RESUMO
The phagocytic clearance of apoptotic cells is essential to prevent chronic inflammation and autoimmunity. The phosphatidylserine-binding protein milk fat globule-EGF factor 8 (MFG-E8) is a major opsonin for apoptotic cells, and MFG-E8(-/-) mice spontaneously develop a lupus-like disease. Similar to human systemic lupus erythematosus (SLE), the murine disease is associated with an impaired clearance of apoptotic cells. SLE is routinely treated with glucocorticoids (GCs), whose anti-inflammatory effects are consentaneously attributed to the transrepression of pro-inflammatory cytokines. Here, we show that the GC-mediated transactivation of MFG-E8 expression and the concomitantly enhanced elimination of apoptotic cells constitute a novel aspect in this context. Patients with chronic inflammation receiving high-dose prednisone therapy displayed substantially increased MFG-E8 mRNA levels in circulating monocytes. MFG-E8 induction was dependent on the GC receptor and several GC response elements within the MFG-E8 promoter. Most intriguingly, the inhibition of MFG-E8 induction by RNA interference or genetic knockout strongly reduced or completely abolished the phagocytosis-enhancing effect of GCs in vitro and in vivo. Thus, MFG-E8-dependent promotion of apoptotic cell clearance is a novel anti-inflammatory facet of GC treatment and renders MFG-E8 a prospective target for future therapeutic interventions in SLE.
Assuntos
Antígenos de Superfície/genética , Antígenos de Superfície/metabolismo , Apoptose/imunologia , Glucocorticoides/metabolismo , Proteínas do Leite/genética , Proteínas do Leite/metabolismo , Fagocitose/imunologia , Animais , Linhagem Celular Tumoral , Humanos , Lúpus Eritematoso Sistêmico/genética , Lúpus Eritematoso Sistêmico/imunologia , Lúpus Eritematoso Sistêmico/metabolismo , Macrófagos/imunologia , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Proteínas Opsonizantes/genética , Proteínas Opsonizantes/metabolismo , Regiões Promotoras Genéticas , Interferência de RNA , RNA Interferente Pequeno , Receptores de Glucocorticoides/metabolismo , Elementos de Resposta/genética , Células U937RESUMO
OBJECTIVE: To study whether Dickkopf (DKK)-1, an inhibitor of wingless (Wnt) signalling, is involved in the fusion of sacroiliac joints. METHODS: Mice transgenic for tumour necrosis factor (TNFtg mice), which develop bilateral sacroiliitis, were treated with vehicle, anti-TNF antibody or anti-DKK1 antibody. Sacroiliac joints were analysed for histological signs of inflammation, bone erosion, osteoclast formation and ankylosis. Moreover, expression of collagen type X, beta-catenin and DKK-1 was assessed by immunohistochemistry. RESULTS: There were no signs of spontaneous ankylosis of the sacroiliac joints in TNFtg mice. TNF blockade effectively reduced inflammation, bone erosion and osteoclast numbers in the sacroiliac joints, but did not lead to ankylosis. Blockade of DKK1 had no effect on inflammatory signs of sacroiliitis, but significantly reduced bone erosions and osteoclast counts. Moreover, DKK1 blockade promoted expression of collagen type X, the formation of hypertrophic chondrocytes and ankylosis of sacroiliac joints. CONCLUSION: DKK1 influences inflammatory remodelling of sacroiliac joints by prevention of joint ankylosis. This may indicate an important role of the Wnt signalling pathway in the structural bone changes of axial joint disease. Although this model does not reflect the entire spectrum of ankylosing spondylitis in humans, it helps to explain the pathophysiological processes of sacroiliac joint ankylosis, which is a hallmark of the spondyloarthritides.