Protein receptor-independent plasma membrane remodeling by HAMLET: a tumoricidal protein-lipid complex

Abstract

A central tenet of signal transduction in eukaryotic cells is that extra-cellular ligands activate specific cell surface receptors, which orchestrate downstream responses. This ‘’protein-centric” view is increasingly challenged by evidence for the involvement of specialized membrane domains in signal transduction. Here, we propose that membrane perturbation may serve as an alternative mechanism to activate a conserved cell-death program in cancer cells. This view emerges from the extraordinary manner in which HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) kills a wide range of tumor cells in vitro and demonstrates therapeutic efficacy and selectivity in cancer models and clinical studies. We identify a ‘’receptor independent” transformation of vesicular motifs in model membranes, which is paralleled by gross remodeling of tumor cell membranes. Furthermore, we find that HAMLET accumulates within these de novo membrane conformations and define membrane blebs as cellular compartments for direct interactions of HAMLET with essential target proteins such as the Ras family of GTPases. Finally, we demonstrate lower sensitivity of healthy cell membranes to HAMLET challenge. These features suggest that HAMLET-induced curvature-dependent membrane conformations serve as surrogate receptors for initiating signal transduction cascades, ultimately leading to cell death.

Bladder cancer therapy without toxicity—A dose‐escalation study of alpha1‐oleate

“…By NMR spectroscopy and molecular modeling, the alpha1 peptide has been shown to retain structural flexibility when bound to oleic acid; features linked to potent membrane effects in tumor cells that initiate the cell death response 17 . In addition to plasma membrane remodeling, the mechanism of action includes internalization of the complex, nuclear translocation and chromatin interactions, inhibiting cell proliferation and preventing repair 15 . Furthermore, broad inhibitory effects on oncogenes such as the Ras GTPases, cancer related signaling pathways and kinase activity and have been identified 15 …”

“…In addition to plasma membrane remodeling, the mechanism of action includes internalization of the complex, nuclear translocation and chromatin interactions, inhibiting cell proliferation and preventing repair 15 . Furthermore, broad inhibitory effects on oncogenes such as the Ras GTPases, cancer related signaling pathways and kinase activity and have been identified 15 …”

“…These complexes acquire tumoricidal activity and have been shown to kill a large number of different tumor cells in vitro, by an apoptosis‐like mechanism. The mechanism of action includes extensive plasma membrane remodeling, internalization of the complex and chromatin interaction, resulting in inhibition of cell proliferation and repair 15 …”

Direct observation of alpha-lactalbumin, adsorption and incorporation into lipid membrane and formation of lipid/protein hybrid structures

“…Besides being an excellent model system for studying protein-membrane interactions and describing specific mechanisms for amphitrophic protein behavior [11], the specific interest in the analysis of α-La-membrane interaction is mainly based on the fact that the bovine and human variant of this protein can alter its biological function and gain a tumoricidal property if partially unfolded and bound to oleic acid [32,[50][51][52]. These complexes are named HAMLET (human α-La made lethal to tumor cells) and BAMLET (bovine α-La made lethal to tumor cells) [50,51] and their interaction with membrane has been shown to be crucial to trigger tumor cell death [32,53,54]. Nevertheless, results remain controversial and a clear unifying picture of membrane and protein structural changes related to the formation of membrane-protein complexes is not clear.…”

Protein-dependent Membrane Interaction of A Partially Disordered Protein Complex with Oleic Acid: Implications for Cancer Lipidomics

“…The efficacy of HAMLET as a selective killer of tumor cells has been documented in vitro and in vivo in several animal models and clinical studies 34 35 36 37 38 . Membrane interactions are essential to trigger tumor cell death and direct effects on membranes are the first step in this process 39 . In vesicular membranes consisting of egg yolk or soybean lipids, HAMLET caused membrane elongation and altered fluidity 40 .…”