Proinsulin conversion to insulin occurs in secretory granules of pancreatic β-cells. This processing has been suggested to require both the endoproteases PC2 and PC3 with each cleaving at only one of the two sites linking the insulin A- and B-chains with C-peptide. To evaluate this in an appropriate cellular setting, conversion of human proinsulin was followed in GH3 (rat pituitary) cells normally unable to convert this prohormone but equipped with the regulated secretory pathway. For this purpose, human proinsulin was expressed in GH3 cells, alone or in combination with PC2 and/or PC3, using recombinant adenoviruses. Cells were infected with the given adenoviruses and 24 h later were pulse-chased. Kinetics of proinsulin conversion were monitored by reverse-phase high-performance liquid chromatography. It was observed that while the two endoproteases do display a preference for a single site of cleavage (PC2 at the A-chain/C-peptide junction; PC3 at the B-chain/Cpeptide junction) and act in a synergistic manner to promote proinsulin conversion, either PC2 or PC3 alone can cleave at both sites to fully convert proinsulin to insulin. These results also show that a cell can be successfully infected by three different recombinant adenoviruses.

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