Mechanism and significance of adipose inflammatory recruitment
Creators: name: Haczeyni, Fahrettin
Publisher: name: The Australian National University
Year: 2015
Subjects: Adipose Tissue
Adipose Inflammation
Insulin Resistance
Diabetes
NAFLD
NASH
Liver
Morphometry
Exercise

description: Adipose tissue has significant roles in whole body energy homeostasis, systemic insulin sensitivity, and lipid metabolism. Increased food intake, physical inactivity, and genetic predisposition lead to over-expansion of adipose tissues. Under constant energy surplus, adipocytes become hypertrophic and adipose tissues undergo hyperplasia. These tissue modifications lead to recruitment of preadipocytes and preadipocyte progenitors (mesenchymal stem cells) into adipogenic lineage, thereby increases the lipid storage capacity of adipose tissues. This keeps circulating blood glucose and fatty acids below toxic levels; however, adipocytes have a saturation point where they cannot store more lipids; when adipocytes are completely engorged with lipids, they start expressing stress signals to recruit inflammation into the tissue. While the mechanisms involved in recruitment of adipose inflammation remain largely unknown, some findings point to “extensive adiposity” as the responsible factor. This thesis focuses on persistent adipose inflammation and its relationship with metabolic comorbidities such as insulin resistance, type 2 diabetes, and particularly, non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). A theme of this research is that adipose tissue is not inert, but as closely linked functional “mini endocrine organs” distributed throughout the body. The current literature regarding structural and functional differences in different adipose tissues is reviewed with a focus on morphometric changes (under energy surplus), and systemic effects of adipose inflammation and dysfunction (development of insulin resistance, NAFLD, etc.). The contributions of muscle activity in bodily energy homeostasis and the close interaction between muscle, adipose, liver and insulin metabolism are discussed, and the implications of “adipose failure” for lipid partitioning into the liver, a key pathway to pathogenesis of NAFLD/NASH is highlighted. The low yield of cellular material and the excessive lipid contamination make it harder to work with adipose compared to liver. The candidate developed experimental protocols for this study, which were optimised, including adipose morphometric analysis, that were used to describe the development and progression of adipose tissue inflammation and fatty liver disease in a mouse model of NASH. We examined the effects of feeding an atherogenic diet (23% fat, 45% carbohydrate, 0.19% cholesterol) on adipose morphometry and the recruitment of inflammatory cells. We studied Alms1 mutant foz/foz mice, which have a profound disturbance of hypothalamic appetite regulation. foz/foz mice fed an atherogenic diet developed adipocyte hypertrophy, adipose inflammation, hyperglycemia, and evidence of NASH. Wildtype (WT) mice fed the same diet developed milder metabolic phenotype compared to foz/foz mice. The phenotypic switch towards a proinflammatory phenotype in enlarged adipocytes, this increasing cellular stress causes recruitment of macrophage crown-like structures (CLSs) into adipose tissue and a higher rate of adipocyte injury/necrosis. Toll-like receptors (TLRs) are innate immune system receptors activated by danger-associated molecular patterns (DAMPs). Necrotic cellular debris contains DAMPs which can stimulate TLR9 signalling. Activation of TLR9, particularly in macrophages, exacerbates adipose inflammation. Other works support a role for TLR4 in adipose inflammation, but the roles of other pattern recognizing receptors are less clear. In this study, we used Tlr9-/- mouse to investigate the contribution of TLR9 signalling, if any, to adipose inflammatory recruitment. Increased calorie intake with atherogenic feeding for 24 weeks, led to inflammation in adipose tissue. TLR9 deletion abolished this effect. Correspondingly, NASH prevalence was much less in Tlr9-/- mice. The farnesoid X receptor (FXR) agonist obeticholic acid (6-ECDCA) improves steatosis in patients with NASH, but protective mechanisms remain unresolved. We therefore investigated the effects of 6-ECDCA (1mg/kg/day) on glucose metabolism, multiple adipose compartments and liver in atherogenic diet-fed foz/foz and WT mice. 6-ECDCA reduced body weight, liver mass and hepatic lipid partitioning with striking improvement of glucose tolerance in WT but not foz/foz mice, in which it had no effect on liver histology. 6-ECDCA limited expansion of adipose tissues in atherogenic diet-fed WT but not foz/foz mice. In addition, 6-ECDCA treatment altered macrophage polarization towards a more anti-inflammatory phenotype in WT mouse adipose compartments but not foz/foz mice. To conclude, 6-ECDCA improves glucose metabolism, adiposity and adipose inflammation in animals with a milder metabolic phenotype. Conversely, 6-ECDCA fails to improve adipose inflammation or hepatic lipid partitioning in profoundly obese mice, and there is no reversal of NASH. These results help explain why 6-ECDCA treatment against human NASH improves steatosis but fails to reverse NASH pathology. Physical inactivity contributes to adverse effects of overnutrition. We studied the effects of an exercise intervention on adipose and liver pathology. From weaning, mice were provided with an in-cage exercise wheel, in which they were calculated to run over 4 km/day. In this study, voluntary exercise protected mice against the metabolic effects of high calorie intake and atherogenic dietary feeding by reducing adipose inflammation and maintaining systemic insulin sensitivity, the latter principally with its effects on muscle. Improvements in fatty liver pathology appear to be one of the benefits conferred by exercise. In the final chapter, the most important findings of earlier experiments are discussed in relation to and how they extend the knowledge in the field. Dysfunction of adipose tissue related to stress and inflammation is of pathologic importance. Limited lipid storage capacity and pro-inflammatory factors released from adipose depots can disrupt normal functioning of other organs. Circulating lipids increase when adipose is inflamed and leading to adverse effects on other tissues, primarily the liver. Meanwhile, inflammation in adipose tissue becomes persistent. Obesity-associated NAFLD/NASH, type 2 diabetes and metabolic syndrome are closely linked to persistent adipose inflammation.