Prohormone-convertase 1 processing enhances post-Golgi sorting of prothyrotropin-releasing hormone-derived peptides

Rat prothyrotropin-releasing hormone (pro-TRH) is endoproteolyzed within the regulated secretory pathway of neuroendocrine cells yielding five TRH peptides and seven to nine other unique peptides. Endoproteolysis is performed by two prohormone convertases, PC1 and PC2. Proteolysis of pro-TRH begins in the trans-Golgi network and forms two intermediates that are then differentially processed as they exit the Golgi and are packaged into immature secretory granules. We hypothesized that this initial endoproteolysis may be necessary for downstream sorting of pro-TRH-derived peptides as it occurs before Golgi exit and thus entry into the regulated secretory pathway. We now report that when pro-TRH is transiently expressed in GH4C1 cells, a neuroendocrine cell line lacking PC1, under pulse-chase conditions release is constitutive and composed of more immature processing intermediates. This is also observed by radioimmunoassay under steady-state conditions. When a mutant form of pro-TRH, which has the dibasic sites of initial processing mutated to glycines, is expressed in AtT20 cells, a neuroendocrine cell line endogenously expressing PC1, both steady-state and pulse-chase experiments revealed that peptides derived from this mutant precursor are secreted in a constitutive fashion. A constitutively secreted form of PC1 does not target pro-TRH peptides to the constitutive secretory pathway but results in sorting to the regulated secretory pathway. These results indicated that initial processing action of PC1 on pro-TRH in the trans-Golgi network, and not a cargo-receptor relationship, is important for the downstream sorting events that result in storage of pro-TRH-derived peptides in mature secretory granules.