Prolonged hyperinsulinemia increases the production of inflammatory cytokines in equine digital lamellae but not in striated muscle.

Hyperinsulinemia is the key feature of equine metabolic syndrome (EMS) which leads to debilitating sequelae. Hyperinsulinemia-associated laminitis (HAL) is one of the major sequelae of EMS, although the pathophysiological mechanisms are not well elucidated. Using an equine model, we hypothesized that expression of inflammatory markers would be increased in digital lamellae and striated muscle following prolonged hyperinsulinemia. Healthy Standardbred horses (5.4 ± 1.9 years) were alternately assigned to a prolonged euglycemic-hyperinsulinemic clamp (pEHC) or control group (n = 4 per group). Following a 48 h pEHC or a 48 h infusion of a balanced electrolyte solution (controls), biopsies were collected from digital lamellar tissue, skeletal muscle and cardiac muscle were obtained. All hyperinsulinemic horses developed laminitis regardless of previous health status at enrollment. Protein expression was quantified via Western blotting. A significant (P < 0.05) upregulation of the protein expression of heat shock protein 90 (HSP90), alpha 2 macroglobulin (A2M) and fibrinogen (α, ß isoforms), as well as inflammatory cytokines including interleukin-1ß were detected in digital lamellae following prolonged hyperinsulinemia. In contrast, protein expression of cytokines and acute phase proteins in heart and skeletal muscle was unchanged following hyperinsulinemia. Upregulation of inflammatory cytokines and acute phase proteins in digital lamellae during prolonged hyperinsulinemia may reveal potential biomarkers and novel therapeutic targets for equine endocrinopathic laminitis. Further, the lack of increase of inflammatory proteins and acute phase proteins in striated muscle following prolonged hyperinsulinemia may highlight potential anti-inflammatory and cardioprotective mechanisms in these insulin-sensitive tissues.

Equine insulin dysregulation causes tissue specific alterations of proinflammatory cytokines and acute phase proteins in a NF-kB independent manner.

Similar to human diabetes, equine metabolic syndrome (EMS) causes insulin dysregulation leading to debilitating sequela including laminitis. The pathophysiological mechanisms underlying EMS and laminitis are not well known. Therefore, using an insulin-resistant equine model, we hypothesized that insulin dysregulation induces an increased expression of inflammatory proteins in a tissue specific manner. Two groups of horses (n = -5/group) were categorized as insulin-resistant (IR) or insulin-sensitive (IS), using a frequently sampled intra-venous glucose tolerance test. Biopsies from skeletal muscle, and visceral and subcutaneous adipose tissues were collected in both groups. Protein expression was quantified via Western blotting in order to investigate HSP90, α 2 macroglobulin (A2M), Fibrinogen α, ß, γ isoforms as well as cytokines, including interleukin-1ß (IL-1ß) and interleukin-6 (IL-6), in muscle and adipose tissues. Protein expression of HSP90, A2M and IL1-ß was significantly greater in visceral adipose tissue of IR horses compared to IS horses. Fibrinogen (α and γ) expression was only significantly increased in subcutaneous adipose tissue of IR group compared to IS group. In contrast, no statistically significant difference in protein expression of proinflammatory cytokines and acute phase proteins was reported in skeletal muscle of IR vs. IS horses. Relative protein expression of total and phospho-NFκB protein expression was not statistically significantly changed in adipose tissues of IR horses compared to IS horses. In conclusion, proinflammatory cytokines and acute phase proteins were upregulated in adipose tissue, but not in skeletal muscle, through an NF-kB independent pathway. Insights from this study could reveal novel biomarkers and potential therapeutic targets for EMS.