Topographic abnormalities of proinsulin to insulin conversion in functioning human insulinomas. Comparison of immunoelectron microscopic and clinical data.

It has been proposed that the major defect in human insulinomas is a decreased hormone storage capacity resulting in uncontrolled release of proinsulin and insulin. By immunoelectron microscopy with monoclonal antibodies we studied the subcellular distribution of proinsulin and insulin in benign and malignant functioning insulinomas of different histology and compared the findings with various clinical and pathohistological parameters. We found that, in contrast to normal B cells, the proinsulin to insulin conversion in insulinomas occurs already in the trans Golgi apparatus but remains incomplete, resulting in the formation of secretory granules containing both proinsulin and insulin. Thus, in functioning insulinomas, sorting into secretory granules is not a prerequisite for hormone conversion. Furthermore, proinsulin and insulin storage and most probably subsequent secretion occurs through the secretory granules via the regulated pathway. A substantial variability for both proinsulin and insulin immunolabeling in secretory granules was found not only in individual tumor cells but also among the insulinomas studied. This observed variability may account for the lack of correlation between pathohistological, immunohistochemical, and clinical parameters in functioning insulinomas.