BAMLET activates a lysosomal cell death program in cancer cells.
A complex of human alpha-lactalbumin and oleic acid (HAMLET) was originally isolated from human milk as a potent anticancer agent. It kills a wide range of transformed cells of various origins while leaving nontransformed healthy cells largely unaffected both in vitro and in vivo. Importantly, purified alpha-lactalbumins from other mammals form complexes with oleic acid that show biological activities similar to that of HAMLET. The mechanism by which these protein-lipid complexes kill tumor cells is, however, largely unknown. Here, we show that complex of bovine alpha-lactalbumin and oleic acid (BAMLET), the bovine counterpart of HAMLET, kills tumor cells via a mechanism involving lysosomal membrane permeabilization. BAMLET shows potent cytotoxic activity against eight cancer cell lines tested, whereas nontransformed NIH-3T3 murine embryonic fibroblasts are relatively resistant. BAMLET accumulates rapidly and specifically in the endolysosomal compartment of tumor cells and induces an early leakage of lysosomal cathepsins into the cytosol followed by the activation of the proapoptotic protein Bax. Ectopic expression of three proteins known to stabilize the lysosomal compartment, i.e. heat shock protein 70 (Hsp70), Hsp70-2, and lens epithelium-derived growth factor, confer significant protection against BAMLET-induced cell death, whereas the antiapoptotic protein Bcl-2, caspase inhibition, and autophagy inhibition fail to do so. These data indicate that BAMLET triggers lysosomal cell death pathway in cancer cells, thereby clarifying the ability of alpha-lactalbumin:oleate complexes to kill highly apoptosis-resistant tumor cells.
Rammer
Paul
P
Apoptosis Department and Centre for Genotoxic Stress Response, Institute for Cancer Biology, Danish Cancer Society, Copenhagen, Denmark.
Groth-Pedersen
Line
L
Kirkegaard
Thomas
T
Daugaard
Mads
M
Rytter
Anna
A
Szyniarowski
Piotr
P
Høyer-Hansen
Maria
M
Povlsen
Lou Klitgaard
LK
Nylandsted
Jesper
J
Larsen
Jakob E
JE
Jäättelä
Marja
M
D000818:Animals; D001343:Autophagy / Q000187:drug effects; D061945:Caspase Inhibitors; D002417:Cattle; D016923:Cell Death / Q000187:drug effects; D045744:Cell Line, Tumor; D019610:Cytoprotection / Q000187:drug effects; D004354:Drug Screening Assays, Antitumor; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism; D007768:Lactalbumin / Q000737:chemistry / Q000494:pharmacology*; D008247:Lysosomes / Q000187:drug effects* / Q000378:metabolism*; D051379:Mice; D009369:Neoplasms / Q000378:metabolism / Q000473:pathology*; D019301:Oleic Acid / Q000737:chemistry / Q000494:pharmacology*; D009829:Oleic Acids / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D051028:bcl-2-Associated X Protein / Q000378:metabolism
D061945:Caspase Inhibitors; C506422:HAMLET complex, human; D018840:HSP70 Heat-Shock Proteins; D036341:Intercellular Signaling Peptides and Proteins; D009829:Oleic Acids; D051028:bcl-2-Associated X Protein; C119380:lens epithelium-derived growth factor; D019301:Oleic Acid; D007768:Lactalbumin