HAMLET



HAMLET

Introduction

HAMLET (human a -lactalbumin made lethal to tumor cells) was first named after the coincidental discovery of the killing of tumor cells when human breast milk fractions were tested to have effects on bacterial attachment to alveolar type II lung carcinoma cells. A casein fraction obtained after low pH precipitation of human milk was determined to be responsible for the lethal impact on tumor cells. The active component of casein fractions was further analyzed by the ion exchange chromatography and the multimeric a -lactalbumin (MAL) was identified. The MAL-containing substance was found to be able to induce apoptosis in a variety of transformed and immature mammalian cells, but not in normal differentiated cells (Hakansson et al., 1995). Strikingly, the native a-lactalbumin (a-LA) proteins had no detectable effect on tumor cells, but the oleic acid (OA, C18:1, 9 cis) was identified as the required co-factor to complex with _-lactalbumin for the killing of cells. Thus, the ethylenediaminetetraacetic acid (EDTA) treatment to remove the chelated calcium ion from the a -LA protein allows the binding of OA that preloaded on an ion exchange matrix for the preparation of HAMLET (Svensson et al., 2003). So far, HAMLET is known as a tumoricidal complex which is comprised of apo a -LA and OA (Pettersson et al., 2006). HAMLET was demonstrated to effectively destroy a great variety of tumor cells ranging from carcinomas of lung, throat, kidney, colon, bladder, prostate, ovaries, melanomas, to glioblastomas of the brain and leukemia (Gustafsson et al., 2005). Remarkably, the normal and well-differentiated cells were tolerant to HAMLET treatments and were much less affected.

Discussion

As the studies focusing on the structure and mechanisms of HAMLET or bovine a -lactalbumin made lethal to tumor cells (BAMLET) go on recently, other molecule complex comprising of protein and oleic acid (OA), including human a - lactalbumin (HLA) orthologues (bovine, equine, porcine, ovine and caprine) (Spolaore et al., 2010) and structural homologue (equine lysozyme) (Wilhelm et al., 2009) were found to have similar structures and functions of HAMLET. We categorize these complexes as the XAMLET, where “X” means the expansion from human to other species, and “A” may refer to other alternative proteins besides the a -LA, like the amyloid beta. In this review, we discussed the preparation, structural analyses, biological activities and the mechanism of XAMLET from recent reports and prospect the application in the related studies.

Structure Of Hamlet

Previous studies have indicated that the HAMLET is a complex comprised of apo a -LA and OA, but the structural basis and the function of HAMLET remained unclear. The structure of the a -LA, major protein component of HAMLET, is of great interest in mediating the cytotoxicity. The a -LA is a small (MW 14.2kDa), acidic (pI 4-5) calcium-binding protein in human milk and functions as a co-enzyme of galactosyltransferase in lactose synthesis (Permyakov et al., 2005). The native a -LA consists of two domains in the entire 122-123 amino acid sequence, a large a -helical domain contains four a - helices (amino acids 5-11, ...