High-molecular-weight insulin-like growth factor 2 : immunohistochemical, ultrastructural and biochemical findings in insulin cells under normal, diabetic, and neoplastic conditions, and its role in non-islet cell tumour hypoglycaemia

Abstract

IGF-2 immunoreactive (IR) cells were identified in the normal adult pancreas of man and rat and occurred exclusively in insulin-producing cells. When frozen human pancreatic tissue was extracted and gel-size-separated, a high-molecular-weight (HMW) IGF-2 peptide could be identified. By using radioimmunoassay (RIA) the HMW IGF-2 was found to be in other fractions than those of insulin/pro-insulin. In order to study the ultrastructural localization of the IGF-2 in normal rats, a pre-embedding technique and immuno-gold-silver staining was used. The IGF-2 immunoreactivity was found to be situated in the halo of a subset of the insulin IR B-granules in normal rats. Insulin immunoreactivity occurred more frequently in the B-granules.

The role of HMW IGF-2 in dysplastic insulin cells was studied in the endocrine pancreas of an animal model of non-insulin-dependent diabetes mellitus, namely the Goto-Kakizaki (GK) rat. At 2 months of age, these rats have both islets of normal structure and 'starfish-shaped' islets. They were characterized by an abundance of collagen fibers and great variations in their numbers of B-granules in the insulin cells. The most striking ultrastructural feature of these cells was the presence of large, usually electron-translucent vesicles. Some of them were well demarcated and contained electron-dense material resembling crinophagic bodies, indicating intracellular degradation of their B-granules. As expected, IGF-2 was found in the insulin cells of both GK and control rats, but RIA revealed a considerably larger amount of HMW IGF-2 peptide in the 2- and 6-month-old GK rats than in the 1-month-old GK ones and in the control rats. In situ hybridization of 3-month-old GK rat pancreas showed increased IGF-2 mRNA expression in the "starfish-shaped" islets compared to in the islets of normal structure, both in diabetic and non-diabetic rats. Then, the presence of HMW IGF-2 in neoplastic insulin and other islet cells was investigated using pancreatic tumours of both endocrine and exocrine types. IGF-2 IR cells were found in 13 out of 14 insulin-producing tumours and in 5 out of 20 other endocrine neoplasms.

Biochemical techniques revealed that the amounts of the IGF-2 IR peptide and its molecular weight forms differed from case to case. By using the Northern blot technique, a 5 kb IGF-2 mRNA transcript was found in an insulinoma metastasis. Two other insulinomas, one with few, the other with an abundance of IGF-2 IR cells, showed intense IGF-2 mRNA signal in the majority of the tumour cells by means of in situ hybridization. In 17 specimens of ductal pancreatic adenocarcinomas, only a few scattered cells were IGF-2 IR but they were interpreted as non-neoplastic. No IGF-2 peptide was found in tumour extracts of the adenocarcinomas. Lastly, HMW IGF-2 was studied in a patient suffering from hypoglycaemia caused by a large, haemangiopericytoma with recurrencies for more than a decade. High concentrations of HMW IGF 2 peptide were found in serum samples and IGF-2 IR tumour cells were detected in biopsy specimens of the neoplasm. A surprising finding was, however, that no such IGF-2 IR cells were observed in the specimens from the first operation. When the in situ hybridization technique was used it could be shown that IGF-2 mRNA expression actually occurred also in the original tumour cells, and that it was increased already 6 years before the onset of the hypoglycaemic symptoms. These observations show that HMW IGF-2 is a clinically important, biologically active member of the insulin family, closely related to normal, dysplastic and neoplastic insulin cells.