Interaction of the Glycoalkaloid Tomatine with DMPC and Sterol Monolayers Studied by Surface Pressure Measurements and Brewster Angle Microscopy

Atomic force microscopy study of the complexation of sterols and the glycoalkaloid α-tomatine in Langmuir-Blodgett monolayers

Glycoalkaloids are secondary metabolites produced by plants that aid in their protection from pathogens and pests. They are known to form 1:1 complexes with 3β-hydroxysterols such as cholesterol causing membrane disruption. So far, the visual evidence showcasing the complexes formed between glycoalkaloids and sterols in monolayers has been mainly restricted to some earlier studies using Brewster angle microscopy which were of low resolution showing the formation of floating aggregates of these complexes. This study is aimed at using atomic force microscopy (AFM) for topographic and morphological analysis of the aggregates of these sterol-glycoalkaloid complexes. Langmuir-Blodgett (LB) transfer of mixed monolayers of the glycoalkaloid α-tomatine, sterols, and lipids in varying molar ratios onto mica followed by AFM examination was performed. The AFM method allowed visualization of the aggregation of sterol-glycoalkaloid complexes at nanometer resolution. While aggregation was observed in mixed monolayers of α-tomatine with cholesterol and in mixed monolayers with coprostanol, no sign of complexation was observed for the mixed monolayers of epicholesterol and α-tomatine, confirming their lack of interaction found in prior monolayer studies. Aggregates were observed in transferred monolayers of ternary mixtures of α-tomatine with cholesterol and the phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or egg sphingomyelin (egg SM). The formation of aggregates was found to be less prevalent for mixed monolayers of DMPC and cholesterol containing α-tomatine than it was for mixed monolayers containing egg SM and cholesterol with α-tomatine. The observed aggregates were generally elongated structures, of a width ranging from about 40-70 nm.

The steroidal alkaloids α-tomatine and tomatidine: Panorama of their mode of action and pharmacological properties

The steroidal glycoalkaloid α-tomatine (αTM) and its aglycone tomatidine (TD) are abundant in the skin of unripe green tomato and present in tomato leaves and flowers. They mainly serve as defensive agents to protect the plant against infections by insects, bacteria, parasites, viruses, and fungi. In addition, the two products display a range of pharmacological properties potentially useful to treat various human diseases. We have analyzed all known pharmacological activities of αTM and TD, and the corresponding molecular targets and pathways impacted by these two steroidal alkaloids. In experimental models, αTM displays anticancer effects, particularly strong against androgen-independent prostate cancer, as well as robust antifungal effects. αTM is a potent cholesterol binder, useful as a vaccine adjuvant to improve delivery of protein antigens or therapeutic oligonucleotides. TD is a much less cytotoxic compound, able to restrict the spread of certain viruses (such as dengue, chikungunya and porcine epidemic diarrhea viruses) and to provide cardio and neuro-protective effects toward human cells. Both αTM and TD exhibit marked anti-inflammatory activities. They proceed through multiple signaling pathways and protein targets, including the sterol C24 methyltransferase Erg6 and vitamin D receptor, both directly targeted by TD. αTM is a powerful regulator of the NFkB/ERK signaling pathway implicated in various diseases. Collectively, the analysis shed light on the multitargeted action of αTM/TD and their usefulness as chemo-preventive or chemotherapeutic agents. A novel medicinal application for αTM is proposed.

Incorporation of simvastatin into lipid membranes: Why deliver a statin in form of inclusion complex with hydrophilic cyclodextrin

In this work, the effects of simvastatin (SIM), (2-hydroxypropyl)-β-cyclodextrin (HPβCD) and their complex (SIM:HPβCD) on the structure and properties of lipid membranes were investigated for the first time by Langmuir technique combined with PM-IRRAS spectroscopy. An improved understanding of the differences of the interactions between free SIM, and SIM in the form of an inclusion complex with HPβCD with the lipid membrane will improve the development of preparation methods for in vivo applications. Monolayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), cholesterol (Chol) and their mixture DMPC:Chol (7:3) served as simple models of one leaflet of the cell membrane. The penetration of well-organized lipid layers by simvastatin lead to their fluidization but the extent of this unwanted effect was smaller when the drug was delivered in the form of the SIM:HPβCD complex. Surface pressure vs. time dependencies showed that the drug encapsulated with cyclodextrin dissociated from the complex upon contact with the lipid layer and the weak interactions between the exterior polar part of the HPβCD and the polar headgroups of the lipid layer facilitated smooth incorporation of the released lipophilic drug into the membrane. At a longer time-scale, the HPβCD ligand released from the complex removed some cholesterol, but not DMPC, from the lipid layer, hence, similarly to the enzyme inhibiting action of statins - it lead to the decrease of the amount of cholesterol in the membrane. Delivery of simvastatin in the form of an inclusion complex with HPβCD is proposed as an approach improving its bioavailability in the cholesterol-lowering therapies.

Production of antifungal saponins in an airlift bioreactor with a cell line transformed from Solanum chrysotrichum and its activity against strawberry phytopathogens

Biotechnology through plant cell cultures in bioreactors is a tool that allows increasing the production of secondary metabolites of commercial interest. The hydrodynamic characterization, in addition to the transfer (OTR) and uptake (OUR) of oxygen through the dynamic method with different aeration rate, were used to see their influence on the production of biomass and saponins. The culture poisoning technique was used to determine the antifungal activity of the SC-2 and SC-3 saponins in vitro. Likewise, the shear or hydrodynamic stress of 273.6 mN/m² were calculated based on the Reynolds Number. The oxygen supply (OTR) was always greater than the demand (OUR) for all the aeration rate evaluated. Dry weight values of 8.6 gDW/L and a concentration of 2.7 mg/L and 187.3 mg/L of the saponins SC-2 and SC-3 respectively were obtained with an air flow of 0.1 vvm. In addition, it was possible to inhibit the growth of phytopathogenic fungi in vitro by up to 93%, while in vivo it was possible to reduce the infections of strawberry seeds inoculated with phytopathogens, obtaining up to 94% of germinated seeds. This information will facilitate the rational operation of the bioreactor culture system that produces secondary metabolites.

Glycoalkaloids: Structure, Properties, and Interactions with Model Membrane Systems

The glycoalkaloids which are secondary metabolites from plants have proven to be of significant interest for their biological properties both in terms of their roles in plant biology and the effects they exhibit when ingested by humans. The main feature of the action of glycoalkaloids is their strong binding to 3β-hydroxysterols, such as cholesterol, to form complexes with the consequence that membrane structure is significantly perturbed, and leakage or release of contents inside cells or liposomes becomes possible. The glycoalkaloids have been studied for their ability to inhibit the growth of cancer cells and in other roles such as vaccine adjuvants and as synergistic agents when combined with other therapeutics. The glycoalkaloids have rich and complex physical behavior when interacting with model membranes for which many aspects are yet to be understood. This review introduces the general properties of glycoalkaloids and aspects of their behavior, and then summarizes their effects against model membrane systems. While there are many glycoalkaloids that have been identified, most physical or biological studies have focused on the readily available ones from tomatoes (α-tomatine), potatoes (α-chaconine and α-solanine), and eggplant (α-solamargine and α-solasonine).

On the Interaction between Digitonin and Cholesterol in Langmuir Monolayers

In this article, we describe the effect of a highly hemolytic saponin, digitonin, on model lipids cholesterol and dipalmitoylphosphatidylcholine (DPPC) using a combination of tensiometric (surface pressure and dilatational surface elasticity), spectroscopic (infrared reflection absorption spectroscopy, IRRAS), microscopic (fluorescence microscopy), and scattering techniques (neutron reflectivity, NR, and grazing incidence X-ray diffraction, GIXD). The monolayers of individual lipids and their 10:9 (mol/mol) mixture were exposed to an aqueous solution of digitonin (10(-4) M) by subphase exchange using a setup developed recently in our laboratory. The results confirm that digitonin can adsorb onto both bare and lipid-covered water-air interfaces. In the case of DPPC, a relatively weak interaction can be observed, but the presence of cholesterol drastically enhances the effect of digitonin. The latter is shown to dissociate the weak cholesterol-DPPC complexes and to bind cholesterol in an additional layer attached to the original lipid monolayer.

Complexation of phospholipids and cholesterol by triterpenic saponins in bulk and in monolayers

The interactions between three triterpene saponins: α-hederin, hederacoside C and ammonium glycyrrhizate with model lipids: cholesterol and dipalmitoylphosphatidylcholine (DPPC) are described. The oleanolic acid-type saponins (α-hederin and hederacoside C) were shown to form 1:1 complexes with lipids in bulk, characterized by stability constants in the range (4.0±0.2)·10(3)-(5.0±0.4)·10(4) M(-1). The complexes with cholesterol are generally stronger than those with DPPC. On the contrary, ammonium glycyrrhizate does not form complexes with any of the lipids in solution. The saponin-lipid interactions were also studied in a confined environment of Langmuir monolayers of DPPC and DPPC/cholesterol with the saponins present in the subphase. A combined monolayer relaxation, surface dilational rheology, fluorescence microscopy and neutron reflectivity (NR) study showed that all three saponins are able to penetrate pure DPPC and mixed DPPC/cholesterol monolayers. Overall, the effect of the saponins on the model lipid monolayers does not fully correlate with the lipid-saponin complex formation in the homogeneous solution. The best correlation was found for α-hederin, for which even the preference for cholesterol over DPPC observed in bulk is well reflected in the monolayer studies and the literature data on its membranolytic activity. Similarly, the lack of interaction of ammonium glycyrrhizate with both lipids is evident equally in bulk and monolayer experiments, as well as in its weak membranolytic activity. The combined bulk and monolayer results are discussed in view of the role of confinement in modulating the saponin-lipid interactions and possible mechanism of membranolytic activity of saponins.

Detailed Analysis of the Surface Area and Elasticity in the Saturated 1,2-Diacylphosphatidylcholine/Cholesterol Binary Monolayer System

The surface pressure-area (π-A) isotherms of DMPC, DPPC, and DSPC/cholesterol binary monolayers were systematically measured with great care to gain insight into the lateral molecular packing in these binary monolayer systems. The average molecular area A and the area elastic modulus C(s)⁻¹ at a given surface pressure were calculated as a function of cholesterol mole fraction x(chol). As a result, data reliable enough for the analysis of detailed phase behavior were obtained. We identified several characteristic phase regions and assigned the phase state in each region on the basis of the deviation of A(x(chol)) and C(s)⁻¹(x(chol)) from ideal additivity. We also estimated the partial molecular areas of DMPC, DPPC, DSPC, and cholesterol in the single-phase regions, where C(s)⁻¹(x(chol)) values fell on an ideal additivity curve. We found that the addition of cholesterol induces the formation of a highly condensed phase where the diacylphosphatidylcholine (diacyl PC) molecule has a surface area even smaller than that in the solid phase, irrespective of the surface pressure and the chain length of diacyl PC. Here, we call the cholesterol-induced condensed phase the CC phase. Furthermore, we demonstrated that the basic features of A(x(chol)) and C(s)⁻¹(x(chol)) profiles can be explained semiquantitatively by assuming the state of vicinity lipids surrounding sparsely distributed cholesterol molecules in the low x(chol) region as a third state of the diacyl PC molecule in addition to the states in the pure diacyl PC monolayer and in the CC phase.

The amphiphilic nature of saponins and their effects on artificial and biological membranes and potential consequences for red blood and cancer cells

Saponins, amphiphiles of natural origin with numerous biological activities, are widely used in the cosmetic and pharmaceutical industry. Some saponins exhibit relatively selective cytotoxic effects on cancer cells but the tendency of saponins to induce hemolysis limits their anticancer potential. This review focused on the effects of saponin activity on membranes and consequent implications for red blood and cancer cells. This activity seems to be strongly related to the amphiphilic character of saponins that gives them the ability to self-aggregate and interact with membrane components such as cholesterol and phospholipids. Membrane interactions of saponins with artificial membrane models, red blood and cancer cells are reviewed with respect to their molecular structures. The review considered the mechanisms of these membrane interactions and their consequences including the modulation of membrane dynamics, interaction with membrane rafts, and membrane lysis. We summarized current knowledge concerning the mechanisms involved in the interactions of saponins with membrane lipids and examined the structure activity relationship of saponins regarding hemolysis and cancer cell death. A critical analysis of these findings speculates on their potential to further develop new anticancer compounds.

Impact of two different saponins on the organization of model lipid membranes

Saponins, naturally occurring plant compounds are known for their biological and pharmacological activity. This activity is strongly related to the amphiphilic character of saponins that allows them to aggregate in aqueous solution and interact with membrane components. In this work, Langmuir monolayer techniques combined with polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and Brewster angle microscopy were used to study the interaction of selected saponins with lipid model membranes. Two structurally different saponins were used: digitonin and a commercial Merck Saponin. Membranes of different composition, namely, cholesterol, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine or 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) were formed at the air/water and air/saponin solution interfaces. The saponin-lipid interaction was characterized by changes in surface pressure, surface potential, surface morphology and PM-IRRAS signal. Both saponins interact with model membranes and change the physical state of membranes by perturbing the lipid acyl chain orientation. The changes in membrane fluidity were more significant upon the interaction with Merck Saponin. A higher affinity of saponins for cholesterol than phosphatidylglycerols was observed. Moreover, our results indicate that digitonin interacts strongly with cholesterol and solubilize the cholesterol monolayer at higher surface pressures. It was shown, that digitonin easily penetrate to the cholesterol monolayer and forms a hydrogen bond with the hydroxyl groups. These findings might be useful in further understanding of the saponin action at the membrane interface and of the mechanism of membrane lysis.

Domain formation and permeabilization induced by the saponin α-hederin and its aglycone hederagenin in a cholesterol-containing bilayer

Saponins and triterpenic acids have been shown to be able to interact with lipid membranes and domains enriched with cholesterol (rafts). How saponins are able to modulate lipid phase separation in membranes and the role of the sugar chains for this activity is unknown. We demonstrate in a binary membrane model composed of DMPC/Chol (3:1 mol/mol) that the saponin α-hederin and its aglycone presenting no sugar chain, the triterpenic acid hederagenin, are able to induce the formation of lipid domains. We show on multilamellar vesicles (MLV), giant unilamellar vesicles (GUV), and supported planar bilayers (SPB) that the presence of sugar units on the sapogenin accelerates domain formation and increases the proportion of sterols within these domains. The domain shape is also influenced by the presence of sugars because α-hederin and hederagenin induce the formation of tubular and spherical domains, respectively. These highly curved structures should result from the induction of membrane curvature by both compounds. In addition to the formation of domains, α-hederin and hederagenin permeabilize GUV. The formation of membrane holes by α-hederin comes along with the accumulation of lipids into nonbilayer structures in SPB. This process might be responsible for the permeabilizing activity of both compounds. In LUV, permeabilization by α-hederin was sterol-dependent. The biological implications of our results and the mechanisms involved are discussed in relation to the activity of saponins and triterpenic acids on membrane rafts, cancer cells, and hemolysis.

Structure-activity relationships of α-, β(1)-, γ-, and δ-tomatine and tomatidine against human breast (MDA-MB-231), gastric (KATO-III), and prostate (PC3) cancer cells

Partial acid hydrolysis of the tetrasaccharide (lycotetraose) side chain of the tomato glycoalkaloid α-tomatine resulted in the formation of four products with three, two, one, and zero carbohydrate side chains, which were separated by high-performance liquid chromatography (HPLC) and identified by thin-layer chromatography (TLC) and liquid chromatography ion-trap time-of-flight mass spectrometry (LCMS-IT-TOF). The inhibitory activities in terms of IC(50) values (concentration that inhibits 50% of the cells under the test conditions) of the parent compound and the hydrolysates, isolated by preparative HPLC, against normal human liver and lung cells and human breast, gastric, and prostate cancer cells indicate that (a) the removal of sugars significantly reduced the concentration-dependent cell-inhibiting effects of the test compounds, (b) PC3 prostate cancer cells were about 10 times more susceptible to inhibition by α-tomatine than the breast and gastric cancer cells or the normal cells, (c) the activity of α-tomatine against the prostate cancer cells was 200 times greater than that of the aglycone tomatidine, and (d) the activity increased as the number of sugars on the aglycone increased, but this was only statistically significant at p < 0.05 for the normal lung Hel299 cell line. The effect of the alkaloids on tumor necrosis factor α (TNF-α) was measured in RAW264.7 macrophage cells. There was a statistically significant negative correlation between the dosage of γ- and α-tomatine and the level of TNF-α. α-Tomatine was the most effective compound at reducing TNF-α. The dietary significance of the results and future research needs are discussed.

Molecular activities, biosynthesis and evolution of triterpenoid saponins

Saponins are bioactive compounds generally considered to be produced by plants to counteract pathogens and herbivores. Besides their role in plant defense, saponins are of growing interest for drug research as they are active constituents of several folk medicines and provide valuable pharmacological properties. Accordingly, much effort has been put into unraveling the modes of action of saponins, as well as in exploration of their potential for industrial processes and pharmacology. However, the exploitation of saponins for bioengineering crop plants with improved resistances against pests as well as circumvention of laborious and uneconomical extraction procedures for industrial production from plants is hampered by the lack of knowledge and availability of genes in saponin biosynthesis. Although the ability to produce saponins is rather widespread among plants, a complete synthetic pathway has not been elucidated in any single species. Current conceptions consider saponins to be derived from intermediates of the phytosterol pathway, and predominantly enzymes belonging to the multigene families of oxidosqualene cyclases (OSCs), cytochromes P450 (P450s) and family 1 UDP-glycosyltransferases (UGTs) are thought to be involved in their biosynthesis. Formation of unique structural features involves additional biosynthetical enzymes of diverse phylogenetic background. As an example of this, a serine carboxypeptidase-like acyltransferase (SCPL) was recently found to be involved in synthesis of triterpenoid saponins in oats. However, the total number of identified genes in saponin biosynthesis remains low as the complexity and diversity of these multigene families impede gene discovery based on sequence analysis and phylogeny. This review summarizes current knowledge of triterpenoid saponin biosynthesis in plants, molecular activities, evolutionary aspects and perspectives for further gene discovery.

Tomatine-containing green tomato extracts inhibit growth of human breast, colon, liver, and stomach cancer cells

Tomato plants ( Lycopersicon esculentum ) synthesize the glycoalkaloids dehydrotomatine and alpha-tomatine, possibly as a defense against bacteria, fungi, viruses, and insects. Six green and three red tomato extracts were investigated for their ability to induce cell death in human cancer and normal cells using a microculture tetrazolium (MTT) assay. Compared to untreated controls, the high-tomatine green tomato extracts strongly inhibited the following human cancer cell lines: breast (MCF-7), colon (HT-29), gastric (AGS), and hepatoma (liver) (HepG2), as well as normal human liver cells (Chang). There was little inhibition of the cells by the three low-tomatine red tomato extracts. Cell death induced by the pure glycoalkaloids dehydrotomatine and alpha-tomatine isolated from green tomatoes and characterized by HPLC, GC, and GC-MS, as well as their respective aglycones tomatidenol and tomatidine, was also evaluated. alpha-Tomatine was highly effective in inhibiting all of the cell lines. Dehydrotomatine, tomatidenol, and tomatidine had little, if any, effect on cell inhibition. The results show that the susceptibility to destruction varies with the nature of the alkaloid and plant extract and the type of cancer cell. These findings extend related observations on the anticarcinogenic potential of glycoalkaloids and suggest that consumers may benefit by eating not only high-lycopene red tomatoes but also green tomatoes containing glycoalkaloids. Possible mechanisms of the anticarcinogenic and other beneficial effects and the significance of the cited observations for breeding improved tomatoes and for the human diet are discussed.

Comparative calorimetric and spectroscopic studies of the effects of cholesterol and epicholesterol on the thermotropic phase behaviour of dipalmitoylphosphatidylcholine bilayer membranes

We carried out comparative differential scanning calorimetric and Fourier transform infrared spectroscopic studies of the effects of cholesterol (Chol) and epicholesterol (EChol) on the thermotropic phase behaviour and organization of dipalmitoylphosphatidylcholine (DPPC) bilayers. EChol is an epimer of Chol in which the axially oriented hydroxyl group of C3 of Chol is replaced by an equatorially oriented hydroxyl group, resulting in a different orientation of the hydroxyl group relative to sterol fused ring system. Our calorimetric studies indicate that the incorporation of EChol is more effective than Chol is in reducing the enthalpy of the pretransition of DPPC. EChol is also initially more effective than Chol in reducing the enthalpies of both the sharp and broad components of the main phase transition of DPPC. However, at higher EChol concentrations (~30-50 mol%), EChol becomes less effective than Chol in reducing the enthalpy and cooperativity of the main phase transition, such that at sterol concentrations of 50 mol%, EChol does not completely abolish the cooperative hydrocarbon chain-melting phase transition of DPPC, while Chol does. However, EChol does not appear to form a calorimetrically detectable crystallite phase at higher sterol concentrations, suggesting that EChol, unlike Chol, may form dimers or lower order aggregates at higher sterol concentrations. Our spectroscopic studies demonstrate that EChol incorporation produces more ordered gel and comparably ordered liquid-crystalline bilayers compared to Chol, which are characterized by increased hydrogen bonding in the glycerol backbone region of the DPPC bilayer. These and other results indicate that monomeric EChol is less miscible in DPPC bilayers than is Chol at higher sterol concentrations, but perturbs their organization to a greater extent at lower sterol concentrations, probably due primarily to the larger effective cross-sectional area of the EChol molecule. Nevertheless, EChol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers.