Functioning human insulinomas

Abstract

Sixty-seven insulinomas were investigated by immunohistochemistry using site-directed antibodies against insulin, proinsulin, chromogranin A, HISL-19, and four proteins directly or indirectly involved in the proteolytic processing of proinsulin: the prohormone convertases PC2 and PC3, carboxypeptidase H (CPH) and 7B2. Results were expressed in a six-grade score according to the frequency of immunoreactive tumour cells. Insulin was expressed by all tumours, appearing in either a diffuse or a polarized pattern and being detected in more than 30% of tumour cells in all cases but three. Proinsulin was also expressed in all tumours, with more than 50% of tumour cells immunoreactive in all cases but 5. It was consistently localized in the Golgi apparatus. In about half the cases, moreover, it also showed diffuse cytoplasmic staining, usually with a very sparse distribution. Trabecular and solid insulinomas did not present specific, homogeneous patterns of insulin immunostaining. However, insulin immunoreactivity was much more abundant in trabecular than in solid neoplasms, being present in virtually all tumour cells (score 6) in 50% and 8% of cases, respectively. Virtually all insulinomas expressed PC2, PC3, CPH and 7B2, usually in 30-100% of tumour cells, with a frequency significantly related to that of insulin. However, detection of PC2 and 7B2 was slightly less frequent than that of PC3 and CPH. In consecutive sections these proteins were found to be mostly co-localized with insulin and chromogranin A but not with proinsulin. They were heavily expressed in all 10 tumours with more than 10% of cells showing cytoplasmic proinsulin immunoreactivity, indicating that the leakage of proinsulin from the Golgi compartment is not associated with faulty expression of converting enzymes and possibly reflects a saturated processing capacity. HISL-19 immunoreactivity was found in both Golgi apparatus and insulin stores, indicating that the relevant antigen is different from all other proteins investigated. These results do not support a defect in expression or localization of proinsulin-processing enzymes in most insulinomas.

The spectrum of endogenous human chromogranin A-derived peptides identified using a modified proteomic strategy

“…While some of the cleavage sites identified are predictable, others, especially those occurring at single basic residue sites, are not. All cleavages identified are consistent with the known specificities of prohormone convertases 1/3 and 2 and carboxypeptidase H, all of which are known to be present and active in both normal and neoplastic beta cells [42]. Although all peptides described were initially isolated from an insulinoma, their virtual universal presence in three additional neuroendocrine tumour types, arising from anatomically discrete tissues and cell types, is highly suggestive of biological relevance.…”

“…As a high degree of CgA processing is not universal in different phenotypes of neuroendocrine cells, the appropriate choice of discrete hepatic metastasis would be one which has arisen from a cell type known to both express and process this protein to a high degree in the non-neoplastic state. The human insulinoma employed in the present study fulfils all of these criteria in that the normal beta cell is known to process CgA actively [16] and tumours arising from this cell type have been shown to express all of the prohormone convertases associated with non-neoplastic beta cells [42]. As CgA and proinsulin/insulin represent the major proteins of beta cell storage granules, the profile of endogenous products of CgA may have specific implications in beta cell function and, by extrapolation, in diabetes mellitus.…”

Carboxypeptidase E promotes cancer cell survival, but inhibits migration and invasion

“…Neuroendocrine cancers such as pheochromocytoma, are some of the least understood forms of the disease [2]. It is well established that various types of (neuro)endocrine tumors, including pheochromocytomas, lung small cell carcinomas [3], insulinomas [4], breast adenocarcinoma [5] and glioblastomas [6] secrete large numbers of hormones and growth factors, as well as their processing enzymes, such as carboxypeptidase E (CPE) and peptidylglycine alpha-amidating mono-oxygenase [7]. Among the prohormone processing enzymes, CPE has been the most studied with respect to tumorigenesis.…”