Apoptosis and Tumor Cell Death in Response to HAMLET (Human α-Lactalbumin Made Lethal to Tumor Cells)

Beyond Milk and Nurture: Breastfeeding's Powerful Impact on Breast Cancer

Breast cancer (BC) stands as a global concern, given its high incidence and impact on women's mortality. This complex disease has roots in various risk factors, some modifiable and others not. Understanding and identifying these factors can be instrumental in both preventing BC and improving survival rates. Remarkably, women's reproductive behaviors have emerged as critical determinants of BC susceptibility. Numerous studies have shed light on how aspects including age of menarche, first pregnancy and menopause along with number of pregnancies, hormone replacement therapies, can influence one's risk of developing BC. Furthermore, the act of breastfeeding and its duration have shown an inverse relationship with BC risk. This review delves into the biological and molecular mechanisms associated with breastfeeding that contribute to BC protection. It highlights the role of endocrine processes triggered by suckling stimulation, the gradual onset of lactational amenorrhea, delayed weaning, reduced lifetime menstrual cycles, chromosomal repair mechanisms, and immunological events throughout the lactation cycle. These insights provide a potential explanation for the protective effects conferred by breastfeeding against breast carcinomas.

The effect of extra virgin olive oil (EVOO) on fat mass and fat-free mass for breastfeeding mothers (0-24 months) in Makassar City, Indonesia

BACKGROUND

Extra Virgin Olive Oil (EVOO) is a fat source classified as monounsaturated fatty acid. Previous studies have shown that 47.5% of breastfeeding mothers have less fat intake when compared to the Adequacy of Nutrition Rate (RDA). EVOO consumption can improve the composition of breast milk. However, no research has been found regarding the body composition of breastfeeding mothers after consuming EVOO. This study aims to determine the effect of EVOO on the body composition of breastfeeding mothers 0-24 months.

MATERIALS AND METHODS

The study was conducted at the Sudiang Raya and Tamalanrea Health Centers in Makassar City with a randomized clinical trial design from January to February 2023. The sample was breastfeeding mothers 0-24 months divided into the intervention group (EVOO and Nutrition Education, n = 17) and the control (Nutrition Education, n = 17). Instruments with Bioelectrical Impedence Analysis tools. Data analysis using t-test.

RESULTS

The sample is breastfeeding mothers aged between ≤19 and >35 years. The results of the analysis showed that the average pre-post test, % fat mass (FM) P = 0.426 and % fat free mass (FFM) P = 0.508 (P > 0.05), meant that there was no significant difference. Between the two groups there was no significant difference at the end of the study, %FM P = 0.469 and %FFM P = 0.529 (P > 0.05).

CONCLUSIONS

In the intervention group that was given EVOO, it was possible to maintain%FM and%FFM in the normal percentage range, while in the control group, there was a decrease from the normal to less/low range.

Assessing the Therapeutic Impacts of HAMLET and FOLFOX on BRAF-Mutated Colorectal Cancer: A Study of Cancer Cell Survival and Mitochondrial Dynamics In Vitro and Ex Vivo

Background and Objectives: Colorectal cancer (CRC) is a major global health challenge. The BRAF V600E mutation, found in 8-12% of CRC patients, exacerbates this by conferring poor prognosis and resistance to therapy. Our study focuses on the efficacy of the HAMLET complex, a molecular substance derived from human breast milk, on CRC cell lines and ex vivo biopsies harboring this mutation, given its previously observed selective toxicity to cancer cells. Materials and Methods: we explored the effects of combining HAMLET with the FOLFOX chemotherapy regimen on CRC cell lines and ex vivo models. Key assessments included cell viability, apoptosis/necrosis induction, and mitochondrial function, aiming to understand the mutation-specific resistance or other cellular response mechanisms. Results: HAMLET and FOLFOX alone decreased viability in CRC explants, irrespective of the BRAF mutation status. Notably, their combination yielded a marked decrease in viability, particularly in the BRAF wild-type samples, suggesting a synergistic effect. While HAMLET showed a modest inhibitory effect on mitochondrial respiration across both mutant and wild-type samples, the response varied depending on the mutation status. Significant differences emerged in the responses of the HT-29 and WiDr cell lines to HAMLET, with WiDr cells showing greater resistance, pointing to factors beyond genetic mutations influencing drug responses. A slight synergy between HAMLET and FOLFOX was observed in WiDr cells, independent of the BRAF mutation. The bioenergetic analysis highlighted differences in mitochondrial respiration between HT-29 and WiDr cells, suggesting that bioenergetic profiles could be key in determining cellular responses to HAMLET. Conclusions: We highlight the potential of HAMLET and FOLFOX as a combined therapeutic approach in BRAF wild-type CRC, significantly reducing cancer cell viability. The varied responses in CRC cell lines, especially regarding bioenergetic and mitochondrial factors, emphasize the need for a comprehensive approach considering both genetic and metabolic aspects in CRC treatment strategies.

Comparative Analysis of the Protein Composition of Goat Milk from French Alpine, Nubian, and Creole Breeds and Holstein Friesian Cow Milk: Implications for Early Infant Nutrition

Simple Summary

Goat’s milk is a food that contains proteins of value for nutrition. The protein profile in the milk of goat breeds is different from that of cow milk, with a lower relative abundance of allergenic proteins. In addition, regardless of the breed, goat milk has beta-casein of type A2 in a more significant proportion than cow milk, which impacts different bioactive peptides hydrolyzed in the milk of the species.

Abstract

Of the diversity of proteins and high digestibility, goat milk will be a food of significant value for infant nutrition. The genetic polymorphisms of milk proteins play an essential role in the different degrees of allergic reactions. This work aimed to identify the proteins and peptides in the composition of goat milk and compare them to those in cow’s milk. The work was performed with goats French Alpine, Nubian, and Creole breeds and Holstein Friesian milking cows at the Universidad Autónoma de Querétaro, Amazcala. We investigated the relative abundance of goat and cow milk protein fractions by SDS-PAGE resolution and the densitometric analysis of gels. The protein alfa-casein was (17.67 ± 0.46) for Creole, (19.18 ± 0.88) French Alpine, (17.35 ± 0.49) Nubian, and (35.92 ± 1.96) Holstein cows. The relative abundance obtained from alfa-casein was statistically different between goats and cows, and this protein was vital because it is a protein related to allergies. On the other hand, the amino acid in position 67 of the beta-casein from three goat breeds is a Proline, so it is assumed that the beta-casein variant of goat milk is an A2-type. The latter has excellent relevance for infant nutrition and differs from cow milk.

Generation of a soluble and stable apoptin-EGF fusion protein, a targeted viral protein applicable for tumor therapy

A promising candidate for tumor targeted toxins is the chicken anemia-derived protein apoptin that induces tumor-specific apoptosis. It was aimed to design a novel apoptin-based targeted toxin by genetic fusion of apoptin with the tumor-directed ligand epidermal growth factor (EGF) using Escherichia coli as expression host. However, apoptin is highly hydrophobic and tends to form insoluble aggregates. Therefore, three different apoptin-EGF variants were generated. The fusion protein hexa-histidine (His)-apoptin-EGF (HAE) was expressed in E. coli and purified under denaturing conditions due to inclusion bodies. The protein solubility was improved by maltose-binding protein (MBP) or glutathione S-transferase. The protein MBP-apoptin-EGF^His (MAEH) was found favorable as a targeted toxin regarding final yield (4-6 mg/L) and stability. MBP was enzymatically removed using clotting factor Xa, which resulted in low yield and poor separation. MAEH was tested on target and non-target cell lines. The targeted tumor cell line A431 showed significant toxicity with an IC₅₀ of 69.55 nM upon incubation with MAEH while fibroblasts and target receptor-free cells remained unaffected. Here we designed a novel EGF receptor targeting drug with high yield, purity and stability.

Identification of a α-helical molten globule intermediate and structural characterization of β-cardiotoxin, an all β-sheet protein isolated from the venom of Ophiophagus hannah (king cobra)

β-Cardiotoxin is a novel member of the snake venom three-finger toxin (3FTX) family. This is the first exogenous protein to antagonize β-adrenergic receptors and thereby causing reduction in heart rates (bradycardia) when administered into animals, unlike the conventional cardiotoxins as reported earlier. 3FTXs are stable all β-sheet peptides with 60-80 amino acid residues. Here, we describe the three-dimensional crystal structure of β-cardiotoxin together with the identification of a molten globule intermediate in the unfolding pathway of this protein. In spite of the overall structural similarity of this protein with conventional cardiotoxins, there are notable differences observed at the loop region and in the charge distribution on the surface, which are known to be critical for cytolytic activity of cardiotoxins. The molten globule intermediate state present in the thermal unfolding pathway of β-cardiotoxin was however not observed during the chemical denaturation of the protein. Interestingly, circular dichroism (CD) and NMR studies revealed the presence of α-helical secondary structure in the molten globule intermediate. These results point to substantial conformational plasticity of β-cardiotoxin, which might aid the protein in responding to the sometimes conflicting demands of structure, stability, and function during its biological lifetime.

In Silico and Biochemical Characterization of Lysozyme-Like Proteins in the Rat

Background

Spermatogenesis and sperm maturation in the male reproductive tract is dictated by a variety of proteins secreted in the testis and epididymis. Though the proteome of these tissues is known, the functional role of many of these proteins remains uncharacterized. In this study, we characterize the rat Lysozyme-like (Lyzl) genes and proteins.

Methods

In silico tools were used to predict the primary, secondary and tertiary structures. Reverse transcription PCR, immunofluorescence and immunoblotting were used to determine the expression pattern. Lysozyme like enzyme activity was assessed by standard assays.

Results

Six rat Lyzl genes namely Lyzl1, Lyzl3, Lyzl4, Lyzl5, Lyzl6 and Lyzl7 were found to be highly conserved among the vertebrates with higher homology to mouse counterparts than with human counterparts. All the LYZL proteins contained the characteristic 4 disulfide bridges similar to c-type lysozyme. Only LYZL 1 and 6, conserved the active site amino acids of the lysozyme. Molecular modeling studies indicated that LYZL proteins exhibit strikingly similar three-dimensional structures among themselves. The secondary structure analysis of the recombinant LYZL proteins indicated the presence of α-helix, β-sheet and random coil with α-helix being the majority. Docking studies indicated the peptidoglycan binding nature of LYZL proteins. All the rat Lyzl mRNA transcripts (Lyzl1, Lyzl3, Lyzl4, Lyzl5, Lyzl6 and Lyzl7) are predominantly expressed in testes though some of them are expressed in tissues other than reproductive tract. Their expression was androgen independent. The rat LYZL proteins are localized in the germinal epithelium and on the spermatozoa. Recombinant LYZL1 and 6 possessed muramidase, isopeptidase and antibacterial activities. The mechanism of antibacterial action of LYZL1 and LYZL6 involved bacterial membrane damage and leakage of cellular contents. Only LYZL1 and 6 possess peptidoglycan binding ability, whereas LYZL3, LYZL4 and LYZL5 possess hyaluronan binding ability suggesting a possible functional divergence of these proteins. LYZL3, LYZL4 and LYZL7 possessed free radical scavenging property, suggesting that they may act as antioxidants.

Conclusion

The divergent properties of LYZL proteins indicate that they may have a role in sperm function, innate immunity and other physiological process as well.

Immunology of breast milk

Summary In the critical phase of immunological immaturity of the newborn, particularly for the immune system of mucous membranes, infants receive large amounts of bioactive components through colostrum and breast milk. Colostrum is the most potent natural immune booster known to science. Breastfeeding protects infants against infections mainly via secretory IgA (SIgA) antibodies, but also via other various bioactive factors. It is striking that the defense factors of human milk function without causing inflammation; some components are even anti-inflammatory. Protection against infections has been well evidenced during lactation against, e.g., acute and prolonged diarrhea, respiratory tract infections, including otitis media, urinary tract infection, neonatal septicemia, and necrotizing enterocolitis. The milk’s immunity content changes over time. In the early stages of lactation, IgA, anti-inflammatory factors and, more likely, immunologically active cells provide additional support for the immature immune system of the neonate. After this period, breast milk continues to adapt extraordinarily to the infant’s ontogeny and needs regarding immune protection and nutrition. The need to encourage breastfeeding is therefore justifiable, at least during the first 6 months of life, when the infant’s secretory IgA production is insignificant.

Oleic acid complex of bovine α-lactalbumin induces eryptosis in human and other erythrocytes by a Ca(2+)-independent mechanism

BACKGROUND

Complexes of oleic acid (OA) with milk α-lactalbumin, received remarkable attention in view of their selective toxicity towards a spectrum of tumors during the last two decades. OA complexes of some structurally related/unrelated proteins are also tumoricidal. Erythrocytes are among the few differentiated cells that are sensitive and undergo hemolysis when exposed to the complexes.

METHODS

The effects of OA complex of bovine α-lactalbumin (Bovine Alpha-lactalbumin Made LEthal to Tumor cells, BAMLET) on human, goat and chicken erythrocytes on calcein leakage, phosphatidylserine exposure, morphological changes and hemolysis were studied by confocal microscopy, FACS analysis, scanning electron microscopy and measuring hemoglobin release.

RESULTS

Erythrocytes exposed to BAMLET undergo eryptosis-like alterations as revealed by calcein leakage, surface phosphatidylserine exposure and transformation to echinocytes at low concentrations and hemolysis when the concentration of the complex was raised. Ca(2+) was not essential and restricted the alterations when included in the medium. The BAMLET-induced alterations in human erythrocytes were prevented by the cation channel inhibitors, amiloride and BaCl2 but not by inhibitors of thiol proteases, sphingomyelinase and by the antioxidant N-acetyl cysteine.

CONCLUSIONS

The work shows for the first time that low concentrations of BAMLET induces eryptosis in erythrocytes by a novel mechanism not requiring Ca(2+) and hemolysis by detergent-like action by the released OA at higher concentrations.

GENERAL SIGNIFICANCE

The study points out to the need for a comprehensive evaluation of the toxicity of OA complexes of α-lactalbumin and other proteins towards erythrocytes and other differentiated cells before being considered for therapy.

Prevention and treatment of colon cancer by peroral administration of HAMLET (human α-lactalbumin made lethal to tumour cells)

BACKGROUND

Most colon cancers start with dysregulated Wnt/β-catenin signalling and remain a major therapeutic challenge. Examining whether HAMLET (human α-lactalbumin made lethal to tumour cells) may be used for colon cancer treatment is logical, based on the properties of the complex and its biological context.

OBJECTIVE

To investigate if HAMLET can be used for colon cancer treatment and prevention. Apc(Min)(/+) mice, which carry mutations relevant to hereditary and sporadic human colorectal tumours, were used as a model for human disease.

METHOD

HAMLET was given perorally in therapeutic and prophylactic regimens. Tumour burden and animal survival of HAMLET-treated and sham-fed mice were compared. Tissue analysis focused on Wnt/β-catenin signalling, proliferation markers and gene expression, using microarrays, immunoblotting, immunohistochemistry and ELISA. Confocal microscopy, reporter assay, immunoprecipitation, immunoblotting, ion flux assays and holographic imaging were used to determine effects on colon cancer cells.

RESULTS

Peroral HAMLET administration reduced tumour progression and mortality in Apc(Min)(/+) mice. HAMLET accumulated specifically in tumour tissue, reduced β-catenin and related tumour markers. Gene expression analysis detected inhibition of Wnt signalling and a shift to a more differentiated phenotype. In colon cancer cells with APC mutations, HAMLET altered β-catenin integrity and localisation through an ion channel-dependent pathway, defining a new mechanism for controlling β-catenin signalling. Remarkably, supplying HAMLET to the drinking water from the time of weaning also significantly prevented tumour development.

CONCLUSIONS

These data identify HAMLET as a new, peroral agent for colon cancer prevention and treatment, especially needed in people carrying APC mutations, where colon cancer remains a leading cause of death.

Anticancer gene transfer for cancer gene therapy

Gene therapy vectors are among the treatments currently used to treat malignant tumors. Gene therapy vectors use a specific therapeutic transgene that causes death in cancer cells. In early attempts at gene therapy, therapeutic transgenes were driven by non-specific vectors which induced toxicity to normal cells in addition to the cancer cells. Recently, novel cancer specific viral vectors have been developed that target cancer cells leaving normal cells unharmed. Here we review such cancer specific gene therapy systems currently used in the treatment of cancer and discuss the major challenges and future directions in this field.

Oleic acid may be the key contributor in the BAMLET-induced erythrocyte hemolysis and tumoricidal action

A chance discovery of the tumoricidal action of a human milk fraction led to the characterization of the active component as oleic acid complex of the α-lactalbumin, which was given the acronym HAMLET. We report in this study that the oleic acid complex of bovine α-lactalbumin (BAMLET) is hemolytic to human erythrocytes as well as to those derived from some other mammals. Indirect immunofluorescence analysis suggested binding of BAMLET to erythrocytes prior to induction of hemolysis. Free OA was hemolytic albeit at higher concentrations, while sodium oleate caused hemolysis at far lower concentrations. Amiloride and BaCl2 offered protection against BAMLET-induced hemolysis suggesting the involvement of a cation leak channel in the process. BAMLET coupled to CNBr-activated Sepharose was not only hemolytic but also tumoricidal to Jurkat and MCF-7 cells in culture. The Sepharose-linked preparation was however not toxic to non-cancerous peritoneal macrophages and primary adipocytes. The tumoricidal action was studied using the MTT-assay while apoptosis induction measured by the annexin V-propidium iodide assay. Repeated incubation of the immobilized BAMLET with erythrocytes depleted oleic acid and decreased the hemolytic activity of the complex. Incubation of MCF-7 and Jurkat cells with OA, soluble or immobilized BAMLET resulted in increase in the uptake of Lyso Tracker Red and Nile red by the cells. The data presented support the contention that oleic acid plays the key role, both in BAMLET-induced hemolysis and tumoricidal action.

Molecular mechanisms of the cytotoxicity of human α-lactalbumin made lethal to tumor cells (HAMLET) and other protein-oleic acid complexes

Although HAMLET (human α-lactalbumin made lethal to tumor cells), a complex formed by human α-lactalbumin and oleic acid, has a unique apoptotic activity for the selective killing of tumor cells, the molecular mechanisms of expression of the HAMLET activity are not well understood. Therefore, we studied the molecular properties of HAMLET and its goat counterpart, GAMLET (goat α-lactalbumin made lethal to tumor cells), by pulse field gradient NMR and 920-MHz two-dimensional NMR techniques. We also examined the expression of HAMLET-like activities of complexes between oleic acid and other proteins that form a stable molten globule state. We observed that both HAMLET and GAMLET at pH 7.5 were heterogeneous, composed of the native protein, the monomeric molten globule-like state, and the oligomeric species. At pH 2.0 and 50 °C, HAMLET and GAMLET appeared in the monomeric state, and we identified the oleic acid-binding site in the complexes by two-dimensional NMR. Rather surprisingly, the binding site thus identified was markedly different between HAMLET and GAMLET. Furthermore, canine milk lysozyme, apo-myoglobin, and β2-microglobulin all formed the HAMLET-like complex with the anti-tumor activity, when the protein was treated with oleic acid under conditions in which their molten globule states were stable. From these results, we conclude that the protein portion of HAMLET, GAMLET, and the other HAMLET-like protein-oleic acid complexes is not the origin of their cytotoxicity to tumor cells and that the protein portion of these complexes plays a role in the delivery of cytotoxic oleic acid molecules into tumor cells across the cell membrane.

Autophagy protein p62/SQSTM1 is involved in HAMLET-induced cell death by modulating apotosis in U87MG cells

HAMLET is a complex of oleic acids and decalcified α-lactalbumin that was discovered to selectively kill tumor cells both in vitro and in vivo. Autophagy is an important cellular process involved in drug-induced cell death of glioma cells. We treated U87MG human glioma cells with HAMLET and found that the cell viability was significantly decreased and accompanied with the activation of autophagy. Interestingly, we observed an increase in p62/SQSTM1, an important substrate of autophagosome enzymes, at the protein level upon HAMLET treatment for short periods. To better understand the functionality of autophagy and p62/SQSTM1 in HAMLET-induced cell death, we modulated the level of autophagy or p62/SQSTM1 with biochemical or genetic methods. The results showed that inhibition of autophagy aggravated HAMLET-induced cell death, whereas activation of authophagy attenuated this process. Meanwhile, we found that overexpression of wild-type p62/SQSTM1 was able to activate caspase-8, and then promote HAMLET-induced apoptosis, whereas knockdown of p62/SQSTM1 manifested the opposite effect. We further demonstrated that the function of p62/SQSTM1 following HAMLET treatment required its C-terminus UBA domain. Our results indicated that in addition to being a marker of autophagy activation in HAMLET-treated glioma cells, p62/SQSTM1 could also function as an important mediator for the activation of caspase-8-dependent cell death.

A unifying mechanism for cancer cell death through ion channel activation by HAMLET

Ion channels and ion fluxes control many aspects of tissue homeostasis. During oncogenic transformation, critical ion channel functions may be perturbed but conserved tumor specific ion fluxes remain to be defined. Here we used the tumoricidal protein-lipid complex HAMLET as a probe to identify ion fluxes involved in tumor cell death. We show that HAMLET activates a non-selective cation current, which reached a magnitude of 2.74±0.88 nA within 1.43±0.13 min from HAMLET application. Rapid ion fluxes were essential for HAMLET-induced carcinoma cell death as inhibitors (amiloride, BaCl₂), preventing the changes in free cellular Na⁺ and K⁺ concentrations also prevented essential steps accompanying carcinoma cell death, including changes in morphology, uptake, global transcription, and MAP kinase activation. Through global transcriptional analysis and phosphorylation arrays, a strong ion flux dependent p38 MAPK response was detected and inhibition of p38 signaling delayed HAMLET-induced death. Healthy, differentiated cells were resistant to HAMLET challenge, which was accompanied by innate immunity rather than p38-activation. The results suggest, for the first time, a unifying mechanism for the initiation of HAMLET’s broad and rapid lethal effect on tumor cells. These findings are particularly significant in view of HAMLET’s documented therapeutic efficacy in human studies and animal models. The results also suggest that HAMLET offers a two-tiered therapeutic approach, killing cancer cells while stimulating an innate immune response in surrounding healthy tissues.

Anti-infectivity of camel polyclonal antibodies against hepatitis C virus in Huh7.5 hepatoma

Purpose

To extend the study of the camel milk proteins which have antiviral activity against HCV, camel naïve polyclonal IgGs, α-lactalbumin were purified from camel milk and their anti-HCV effect was examined using PBMCs and Huh7.5 cell-lines. They were compared with the activity of human polyclonal IgGs and camel lactoferrin and casein.

Material and methods

Three types of experiments were performed on PBMCs and HuH7.5 cell. HCV was directly incubated with the purified proteins and then mixed with both cell types, or the proteins were incubated with the cells and then exposed to HCV, or the HCV pre-infected cells were treated with the proteins to inhibit intracellular replication. The proteins were added to cells or virus at different concentrations and time intervals.

Results

Pretreated PBMCs and Huh7.5 cells with milk proteins were not protected when exposed to HCV infection. The direct interaction between HCV and camel IgGs and camel lactoferrin (cLf) led to a complete inhibition of HCV entry into cells, while casein, α-lactalbumin and human IgGs failed to inhibit HCV entry at any tested concentration. Camel IgGs showed ability to recognize HCV peptides with a significant titer (12 × 10³) in comparison with human IgGs which failed to do it. Camel lactoferrin was capable of inhibiting the intracellular HCV replication at concentrations of 0.25-1.25 mg/ml.

Conclusion

Camel milk naïve polyclonal IgGs isolated from camel milk could inhibit the HCV infectivity and demonstrated strong signal against its synthetic peptides. Lactoferrin inhibit the HCV infectivity started from 0.25 mg/ml. However, α-lactalbumin, human IgGs and casein failed to demonstrate any activity against HCV infectivity.

HAMLET forms annular oligomers when deposited with phospholipid monolayers

Recently, the anticancer activity of human α-lactalbumin made lethal to tumor cells (HAMLET) has been linked to its increased membrane affinity in vitro, at neutral pH, and ability to cause leakage relative to the inactive native bovine α-lactalbumin (BLA) protein. In this study, atomic force microscopy resolved membrane distortions and annular oligomers (AOs) produced by HAMLET when deposited at neutral pH on mica together with a negatively charged lipid monolayer. BLA, BAMLET (HAMLET's bovine counterpart) and membrane-binding Peptide C, corresponding to BLA residues 75-100, also form AO-like structures under these conditions but at higher subphase concentrations than HAMLET. The N-terminal Peptide A, which binds to membranes at acidic but not at neutral pH, did not form AOs. This suggests a correlation between the capacity of the proteins/peptides to integrate into the membrane at neutral pH-as observed by liposome content leakage and circular dichroism experiments-and the formation of AOs, albeit at higher concentrations. Formation of AOs, which might be important to HAMLET's tumor toxic action, appears related to the increased tendency of the protein to populate intermediately folded states compared to the native protein, the formation of which is promoted by, but not uniquely dependent on, the oleic acid molecules associated with HAMLET.

Applications of site-specific labeling to study HAMLET, a tumoricidal complex of α-lactalbumin and oleic acid

BACKGROUND

Alpha-lactalbumin (α-LA) is a calcium-bound mammary gland-specific protein that is found in milk. This protein is a modulator of β1,4-galactosyltransferase enzyme, changing its acceptor specificity from N-acetyl-glucosamine to glucose, to produce lactose, milk's main carbohydrate. When calcium is removed from α-LA, it adopts a molten globule form, and this form, interestingly, when complexed with oleic acid (OA) acquires tumoricidal activity. Such a complex made from human α-LA (hLA) is known as HAMLET (Human A-lactalbumin Made Lethal to Tumor cells), and its tumoricidal activity has been well established.

METHODOLOGY/PRINCIPAL FINDINGS

In the present work, we have used site-specific labeling, a technique previously developed in our laboratory, to label HAMLET with biotin, or a fluoroprobe for confocal microscopy studies. In addition to full length hLA, the α-domain of hLA (αD-hLA) alone is also included in the present study. We have engineered these proteins with a 17-amino acid C-terminal extension (hLA-ext and αD-hLA-ext). A single Thr residue in this extension is glycosylated with 2-acetonyl-galactose (C2-keto-galactose) using polypeptide-α-N-acetylgalactosaminyltransferase II (ppGalNAc-T2) and further conjugated with aminooxy-derivatives of fluoroprobe or biotin molecules.

CONCLUSIONS/SIGNIFICANCE

We found that the molten globule form of hLA and αD-hLA proteins, with or without C-terminal extension, and with and without the conjugated fluoroprobe or biotin molecule, readily form a complex with OA and exhibits tumoricidal activity similar to HAMLET made with full-length hLA protein. The confocal microscopy studies with fluoroprobe-labeled samples show that these proteins are internalized into the cells and found even in the nucleus only when they are complexed with OA. The HAMLET conjugated with a single biotin molecule will be a useful tool to identify the cellular components that are involved with it in the tumoricidal activity.

Naturally occurring, tumor-specific, therapeutic proteins

The emerging approach to cancer treatment known as targeted therapies offers hope in improving the treatment of therapy-resistant cancers. Recent understanding of the molecular pathogenesis of cancer has led to the development of targeted novel drugs such as monoclonal antibodies, small molecule inhibitors, mimetics, antisense and small interference RNA-based strategies, among others. These compounds act on specific targets that are believed to contribute to the development and progression of cancers and resistance of tumors to conventional therapies. Delivered individually or combined with chemo- and/or radiotherapy, such novel drugs have produced significant responses in certain types of cancer. Among the most successful novel compounds are those which target tyrosine kinases (imatinib, trastuzumab, sinutinib, cetuximab). However, these compounds can cause severe side-effects as they inhibit pathways such as epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor, which are also important for normal functions in non-transformed cells. Recently, a number of proteins have been identified which show a remarkable tumor-specific cytotoxic activity. This toxicity is independent of tumor type or specific genetic changes such as p53, pRB or EGFR aberrations. These tumor-specific killer proteins are either derived from common human and animal viruses such as E1A, E4ORF4 and VP3 (apoptin) or of cellular origin, such as TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and MDA-7 (melanoma differentiation associated-7). This review aims to present a current overview of a selection of these proteins with preferential toxicity among cancer cells and will provide an insight into the possible mechanism of action, tumor specificity and their potential as novel tumor-specific cancer therapeutics.

Structure and function of human α-lactalbumin made lethal to tumor cells (HAMLET)-type complexes

Human α-lactalbumin made lethal to tumor cells (HAMLET) and equine lysozyme with oleic acid (ELOA) are complexes consisting of protein and fatty acid that exhibit cytotoxic activities, drastically differing from the activity of their respective proteinaceous compounds. Since the discovery of HAMLET in the 1990s, a wealth of information has been accumulated, illuminating the structural, functional and therapeutic properties of protein complexes with oleic acid, which is summarized in this review. In vitro, both HAMLET and ELOA are produced by using ion-exchange columns preconditioned with oleic acid. However, the complex of human α-lactalbumin with oleic acid with the antitumor activity of HAMLET was found to be naturally present in the acidic fraction of human milk, where it was discovered by serendipity. Structural studies have shown that α-lactalbumin in HAMLET and lysozyme in ELOA are partially unfolded, 'molten-globule'-like, thereby rendering the complexes dynamic and in conformational exchange. HAMLET exists in the monomeric form, whereas ELOA mostly exists as oligomers and the fatty acid stoichiometry varies, with HAMLET holding an average of approximately five oleic acid molecules, whereas ELOA contains a considerably larger number (11- 48). Potent tumoricidal activity is found in both HAMLET and ELOA, and HAMLET has also shown strong potential as an antitumor drug in different in vivo animal models and clinical studies. The gain of new, beneficial function upon partial protein unfolding and fatty acid binding is a remarkable phenomenon, and may reflect a significant generic route of functional diversification of proteins via varying their conformational states and associated ligands.

Recombinant analogs of a novel milk pro-apoptotic peptide, lactaptin, and their effect on cultured human cells

We recently isolated and characterized a human milk peptide, lactaptin, which induced apoptosis of cultured human MCF-7 cells. Lactaptin was identified as a proteolytic fragment of human kappa-casein. Here, we generated two recombinant analogs of the peptide, RL1 and RL2, containing truncated and complete amino acid sequences of lactaptin, respectively. Analogs were produced in E.coli, purified and assayed for biological activity on cultured human MCF-7 cells. RL1 was shown to induce only a small decrease in cell viability, whereas RL2 lowered the viability of MCF-7 cells by 60%. This reduction in MCF-7 cell viability was associated with apoptosis, which was indicated by phosphatidilserine externalization and caspase-7 activation. The viability of A549 and Hep-2 cells was also reduced by RL2, albeit to a lesser degree than seen with MCF-7 cells; this reduced viability was not accompanied by apoptosis. Non-malignant human mesenchymal stem cells (MSC) were completely resistant to RL2 action.

Anticancer genes: inducers of tumour-specific cell death signalling

Recent studies have revealed a new class of genes encoding proteins with specific anticancer activity. Upon ectopic expression, these factors cause cell death specifically in tumour cells by apoptosis, autophagy or mitotic catastrophe, yet normal cells are spared. Some of these genes or their encoded proteins are in clinical development and show promising results, and their signalling pathways are currently under intense investigation. Defining these genes as anticancer genes, we review what is known about their functions, the specific cell death signals they induce and the status of cancer therapy approaches that emulate their function. Systematic screening for such anticancer genes might lead to the identification of a repertoire of signalling pathways directed against cellular alterations that are specific for tumour cells.

Proteins selectively killing tumor cells

All human cells have a genetic program that upon activation will cause cell death, named apoptosis. Cancer cells can grow due to unbalances in proliferation, cell cycle regulation and their apoptosis machinery: genomic mutations resulting in non-functional pro-apoptosis proteins or over-expression of anti-apoptosis proteins form the basis of tumor formation. Surprisingly, lessons learned from viruses show that cancer cannot be regarded simply as the opposite of apoptosis. For instance, adenovirus can only transform cells when both its anti- and pro-apoptotic proteins are produced. Oncolytic viruses are known to replicate selectively in tumor cells resulting in cell death. Proteins derived from viruses, i.e. chicken anemia virus (CAV)-derived apoptosis-inducing protein (apoptin), adenovirus early region 4 open reading frame (E4orf4) and parvovirus-H1 derived non-structural protein 1 (NS1), the human alpha-lactalbumin made lethal to tumor cells (HAMLET), which is present in human milk or the human cytokines melanoma differentiation-associated gene-7 (mda-7) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) have all the ability to induce tumor-selective apoptosis. The tumor-selective apoptosis-inducing proteins seem to interact with transforming survival processes, which can become redirected by these proteins into cell death. Transformation-related processes have been identified, which seem to be crucial for the tumor-selectively killing activity of these proteins. For instance, the transformation-related protein phosphatase 2A (PP2A) plays a role in the induction of tumor-selective apoptosis. The proteins mda-7, TRAIL and HAMLET are already successfully tested in first clinical trials. Proteins harboring tumor-selective apoptosis characteristics represent, therefore, a therapeutic potential and a tool for unraveling tumor-related processes. Fundamental molecular and (pre)clinical therapeutic studies of the various tumor-selective apoptosis-inducing proteins apoptin, E4orf4, HAMLET, mda-7, NS1, TRAIL and related proteins will be discussed.

Apoptin, a tumor-selective killer

Apoptin, a small protein from chicken anemia virus, has attracted great attention, because it specifically kills tumor cells while leaving normal cells unharmed. The subcellular localization of apoptin appears to be crucial for this tumor-selective activity. In normal cells, apoptin resides in the cytoplasm, whereas in cancerous cells it translocates into the nucleus. The nuclear translocation of apoptin is largely controlled by its phosphorylation. In tumor cells, apoptin causes the nuclear accumulation of survival kinases including Akt and is phosphorylated by CDK2. Thereby, apoptin redirects survival signals into cell death responses. Apoptin also binds as a multimeric complex to DNA and interacts with several nuclear targets, such as the anaphase-promoting complex, resulting in a G2/M phase arrest. The proapoptotic signal of apoptin is then transduced from the nucleus to cytoplasm by Nur77, which triggers a p53-independent mitochondrial death pathway. In this review, we summarize recent discoveries of apoptin's mechanism of action that might provide intriguing insights for the development of novel tumor-selective anticancer drugs.

JNK-mediated phosphorylation of paxillin in adhesion assembly and tension-induced cell death by the adenovirus death factor E4orf4

The adenovirus type 2 Early Region 4 ORF4 (E4orf4) protein induces a caspase-independent death program in tumor cells involving changes in actin dynamics that are functionally linked to cell killing. Because an increase in myosin II-based contractility is needed for the death of E4orf4-expressing cells, we have proposed that alteration of cytoskeletal tension is part of the signals engaging the death pathway. Yet the mechanisms involved are poorly defined. Herein, we show that the Jun N-terminal kinase JNK is activated in part through a pathway involving Src, Rho, and ROCK (Rho kinase) and contributes to dysregulate adhesion dynamics and to kill cells in response to E4orf4. JNK supports the formation of atypically robust focal adhesions, which are bound to the assembly of the peculiar actomyosin network typifying E4orf4-induced cell death and which are required for driving nuclear condensation. Remarkably, the dramatic enlargement of focal adhesions, actin remodeling, and cell death all rely on paxillin phosphorylation at Ser-178, which is induced by E4orf4 in a JNK-dependent way. Furthermore, we found that Ser-178-paxillin phosphorylation is necessary to decrease adhesion turnover and to enhance the time residency of paxillin at focal adhesions, promoting its recruitment from an internal pool. Our results indicate that perturbation of tensional homeostasis by E4orf4 involves JNK-regulated changes in paxillin adhesion dynamics that are required to engage the death pathway. Moreover, our findings support a role for JNK-mediated paxillin phosphorylation in adhesion growth and stabilization during tension signaling.