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.


RESULTS
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
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.
Mossberg
Ann-Kristin
AK
Section of Microbiology, Immunology, and Glycobiology, Institute of Laboratory Medicine, Lund University, Lund, Sweden.
Puchades
Maja
M
Halskau
Øyvind
Ø
Baumann
Anne
A
Lanekoff
Ingela
I
Chao
Yinxia
Y
Martinez
Aurora
A
Svanborg
Catharina
C
Karlsson
Roger
R
D000818:Animals; D017209:Apoptosis; D001667:Binding, Competitive; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism*; D006224:Cricetinae; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007768:Lactalbumin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism*; D008563:Membrane Lipids / Q000737:chemistry; D021962:Membrane Microdomains / Q000737:chemistry / Q000378:metabolism; D018613:Microscopy, Confocal; D009154:Mutation; D019301:Oleic Acid / Q000737:chemistry / Q000378:metabolism*; D016716:PC12 Cells; D017510:Protein Folding; D051381:Rats; D020349:Surface Plasmon Resonance
D008051:Lipid Bilayers; D008563:Membrane Lipids; D019301:Oleic Acid; D007768:Lactalbumin