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The sodium bicarbonate cotransporter (NBC1) mediates bicarbonate reabsorption in the renal proximal tubule. NBC1 activity is stimulated by 10% CO2, however, the mechanism is poorly understood. Here, we examined the mechanism of NBC1 regulation by 10% CO2 using an immortalized human proximal tubule cell line (HK2). In cells exposed to 10% CO2, the cotransporter activity (measured as deltapH/min) increased within minutes and this increase was maintained for 6 to 24 h. Early NBC1 stimulation was accompanied by increased NBC1 phosphorylation. Basolateral membrane NBC1 protein increased by 30 min and reached a maximum at 6 h. Increased NBC activity at 6 h was accounted for by increased NBC exocytosis to the basolateral membrane and not by decreased endocytosis. Latruncullin B (an actin cytoskeleton inhibitor) did not prevent CO2-induced stimulation, while nocodazole (a microtubule-disrupting agent) abrogated the stimulatory effect of 10% CO2. A significant increase in NBC1 mRNA expression level was observed at 6 h and maintained for 24 h. Total NBC1 protein increased at 12 to 24 h with 10% CO2 incubation and this effect was blocked by cycloheximide. In summary, the present study demonstrates that early activation of NBC1 activity by 10% CO2 was mediated by NBC1 phosphorylation. The stimulation of cotransporter activity observed at 6 h was due to exocytosis, while the late effect starting from 12 h was accounted for by increased protein synthesis.

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A full-length mouse cDNA clone encoding KLF4 was initially isolated from a NIH3T3 cDNA library by reduced stringency screening with a DNA probe containing the zinc finger region of an immediate early gene product, Zif268 or Egr1. A distinct feature of the KLF4 gene is the highly GC-rich nature of the sequence near its 5'-end. Thus, the G+C content of the 1000 nt 5'-flanking region is 67% and that of the 5'-untranslated region is 63%. Moreover, the bulk of the GC residues are concentrated in the region between nt -600 and +300 of the gene where the G+C content is 82%. The gene has four exons, each containing a portion of the translated region. TranscriptionThe open-reading frame of the Klf4 gene encodes a polypeptide of 483 amino acids with a predicted molecular weight of 53 kDa. The entire polypeptide sequence of Klf4 with the exception of the first amino acid is encoded by exons 2, 3 and 4. Protein DescriptionKLF4 encodes a polypeptide of 483 aa and similar to KLFs, contains three Kruppel-type zinc fingers in the very C-terminal end. The region immediately N terminal to the three zinc fingers is a 20-aa peptide containing a cluster of basic aa residues, which is essential for the nuclear localization of the protein. ExpressionKLF4 is a nuclear protein whose cellular address depends on two nuclear localization signals. Expression of the KLF4 gene is developmentally regulated, with a higher level of expression occurring toward the later stage of fetal development. In adults, KLF4 is highly enriched in epithelial tissues, including the skin, lung, and intestine. In the intestinal tract, KLF4 is predominantly present in the terminally differentiated, postmitotic epithelial cells lining the villus border of the small intestine and the upper crypt region of the large intestine.The structure of the murine KLF4 gene. The four exons of the murine GKLF gene are identified by Roman numerals. The translated region or open reading frame is depicted in black. The locations of restriction sites for several endonucleases are labeled: Nc, NcoI; N,