein was matched with that of glycogen phosphorylase (GP). The proteolytic cleavage pattern of the 97 kDa protein was identical with that of GP. Furthermore, [(125)I-Tyr(3)]mastoparan specifically bound to GP. Interestingly, mastoparan-induced Ca(2+) release was inhibited by exogenous addition of GP-a, and mastoparan dissociated GP from HSR. These results indicate that the 97 kDa mastoparan-binding protein is GP, which negatively regulates Ca(2+) release from HSR. There may be a functional cross-talk between Ca(2+) release from HSR and glycogenolysis for energy supply mediated through GP in skeletal muscles.

Inhibition of the type 1 inositol 1,4,5-trisphosphate receptor by 2-aminoethoxydiphenylborate

2-Aminoethoxydiphenylborate (2-APB) inhibits the extent of inositol 1,4,5-trisphosphate (InsP(3))-induced Ca(2+) release from cerebellar microsomes with a potency that is dependent upon the InsP(3) concentration used. At high InsP(3) concentrations (10 microM), the concentration of 2-APB required to cause half-maximal InsP(3)-induced Ca(2+) release (IC(50)) was greater than 1 mM, while at 0.25 microM InsP(3) this reduced to 220 microM. The fact that the inhibition of the extent of InsP(3)-induced Ca(2+) release (IICR) by 2-APB was not restored to control levels by high concentrations of InsP(3), in addition to the fact 2-APB did not substantially inhibit [3H]InsP(3) binding to its receptor, indicates that the inhibition is not competitive in nature. Since the cooperativity of IICR as a function of InsP(3) was reduced in the presence of 2-APB (Hill coefficient changing from 1.9 in the absence of 2-APB to 1.4 in the presence of 1 mM 2-APB), this suggests that it is acting as an allosteric inhibitor. 2-APB also reduces the rate constants for IICR. In cerebellar microsomes this release process is biphasic in nature, with a fast and slow phase. 2-APB appears particularly to affect the fast-phase component. Although 2-APB does not inhibit the ryanodine receptor, it does inhibit the Ca(2+) ATPase activity as well store-operated Ca(2+) entry channels, which may limit its use as a specific membrane permeant InsP(3) receptor inhibitor.

The mechanism of inhibition of the sarco/endoplasmic reticulum Ca2+ ATPase by paxilline

Paxilline, an indole alkaloid mycotoxin from Penicillium paxilli, is an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA). Paxilline inhibited differing isoforms of SERCA with IC50s between 5 and 50 microM. It inhibited more potently the purified Ca2+ ATPase activity from skeletal muscle with an IC50 of 5 microM. Detailed effects of this inhibitor on the Ca2+ and ATP dependence upon activity indicate that it affects the high-affinity Ca2+-binding (E1) form of the ATPase. In addition, paxilline is a "competitive" inhibitor with respect to high concentrations of ATP, increasing the regulatory binding site K(m), without affecting the catalytic binding site K(m). At higher concentrations, paxilline inhibits phosphoenzyme formation from ATP and inorganic phosphate, without affecting nucleotide binding. We therefore suggest that paxilline has two effects on the Ca2+ ATPase. At lower concentrations (5-10 microM), paxilline inhibits the ATP-dependent acceleration of Ca2+ release from the phosphoenzyme and/or phosphoenzyme decay. At higher concentrations, paxilline inhibits phosphoenzyme formation.

The inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase by macrocyclic lactones and cyclosporin A

The pharmacology of macrocyclic lactones is varied, with many beneficial effects in treating disease processes. FK-506, rapamycin and ascomycin have been utilized as immunosuppressant agents. Ivermectin is typically used to treat parasitic worm infections in mammals. Another immunosuppressant, cyclosporin A, is a cyclic oligotide that has similar immunosuppressant properties to those exerted by macrocyclic lactones. Here we report on the inhibition by these compounds of sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase (SERCA) Ca(2+) pumps. Ivermectin, cyclosporin A and rapamycin all inhibited the skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1). In addition, although ivermectin inhibited brain microsomal endoplasmic reticulum (type 2b) Ca(2+)-ATPase, cyclosporin A and rapamycin did not. As cyclosporin A also did not inhibit cardiac Ca(2+)-ATPase activity, this would suggest that it could be an isoform-specific inhibitor. Ivermectin was shown to be the most potent Ca(2+)-ATPase inhibitor of the macrocyclic lactones (IC(50)=7 microM). It appears to show a 'competitive' inhibition with respect to high concentrations of ATP by increasing the regulatory binding site K(m) but without affecting the catalytic site K(m). In addition, ivermectin stabilizes the ATPase in an E1 conformational state, and inhibits Ca(2+) release from the enzyme during turnover. This would suggest that ivermectin inhibits Ca(2+) release from the luminal binding sites of the phosphoenzyme intermediate, a step that is known to be accelerated by high [ATP].

Inhibition of SERCA Ca2+ pumps by 2-aminoethoxydiphenyl borate (2-APB). 2-APB reduces both Ca2+ binding and phosphoryl transfer from ATP, by interfering with the pathway leading to the Ca2+-binding sites

2-Aminoethoxydiphenyl Borate (2-APB) has been extensively used recently as a membrane permeable modulator of inositol-1,4,5-trisphosphate-sensitive Ca2+ channels and store-operated Ca2+ entry. Here, we report that 2-APB is also an inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ pumps, and additionally increases ion leakage across the phospholipid bilayer. Therefore, we advise caution in the interpretation of results when used in Ca2+ signalling experiments. The inhibition of 2-APB on the SERCA Ca2+ pumps is isoform-dependent, with SERCA 2B being more sensitive than SERCA 1A (IC50 values for inhibition being 325 and 725 micro m, respectively, measured at pH 7.2). The Ca2+-ATPase is also more potently inhibited at lower pH (IC50 = 70 micro m for SERCA1A at pH 6). 2-APB decreases the affinity for Ca2+ binding to the ATPase by more than 20-fold, and also inhibits phosphoryl transfer from ATP (by 35%), without inhibiting nucleotide binding. Activity studies performed using mutant Ca2+-ATPases show that Tyr837 is critical for the inhibition of activity by 2-APB. Molecular modeling studies of 2-APB binding to the Ca2+ ATPase identified two potential binding sites close to this residue, near or between transmembrane helices M3, M4, M5 and M7. The binding of 2-APB to these sites could influence the movement of the loop between M6 and M7 (L6-7), and reduce access of Ca2+ to their binding sites.

Novel mastoparan analogs induce differential secretion from mast cells

Cationic amphiphilic peptides stimulate secretion via a receptor-independent action upon G proteins. We have previously utilized chimeric analogs of mastoparan (MP), including galparan (galanin(1-13)-MP ), as molecular probes of secretion. Here, we further resolve the structure-activity relationship of peptidyl secretagogs, including rationally designed chimeric MP analogs. The secretory efficacies of 10 MP analogs were significantly higher than 45 unrelated basic peptides. Comparative studies identified MP analogs that are differential secretagogs for 5-hydroxytryptamine (5-HT) and beta-hexosaminidase. Peptide-induced activation of phospholipase D (PLD), an enzyme intimately involved in regulated exocytosis [5], correlated with the secretion of beta-hexosaminidase but not 5-HT. Thus, these data indicate that different mechanisms are responsible for the exocytosis of 5-HT and beta-hexosaminidase, respectively. Moreover, mastoparan analogs are novel tools for probing the molecular details of exocytosis and other biological phenomena.

Curcumin: a new cell-permeant inhibitor of the inositol 1,4,5-trisphosphate receptor

Curcumin (diferuoylmethane or 1,7-bis (4-hydroxy-3-methoxyphenol)-1,6-hepatadiene-3,5-dione) is the active ingredient of the spice turmeric. Curcumin has been shown to have a number of pharmacological and therapeutic uses. This study shows that curcumin is a potent inhibitor of the inositol 1,4,5-trisphosphate-sensitive Ca2+ channel (InsP3 receptor). In porcine cerebellar microsomes, the extent of InsP3-induced Ca2+ release (IICR) is almost completely inhibited by 50 microM curcumin (IC50 = 10 microM). As the extent of IICR cannot be restored back to control levels by the addition of excess InsP3 and since it has little effect on [3H]InsP3 binding to cerebellar microsomes, this inhibition is likely to be non-competitive in nature. IICR in cerebellar microsomes is biphasic consisting of a fast and slow component. The rate constants for the two components are both reduced by curcumin to similar extents (by about 70% of control values at 40 microM curcumin). In addition, curcumin also reduces agonist (ATP)-stimulated Ca2+ mobilization from intact HL-60 cells, indicating that curcumin is cell permeant. However, since it also affects intracellular Ca2+ pumps and possibly ryanodine receptors, it may lead to complex Ca2+ transient responses within cells, which may well explain some of its putative therapeutic properties.

Inhibition of the SERCA Ca2+ pumps by curcumin. Curcumin putatively stabilizes the interaction between the nucleotide-binding and phosphorylation domains in the absence of ATP

Curcumin is a compound derived from the spice, tumeric. It is a potent inhibitor of the SERCA Ca2+ pumps (all isoforms), inhibiting Ca2+-dependent ATPase activity with IC50 values of between 7 and 15 microm. It also inhibits ATP-dependent Ca2+-uptake in a variety of microsomal membranes, although for cerebellar and platelet microsomes, a stimulation in Ca2+ uptake is observed at low curcumin concentrations (<10 microm). For the skeletal muscle isoform of the Ca2+ pump (SERCA1), the inhibition of curcumin is noncompetitive with respect to Ca2+, and competitive with respect to ATP at high curcumin concentrations ( approximately 10-25 microm). This was confirmed by ATP binding studies that showed inhibition in the presence of curcumin: ATP-dependent phosphorylation was also reduced. Experiments with fluorescein 5'-isothiocyanate (FITC)-labelled ATPase also suggest that curcumin stabilizes the E1 conformational state. The fact that FITC labels the nucleotide binding site of the ATPase (precluding ATP from binding), and the fact that curcumin affects FITC fluorescence indicate that curcumin must be binding to another site within the ATPase that induces a conformational change to prevent ATP from binding. This observation is interpreted, with the aid of recent structural information, as curcumin stabilizing the interaction between the nucleotide-binding and phosphorylation domains, precluding ATP binding.

Estrogenic alkylphenols induce cell death by inhibiting testis endoplasmic reticulum Ca(2+) pumps

Industrial alkylphenols in the environment may act as "xenoestrogens" to disrupt testicular development and decrease male fertility. Amongst possible targets for these compounds are testicular Sertoli cells, which nurture the developing sperm cells. We demonstrate that SERCA 2 and 3 Ca(2+) pumps are relatively abundant in rat testis microsomal membranes, and also in Sertoli, myoid, and TM4 cells (a Sertoli cell line). A number of estrogenic alkylphenols such as nonylphenol, octylphenol, bisphenol A, and butylated hydroxytoluene all inhibit testicular Ca(2+) ATPase in the low micromolar concentration range. These agents also mobilize intracellular Ca(2+) in intact TM4 cells in a manner consistent with the inhibition of ER Ca(2+) pumps. Alkylphenols dramatically decrease the viability of TM4 cells, an effect that is reversed by either a caspase inhibitor or by BAPTA, and is therefore consistent with Ca(2+)-dependent cell death via apoptosis. We postulate that alkylphenols disrupt testicular development by inhibiting ER Ca(2+) pumps, thus disturbing testicular Ca(2+) homeostasis.

Himehabu lectin, a novel inducer of Ca2+-release from the venom of the snake Trimeresurus okinavensis, in sarcoplasmic reticulum

The lectin himehabu lectin (HHL) has recently been isolated from crude venom of the snake Trimeresurus okinavensis. Ca2+ -electrode and fluorescent Ca2+ -indicator experiments showed that HHL induced release of Ca2+ from the heavy fraction of skeletal muscle sarcoplasmic reticulum (HSR). The release of Ca2+ induced by caffeine from HSR was abolished by ryanodine, Mg2+ and ruthenium red, typical inhibitors of Ca2+ -release channels, whereas that induced by HHL was only partially reduced by these inhibitors. HHL, unlike caffeine, had no effect on [3H]ryanodine binding to HSR. These results suggest that HHL induces release of Ca2+ which is at least partially mediated through Ca2+ -release channels with novel pharmacological properties.

Effects of vasopressin-mastoparan chimeric peptides on insulin release and G-protein activity

Two chimeric peptides, consisting of the linear vasopressin receptor V1 antagonist PhAc-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr, in the N-terminus and mastoparan in the C-terminus connected directly (M375) or via 6-aminohexanoic acid (M391), have been synthesised. At 10 microM concentration, these novel peptides increased insulin secretion from isolated rat pancreatic islet cells 18-26-fold at 3.3 mM glucose and 3.5-5-fold at 16.7 mM glucose. PTX pretreatment of the islets decreased the peptide-induced insulin release. M375 and M391 bind to V1a vasopressin receptors with affinities lower than the unmodified vasopressin antagonist, but with K(D) values of 3.76 nM and 9.02 nM, respectively, both chimeras are high affinity ligands. The GTPase activity and GTPgammaS binding in the presence of these peptides has been characterised in Rin m5F cells. Comparison of the influence of the peptides M375 and M391 on GTPase activity in native and pertussis toxin-treated cells suggests a selective activation of G alpha(i)/G alpha(o) subunits, combined with a suppression of other GTPases, primarily G alpha(s). However, the GTPgammaS binding data show that the peptides retain some of the activating property even in PTX-treated cell membranes. In conclusion, the conjugation of mastoparan with the V1a receptor antagonists produce peptides with properties different from the parent peptides that could be used to elucidate the role of different G proteins in insulin release.

The mechanism of inhibition of the Ca2+-ATPase by mastoparan. Mastoparan abolishes cooperative ca2+ binding

The amphiphilic peptide mastoparan, isolated from wasp venom, is a potent inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. At pH 7. 2, ATPase activity is inhibited with an inhibitory constant (Ki) of 1 +/- 0.13 microM. Mastoparan shifts the E2-E1 equilibrium toward E1 and may affect the regulatory ATP binding site. The peptide also decreases the affinity of the ATPase for Ca2+ and abolishes the cooperativity of Ca2+ binding. In the presence of mastoparan, the two Ca2+ ions bind independently of one another. Our results appear to support the model that describes the relationship between the two Ca2+ binding sites as "side-by-side," because this model allows the possibility of independent Ca2+ entry to the two sites. Mastoparan shifts the steady-state equilibrium between E1'Ca2 and E1'Ca2.P toward E1'Ca2.P, by possibly affecting the conformational change that follows ATP binding. The peptide also causes a reduction in the levels of phosphoenzyme formed from [32P]Pi. Some analogues of mastoparan were also tested and were found to cause inhibition of the Ca2+-ATPase in the range of 2-4 microM. The inhibitory action of mastoparan and its analogues appears dependent on their ability to form alpha-helices in membranes.