Glucocorticoid regulation and glycosylation of mouse intestinal type IIb Na-P_icotransporter during ontogeny

Abstract

We sought to characterize expression of an apically expressed intestinal Na-P(i) cotransporter (Na-P(i)-IIb) during mouse ontogeny and to assess the effects of methylprednisolone (MP) treatment. In control mice, Na-P(i) uptake by intestinal brush-border membrane vesicles was highest at 14 days of age, lower at 21 days, and further reduced at 8 wk and 8-9 mo of age. Na-P(i)-IIb mRNA and immunoreactive protein levels in 14-day-old animals were markedly higher than in older groups. MP treatment significantly decreased Na-P(i) uptake and Na-P(i)-IIb mRNA and protein expression in 14-day-old mice. Additionally, the size of the protein was smaller in 14-day-old mice. Deglycosylation of protein from 14-day-old and 8-wk-old animals with peptide N-glycosidase reduced the molecular weight to the predicted size. We conclude that intestinal Na-P(i) uptake and Na-P(i)-IIb expression are highest at 14 days and decrease with age. Furthermore, MP treatment reduced intestinal Na-P(i) uptake approximately threefold in 14-day-old mice and this reduction correlates with reduced Na-P(i)-IIb mRNA and protein expression. We also demonstrate that Na-P(i)-IIb is an N-linked glycoprotein and that glycosylation is age dependent.

Regulation of intestinal NaPi-IIb cotransporter gene expression by estrogen

“…Tissue sections (6-10 m) from both tissues were prepared by the Histopathology Core Service at the University of Arizona (Tucson, AZ). Immunohistochemical staining and detection was performed as previously described (2). NaPi-IIb antiserum was reacted with sections for 60 min at a 1:500 (for intestinal sections) or a 1:4,000 (for lung sections) dilution in PBS.…”

“…Intestinal Pi absorption through the NaPi-IIb cotransporter is regulated by various physiological effectors. Epidermal growth factor (EGF) and glucocorticoids inhibit intestinal sodium-dependent Pi (Na/Pi) absorption and NaPi-IIb gene expression (2,31), whereas 1,25(OH) 2 vitamin D 3 and dietary Pi deprivation stimulate intestinal Na/Pi absorption and NaPi-IIb gene expression (17,30).…”

“…Estrogen is involved not only in the regulation of Ca 2ϩ absorption (3,13) but also in the maintenance of bone density (3,8) and in the modulation of 1,25(OH) 2 vitamin D 3 synthesis (4,8,20,23,27). Although 1,25(OH) 2 vitamin D 3 is known to regulate intestinal Pi absorption, there is no evidence to date demonstrating estrogen regulation via NaPi-IIb cotransporter expression. Thus the current studies were designed to investigate the possible link between estrogen and intestinal NaPi-IIb gene expression.…”

NF1 transcriptional factor(s) is required for basal promoter activation of the human intestinal NaPi-IIb cotransporter gene

“…Type II Na ϩ -P i cotransporters (NaPi-II) are largely responsible for renal and intestinal transport of P i in the human body, with the IIa and IIb isoforms being most important in the kidney and intestine, respectively. The NaPiIIa cotransporter is localized on the brush-border membrane of the renal proximal tubules and is regulated by dietary P i levels (6,28,30), parathyroid hormone (15,21), and 1,25(OH) 2 -vitamin D 3 (27). The NaPi-IIb cotransporter is localized on the apical membrane of intestinal enterocytes and is also regulated by dietary P i levels and vitamin D 3 (11,12,32).…”

“…In addition, recent studies have shown that other physiological factors influence intestinal P i uptake by altering NaPi-IIb cotransporter gene expression. For example, EGF and glucocorticoids decrease NaPi-IIb gene expression, which corresponds to a decrease in intestinal Na-dependent P i absorption (3,34). Conversely, vitamin D 3 (11,32) and estrogen (36) stimulate NaPiIIb gene expression, which corresponds to an increase in intestinal Na-dependent P i absorption.…”