Cytokines use different intracellular mechanisms to upregulate the membrane expression of CX 3 CR1 in human monocytes

Abstract

Membrane expression of fractalkine (CX 3 CL1)-receptor (CX 3 CR1) is relevant in monocytes (Mo) because CX 3 CR1-CX 3 CL1 interactions might participate on both, homeostatic and pathologic conditions. We have previously demonstrated that CX 3 CR1 levels are decreased during culture and when Mo are differentiated into dendritic cells, but enhanced when differentiated into macrophages. Regarding soluble factors, lipopolysaccharide (LPS) accelerated the loss of CX 3 CR1, while interleukin (IL)-10 and Interferon-gamma (IFN-γ) prevented it. However, the comprehensive knowledge about the intracellular pathways that underlay the level of CX 3 CR1 expression in Mo is still incomplete. In the current work, we studied the effect of anti-inflammatory cytokines (IL-4, IL-13, IL-10), alone or together with IFN- γ on CX 3 CR1 expression. We found that only IL-10 and IFN-γ separately were able to prevent CX 3 CR1 down-modulation during culture of human Mo. Besides, Mo incubated with IL-10 plus IFN-γ showed the highest CX 3 CR1 expression by cell, suggesting cooperation between two different mechanism used by both cytokines. By studying intracellular mechanisms triggered by IL-10 and IFN-γ, we demonstrated that they specifically induced PI3K-dependent serine-phosphorylation of signal transducer and activator of transcription (STAT)3 or STAT1, respectively. Moreover, chemical inhibitors of STAT1 or STAT3 abrogated IFN-γ or IL-10 effects on CX 3 CR1 expression. Strikingly, only IL-10 increased CX 3 CR1 mRNA level, as consequence of augmenting mRNA stability. CX 3 CR1 mRNA increase was PI3K-dependent, supporting the causal link between the action of IL-10 at the CX 3 CR1 transcript and CX 3 CR1 protein level on Mo. Thus, both cytokines up-regulate CX 3 CR1 expression on human Mo by different intracellular mechanisms.