Altered islet amyloid polypeptide (amylin) gene expression in rat models of diabetes

Abstract

Summary. The response of the islet amyloid polypeptide gene to chronic dexamethasone treatment in adult rats was investigated. After 12 daily injections, rats were severely underweight and fasting blood glucose levels were elevated. When pancreatic mRNA was analysed, a 16-fold elevation in islet amyloid polypeptide mRNA was observed with only a fourfold increase in insulin mRNA levels. Pancreatic islet amyloid polypeptide and insulin mRNA levels were also determined 12 days after streptozotocin treatment. In these rats, which were not severely diabetic, the reduction in islet amyloid polypeptide mRNA levels was sixfold less than the reduction in insulin mRNA levels. In both these models of diabetes the ratio of islet amyloid polypeptide to insulin mRNA levels was raised. This would not be expected if the physiological role of islet amyloid polypeptide is as a simple hyperglycaemic agent opposing insulin action or release.Key words: Islet amyloid polypeptide, amylin, Type 2 (noninsulin-dependent) diabetes mellitus, mRNA, streptozotocin, dexamethasone, rat.A 37 amino acid peptide with homology to calcitonin gene related peptide (CGRP) has recently been characterised from the pancreatic amyloid deposits of a Type 2 (non-insulin-dependent) diabetic patient [1] and from an insulinoma [2]. This peptide is now known as islet amyloid polypeptide (IAPP) [2], or amylin [3]. IAPP opposes the action of insulin on isolated muscle preparations [3] and inhibits insulin release from rat islets in vitro [4]. Using a cloned DNA probe representing the rat IAPP coding sequence, we have recently found IAPP to be present in the stomach as well as the pancreas [5]. A variety of evidence thus suggests that this peptide may play an important role in the control of carbohydrate metabolism. We therefore decided to study the expression of the IAPP gene in the rat pancreas after treatment with agents known to affect carbohydrate metabolism. In particular, we wanted to know whether IAPP and insulin levels could be regulated independently and whether induced peripheral insulin resistance would lead to changes in the pancreas which might have relevance to amyloid deposition in man. Materials and methods Drug treatmentsAdult male Wistar rats of 125-150g were divided into 3 groups: (1) controls (n = 5) fed ad libitum on standard rat chow, (2) dexamethasone (n = 5) 2 mg/kg injected i.p. each day for 12 days, and (3) streptozotocin treatment (n =24) 60 mg/kg injected i.v. 12 days prior to analysis. After treatment, rats were starved overnight and killed by cervical dislocation after ether anaesthesia. Blood was taken for glucose measurements and the pancreas was dissected and frozen under liquid nitrogen for RNA extraction. RNA analysisTotal RNA was prepared from the frozen tissues and analysed by Northern blotting with rat IAPP and insulin probes as previously [5]. However, to minimise handling, polyadenylated RNA was not purified in the present experiments and 30 ~g of total RNA was loaded onto each track of these gels. Stringent washing conditi...

Islet amyloid polypeptide in the islets of Langerhans: friend or foe?

“…In streptozotocin-diabetic rats, IAPP and insulin mRNA expressions are reduced to 24 and 15 % of that in controls at day 7 [66], as examined by in situ hybridisation, thereby confirming the initial observations [55]. The phenomenon is neither species-specific nor agent-specific because a similar regulation was observed in alloxan-treated mice [66].…”

“…2). In situ hybridisation to IAPP and insulin mRNA confirmed that IAPP expression is more highly up-regulated than that of insulin after dexamethasone treatment for 12 days [55,62]. Whereas IAPP mRNA expression is increased at the cellular level, insulin mRNA expression is actually down regulated by dexamethasone treatment.…”

“…Initially it was found that IAPP mRNA expression is fourfold higher than insulin mRNA expression after dexamethasone treatment [55]. In addition, after streptozotocin-induced diabetes in rats, IAPP mRNA expression was found to be sixfold less reduced than that of insulin mRNA [55]. Although insulin expression in absolute terms is likely to vastly exceed that of IAPP under the examined conditions [56±58], the ratio of IAPP mRNA to insulin mRNA is increased.…”

β-Cell Deficit and Increased β-Cell Apoptosis in Humans With Type 2 Diabetes

“…This mechanism could certainly compound the effects of IAPP oligomers once hyperglycemia supervenes. Since hyperglycemia and insulin resistance increase the expression rate of IAPP to a greater extent than insulin (53,54), hyperglycemia might also further enhance apoptosis due to IAPP oligomers.…”

Glucocorticoid treatment and endocrine pancreas function: implications for glucose homeostasis, insulin resistance and diabetes

“…With this enhanced hormonal release, impaired intracellular IAPP processing may initiate the process of amylin aggregation. For instance, dexamethasone treatment for up to 12 days led to increased levels of both proinsulin and Iapp mRNA in rat islets (Bretherton-Watt et al 1989, Koranyi et al 1992. Similarly, both enhanced plasma amylin levels and amylin secretion from isolated pancreata were found in dexamethasone-induced IR rats (Pieber et al 1993, Mulder et al 1995.…”