Processing of pro-PTHRP by the prohormone convertase, furin: effect on biological activity

We have examined the conversion of parathyroid hormone-related peptide (PTHRP) from its NH2-terminally extended precursor pro-PTHRP. Within pro-PTHRP, an amino acid sequence exists that can serve as a substrate for the prohormone convertase, furin. To evaluate the potential role of furin in processing of this entity, we expressed pro-PTHRP in COS-7 cells, which normally produce this enzyme. Transiently transfected COS-7 cells secreted high levels of PTHRP into conditioned culture medium. Cotransfection of these cells with antisense furin cDNA resulted in marked inhibition of furin mRNA expression and secretion of an NH2-terminal fragment of pro-PTHRP, which comigrated with synthetic pro-PTHRP-(-12-->+36) on gel-permeation high-pressure liquid chromatography. In an adenylate cyclase bioassay, condition medium containing this fragment and synthetic pro-PTHRP-(-12-->+36) both exhibited lower potency than synthetic PTHRP-(1-36) and conditioned medium containing PTHRP produced by COS-7 cells in the absence of antisense furin. These results demonstrate the capacity of furin to convert pro-PTHRP to a more active product and suggest a role for this enzyme in the normal intracellular processing of this hormone.