Thyroid hormones regulate phosphate homoeostasis through transcriptional control of the renal type IIa sodium-dependent phosphate co-transporter (Npt2a) gene.
The type IIa renal sodium-dependent phosphate (Na/Pi) co-transporter Npt2a is implicated in the control of serum phosphate levels. It has been demonstrated previously that renal Npt2a protein and its mRNA expression are both up-regulated by the thyroid hormone T3 (3,3',5-tri-iodothyronine) in rats. However, it has never been established whether the induction was mediated by a direct effect of thyroid hormones on the Npt2a promoter. To address the role of Npt2a in T3-dependent regulation of phosphate homoeostasis and to identify the molecular mechanisms by which thyroid hormones modulate Npt2a gene expression, mice were rendered pharmacologically hypo- and hyper-thyroid. Hypothyroid mice showed low levels of serum phosphate and a marked decrease in renal Npt2a protein abundance. Importantly, we also showed that Npt2a-deficient mice had impaired serum phosphate responsiveness to T3 compared with wild-type mice. Promoter analysis with a luciferase assay revealed that the transcriptional activity of a reporter gene containing the Npt2a promoter and intron 1 was dependent upon TRs (thyroid hormone receptors) and specifically increased by T3 in renal cells. Deletion analysis and EMSAs (electrophoretic mobility-shift assays) determined that there were unique TREs (thyroid-hormone-responsive elements) within intron 1 of the Npt2a gene. These results suggest that Npt2a plays a critical role as a T3-target gene, to control phosphate homoeostasis, and that T3 transcriptionally activates the Npt2a gene via TRs in a renal cell-specific manner.
Ishiguro
Mariko
M
Department of Clinical Nutrition, Institute of Health Biosciences, University of Tokushima Graduate School, Kuramoto-Cho 3-18-15, Tokushima City 770-8503, Japan.
Yamamoto
Hironori
H
Masuda
Masashi
M
Kozai
Mina
M
Takei
Yuichiro
Y
Tanaka
Sarasa
S
Sato
Tadatoshi
T
Segawa
Hiroko
H
Taketani
Yutaka
Y
Arai
Hidekazu
H
Miyamoto
Ken-Ichi
K
Takeda
Eiji
E
D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D004285:Dogs; D024202:Electrophoretic Mobility Shift Assay; D005260:Female; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006706:Homeostasis; D006801:Humans; D007668:Kidney / Q000166:cytology / Q000378:metabolism*; D008156:Luciferases / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D010710:Phosphates / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D051381:Rats; D011988:Receptors, Thyroid Hormone / Q000378:metabolism; D020218:Response Elements; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000502:physiology*; D014158:Transcription, Genetic / Q000187:drug effects*; D015533:Transcriptional Activation; D014284:Triiodothyronine / Q000494:pharmacology*
D010710:Phosphates; D011988:Receptors, Thyroid Hormone; C493555:Slc34a1 protein, mouse; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa; D014284:Triiodothyronine; D008156:Luciferases