RESUMEN
Phospholipid scramblase induces nonspecific bidirectional movement of phospholipids across the membrane during cell activation and has been proposed to mediate the appearance of phosphatidylserine (PS) in the plasma membrane outer leaflet during apoptosis, a cell surface change that is critical for apoptotic cell removal. We report here that protein kinase C (PKC) delta plays an important role in activated transbilayer movement of phospholipids and surface PS exposure by directly enhancing the activity of phospholipid scramblase. Specific inhibition of PKCdelta by rottlerin prevented both apoptosis- and activation-induced scramblase activity. PKCdelta was either selectively cleaved and activated in a caspase 3-dependent manner (during apoptosis) or translocated to the plasma membrane (in stimulated cells) and could directly phosphorylate scramblase immunoprecipitated from Jurkat cells. Furthermore, reconstitution of PKCdelta and scramblase, but not scramblase or PKCdelta alone in Chinese hamster ovary cells demonstrated enhanced scramblase activity.
Asunto(s)
Apoptosis , Proteínas Portadoras/metabolismo , Isoenzimas/metabolismo , Proteínas de la Membrana/metabolismo , Proteínas de Transferencia de Fosfolípidos , Proteína Quinasa C/metabolismo , Secuencia de Aminoácidos , Animales , Células CHO , Cricetinae , Humanos , Células Jurkat , Membrana Dobles de Lípidos , Datos de Secuencia Molecular , Proteína Quinasa C-delta , TransfecciónRESUMEN
During apoptosis, phosphatidylserine (PS) is moved from the plasma membrane inner leaflet to the outer leaflet where it triggers recognition and phagocytosis of the apoptotic cell. Although the mechanisms of PS appearance during apoptosis are not well understood, it is thought that declining activity of the aminophospholipid translocase and calcium-mediated, nonspecific flip-flop of phospholipids play a role. As previous studies in the erythrocyte ghost have shown that polyamines can alter flip-flop of phospholipids, we asked whether alterations in cellular polyamines in intact cells undergoing apoptosis would affect PS appearance, either by altering aminophospholipid translocase activity or phospholipid flip-flop. Cells of the human leukemic cell line, HL-60, were incubated with or without the ornithine decarboxylase inhibitor, difluoromethylornithine (DFMO), and induced to undergo apoptosis by ultraviolet irradiation. Whereas DFMO treatment resulted in profound depletion of putrescine and spermidine (but not spermine), it had no effect on caspase activity, DNA fragmentation, or plasma membrane vesiculation, typical characteristics of apoptosis. Notably, DFMO treatment prior to ultraviolet irradiation did not alter the decline in PS inward movement by the aminophospholipid translocase as measured by the uptake of 6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aminocaproyl] (NBD)-labeled PS detected in the flow cytometer. Conversely, the appearance of endogenous PS in the plasma membrane outer leaflet detected with fluorescein isothiocyanate-labeled annexin V and enhanced phospholipid flip-flop detected by the uptake of 1-palmitoyl-1-[6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)aminocaproyl]-sn-glycero-3-phosphocholine (NBD-PC) seen during apoptosis were significantly inhibited by prior DFMO treatment. Importantly, replenishment of spermidine, by treatment with exogenous putrescine to bypass the metabolic blockade by DFMO, restored both enhanced phospholipid flip-flop and appearance of PS during apoptosis. Such restoration was seen even in the presence of cycloheximide but was not seen when polyamines were added externally just prior to assay. Taken together, these data show that intracellular polyamines can modulate PS appearance resulting from nonspecific flip-flop of phospholipids across the plasma membrane during apoptosis.
Asunto(s)
Apoptosis/fisiología , Poliaminas Biogénicas/fisiología , Lípidos de la Membrana/metabolismo , Fosfolípidos/metabolismo , Secuencia de Bases , Poliaminas Biogénicas/metabolismo , Cartilla de ADN , Eflornitina/farmacología , Inhibidores Enzimáticos/farmacología , Células HL-60 , Humanos , Inhibidores de la Ornitina DescarboxilasaRESUMEN
Phosphatidylserine (PS), ordinarily sequestered in the plasma membrane inner leaflet, appears in the outer leaflet during apoptosis, where it triggers non-inflammatory phagocytic recognition of the apoptotic cell. The mechanism of PS appearance during apoptosis is not well understood but has been associated with loss of aminophospholipid translocase activity and nonspecific flip-flop of phospholipids of various classes. The human leukemic cell line HL-60, the T cell line Jurkat, and peripheral blood neutrophils, undergoing apoptosis induced either with UV irradiation or anti-Fas antibody, were probed in the cytofluorograph for (i) surface PS using fluorescein isothiocyanate-labeled annexin V, (ii) PS uptake by the aminophospholipid translocase using [6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino] caproyl] (NBD)-labeled PS, (iii) nonspecific uptake of phospholipids (as a measure of transbilayer flip-flop) using NBD-labeled phosphatidylcholine, and (iv) the appearance of hypodiploid DNA. In all three types of cells undergoing apoptosis, the appearance of PS followed loss of aminophospholipid translocase and was accompanied by nonspecific phospholipid flip-flop. Importantly, however, in the absence of extracellular calcium, the appearance of PS was completely inhibited despite DNA fragmentation and loss of aminophospholipid translocase activity, the latter demonstrating that loss of the translocase is insufficient for PS appearance during apoptosis. Furthermore, while both the appearance of PS and nonspecific phospholipid uptake demonstrated identical extracellular calcium requirements with an ED50 of nearly 100 microM, the magnitude of PS appearance depended on the level of aminophospholipid translocase activity. Taken together, the data strongly suggest that while nonspecific flip-flop is the driving event for PS appearance in the plasma membrane outer leaflet, aminophospholipid translocase activity ultimately modulates its appearance.