Previous studies have suggested that MCP-1 is an adipokine whose expression is increased in obese animals (11
). However, it has remained unclear whether MCP-1 contributes to development of the insulin resistance and hepatic steatosis associated with obesity. We have now shown that expression of an MCP-1 transgene in adipose tissue under the control of the aP2
gene promoter was sufficient to induce macrophage infiltration into adipose tissue, insulin resistance, and increased hepatic triglyceride content in mice. Furthermore, disruption of the MCP-1
gene reduced the extents of macrophage accumulation in adipose tissue, insulin resistance, and hepatic steatosis associated with obesity. In addition, acute inhibition of MCP-1 by expression of a dominant-negative mutant also ameliorated insulin resistance and hepatic steatosis in obese mice. Our results indicate that the increased expression of MCP-1 in adipose tissue that is associated with obesity plays an important role in the pathogenesis of insulin resistance, macrophage infiltration into adipose tissue, and hepatic steatosis.
We found that the MCP-1
gene is expressed in 3T3-L1 adipocytes and in adipocytes of obese mice. Furthermore, macrophage infiltration was apparent in adipose tissue of transgenic mice that overexpress MCP-1 in adipocytes. In contrast, the macrophage accumulation in adipose tissue induced by feeding on a high-fat diet in normal mice was inhibited in MCP-1 homozygous KO mice. These results suggest that secretion of MCP-1 from adipocytes directly triggers the recruitment of macrophages to adipose tissue. The infiltrated macrophages may in turn secrete a variety of chemokines and other cytokines that further promote a local inflammatory response and affect gene expression in adipocytes, resulting in systemic insulin resistance. However, given that 3 types of transgene controlled by the aP2
gene promoter were shown to be expressed in macrophages of mice (27
), we cannot exclude the possibility that the MCP-1 transgene was expressed in macrophages of our MCP-1 Tg-B mice and that the MCP-1 produced by these cells contributed to the infiltration of macrophages into the adipose tissue of these animals. Nevertheless, the observation that MCP-1 Tg-B mice manifest a similar phenotype to obese mice in terms of insulin sensitivity and hepatic steatosis suggests that MCP-1 expression in adipose tissue plays an important role in the insulin resistance and hepatic steatosis associated with obesity.
During the preparation of this manuscript, a report was published that high-fat feeding–induced obesity, adipose tissue macrophage infiltration, adipose tissue inflammation, systemic insulin resistance, and hepatic steatosis were decreased in CCR2 KO mice (28
). These phenotypes of CCR2 KO mice were very similar to those of our MCP-1 KO mice, except for adipocyte size, which was larger in CCR2 KO mice than in controls in high-fat diet feeding. The reason for this difference in adipocyte size was not clear. Complex and redundant interactions are known between chemokines and their receptors. CCR2 also recognizes MCP-2 (29
), MCP-3 (30
), and MCP-4 (31
) as well as MCP-1. On the other hand, MCP-1 is the strongest ligand for CCR2 (32
), although it also shows low affinity for CCR11 (33
). The similar phenotypes obtained with MCP-1 and CCR2 KO mice suggest that the MCP-1–CCR2 pathway is actually important for obesity-induced insulin resistance and hepatic steatosis among these redundant chemokine signaling pathways.
Our results obtained with MCP-1 transgenic mice, MCP-1 homozygous KO mice fed a high-fat diet, and obese mice expressing a dominant-negative mutant of MCP-1 support the notion that MCP-1 is responsible for the hepatic insulin resistance and steatosis observed in MCP-1 transgenic mice and obese mice. The increased hepatic glucose production and steatosis apparent in these animals may be explained at least in part by the associated changes in hepatic mRNA levels both for PEPCK
, key enzymes of gluconeogenesis, and for SREBP-1c
, an important transcription factor for lipid synthesis. Given that hepatic expression of MCP-1 and infiltration of macrophages into the liver were not detected in MCP-1 Tg-B mice and that the MCP-1 receptor CCR2 is expressed in the liver (34
), MCP-1 secreted from adipose tissue into the circulation of MCP-1 Tg-B mice may increase the hepatic expression of PEPCK
, and SREBP-1c
genes by interacting with CCR2 in the liver. Furthermore, given that hepatic steatosis is frequently associated with hepatic insulin resistance, the increase in the abundance of SREBP-1c
mRNA in the liver may be responsible for the increases in PEPCK
gene expression. However, overexpression of SREBP-1c
was previously found to suppress the hepatic expression of PEPCK
genes both in vitro and in vivo (35
), suggesting that MCP-1 may increase the abundance of SREBP-1c
mRNA and that of PEPCK
mRNAs by independent mechanisms. The precise mechanisms by which MCP-1 induces hepatic insulin resistance and steatosis require further investigation. Serum FFA levels were significantly increased in MCP-1 Tg-B mice and decreased in MCP-1 KO mice fed a high-fat diet compared with corresponding control animals. These changes in serum FFA level might also contribute to the associated changes in hepatic triglyceride content and insulin resistance (36
Our results with various mouse models have shown that MCP-1 links obesity to insulin resistance, hepatic steatosis, and macrophage infiltration into adipose tissue. Recent studies have also associated MCP-1 with metabolic state in humans. The abundance of MCP-1
mRNA in subcutaneous adipose tissue was found to correlate significantly with both plasma MCP-1 and BMI, and the plasma MCP-1 level also correlated directly with BMI (37
). In addition, the plasma concentration of MCP-1 was significantly associated with markers of the metabolic syndrome, including insulin resistance, type 2 diabetes, hypertension, obesity, waist/hip ratio, and increased serum triglyceride concentration (38
). MCP-1 expression is higher in visceral than in subcutaneous human adipose tissue (39
), and expression of a macrophage antigen in human subcutaneous adipose tissue was significantly correlated with both BMI and adipocyte size (15
). These observations are consistent with the hypothesis that MCP-1 contributes both to the recruitment of macrophages to adipose tissue and to the development of insulin resistance in humans through a mechanism similar to that shown by the present study to be operative in mice. Furthermore, treatment with the insulin-sensitizing agent rosiglitazone resulted in a reduction in plasma MCP-1 concentration in both obese nondiabetic and obese diabetic individuals (40
). We have also shown that administration of troglitazone to mice with obesity induced by a high-fat diet inhibited both the upregulation of MCP-1
gene expression in adipose tissue and the infiltration of macrophages into adipose tissue (H. Kanda, unpublished observations). Given that PPARγ is expressed in mature macrophages (41
), it is possible that thiazolidinediones reduce the plasma concentration of MCP-1 by inhibiting MCP-1 expression in macrophages recruited to adipose tissue.
Obesity, especially visceral obesity, is an important feature of metabolic syndrome, which is also characterized by insulin resistance, glucose intolerance, dyslipidemia, and hypertension. Our present data suggest that MCP-1 links obesity and insulin resistance through induction of an inflammatory response in adipose tissue. We have shown that inhibition of MCP-1 function ameliorated both insulin resistance and hepatic steatosis as well as reduced the extent of macrophage infiltration into adipose tissue of obese mice. Our results thus suggest that MCP-1 plays an important role in the pathogenesis of metabolic syndrome and that inhibition of the interaction of MCP-1 with CCR2 might provide the basis for development of new therapies for this syndrome.