Effects of acute hyperinsulinemia on inflammatory proteins in horses

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, β isoforms), as well as inflammatory cytokines including interleukin-1β 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.

The effects of obesity and insulin dysregulation on mare reproduction, pregnancy, and foal health: a review

Obesity is a growing welfare concern in modern equine populations and predisposes horses to disturbances in energy metabolism such as insulin dysregulation. However, equine metabolic syndrome has only been recognized in recent decades. Functioning energy metabolism is pivotal to normal body homeostasis and affects essentially all organ systems, including reproduction. Previous literature suggests that obesity has an effect not only on the reproductive processes in mares but also on offspring health, predisposing the offspring to later-onset orthopedic and metabolic problems. This review focuses on the effects of obesity, insulin dysregulation and hyperinsulinemia on the reproductive functions of mares and the implications on foal health before and after birth. The points of interest are the cyclicity and ovarian function, uterine environment, gestation, the postpartum period, and the newborn foal. The aim is to review the current state of knowledge, and identify outstanding questions that could stimulate future research. This topic is important not only from the equine industry and production perspective but is also relevant for the welfare of future populations and individuals.

Measurement of Plasma Resistin Concentrations in Horses with Metabolic and Inflammatory Disorders

Simple Summary

Obesity and its associated complications, such as metabolic syndrome, are an increasing problem in both humans and horses in the developed world. Adipose tissue is a key endocrine organ that communicates with other organs by multiple endocrine substances called adipokines. There is evidence to suggest that adipokines may contribute to the regulation of biological processes, such as metabolism, immunity, and inflammation. The aim of this study was to investigate the usefulness of one of these adipokines in horses, resistin, and its relationship with insulin dysregulation (ID) and inflammation. Seventy-two horses, included in one of the four following groups, were studied: healthy controls, horses with inflammatory conditions, horses with mild, and horses with severe ID. Plasma resistin concentrations were significantly different between groups, and the highest values were recorded in the inflammatory and severe ID groups. The lack of correlation of resistin with basal insulin concentration and the significant correlation of resistin with the inflammatory marker serum amyloid A suggest that, as is the case in humans, plasma resistin concentrations in horses are predominantly related to inflammatory conditions and not to ID.

Abstract

Obesity and its associated complications, such as metabolic syndrome, are an increasing problem in both humans and horses in the developed world. The expression patterns of resistin differ considerably between species. In rodents, resistin is expressed by adipocytes and is related to obesity and ID. In humans, resistin is predominantly produced by inflammatory cells, and resistin concentrations do not reflect the degree of obesity, although they may predict cardiovascular outcomes. The aim of this study was to investigate the usefulness of resistin and its relationship with ID and selected indicators of inflammation in horses. Seventy-two horses, included in one of the four following groups, were studied: healthy controls (C, n = 14), horses with inflammatory conditions (I, n = 21), horses with mild ID (ID1, n = 18), and horses with severe ID (ID2, n = 19). Plasma resistin concentrations were significantly different between groups and the higher values were recorded in the I and ID2 groups (C: 2.38 ± 1.69 ng/mL; I: 6.85 ± 8.38 ng/mL; ID1: 2.41 ± 2.70 ng/mL; ID2: 4.49 ± 3.08 ng/mL). Plasma resistin was not correlated with basal insulin concentrations. A significant (r = 0.336, p = 0.002) correlation was found between resistin and serum amyloid A. Our results show that, as is the case in humans, plasma resistin concentrations in horses are predominantly related to inflammatory conditions and not to ID. Horses with severe ID showed an elevation in resistin that may be secondary to the inflammatory status associated with metabolic syndrome.

Morphometric, metabolic, and inflammatory markers across a cohort of client-owned horses and ponies on the insulin dysregulation spectrum

In human metabolic syndrome and type II diabetes, methylglyoxal (MG), D-lactate, and several cytokines have been recognized as biomarkers of important metabolic and inflammatory processes. Equine metabolic syndrome (EMS) shares many similarities with these human counterparts. The objectives of this cross-sectional study were to compare body condition score (BCS), cresty neck score (CNS), resting insulin, MG, D-lactate, L-lactate, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and monocyte chemoattractant protein-1 (MCP-1) between horses with and without insulin dysregulation, as classified via combined glucose and insulin test (CGIT). 32 client-owned horses were included. History and morphometric data such as BCS and CNS were recorded. Subjects with abnormalities on physical examination or CBC, elevated ACTH or incomplete information were excluded. Baseline serum or plasma concentrations of biomarkers were tested via commercial ELISA or colorimetric assays. Characteristics of insulin dysregulated and insulin sensitive horses were compared by univariate analysis and forward logistic regression. 12 (38%) of the 32 horses were classified as insulin dysregulated. No significant difference between the 2 groups was found for age, BCS, baseline glucose, triglycerides, MG, D-lactate, L-lactate, TNF-α, IL-6, and MCP-1. Baseline insulin was significantly associated with insulin dysregulation in univariate analysis (P = 0.02), but not in the final model. Horses with CNS ≥ 3 had 11.3 times higher odds of having insulin dysregulation (OR 11.3, 95% C.I. 2.04 - 63.08, P = 0.006). In this population, horses with mild-moderate signs of EMS presented similar metabolic and inflammatory profiles to non-insulin dysregulated controls.

Metabolic changes induced by oral glucose tests in horses and their diagnostic use

Background

Little is known about the implications of hyperinsulinemia on energy metabolism, and such knowledge might help understand the pathophysiology of insulin dysregulation.

Objectives

Describe differences in the metabolic response to an oral glucose test, depending on the magnitude of the insulin response.

Animals

Twelve Icelandic horses in various metabolic states.

Methods

Horses were subjected to 3 oral glucose tests (OGT; 0.5 g/kg body weight glucose). Basal, 120 and 180 minutes samples were analyzed using a combined liquid chromatography tandem mass spectrometry and flow injection analysis tandem mass spectrometry metabolomic assay. Insulin concentrations were measured using an ELISA. Analysis was performed using linear models and partial least‐squares regression.

Results

The kynurenine : tryptophan ratio increased over time during the OGT (adjusted P‐value = .001). A high insulin response was associated with lower arginine (adjusted P‐value = .02) and carnitine (adjusted P‐value = .03) concentrations. A predictive model using only baseline samples performed well with as few as 7 distinct metabolites (sensitivity, 86%; 95% confidence interval [CI], 81%‐90%; specificity, 88%; 95% CI, 84%‐92%).

Conclusions and Clinical Importance

Our results suggest induction of low‐grade inflammation during the OGT. Plasma arginine and carnitine concentrations were lower in horses with high insulin response and could constitute potential therapeutic targets. Development of screening tools to identify insulin‐dysregulated horses using only baseline blood sample appears promising.

Effects of advanced age and pituitary pars intermedia dysfunction on components of the acute phase reaction in horses

Age, neurodegenerative disorders, and dysfunction of insulin secretion may be correlated with increased systemic concentrations of acute phase markers. Thus, the study aimed to determine the effect of age, pituitary pars intermedia dysfunction (PPID), and insulin dysregulation (ID) associated with PPID, on markers of the acute phase reaction. Twenty-nine mix-breed horses of both sexes were classified into groups: (1) healthy adult controls, (2) healthy non-PPID geriatric horses, (3) PPID ID+ horses, and (4) PPID ID- horses. Whole blood proinflammatory cytokine gene expression and serum concentrations of pro- and anti-inflammatory cytokines and acute phase proteins were measured. The data were analyzed using the Mann-Whitney U-test, and correlations between groups of data were assessed using Spearman's correlation coefficient. The tests were statistically significant if P < 0.05. No differences in the whole blood cytokine gene expression, serum cytokine concentrations, or acute phase proteins were noted between the groups. In the PPID ID group, there was a strong correlation between the ACTH concentration after the administration of thyrotropin-releasing hormone and the expression of IL-8 (r = 0.941; P = 0.0321). In the PPID ID+ group, there was a strong correlation between basal insulin concentrations and serum amyloid A (SAA; r = 0.936; P = 0.0083) as well as between postprandial insulin concentrations and SAA (r = 0.965; P = 0.001). These data suggest that neurodegeneration in horses moderately affects circulating markers of inflammation and that ID in horses with PPID influences acute phase inflammatory markers.

Effects of equine metabolic syndrome on inflammation and acute-phase markers in horses

Obesity and metabolic disorders are associated with systemic low-grade chronic inflammation, both in humans and animals. The aim of the study is to assess the effects of obesity and hyperinsulinemia on individual components of the acute-phase reaction in equine metabolic syndrome (EMS) horses. Eight mixed-breed EMS and six control, age-matched horses of both sexes were included in the study. Animals were classified as EMS or control based on the assessment of BCS, cresty neck score, and basal insulin >50 μU/mL and/or insulin responses to the oral sugar test (OST) >60 μU/mL. Peripheral venous blood was collected. The expression of proinflammatory cytokines, the concentration of circulating cytokines, and acute-phase proteins (serum amyloid A, C-reactive protein, haptoglobin, activin A, and procalcitonin) were measured. The data were analyzed using the Mann-Whitney test, whereas correlations were examined using Spearman's correlation coefficient. The tests were statistically significant if P ≤ 0.05. There were no differences in cytokine gene expression, circulating cytokine concentrations, or concentrations of acute-phase proteins between the EMS and the control groups. There was a strong correlation between the basal concentration of insulin and the serum concentrations of IL-6 (r = 0.71, P < 0.05). Activin A was positively correlated with post-OST insulin concentrations (r = 0.707, P = 0.05), indicating that this marker of inflammation could warrant further investigation in horses with EMS.

Preliminary analysis of the FAM174A gene suggests it lacks a strong association with equine metabolic syndrome in ponies

Equine metabolic syndrome (EMS) describes a group of risk factors, including obesity and insulin dysregulation (hyperinsulinemia and/or insulin resistance), that can lead to the development of the debilitating hoof disease laminitis. Although the underlying mechanisms of EMS are not fully understood, a genetic component has been reported, and an 11 guanine polymorphism located at the FAM174A gene has been identified as a risk locus for the syndrome in Arabian horses. To examine associations between the FAM174A risk allele and the clinical signs of EMS, the allele was examined in an Australian cohort of ponies (n = 20) with known metabolic status. The 11 guanine polymorphism was identified in only 3 of 13 ponies with EMS, and no significant association could be made between the risk loci and morphometric measurements associated with obesity (BCS [P = 0.21], cresty neck score [P = 0.58], basal triglyceride concentration [P = 0.85], and adiponectin concentration [P = 0.48]), or insulin dysregulation (insulin dysregulation status [P = 0.35] and serum insulin concentration during an oral glucose test [P = 0.44]). These results suggest that the FAM174A 11 guanine homopolymer allele is unlikely to be a singular key gene polymorphism associated with EMS in ponies. However, due to the small number of ponies identified with the polymorphism, further study of the FAM174A risk allele in a larger cohort of horses and ponies of uniform breed would be useful.

ECEIM consensus statement on equine metabolic syndrome

Equine metabolic syndrome (EMS) is a widely recognized collection of risk factors for endocrinopathic laminitis. The most important of these risk factors is insulin dysregulation (ID). Clinicians and horse owners must recognize the presence of these risk factors so that they can be targeted and controlled to reduce the risk of laminitis attacks. Diagnosis of EMS is based partly on the horse's history and clinical examination findings, and partly on laboratory testing. Several choices of test exist which examine different facets of ID and other related metabolic disturbances. EMS is controlled mainly by dietary strategies and exercise programs that aim to improve insulin regulation and decrease obesity where present. In some cases, pharmacologic aids might be useful. Management of an EMS case is a long‐term strategy requiring diligence and discipline by the horse's carer and support and guidance from their veterinarians.

The Effects of Various Levels of Docosahexaenoic Acid on Inflammatory Markers in Conditioned Horses During Lactate Threshold Tests

Exercise stimulates the release of inflammatory cytokines and supplementation with n-3 fatty acids reduces inflammation. The effects of different doses of docosahexaenoic acid (DHA) on inflammation in polo horses submitted to field lactate threshold tests (LT) were analyzed. We hypothesized that higher doses of DHA would reduce postexercise inflammation. Twenty polo horses were assigned to different treatments: control group fed (n = 5) grain and hay, 3 treatment groups (n = 5) fed 10, 20, or 50 g/day of DHA with grain and free choice hay during 60 days. Horses underwent LT tests before start, 30, and 60 days of supplementation. Blood samples were taken at rest for blood cytokine expression (CEx), plasma cytokine enzyme-linked immunosorbent assay (CEL), fatty acid, vitamin E, and creatine kinase (CK) analysis, after LT for CEx analysis (interferon gamma, tumor necrosis factor alpha [TNF-α], interleukin-1 [IL-1], interleukin-6 [IL-6], interleukin-10 [IL-10]), CEL, and CK analysis. Effects of treatment, time, and exercise were analyzed by analysis of variance, significant results compared by least square means analysis, and significance set at P < .05. There was a dose-dependent increase in plasma DHA, and highest arachidonic acid was found in 20 and 50 g. Vitamin E was lowest in 20 and 50 g. LT did not change IL-6, downregulated IL-1 and TNF-α, upregulated IL-10, and interferon gamma. The 10 g led to postexercise downregulation of interferon gamma and IL-10 CEx compared to other treatments. A lack of antioxidants in the supplements may have led to the absence of treatment effects in the 20 and 50 g. 10 g DHA helped moderate postexercise inflammation.

Paradigm shifts in understanding equine laminitis

Laminitis, one of the most debilitating conditions of all equids, is now known to be the result of several systemic disease entities. This finding, together with other recent developments in the field of laminitis research, have provoked a rethink of our clinical and research strategies for this condition. First, laminitis is now considered to be a clinical syndrome associated with systemic disease (endocrine disease, sepsis or systemic inflammatory response syndrome, SIRS) or altered weight bearing rather than being a discrete disease entity. Next, laminitis associated with endocrine disease (endocrinopathic laminitis) is now believed to be the predominant form in animals presenting (primarily) for lameness. Third, the designation of laminitis as a primary and severe basement membrane pathology now requires revision. Instead, current data now proposes a variable subclinical phase associated with gross changes in the hoof capsule, with stretching and elongation of the lamellar cells an early and key event in the pathophysiology. These findings have fuelled new mechanistic hypotheses and research directions that will be discussed, together with their implications for future clinical management.

Simultaneous detection and quantification of six equine cytokines in plasma using a fluorescent microsphere immunoassay (FMIA)

Cytokines are cell signalling proteins that mediate a number of different physiological responses. They are also biomarkers for inflammatory conditions and potential diagnostic references for diseases. Until recently, simultaneous quantification of cytokine profiles had not been possible. Now however, fluorescent microsphere immunoassays (FMIA) are able to measure multiple cytokines in a single sample. The following pro-inflammatory and anti-inflammatory cytokines were quantified in equine plasma and serum samples: interleukin (IL)-2, IL-4, IL-6, IL-10, interferon (IFN)-γ, tumor necrosis factor (TNF)-α.

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The objective of this study was to quantify six equine cytokines simultaneously using the BioPlex^® 200 system in equine EDTA-plasma and serum.

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It demonstrates an increased number of detectable cytokines over published studies.

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This technology has the advantage of reduced sample volume and assay time compared to traditional sandwich ELISAs.

The production and distribution of IL-6 and TNF-a in subcutaneous adipose tissue and their correlation with serum concentrations in Welsh ponies with equine metabolic syndrome

A main symptom of equine metabolic syndrome (EMS) in ponies is pathological obesity characterized by abnormal accumulation of fat deposits and inflammation. In this study, we analyzed the expression of two pro-inflammatory cytokines, interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), in subcutaneous adipose tissue and the correlation with serum concentrations in peripheral blood of Welsh ponies. Based on clinical examination findings, the animals were divided into two groups: ponies affected with EMS (n = 8) and obese ponies (n = 8). The adipose tissue was examined using immunohistochemical analysis while concentrations IL-6 and TNF-α were measured using enzyme-linked immunosorbent assays (ELISAs). Additionally, histological characterization of the adipose tissue was performed. The results obtained showed that IL-6 expression in adipose tissue biopsies derived from animals with EMS was enhanced while TNF-α levels of both groups were comparable. Compared to the obese ponies, EMS animals also had significantly elevated levels of serum IL-6 and TNF-α. Histological analysis revealed macrophage infiltration and fibrosis in adipose tissue preparations from the EMS group. These data suggest that IL-6 may play a key role in the course of EMS in Welsh ponies. Our findings also demonstrated that analysis of pro-inflammatory cytokines levels in serum may serve as an additional tool for diagnosing EMS.

Insulin dysregulation

Abnormalities of insulin metabolism include hyperinsulinaemia and insulin resistance, and these problems are collectively referred to as insulin dysregulation in this review. Insulin dysregulation is a key component of equine metabolic syndrome: a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses, ponies and donkeys. Insulin dysregulation can also accompany prematurity and systemic illness in foals. Causes of insulin resistance are discussed, including pathological conditions of obesity, systemic inflammation and pituitary pars intermedia dysfunction, as well as the physiological responses to stress and pregnancy. Most of the discussion of insulin dysregulation to date has focused on insulin resistance, but there is increasing interest in hyperinsulinaemia itself and insulin responses to feeding. An oral sugar test or in-feed oral glucose tolerance test can be performed to assess insulin responses to dietary carbohydrates, and these tests are now recommended for use in clinical practice. Incretin hormones are likely to play an important role in postprandial hyperinsulinaemia and are the subject of current research. Insulin resistance exacerbates hyperinsulinaemia, and insulin sensitivity can be measured by performing a combined glucose-insulin test or i.v. insulin tolerance test. In both of these tests, exogenous insulin is administered and the rate of glucose uptake into tissues measured. Diagnosis and management of hyperinsulinaemia is recommended to reduce the risk of laminitis. The term insulin dysregulation is introduced here to refer collectively to excessive insulin responses to sugars, fasting hyperinsulinaemia and insulin resistance, which are all components of equine metabolic syndrome.

Toll-like receptor and pro-inflammatory cytokine expression during prolonged hyperinsulinaemia in horses: implications for laminitis

Equine laminitis, a disease of the lamellar structure of the horse's hoof, can be incited by numerous factors that include inflammatory and metabolic aetiologies. However, the role of inflammation in hyperinsulinaemic laminitis has not been adequately defined. Toll-like receptor (TLR) activation results in up-regulation of inflammatory pathways and the release of pro-inflammatory cytokines, including interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-α), and may be a pathogenic factor in laminitis. The aim of this study was to determine whether TLR4 expression and subsequent pro-inflammatory cytokine production is increased in lamellae and skeletal muscle during equine hyperinsulinaemia. Standardbred horses were treated with either a prolonged, euglycaemic hyperinsulinaemic clamp (p-EHC) or a prolonged, glucose infusion (p-GI), which induced marked and moderate hyperinsulinaemia, respectively. Age-matched control horses were treated simultaneously with a balanced electrolyte solution. Treated horses developed clinical (p-EHC) or subclinical (p-GI) laminitis, whereas controls did not. Skeletal muscle and lamellar protein extracts were analysed by Western blotting for TLR4, IL-6, TNF-α and suppressor of cytokine signalling 3 (SOCS3) expression. Lamellar protein expression of TLR4 and TNF-α, but not IL-6, was increased by the p-EHC, compared to control horses. A significant positive correlation was found between lamellar TLR4 and SOCS3. Skeletal muscle protein expression of TLR4 signalling parameters did not differ between control and p-EHC-treated horses. Similarly, the p-GI did not result in up-regulation of lamellar protein expression of any parameter. The results suggest that insulin-sensitive tissues may not accurately reflect lamellar pathology during hyperinsulinaemia. While TLR4 is present in the lamellae, its activation appears unlikely to contribute significantly to the developmental pathogenesis of hyperinsulinaemic laminitis. However, inflammation may have a role to play in the later stages (e.g., repair or remodelling) of the disease.

The effect of exercise on plasma concentrations of inflammatory markers in normal and previously laminitic ponies

REASONS FOR PERFORMING STUDY

The mechanisms underlying predisposition to pasture-associated laminitis remain unclear; chronic inflammation is implicated, and this may be exacerbated by physical inactivity.

OBJECTIVES

To determine whether exercise affects the inflammatory profile of normal and previously laminitic ponies.

STUDY DESIGN

Prospective case-control study.

METHODS

The short (1 day) and longer term (14 days) effects of low intensity (10 min walking and 5 min trotting) exercise on plasma inflammatory marker concentrations in normal (NL) and previously laminitic (PL) nonobese ponies (n = 6/group) was determined. Plasma concentrations of TNF-α, serum amyloid A (SAA), haptoglobin, insulin, adiponectin and fibrinogen were assayed by validated/standard methods. Data were analysed using a linear mixed effects model.

RESULTS

Before exercise, plasma [adiponectin] was significantly (P = 0.0001) lower in PL (mean ± s.d. 2.4 ± 0.1 ng/l) than in NL (4.03 ± 0.2 ng/l), but exercise had no effect. Previous laminitis and exercise had no effect on plasma [TNF-α] or [fibrinogen]. Serum amyloid A concentrations in all ponies were significantly (P = 0.00001) reduced after longer term exercise compared to Day 1 values. Plasma [haptoglobin] was significantly (P = 0.00001) higher in PL compared to NL on Day 1. This difference was no longer apparent after longer term exercise, such that [haptoglobin] in PL had decreased to concentrations similar to NL. Following short-term exercise, all ponies had an initial decrease in serum [insulin] immediately after exercise, followed by an increase peaking 10 min after exercise cessation, before returning to pre-exercise values. On Day 14 these fluctuations were significantly (P = 0.001) reduced in all ponies.

CONCLUSIONS

Fourteen days of low intensity exercise significantly decreased [SAA] in all ponies and plasma [haptoglobin] in PL such that it was no longer increased compared to NL. Regular low intensity exercise appears to have an anti-inflammatory effect, which is possibly greater in PL and so may be beneficial in reducing this putative risk factor in pasture-associated laminitis.

A review of recent advances and current hypotheses on the pathogenesis of acute laminitis

With the increasing number of studies being published on the different experimental models used to induce and study acute laminitis, the pathophysiological events associated with these various models (i.e. starch overload, oligofructose overload, black walnut extract and hyperinsulinaemia) can be compared more realistically. Within this review, the mechanisms for metabolic vs. inflammatory laminitis are discussed, and the question of how pasture laminitis may fit into any of the proposed mechanisms is addressed.

A Potential Role for Pro-Inflammatory Cytokines in the Development of Insulin Resistance in Horses

Insulin resistance is a metabolic condition involving reduced sensitivity of insulin-sensitive tissues to insulin-induced glucose disposal, including adipose tissue, skeletal muscle, and liver. Insulin resistance occurs in overweight and obese horses, and may increase risk for the development of laminitis. The development of insulin resistance is thought to occur in response to increased production of pro-inflammatory cytokines by adipose tissue in obesity, that then have an inhibitory effect on insulin signaling pathways in multiple tissues. This article reviews current knowledge of the involvement of pro-inflammatory cytokines in the development of insulin resistance in horses and uses data from other species to provide context.

Understanding the mechanisms involved in the development of insulin resistance in horses should enable development of effective treatment and prevention strategies. Current knowledge of these mechanisms is based upon research in obese humans and rodents, in which there is evidence that the increased production of pro-inflammatory cytokines by adipose tissue negatively influences insulin signaling in insulin-responsive tissues. In horses, plasma concentrations of the cytokine, tumor necrosis factor-α, have been positively correlated with body fatness and insulin resistance, leading to the hypothesis that inflammation may reduce insulin sensitivity in horses. However, little evidence has documented a tissue site of production and a direct link between inflammation and induction of insulin resistance has not been established. Several mechanisms are reviewed in this article, including the potential for macrophage infiltration, hyperinsulinemia, hypoxia, and lipopolysaccharide to increase pro-inflammatory cytokine production by adipose tissue of obese horses. Clearly defining the role of cytokines in reduced insulin sensitivity of horses will be a very important step in determining how obesity and insulin resistance are related.