RESUMEN
MicroRNAs have been implicated as important mediators of cancer cell homeostasis, and accumulating data suggest compelling roles for them in the apoptosis pathway. X-linked inhibitor of apoptosis protein (XIAP) is a potent caspase inhibitor and an important barrier to apoptotic cell death, but the mechanisms that determine the diverse range of XIAP expression seen in cancer remains unclear. In this study, we present evidence that miR-24 directly targets the 3'UTR of the XIAP messenger RNA (mRNA) to exert translational repression. Using a heuristic algorithm of bioinformatics analysis and in vitro screening, we identified miR-24 as a candidate regulator of XIAP expression. Array comparative genomic hybridization and spectral karyotype analysis reveal that genomic copy number loss at the miR-24 locus is concordant with the loss of endogenous miR-24 in cancer cells. Using a luciferase construct of the XIAP 3'UTR, we showed that miR-24 specifically coordinates to the XIAP mRNA. Interference with miR-24's binding of the critical seed region, resulting from site-directed mutagenesis of the 3'UTR, significantly abrogated miR-24's effects on XIAP expression. Moreover, miR-24 overexpression can overcome apoptosis resistance in cancer cells via downregulation of XIAP expression, and the resulting cancer cell death induced by tumor necrosis factor-related apoptosis-inducing ligand is executed by the canonical caspase-mediated apoptosis pathway. In summary, our data suggest a novel mechanism by which miR-24 directly modulates XIAP expression level and consequently the apoptosis threshold in cancer cells.
Asunto(s)
Apoptosis/fisiología , MicroARNs/genética , Neoplasias/genética , Neoplasias/patología , Proteína Inhibidora de la Apoptosis Ligada a X/metabolismo , Regiones no Traducidas 3' , Apoptosis/genética , Procesos de Crecimiento Celular/genética , Procesos de Crecimiento Celular/fisiología , Línea Celular Tumoral , Regulación hacia Abajo , Células HeLa , Humanos , MicroARNs/metabolismo , Neoplasias/metabolismo , Transfección , Proteína Inhibidora de la Apoptosis Ligada a X/genéticaAsunto(s)
Antígenos de Superficie/farmacología , Macrófagos/efectos de los fármacos , Macrófagos/metabolismo , Proteínas de la Leche/farmacología , Fagocitosis/efectos de los fármacos , Fosfatidilserinas/metabolismo , Extensiones de la Superficie Celular/efectos de los fármacos , Extensiones de la Superficie Celular/metabolismo , Humanos , Células Jurkat , Macrófagos/citologíaRESUMEN
Since the (re)discovery of cytochrome c (cyt c) in the early 1920s and subsequent detailed characterization of its structure and function in mitochondrial electron transport, it took over 70 years to realize that cyt c plays a different, not less universal role in programmed cell death, apoptosis, by interacting with several proteins and forming apoptosomes. Recently, two additional essential functions of cyt c in apoptosis have been discovered that are carried out via its interactions with anionic phospholipids: a mitochondria specific phospholipid, cardiolipin (CL), and plasma membrane phosphatidylserine (PS). Execution of apoptotic program in cells is accompanied by substantial and early mitochondrial production of reactive oxygen species (ROS). Because antioxidant enhancements protect cells against apoptosis, ROS production was viewed not as a meaningless side effect of mitochondrial disintegration but rather playing some - as yet unidentified - role in apoptosis. This conundrum has been resolved by establishing that mitochondria contain a pool of cyt c, which interacts with CL and acts as a CL oxygenase. The oxygenase is activated during apoptosis, utilizes generated ROS and causes selective oxidation of CL. The oxidized CL is required for the release of pro-apoptotic factors from mitochondria into the cytosol. This redox mechanism of cyt c is realized earlier than its other well-recognized functions in the formation of apoptosomes and caspase activation. In the cytosol, released cyt c interacts with another anionic phospholipid, PS, and catalyzes its oxidation in a similar oxygenase reaction. Peroxidized PS facilitates its externalization essential for the recognition and clearance of apoptotic cells by macrophages. Redox catalysis of plasma membrane PS oxidation constitutes an important redox-dependent function of cyt c in apoptosis and phagocytosis. Thus, cyt c acts as an anionic phospholipid specific oxygenase activated and required for the execution of essential stages of apoptosis. This review is focused on newly discovered redox mechanisms of complexes of cyt c with anionic phospholipids and their role in apoptotic pathways in health and disease.