Genetically engineered proinsulin constitutively processed and secreted as mature, active insulin

The conversion of human proinsulin to insulin occurs only in specialized cells which contain the appropriate processing enzymes. To allow proinsulin processing to occur in a wide variety of cell types, we engineered human proinsulin to be cleaved in the constitutive secretory pathway. Using site-directed mutagenesis, we have introduced furin consensus cleavage sequences (Arg-X-Lys-Arg) into the human proinsulin cDNA. These mutations allowed for efficient proteolytic maturation of human proinsulin to insulin within cells containing only a constitutive pathway of secretion. Additionally, a naturally occurring mutation (histidine B10 to aspartic acid) yields a form of human insulin which accumulates 10- to over 100-fold more mature insulin when compared to the mutants lacking this change. Engineering furin-specific cleavage sites into each junction of the human proinsulin cDNA results in the secretion of peptides that display the expected molecular weights for the A and B chains of insulin. The accumulation of mature, processed, human insulin was measured in the supernatants by radioimmunoassay, and the bioactivity of this molecule was measured by its ability to stimulate autophosphorylation of the insulin receptor. Our results suggest that any cell type might be engineered to produce mature, active, and stable insulin in the constitutive pathway of secretion.