Background: Low urinary excretion of citrate is a major risk in Thai kidney stone patients. Reabsorption of citrate at renal proximal tubules requires sodium-dicarboxylate cotransporter-1 (NaDC-1).
Objectives: We investigated the expression of NaDC-1 in stone-containing kidneys and evaluated the association of NaDC-1 expression with urine pH. Expression of NaDC-1 protein in acid-treated human proximal renal cells (HK-2 cells) was also studied.
Methods: Twenty-four patients with nephrolithiasis aged 50.61 ± 13.30 years (9 males, 15 females) were recruited. Twenty-four hours urine samples were collected from all patients. Expression of NaDC-1 in renal tissues and HK-2 cells was determined by immunohistochemistry and Western blotting, respectively.
Results: NaDC-1 was expressed mainly in proximal renal tubular cells. Tubular cells in the medullary region were weakly positive for NaDC-1. The intensity of NaDC-1 expression varied among nephrolithic renal tissues and was categorized into weak (6/24, 25%), intermediate (10/24, 42%), and high (8/24, 33%) expression. A trend of decreased urine pH in patients with increased NaDC-1 expression was observed. When the expression of NaDC-1 was recategorized into low (16/24) and high (8/24) expression, patients with high NaDC-1 expression had significantly lower urine pH than those with low NaDC-1 expression. Acid-treated HK-2 cells (pH 6.8) showed significantly higher expression of NaDC-1 compared with the control nontreated cells (pH 7.4). Significant association between urinary citrate and urine pH was not found. Also, significant association between urinary citrate and intrarenal NaDC-1 expression was not revealed.
Conclusion: NaDC-1 was principally expressed in proximal renal tubules of stone-bearing kidneys. High expression of NaDC-1 was associated with low urine pH. To our knowledge, this is the first report of NaDC-1 expression in the kidneys of nephrolithiasis patients. We experimentally confirmed that acid conditions upregulated the expression of NaDC-1 in the human proximal tubular cells.
1. Fakheri RJ, Goldfarb DS. Ambient temperature as a contributor to kidney stone formation: implications of global warming. Kidney Int. 2011; 79:1178-85.
2. Yanagawa M, Kawamura J, Onishi T, Soga N, Kameda K, Sriboonlue P, et al. Incidence of urolithiasis in northeast Thailand. Int J Urol. 1997; 4:537-40.
3. Rule AD, Krambeck AE, Lieske JC. Chronic kidney disease in kidney stone formers. Clin J Am Soc Nephrol. 2011; 6:2069-75.
4. Zacchia M, Preisig P. Low urinary citrate: an overview. J nephrol. 2010; 23 Suppl 16:S49-56.
5. Hamm LL, Hering-Smith KS. Pathophysiology of hypocitraturic nephrolithiasis. Endocrinol Metab Clin North Am. 2002; 31:885-93.
6. Pak CY. Etiology and treatment of urolithiasis. Am J Kidney Dis. 1991; 18:624-37.
7. Pak CY. Citrate and renal calculi: an update. Miner Electrolyte Metab. 1994; 20:371-7.
8. Aruga S, Wehrli S, Kaissling B, Moe OW, Preisig PA, Pajor AM, et al. Chronic metabolic acidosis increases NaDC-1 mRNA and protein abundance in rat kidney. Kidney Int. 2000; 58:206-15.
9. Hamm LL. Renal handling of citrate. Kidney Int. 1990; 38:728-35.
10. Pajor AM. Molecular cloning and functional expression of a sodium-dicarboxylate cotransporter from human kidney. Am J Physiol. 1996; 270:F642-8.
11. Pajor AM, Sun N. Functional differences between rabbit and human Na(+)-dicarboxylate cotransporters, NaDC-1 and hNaDC-1. Am J Physiol. 1996; 271: F1093-9.
12. Okamoto N, Aruga S, Matsuzaki S, Takahashi S, Matsushita K, Kitamura T. Associations between renal sodium-citrate cotransporter (hNaDC-1) gene polymorphism and urinary citrate excretion in recurrent renal calcium stone formers and normal controls. Int J Urol. 2007; 14:344-9.
13. Youngjermchan P, Pumpaisanchai S, Ratchanon S, Pansin P, Tosukhowong P, Tungsanga K, et al. Hypocitraturia and hypokaliuria: major metabolic risk factors for kidney stone disease. Chula Med J. 2006; 50:605-21.
14. Domrongkitchaiporn S, Stitchantrakul W, Kochakarn W. Causes of hypocitraturia in recurrent calcium stone formers: focusing on urinary potassium excretion. Am J Kidney Dis. 2006; 48:546-54.
15. Tungsanga K, Sriboonlue P, Futrakul P, Yachantha C, Tosukhowong P. Renal tubular cell damage and oxidative stress in renal stone patients and the effect of potassium citrate treatment. Urol Res. 2005; 33:65-9.
16. Aruga S, Pajor AM, Nakamura K, Liu L, Moe OW, Preisig PA, et al. OKP cells express the Na-dicarboxylate cotransporter NaDC-1. Am J Physiol Cell Physiol. 2004; 287:C64-72.
17. Pajor AM. Citrate transport by the kidney and intestine. Semin Nephrol. 1999; 19:195-200.
18. Boonla C, Wunsuwan R, Tungsanga K, Tosukhowong P. Urinary 8-hydroxydeoxyguanosine is elevated in patients with nephrolithiasis. Urol Res. 2007; 35: 185-91.