DIFFERENTIAL EXPRESSION OF VACUOLAR-TYPE H-ATPASE BETWEEN NORMAL HUMAN PANCREATIC-ISLET B-CELLS AND INSULINOMA CELLS()

Recent studies have shown that bafilomycin A(1)-sensitive vacuolar typ e H+-ATPase (V-ATPase) is responsible for the acidification of intrace llular compartments in eukaryotic cells. B-cells of pancreatic islets also are known to include acidifying secretory vesicles which are the major cellular site of proinsulin to insulin conversion. This study wa s designed to examine immunohistochemically the level of V-ATPase prot ein expression in normal pancreas (five cases) and benign insulinoma ( six cases), using mouse monoclonal antibody raised against the 116 kDa subunit of human V-ATPase. Light microscopic immunohistochemistry rev ealed that moderate to marked V-ATPase expression was observed in norm al islet B-cells, while insulinoma cells in each case expressed V-ATPa se faintly or not at all. By immunoelectron microscopy, the majority o f secretory vesicles in insulinoma cells did not express V-ATPase prot ein at their endomembranes, although mild to marked V-ATPase expressio n was noted at the endomembrane of secretory vesicles in normal islet B-cells. Thus, differential expression of V-ATPase protein at the endo membrane of secretory vesicles was observed between normal islet B-cel ls and insulinoma cells. These findings suggest that the reduced activ ity of V-ATPase per insulin secretory vesicle in insulinoma cells have a profound effect on the efficiency of proteolytic cleavage of proins ulin.