Equine insulin receptor and insulin-like growth factor-1 receptor expression in digital lamellar tissue and insulin target tissues

Endocrinopathic Laminitis

Endocrinopathic laminitis (EL) primarily occurs because of insulin dysregulation (ID) mediated through downstream effects of insulin on IGF-1R in lamellar tissues. There is likely contributing vascular and metabolic dysfunction within the lamellae, but EL is relatively non-inflammatory. EL is associated with lamellar stretching, proliferation, and failure, ultimately causing failure of the suspensory apparatus of the distal phalanx. Proper education regarding mitigating risk factors makes this a largely preventable cause of laminitis. Annual hoof evaluation plus screening geriatric horses for pituitary pars intermedia dysfunction and ID, and younger horses for ID, can significantly decrease the incidence of this devastating condition.

An investigation of the equine epidermal growth factor system during hyperinsulinemic laminitis

Equine laminitis is a disease of the digital epidermal lamellae typified by epidermal cell proliferation and structural collapse. Most commonly the disease is caused by hyperinsulinemia, although the pathogenesis is incompletely understood. Insulin can activate the epidermal growth factor (EGF) system in other species and the present study tested the hypothesis that upregulation of EGF receptor (EGFR) signalling is a key factor in laminitis pathophysiology. First, we examined lamellar tissue from healthy Standardbred horses and those with induced hyperinsulinemia and laminitis for EGFR distribution and quantity using immunostaining and gene expression, respectively. Phosphorylation of EGFR was also quantified. Next, plasma EGF concentrations were compared in healthy and insulin-infused horses, and in healthy and insulin-dysregulated ponies before and after feeding. The EGFR were localised to the secondary epidermal lamellae, with stronger staining in parabasal, rather than basal, cells. No change in EGFR gene expression occurred with laminitis, although the receptor showed some phosphorylation. No difference was seen in EGF concentrations in horses, but in insulin-dysregulated ponies mean, post-prandial EGF concentrations were almost three times higher than in healthy ponies (274 ± 90 vs. 97.4 ± 20.9 pg/mL, P = 0.05). Although the EGFR does not appear to play a major pathogenic role in hyperinsulinemic laminitis, the significance of increased EGF in insulin-dysregulated ponies deserves further investigation.

Characterization of insulin and IGF-1 receptor binding in equine liver and lamellar tissue: implications for endocrinopathic laminitis

Although it is well established that equine laminitis can be triggered by extreme hyperinsulinemia, the mechanism of insulin action is not known. High concentrations of insulin lead to separation of the weight-bearing apparatus from the hoof wall and are associated with an increased cycle of cell death and proliferation in the lamellae. Gene expression and immunohistochemistry studies have indicated that the lamellae are sparsely populated with insulin receptors, whereas IGF-1 receptors (IGF-1R) are abundant, suggesting that the action of insulin may be mediated by insulin binding to the IGF-1R. To investigate this possibility, cell membrane fragments containing IGF-1R were extracted from the livers of 6 horses and the lamellae of >50 horses euthanized for nonresearch purposes at an abattoir. Radioligand-binding studies using ¹²⁵I-IGF-1 and ¹²⁵I-insulin confirmed an abundance of high-affinity IGF-1R in the liver (K_D 0.11 nM, B_max 223 fmol/mg protein) and lamellae (K_D 0.16 nM, B_max 243 fmol/mg protein). However, the affinity of insulin for binding to the lamellar IGF-1R (K_i 934 nM) was >5,800 fold less than that of IGF-1, suggesting that insulin is unlikely to bind to equine IGF-1R at physiological concentrations. Although insulin receptors could be detected in the liver (K_D 0.48 nM, B_max 123 fmol/mg protein), they were barely detectable in lamellae (estimated B_max 14 fmol/mg protein). There was no evidence to support the presence of insulin/IGF-1 hybrid receptors in either tissue. These findings suggest that insulin does not act directly through IGF-1 receptors and that an alternative theory is required to explain the mechanism of insulin action in laminitis.

The effect of insulin on equine lamellar basal epithelial cells mediated by the insulin-like growth factor-1 receptor

Background

In horses and ponies, insulin dysregulation leading to hyperinsulinemia may be associated with increased risk of laminitis, and prolonged infusion of insulin can induce the condition. It is unclear whether insulin may have a direct or indirect effect on the lamellar tissues. Insulin is structurally related to insulin-like growth factor (IGF-1), and can bind the IGF-1 receptor, albeit at a lower affinity than IGF-1.

Methods

Immunohistochemistry was performed on formalin-fixed lamellar tissue sections from six normal horses, euthanised for non-research purposes, using an anti-IGF-1 receptor antibody. In further studies, lamellar epithelial cells were obtained by collagenase digestion from the hooves of 18 normal horses, also euthanised for non-research purposes, and incubated for 48 h in the presence of insulin (0–2,000 m IU/ml). The increase in cell numbers was determined using a cell proliferation assay, and compared to the effect of zero insulin using one-way ANOVA.

Results

Immunohistochemistry demonstrated IGF-1 receptors on lamellar epidermal epithelial cells. With cultured cells, insulin caused a concentration-dependent increase in cell proliferation compared to untreated cells (maximal effect 63.3 ± 12.8% more cells after 48 h with 1,000 m IU/ml insulin; P < 0.01). Co-incubation with a blocking antibody against the IGF-1 receptor significantly inhibited the proliferative effect of insulin (P < 0.01).

Discussion

These results demonstrate that IGF-1 receptors are present on lamellar epithelial cells. At high physiological concentrations, insulin may activate these cells, by a mechanism involving IGF-1 receptors, resulting in a proliferative effect. This mechanism could help to explain the link between hyperinsulinemia and laminitis.

Association between hyperinsulinaemia and laminitis severity at the time of pituitary pars intermedia dysfunction diagnosis

BACKGROUND

Hyperinsulinaemia is the suspected component of insulin dysregulation having the strongest association with laminitis and occurs variably in equids with pituitary pars intermedia dysfunction (PPID).

OBJECTIVES

We hypothesised that magnitude of hyperinsulinaemia correlates with laminitis severity in PPID-affected equids. Furthermore, we hypothesised that owners can be unaware of chronic endocrinopathic laminitis.

STUDY DESIGN

Cross-sectional study.

METHODS

Serum insulin concentrations, owner-reported laminitis history and radiographic evidence of laminitis were determined in 38 client-owned horses and ponies with confirmed PPID. Laminitis severity was classified into four categories (normal [nonlaminitic], mild, moderate or severe laminitis) based on degree of distal phalangeal rotation. Animals were also categorised as normoinsulinaemic (<20 μU/ml), mildly hyperinsulinaemic (20-50 μU/ml) and severely hyperinsulinaemic (>50 μU/ml). One-way ANOVA, t tests and Fisher's exact tests were performed.

RESULTS

While owners reported laminitis in 37% of animals, 76% were laminitic based on study criteria (P = 0.01). Owners reported laminitis more frequently in hyperinsulinaemic vs. normoinsulinaemic animals; recognition increased with severity of hyperinsulinaemia (P = 0.03). Mean insulin concentrations were higher in equids with moderate to severe radiographic laminitis (geometric mean 74.1, 95% confidence interval (CI) 38.4-143.1 uU/ml) vs. those classified radiographically as normal to mild (31.9, 95% CI 21.1-48.1 uU/ml P = 0.03).

MAIN LIMITATIONS

Dynamic insulin testing was not performed; some normoinsulinaemic animals might have had subtle insulin dysregulation.

CONCLUSIONS

Although radiographic abnormalities were present in most animals at the time of PPID diagnosis, chronic laminitis remained unrecognised by many owners. Owner awareness of laminitis increased with severity of hyperinsulinaemia and higher insulin concentrations were detected in association with more severe radiographic changes. The Summary is available in Chinese - See Supporting Information.

Ex vivo effects of insulin on the structural integrity of equine digital lamellae

BACKGROUND

Laminitis has a considerable impact on the equine industry. Endocrinopathic laminitis is the most common form and affected horses often have hyperinsulinaemia due to an underlying metabolic disorder.

OBJECTIVES

The aim of this study was to determine if insulin weakens the structural integrity of digital lamellae and to develop an ex vivo model for the study of hyperinsulinaemia-induced lamellar failure.

STUDY DESIGN

Ex vivo experiment.

METHODS

Biomechanical testing was used to assess the structural integrity of lamellar explants exposed to either medium alone (control) or medium supplemented with insulin. Lamellar explants comprised of hoof wall, lamellar tissue and distal phalanx were harvested from four adult horses with no evidence of inflammatory disease or pre-existing disease of the digit. Following an equilibration period, explants were incubated in medium or medium supplemented with insulin (2.5 μg/ml) for 8 h prior to biomechanical testing to obtain load (N), stress (MPa), elongation to failure (mm), and Young's modulus (MPa) for each explant. Significant differences were assessed using a mixed linear model with horses as a random factor and control or insulin-treated group as a fixed factor.

RESULTS

Lamellar explants incubated in medium supplemented with insulin failed at significantly lower load (P = 0.0001) and lower stress (P = 0.001) and had greater elongation to failure (P = 0.02).

MAIN LIMITATIONS

In addition to the ex vivo nature of the study, location-dependent variability in explant structural integrity and variable diffusion of nutrients due to explant size may have been limitations. However, the study design attempted to account for these limitations through random assignment of explants to treatment groups independent of location and by evaluating stress to failure.

CONCLUSIONS

Insulin weakens the structural integrity of equine lamellar explants and an ex vivo model for evaluation of hyperinsulinaemia-induced lamellar failure was established. The summary is available in Spanish - see Supporting Information.

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.

Prospective cohort study evaluating risk factors for the development of pasture-associated laminitis in the United Kingdom

BACKGROUND

Certain individuals appear to be predisposed to recurrent pasture-associated laminitis. Previous studies have predominantly investigated risk factors only after disease occurrence.

OBJECTIVES

To investigate risk factors for pasture-associated laminitis prior to disease occurrence.

STUDY DESIGN

Prospective cohort study.

METHODS

Non-laminitic ponies aged ≥7 years were recruited. Body condition score (BCS), height, weight, crest height and thickness were measured and an overnight dexamethasone suppression test performed. Plasma or serum adiponectin, leptin, triglyceride, basal insulin, insulin post-dexamethasone, insulin-like growth factor 1 (IGF-1), IGF binding protein 1 (IGFBP-1), IGFBP-3, C-reactive protein, von Willebrand's factor, soluble E-selectin and P-selectin concentrations were assayed. Follow-up data were obtained from owners annually for 3 years to ascertain occurrences of veterinarian-diagnosed pasture-associated laminitis. Data were analysed by multivariate logistic regression. Receiver operating characteristic (ROC) curves analysis was performed for significant risk factors and cut-off values determined.

RESULTS

A total of 446 animals with a median (interquartile range) age of 15 (10-20) years were recruited. Of these, 50.4% were mares and 49.6% were geldings. The most common breeds were Welsh (36.4%), Shetland (17.0%) and cob (9.4%). Overall, 72.2% of animals were overweight/obese (BCS 7-9/9), 27.3% were of ideal weight (BCS 4-6/9) and 0.5% were underweight (BCS 1-3/9). After 1, 2 and 3 years, respectively, 18 (4.0%), 30 (6.7%) and 44 (9.9%) animals were reported to have had laminitis. Plasma adiponectin, and serum basal (insulin) and (insulin) post-dexamethasone levels were significantly (P ≤ 0.05) associated with laminitis occurrence cumulatively after 1, 2 and 3 years. Use of the area under the ROC curves to distinguish animals that did and did not develop laminitis showed good (basal [insulin] after 1 year), fair (all others) or poor ([insulin] post-dexamethasone) levels of accuracy.

MAIN LIMITATIONS

Animals were evaluated at a single time point and biomarkers were assayed using single assays.

CONCLUSIONS

Risk factors for future laminitis prior to disease occurrence include low plasma adiponectin and high serum basal insulin or insulin post-dexamethasone concentrations.

Prolonged hyperinsulinemia affects metabolic signal transduction markers in a tissue specific manner

Insulin dysregulation is common in horses although the mechanisms of metabolic dysfunction are poorly understood. We hypothesized that insulin signaling in striated (cardiac and skeletal) muscle and lamellae may be mediated through different receptors as a result of receptor content, and that transcriptional regulation of downstream signal transduction and glucose transport may also differ between tissues sites during hyperinsulinemia. Archived samples from horses treated with a prolonged insulin infusion or a balanced electrolyte solution were used. All treated horses developed marked hyperinsulinemia and clinical laminitis. Protein expression was compared across tissues for the insulin receptor and insulin-like growth factor 1 receptor (IGF-1R) by immunoblotting. Gene expression of metabolic insulin-signaling markers (insulin receptor substrate 1, Akt2, and glycogen synthase kinase 3 beta [GSK-3β]) and glucose transport (basal glucose transporter 1 and insulin-sensitive glucose transporter 4) was evaluated using real-time reverse transcription polymerase chain reaction. Lamellar tissue contained significantly more IGF-1R protein than skeletal muscle, indicating the potential significance of IGF-1R signaling for this tissue. Gene expression of the selected markers of insulin signaling and glucose transport in skeletal muscle and lamellar tissues was unaffected by prolonged hyperinsulinemia. In contrast, the significant upregulation of Akt2, GSK-3β, GLUT1, and GLUT4 gene expression in cardiac tissue suggested that the prolonged hyperinsulinemia induced an increase in insulin sensitivity and a transcriptional activation of glucose transport. Responses to insulin are tissue-specific, and extrapolation of data across tissue sites is inappropriate.

[Reference ranges of insulin, insulin like growth factor-1 and adrenocorticotropic hormone in ponies]

OBJECTIVES

The aim of this study was to validate a chemiluminescence immunometric assay using the IMMULITE 2000® for the determination of adrenocorticotropic hormone (ACTH), insulin and insulin-like growth factor-1 (IGF-1) from which reference ranges were calculated for ponies.

MATERIAL AND METHODS

Blood samples of 130 ponies aged 3-32 years were collected in the afternoon. The reference ranges were calculated according to the Guideline EP28-A3C of the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) from 2010.

RESULTS

The determined intraday precision for insulin was 3.28%, for ACTH 3.35% and for IGF-1 1.84%. The interday precision (insulin: 3.45%; IGF-1: 2.89%; ACTH: 9.77%) was determined on three consecutive days, where the samples were stored at 4   °C. There was no significant loss of insulin activity nor of ACTH or IGF-1 concentration during this time. The reference ranges of insulin and IGF-1 (2.0-21.2 mU/l and 50.2-357.2 mU/l, respectively) were age independent, whereas for ACTH, an age-dependent reference range was established. Ponies aged 3-12 years had a significantly lower reference range (4.2-19.8 pg/ml) than ponies aged 13-32 years (5.0-22.6 pg/ml).

CONCLUSION

The method used is suitable for the determination of ACTH, IGF-1 and insulin in serum of ponies, but the lowest detection limit for insulin is 2.0 mU/l.

CLINICAL RELEVANCE

The calculated reference ranges of insulin and ACTH are helpful for the diagnosis and clinical monitoring of equine metabolic syndrome and pituitary pars intermedia dysfunction (PPID).