RESUMEN
Bile acids (BAs) are the end products of cholesterol metabolism. One of the critical steps in their biosynthesis involves the isomerization of the 3ß-hydroxyl (-OH) group on the cholestane ring to the common 3α-configuration on BAs. BAs are actively recaptured from the small intestine by the human Apical Sodium-dependent Bile Acid Transporter (hASBT) with high affinity and capacity. Previous studies have suggested that no particular hydroxyl group on BAs is critical for binding or transport by hASBT, even though 3ß-hydroxylated BAs were not examined. The aim of this study was to elucidate the role of the 3α-OH group on BAs binding and translocation by hASBT. Ten 3ß-hydroxylated BAs (Iso-bile acids, iBAs) were synthesized, characterized, and subjected to hASBT inhibition and uptake studies. hASBT inhibition and uptake kinetics of iBAs were compared to that of native 3α-OH BAs. Glycine conjugates of native and isomeric BAs were subjected to molecular dynamics simulations to identify topological descriptors related to binding and translocation by hASBT. Iso-BAs bound to hASBT with lower affinity and exhibited reduced translocation than their respective 3α-epimers. Kinetic data suggests that, in contrast to native BAs where hASBT binding is the rate-limiting step, iBAs transport was rate-limited by translocation and not binding. Remarkably, 7-dehydroxylated iBAs were not hASBT substrates, highlighting the critical role of 7-OH group on BA translocation by hASBT, especially for iBAs. Conformational analysis of gly-iBAs and native BAs identified topological features for optimal binding as: concave steroidal nucleus, 3-OH "on-" or below-steroidal plane, 7-OH below-plane, and 12-OH moiety toward-plane. Our results emphasize the relevance of the 3α-OH group on BAs for proper hASBT binding and transport and revealed the critical role of 7-OH group on BA translocation, particularly in the absence of a 3α-OH group. Results have implications for BA prodrug design.
Asunto(s)
Ácidos y Sales Biliares/química , Ácidos y Sales Biliares/metabolismo , Transportadores de Anión Orgánico Sodio-Dependiente/química , Transportadores de Anión Orgánico Sodio-Dependiente/metabolismo , Simportadores/química , Simportadores/metabolismo , Ácidos y Sales Biliares/síntesis química , Transporte Biológico , Humanos , Hidroxilación , Conformación Molecular , Transportadores de Anión Orgánico Sodio-Dependiente/antagonistas & inhibidores , Unión Proteica , Simportadores/antagonistas & inhibidoresRESUMEN
PURPOSE: To explore the involvement of transmembrane domain (TM) 7 of the human apical sodium-dependent bile acid transporter (hASBT) on bile acid (BA) binding/translocation, using two electrophilic BA derivatives as molecular probes. METHODS: Two electrophilic derivatives of chenodeoxycholic acid (CDCA) were designed, synthesized and evaluated for their ability to inactivate hASBT, and the human organic cation/carnitine transporter (hOCTN2) as a control (i.e. a non-BA transporting model). The ability of electrophilic derivatives to interact with hASBT was evaluated by 2-aminoethyl-methanethiosulfonate (MTSEA)-biotin labeling of thiol groups in TM7 cysteine mutants. RESULTS: Unlike native BAs, the electrophilic CDCA derivatives specifically inactivated hASBT, but not hOCTN2, and inhibited hASBT in a time- and concentration-dependent fashion. Preincubation of hASBT Cys-mutants in the exofacial half of TM7 with reactive electrophilic probes blocked transporter biotinylation by MTSEA-biotin, similar to 2-(trimethylammonium)ethyl-methanethiosulfonate (MTSET) blocking. This blocking pattern differed from that produced by native BAs, which exposed exofacial TM7 residues, thereby increasing staining. CONCLUSION: Kinetic and biochemical data indicate these novel electrophilic BAs are potent and specific irreversible inhibitors of hASBT and offer new evidence about the role of TM7 in binding/translocation of bile acids.
Asunto(s)
Ácidos y Sales Biliares/metabolismo , Ácido Quenodesoxicólico/análogos & derivados , Ácido Quenodesoxicólico/farmacología , Sondas Moleculares/química , Sondas Moleculares/farmacología , Transportadores de Anión Orgánico Sodio-Dependiente/antagonistas & inhibidores , Transportadores de Anión Orgánico Sodio-Dependiente/metabolismo , Simportadores/antagonistas & inhibidores , Simportadores/metabolismo , Secuencia de Aminoácidos , Animales , Ácidos y Sales Biliares/química , Transporte Biológico/efectos de los fármacos , Línea Celular , Humanos , Datos de Secuencia Molecular , Transportadores de Anión Orgánico Sodio-Dependiente/química , Transportadores de Anión Orgánico Sodio-Dependiente/genética , Proteínas de Transporte de Catión Orgánico/antagonistas & inhibidores , Proteínas de Transporte de Catión Orgánico/metabolismo , Mutación Puntual , Estructura Terciaria de Proteína , Miembro 5 de la Familia 22 de Transportadores de Solutos , Simportadores/química , Simportadores/genéticaRESUMEN
Ascorbic acid (AA) is best known for its role as an essential nutrient in humans and other species. As the brain does not synthesize AA, high levels are achieved in this organ by specific uptake mechanisms, which concentrate AA from the bloodstream to the CSF and from the CSF to the intracellular compartment. Two different isoforms of sodium-vitamin C co-transporters (SVCT1 and SVCT2) have been cloned. Both SVCT proteins mediate high affinity Na(+)-dependent L-AA transport and are necessary for the uptake of vitamin C in many tissues. In the adult brain the expression of SVCT2 was observed in the hippocampus and cortical neurons by in situ hybridization; however, there is no data regarding the expression and distribution of this transporter in the fetal brain. The expression of SVCT2 in embryonal mesencephalic neurons has been shown by RT-PCR suggesting an important role for vitamin C in dopaminergic neuronal differentiation. We analyze SVCT2 expression in human and rat developing brain by RT-PCR. Additionally, we study the normal localization of SVCT2 in rat fetal brain by immunohistochemistry and in situ hybridization demonstrating that SVCT2 is highly expressed in the ventricular and subventricular area of the rat brain. SVCT2 expression and function was also confirmed in neurons isolated from brain cortex and cerebellum. The kinetic parameters associated with the transport of AA in cultured neurons and neuroblastoma cell lines were also studied. We demonstrate two different affinity transport components for AA in these cells. Finally, we show the ability of different flavonoids to inhibit AA uptake in normal or immortalized neurons. Our data demonstrates that brain cortex and cerebellar stem cells, neurons and neuroblastoma cells express SVCT2. Dose-dependent inhibition analysis showed that quercetin inhibited AA transport in cortical neurons and Neuro2a cells.