RESUMO
Annexin A1 is an intracellular calcium/phospholipid-binding protein that is involved in membrane organization and the regulation of the immune system. It has been attributed an anti-inflammatory role at various control levels, and recently we could show that annexin A1 externalization during secondary necrosis provides an important fail-safe mechanism counteracting inflammatory responses when the timely clearance of apoptotic cells has failed. As such, annexin A1 promotes the engulfment of dying cells and dampens the postphagocytic production of proinflammatory cytokines. In our current follow-up study, we report that exposure of annexin A1 during secondary necrosis coincided with proteolytic processing within its unique N-terminal domain by ADAM10. Most importantly, we demonstrate that the released peptide and culture supernatants of secondary necrotic, annexin A1-externalizing cells induced chemoattraction of monocytes, which was clearly reduced in annexin A1- or ADAM10-knockdown cells. Thus, altogether our findings indicate that annexin A1 externalization and its proteolytic processing into a chemotactic peptide represent final events during apoptosis, which after the transition to secondary necrosis contribute to the recruitment of monocytes and the prevention of inflammation.
Assuntos
Proteínas ADAM/imunologia , Secretases da Proteína Precursora do Amiloide/imunologia , Anexina A1/imunologia , Fatores Quimiotáticos/imunologia , Quimiotaxia/imunologia , Proteínas de Membrana/imunologia , Monócitos/imunologia , Proteólise , Transdução de Sinais/imunologia , Proteínas ADAM/genética , Proteínas ADAM/metabolismo , Proteína ADAM10 , Secretases da Proteína Precursora do Amiloide/genética , Secretases da Proteína Precursora do Amiloide/metabolismo , Anexina A1/genética , Anexina A1/metabolismo , Fatores Quimiotáticos/genética , Fatores Quimiotáticos/metabolismo , Quimiotaxia/genética , Técnicas de Silenciamento de Genes , Células HL-60 , Humanos , Células Jurkat , Proteínas de Membrana/genética , Proteínas de Membrana/metabolismo , Monócitos/metabolismo , Monócitos/patologia , Necrose/genética , Necrose/imunologia , Necrose/metabolismo , Estrutura Terciária de Proteína , Transdução de Sinais/genética , Células U937RESUMO
The engulfment of apoptotic cells is of crucial importance for tissue homeostasis in multicellular organisms. A failure of this process results in secondary necrosis triggering proinflammatory cytokine production and autoimmune disease. In the present study, we investigated the role of annexin A1, an intracellular protein that has been implicated in the efficient removal of apoptotic cells. Consistent with its function as bridging protein in the phagocyte synapse, opsonization of apoptotic cells with purified annexin A1 strongly enhanced their phagocytic uptake. A detailed analysis, however, surprisingly revealed that annexin A1 was hardly exposed to the cell surface of primary apoptotic cells, but was strongly externalized only on secondary necrotic cells. Interestingly, while the exposure of annexin A1 failed to promote the uptake of these late secondary necrotic cells, it efficiently prevented induction of cytokine production in macrophages during engulfment of secondary necrotic cells. Our results therefore suggest that annexin A1 exposure during secondary necrosis provides an important failsafe mechanism counteracting inflammatory responses, even when the timely clearance of apoptotic cells has failed.
