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Identification and Functional Characterization of Adipocyte-Derived CXCL12 for Macrophage Recruitment and Insulin Resistance

Identification and Functional Characterization of Adipocyte-Derived CXCL12 for Macrophage Recruitment and Insulin Resistance
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Adipose tissue is a crucial site for the generation of inflammatory responses and mediators in metabolic syndrome. In addition to the intrinsic properties of adipocytes in energy storage and metabolic homeostasis, adipose tissue serves as a key area for the interaction of adipocytes with other factors of the immune system. Macrophages are infiltrated into adipose tissue under obese conditions and these macrophage-adipose complex synergistically secretes pro-inflammatory cytokines exert to induction of insulin resistance. It has been suggested that expanding adipocytes or neighboring pre-adipocytes could start to produce chemotactic signals inducing to macrophage recruitment, and this event is linked to systemic inflammation and insulin resistance. Therefore, studies on macrophage recruitment factors can provide critical information about diagnosis and treatment for diabetes and metabolic syndrome. This study identified CXCL12 (C-X-C motif ligand 12), as a monocyte recruitment factor, from 3T3-L1 adipocyte conditioned media by using an integrated method involving sequential HPLC, cell-based activity assay and mass analysis. Diet-induced obese mice demonstrated a robust increase of CXCL12 expression in white adipose tissue. Treatment of obese mice with CXCL12 receptor (CXCR4) antagonist reduced macrophage accumulation and production of pro-inflammatory cytokines in white adipose tissue, and improved systemic insulin sensitivity. Therefore CXCL12 is a required factor for the establishment of obesity-induced adipose tissue inflammation and systemic insulin resistance. To determine whether a surplus of pro-inflammatory cytokine upregulated in obese state could be the stimuli for CXCL12 expression, this study tested TNF-α as a representative pro-inflammatory cytokine which systemically upregulated in obese state. This study found TNF-α upregulated CXCL12 expression in 3T3-L1 adipocytes. Treatment of inhibitors for TNF-α downstream signaling molecules such as NF-κB and JNK revealed that TNF-α-induced CXCL12 expression depended on NF-κB as well as JNK activation. Accordingly, this study found CXCL12 as a new adipokine and revealed the involvement of CXCL12-CXCR4 axis in obesity induced insulin resistance.
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