Interactions between the prohormone convertase 2 promoter and the thyroid hormone receptor

The majority of prohormones are cleaved at paired basic residues to generat e bioactive hormones by prohormone convertases (PCs). As PC1 and PC2, two n euroendocrine-specific PCs, appear to be the key enzymes capable of process ing a variety of prohormones, alterations of PC2 and/or PC1 levels will pro bably have a profound effect on hormonal homeostasis. We investigated the r egulation of PC2 messenger RNA (mRNA) by thyroid hormone using GH(3) cells to demonstrate that T-3 negatively regulated PC2 mRNA levels in a dose- and time-dependent fashion. Functional analysis of progressive 5'-deletions of the human (h) PC2 promoter luciferase constructs in GH, cells demonstrated that the regulation probably occurs at the transcriptional level, and that putative negative thyroid hormone response elements were located within th e region from -44 to +137 bp relative to the transcriptional start site. Tr ansient transfections in JEG-3 cells and COS-1 cells showed that the suppre ssive effect of T-3 was equally mediated by the thyroid hormone receptor (T R) isoforms TR alpha 1 and TR beta 1. Electrophoretic mobility shift assays using purified TR alpha 1 and retinoid X receptor-beta protein as well as GH(3) nuclear extracts showed that regions from +51 to +71 bp and from +118 to +137 bp of the hPC2 promoter bind to TR alpha 1 as both a monomer and a homodimer and with TR alpha 1/retinoid X receptor-beta as a heterodimer. F inally, the in vivo regulation of pituitary PC2 mRNA by thyroid status was demonstrated in rats. These results demonstrate that T-3 negatively regulat es PC2 expression at the transcriptional level and that functional negative thyroid hormone response elements exist in the hPC2 promoter. We postulate that the alterations of PC2 activity may mediate some of the pathophysiolo gical consequences of hypo- or hyperthyroidism.