Congenital chloride diarrhea (CLD) is a recessively inherited disorder characterized by massive loss of chloride in stool. We previously identified mutations in the downregulated in adenoma ( DRA) gene in patients with CLD and demonstrated that DRA encodes an intestine-specific sulfate transporter. To determine whether DRA is an intestinal chloride transporter and how mutations affect transport, Xenopus oocytes were injected with wild-type and mutagenized DRA cRNA and uptake of Cl− and[Formula: see text] was assayed. Both Cl− and[Formula: see text] were transported by wild-type DRA and an outwardly directed pH gradient stimulated Cl− uptake, consistent with Cl−/OH−exchange. Among three mutants, C307W transported both anions as effectively as wild-type, whereas transport activity was lost in V317del and the double mutant identified in 32 of 32 Finnish CLD patients. We conclude that DRA is a chloride transporter defective in CLD and that V317del is a functional mutation and C307W a silent polymorphism.
The majority of veteran patients are not suitable candidates for HCV treatment because of substance abuse, psychiatric disease, and comorbid medical disease, and many who are candidates decline therapy. Multidisciplinary collaboration is needed to overcome barriers to HCV therapy in this population.
A gene has been described, Down Regulated in Adenoma (dra), which is expressed in normal colon but is absent in the majority of colon adenomas and adenocarcinomas. However, the function of this protein is unknown. Because of sequence similarity to a recently cloned membrane sulfate transporter in rat liver, the transport function of Dra was examined. We established that dra encodes for a Na(+)-independent transporter for both sulfate and oxalate using microinjected Xenopus oocytes as an assay system. Sulfate transport was sensitive to the anion exchange inhibitor DIDS (4,4'-diisothiocyano-2,2' disulfonic acid stilbene). Using an RNase protection assay, we found that dra mRNA expression is limited to the small intestine and colon in mouse, therefore identifying Dra as an intestine-specific sulfate transporter. dra also had a unique pattern of expression during intestinal development. Northern blot analysis revealed a low level of expression in colon at birth with a marked increase in the first 2 postnatal weeks. In contrast, there was a lower, constant level of expression in small intestine in the postnatal period. Caco-2 cells, a colon carcinoma cell line that differentiates over time in culture, demonstrated a marked induction of dra mRNA as cells progressed from the preconfluent (undifferentiated) to the postconfluent (differentiated) state. These results show that Dra is an intestine-specific Na(+)-independent sulfate transporter that has differential expression during colonic development. This functional characterization provides the foundation for investigation of the role of Dra in intestinal sulfate transport and in the malignant phenotype.
Intrahepatic cholestasis in the setting of extrahepatic bacterial infection has been attributed to the effects of endotoxin and cytokines such as tumor necrosis factor-alpha (TNF-alpha) on bile acid transport. To define the mechanism of sepsis-associated cholestasis, taurocholate transport was examined in basolateral (bLPM) and canalicular (cLPM) rat liver plasma membrane vesicles derived from control and endotoxin [lipopolysaccharide (LPS)]-treated animals and in plasma membrane vesicles prepared after TNF-alpha treatment. Na(+)-dependent [3H]taurocholate uptake and both membrane-potential-dependent and ATP-dependent [3H]taurocholate transport were reduced in bLPM and cLPM vesicles, respectively, after LPS treatment. In membrane vesicles from TNF-alpha-treated animals, Na(+)-dependent [3H]taurocholate uptake was also reduced. Northern blot hybridization, using cDNA probes for the putative sinusoidal bile acid transporter (Ntcp) and canalicular ecto-adenosinetriphosphatase, demonstrated decreased mRNA levels after LPS and TNF-alpha treatment. Immunoblot analysis of membrane extracts from LPS-treated animals revealed decreased levels of these putative bile acid transporters. Impaired bile acid transport at the sinusoidal and canalicular membrane domains by these and other mediators of the inflammatory response may account for sepsis-associated cholestasis.
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