title: Direct in vitro evidence of extracellular Ca2+-induced amino-terminal truncation of human parathyroid hormone (1-84) by human parathyroid cells.
abstract: BACKGROUND
  Although serum calcium (Ca2+) concentration regulates the generation of amino-terminally (N-terminally) truncated forms of human PTH (hPTH) degraded from (1-84)hPTH, no studies have yet reported whether the parathyroid gland itself is responsible for this process.
  
  
  OBJECTIVE
  Our objective was to determine the site of N-terminal truncation and its roles in PTH metabolism in parathyroid cells in vitro.
  
  
  METHODS
  The effect of extracellular Ca2+ concentration was examined on N-terminal truncation in primary cultured parathyroid cells. The parathyroid glands were obtained from the patients with primary and uremia-associated secondary hyperparathyroidisms who underwent therapeutic parathyroidectomies.
  
  
  RESULTS
  The N-terminally truncated fragments were detectable with commercially available intact PTH (I-PTH) assays, but not with the bio-intact PTH (Bio-PTH) assay, which detected only the (1-84)hPTH. HPLC revealed that generation of N-terminally truncated fragments detectable by I-PTH increased with extracellular Ca2+ concentration. Suppression of PTH secretion by increasing the extracellular Ca2+ concentration was more evident with the Bio-PTH assay than with the I-PTH assay for both cultured parathyroid cells prepared from parathyroid adenomas and uremia-associated secondary hyperparathyroidism. The Bio-PTH/I-PTH ratio, which is the ratio of (1-84)hPTH to the sum of (1-84)hPTH and N-terminally truncated fragments, decreased in response to increases in extracellular Ca2+.
  
  
  CONCLUSIONS
  These findings suggest that the N-terminal truncation is regulated by extracellular Ca2+ concentration and works to suppress the generation of (1-84)hPTH in parathyroid cells.
authors.affiliation: Department of Metabolism, Endocrinology, and Molecular Medicine, Osaka City University Graduate School of Medicine, 1-4-3, Asahi-machi, Abeno-ku, Osaka 545-8585, Japan.
authors.forename: Takehisa
authors.initials: T
authors.lastname: Kawata
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Imanishi
authors.forename: Keisuke
authors.initials: K
authors.lastname: Kobayashi
authors.forename: Naoyoshi
authors.initials: N
authors.lastname: Onoda
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Takemoto
authors.forename: Hideki
authors.initials: H
authors.lastname: Tahara
authors.forename: Senji
authors.initials: S
authors.lastname: Okuno
authors.forename: Eiji
authors.initials: E
authors.lastname: Ishimura
authors.forename: Takami
authors.initials: T
authors.lastname: Miki
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Ishikawa
authors.forename: Masaaki
authors.initials: M
authors.lastname: Inaba
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Nishizawa
mesh_terms: D002118:Calcium / Q000494:pharmacology*; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D006801:Humans; D006962:Hyperparathyroidism, Secondary / Q000378:metabolism; D010280:Parathyroid Glands / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000378:metabolism*; D010446:Peptide Fragments / Q000737:chemistry / Q000302:isolation & purification; D014511:Uremia / Q000150:complications
chemical_list: D010281:Parathyroid Hormone; D010446:Peptide Fragments; D002118:Calcium