Mechanisms underlying the long-term regulation of NHE3 by parathyroid hormone

Abstract

Bezerra CN, Girardi AC, Carraro-Lacroix LR, Rebouças NA. Mechanisms underlying the long-term regulation of NHE3 by parathyroid hormone. Am J Physiol Renal Physiol 294: F1232-F1237, 2008. First published March 5, 2008 doi:10.1152/ajprenal.00025.2007The activity of the Na ϩ /H ϩ exchanger NHE3 is regulated by a number of factors including parathyroid hormone (PTH). In the current study, we used a renal epithelial cell line, the opossum kidney (OKP) cell, to elucidate the mechanisms underlying the long-term effects of PTH on NHE3 transport activity and expression. We observed that NHE3 activity was reduced 6 h after addition of PTH, and this reduction persisted almost unaltered after 24 h. The decrease in activity was associated with diminished NHE3 cell surface expression at 6, 16, and 24 h after PTH addition, total cellular NHE3 protein at 16 and 24 h, and NHE3 mRNA abundance at 24 h. The lower levels of NHE3 mRNA were associated to a small, but significant, decrease in mRNA stability. Additionally, by analyzing the rat NHE3 gene promoter activity in OKP cells, we verified that the regulatory region spanning the segment Ϫ152 to ϩ55 was mildly reduced under the influence of PTH. This effect was completely abolished by the presence of the PKA inhibitor KT 5720. In conclusion, long-term exposure to PTH results in reduction of NHE3 mRNA levels due to a PKA-dependent inhibitory effect on the NHE3 promoter and a small reduction of mRNA half-life, and decrease in the total amount of protein which is preceded by endocytosis of the apical surface NHE3. The decreased NHE3 expression is likely to be responsible for the reduction of sodium, bicarbonate, and fluid reabsorption in the proximal tubule consistently perceived in experimental models of PTH disorders. Naϩ /H ϩ exchange; proximal tubule; hyperparathyroidism; surface expression; promoter activity THE HUMAN BODY has a remarkable ability to maintain precise control of its acid-base and volume status. Although multiple mechanisms throughout the body contribute to this process, the renal proximal tubule plays a fundamental role, since it is responsible for reabsorption of the majority of the filtered sodium, bicarbonate, chloride, and water. Two-thirds of the transcellular NaHCO 3 reabsorption are mediated by proton secretion through the apical membrane Na ϩ /H ϩ exchanger, NHE3 (4, 21), and most of NaCl reabsorption is indirectly related to it. This transporter thereby plays an important role in the maintenance of fluid, electrolyte, and acid-base balance, and its activity is regulated in response to a wide variety of acute and chronic changes in physiological parameters (29,(31)(32)(33).The kidney is one of the major target organs for parathyroid hormone (PTH). PTH, the primary regulator of serum calcium and phosphate homeostasis, acts on the proximal tubule, the thick ascending limb, and the distal convoluted tubule to alter urinary electrolyte and fluid excretion (1,2,7,14,15,17). PTH has a potent inhibitory effect on NHE3. The acute inhibition of NHE3 by PTH has be...

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO3− secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

“…Moreover, PTH inhibited the tubular Na ϩ and HCO 3 Ϫ reabsorption in the renal proximal tubule (6,7,16), which functioned as part of body Na ϩ and acid-base balance, respectively. Despite a number of studies pertaining to ion regulatory action of PTH on the kidney, virtually nothing is currently known regarding this PTH effect on intestinal epithelium, which is another important regulatory site for ion homeostasis.…”

Regulation of Na+/H+ Exchanger Isoform 3 by Protein Kinase A in the Renal Proximal Tubule

“…Long term inhibition of NHE3 by PTH is associated with a reduction on NHE3 protein and mRNA levels. PTH also provokes a mild inhibitory effect on NHE3 promoter that seems to be PKA-dependent (Bezerra et al, 2008).…”

“…PTH is primarily involved in modulation of serum calcium and phosphate homeostasis but also acts on the proximal tubule, the thick ascending limb, and the distal convoluted tubule to alter urinary electrolyte and fluid excretion. The inhibitory effect of PTH on renal proximal mRNA and protein abundance Collazo et al, 2000;Zhang et al, 1999;Girardi et al, 2000;Bezerra et al, 2008) Dopamine  NHE3-mediated Na + /H + exchange due to increased endocytosis caused by increased PKA-mediated NHE3 phosphorylation (Gomes & Soares-da-Silva, 2004;Hu et al, 2001;Bacic et al, 2003) Glucagon-like peptide 1  NHE3 activity due to increased NHE3 phosphorylation (Crajoinas et al, 2011;Carraro-Lacroix et al, 2009) Glucagon Acutely - NHE via a PKAdependent pathway Chronically - NHE3 mRNA and protein expression at the plasma membrane (Amemiya et al, 2002) Guanylins  NHE3-mediated Na + /H + due to increased levels of NHE3 PKAdependent phosphorylation and reduction of the exchanger at the plasma membrane (Amorim et al, 2006; Lessa LM, Girardi AC, and Malnic G, unpublished observations)…”

“…The chronic effect of PTH on NHE3 regulation has also been evaluated (Girardi et al, 2000;Bezerra et al, 2008). Long term inhibition of NHE3 by PTH is associated with a reduction on NHE3 protein and mRNA levels.…”

Essential regulatory elements for NHE3 gene transcription in renal proximal tubule cells

“…In parallel to acute regulatory mechanisms of NHE3 activity, the expression of this protein is also adjusted to different chronic physiological or pathological conditions. Chronic metabolic acidosis (4-6), reduced extracellular volume (7), exposure to angiotensin II (8,9), insulin (10), glucocorticoids (10)(11)(12), thyroid (13), or parathyroid hormones (14,15) are all conditions that can alter NHE3 expression. The regulated steps that result in changes of NHE3-mRNA levels in these different states can occur at any point from gene transcription to the final mRNA degradation in the cytoplasm.…”