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

Proglucagon is processed differentially in pancreatic ␣-cells and intestinal endocrine L cells to release either glucagon or glucagon-like peptide-1-(7-36amide) (tGLP-1), two peptide hormones with opposing biological actions.Previous studies have demonstrated that the prohormone convertase PC2 is responsible for the processing of proglucagon to glucagon, and have suggested that the related endoprotease PC3 is involved in the formation of tGLP-1.To understand better the biosynthetic pathway of tGLP-1, proglucagon processing was studied in the mouse pituitary cell line AtT-20, a cell line that mimics the intestinal pathway of proglucagon processing and in the rat insulinoma cell line INS-1.In both of these cell lines, proglucagon was initially cleaved to glicentin and the major proglucagon fragment (MPGF) at the interdomain site Lys 70 -Arg 71 .In both cell lines, MPGF was cleaved successively at the monobasic site Arg 77 and then at the dibasic site Arg 109 -Arg 110 , thus releasing tGLP-1, the cleavages being less extensive in INS-1 cells.Glicentin was completely processed to glucagon in INS-1 cells, but was partially converted to oxyntomodulin and very low levels of glucagon in AtT-20 cells in the face of generation of tGLP-1.Adenovirusmediated co-expression of PC3 and proglucagon in GH 4 C 1 cells (normally expressing no PC2 or PC3) resulted in the formation of tGLP-1, glicentin, and oxyntomodulin, but no glucagon.When expressed in ␣TC1-6 (transformed pancreatic ␣-cells) or in rat primary pancreatic ␣-cells in culture, PC3 converted MPGF to tGLP-1.Finally, GLP-1-(1-37) was cleaved to tGLP-1 in vitro by purified recombinant PC3.Taken together, these results indicate that PC3 has the same specificity as the convertase that is responsible for the processing of proglucagon to tGLP-1, glicentin and oxyntomodulin in the intestinal L cell, and it is concluded that this enzyme is thus able to act alone in this processing pathway.A large number of proteins and peptides are synthesized as larger precursors that are proteolytically processed during their intracellular transport to produce the biologically active molecules.The hallmark of this intracellular maturation, which takes place relatively late in the secretory pathway, is its high specificity for sites containing basic residues.The specific