Prohormone convertases PC2 and PC3, yeast Kex2-family endoproteases specific to the regulated secretory pathway, cleave proinsulin to insulin in the secretory granules of pancreatic cells. The well-differentiated cell line MIN6 expresses PC2 and PC3 and another regulated secretory pathway-specific protein chromogranin A. Furin, another yeast Kex2 endoprotease, exists in the trans-Golgi networks of many cell types. The cell line RINm5F (a cell line that is less differentiated than the MIN6 cell line) does not express the regulated pathway-specific proteins, but strongly expresses furin. We suspected that furin expression may cause the decrement of regulated secretory pathway-specific proteins. To test this hypothesis, we expressed a furin cDNA with a metallothionein promoter in MIN6 cells. With Zn stimulation of furin expression, the messages of PC2, PC3, and chromogranin A decreased, and the processing of proinsulin to mature insulin became less efficient. The furin-expressing MIN6 cells exhibited less insulin content and weakened insulin secretion in response to a high glucose concentration. The conditioned medium from furin-expressing MIN6 cells also exerted a decrease of PC2 and PC3 expression in unaltered MIN6 cells. Thus, proteins cleaved by furin inside the cells or by truncated furin shed into the culture medium appear to cause decreased PC2 and PC3 expression, insulin content, and glucose-responsive insulin secretion in MIN6 cells.

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