Liprotides made of α-lactalbumin and cis fatty acids form core-shell and multi-layer structures with a common membrane-targeting mechanism

The mosquito protein AEG12 displays both cytolytic and antiviral properties via a common lipid transfer mechanism

The mosquito protein AEG12 is up-regulated in response to blood meals and flavivirus infection though its function remained elusive. Here, we determine the three-dimensional structure of AEG12 and describe the binding specificity of acyl-chain ligands within its large central hydrophobic cavity. We show that AEG12 displays hemolytic and cytolytic activity by selectively delivering unsaturated fatty acid cargoes into phosphatidylcholine-rich lipid bilayers. This property of AEG12 also enables it to inhibit replication of enveloped viruses such as Dengue and Zika viruses at low micromolar concentrations. Weaker inhibition was observed against more distantly related coronaviruses and lentivirus, while no inhibition was observed against the nonenveloped virus adeno-associated virus. Together, our results uncover the mechanistic understanding of AEG12 function and provide the necessary implications for its use as a broad-spectrum therapeutic against cellular and viral targets.

α-Lactalbumin, Amazing Calcium-Binding Protein

α-Lactalbumin (α-LA) is a small (Mr 14,200), acidic (pI 4–5), Ca²⁺-binding protein. α-LA is a regulatory component of lactose synthase enzyme system functioning in the lactating mammary gland. The protein possesses a single strong Ca²⁺-binding site, which can also bind Mg²⁺, Mn²⁺, Na⁺, K⁺, and some other metal cations. It contains several distinct Zn²⁺-binding sites. Physical properties of α-LA strongly depend on the occupation of its metal binding sites by metal ions. In the absence of bound metal ions, α-LA is in the molten globule-like state. The binding of metal ions, and especially of Ca²⁺, increases stability of α-LA against the action of heat, various denaturing agents and proteases, while the binding of Zn²⁺ to the Ca²⁺-loaded protein decreases its stability and causes its aggregation. At pH 2, the protein is in the classical molten globule state. α-LA can associate with membranes at neutral or slightly acidic pH at physiological temperatures. Depending on external conditions, α-LA can form amyloid fibrils, amorphous aggregates, nanoparticles, and nanotubes. Some of these aggregated states of α-LA can be used in practical applications such as drug delivery to tissues and organs. α-LA and some of its fragments possess bactericidal and antiviral activities. Complexes of partially unfolded α-LA with oleic acid are cytotoxic to various tumor and bacterial cells. α-LA in the cytotoxic complexes plays a role of a delivery carrier of cytotoxic fatty acid molecules into tumor and bacterial cells across the cell membrane. Perhaps in the future the complexes of α-LA with oleic acid will be used for development of new anti-cancer drugs.

Structures and mechanisms of formation of liprotides

Many proteins form complexes called liprotides with oleic acid and other cis-fatty acids under conditions where the protein is partially unfolded. The complexes vary in structure depending on the ratio of protein and lipid, but the most common structural organization is the core-shell structure, in which a layer of dynamic, partially unfolded and extended proteins surrounds a micelle-like fatty acid core. This structure, first reported for α-lactalbumin together with OA, resembles complexes formed between proteins and anionic surfactants like SDS. Liprotides first rose to fame through their anti-carcinogenic properties which still remains promising for topical applications though not yet implemented in the clinic. In addition, liprotides show potential in drug delivery thanks to the ability of the micelle core to solubilize and stabilize hydrophobic compounds, though applications are challenged by their sensitivity to acidic pH and dynamic exchange of lipids which makes them easy prey for serum "hoovers" such as albumin. However, liprotides are also of fundamental interest as a generic "protein complex structure", demonstrating the many and varied structural consequences of protein-lipid interactions. Here we provide an overview of the different types of liprotide complexes, ranging from quasi-native complexes via core-shell structures to multi-layer structures, and discuss the many conditions under which they form. Given the many variable types of complexes that can form, rigorous biophysical analysis (stoichiometry, shape and structure of the complexes) remains crucial for a complete understanding of the mechanisms of action of this fascinating group of protein-lipid complexes both in vitro and in vivo.

Evaluation of Milk Colostrum Derived Lactoferrin of Sahiwal (Bos indicus) and Karan Fries (Cross-Bred) Cows for Its Anti-Cancerous Potential

Lactoferrin (Lf) is an iron-binding glycoprotein protein known to have immune-modulatory role and recently, its anticancerous effect against different cancer cell types was emphasized. In the present investigation, a comparative evaluation of anticancer potential of colostrum-derived lactoferrin from Indian native zebu cow (Sahiwal, SAC), crossbred (Karan Fries, KFC) and commercially available (C-Lf) lactoferrin from exotic cow using cellular models was made. A protocol was standardized successfully to purify Lf protein from colostrum of both breeds using HPLC and purity was confirmed by LC-MS. A standardized dose of 750 µg/mL Lf was used to treat two cell types MDA-MB-231 and MCF-7 with Lf from three different sources; SAC-Lf, KFC-Lf and C-Lf for 48 h and 72 h. Different cellular parameters including cytotoxicity, viability, apoptosis and cell proliferation were determined. Comparatively, Lf from commercial source (C-Lf) had maximum effect in both cell types followed by SAC-Lf and KFC-Lf. Further, transcriptional changes in genes associated with apoptosis (Bax and Bcl-2), tumor progression (p53, p21, CD44 and NF-κβ) and survival (survivin) were evaluated in Lf treatment. The overall results strongly emphasized to the fact that Lf purified from cow colostrum has the capacity to inhibit the in vitro growth of cancerous cell lines albeit to a varied extent.

Temperature, pH, and Molecular Packing Effects on the Penetration of Oleic Acid Monolayer by α-Lactalbumin

Recently, we reported on the interfacial behavior of mixed oleic acid (OA)-α-lactalbumin monolayer and its relevance in the formation of tumoricidal HAMLET (human α-lactalbumin made lethal to tumor cells)-like complex. This complex is probably formed in the gastrointestinal tract, but it has not been proved so far. The molecular base and the underlying physicochemical forces leading to such complexation remain to be known as well. There are also several other issues related with the complex stoichiometry that need to be fully explained. This study provides insight into the mechanism of temperature, pH, and physical state of monolayer-dependent binding of OA by the milk protein- apo-α-lactalbumin. Using the Langmuir and Langmuir-Blodgett approaches, we investigated the interactions between the OA monolayer and the apo-bovine α-lactalbumin (BLA III) at different pH, temperatures, and molecular packing. We found that the most favorable conditions for the formation of mixed OA-BLA III film are relevant to the gastric environment. The stabilization of OA-BLA III at the interface is associated with the conformational changes of protein in the presence of fatty acids induced by low pH and high temperature in the expanded monolayer. Our approach helps to understand the molecular mechanism of HAMLET/bovine α-lactalbumin made lethal to tumor cells formation in vivo.

A Protein Complex from Human Milk Enhances the Activity of Antibiotics and Drugs against Mycobacterium tuberculosis

Mycobacterium tuberculosis, the causative agent of human tuberculosis (TB), has surpassed HIV/AIDS as the leading cause of death from a single infectious agent. The increasing occurrence of drug-resistant strains has become a major challenge for health care systems and, in some cases, has rendered TB untreatable. However, the development of new TB drugs has been plagued with high failure rates and costs. Alternative strategies to increase the efficacy of current TB treatment regimens include host-directed therapies or agents that make M. tuberculosis more susceptible to existing TB drugs. In this study, we show that HAMLET, an α-lactalbumin-oleic acid complex derived from human milk, has bactericidal activity against M. tuberculosis HAMLET consists of a micellar oleic acid core surrounded by a shell of partially denatured α-lactalbumin molecules and unloads oleic acid into cells upon contact with lipid membranes. At sublethal concentrations, HAMLET potentiated a remarkably broad array of TB drugs and antibiotics against M. tuberculosis For example, the minimal inhibitory concentrations of rifampin, bedaquiline, delamanid, and clarithromycin were decreased by 8- to 16-fold. HAMLET also killed M. tuberculosis and enhanced the efficacy of TB drugs inside macrophages, a natural habitat of M. tuberculosis Previous studies showed that HAMLET is stable after oral delivery in mice and nontoxic in humans and that it is possible to package hydrophobic compounds in the oleic acid core of HAMLET to increase their solubility and metabolic stability. The potential of HAMLET and other liprotides as drug delivery and sensitization agents in TB chemotherapy is discussed here.

A biophysical study on the mechanism of interactions of DOX or PTX with α-lactalbumin as a delivery carrier

Doxorubicin and paclitaxel, two hydrophobic chemotherapeutic agents, are used in cancer therapies. Presence of hydrophobic patches and a flexible fold could probably make α-Lactalbumin a suitable carrier for hydrophobic drugs. In the present study, a variety of thermodynamic, spectroscopic, computational, and cellular techniques were applied to assess α-lactalbumin potential as a carrier for doxorubicin and paclitaxel. According to isothermal titration calorimetry data, the interaction between α-lactalbumin and doxorubicin or paclitaxel is spontaneous and the K (M^-1) value for the interaction of α-lactalbumin and paclitaxel is higher than that for doxorubicin. Differential scanning calorimetry and anisotropy results indicated formation of α-lactalbumin complexes with doxorubicin or paclitaxel. Furthermore, molecular docking and dynamic studies revealed that TRPs are not involved in α-Lac's interaction with Doxorubicin while TRP 60 interacts with paclitaxel. Based on Pace analysis to determine protein thermal stability, doxorubicin and paclitaxel induced higher and lower thermal stability in α-lactalbumin, respectively. Besides, fluorescence lifetime measurements reflected that the interaction between α-lactalbumin with doxorubicin or paclitaxel was of static nature. Therefore, the authors hypothesized that α-lactalbumin could serve as a carrier for doxorubicin and paclitaxel by reducing cytotoxicity and apoptosis which was demonstrated during our in vitro cell studies.

BAMLET kills chemotherapy-resistant mesothelioma cells, holding oleic acid in an activated cytotoxic state

Malignant pleural mesothelioma is an aggressive cancer with poor prognosis. Here we have investigated in vitro efficacy of BAMLET and BLAGLET complexes (anti-cancer complexes consisting of oleic acid and bovine α-lactalbumin or β-lactoglobulin respectively) in killing mesothelioma cells, determined BAMLET and BLAGLET structures, and investigated possible biological mechanisms. We performed cell viability assays on 16 mesothelioma cell lines. BAMLET and BLAGLET having increasing oleic acid content inhibited human and rat mesothelioma cell line proliferation at decreasing doses. Most of the non-cancer primary human fibroblasts were more resistant to BAMLET than were human mesothelioma cells. BAMLET showed similar cytotoxicity to cisplatin-resistant, pemetrexed-resistant, vinorelbine-resistant, and parental rat mesothelioma cells, indicating the BAMLET anti-cancer mechanism may be different to drugs currently used to treat mesothelioma. Cisplatin, pemetrexed, gemcitabine, vinorelbine, and BAMLET, did not demonstrate a therapeutic window for mesothelioma compared with immortalised non-cancer mesothelial cells. We demonstrated by quantitative PCR that ATP synthase is downregulated in mesothelioma cells in response to regular dosing with BAMLET. We sought structural insight for BAMLET and BLAGLET activity by performing small angle X-ray scattering, circular dichroism, and scanning electron microscopy. Our results indicate the structural mechanism by which BAMLET and BLAGLET achieve increased cytotoxicity by holding increasing amounts of oleic acid in an active cytotoxic state encapsulated in increasingly unfolded protein. Our structural studies revealed similarity in the molecular structure of the protein components of these two complexes and in their encapsulation of the fatty acid, and differences in the microscopic structure and structural stability. BAMLET forms rounded aggregates and BLAGLET forms long fibre-like aggregates whose aggregation is more stable than that of BAMLET due to intermolecular disulphide bonds. The results reported here indicate that BAMLET and BLAGLET may be effective second-line treatment options for mesothelioma.

Dynamic content exchange between liprotides

Liprotides are complexes composed of partially denatured proteins and fatty acids in which the fatty acids form a micelle-like core surrounded by a shell of proteins. Liprotides, composed of α-lactalbumin (aLA) and oleic acid (OA), are similar in components and cytotoxicity to the original HAMLET protein-fatty acid complex. Liprotides composed of aLA and OA kill tumor cells by transferring the OA component to, and thus destabilizing, the cell membrane. Here we investigate liprotides' dynamics of transfer of contents between themselves and membranes using the hydrophobic fluorescent probe pyrene. We find that pyrene incorporated into liprotides is exchanged between liprotides within the dead time of a stopped-flow instrument, while the transfer to membranes occurs within 20s. Transfer kinetics was not affected by the presence of the membrane stabilizing lipid cholesterol. Thus, transfer is a remarkably rapid process which illustrates liprotides' efficacy as transporters of hydrophobic compounds.

Liprotides assist in folding of outer membrane proteins

Proteins and lipids can form complexes called liprotides, in which the partially denatured protein forms a shell encasing a lipid core. This effectively stabilizes a lipid micelle in an aqueous solvent and suggests that liprotides may provide a suitable vessel for membrane proteins. Accordingly we have investigated if liprotides consisting of α-lactalbumin and oleate could aid folding of four different outer membrane proteins (OMPs) tOmpA, PagP, BamA, and OmpF. tOmpA was able to fold in the presence of the liprotide, and folding did not occur if only oleate or α-lactalbumin were added. Although the liprotides did not fold the other three OMPs on its own, it was able to assist their folding in the presence of vesicles. Incubation with liprotides before folding into vesicles increased the folding yield of the outer membrane proteins to a level higher than using micelles of the non-ionic surfactant DDM. Even though the liprotide was stable at both high urea concentrations and high pH, it failed to efficiently fold OmpA at high pH. Instead, optimal folding was seen at pH 8-9, suggesting that important changes in the liprotide occurred when increasing the pH. We conclude that an otherwise folding-inactive fatty acid can be activated when presented by a liprotide and thereby work as an in vitro chaperone for outer membrane proteins.

Liprotides kill cancer cells by disrupting the plasma membrane

HAMLET (human α-lactalbumin made lethal to tumour cells) is a complex of α-lactalbumin (aLA) and oleic acid (OA) which kills transformed cells, while leaving fully differentiated cells largely unaffected. Other protein-lipid complexes show similar anti-cancer potential. We call such complexes liprotides. The cellular impact of liprotides, while intensely investigated, remains unresolved. To address this, we report on the cell-killing mechanisms of liprotides prepared by incubating aLA with OA for 1 h at 20 or 80 °C (lip20 and lip80, respectively). The liprotides showed similar cytotoxicity against MCF7 cells, though lip80 acts more slowly, possibly due to intermolecular disulphide bonds formed during preparation. Liprotides are known to increase the fluidity of a membrane and transfer OA to vesicles, prompting us to focus on the effect of liprotides on the cell membrane. Extracellular Ca²⁺ influx is important for activation of the plasma membrane repair system, and we found that removal of Ca²⁺ from the medium enhanced the liprotides’ killing effect. Liprotide cytotoxicity was also increased by knockdown of Annexin A6 (ANXA6), a protein involved in plasma membrane repair. We conclude that MCF7 cells counteract liprotide-induced membrane permeabilization by activating their plasma membrane repair system, which is triggered by extracellular Ca²⁺ and involves ANXA6.

Formation mechanism of α-lactalabumin/oleic acid complex characterized by 2D correlation analysis

Partially unfolded α-lactalbumin (ALA) forms a complex with oleic acid (OA) that exhibits cytotoxic activity. In this study, for the first time, the pH-induced formation mechanism for ALA/OA complexes with two different molar ratios was investigated at the molecular level. For a deeper understanding of the formation mechanism of the two different ALA/OA complexes with decreasing pH, principal component analysis (PCA) and two-dimensional (2D) correlation spectroscopy were used to examine the pH-dependent IR spectra of ALA/OA complexes. By tracking the secondary structural variations in the ALA/OA complexes with decreasing pH, we successfully elucidated the formation mechanism of the ALA/OA complexes at the molecular level. The results showed that the secondary structures of theses complexes exhibited the greatest change between pH4 and pH3.5 and that the components that mainly contributed to the pH-induced transition from the N-state to the A-state were dissimilar in the two different ALA/OA complexes.

Divergent Anticancer Activity of Free and Formulated Camel Milk α-Lactalbumin

Alpha-lactalbumin (α-LA), a small milk calcium-binding globular protein, is known to possess noticeable anticancer activity, which is determined by the ability of this protein to form complexes with oleic acid (OA). To date, in addition to human and bovine α-LA, the ability to form such anti-tumor complexes with OA was described for goat and camel α-LA. Although the mechanisms of the anticancer activity of human and bovine α-LA are already well-studied, little is currently known about the anticancer action of this camel protein. The goal of this study was to fill this gap and to analyze the anticancer and pro-apoptotic activities of camel α-LA in its free form (α-cLA) and as an OA-containing complex (OA-α-cLA) using four human cancer cell lines, including Caco-2 colon cancer cells, PC-3 prostate cancer cells, HepG-2 hepatoma cells, and MCF-7 breast cancer cells as targets. The anti-tumor activities of OA-α-cLA and α-cLA were analyzed using MTT test, annexin/PI staining, cell cycle analysis, nuclear staining, and tyrosine kinase (TK) inhibition methods. We show here that the OA-α-cLA complex does not affect normal cells but has noticeable anti-cancer activity, especially against MCF-7 cells, thus boosting the anticancer activity of α-cLA and improving the selectivity of OA. The OA-α-cLA complex mediated cancer cell death via selective induction of apoptosis and cell-cycle arrest at lower IC₅₀ than that of free α-cLA by more than two folds. However, OA induced apoptosis at higher extent than OA-α-cLA and α-cLA. OA also caused unselective apoptosis-dependent cell death in both normal and cancer cells to a similar degree. The apoptosis and cell-cycle arresting effect of OA-α-cLA may be attributed to the TK inhibition activity of OA. Therefore, OA-α-cLA serves as efficient anticancer complex with two functional components, α-cLA and OA, possessing different activities. This study declared the effectiveness of OA-α-cLA complex as a promising entity with anticancer activity, and these formulated OA-camel protein complexes constitute an auspicious approach for cancer remedy, particularly for breast cancer.

Neutron scattering shows a droplet of oleic acid at the center of the BAMLET complex

The anti-cancer complex, Bovine Alpha-lactalbumin Made LEthal to Tumors (BAMLET), has intriguing broad-spectrum anti-cancer activity. Although aspects of BAMLET's anti-cancer mechanism are still not known, it is understood that it involves the oleic acid or oleate component of BAMLET being preferentially released into cancer cell membranes leading to increased membrane permeability and lysis. The structure of the protein component of BAMLET has previously been elucidated by small angle X-ray scattering (SAXS) to be partially unfolded and dramatically enlarged. However, the structure of the oleic acid component of BAMLET and its disposition with respect to the protein component was not revealed as oleic acid has the same X-ray scattering length density (SLD) as water. Employing the difference in the neutron SLDs of hydrogen and deuterium, we carried out solvent contrast variation small angle neutron scattering (SANS) experiments of hydrogenated BAMLET in deuterated water buffers, to reveal the size, shape, and disposition of the oleic acid component of BAMLET. Our resulting analysis and models generated from SANS and SAXS data indicate that oleic acid forms a spherical droplet of oil incompletely encapsulated by the partially unfolded protein component. This model provides insight into the anti-cancer mechanism of this cache of lipid. The model also reveals a protein component "tail" not associated with the oleic acid component that is able to interact with the tail of other BAMLET molecules, providing a plausible explanation of how BAMLET readily forms aggregates. Proteins 2017; 85:1371-1378. © 2017 Wiley Periodicals, Inc.