Proprotein convertases (PC1/PC3 and PC2) in normal and neoplastic human tissues: their use as markers of neuroendocrine differentiation.

Abstract

By immunocytochemistry and immunoblotting, we examined normal and neoplastic human tissues with polyclonal antibodies raised against selected peptide regions of proprotein convertase-2 and -3 (PC2 and PC3), two proteases that have been shown to selectively cleave neuroendocrine precursor molecules at pairs of basic residues. Immunoreactivity for both enzymes was detected in neuroendocrine cells of pituitary, gut, pancreas, thyroid, and adrenals and in tumors thereof, but was absent in thyroid follicular cells, parathyroids, adrenal cortex, testes, and a number of nonneuroendocrine tissues, both normal and tumorous. Although both PCs were virtually universal concomitants of the neuroendocrine system, cells with a neural phenotype (e.g. pheochromocytes and Merkel cells) predominantly contained PC2, whereas classic endocrine cells contained mostly PC3. PC3 immunoreactive cells were abundant all along the gastrointestinal tract, whereas PC2 was highly expressed only in the pyloric antrum and proximal third of duodenum. Double immunostaining experiments revealed colocalization of PC3 with virtually all gastrointestinal peptides, whereas PC2 immunoreactivity was mostly expressed in gastrin, cholecystokinin, and somatostatin cells. Noticeably, the proportion of glucagon-producing cells immunoreactive for PC3 was high in the gut and low in pancreatic islets and glucagonomas, whereas the reverse occurred for PC2. At the ultrastructural level, immunostaining was confined to the mature dense core granules, the site of storage of granins and peptide hormones. With the exception of parathyroid cells, PC2 and/or PC3 expression correlated with the occurrence of granins, canonical markers of the secretory granules. Immunoblotting experiments confirmed the identity of the immunocytochemical reactivities. It is concluded that PC2 and PC3 are highly sensitive markers of neuroendocrine differentiation and have distinct distribution patterns, and that antibodies to these enzymes may play an important role in the analysis of tumors.

Differential Processing of Proglucagon by the Subtilisin-like Prohormone Convertases PC2 and PC3 to Generate either Glucagon or Glucagon-like Peptide

“…This view is supported by recent findings that pancreatic ␣ cells express high levels of PC2 and low levels, if any, of PC3 (17,23,24), whereas intestinal L cells contain immunoreactive PC3 but not PC2 (25). This suggests that PC2 generates the pancreatic phenotype while PC3 may be largely responsible for the intestinal phenotype.…”

Defective prohormone processing and altered pancreatic islet morphology in mice lacking active SPC2

“…Instead, SS-28 is seen to be present as a major end-product of prosomatostatin processing in the SPC2 Ϫ͞Ϫ extracts. These results indicate that cleavage at the single Arg 64 residue occurs normally, probably catalyzed by another convertase, whereas cleavage at the Arg-Lys pair (positions 77 and 78 in prosomatostatin) to yield SS-14 (and SS-28 [1][2][3][4][5][6][7][8][9][10][11][12][13][14] ) requires SPC2. Although SPC3 can also cleave at this position (34), only SPC2 can be detected in delta cells by immunostaining (49).…”

“…SPC2 and SPC3 are expressed almost exclusively in neuroendocrine cells throughout the brain, gut, pancreatic islets, and other endocrine tissues of the body (6)(7)(8)(9). Their acidic pH optima, requirement for calcium ions, and ability to be sorted into dense-core vesicles of the regulated secretory pathway are consistent with their putative role as the major endoproteases responsible for the maturation of a large number of hormone and neuropeptide precursors throughout the organism (10)(11)(12)(13)(14)(15).…”

Role of the Prohormone Convertase PC3 in the Processing of Proglucagon to Glucagon-like Peptide 1

“…1). The first indication that this differential processing may result from the expression of different convertases in ␣-and L cells came from the observation that pancreatic ␣-cells express high levels of PC2 but no detectable PC3 (8,(33)(34)(35)(36), whereas intestinal L cells contain immunoreactive PC3 but no PC2 (37). The pancreatic pathway of proglucagon processing has been determined in ␣TC1-6 cells, a transformed cell line processing proglucagon in a manner that faithfully reflects the parental pancreatic ␣-cell from which it has been derived (8).…”