Final Report Summary - ADDIO (Role of Adipose DPP4 Deletion in Diet-Induced Obesity)
Obesity, insulin resistance and type 2 diabetes are a cluster of metabolic diseases whose proportions are turning epidemic. In the search for understanding the mechanisms underlying these diseases, adipose tissue (AT) has been as a real endocrine organ releasing very diverse bioactive factors named adipokines. During an in-depth characterization of the factors released from human adipocytes, our group identified dipeptidyl peptidase 4 (DPP4), as a novel factor potentially linking obesity to the metabolic syndrome. DPP4 is a well-known therapeutic target in type 2 diabetes, since this enzyme cleaves and inactivates important substrates such as the incretins GIP and GLP1. Incretins are hormones that cause an increase in the amount of insulin released after eating, therefore stimulating a decrease in blood glucose levels. Therefore, DPP4 inhibitors are currently employed as anti-diabetic agents to prolong the insulinotropic effect of incretins. In order to characterize the impact of adipose-tissue derived DPP-4 we generated a unique animal model, an AT specific DPP4 KO mouse. In this model, we have explored the impact of AT-derived DPP4 in diet-induced obesity. Wild type (WT) and KO animal were given whether chow or 60% high fat diet (HFD). 24 weeks after the diet started body composition, glucose and insulin tolerance, AT inflammation and hepatic trygliceride content were analysed (See figure 1).
Figure 1. Schematic diagram depicting the experimental design of the project. Mice with deletion of the DPP4 gene in AT were generated using the cre/loxP system under control of the aP2 promoter on a C57BL/6J background. Body composition was analyzed every 4 weeks by NMR. ipGTT, ipITT were assessed at week 20. Blood parameters and tissue analysis were done at end point.
DPP4 expression in mature adipocytes from KO mice was significantly reduced up to 65 % with unaffected expression in non-adipocyte cells within AT. Serum DPP4 was significantly lower in KO animals on both diets. KO mice gained significantly more weight, fat and lean mass on HFD. The genotype exerted no effect on energy expenditure, respiratory quotient and spontaneous physical activity.
At systemic level, both glucose and insulin tolerance (assessed by intraperitoneal tests) and clamps were affected by HFD but not by the genotype. However, other markers of insulin resistance such as fasting insulin and HOMA-IR were significantly lower in KO mice on HFD. Cholesterol was reduced in both KO mice on chow and on HFD compared to WT, but triglycerides were similar. In this line, trygliceride content in the liver did not differ between the WT and KO animals.
Within adipose tissue, we analyzed adipocyte size as a surrogate marker of insulin resistance. Interestingly, within the HFD group, the KO mice displayed a marked shift in the adipocyte size distribution towards smaller adipocytes. This effect was more dramatic in the subcutaneous fat compared to the visceral depot. AT inflammation plays a pivotal role in the development of insulin resistance, therefore we assessed the expression of the M2 anti-inflammatory macrophage markers vs. M1 pro-inflammatory markers. In both fat depots, differentiation markers remained similar whereas the anti-inflammatory M2 macrophage markers macrophage mannose receptor 1 and interleukin (IL)-10 were significantly upregulated in KO animals in both depots compared to WT under HFD. Nevertheless, in the visceral fat, the pro-inflammatory markers, IL-6 and monocyte chemotactic protein-1 were significantly increased in KO mice compared to WT animals under HFD. Macrophage infiltration in the AT was analyzed by counting galectin-3 positive crown-like structures by immunostaining. Crown-like structures (CLS) are formed as a result of macrophages infiltrating into AT to reabsorbe dead adipocytes. Under HFD, the KO animals displayed significantly enhanced CLS in the visceral depot. Serum DPP4 correlated significantly with adipocyte size in both subcutaneous and visceral fat, while negatively with serum adiponectin.
In conclusion, our model proves that AT is an important source of DPP4 in mice. The higher increase in body weight and fat mass under HFD challenge observed in the KO animals, is not followed by an increased impairment of glucose tolerance. Taken into account that KO animals display smaller adipocytes under HFD, these findings point towards a beneficial role for DPP4 deletion in adipose tissue remodeling during HFD, specially within the subcutaneous fat, where a more anti-inflammatory profile is observed under HFD (as summarized in figure 2). Nevertheless, future studies will help to clarify how DPP4 deletion in AT can be translated into a more efficient protection against in metabolic diseases. This is of special interest in the case of obese type 2 diabetic patients currently treated with DPP4 inhibitors.
Figure 2. Schematic diagram depicting the main outcomes of the ADDIO project.
Figure 1. Schematic diagram depicting the experimental design of the project. Mice with deletion of the DPP4 gene in AT were generated using the cre/loxP system under control of the aP2 promoter on a C57BL/6J background. Body composition was analyzed every 4 weeks by NMR. ipGTT, ipITT were assessed at week 20. Blood parameters and tissue analysis were done at end point.
DPP4 expression in mature adipocytes from KO mice was significantly reduced up to 65 % with unaffected expression in non-adipocyte cells within AT. Serum DPP4 was significantly lower in KO animals on both diets. KO mice gained significantly more weight, fat and lean mass on HFD. The genotype exerted no effect on energy expenditure, respiratory quotient and spontaneous physical activity.
At systemic level, both glucose and insulin tolerance (assessed by intraperitoneal tests) and clamps were affected by HFD but not by the genotype. However, other markers of insulin resistance such as fasting insulin and HOMA-IR were significantly lower in KO mice on HFD. Cholesterol was reduced in both KO mice on chow and on HFD compared to WT, but triglycerides were similar. In this line, trygliceride content in the liver did not differ between the WT and KO animals.
Within adipose tissue, we analyzed adipocyte size as a surrogate marker of insulin resistance. Interestingly, within the HFD group, the KO mice displayed a marked shift in the adipocyte size distribution towards smaller adipocytes. This effect was more dramatic in the subcutaneous fat compared to the visceral depot. AT inflammation plays a pivotal role in the development of insulin resistance, therefore we assessed the expression of the M2 anti-inflammatory macrophage markers vs. M1 pro-inflammatory markers. In both fat depots, differentiation markers remained similar whereas the anti-inflammatory M2 macrophage markers macrophage mannose receptor 1 and interleukin (IL)-10 were significantly upregulated in KO animals in both depots compared to WT under HFD. Nevertheless, in the visceral fat, the pro-inflammatory markers, IL-6 and monocyte chemotactic protein-1 were significantly increased in KO mice compared to WT animals under HFD. Macrophage infiltration in the AT was analyzed by counting galectin-3 positive crown-like structures by immunostaining. Crown-like structures (CLS) are formed as a result of macrophages infiltrating into AT to reabsorbe dead adipocytes. Under HFD, the KO animals displayed significantly enhanced CLS in the visceral depot. Serum DPP4 correlated significantly with adipocyte size in both subcutaneous and visceral fat, while negatively with serum adiponectin.
In conclusion, our model proves that AT is an important source of DPP4 in mice. The higher increase in body weight and fat mass under HFD challenge observed in the KO animals, is not followed by an increased impairment of glucose tolerance. Taken into account that KO animals display smaller adipocytes under HFD, these findings point towards a beneficial role for DPP4 deletion in adipose tissue remodeling during HFD, specially within the subcutaneous fat, where a more anti-inflammatory profile is observed under HFD (as summarized in figure 2). Nevertheless, future studies will help to clarify how DPP4 deletion in AT can be translated into a more efficient protection against in metabolic diseases. This is of special interest in the case of obese type 2 diabetic patients currently treated with DPP4 inhibitors.
Figure 2. Schematic diagram depicting the main outcomes of the ADDIO project.