Characterization of the insulin-signaling pathway in lacrimal and salivary glands of rats

The Role of Insulin-like Growth Factor (IGF) System in the Corneal Epithelium Homeostasis-From Limbal Epithelial Stem Cells to Therapeutic Applications

The corneal epithelium, comprising three layers of cells, represents the outermost portion of the eye and functions as a vital protective barrier while concurrently serving as a critical refractive structure. Maintaining its homeostasis involves a complex regenerative process facilitated by the functions of the lacrimal gland, tear film, and corneal nerves. Crucially, limbal epithelial stem cells located in the limbus (transitional zone between the cornea and the conjunctiva) are instrumental for the corneal epithelium integrity by replenishing and renewing cells. Re-epithelialization failure results in persistent defects, often associated with various ocular conditions including diabetic keratopathy. The insulin-like growth factor (IGF) system is a sophisticated network of insulin and other proteins essential for numerous physiological processes. This review examines its role in maintaining the corneal epithelium homeostasis, with a special focus on the interplay with corneal limbal stem cells and the potential therapeutic applications of the system components.

INitial Steps of Insulin Action in Parotid Glands of Male Wistar Rats

The parotid gland is the largest salivary gland. It produces watery saliva, rich in proteins (amylase, lysozymes, and antibodies). Due to the gland's morphological cytoarchitecture composed of only serous acini, it contributes almost 50% of total salivary volume upon stimulation. It has been reported that the prevalence of saliva secretion impairments, periodontitis, delayed wound healing, and xerostomia increase in diabetic patients. Herein we evaluated the acute effects of insulin on insulin receptor phosphorylation status and its substrates IRS-1 and IRS-2 in the parotid glands of adult male Wistar rats, using Western blot analyses. We confirmed an acute effect of insulin on IR/IRS/PI3K/Akt and MAPK intracellular pathway activation in the parotid glands of male Wistar rats similar to the classical metabolic targets of the hormone, like the liver.

Insulin Promotes Corneal Nerve Repair and Wound Healing in Type 1 Diabetic Mice by Enhancing Wnt/β-Catenin Signaling

The insulin and Wnt signaling pathways are involved in cell proliferation, tissue homeostasis, and tumorigenesis. However, their interrelationship in the pathophysiological process of diabetic corneal injury remains unclear. In this study, the role of insulin in the diabetic cornea was investigated in vitro, using cultured TKE2 cells and trigeminal ganglion neurons, and in vivo, by assessing corneal wound-healing responses in diabetic mice. A selective Wnt antagonist (XAV-939) and activator (BML-284) were used to regulate the interactions between insulin and the Wnt pathway. The results demonstrated that insulin promoted corneal epithelial wound healing and sensation recovery, whereas the expression of molecules involved in the Wnt/β-catenin pathway was also up-regulated in the injured corneal epithelium. However, XAV-939 limited the insulin-induced epithelial and corneal nerve repair. By contrast, BML-284 treatment promoted the healing of the corneal epithelium and corneal nerve repair in diabetic mice. These results indicate that insulin, via Wnt signaling, contributes to diabetic corneal epithelial wound healing and nerve injury recovery and is, therefore, a potential protective factor for diabetic corneal epithelial wounds and nerve injury.

Insulin receptor preserves mitochondrial function by binding VDAC1 in insulin insensitive mucosal epithelial cells

Unlike many epithelial tissues, the corneal epithelium is insulin insensitive, meaning it does not require insulin for glucose uptake. In this study, we show that insulin differentially regulates mitochondrial respiration in two human mucosal epithelial cell types: insulin-insensitive corneal epithelial cells and insulin-sensitive bronchial epithelial cells. In both cell types, insulin blocks glycogen synthase kinase beta (GSK3β) activity. In the corneal epithelium however, insulin selectively regulates PTEN-induced kinase 1 (PINK-1)-mediated mitophagy and mitochondrial accumulation of insulin receptor (INSR). While insulin blocked basal levels of PINK-1-mediated mitophagy in bronchial epithelial cells, mitochondrial trafficking of INSR was not detectable. We further show that in corneal epithelia, INSR interacts with the voltage-dependent anion channel-1 (VDAC1) in mitochondria and that INSR knockdown triggers robust mitochondrial fragmentation, alterations in mitochondrial polarization, and blocks the induction of PINK-1-mediated mitophagy. Collectively, these data demonstrate that INSR interacts with VDAC1 to mediate mitochondrial stability. We also demonstrate unique interactions between VDAC1 and other receptor tyrosine kinases, indicating a novel role for this family of receptors in mitochondria.

Prospective insulin-based ophthalmic delivery systems for the treatment of dry eye syndrome and corneal injuries

The presence of insulin (INS) receptors on the ocular surface (OS) and lacrimal gland (LG), and the high prevalence of dry eye syndrome (DES) and corneal lesions in diabetic patients suggest that INS is relevant for OS homeostasis and wound healing. The study aims at developing delivery systems for the topical administration of INS to the OS in order to improve INS local bioavailability and evaluate the influence of the delivery systems on DES in diabetic rats (DM) (n = 05/group). Chitosan microparticles (MP), chitosan/poloxamer gel (GEL) and MP-loaded GEL (GELMP), with or without INS were developed. Formulations were instilled into the eyes of diabetic rats (DM) for 15 days and the tear fluid volume, corneal cells morphology and cornea thickness were assessed and compared with an aqueous dispersion of INS (DISP-INS). All delivery systems had pH of about 6, osmolality suitable for topical application and positive zeta potential. The MPs with or without INS had sizes close to 4 μm, spherical morphology and INS encapsulation efficiency of 77 ± 6%. DISP-INS and GELMP-INS formulations produced tear secretion amounts significantly higher than those receiving formulations containing no INS and similar to healthy animals. Cornea surface impression cytology showed that treatment with INS-delivery systems and not DISP-INS almost normalized cells morphology. Treatment with GELMP-INS increased INS by 2.5 in the LG and eyeball as compared to the groups treated with GEL-INS and MP-INS, while treatment with DISP-INS left no traces of drug in the eye after treatment termination. GEL and GELMP containing INS were also able to normalize the thickness of the corneal epithelia. In conclusion, GELMP-INS normalized tear fluid volume, corneal thickness, protected corneal cells morphology and increased ocular bioavailability of INS, making it a promising treatment strategy for DES and corneal lesions.

Human amylase gene copy number variation as a determinant of metabolic state

INTRODUCTION

Humans have multiple genes encoding amylase that are broadly divided into salivary (AMY1) and pancreatic (AMY2) genes. They exhibit some of the greatest copy numbers of any human gene, an expansion possibly driven by increased dietary starch intake. Within the population, amylase gene copy number is highly variable and there is evidence of an inverse association between AMY1 copy number and BMI.

AREAS COVERED

We examine the evidence for the link between AMY1 and BMI, its potential mechanisms, and the metabolic effects of salivary and pancreatic amylase, both in the gastrointestinal tract and the blood

EXPERT COMMENTARY

Salivary amylase may influence postprandial 'cephalic phase' insulin release, which improves glucose tolerance, while serum amylase may have insulin-sensitizing properties. This could explain the favorable metabolic status associated with higher AMY1 copy number. The association with BMI is harder to explain and is potentially mediated by increased flux of undigested starch into the ileum, with resultant effects on short-chain fatty acids (SCFAs), changes in gut microbiota and effects on appetite and energy expenditure in those with low copy number. Future research on the role of amylase as a determinant of metabolic health and BMI may lead to novel therapies to target obesity.

TFOS DEWS II Sex, Gender, and Hormones Report

One of the most compelling features of dry eye disease (DED) is that it occurs more frequently in women than men. In fact, the female sex is a significant risk factor for the development of DED. This sex-related difference in DED prevalence is attributed in large part to the effects of sex steroids (e.g. androgens, estrogens), hypothalamic-pituitary hormones, glucocorticoids, insulin, insulin-like growth factor 1 and thyroid hormones, as well as to the sex chromosome complement, sex-specific autosomal factors and epigenetics (e.g. microRNAs). In addition to sex, gender also appears to be a risk factor for DED. "Gender" and "sex" are words that are often used interchangeably, but they have distinct meanings. "Gender" refers to a person's self-representation as a man or woman, whereas "sex" distinguishes males and females based on their biological characteristics. Both gender and sex affect DED risk, presentation of the disease, immune responses, pain, care-seeking behaviors, service utilization, and myriad other facets of eye health. Overall, sex, gender and hormones play a major role in the regulation of ocular surface and adnexal tissues, and in the difference in DED prevalence between women and men. The purpose of this Subcommittee report is to review and critique the nature of this role, as well as to recommend areas for future research to advance our understanding of the interrelationships between sex, gender, hormones and DED.

Tear Film and Ocular Surface Evaluation in Gestational Diabetes Mellitus

BACKGROUND

Dry eye syndrome is one of the complaints of diabetic patients. The aim of the present study was to evaluate the tear functions in pregnant women with gestational diabetes mellitus (GDM) using tests: Schirmer, tear break-up time (TBUT), and tear film osmolarity (TFO) tests and the Ocular Surface Disease Index score (OSDI).

METHODS

Pregnant women with GDM (Group 1, n=46) and healthy pregnant women (Group 2, n=36) were enrolled. Initially, all participants were asked to answer the OSDI and then they underwent a detailed ophthalmic examination including Schirmer, TBUT, and TFO tests. The individuals with ocular or systemic disorders that might affect the tear function tests and who were using topical medications were excluded.

RESULTS

The results of Schirmer, TBUT, TFO tests and OSDI scores were 11.20±4.93 mm, 5.59±2.16 sec, 309.65±14.80 mOsm/L, and 9.59 ± 9.69 in Group 1, respectively, and 12.33±5.33 mm, 5.67±2.68 sec, 308.36±16.00 mOsm/L, and 10.62±8.66 in Group 2, respectively. There was no significant difference in any of the tear function tests and OSDI scores between the study groups (p>0.05).

CONCLUSION

GDM seems to have no negative effects on tear function tests. This may be due to a lack of duration of hyperglycemia long enough to affect the tear function tests of pregnant women.

The Emerging Role of the Insulin-like Growth Factors in Oral Biology

The insulin-like growth factors (IGF) are a family of growth factors, receptors and binding proteins that are involved in numerous growth and differentiation processes, as well as in various pathological conditions. The aim of this review is to summarize data that has been accumulating in recent years linking the IGF system to a number of physiological and pathological oral processes. The IGF system fulfills an important role in growth and development of teeth, mandible, maxillae, and tongue. It has been postulated that IGF-I may be of great value in the treatment of periodontal defects and in tissue healing. Furthermore, IGF-II has been shown to be overexpressed in salivary gland adenomas, suggesting that aberrant IGF signaling may be a key factor in the etiology of oral malignancies. Understanding the role and regulation of IGF system components in salivary glands and other oral structures will be of significant basic and clinical relevance.

Long-term dexamethasone treatment alters the histomorphology of acinar cells in rat parotid and submandibular glands

Glucocorticoids (GCs) induce insulin resistance (IR), a condition known to alter oral homeostasis. This study investigated the effects of long-term dexamethasone administration on morphofunctional aspects of salivary glands. Male Wistar rats received daily injections of dexamethasone [0.1 mg/kg body weight (b.w.), intraperitoneally] for 10 days (DEX), whereas control rats received saline. Subsequently, glycaemia, insulinaemia, insulin secretion and salivary flow were analysed. The parotid and submandibular glands were collected for histomorphometric evaluation and Western blot experiments. The DEX rats were found to be normoglycaemic, hyperinsulinaemic, insulin resistant and glucose intolerant (P < 0.05). DEX rat islets secreted more insulin in response to glucose (P < 0.05). DEX rats had significant reductions in the masses of the parotid (29%) and submandibular (16%) glands (P < 0.05) that was associated with reduced salivary flux rate. The hypotrophy in both glands observed in the DEX group was associated with marked reduction in the volume of the acinar cells in these glands of 50% and 26% respectively (P < 0.05). The total number of acinar cells was increased in the submandibular glands of the DEX rats (P < 0.05) but not in the parotid glands. The levels of proteins related to insulin and survival signalling in both glands did not differ between the groups. In conclusion, the long-term administration of dexamethasone caused IR, which was associated with significant reductions in both mass and flux rate of the salivary glands. The parotid and submandibular glands exhibited reduced acinar cell volume; however, the submandibular glands displayed acinar hyperplasia, indicating a gland-specific response to GCs. Our data emphasize that GC-based therapies and insulin-resistant states have a negative impact on salivary gland homeostasis.

Identification of orexins and cognate receptors in the lacrimal gland of sheep

The aim of the present work was to study, by means of immunohistochemical and RT-PCR techniques, the presence and distribution of immunopositivity for orexin A and B (OXA and OXB) and orexin type 1 and 2 receptors (OX(1)R and OX(2)R) in the lacrimal gland of sheep as well as the gene expressions for prepro-orexin (PPOX) and cognate receptors. In serial sections, positive staining for OXA and OXB were localized in the same nervous fibers within the connective tissue septa. Positive staining for OX(1)R was evidenced in the wall of small arteries while that for OX(2)R was observed in the secretory portion of the acinar gland cells with a characteristic localization in the apical cytoplasm. RT-PCR analysis showed the presence of transcripts for PPOX, OX(1)R and OX(2)R in the sheep lacrimal gland; the gene expression of OX(1)R was two-fold greater (p<0.01) than that of OX(2)R. Taken together the present findings raise intriguing questions on the potential role of the orexinergic system in the regulation of lacrimal gland functions that require further investigations.

Calorie restriction: A new therapeutic intervention for age-related dry eye disease in rats

A decrease in lacrimal gland secretory function is closely related to aging and leads to an increased prevalence of dry eye syndrome. Since calorie restriction (CR) is considered to prevent functional decline of various organs due to aging, we hypothesized that CR could prevent age-related lacrimal dysfunction. Six-month-old male Fischer 344 rats were randomly divided into ad libitum (AL) and CR (-35%) groups. After 6months of CR, tear function was examined under conscious state. After euthanasia, lacrimal glands were subjected to histological examination, tear protein secretion stimulation test with Carbachol, and assessment of oxidative stress with 8-hydroxy-2 deoxyguanosine (8-OHdG) and 4-hydroxynonenal (HNE) antibodies. CR significantly improved tear volume and tended to increase tear protein secretion volume after stimulation with Carbachol compared to AL. The acinar unit density was significantly higher in the CR rats compared to AL rats. Lacrimal glands in the CR rats showed a lesser degree of interstitial fibrosis. CR reduced the concentration of 8-OHdG and the extent of staining with HNE in the lacrimal gland, compared to AL. Furthermore, our electron microscopic observations showed that mitochondrial structure of the lacrimal gland obtained from the middle-aged CR rats was preserved in comparison to the AL rats. Collectively, these results demonstrate for the first time that CR may attenuate oxidative stress related damage in the lacrimal gland with preservation of lacrimal gland functions. Although molecular mechanism(s) by which CR maintains lacrimal gland function remains to be resolved, CR might provide a novel therapeutic strategy for treating dry eye syndrome.

Influence of insulin treatment on the lacrimal gland and ocular surface of diabetic rats

Previous studies have observed changes in the lacrimal gland and ocular surface related to diabetes mellitus and related it to insulin resistance or insufficiency and oxidative damage. The aim of this study was to evaluate whether insulin treatment inhibits those changes. Diabetes was induced in male Wistar rats with a single intravenous injection of streptozotocin and a subgroup was treated with insulin. After 5 and 10 weeks, the three groups (n = 5-10/group/experimental procedure) were compared for biochemical, functional, and histological parameters. After 5 weeks, changes in morphology and increased numbers of lipofucsin-like inclusions were observed in lacrimal glands of diabetic but not insulin-treated rats. After 5 weeks, malonaldehyde and total peroxidase activity were significantly higher in diabetic rats, but similar to control in insulin-treated diabetic rats (P = 0.03, P = 0.02, respectively). Our data indicate that diabetes induces histological alterations in lacrimal gland and suggests that hyperglycemia-related oxidative stress may participate in diabetic dry eye syndrome. Prevention by insulin replacement suggests direct hormone action and/or benefit by early sub optimal metabolic control.

Aspirin prevents diabetic oxidative changes in rat lacrimal gland structure and function

The aim of this study is to evaluate whether aspirin reduces Diabetis Mellitus (DM) oxidative damage in the lacrimal gland (LG), and ocular surface (OS). Ten weeks after streptozotocin induced DM and aspirin treatment, LG and OS of rats were compared for tear secretion, hidtology, peroxidase activity, and expression of uncoupling proteins (UCPs). DM reduction of tear secretion was prevented by aspirin (P < 0.01). Alterations of LG morphology and increased numbers of lipofucsin-like inclusions were observed in diabetic but not in aspirin-treated diabetic rats. Peroxidase activity levels were higher and UCP-2 was reduced in DM LG but not in aspirin treated (P = 0.0025 and P < 0.05, respectively). The findings prevented by aspirin indicate a direct inhibitory effect on oxidative pathways in LG and their inflammatory consequences, preserving the LG structure and function against hyperglycemia and/or insulin deficiency damage.

Tearful relationships? Sex, hormones, the lacrimal gland, and aqueous-deficient dry eye

Sex and the endocrine system exert a significant influence on the physiology and pathophysiology of the lacrimal gland. The purpose of this article is to briefly review the nature and magnitude of these interactions between sex, hormones and lacrimal tissue, and to address how they may relate to the pathogenesis of aqueous-deficient dry eye. Towards this end, this article has a 3-fold approach: first, to summarize the influence of androgens, estrogens, glucocorticoids, mineralocorticoids, retinoic acid, prolactin, alpha-melanocyte stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, follicle-stimulating hormone, growth hormone, thyroid-stimulating hormone, arginine vasopressin, oxytocin, thyroxine, parathyroid hormone, insulin, glucagon, melatonin, human chorionic gonadotropin and cholecystokinin on the structure and function of the lacrimal gland; second, to discuss the mechanism of action of each hormone on lacrimal tissue; and third, to discuss the clinical relevance of the endocrine-lacrimal gland interrelationship, with a particular focus on each hormone's role (i.e. if relevant) in the development of aqueous-tear deficiency.

Extra-pancreatic insulin production in RAt lachrymal gland after streptozotocin-induced islet β-cells destruction

Previous work has revealed that insulin is secreted in the tear film; its mRNA is expressed in the lachrymal gland (LG) and its receptor in tissues of the ocular surface. To test the hypothesis of insulin production in the LG, we compared normal and diabetic rats for: (1) the presence of insulin and C-peptide, (2) glucose- and carbachol-induced insulin secretion ex-vivo, and (3) biochemical and histological characteristics of diabetic LG that would support this possibility. Four weeks after streptozotocin injection, blood and tears were collected from streptozotocin-diabetic male Wistar rats. Insulin levels in the tear film rose after glucose stimulation in diabetic rats, but remained unchanged in the blood. Ex vivo static secretion assays demonstrated that higher glucose and 200 microM carbachol significantly increased mean insulin levels from LG samples of both groups. Insulin and C-peptide were expressed in LG of diabetic rats as determined by RIA. Comparable synaptophysin immune staining and peroxidase activity in the LG of both groups suggest that the structure and function of these tissues were maintained. These findings provide evidence of insulin production by LG. Higher expression of reactive oxygen species scavengers may prevent oxidative damage to LG compared to pancreatic beta-cells.

Reduced tear production in three canine endocrinopathies

OBJECTIVES

Previous reports have suggested that hypothyroid and diabetic patients can be predisposed to keratoconjunctivitis sicca. This study aimed to measure tear production in dogs with diabetes, hypothyroidism and hyperadrenocorticism using the Schirmer tear test and to compare these results with Schirmer tear test values for a group of normal dogs.

METHODS

Schirmer tear tests were performed on 16 dogs with hyperadrenocorticism, 18 with diabetes and 12 with hypothyroidism together with 100 control dogs. Corneal sensitivity was also measured in 12 of the 18 diabetic dogs with a Cochet Bonnet aesthesiometer and compared with age- and breed-matched normal dogs.

RESULTS

Schirmer tear test values in dogs with hypothyroidism, hyperadrenocorticism and diabetes were 12.3+/-3.2, 14.0+/-4.0 and 12.3+/-5.3 mm/minutes, respectively. Schirmer tear test values were significantly lower than that for the control group (19.6+/-4.2 mm/minutes) in all dogs with an endocrinopathy. Only in two hypothyroid dogs and three diabetics, this was manifested as profound keratoconjunctivitis sicca with Schirmer tear test value lower than 5 mm/minutes. Diabetic dogs had significantly reduced corneal sensitivity compared with a matched set of control dogs.

CLINICAL SIGNIFICANCE

This study shows a significant reduction in tear production in animals with diabetes mellitus, hypothyroidism and hyperadrenocorticism. Further research is needed to elucidate the mechanisms by which this reduction in tear production occurs. Assessment of tear production should be undertaken in animals diagnosed with these endocrinopathies, as these animals may progress to clinical keratoconjunctivitis sicca.

Increased expression of advanced glycation end-products and their receptor, and activation of nuclear factor kappa-B in lacrimal glands of diabetic rats

AIMS/HYPOTHESIS

To assess the involvement of the AGE-specific receptor (AGER, also known as RAGE) axis and nuclear factor kappa-B (NFKB, also known as NF-kappaB) activation in the development of lacrimal gland and tear film dysfunction in diabetes, the present study evaluated: (1) lacrimal gland and tear film alterations in diabetic rats; and (2) the expression of AGE, AGER and NFKB in ocular tissues of normoglycaemic and diabetic rats.

MATERIALS AND METHODS

Diabetes was induced in male Wistar rats with intravenous streptozotocin. Tear secretion parameters were measured and NFKB expression was evaluated in lacrimal glands of control and diabetic rats by western blot. Immunohistochemistry with confocal microscopy was used to assess AGE, AGER and NFKB expression in lacrimal glands of both groups.

RESULTS

Lacrimal gland weight and tear film volume were lower in diabetic than in control rats (p=0.01 and 0.02, respectively). IL1B and TNF concentrations in tears were higher in diabetic than in control rats (p=0.007 and 0.02, respectively). NFKB protein was identified in rat cornea, conjunctiva and lacrimal glands. AGE, AGER and NFKB expression were greater in lacrimal glands of diabetic than in those of control rats.

CONCLUSIONS/INTERPRETATION

Diabetes induces significant alterations in rat lacrimal gland structure and secretion. The higher expression of AGE, AGER and NFKB in lacrimal glands of diabetic rats suggests that these factors are involved in signalling and in subsequent inflammatory alterations related to dry eye in diabetes mellitus.

Effect of inflammation on lacrimal gland function

The lacrimal gland is the main contributor to the aqueous layer of the tear film. It secretes proteins, electrolytes and water, which helps to nourish and protect the ocular surface. Lacrimal gland secretion is primarily under neural control, which is achieved through a neural reflex arc. Stimuli to the ocular surface activate afferent sensory nerves in the cornea and conjunctiva. This in turn activates efferent parasympathetic and sympathetic nerves in the lacrimal gland to stimulate secretion. Sex steroid hormones are also important regulators of lacrimal gland functions. A decrease or lack of lacrimal gland secretion is the leading cause of aqueous tear deficient dry eye syndrome (DES). It has been suggested that DES is an inflammatory disorder that affects the ocular surface and the lacrimal gland. In several pathological instances, the lacrimal gland can become a target of the immune system and show signs of inflammation. This can result from autoimmune diseases (Sjögren's syndrome), organ transplantation (graft versus host disease), or simply as a result of aging. The hallmarks of lacrimal gland inflammation are the presence of focal lymphocytic infiltrates and increased production of proinflammatory cytokines. The mechanisms leading to lacrimal gland dysfunction are still poorly understood. Apoptosis, production of autoantibodies, hormonal imbalance, alterations in signaling molecules, neural dysfunction, and increased levels of proinflammatory cytokines have been proposed as possible mediators of lacrimal gland insufficiency in disease states.

Insulin secretion by rat lacrimal glands: effects of systemic and local variables

To understand the secretory mechanisms and physiological role of insulin in the tear film, the present study examined 1) the time course of insulin secretion in the tear film under glucose intravenous stimulation, 2) the glucose- and carbachol-induced insulin secretion from isolated lacrimal gland (LG), 3) the effect of insulin on glucose consumption by the cornea, and 4) the expression of insulin, pancreatic duodenal homeobox-1 (PDX-1), and glucose transport proteins (GLUTs) in LG tissue. The insulin level in the tear film of 8-wk-old male Wistar rats increased from 0.6 +/- 0.45 to 3.7 +/- 1.3 ng/ml in the initial minutes after glucose stimulation. In vitro assays demonstrated that higher glucose concentrations from 2.8 to 16.7 mM, 200 microM carbachol, or 40 mM KCl significantly increased insulin secretion from lacrimal glands compared with controls, but did not detect C-peptide as measured by RIA. Glucose consumption by corneal tissue, evaluated by radiolabeled D-[U-14C]glucose uptake, was 24.07 +/- 0.61 and was enhanced to 31.63 +/- 3.15 nmol x cornea(-1) x 2 h(-1) in the presence of 6 nM insulin (P = 0.033) and to 37.5 +/- 3.7 nmol x cornea(-1) x 2 h(-1) in the presence of 11.2 mM glucose (P = 0.015). Insulin and PDX-1 mRNA was detected in LG. Insulin was located in the apical areas of acinar cells by immunoperoxidase and the expression of GLUT-1, but not PDX-1, was confirmed by Western blot. These findings suggest that insulin secretion in the tear film is influenced by local stimuli such as nutrient and neural inputs and that this hormone plays a metabolic role in ocular surface tissues. These data also indicate that under normal conditions the insulin secreted by LG is stored, but it is not clear that is locally produced in the LG.

The influence of ageing on the insulin signalling system in rat lacrimal and salivary glands

PURPOSE

Ageing adversely affects the structure and function of lacrimal and salivary glands (LG and SG) and leads to marked insulin resistance that correlates with reduced insulin signal transduction. The aim of this study was to investigate whether ageing affects insulin signal transduction in LG and SG in vivo.

METHODS

Male Wistar rats aged 20 months and 2 months (control group) were compared (n=8/group). Samples were removed under anaesthesia after i.v. injection of insulin, homogenized, immunoprecipitated with anti-insulin receptor (IR), Shc and STAT-1 antibodies and immunoblotted with antiphosphotyrosine antibody.

RESULTS

The 20-month-old rats were significantly hyperinsulinaemic and presented a reduced rate of blood glucose disappearance in response to insulin, compared to the 2-month-old rats. The level of phosphorylation determined by densitometry in the older group of rats showed that ageing significantly reduced insulin-induced IR phosphorylation in LG and SG and STAT-1 phosphorylation in SG, compared to in the control group, but did not alter Shc phosphorylation.

CONCLUSIONS

Ageing influences insulin signal transduction in the LG and SG of rats. Considering the major anabolic actions of insulin, these observations may help to explain the mechanisms of LG and SG dysfunctions observed in ageing.

IGF-1 and insulin receptor expression in the minor salivary gland tissues of Sjögren's syndrome and mucoceles--immunohistochemical study

OBJECTIVE

To investigate the expression of IGF-1 receptors and insulin receptors on the minor salivary gland (MSG) tissues of patients diagnosed with Sjögren's syndrome (SS) and normal salivary gland tissue surrounding mucoceles.

SUBJECTS AND METHODS

Five MSG tissue sections from SS and seven from mucocele patients were stained immunohistochemically using antibody to IGF-1 receptor and insulin receptor in a horse radish peroxidase and DAB system.

RESULTS

The expression of the insulin receptor was increased in the SS sections compared with controls, while the insulin-like growth factor-1 receptor was more intensely expressed in the controls.

CONCLUSION

The presence of differential expression of receptors for IGF and insulin might suggest a possible role of these growth factors in the pathogenesis of SS.