RESUMO
Human α-lactalbumin made lethal to tumor cells (HAMLET) is the first member in a new family of protein-lipid complexes that kills tumor cells with high selectivity. The protein component of HAMLET is α-lactalbumin, which in its native state acts as a substrate specifier in the lactose synthase complex, thereby defining a function essential for the survival of lactating mammals. In addition, α-lactalbumin acquires tumoricidal activity after partial unfolding and binding to oleic acid. The lipid cofactor serves the dual role as a stabilizer of the altered fold of the protein and a coactivator of specific steps in tumor cell death. HAMLET is broadly tumoricidal, suggesting that the complex identifies conserved death pathways suitable for targeting by novel therapies. Sensitivity to HAMLET is defined by oncogene expression including Ras and c-Myc and by glycolytic enzymes. Cellular targets are located in the cytoplasmic membrane, cytoskeleton, mitochondria, proteasomes, lysosomes and nuclei, and specific signaling pathways are rapidly activated, first by interactions of HAMLET with the cell membrane and subsequently after HAMLET internalization. Therapeutic effects of HAMLET have been demonstrated in human skin papillomas and bladder cancers, and HAMLET limits the progression of human glioblastomas, with no evidence of toxicity for normal brain or bladder tissue. These findings open up new avenues for cancer therapy and the understanding of conserved death responses in tumor cells.
Assuntos
Glioblastoma , Lactalbumina/administração & dosagem , Terapia de Alvo Molecular , Ácidos Oleicos/administração & dosagem , Neoplasias Cutâneas , Neoplasias da Bexiga Urinária , Morte Celular/efeitos dos fármacos , Glioblastoma/tratamento farmacológico , Glioblastoma/metabolismo , Humanos , Lactalbumina/química , Lactalbumina/metabolismo , Lactose Sintase/química , Lactose Sintase/metabolismo , Ácido Oleico/química , Ácido Oleico/metabolismo , Ácidos Oleicos/química , Ácidos Oleicos/metabolismo , Transdução de Sinais , Neoplasias Cutâneas/tratamento farmacológico , Neoplasias Cutâneas/metabolismo , Neoplasias da Bexiga Urinária/tratamento farmacológico , Neoplasias da Bexiga Urinária/metabolismoRESUMO
HAMLET, a complex of partially unfolded alpha-lactalbumin and oleic acid, kills a wide range of tumor cells. Here we propose that HAMLET causes macroautophagy in tumor cells and that this contributes to their death. Cell death was accompanied by mitochondrial damage and a reduction in the level of active mTOR and HAMLET triggered extensive cytoplasmic vacuolization and the formation of double-membrane-enclosed vesicles typical of macroautophagy. In addition, HAMLET caused a change from uniform (LC3-I) to granular (LC3-II) staining in LC3-GFP-transfected cells reflecting LC3 translocation during macroautophagy, and this was blocked by the macroautophagy inhibitor 3-methyladenine. HAMLET also caused accumulation of LC3-II detected by Western blot when lysosomal degradation was inhibited suggesting that HAMLET caused an increase in autophagic flux. To determine if macroautophagy contributed to cell death, we used RNA interference against Beclin-1 and Atg5. Suppression of Beclin-1 and Atg5 improved the survival of HAMLET-treated tumor cells and inhibited the increase in granular LC3-GFP staining. The results show that HAMLET triggers macroautophagy in tumor cells and suggest that macroautophagy contributes to HAMLET-induced tumor cell death.
Assuntos
Autofagia/efeitos dos fármacos , Lactalbumina/farmacologia , Ácidos Oleicos/farmacologia , Apoptose/efeitos dos fármacos , Proteínas Reguladoras de Apoptose/análise , Proteínas Reguladoras de Apoptose/genética , Proteínas Reguladoras de Apoptose/fisiologia , Proteína 5 Relacionada à Autofagia , Proteína Beclina-1 , Linhagem Celular Tumoral , Humanos , Proteínas de Membrana/análise , Proteínas de Membrana/genética , Proteínas de Membrana/fisiologia , Proteínas Associadas aos Microtúbulos/genética , Proteínas Associadas aos Microtúbulos/metabolismo , Proteínas Associadas aos Microtúbulos/fisiologia , Mitocôndrias/efeitos dos fármacos , Proteínas Quinases/análise , RNA Mensageiro/análise , Serina-Treonina Quinases TORRESUMO
Ion channels and ion fluxes control many aspects of tissue homeostasis. During oncogenic transformation, critical ion channel functions may be perturbed but conserved tumor specific ion fluxes remain to be defined. Here we used the tumoricidal protein-lipid complex HAMLET as a probe to identify ion fluxes involved in tumor cell death. We show that HAMLET activates a non-selective cation current, which reached a magnitude of 2.74±0.88 nA within 1.43±0.13 min from HAMLET application. Rapid ion fluxes were essential for HAMLET-induced carcinoma cell death as inhibitors (amiloride, BaCl2), preventing the changes in free cellular Na(+) and K(+) concentrations also prevented essential steps accompanying carcinoma cell death, including changes in morphology, uptake, global transcription, and MAP kinase activation. Through global transcriptional analysis and phosphorylation arrays, a strong ion flux dependent p38 MAPK response was detected and inhibition of p38 signaling delayed HAMLET-induced death. Healthy, differentiated cells were resistant to HAMLET challenge, which was accompanied by innate immunity rather than p38-activation. The results suggest, for the first time, a unifying mechanism for the initiation of HAMLET's broad and rapid lethal effect on tumor cells. These findings are particularly significant in view of HAMLET's documented therapeutic efficacy in human studies and animal models. The results also suggest that HAMLET offers a two-tiered therapeutic approach, killing cancer cells while stimulating an innate immune response in surrounding healthy tissues.
Assuntos
Morte Celular/fisiologia , Canais Iônicos/metabolismo , Lactalbumina/metabolismo , Ácidos Oleicos/metabolismo , Transporte Biológico , Cálcio/metabolismo , Morte Celular/efeitos dos fármacos , Linhagem Celular Tumoral , Análise por Conglomerados , Perfilação da Expressão Gênica , Humanos , Imunidade Inata , Espaço Intracelular/metabolismo , Canais Iônicos/antagonistas & inibidores , Lactalbumina/imunologia , Ácidos Oleicos/imunologia , Fosforilação , Potássio/metabolismo , Transdução de Sinais , Sódio/metabolismo , Proteínas Quinases p38 Ativadas por Mitógeno/metabolismoRESUMO
Cell adhesion is tightly regulated by specific molecular interactions and detachment from the extracellular matrix modifies proliferation and survival. HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) is a protein-lipid complex with tumoricidal activity that also triggers tumor cell detachment in vitro and in vivo, suggesting that molecular interactions defining detachment are perturbed in cancer cells. To identify such interactions, cell membrane extracts were used in Far-western blots and HAMLET was shown to bind α-actinins; major F-actin cross-linking proteins and focal adhesion constituents. Synthetic peptide mapping revealed that HAMLET binds to the N-terminal actin-binding domain as well as the integrin-binding domain of α-actinin-4. By co-immunoprecipitation of extracts from HAMLET-treated cancer cells, an interaction with α-actinin-1 and -4 was observed. Inhibition of α-actinin-1 and α-actinin-4 expression by siRNA transfection increased detachment, while α-actinin-4-GFP over-expression significantly delayed rounding up and detachment of tumor cells in response to HAMLET. In response to HAMLET, adherent tumor cells rounded up and detached, suggesting a loss of the actin cytoskeletal organization. These changes were accompanied by a reduction in ß1 integrin staining and a decrease in FAK and ERK1/2 phosphorylation, consistent with a disruption of integrin-dependent cell adhesion signaling. Detachment per se did not increase cell death during the 22 hour experimental period, regardless of α-actinin-4 and α-actinin-1 expression levels but adherent cells with low α-actinin levels showed increased death in response to HAMLET. The results suggest that the interaction between HAMLET and α-actinins promotes tumor cell detachment. As α-actinins also associate with signaling molecules, cytoplasmic domains of transmembrane receptors and ion channels, additional α-actinin-dependent mechanisms are discussed.
Assuntos
Actinina/metabolismo , Lactalbumina/metabolismo , Neoplasias/metabolismo , Neoplasias/patologia , Ácidos Oleicos/metabolismo , Actinina/química , Actinas/metabolismo , Sequência de Aminoácidos , Sítios de Ligação , Adesão Celular/efeitos dos fármacos , Morte Celular/efeitos dos fármacos , Extratos Celulares , Linhagem Celular Tumoral , Sobrevivência Celular/efeitos dos fármacos , Proteína-Tirosina Quinases de Adesão Focal/metabolismo , Humanos , Integrina beta1/metabolismo , Modelos Biológicos , Modelos Moleculares , Dados de Sequência Molecular , Neoplasias/enzimologia , Peptídeos/química , Peptídeos/metabolismo , Peptídeos/farmacologia , Ligação Proteica/efeitos dos fármacos , Mapeamento de Interação de Proteínas , Transporte Proteico/efeitos dos fármacos , Transdução de Sinais/efeitos dos fármacosRESUMO
BACKGROUND: Proteasomes control the level of endogenous unfolded proteins by degrading them in the proteolytic core. Insufficient degradation due to altered protein structure or proteasome inhibition may trigger cell death. This study examined the proteasome response to HAMLET, a partially unfolded protein-lipid complex, which is internalized by tumor cells and triggers cell death. METHODOLOGY/PRINCIPAL FINDINGS: HAMLET bound directly to isolated 20S proteasomes in vitro and in tumor cells significant co-localization of HAMLET and 20S proteasomes was detected by confocal microscopy. This interaction was confirmed by co-immunoprecipitation from extracts of HAMLET-treated tumor cells. HAMLET resisted in vitro degradation by proteasomal enzymes and degradation by intact 20S proteasomes was slow compared to fatty acid-free, partially unfolded alpha-lactalbumin. After a brief activation, HAMLET inhibited proteasome activity in vitro and in parallel a change in proteasome structure occurred, with modifications of catalytic (beta1 and beta5) and structural subunits (alpha2, alpha3, alpha6 and beta3). Proteasome inhibition was confirmed in extracts from HAMLET-treated cells and there were indications of proteasome fragmentation in HAMLET-treated cells. CONCLUSIONS/SIGNIFICANCE: The results suggest that internalized HAMLET is targeted to 20S proteasomes, that the complex resists degradation, inhibits proteasome activity and perturbs proteasome structure. We speculate that perturbations of proteasome structure might contribute to the cytotoxic effects of unfolded protein complexes that invade host cells.
Assuntos
Morte Celular/fisiologia , Lactalbumina/metabolismo , Ácidos Oleicos/metabolismo , Complexo de Endopeptidases do Proteassoma , Linhagem Celular Tumoral , Inibidores de Cisteína Proteinase/metabolismo , Humanos , Lactalbumina/química , Leupeptinas/metabolismo , Modelos Moleculares , Ácido Oleico/metabolismo , Complexo de Endopeptidases do Proteassoma/química , Complexo de Endopeptidases do Proteassoma/metabolismo , Análise Serial de Proteínas , Conformação Proteica , Dobramento de Proteína , Subunidades Proteicas/química , Subunidades Proteicas/genética , Subunidades Proteicas/metabolismoRESUMO
HAMLET (human alpha-lactalbumin made lethal to tumor cells) is a tumoricidal complex consisting of partially unfolded protein and fatty acid and was first identified in casein fractions of human breast milk. The complex can be produced from its pure components through a modified chromatographic procedure where preapplied oleic acid binds with partially unfolded alpha-lactalbumin on the stationary phase in situ. Because native alpha-lactalbumin itself cannot trigger cell death, HAMLET's remarkable tumor-selective cytotoxicity has been strongly correlated with the conformational change of the protein upon forming the complex, but whether a recovery to the native state subsequently occurs upon entering the tumor cell is yet unclear. To this end, we utilize a recombinant variant of human alpha-lactalbumin in which all eight cysteine residues are substituted for alanines (rHLA(all-Ala)), rendering the protein nonnative and biologically inactive under all conditions. The HAMLET analogue formed from the complex of rHLA(all-Ala) and oleic acid (rHLA(all-Ala)-OA) exhibited equivalent strong tumoricidal activity against lymphoma and carcinoma cell lines and was shown to accumulate within the nuclei of tumor cells, thus reproducing the cellular trafficking pattern of HAMLET. In contrast, the fatty acid-free rHLA(all-Ala) protein associated with the tumor cell surface but was not internalized and lacked any cytotoxic activity. Structurally, whereas HAMLET exhibited some residual native character in terms of NMR chemical shift dispersion, rHLA(all-Ala)-OA showed significant differences to HAMLET and, in fact, was found to be devoid of any tertiary packing. The results identify alpha-lactalbumin as a protein with strikingly different functions in the native and partially unfolded states. We posit that partial unfolding offers another significant route of functional diversification for proteins within the cell.
Assuntos
Antineoplásicos/química , Antineoplásicos/farmacologia , Lactalbumina/química , Lactalbumina/farmacologia , Ácidos Oleicos/química , Ácidos Oleicos/farmacologia , Substituição de Aminoácidos , Antineoplásicos/metabolismo , Linhagem Celular Tumoral , Núcleo Celular/química , Sobrevivência Celular/efeitos dos fármacos , Cisteína/genética , Células Epiteliais/efeitos dos fármacos , Humanos , Lactalbumina/genética , Lactalbumina/metabolismo , Linfócitos/efeitos dos fármacos , Modelos Moleculares , Mutagênese Sítio-Dirigida , Ressonância Magnética Nuclear Biomolecular , Ácidos Oleicos/genética , Ácidos Oleicos/metabolismo , Estrutura Terciária de ProteínaRESUMO
By changing the three-dimensional structure, a protein can attain new functions, distinct from those of the native protein. Amyloid-forming proteins are one example, in which conformational change may lead to fibril formation and, in many cases, neurodegenerative disease. We have proposed that partial unfolding provides a mechanism to generate new and useful functional variants from a given polypeptide chain. Here we present HAMLET (Human Alpha-lactalbumin Made LEthal to Tumor cells) as an example where partial unfolding and the incorporation of cofactor create a complex with new, beneficial properties. Native alpha-lactalbumin functions as a substrate specifier in lactose synthesis, but when partially unfolded the protein binds oleic acid and forms the tumoricidal HAMLET complex. When the properties of HAMLET were first described they were surprising, as protein folding intermediates and especially amyloid-forming protein intermediates had been regarded as toxic conformations, but since then structural studies have supported functional diversity arising from a change in fold. The properties of HAMLET suggest a mechanism of structure-function variation, which might help the limited number of human protein genes to generate sufficient structural diversity to meet the diverse functional demands of complex organisms.