Obesity is characterized as a state of chronic low-grade inflammation, which originates in adipose tissue from adipocyte dysfunction as a consequence of adipocyte hypertrophy. Although rodent models have been pivotal to understanding the etiology of obesity and its co-morbidities, and the roles of specific genes, distinct differences between rodents and humans underscore the need for alternative comparative models. Swine are an attractive comparative model because of similar physiology and anatomy with humans. Ossabaw swine are of particular interest due to their "thrifty" genotype and predisposition to multiple risk factors for the metabolic syndrome.

Saturated fatty acids have the distinct ability to induce inflammation and insulin resistance via activation of toll-like receptor 4, while n-3 fatty acids generally reduce inflammation and restore insulin sensitivity. Therefore, we first used Ossabaw swine to investigate the effects of high saturated fat and high saturated fat plus n-3 fatty acids on adipose tissue macrophages, inflammation, and insulin resistance. In this study, despite severe obesity, Ossabaw swine did not develop systemic inflammation. Consumption of the high palm oil diet did decrease serum adiponectin, and this decrease was attenuated by the addition of n-3 fatty acids to the diet. Additionally, we determined that the majority of adipose tissue macrophages are of an anti-inflammatory phenotype. High dietary palm oil did increase the percentage of inflammatory macrophages, and this increase was attenuated by the addition of n-3 fatty acids. Finally, the consumption of high palm oil induced insulin resistance, which was attenuated by the addition of n-3 fatty acids. Thus, in Ossabaw swine, morbid obesity induces insulin resistance, but insulin resistance is not linked to inflammation.

Obesity in humans and rodents causes an increase in subcutaneous adipocyte size of ~2 to 3 fold. Ossabaw subcutaneous adipocyte size was increased ~4.3 fold due to the consumption of a diet high in fat. Because of marked adipocyte hypertrophy coupled with the lack of inflammation in obese Ossabaw adipocytes, we sought to determine if Ossabaw adipocytes were protected against saturated fatty acid-induced inflammation. Therefore, we investigated the differences between Ossabaw and Yorkshire stromal vascular cell-derived adipocytes with respect to differentiation and palmitate-induced inflammation and insulin resistance. In this study, we determined that Ossabaw adipocytes express more peroxisome proliferator-activated receptor gamma, have more glycerol-3-phosphate dehydrogenase activity, and higher rates of clonal expansion than Yorkshire adipocytes. These finding indicate that Ossabaw adipocytes differentiate to a greater extent than Yorkshire adipocytes. Palmitate decreased insulin-stimulated glucose uptake in both adipocyte genotypes. However, Ossabaw adipocytes had decreased basal and insulin-stimulated glucose uptake compared with Yorkshire adipocytes, although the fold increase was not different with insulin stimulation. With respect to inflammation, palmitate induced nuclear factor kappa B activation equally in both adipocyte genotypes, but only Yorkshire adipocytes secreted interleukin 8 in response to palmitate. Consistent with this, there were no differences in serum or adipose tissue IL-8 concentrations between lean and obese Ossabaw swine. Interleukin 8 induces neutrophil chemotaxis into tissues. Neutrophil infiltration into adipose tissue may be important in the early stages of obesity. These data suggest that differences in Ossabaw adipocytes compared with Yorkshire adipocytes may be instrumental in protecting against inflammation during high fat diet-induced obesity.

Finally, we evaluated global gene expression in subcutaneous and visceral adipose tissue of restricted and ad libitum fed Ossabaw swine by using an Affymetrix microarray. Microarray analysis revealed no differentially expressed genes between lean and obese Ossabaw swine. This result coincides with no metabolic alterations induced by obesity relating to glucose homeostasis or serum lipids. There were a total of 1494 genes differentially expressed by adipose tissue depot, of which 777 were annotated. DAVID was used to identify functional categories that contained over-represented genes that were differentially expressed by depot. Extracellular space was identified as a gene ontology cellular component category. Two KEGG pathways were identified by DAVID. These were arachidonic acid metabolism and chemokine signaling pathway.

Collectively, we have demonstrated that dietary factors influence the development of metabolic alterations in Ossabaw swine as evidenced by the induction of insulin resistance by high dietary palm oil, the attenuation of insulin resistance by the addition of n-3 fatty acids, and the lack of differentially expressed genes in the adipose tissue of obese Ossabaw swine when consuming a diet without added fat. Additionally, even in the presence of insulin resistance, there was minimal evidence of inflammation beyond the slight increase in the inflammatory CD16^-CD14⁺ macrophages in the adipose tissue of Ossabaw swine fed a high dietary palm oil. Palmitate-induced interleukin 8 secretion by Yorkshire adipocytes, but not Ossabaw adipocytes, further supports that Ossabaw swine are resistant to obesity-linked inflammation. The ability of Ossabaw swine to expand adipose tissue may afford protection against the induction of inflammation by diet-induced obesity. Further research into the role of adipose expansion and protection against inflammation is warranted.