Direct in vitro evidence of extracellular Ca2+-induced amino-terminal truncation of human parathyroid hormone (1-84) by human parathyroid cells.
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.
Kawata
Takehisa
T
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.
Imanishi
Yasuo
Y
Kobayashi
Keisuke
K
Onoda
Naoyoshi
N
Takemoto
Yoshiaki
Y
Tahara
Hideki
H
Okuno
Senji
S
Ishimura
Eiji
E
Miki
Takami
T
Ishikawa
Tetsuro
T
Inaba
Masaaki
M
Nishizawa
Yoshiki
Y
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
D010281:Parathyroid Hormone; D010446:Peptide Fragments; D002118:Calcium