Functioning human insulinomas - An immunohistochemical analysis of intracellular insulin processing

Sixty-seven insulinomas were investigated by immunohistochemistry using sit e-directed antibodies against insulin, proinsulin, chromogranin A, HISL-19, and four proteins directly or indirectly involved in the proteolytic proce ssing of proinsulin: the prohormone convertases PC2 and PC3, carboxypeptida se H (CPH) and 7B2. Results were expressed in a six-grade score according t o the frequency of immunoreactive tumour cells. Insulin was expressed by al l tumours, appearing in either a diffuse or a polarized pattern and being d etected 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 immu noreactive in all cases but 5. It was consistently localized in the Golgi a pparatus. In about half the cases, moreover, it also showed diffuse cytopla smic staining, usually with a very sparse distribution. Trabecular and soli d insulinomas did not present specific, homogeneous patterns of insulin imm unostaining. However, insulin immunoreactivity was much more abundant in tr abecular than in solid neoplasms, being present in virtually all tumour cel ls (score 6) in 50% and 8% of cases, respectively. Virtually all insulinoma s expressed PC2, PC3, CPH and 7B2, usually in 30-100% of tumour cells, with a frequency significantly related to that of insulin. However, detection o f PC2 and 7B2 was slightly less frequent than that of PC3 and CPH. In conse cutive sections these proteins were found to be mostly co-localized with in sulin and chromogranin A but not with proinsulin. They were heavily express ed in all 10 tumours with more than 10% of cells showing cytoplasmic proins ulin immunoreactivity, indicating that the leakage of proinsulin from the G olgi compartment is not associated with faulty expression of converting enz ymes and possibly reflects a saturated processing capacity. HISL-19 immunor eactivity 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 pr oinsulin-processing enzymes in most insulinomas.