PACE4 - A SUBTILISIN-LIKE ENDOPROTEASE PREVALENT IN THE ANTERIOR-PITUITARY AND REGULATED BY THYROID STATUS

Low stringency screening of a rat hypothalamic complementary DNA libra ry for additional members of the subtilisin-like prohormone convertase (PC) family identified rat PACE4, which is 90% identical to human PAC E4 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 cataly tic site triad, an Arg-Gly-Asp potential integrin binding site, and th ree potential sites for N-linked glycosylation. Rat PACE4 has a long C OOH-terminal region, which is very rich in Cys residues (15%). The uni que signal sequence of rat PACE4 mediates translocation across microso mal membranes during in vitro translation and secretion of PACE4 from stably transfected fibroblast cells. Rat PACE4 has a tissue and cell l ine distribution unlike any reported PC, including human PACE4, with h igh expression in the anterior pituitary and readily detectable expres sion in several brain regions, the atrium, and the ventricle; negligib le PACE4 messenger RNA (mRNA) is detected in neurointermediate pituita ry and many nonneuroendocrine tissues. PACE4 mRNA is prevalent in Buff alo rat liver and GH(3) cells and present at low levels in AtT-20 cell s, whereas it is undetectable in several other cell lines. In situ hyb ridization coupled with immunocytochemistry revealed that PACE4 is pro duced by somatotropes, mammotropes, and corticotropes, whereas less PA CE4 mRNA was detected in thyrotropes. PACE4 mRNA levels in anterior pi tuitary are strikingly regulated by thyroid status, with more than a 1 0-fold increase seen from hypothyroid to hyperthyroid animals. The pre valence of PACE4 in anterior pituitary and the striking effect of thyr oid status on PACE4 expression suggest a specific role for PACE4 in pr ocessing neuroendocrine peptides.