PACE4: a subtilisin-like endoprotease prevalent in the anterior pituitary and regulated by thyroid status

Low stringency screening of a rat hypothalamic complementary DNA library for additional members of the subtilisin-like prohormone convertase (PC) family identified rat PACE4, which is 90% identical to human PACE4 in amino acid sequence, with much lower similarity to rat PC1, PC2, furin, PC4, or PC6. The rat PACE4 sequence has the Asp-His-Ser catalytic site triad, an Arg-Gly-Asp potential integrin binding site, and three potential sites for N-linked glycosylation. Rat PACE4 has a long COOH-terminal region, which is very rich in Cys residues (15%). The unique signal sequence of rat PACE4 mediates translocation across microsomal membranes during in vitro translation and secretion of PACE4 from stably transfected fibroblast cells. Rat PACE4 has a tissue and cell line distribution unlike any reported PC, including human PACE4, with high expression in the anterior pituitary and readily detectable expression in several brain regions, the atrium, and the ventricle; negligible PACE4 messenger RNA (mRNA) is detected in neurointermediate pituitary and many nonneuroendocrine tissues. PACE4 mRNA is prevalent in Buffalo rat liver and GH3 cells and present at low levels in AtT-20 cells, whereas it is undetectable in several other cell lines. In situ hybridization coupled with immunocytochemistry revealed that PACE4 is produced by somatotropes, mammotropes, and corticotropes, whereas less PACE4 mRNA was detected in thyrotropes. PACE4 mRNA levels in anterior pituitary are strikingly regulated by thyroid status, with more than a 10-fold increase seen from hypothyroid to hyperthyroid animals. The prevalence of PACE4 in anterior pituitary and the striking effect of thyroid status on PACE4 expression suggest a specific role for PACE4 in processing neuroendocrine peptides.