European Journal of Endocrinology (2010) 163 551–558 ISSN 0804-4643 CLINICAL STUDY Proinsulin levels in patients with pancreatic diabetes are associated with functional changes in insulin secretion rather than pancreatic b-cell area Thomas G K Breuer1, Bjoern A Menge1, Matthias Banasch1, Waldemar Uhl2, Andrea Tannapfel3, Wolfgang E Schmidt1, Michael A Nauck4 and Juris J Meier1 Departments of 1Medicine I and 2Surgery, St Josef-Hospital, Ruhr-University Bochum, Gudrunstrasse 56, 44791 Bochum, Germany, 3Department of Pathology, Ruhr-University Bochum, 44789 Bochum, Germany and 4Diabeteszentrum Bad Lauterberg, 37431 Bad Lauterberg, Germany (Correspondence should be addressed to J J Meier; Email: [email protected]) Abstract Introduction: Hyperproinsulinaemia has been reported in patients with type 2 diabetes. It is unclear whether this is due to an intrinsic defect in b-cell function or secondary to the increased demand on the b-cells. We investigated whether hyperproinsulinaemia is also present in patients with secondary diabetes, and whether proinsulin levels are associated with impaired b-cell area or function. Patients and methods: Thirty-three patients with and without diabetes secondary to pancreatic diseases were studied prior to pancreatic surgery. Intact and total proinsulin levels were compared with the pancreatic b-cell area and measures of insulin secretion and action. Results: Fasting concentrations of total and intact proinsulin were similar in patients with normal, impaired (including two cases of impaired fasting glucose) and diabetic glucose tolerance (PZ0.58 and PZ0.98 respectively). There were no differences in the total proinsulin/insulin or intact proinsulin/insulin ratio between the groups (PZ0.23 and PZ0.71 respectively). There was a weak inverse association between the total proinsulin/insulin ratio and pancreatic b-cell area (r2Z0.14, PZ0.032), whereas the intact proinsulin/insulin ratio and the intact and total proinsulin levels were unrelated to b-cell area. However, a strong inverse relationship between homeostasis model assessment index of b-cell function and both the total and the intact proinsulin/insulin ratio was found (r2Z0.55 and r2Z0.48 respectively). The association of insulin resistance (IR) with intact proinsulin was much weaker than the correlation with fasting insulin. Conclusions: Hyperproinsulinaemia is associated with defects in insulin secretion rather than a reduction in b-cell area. The weak association between intact proinsulin and IR argues against the usefulness of this parameter in clinical practice. European Journal of Endocrinology 163 551–558 Introduction individuals (6, 7), although this finding has not been confirmed by others (8, 9). Nevertheless, the determina- In health, glucose homoeostasis is primarily controlled tion of intact proinsulin as a marker for the diagnosis of by the secretion of insulin from pancreatic b-cells (1). insulin resistance (IR) has been widely advocated (10). About 80–90% of the total circulating insulin concen- Two major hypotheses have been expounded to trations are derived from mature, biologically fully explain the hyperproinsulinaemia in type 2 diabetes: active, insulin (2). In addition, a small percentage of some investigators have speculated that the increased immature insulin precursors and incomplete proinsulin release of proinsulin in such patients might result from cleavage products, such as intact proinsulin, des an intrinsic defect in proinsulin processing, leading to (31,32)-proinsulin and des (64,65)-proinsulin, are an increased release of immature insulin precursors and being co-secreted along with mature insulin. While in thus contributing to the impairment in b-cell function healthy, non-diabetic subjects, these precursors consti- in type 2 diabetes (5). Alternatively, hyperproinsulinae- tute only a minor fraction of the total amount of mia may be caused by an increased secretory demand secreted insulin, the release of proinsulin is significantly on the b-cells, leading to depletion of the ‘readily increased in patients with type 2 diabetes as well as in releasable’ insulin granule pool and mobilisation of individuals with impaired glucose tolerance (IGT) (2–5). insulin granules from the ‘reserve pool’, which is A couple of recent studies have also reported an thought to contain greater amounts of immature increase in intact proinsulin levels in insulin-resistant insulin precursors (11). The latter explanation would q 2010 European Society of Endocrinology DOI: 10.1530/EJE-10-0330 Online version via www.eje-online.org Downloaded from Bioscientifica.com at 10/01/2021 10:51:11AM via free access 552 T G K Breuer and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2010) 163 be supported by the fact that the ratio of proinsulin intact proinsulin levels were carried out, and the over insulin was also increased in humans after a fractional b-cell area of the resected pancreas was hemipancreatectomy (12). measured. The study protocol was approved by the In order to address the aetiology of increased ethics committee of the Ruhr-University Bochum proinsulin release in patients with type 2 diabetes, (registration number 2528). All patients provided two studies have compared the proinsulin/insulin ratios written informed consent prior to study enrolment. after a transient inhibition of insulin secretion aiming to induce b-cell rest (13, 14). Both studies showed clear reductions in relative proinsulin secretion after somato- Patients statin infusion, but only one study could demonstrate a full restoration of the proinsulin/insulin ratio to A total of 33 patients (17 males and 16 females) similar levels as found in non-diabetic individuals (13). undergoing pancreatic resections in the Department of Therefore, the underlying causes for the hyperpro- Surgery, St Josef-Hospital, Ruhr-University Bochum, insulinaemia in type 2 diabetes are still elusive. between the years 2004 and 2007 were included. Furthermore, although some studies have demon- Amongst those, 17 patients had been diagnosed with strated an association of proinsulin/insulin ratio with chronic pancreatitis, 13 underwent surgery for the various measures of insulin secretion, its relationship removal of benign pancreatic adenomas and 3 patients with b-cell mass has not yet been examined. Thus, underwent partial pancreatectomy because of tumours whether this association is due to a reduction in b-cell of the ampulla of Vater. The clinical diagnoses of number or a functional impairment in insulin release chronic pancreatitis, pancreatic carcinoma, pancreatic remains to be elucidated. adenoma or ampullary cancer were confirmed by an An alternative way to address whether the hyper- independent pathologist in all cases. Diabetes was proinsulinaemia in patients with type 2 diabetes is due previously known in six patients (treated with insulin to an intrinsic defect in proinsulin processing or an in four cases, glimepiride in one case and diet in one increased demand on the b-cells is to determine case), whereas the other patients had no history of proinsulin levels in patients with diabetes due to other known diabetes. A detailed description of the patient aetiologies than type 2 diabetes. Furthermore, by characteristics has been previously provided (15). comparing proinsulin concentrations with pancreatic b-cell area and various measures of insulin secretion, it should be possible to ascribe the development of Experimental procedures hyperinsulinaemia to defects in either b-cell mass or The experiments were performed in the morning after the functional regulation of insulin secretion. an overnight fast with subjects in a supine position Therefore, in the present study, total and intact throughout the experiments as described (15). All other proinsulin concentrations as well as various established concomitant medication was withdrawn since the measures of insulin secretion were determined in evening of the preceding day. The experiments were patients with and without diabetes secondary to started by the ingestion of the oral glucose load (75 g pancreatic disorders immediately prior to pancreatic glucose in 300 ml) over 5 min, and capillary and resections, and the fractional b-cell area was venous blood samples were drawn at tZK5, 0, 15, determined in the resected pancreatic tissue. By these 30, 60, 90, 120, 150, 180, 210 and 240 min. Capillary means, we addressed the following questions: i) Do blood samples (w100 ml) were added to NaF (Microvette patients with diabetes due to pancreatic disorders exhibit disproportionate hyperproinsulinaemia CB 300; Sarstedt, Nu¨ mbrecht, Germany) for the compared to non-diabetic individuals? ii) Are increased immediate measurement of glucose. Venous blood was proinsulin levels in these patients associated with drawn into chilled tubes containing EDTA and aprotinin defects in pancreatic b-cell area or b-cell function? iii) (Trasylol; 20 000 KIU/ml, 200 ml per 10 ml blood; Do intact proinsulin levels predict IR in lean patients Bayer AG) and kept on ice. After centrifugation with and without diabetes due to pancreatic disorders? at 4 8C, plasma for hormone analyses was kept frozen at K28 8C. Patients and methods Measurements Glucose was measured as described (15) using a glucose Study design oxidase method with a Glucose Analyser 2 (Beckman Thirty-three patients undergoing pancreatic surgery for Instruments, Munich, Germany). chronic pancreatitis, benign pancreatic adenomas or Insulin was measured as described (15) using an papillary tumours necessitating partial