The central role of JNK in the pathogenesis of obesity, insulin resistance, and type 2 diabetes was first suggested by the demonstration that deletion of the JNK1
) leads to decreased adiposity and significant improvements in insulin sensitivity in both dietary and genetic (ob/ob
) mouse models of obesity (8
). Subsequent studies using targeted gene deletion (12
) or reciprocal adoptive transfer (13
) approaches had provided evidence for the contribution of JNK1
inactivation in the adipocytes (14
), skeletal muscles (12
), and myeloid cells, such as macrophages (14
), to the protection against HFD-induced insulin resistance seen in the JNK1−/−
mice. However, none of these subsequent studies with these approaches for selective JNK1
deletion could reproduce the protection from obesity evident in the conventional JNK1−/−
mice. Similarly, the adenovirus-mediated hepatic overexpression of a dominant-negative (dn) form of JNK (11
) also led to the improvement in whole-body insulin resistance, largely secondary to a reduction in hepatic glucose production. Again, this model of JNK inactivation in the liver, albeit only for 2 weeks, had no effect on weight gain in the treated animals. The identity of the cell type responsible for the resistance to obesity in the JNK−/−
mice remains unknown. In this study, we have demonstrated that in the ap2-dn-JNK
transgenic mice, selective inactivation of both JNK1 and JNK2 in the adipose tissue and macrophages can reproduce all the beneficial metabolic effects observed in the JNK1−/−
mice, including protection from HFD-induced adiposity.
It should be noted that the resistance to obesity, together with enhanced insulin signaling, was also demonstrated in the mice with heterozygous JNK1
deletion together with complete JNK2
deletion (JNK+/− JNK2−/−
). The authors concluded that the most critical determinant of insulin sensitivity, inflammation, and overall weight gain in the mice on HFD is likely to be total JNK kinase activity. The findings of our study suggest that the selective suppression of total JNK kinase activity, including inactivation of both JNK1 and JNK2, in adipose tissue, resulting from the combined downregulation of JNK in adipocytes and macrophages, is sufficient to reproduce the beneficial effects observed in the JNK1−/−
or JNK+/− JNK2−/−
mice. Previous studies have demonstrated considerable overlap in the biology and functions of the macrophages and adipocytes in obesity, which appear to be important for the high level of coordination between the inflammatory and metabolic pathways (2
). Our study would support the importance of a cross-talk between the macrophages and adipocytes in mediating the effect of the JNK activation in diet-induced obesity. This information is of potential importance in drug development programs using JNK as a therapeutic target for obesity and type 2 diabetes.
In this study, the ap2-dn-JNK
transgenic mice had decreased adipocyte size, which was also seen in the JNK1−/−
). These transgenic mice had similar caloric intake as their wild-type littermates but had an increased rate of oxygen consumption and energy expenditure, findings that were also proposed to explain for the resistance to obesity in the JNK1−/−
). A reduction in the respiration exchange rate in the ap2-dn-JNK
transgenic mice, suggesting a preferential use of fat as a fuel source, provided another possible mechanism for their reduction in adiposity and triglyceride content in adipocytes. On the other hand, an impairment in adipocyte differentiation did not appear to play a significant role in the reduced fat mass in these transgenic animals.
The mechanism whereby inactivation of JNK in adipose tissue leads to increased energy expenditure remains unknown at this stage. In rodents, BAT is the primary site for energy dissipation (23
). In diet-induced obesity, inflammation also occurs in BAT (24
), which might impair its thermogenic functions by inducing mitochondria dysfunction. In our transgenic mice, the dn JNK is also expressed in high abundance in BAT. Therefore, it is possible that inactivation of JNK in BAT can restore the thermogenic activities impaired by the HFD. This hypothesis is currently under investigation in our laboratory.
On the HFD, the ap2-dn-JNK
transgenic mice in this study had improved glucose tolerance and insulin sensitivity compared with their wild-type littermates. Their improvement in insulin sensitivity was accompanied by enhanced insulin-stimulated glucose uptake in the skeletal muscles, as well as reduced glucose production in response by the liver to pyruvate challenge. They were also protected against HFD-induced hepatosteatosis and had reduced expression of the genes involved in gluconeogenesis, compared with HFD-fed wild-type littermates. In a previous study, mice with selective adipocyte deletion of the JNK1
gene also exhibited a protection against hepatosteatosis and hepatic insulin resistance but had no improvement in skeletal muscle insulin action. Furthermore, in the adipocyte-specific JNK1−/−
mice, the degree of macrophage infiltration in the adipose tissue was similar to wild-type mice and only the production of IL-6, but not TNF-α, was reduced relative to the wild-type mice. It is obvious that the ap2-dn-JNK
transgenic mice in our study exhibited a much more marked protection against the inflammatory changes induced by the HFD. Not only was there a marked reduction in macrophage infiltration in the adipose tissue, there was also evidence of reduced expression of various proinflammatory cytokines and hormones, including TNF-α, MCP-1, A-FABP, and leptin in addition to IL-6. On the other hand, the circulating level of the anti-inflammatory adipokine, adiponectin, was increased. JNK activation on the HFD was also reduced in the liver and skeletal muscle, probably secondary to the reduced circulating levels of JNK-activating adipokines, such as TNF-α (9
) and A-FABP (20
), or to the decreased lipid contents in these tissues. These data suggest that the suppression of JNK activation in both the adipocytes and macrophages is required for the protection against HFD-induced insulin resistance in the skeletal muscles, probably consequent to the amelioration of adipose tissue inflammation and adipokine dysregulation.
In conclusion, this study has provided the first evidence that selective suppression of both JNK1 and JNK2 activation in adipose tissue and macrophages can protect against diet-induced obesity, through an increase in energy expenditure and fat utilization. Our data also support the importance of the cross-talk between the inflammatory and metabolic pathways mediated by macrophages and adipocytes in the development of obesity, insulin resistance, and type 2 diabetes.