Conversion of alpha-lactalbumin to a protein inducing apoptosis

Conserved features of cancer cells define their sensitivity to HAMLET-induced death; c-Myc and glycolysis

HAMLET is the first member of a new family of tumoricidal protein-lipid complexes that kill cancer cells broadly, while sparing healthy, differentiated cells. Many and diverse tumor cell types are sensitive to the lethal effect, suggesting that HAMLET identifies and activates conserved death pathways in cancer cells. Here, we investigated the molecular basis for the difference in sensitivity between cancer cells and healthy cells. Using a combination of small-hairpin RNA (shRNA) inhibition, proteomic and metabolomic technology, we identified the c-Myc oncogene as one essential determinant of HAMLET sensitivity. Increased c-Myc expression levels promoted sensitivity to HAMLET and shRNA knockdown of c-Myc suppressed the lethal response, suggesting that oncogenic transformation with c-Myc creates a HAMLET-sensitive phenotype. Furthermore, HAMLET sensitivity was modified by the glycolytic state of tumor cells. Glucose deprivation sensitized tumor cells to HAMLET-induced cell death and in the shRNA screen, hexokinase 1 (HK1), 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 1 and hypoxia-inducible factor 1α modified HAMLET sensitivity. HK1 was shown to bind HAMLET in a protein array containing ∼8000 targets, and HK activity decreased within 15 min of HAMLET treatment, before morphological signs of tumor cell death. In parallel, HAMLET triggered rapid metabolic paralysis in carcinoma cells. Tumor cells were also shown to contain large amounts of oleic acid and its derivatives already after 15 min. The results identify HAMLET as a novel anti-cancer agent that kills tumor cells by exploiting unifying features of cancer cells such as oncogene addiction or the Warburg effect.

A novel method for preparation of HAMLET-like protein complexes

Some natural proteins induce tumor-selective apoptosis. α-Lactalbumin (α-LA), a milk calcium-binding protein, is converted into an antitumor form, called HAMLET/BAMLET, via partial unfolding and association with oleic acid (OA). Besides triggering multiple cell death mechanisms in tumor cells, HAMLET exhibits bactericidal activity against Streptococcus pneumoniae. The existing methods for preparation of active complexes of α-LA with OA employ neutral pH solutions, which greatly limit water solubility of OA. Therefore these methods suffer from low scalability and/or heterogeneity of the resulting α-LA - OA samples. In this study we present a novel method for preparation of α-LA - OA complexes using alkaline conditions that favor aqueous solubility of OA. The unbound OA is removed by precipitation under acidic conditions. The resulting sample, bLA-OA-45, bears 11 OA molecules and exhibits physico-chemical properties similar to those of BAMLET. Cytotoxic activities of bLA-OA-45 against human epidermoid larynx carcinoma and S. pneumoniae D39 cells are close to those of HAMLET. Treatment of S. pneumoniae with bLA-OA-45 or HAMLET induces depolarization and rupture of the membrane. The cells are markedly rescued from death upon pretreatment with an inhibitor of Ca(2+) transport. Hence, the activation mechanisms of S. pneumoniae death are analogous for these two complexes. The developed express method for preparation of active α-LA - OA complex is high-throughput and suited for development of other protein complexes with low-molecular-weight amphiphilic substances possessing valuable cytotoxic properties.

Apoptosis-like death in bacteria induced by HAMLET, a human milk lipid-protein complex

Background

Apoptosis is the primary means for eliminating unwanted cells in multicellular organisms in order to preserve tissue homeostasis and function. It is characterized by distinct changes in the morphology of the dying cell that are orchestrated by a series of discrete biochemical events. Although there is evidence of primitive forms of programmed cell death also in prokaryotes, no information is available to suggest that prokaryotic death displays mechanistic similarities to the highly regulated programmed death of eukaryotic cells. In this study we compared the characteristics of tumor and bacterial cell death induced by HAMLET, a human milk complex of alpha-lactalbumin and oleic acid.

Methodology/Principal Findings

We show that HAMLET-treated bacteria undergo cell death with mechanistic and morphologic similarities to apoptotic death of tumor cells. In Jurkat cells and Streptococcus pneumoniae death was accompanied by apoptosis-like morphology such as cell shrinkage, DNA condensation, and DNA degradation into high molecular weight fragments of similar sizes, detected by field inverse gel electrophoresis. HAMLET was internalized into tumor cells and associated with mitochondria, causing a rapid depolarization of the mitochondrial membrane and bound to and induced depolarization of the pneumococcal membrane with similar kinetic and magnitude as in mitochondria. Membrane depolarization in both systems required calcium transport, and both tumor cells and bacteria were found to require serine protease activity (but not caspase activity) to execute cell death.

Conclusions/Significance

Our results suggest that many of the morphological changes and biochemical responses associated with apoptosis are present in prokaryotes. Identifying the mechanisms of bacterial cell death has the potential to reveal novel targets for future antimicrobial therapy and to further our understanding of core activation mechanisms of cell death in eukaryote cells.

HAMLET binding to α-actinin facilitates tumor cell detachment

Cell adhesion is tightly regulated by specific molecular interactions and detachment from the extracellular matrix modifies proliferation and survival. HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) is a protein-lipid complex with tumoricidal activity that also triggers tumor cell detachment in vitro and in vivo, suggesting that molecular interactions defining detachment are perturbed in cancer cells. To identify such interactions, cell membrane extracts were used in Far-western blots and HAMLET was shown to bind α-actinins; major F-actin cross-linking proteins and focal adhesion constituents. Synthetic peptide mapping revealed that HAMLET binds to the N-terminal actin-binding domain as well as the integrin-binding domain of α-actinin-4. By co-immunoprecipitation of extracts from HAMLET-treated cancer cells, an interaction with α-actinin-1 and -4 was observed. Inhibition of α-actinin-1 and α-actinin-4 expression by siRNA transfection increased detachment, while α-actinin-4-GFP over-expression significantly delayed rounding up and detachment of tumor cells in response to HAMLET. In response to HAMLET, adherent tumor cells rounded up and detached, suggesting a loss of the actin cytoskeletal organization. These changes were accompanied by a reduction in β1 integrin staining and a decrease in FAK and ERK1/2 phosphorylation, consistent with a disruption of integrin-dependent cell adhesion signaling. Detachment per se did not increase cell death during the 22 hour experimental period, regardless of α-actinin-4 and α-actinin-1 expression levels but adherent cells with low α-actinin levels showed increased death in response to HAMLET. The results suggest that the interaction between HAMLET and α-actinins promotes tumor cell detachment. As α-actinins also associate with signaling molecules, cytoplasmic domains of transmembrane receptors and ion channels, additional α-actinin-dependent mechanisms are discussed.

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.

Lactosomes: structural and compositional classification of unique nanometer-sized protein lipid particles of human milk

Milk fat globules (MFGs) are accepted primarily as triacylglycerol delivery systems. The identification of nanometer-sized lipid-protein particles termed "lactosomes" that do not contain triacylglycerol raises the question of their possible functions. MFGs were isolated by slow centrifugation, and lactosomes were isolated by ultracentrifugation at a density equivalent to plasma high-density lipoproteins (HDL) (d > 1.063 g/mL) from human milk obtained from six volunteers at different lactation stages. Isolated lactosomes were analyzed and compared with MFGs for their size distribution, lipidome, proteome, and functional activity. Lactosomes from early milk, day 8, were found to be similar in size as those from mature milk >28 days, averaging ∼ 25 nm in diameter. In total, 97 nonredundant proteins were identified in the MFG and lactosome fractions, 46 of which were unique to the MFG fraction and 29 of which were unique to the lactosome fraction. The proteins identified in the lactosome and MFG fractions were enriched with proteins identified with immunomodulatory pathways. Unlike MFGs and GM1-laden reconstituted HDL that served as a positive control, lactosomal binding capacity to cholera toxin was weak. Lipidomic analyses found that lactosomes were devoid of triacylglycerol and gangliosides, unlike MFGs, but rich in a variety of phospholipid species. The data found differences in structure, composition, and function between lactosomes and MFG, suggesting that these two particles are derived from different biosynthetic and/or secretory pathways. The results reveal a bioactive lipid-protein, nanometer-length scale particle that is secreted into milk not to supply energy to the infant but to play unique, protective, and regulatory roles.

Label-free liquid chromatography-tandem mass spectrometry analysis with automated phosphopeptide enrichment reveals dynamic human milk protein phosphorylation during lactation

Protein phosphorylation is a critical posttranslational modification that affects cell-cell signaling and protein function. However, quantifying the relative site-specific changes of phosphorylation occupancies remains a major issue. An online enrichment of phosphopeptides using titanium dioxide incorporated in a microchip liquid chromatography device was used to analyze trypsin-digested human milk proteins with mass spectrometry. The method was validated with standards and used to determine the dynamic behavior of protein phosphorylation in human milk from the first month of lactation. α-Casein, β-casein, osteopontin, and chordin-like protein 2 phosphoproteins were shown to vary during this lactation time in an independent manner. In addition, changes in specific regions of these phosphoproteins were found to vary independently. Novel phosphorylation sites were discovered for chordin-like protein 2, α-lactalbumin, β-1,4-galactosyl transferase, and poly-Ig (immunoglobulin) receptor. Coefficients of variation for the quantitation were comparable to those in other contemporary approaches using isotopically labeled peptides, with a median value of 11% for all phosphopeptide occupancies quantified.

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.

Effect of denaturation of alpha-lactalbumin on the formation of BAMLET (bovine alpha-lactalbumin made lethal to tumor cells)

A complex of alpha-lactalbumin with oleic acid, also known as HAMLET/BAMLET (human/bovine alpha-lactalbumin made lethal to tumor cells), causes apoptosis-like death in tumor cells but has little effect on healthy differentiated cells. The aim of this study was to examine whether irreversible denaturation of alpha-lactalbumin is detrimental to the formation and cytotoxicity of BAMLET. Commercial bovine holo alpha-lactalbumin (1-4% w/v) was heated at 80 degrees C for up to 100 min. With an increasing concentration of protein, the denaturation of alpha-lactalbumin proceeded faster, and aggregation became more extensive. Native and sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that a high proportion of the aggregates was linked by disulfide bonds. BAMLET was prepared from native and heat-treated alpha-lactalbumin according to a previously described chromatographic method. Despite the high content of denatured and aggregated alpha-lactalbumin in the heat-treated samples, their conversion into BAMLET was not negatively affected, resulting in BAMLET complexes partly composed of covalently linked aggregates of alpha-lactalbumin. The cytotoxicity of all prepared BAMLET samples was comparable to that of the control sample prepared from native alpha-lactalbumin (LD(50) = 34.6 +/- 2.7 mumol L(-1)). It was concluded that alpha-lactalbumin is not required to be in its native conformation for the conversion into its biologically active BAMLET complex.

HAMLET interacts with lipid membranes and perturbs their structure and integrity

BACKGROUND

Cell membrane interactions rely on lipid bilayer constituents and molecules inserted within the membrane, including specific receptors. HAMLET (human alpha-lactalbumin made lethal to tumor cells) is a tumoricidal complex of partially unfolded alpha-lactalbumin (HLA) and oleic acid that is internalized by tumor cells, suggesting that interactions with the phospholipid bilayer and/or specific receptors may be essential for the tumoricidal effect. This study examined whether HAMLET interacts with artificial membranes and alters membrane structure.

METHODOLOGY/PRINCIPAL FINDINGS

We show by surface plasmon resonance that HAMLET binds with high affinity to surface adherent, unilamellar vesicles of lipids with varying acyl chain composition and net charge. Fluorescence imaging revealed that HAMLET accumulates in membranes of vesicles and perturbs their structure, resulting in increased membrane fluidity. Furthermore, HAMLET disrupted membrane integrity at neutral pH and physiological conditions, as shown by fluorophore leakage experiments. These effects did not occur with either native HLA or a constitutively unfolded Cys-Ala HLA mutant (rHLA(all-Ala)). HAMLET also bound to plasma membrane vesicles formed from intact tumor cells, with accumulation in certain membrane areas, but the complex was not internalized by these vesicles or by the synthetic membrane vesicles.

CONCLUSIONS/SIGNIFICANCE

The results illustrate the difference in membrane affinity between the fatty acid bound and fatty acid free forms of partially unfolded HLA and suggest that HAMLET engages membranes by a mechanism requiring both the protein and the fatty acid. Furthermore, HAMLET binding alters the morphology of the membrane and compromises its integrity, suggesting that membrane perturbation could be an initial step in inducing cell death.

Comparative study on heat stability of camel and bovine apo and holo α-lactalbumin

The stability of camel α-lactalbumin (α-la) against heat denaturation was measured, using circular dichroism (CD) and fluorescence spectroscopy, as well as differential scanning calorimetry (DSC). The experiments were performed in the presence of saturating concentrations of calcium as well as in the presence of EDTA, yielding to the apo form of α-la. The change in heat capacity (ΔCp) suggests a greater contribution of hydrophobic interactions to the stability of holo camel α-la than in its bovine counterpart. Overall the results obtained in this study suggest a greater stability of camel α-la than the bovine protein in both holo and apo states. Also CD experiments showed similar secondary structure for camel and bovine α-la and secondary structure of camel α-la was better preserved than that of bovine α-la during heat denaturation. The differences in thermal stability between the proteins from two species can be primarily ascribed to the difference in the quantity of hydrophobic interactions involved in their folding.

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.

Glycation and phosphorylation of alpha-lactalbumin by dry heating: effect on protein structure and physiological functions

Alpha-lactalbumin (alpha-LA) was glycated with maltopentaose (MP) through the Maillard reaction (MP-alpha-LA) and subsequently phosphorylated by dry heating in the presence of pyrophosphate to investigate its structure and physiological functions. Glycation occurred effectively, and the sugar content of alpha-LA increased by approximately 22.3% through the Maillard reaction. The phosphorylation of MP-alpha-LA was enhanced with an increase in the dry-heating time from 1 to 5 d, and the phosphorous content of MP-alpha-LA increased by approximately 1.01% by dry heating at pH 4.0 and 85 degrees C for 5 d in the presence of pyrophosphate. The electrophoretic mobility of alpha-LA increased with an increase in the phosphorylation level. The circular dichroism spectra showed that the change in the secondary structure of the alpha-LA molecule by glycation and subsequent phosphorylation was slight. However, the Trp fluorescence intensity was increased by phosphorylation after glycation. In addition, the differential scanning calorimetry thermograms of alpha-LA showed that the denaturation temperature of MP-alpha-LA was decreased by phosphorylation. These results indicated that molten (partially unfolded) conformations of alpha-LA were formed by dry heating in the presence of pyrophosphate after glycation. The anti-alpha-LA antibody response was significantly reduced by glycation and subsequent phosphorylation. The suppressive effect of alpha-LA on the production of proinflammatory cytokines such as IL-6 and tumor necrosis factor-alpha from THP-1 cells after stimulation with lipopolysaccharide was significantly enhanced by glycation with MP and was further enhanced by phosphorylation after glycation. The Ca phosphate-solubilizing ability of alpha-LA was enhanced by phosphorylation. The apoptotic activity of alpha-LA was reduced by glycation and subsequent phosphorylation. These results suggest that phosphorylation by dry heating in the presence of pyrophosphate after glycation with MP through the Maillard reaction is a useful method for improvement of the physiological functions of alpha-LA.

The human alpha-lactalbumin molten globule: comparison of structural preferences at pH 2 and pH 7

Structural investigations of molten globules provide an important contribution towards understanding protein folding pathways. A close similarity between equilibrium molten globule states and kinetic species observed during refolding has been reported for several proteins. However, the experimental conditions, and in particular the pH, under which the equilibrium and kinetic species are studied often differ significantly. For human α-lactalbumin (α-LA), the equilibrium molten globule is most often studied at pH 2, the so-called A-state, while kinetic refolding experiments are performed at neutral pH. α-LA contains a large number of acidic amino acid residues that may influence the properties of the molten globule differently at low and neutral pH. In this study, we investigate the structural preferences of the α-LA molten globule at pH 7 at the level of individual residues using nuclear magnetic resonance spectroscopy and compare these data with previous results obtained at pH 2. We show that differences exist in the conformational ensemble that describes the α-LA molten globule at these two pH values. The molten globule at pH 7 is generally less stable than that at the low pH A-state. Most notable are differences in the stability of structure for the C-helix and the calcium-binding loop that precedes it and differences in the contribution of long-range hydrophobic contacts between the N-terminal and C-terminal regions of the α-domain to the stability of the molten globule. Our results are discussed in the context of previous studies of the α-LA molten globule and can be used to reconcile apparent discrepancies in published data relating to the C-helix. In the light of our results, the low pH A-state may not be the best model for the kinetic molten globule observed during refolding of α-LA. The pH-dependent effects reported here for α-LA may be of relevance in comparisons of equilibrium and kinetic molten globules of other proteins.

The bovine lactation genome: insights into the evolution of mammalian milk

Comparison of milk protein and mammary genes in the bovine genome with those from other mammals gives insights into the evolution of lactation.

Background

The newly assembled Bos taurus genome sequence enables the linkage of bovine milk and lactation data with other mammalian genomes.

Results

Using publicly available milk proteome data and mammary expressed sequence tags, 197 milk protein genes and over 6,000 mammary genes were identified in the bovine genome. Intersection of these genes with 238 milk production quantitative trait loci curated from the literature decreased the search space for milk trait effectors by more than an order of magnitude. Genome location analysis revealed a tendency for milk protein genes to be clustered with other mammary genes. Using the genomes of a monotreme (platypus), a marsupial (opossum), and five placental mammals (bovine, human, dog, mice, rat), gene loss and duplication, phylogeny, sequence conservation, and evolution were examined. Compared with other genes in the bovine genome, milk and mammary genes are: more likely to be present in all mammals; more likely to be duplicated in therians; more highly conserved across Mammalia; and evolving more slowly along the bovine lineage. The most divergent proteins in milk were associated with nutritional and immunological components of milk, whereas highly conserved proteins were associated with secretory processes.

Conclusions

Although both copy number and sequence variation contribute to the diversity of milk protein composition across species, our results suggest that this diversity is primarily due to other mechanisms. Our findings support the essentiality of milk to the survival of mammalian neonates and the establishment of milk secretory mechanisms more than 160 million years ago.

Changes in proteasome structure and function caused by HAMLET in tumor cells

Background

Proteasomes control the level of endogenous unfolded proteins by degrading them in the proteolytic core. Insufficient degradation due to altered protein structure or proteasome inhibition may trigger cell death. This study examined the proteasome response to HAMLET, a partially unfolded protein-lipid complex, which is internalized by tumor cells and triggers cell death.

Methodology/Principal Findings

HAMLET bound directly to isolated 20S proteasomes in vitro and in tumor cells significant co-localization of HAMLET and 20S proteasomes was detected by confocal microscopy. This interaction was confirmed by co-immunoprecipitation from extracts of HAMLET-treated tumor cells. HAMLET resisted in vitro degradation by proteasomal enzymes and degradation by intact 20S proteasomes was slow compared to fatty acid-free, partially unfolded α-lactalbumin. After a brief activation, HAMLET inhibited proteasome activity in vitro and in parallel a change in proteasome structure occurred, with modifications of catalytic (β1 and β5) and structural subunits (α2, α3, α6 and β3). Proteasome inhibition was confirmed in extracts from HAMLET-treated cells and there were indications of proteasome fragmentation in HAMLET-treated cells.

Conclusions/Significance

The results suggest that internalized HAMLET is targeted to 20S proteasomes, that the complex resists degradation, inhibits proteasome activity and perturbs proteasome structure. We speculate that perturbations of proteasome structure might contribute to the cytotoxic effects of unfolded protein complexes that invade host cells.

Cytotoxic aggregates of alpha-lactalbumin induced by unsaturated fatty acid induce apoptosis in tumor cells

The effects of three fatty acids on cytotoxic aggregate formation of Ca(2+)-depleted bovine alpha-lactalbumin (apo-BLA) have been studied by UV absorbance spectroscopy and transmission electron microscopy. The experimental results demonstrate that two unsaturated fatty acids, oleic acid and linoleic acid, and one saturated fatty acid, stearic acid, induce the intermediate of apo-BLA at pH 4.0-4.5 to form amorphous aggregates in time- and concentration-dependent manners. These aggregates are dissolved under physiological conditions at 37 degrees C and further characterized by fluorescence spectroscopy, circular dichroism and time-of-flight mass spectrometry. Our data here indicate that the structural characteristics of these aggregates are similar to those of HAMLET/BAMLET (human/bovine alpha-lactalbumin made lethal to tumor cells), a complex of the partially unfolded alpha-lactalbumin with oleic acid. Cell viability experiments indicate the aggregates of apo-BLA induced by oleic acid and linoleic acid show significant dose-dependent cytotoxicity to human lung tumor cells of A549 but those induced by stearic acid have no toxicity to tumor cells. Furthermore, the cytotoxic aggregates of apo-BLA induced by both unsaturated fatty acids induce apoptosis of human lung cancer cell line A549, suggesting that such cytotoxic aggregates of apo-BLA could be potential antitumor drugs. The present study provides insight into the mechanism of fatty acid-dependent oligomerization and cytotoxicity of alpha-lactalbumin, and will be helpful in the understanding of the molecular mechanism of HAMLET/BAMLET formation.

Isolation of ORCTL3 in a novel genetic screen for tumor-specific apoptosis inducers

We have established a systematic high-throughput screen for genes that cause cell death specifically in transformed tumor cells. In a first round of screening cDNAs are detected that induce apoptosis in a transformed human cell line. Positive genes are subsequently tested in a synthetic lethal screen in normal cells versus their isogenic counterparts that have been transformed by a particular oncogene. In this way, ORCTL3 was found to be inactive in normal rat kidney cells (NRK), but to induce apoptosis in NRK cells transformed by oncogenic H-ras. ORCTL3 also causes cell death in v-src-transformed cells and in various human tumor cell lines but not in normal cells or untransformed cell lines. While ORCTL3 is a member of the organic-cation transporter gene family, our data indicate that this gene induces apoptosis independently of its putative transporter activity. Rather, various lines of evidence suggest that ORCTL3 brings about apoptosis via an ER stress mediated mechanism. Finally, we detected ORCTL3 to be down-regulated in human kidney tumors.

Can misfolded proteins be beneficial? The HAMLET case

By changing the three-dimensional structure, a protein can attain new functions, distinct from those of the native protein. Amyloid-forming proteins are one example, in which conformational change may lead to fibril formation and, in many cases, neurodegenerative disease. We have proposed that partial unfolding provides a mechanism to generate new and useful functional variants from a given polypeptide chain. Here we present HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) as an example where partial unfolding and the incorporation of cofactor create a complex with new, beneficial properties. Native alpha-lactalbumin functions as a substrate specifier in lactose synthesis, but when partially unfolded the protein binds oleic acid and forms the tumoricidal HAMLET complex. When the properties of HAMLET were first described they were surprising, as protein folding intermediates and especially amyloid-forming protein intermediates had been regarded as toxic conformations, but since then structural studies have supported functional diversity arising from a change in fold. The properties of HAMLET suggest a mechanism of structure-function variation, which might help the limited number of human protein genes to generate sufficient structural diversity to meet the diverse functional demands of complex organisms.

Beneficial health effects of milk and fermented dairy products--review

Milk is a complex physiological liquid that simultaneously provides nutrients and bioactive components that facilitate the successful postnatal adaptation of the newborn infant by stimulating cellular growth and digestive maturation, the establishment of symbiotic microflora, and the development of gut-associated lymphoid tissues. The number, the potency, and the importance of bioactive compounds in milk and especially in fermented milk products are probably greater than previously thought. They include certain vitamins, specific proteins, bioactive peptides, oligosaccharides, organic (including fatty) acids. Some of them are normal milk components, others emerge during digestive or fermentation processes. Fermented dairy products and probiotic bacteria decrease the absorption of cholesterol. Whey proteins, medium-chain fatty acids and in particular calcium and other minerals may contribute to the beneficial effect of dairy food on body fat and body mass. There has been growing evidence of the role that dairy proteins play in the regulation of satiety, food intake and obesity-related metabolic disorders. Milk proteins, peptides, probiotic lactic acid bacteria, calcium and other minerals can significantly reduce blood pressure. Milk fat contains a number of components having functional properties. Sphingolipids and their active metabolites may exert antimicrobial effects either directly or upon digestion.

Interaction of human alpha-lactalbumin with fatty acids: determination of binding parameters

The interaction of holo- and apo-forms of human alpha-lactalbumin with fatty acids was studied by a partition equilibrium method. Apo-alpha-lactalbumin, obtained by treatment with EDTA, displays one binding site for fatty acids, the association constants for oleic and palmitic acids being 1.9.10(6) and 4.2.10(5) M(-1), respectively. However, holo-alpha-lactalbumin was unable to bind fatty acids as measured by this technique. Likewise, no fatty acids bound to holo-alpha-lactalbumin, isolated using nondenaturing conditions, were detected by gas chromatography. These results demonstrate that the conformational change induced in alpha-lactalbumin by the removal of calcium enables the protein to interact with fatty acids.