The Relationship of Infiltrative Leukocytes to Adipose Tissue Chemokine Expression and Insulin Resistance in Diet-Induced Obesity in Horses
Equine metabolic syndrome is characterized by obesity (generalized or regional adiposity), insulin resistance, and increased risk of laminitis. In obesity in other species, fat depots function as "inflammatory organs" in which the production of chemokines such as monocyte chemoattractant protein (MCP) isoforms results in infiltration of the adipose tissue by macrophages. This abnormal milieu of macrophages and adipocytes is reportedly responsible for the production of inflammatory molecules including proinflammatory cytokines and plasminogen activator inhibitor-1 (PAI-1); whereas as the inflammatory molecules can be produced by both adipocytes and macrophages, the majority of PAI-1 is reported to be produced by macrophages. The release of these proinflammatory cytokines and PAI-1 into the circulation contributes to the decrease in insulin sensitivity observed in this syndrome. In a companion study of equine diet-induced obesity, twelve Arabian horses were documented to undergo an ~20% increase in bodyweight and a marked decrease in insulin sensitivity after being fed 200% of daily digestible energy requirements for 4 months. However, in samples of subcutaneous adipose tissue obtained before and after weight gain, we found minimal evidence of abnormal proinflammatory cytokine expression in the face of the marked decrease in insulin sensitivity. As these data are in contrast to findings in human obesity studies, and the presence or role of macrophages in adipose tissue in equine metabolic syndrome has not been previously determined, the present study investigated events concerning monocyte recruitment and macrophage presence in adipose tissue in horses with obesity-related insulin resistance. In biopsy samples of nuchal crest subcutaneous fat obtained from the twelve Arabian horses prior to and following weight gain, mRNA concentrations of MCP-1, MCP-2, and PAI-1 were quantified via real-time quantitative PCR. Formalin-fixed adipose tissue was processed for immunohistochemistry using an anti-CD13 primary antibody (surface marker for cells of myeloid lineage) for quantification of leukocytes pre and post weight gain. No CD13-positive cells were noted in any of the adipose tissue samples. However, the expression of MCP-2 and PAI-1 within adipose tissue increased (P<0.05, Mann-Whitney U test) after weight gain, coincident with the decrease in insulin sensitivity. The results of this study suggest that the adipose tissue becomes a source of inflammatory mediators in the horse, but that unlike findings in human obesity, adipocytes may be the primary source of inflammatory mediator expression, minimally influenced by monocyte infiltration. However, other fat depots, including visceral adipose tissue, must be examined prior to generalizing these results to all fat depots in the horse.