The equine metabolic syndrome: studies on the pathophysiology of obesity, insulin resistance, and laminitis in equids
Insulin resistance has been associated with predisposition to laminitis in horses and ponies; in humans, adipose tissue (AT) inflammation is associated with IR. The first series of studies reported here characterized inflammatory gene expression and leukocyte infiltration in several AT depots of insulin-resistant (IR) and insulin-sensitive (IS) horses. No differences in expression of TNF-a, IL-1ß, IL-6, PAI-1, or MCP-1 were noted between IR and IS groups for each depot. However, when data from IR and IS groups were combined for each depot, the expression of IL-1ß (p=0.009) and IL-6 (p=0.023) was significantly higher in nuchal ligament AT, suggesting that this depot is more likely to adopt an inflammatory phenotype than others. The results also suggested a role for MCP-2 as a macrophage chemoattractant in equine AT; however, there was no difference in total leukocyte content between depots. Within the digital laminae, vascular dysfunction and inflammation have been reported to play roles in laminitis associated with equine IR. The purpose of the next series of studies was to characterize laminar insulin receptor (IRc) expression and inflammation in ponies subjected to a dietary carbohydrate challenge. Laminar keratinocytes did not show significant expression of IRc under basal or fed conditions. Up-regulation of IRc expression in the laminar vasculature was observed acutely in response to carbohydrate challenge and accompanied hyperinsulinemia. Primary inflammatory events (including laminar inflammatory gene expression and leukocyte infiltration) were not associated with dietary carbohydrate challenge and do not appear central to EMS-associated laminitis. The purpose of the next study was to characterize the cellular localization and activation state of AMPK in liver, skeletal muscle, and digital laminae of ponies subjected to a dietary carbohydrate challenge. Western blot analysis for phospho(P)-AMPK demonstrated decreased laminar P-AMPK concentrations upon challenge with dietary carbohydrate (p = 0.01). In contrast, P-AMPK concentrations were unchanged in skeletal muscle (p = 0.33), and there was a trend for increased AMPK activation in the liver in obese ponies in response to dietary carbohydrate (p = 0.13). Unchanged or increased P-AMPK concentrations with increased caloric intake suggest IR in skeletal muscle and liver; the decreased laminar P-AMPK concentrations with CHO challenge indicate that the laminae remain IS. Laminar dysfunction in EMS is more likely due to local effects of hyperinsulinemia, not local IR/energy failure. The purpose of the final study was to characterize the morphology of the equine endocrine pancreas in response to dietary carbohydrate challenge. No effect of dietary carbohydrate was noted on ß-cell surface area (p = 0.12). The change in serum insulin concentration was significantly greater in the high carbohydrate-fed ponies than in controls (403.8 +/- 317.1 mIU/L vs. 1.00 +/- 4.03 mIU/L; p = 0.002); however, this variable was not correlated with total islet surface area (r = 0.32; p = 0.17) or ß-cell surface area (r= 0.25; p = 0.3). It will be important to assess both ß-cell function and insulin clearance mechanisms in future studies to delineate the mechanism(s) of hyperinsulinemia in EMS.
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