CX3CL1, also known as fractalkine, is a chemokine with chemotactic activity for monocytes, T cells, and NK cells in the development of numerous inflammatory conditions in obesity-associated chronic complications such as atherosclerosis, insulin resistance and Type 2 diabetes
[36,37,38][30][31][32]. For instance, the elevated CX3CL1 level has been reported in the blood of patients with type 2 diabetes and obesity
[37][31]. CX3CL1 has been demonstrated to promote monocyte adhesion to human adipocytes as well as to evoke adipose tissue inflammation
[37][31]. A study has shown that CX3CL1-mediating early recruitment of microglia induced by HFD might contribute to the induction of hypothalamic inflammatory response and subsequently the impairment of glucose tolerance and adiposity in experimental obesity
[39][33]. Nevertheless, the role of the CX3CL1-CX3CR1 system in obesity-associated adipose tissue inflammation and insulin resistance remains controversial. For instance,
CX3CR1 deficient mice were protected against the development of HFD-induced obesity and WAT inflammation
[40][34]. Deficiency of CX3CL1-CX3CR1 signaling resulted in the reduction of M2-polarized macrophage migration and an M1-dominant shift of macrophages within WAT. Moreover, mice lacking
CX3CR1 expression with HFD feeding displayed the reduced expression of proinflammatory cytokines and improved the profile of proteins involved in lipid metabolism and thermogenesis in BAT
[41][35]. However, CX3CL1 administration in vivo has been reported to have a contradictory effect in diminishing glucose intolerance and insulin resistance
[42][36]. Lee and colleagues also showed that
CX3CR1−/− mice developed glucose intolerance with diminished insulin secretion on both regular chow and HFD.
CX3CR1 deletion also promoted proinflammatory macrophage accumulation in adipose tissue and liver as well as insulin resistance
[38][32]. Moreover, Nagashimada et al. reported that glucose intolerance, insulin resistance, and hepatic steatosis induced by HFD-induced obesity or leptin deficiency were exacerbated in
CX3CR1−/− mice. Thus, these contradictory observations implicate the dualistic role of the CX3CL1-CX3CR1 signaling in obesity-associated dysregulated glucose metabolism and adipose tissue inflammation. However, the underlying mechanism remains unclear.