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Proparathyroid Hormone Processing by the Proprotein Convertase-7: Comparison with Furin and Assessment of Modulation of Parathyroid Convertase Messenger Ribonucleic Acid Levels by Calcium and 1,25-Dihydroxyvitamin D₃¹

Departments of Medicine (L.C., H.P.J.B., Y.H., G.N.H.), Physiology (G.N.H.), and Human Genetics (G.N.H.), McGill University and Royal Victoria Hospital, Montréal, Québec H3A 1A1, Canada; and the J. A. DeSeve Laboratory of Biochemical Neuroendocrinology (N.G.S.), Clinical Research Institute of Montréal, Université de Montréal, Montréal, Québec H2W 1R7, Canada

Address all correspondence and requests for reprints to: Geoffrey N. Hendy, Ph.D., Calcium Research Laboratory, Room H4.67, Royal Victoria Hospital, 687 Pine Avenue West, Montréal, Québec H3A 1A1, Canada. E-mail: gnhendy{at}med.mcgill.ca

We previously showed that the processing of proparathyroid hormone (proPTH) to PTH was accomplished most efficiently by furin (17). Colocalization studies demonstrated that furin is expressed in the parathyroid, whereas proprotein convertase (PC)1 and PC2 are not. Since that time, another member of the PC family, called PC7, has been identified. Here we show, using coinfection studies, that PC7, as well as furin, can appropriately cleave PTH from proPTH. ProPTH and PTH were purified from cell extracts by reversed-phase HPLC and were identified by Western blot analysis and delayed extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Colocalization studies, using Northern blot and reverse transcriptase-PCR analyses, showed that PC7 messenger RNA (mRNA) is expressed in the parathyroid gland. Therefore, PC7, like furin, has the potential to be involved in the physiological processing of proPTH to PTH. The two major regulators of parathyroid cell synthetic and secretory activity are the extracellular fluid calcium and 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels. We investigated whether either of these agents might modulate processing of proPTH to PTH by altering parathyroid convertase gene expression. In both in vitro and in vivo systems in which regulation of PTH mRNA levels were clearly apparent, there was no effect of either calcium or 1,25(OH)₂D₃ on parathyroid furin or PC7 mRNA levels. This is in contrast to the processing of proinsulin to insulin in the pancreatic ß-cell, which is up-regulated by glucose stimulation of PC1 and PC2 synthesis.

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