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Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse^1,2

M. M. Vick^*, A. A. Adams^*, B. A. Murphy^*, D. R. Sessions^*, D. W. Horohov^*, R. F. Cook^*, B. J. Shelton and B. P. Fitzgerald^*,3

^* Department of Veterinary Science, Maxwell Gluck Equine Research Center, University of Kentucky, Lexington 40546; and and Departments of Internal Medicine and Biostatistics, University of Kentucky, Lexington 40504

³ Corresponding author: bfitz{at}uky.edu

Recent studies associate obesity and insulin resistance in horses with development of abnormal reproductive function and debilitating laminitis. The factors contributing to insulin resistance in obese horses are unknown. However, human studies provide evidence that elevated inflammatory cytokines such as tumor necrosis factor (TNF), IL1, and IL6 play direct roles in development of obesity-associated insulin resistance. Thus, inflammation may be a key link between obesity and insulin resistance in horses. The aim of the current investigation was to examine possible relationships between obesity, inflammatory cytokines, and insulin sensitivity (IS) in the horse. Age was recorded and BCS and percent body fat (% FAT) were determined as measures of obesity in 60 mares. In addition, blood mRNA expression of IL1, IL6, and TNF and circulating concentrations of TNF protein (TNFp) were determined in each mare. Finally, fasted concentrations of insulin were determined, and IS was determined using the hyperinsulinemic, euglycemic clamp. Significant correlations between several variables provided evidence for the design of 4 population regression models to estimate relationships between measures of obesity, inflammatory factors, and IS in the sample population. The results of these analyses revealed that IS decreased as BCS and % FAT increased (P < 0.001) in the sample population. Additionally, increased IL1 (P < 0.05) and TNFp (P < 0.01) were associated with decreased IS. However, increased TNF (P < 0.001) was associated with decreased IS only in mares 20 yr of age and older. Increased BCS and % FAT were associated with increased expression of TNF (P = 0.053) and IL1 (P < 0.05), and increased TNFp (P < 0.05). Surprisingly, increased BCS and % FAT were associated with decreased IL6 expression (P = 0.05) in mares <20 yr of age. Finally, evaluation of the influence of obesity and inflammatory cytokines on IS within the same model suggested that BCS and % FAT (P < 0.001) with TNF [mRNA (P = 0.07) and protein (P < 0.05)] are inversely associated with IS independently of one another. Combined, these results provide the first evidence associating obesity with increased inflammatory factors in the horse. Furthermore, the results suggest that an interrelationship exists among obesity, inflammatory cytokines, and IS in the horse and emphasize the need for further studies to elucidate the nature of these relationships.

Key Words: interleukin-1 • interleukin-6 • inflammation • insulin resistance • tumor necrosis factor

² The authors would like to thank Rhonda Van Dyke for her assistance with statistical analysis and the farm crew of Main Chance farm at the University of Kentucky, Lexington, for their extensive assistance with animal handling.