Parathyroid hormone (PTH) decreases sodium-phosphate cotransporter type IIa (NpT2a) mRNA stability.
The acute inhibitory effects of parathyroid hormone (PTH) on proximal tubule Na(+)-K(+)-ATPase (Na-K) and sodium-dependent phosphate (NaPi) transport have been extensively studied, while little is known about the chronic effects of PTH. Patients with primary hyperparathyroidism, a condition characterized by chronic elevations in PTH, exhibit persistent hypophosphatemia but not significant evidence of salt wasting. We postulate that chronic PTH stimulation results in differential desensitization of PTH responses. To address this hypothesis, we compared the effects of chronic PTH stimulation on Na-P(i) cotransporter (Npt2a) expression and Na-K activity and expression in Sprague Dawley rats, transgenic mice featuring parathyroid-specific cyclin D1 overexpression (PTH-D1), and proximal tubule cell culture models. We demonstrated a progressive decrease in brush-border membrane (BBM) expression of Npt2a from rats treated with PTH for 6 h or 4 days, while Na-K expression and activity in the basolateral membranes (BLM) exhibited an initial decrease followed by recovery to control levels by 4 days. Npt2a protein expression in PTH-D1 mice was decreased relative to control animals, whereas levels of Na-K, NHERF-1, and PTH receptor remained unchanged. In PTH-D1 mice, NpT2a mRNA expression was reduced by 50% relative to control mice. In opossum kidney proximal tubule cells, PTH decreased Npt2a mRNA levels. Both actinomycin D and cycloheximide treatment prevented the PTH-mediated decrease in Npt2a mRNA, suggesting that the PTH response requires transcription and translation. These findings suggest that responses to chronic PTH exposure are selectively regulated at a posttranscriptional level. The persistence of the phosphaturic response to PTH occurs through posttranscriptional mechanisms.
Robley Rex Veterans Affairs Medical Center, Louisville, KY 40206, USA.
Rebecca D
RD
Murray
Kristine
K
Holthouser
Barbara J
BJ
Clark
Sarah A
SA
Salyer
Michelle T
MT
Barati
Syed J
SJ
Khundmiri
Eleanor D
ED
Lederer
D000818:Animals; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000235:genetics / Q000378:metabolism; D004195:Disease Models, Animal; D017674:Hypophosphatemia / Q000235:genetics* / Q000378:metabolism; D007672:Kidney Cortex / Q000166:cytology / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000502:physiology*; D051379:Mice; D008822:Mice, Transgenic; D009893:Opossums; D010281:Parathyroid Hormone / Q000378:metabolism* / Q000494:pharmacology; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional / Q000187:drug effects / Q000502:physiology; D020871:RNA Stability / Q000187:drug effects / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D044168:Receptor, Parathyroid Hormone, Type 1 / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000235:genetics* / Q000378:metabolism
C530105:Ccnd1 protein, mouse; D010281:Parathyroid Hormone; D010750:Phosphoproteins; D012333:RNA, Messenger; D044168:Receptor, Parathyroid Hormone, Type 1; C493555:Slc34a1 protein, mouse; C493556:Slc34a1 protein, rat; D017923:Sodium-Hydrogen Exchangers; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa; C093376:sodium-hydrogen exchanger regulatory factor; D019938:Cyclin D1