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

Abstract

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.

Breastfeeding and the Risk of Childhood Hodgkin Lymphoma: A Systematic Review and Meta-analysis

“…High levels of human soluble TNF-related apoptosis-inducing ligand were found in human colostrum and breast milk, which is an immunoactive substance that plays a key role in controlling apoptosis and cell proliferation in various organs and tissues (Davanzo et al, 2013). Human alpha-lactalbumin made lethal to tumor cells, which is a substance with anticancer activity in human milk, has been reported (Mossberg et al, 2010).…”

The Alpha-Lactalbumin/Oleic Acid Complex and Its Cytotoxic Activity

“…However, the finding should have no major implications on HAMLET studies, as native monomeric -LA is unable to reach the cell nuclei of intact tumor cells [3,5,31].…”

“…Alpha-lactalbumin folding occurs in the lumen of the endoplasmatic reticulum and the protein is subsequently transported to the membrane surface of the Golgi apparatus, where it interacts with galactosyltransferase creating the lactose synthase complex [13,31]. In the absence of -lactalbumin, the enzyme is a non-specific galactosyltransferase involved in transferring galactosyl groups from UDP-galactose to a range of different substrates [13,31].…”

“…In the absence of -lactalbumin, the enzyme is a non-specific galactosyltransferase involved in transferring galactosyl groups from UDP-galactose to a range of different substrates [13,31]. Alpha-lactalbumin functions as a substrate modifier, increasing specificity and affinity of the galactosyltransferase for glucose, when catalyzing the final step of lactose production in a lactating mammary gland [7,32].…”

Structural Origin of ELOA Toxicity – Implication for HAMLET-Type Protein Complexes with Oleic Acid

“…Most notably, the newly obtained features include the ability to specifically kill tumor cells while sparing the healthy, normally functioning ones, as it is the case with human or bovine α-lactalbumin made lethal to tumor cells (HAMLET or BAMLET) [1,2] or to indiscriminately induce cell death in all tested cell lines, as it is the case with equine lysozyme (EL) complex with oleic acid (ELOA) [3,4]. While extensive information has been accumulated on the structural, functional and therapeutic properties of protein complexes with OA, many questions remain still unanswered, such as what is the structural origin of their toxicity, what are the specific targets at the cell surface and/or the cellular interior, what are the mechanisms of cellular uptake?…”