Regulation of the expression of the hepatocellular sulfate–oxalate exchanger SAT-1 (SLC26A1) by glyoxylate: A metabolic link between liver and kidney?
Authors: Bruno Stieger
Journal: Journal of Hepatology (2010)

Glyoxylate is a substrate of the sulfate-oxalate exchanger, sat-1, and increases its expression in HepG2 cellsJournal of HepatologyVol. 54Issue 3PreviewHyperoxaluria is a major problem causing nephrolithiasis. Little is known about the regulation of oxalate transport from the liver, the main organ for oxalate synthesis, into the circulation. Since the sulfate anion transporter-1(sat-1) is present in the sinusoidal membrane of hepatocytes and translocates oxalate, its impact on increased oxalate synthesis was studied. Full-Text PDF The liver and kidney are the key organs in detoxification of endogenous and exogenous substances from the body. Detoxification of lipophilic endogenous substances and lipophilic xenobiotics involves biotransformation. This process starts with functionalization of the molecule, often by a member of the family of cytochrome P450 enzymes, followed by conjugation with a hydrophilic molecule, e.g., by glucuronidation or sulfonation. In the case of sulfonation, the sulfate donor for sulfotransferases is 3′-phosphoadenosine 5′-phosphosulfate, which is synthesized from sulfate and ATP [[9]Kauffman F.C. Sulfonation in pharmacology and toxicology.Drug Metab Rev. 2004; 36: 823-843Crossref PubMed Scopus (102) Google Scholar]. Hence, ongoing sulfonation requires supply of sulfate as inorganic anion. Furthermore, sulfation is an important post-translational modification of proteins. In the body, sulfate transport is mediated by two different types of transporters: secondary active sodium-dependent transporters and sodium-independent sulfate/anion exchangers [[17]Morris M.E. Murer H. Molecular mechanisms in renal and intestinal sulfate (re)absorption.J Membr Biol. 2001; 181: 1-9Crossref PubMed Scopus (19) Google Scholar]. The role of sulfate transporters in detoxification is highlighted by the observation that mice with a disrupted gene for the sodium-dependent sulfate transporter NaS1 (Slc13a1) or the sodium-independent sulfate anion transporter Sat-1 (Slc26a1) display hypersulfaturia, hyposulfatemia, and increased sensitivity to acetaminophen induced liver injury [5Dawson P.A. Russell C.S. Lee S. McLeay S.C. van Dongen J.M. Cowley D.M. et al.Urolithiasis and hepatotoxicity are linked to the anion transporter Sat1 in mice.J Clin Invest. 2010; 120: 706-712Crossref PubMed Scopus (82) Google Scholar, 14Lee S. Dawson P.A. Hewavitharana A.K. Shaw P.N. Markovich D. Disruption of NaS1 sulfate transport function in mice leads to enhanced acetaminophen-induced hepatotoxicity.Hepatology. 2006; 43: 1241-1247Crossref PubMed Scopus (38) Google Scholar], while the role of sulfate transporters in post-translational protein modification is evidenced by lethal and nonlethal disorders in patients with mutations in sulfate transporters [10Kere J. Overview of the SLC26 family and associated diseases.Novartis Found Symp. 2006; 273 (discussion 11–8, 261–4): 2-11Crossref PubMed Google Scholar, 12Lee A. Dawson P.A. Markovich D. NaSi-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters.Int J Biochem Cell Biol. 2005; 37: 1350-1356Crossref PubMed Scopus (35) Google Scholar, 20Rossi A. Superti-Furga A. Mutations in the diastrophic dysplasia sulfate transporter (DTDST) gene (SLC26A2): 22 novel mutations, mutation review, associated skeletal phenotypes, and diagnostic relevance.Hum Mutat. 2001; 17: 159-171Crossref PubMed Scopus (149) Google Scholar] and an impaired intestinal barrier function in mice with a disrupted gene for NaS1 [[4]Dawson P.A. Huxley S. Gardiner B. Tran T. McAuley J.L. Grimmond S. et al.Reduced mucin sulfonation and impaired intestinal barrier function in the hyposulfataemic NaS1 null mouse.Gut. 2009; 58: 910-919Crossref PubMed Scopus (81) Google Scholar]. Sat1 is the founding member of the SLC26A family, was cloned from rat liver [[1]Bissig M. Hagenbuch B. Stieger B. Koller T. Meier P.J. Functional expression cloning of the canalicular sulfate transport system of rat hepatocytes.J Biol Chem. 1994; 269: 3017-3021Abstract Full Text PDF PubMed Google Scholar] and is localized to the basolateral plasma membrane of hepatocytes and kidney proximal tubular cells, respectively [8Karniski L.P. Lotscher M. Fucentese M. Hilfiker H. Biber J. Murer H. Immunolocalization of sat-1 sulfate/oxalate/bicarbonate anion exchanger in the rat kidney.Am J Physiol. 1998; 275: F79-F87PubMed Google Scholar, 19Quondamatteo F. Krick W. Hagos Y. Kruger M.H. Neubauer-Saile K. Herken R. et al.Localization of the sulfate/anion exchanger in the rat liver.Am J Physiol Gastrointest Liver Physiol. 2006; 290: G1075-G1081Crossref PubMed Scopus (24) Google Scholar]. It acts as an anion exchanger and transports, in addition to sulfate, also chloride, bicarbonate, and oxalate in an electroneutral manner [1Bissig M. Hagenbuch B. Stieger B. Koller T. Meier P.J. Functional expression cloning of the canalicular sulfate transport system of rat hepatocytes.J Biol Chem. 1994; 269: 3017-3021Abstract Full Text PDF PubMed Google Scholar, 11Krick W. Schnedler N. Burckhardt G. Burckhardt B.C. Ability of sat-1 to transport sulfate, bicarbonate, or oxalate under physiological conditions.Am J Physiol Renal Physiol. 2009; 297: F145-F154Crossref PubMed Scopus (27) Google Scholar]. The oxalate pool is fed by dietary oxalate and as such absorbed to a minor extent in the intestine [[6]Hodgkinson A. Zarembski P.M. Oxalic acid metabolism in man: a review.Calcif Tissue Res. 1968; 2: 115-132Crossref PubMed Scopus (48) Google Scholar]. More important is endogenous oxalate, which is a metabolic end product, mainly derived from ascorbic acid and glycine [[6]Hodgkinson A. Zarembski P.M. Oxalic acid metabolism in man: a review.Calcif Tissue Res. 1968; 2: 115-132Crossref PubMed Scopus (48) Google Scholar]. Elimination of oxalate from the body is complex, but seems to occur predominantly via the kidney [[15]Marengo S.R. Romani A.M. Oxalate in renal stone disease: the terminal metabolite that just won’t go away.Nat Clin Pract Nephrol. 2008; 4: 368-377Crossref PubMed Scopus (78) Google Scholar] and to a minor extent by the gut. The role of the gut in oxalate homeostasis gains importance with progression of renal failure [[3]Costello J.F. Smith M. Stolarski C. Sadovnic M.J. Extrarenal clearance of oxalate increases with progression of renal failure in the rat.J Am Soc Nephrol. 1992; 3: 1098-1104PubMed Google Scholar] and is highlighted by the observation that patients with a short bowel have a considerable risk of developing calcium oxalate renal stones [[18]Nightingale J.M. Hepatobiliary, renal and bone complications of intestinal failure.Best Pract Res Clin Gastroenterol. 2003; 17: 907-929Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]. Urolithiasis is also observed in mice with a disrupted Sat1 gene [[5]Dawson P.A. Russell C.S. Lee S. McLeay S.C. van Dongen J.M. Cowley D.M. et al.Urolithiasis and hepatotoxicity are linked to the anion transporter Sat1 in mice.J Clin Invest. 2010; 120: 706-712Crossref PubMed Scopus (82) Google Scholar]. Therefore, Sat1 may not only be an important transporter involved in sulfate homeostasis but also plays a critical role in the proper excretion of oxalate from the body. Sat-1, therefore, seems to play a broad role in detoxification. Sulfate homeostasis is predominantly, if not exclusively, regulated via renal elimination [[16]Markovich D. Physiological roles and regulation of mammalian sulfate transporters.Physiol Rev. 2001; 81: 1499-1533Crossref PubMed Scopus (214) Google Scholar], which also applies to oxalate [[15]Marengo S.R. Romani A.M. Oxalate in renal stone disease: the terminal metabolite that just won’t go away.Nat Clin Pract Nephrol. 2008; 4: 368-377Crossref PubMed Scopus (78) Google Scholar]. It is evident from these physiologic and pathophysiologic aspects that understanding the mechanisms of regulation of the transporters involved in sulfate and oxalate homeostasis is clinically highly relevant. Studies of the genes for sulfate transporters revealed the presence of response elements for the thyroid hormone T3 in the promoters of human and mouse Sat1 and NaS1, but only the mouse promoter can be activated by T3 [[12]Lee A. Dawson P.A. Markovich D. NaSi-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters.Int J Biochem Cell Biol. 2005; 37: 1350-1356Crossref PubMed Scopus (35) Google Scholar]. In addition, vitamin D and glucocorticoid response elements are present in the promoter of mouse NaS1 [[12]Lee A. Dawson P.A. Markovich D. NaSi-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters.Int J Biochem Cell Biol. 2005; 37: 1350-1356Crossref PubMed Scopus (35) Google Scholar]. Very interestingly, the human NaS1 promoter can be transactivated by 3-methylcholantrene, which further supports the concept of an important role of sulfate transporters in detoxification [[12]Lee A. Dawson P.A. Markovich D. NaSi-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters.Int J Biochem Cell Biol. 2005; 37: 1350-1356Crossref PubMed Scopus (35) Google Scholar]. In addition, mouse NaS1 is regulated by the farnesoid X receptor α (FXRα). Interestingly, the IR-1 element responsible for this regulation is located in intron 2 of mouse NaS1 [[13]Lee H. Hubbert M.L. Osborne T.F. Woodford K. Zerangue N. Edwards P.A. Regulation of the sodium/sulfate co-transporter by farnesoid X receptor alpha.J Biol Chem. 2007; 282: 21653-21661Crossref PubMed Scopus (6) Google Scholar]. The physiologic relevance of this finding is illustrated by the observation that mice with a disrupted gene of FXRα display hypersulfaturia concomitant with reduced mRNA levels for Sat1 in the intestine and kidney. Interestingly, serum sulfate levels were not statistically different between wild type and knock out animals [[13]Lee H. Hubbert M.L. Osborne T.F. Woodford K. Zerangue N. Edwards P.A. Regulation of the sodium/sulfate co-transporter by farnesoid X receptor alpha.J Biol Chem. 2007; 282: 21653-21661Crossref PubMed Scopus (6) Google Scholar], while mice lacking the vitamin D receptor have both, hypersulfaturia and hyposulfatemia [[2]Bolt M.J. Liu W. Qiao G. Kong J. Zheng W. Krausz T. et al.Critical role of vitamin D in sulfate homeostasis: regulation of the sodium-sulfate cotransporter by 1,25-dihydroxyvitamin D3.Am J Physiol Endocrinol Metab. 2004; 287: E744-E749Crossref PubMed Scopus (16) Google Scholar]. In this issue of the Journal of Hepatology, a study identifies glyoxylate as a substrate of Sat-1 and demonstrates a role of glyoxylate in the regulation of Sat-1 expression [[21]Schnedler N. Burckhardt G. Burckhardt B.C. Glyoxylate is a substrate of the sulfate–oxalate exchanger, Sat-1, and increases its expression HepG2 cells.J Hepatol. 2011; 54: 513-520Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar]. To demonstrate Sat-1 mediated glyoxylate transport the authors expressed rat Sat-1 in Xenopus laevis oocytes and performed uptake as well as efflux experiments of radioactively labelled sulfate and oxalate, which were combined with inhibition and trans-stimulation experiments. Regulation of SAT-1 was studied in the human hepatoma cell line HepG2 cultured in the presence of various SAT-1 substrates at the mRNA and protein level. While sulfate did not affect the expression of the three SAT-1 splice variants, glyoxylate up-regulated all three splice variants markedly, while oxalate only up-regulated splice variants 1 and 3. Glyoxylate lead to a marked increase of SAT-1 protein expression in HepG2 cells in conjunction with an up-regulation of sulfate uptake into HepG2 cells. Taken together, the data clearly show that in the model human hepatocyte cell line HepG2, the expression of SAT-1 is controlled by intracellular glyoxylate levels. Endogenous oxalate is primarily produced in the liver and, in addition, in erythrocytes [[15]Marengo S.R. Romani A.M. Oxalate in renal stone disease: the terminal metabolite that just won’t go away.Nat Clin Pract Nephrol. 2008; 4: 368-377Crossref PubMed Scopus (78) Google Scholar]. The metabolic precursor for oxalate is glyoxylate, which by itself is a metabolic intermediate from various substances, such as hydroxyproline or glycolate. In the liver, glyoxylate concentration in hepatocytes is kept low by alanine–glyoxylate aminotransferase, which is located in peroxisomes. This enzyme depends on vitamin B6 for full activity and converts glyoxylate into glycine [[15]Marengo S.R. Romani A.M. Oxalate in renal stone disease: the terminal metabolite that just won’t go away.Nat Clin Pract Nephrol. 2008; 4: 368-377Crossref PubMed Scopus (78) Google Scholar]. In patients with mutations in the gene coding for alanine–glyoxylate aminotransferase, glyoxylate concentration in hepatocytes raises and consequently, glyoxylate is converted into oxalate, excreted from hepatocytes into blood and eliminated via the kidney where it leads to hyperoxaluria [15Marengo S.R. Romani A.M. Oxalate in renal stone disease: the terminal metabolite that just won’t go away.Nat Clin Pract Nephrol. 2008; 4: 368-377Crossref PubMed Scopus (78) Google Scholar, 22Wanders R.J. Waterham H.R. Peroxisomal disorders: the single peroxisomal enzyme deficiencies.Biochim Biophys Acta. 2006; 1763: 1707-1720Crossref PubMed Scopus (203) Google Scholar]. Such patients with primary hyperoxaluria type 1 develop kidney disease, i.e., nephrolithiasis. Oxalate has a very limited solubility. It may not only be toxic to kidney cells [[7]Jonassen J.A. Kohjimoto Y. Scheid C.R. Schmidt M. Oxalate toxicity in renal cells.Urol Res. 2005; 33: 329-339Crossref PubMed Scopus (58) Google Scholar] but potentially also to hepatocytes. Consequently, an urgent need may exist for hepatocytes to efficiently dispose of the excess of oxalate formed as a consequence of primary hyperoxaluria type I. The authors of the present study have observed that glyoxylate, the direct metabolic precursor of oxalate, up-regulates the expression of the oxalate efflux transporter SAT-1 in the human liver cell line HepG2 [[21]Schnedler N. Burckhardt G. Burckhardt B.C. Glyoxylate is a substrate of the sulfate–oxalate exchanger, Sat-1, and increases its expression HepG2 cells.J Hepatol. 2011; 54: 513-520Abstract Full Text Full Text PDF PubMed Scopus (16) Google Scholar]. It is, therefore, conceivable that the same regulation occurs in vivo in hepatocytes and that SAT-1 protein and function are up-regulated in patients suffering from primary hyperoxaluria type I. This hypothesis is supported by the observation that patients suffering from primary hyperoxaluria type I do not present with overt liver disease. The work of Schnedler et al. has added an additional piece of evidence that SAT-1, which was originally identified as a sulfate transporter subject to inhibition by oxalate and 4,4′-diisothiocyano-2,2′-disulfonic acid stilbene, plays an important role in detoxification. Based on its functional properties, Sat-1 was proposed to reside in the canalicular plasma membrane of rat hepatocytes [[1]Bissig M. Hagenbuch B. Stieger B. Koller T. Meier P.J. Functional expression cloning of the canalicular sulfate transport system of rat hepatocytes.J Biol Chem. 1994; 269: 3017-3021Abstract Full Text PDF PubMed Google Scholar], but was subsequently immunologically localized to the basolateral plasma membrane of hepatocytes [[19]Quondamatteo F. Krick W. Hagos Y. Kruger M.H. Neubauer-Saile K. Herken R. et al.Localization of the sulfate/anion exchanger in the rat liver.Am J Physiol Gastrointest Liver Physiol. 2006; 290: G1075-G1081Crossref PubMed Scopus (24) Google Scholar]. There, it mediates the uptake of sulfate, a key constituent of drug metabolism in exchange for oxalate, a metabolic end product destined for renal elimination. Schnedler and co-workers now identified a novel mechanism for the regulation of SAT-1 expression by glyoxylate, a metabolic precursor of oxalate. Consequently, SAT-1 has now moved into the spotlight as an additional element contributing to hyperoxaluria in patients with an inherited defect of glyoxylate metabolism. It will be interesting to see, whether SAT-1 is up-regulated in liver biopsies obtained from patients needing a combined liver and kidney transplant due to hyperoxaluria type I. Furthermore, the elucidation of the molecular mechanisms of the regulation of Sat-1 expression will certainly help to better understand the interplay of SAT-1 with phase II enzymes during drug metabolism. It might also shed light on the potential involvement of SAT-1 in the hepatic metabolism of toxins, such as for example after ethylene glycol intoxication, which is associated with renal failure. Finally, while a considerable amount of knowledge about hepatocellular sulfate uptake has been worked out both at the functional and molecular level, the molecular identity of the canalicular sulfate–bicarbonate exchanger releasing sulfate into primary bile remains elusive and awaits identification. The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.