Characterization of Na+/HCO-3 cotransporter isoform NBC-3.
Na+-HCO-3 cotransporters mediate the transport of HCO-3 into or out of the cell. Two Na+-HCO-3 cotransporters (NBC) have been identified previously, which are referred to as NBC-1 and NBC-2. A cDNA library from uninduced human NT-2 cells was screened with an NBC-2 cDNA probe. Several clones were identified and isolated. Sequence analysis of these clones identified a partial coding region (2 kb) of a novel NBC (called here NBC-3), which showed 53% and 72% identity with NBC-1 and NBC-2, respectively. Northern blot analysis revealed that NBC-3 encodes a 4.4-kb mRNA with a tissue distribution pattern distinct from NBC-1 and NBC-2. NBC-3 is highly expressed in brain and spinal column, with moderate levels in trachea, thyroid, and kidney. In contrast with NBC-1, NBC-3 shows low levels of expression in pancreas and kidney cortex. In the kidney, NBC-3 expression is predominantly limited to the medulla. Cultured mouse inner medullary collecting duct (mIMCD-3) cells showed high levels of NBC-1 and low levels of NBC-3 mRNA expression. Subjecting the mutagenized mIMCD-3 cells to sublethal acid stress decreased the mRNA expression of NBC-1 by approximately 90% but increased the Na+-dependent HCO-3 cotransport activity by approximately 7-fold (as assayed by DIDS-sensitive, Na+-dependent, HCO-3-mediated intracellular pH recovery). This increase was associated with approximately 5.5-fold enhancement of NBC-3 mRNA levels. NBC showed significant affinity for Li+ in the mutant but not the parent mIMCD-3 cells. On the basis of the widespread distribution of NBC-3, we propose that this isoform is likely involved in cell pH regulation by transporting HCO-3 from blood to the cell. We further propose that enhanced expression of NBC-3 in severe acid stress could play an important role in cell survival by mediating the influx of HCO-3 into the cells.
Amlal
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Division of Nephrology, Department of Internal Medicine, University of Cincinnati, Cincinnati, Ohio 45267-0585, USA.
Burnham
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Soleimani
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D000143:Acids / Q000494:pharmacology; D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D001483:Base Sequence / Q000235:genetics; D001921:Brain / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D018076:DNA, Complementary / Q000235:genetics; D006801:Humans; D007668:Kidney / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D012333:RNA, Messenger / Q000378:metabolism; D027982:Sodium-Bicarbonate Symporters; D013116:Spinal Cord / Q000378:metabolism; D014018:Tissue Distribution / Q000502:physiology
D000143:Acids; D002352:Carrier Proteins; D018076:DNA, Complementary; D012333:RNA, Messenger; C498903:SLC4A8 protein, human; D027982:Sodium-Bicarbonate Symporters