Functional Reconstitution of Human OATP1B1 and OATP1B3 in Proteoliposomes: Effect of Bicarbonate Gradients and Substrate-specific Stimulation on Transport
The study identifies bicarbonate gradients as key drivers for the transport of substrates by OATP1B1 and OATP1B3, challenging previous beliefs about glutathione's role.
Where it sits
this study against the rest of the glutathione (gsh) corpusSummary and findings
This study investigates the transport kinetics of human OATP1B1 and OATP1B3 in a reconstituted proteoliposome system. It identifies an outwardly directed bicarbonate gradient as a driving force for substrate uptake and finds no evidence supporting reduced glutathione efflux as a driving force. The study also explores the substrate-specific effect of epigallocatechin gallate on OATP1B3 transport.
Abstract
The hepatic organic anion transporting polypeptides OATP1B1 and OATP1B3 are critical for the clearance of widely prescribed drugs, hepatotoxins, and endogenous metabolites. However, the thermodynamic driving force governing their transport has remained a subject of debate due to the complexity of cellular models. Here, we report a defined, reconstituted proteoliposome system to study the transport kinetics of isolated human OATP1B1 and OATP1B3. Through direct functional assays, real-time counter-ion tracking, and a novel ABCG2-OATP1B1 functional co-reconstitution, we identify an outwardly directed bicarbonate gradient as a thermodynamic driving force for concentrative substrate uptake. In contrast we find no evidence for the proposed role of reduced glutathione efflux as a possible driving force. Targeted mutagenesis experiments revealed that the conserved TM3 residue His115 is needed to couple the bicarbonate gradient to concentrative substrate uptake, mechanistically linking substrate antiport to local pH-sensitive bicarbonate processing. We also investigated the substrate-specific stimulatory effect of the green tea catechin epigallocatechin gallate (EGCG) on OATP1B3 transport, confirming that this effect is intrinsic to OATP1B3, substrate specific, and does not need the involvement of other cellular proteins. Our findings define the energetic basis of OATP1B1 and OATP1B3 hepatic xenobiotic clearance.
Background
The study addresses the transport mechanisms of hepatic organic anion transporting polypeptides OATP1B1 and OATP1B3, which are essential for drug clearance and metabolite transport. Previous research has debated the thermodynamic forces driving their transport due to complex cellular models. This study aims to clarify these mechanisms using a defined proteoliposome system.
Methods
The study utilized a reconstituted proteoliposome system to analyze the transport kinetics of OATP1B1 and OATP1B3. Specific assays included real-time counter-ion tracking and targeted mutagenesis experiments. The focus was on the role of bicarbonate gradients and substrate-specific effects on transport.
Results
The primary finding indicates that an outwardly directed bicarbonate gradient serves as a thermodynamic driving force for substrate uptake. The study also confirmed that the His115 residue is essential for this coupling. No significant role for reduced glutathione efflux was identified.
Interpretation
These findings contribute to understanding the energetic basis of hepatic xenobiotic clearance by OATP1B1 and OATP1B3. The effect sizes related to bicarbonate gradients may be significant in a biochemical context, but the clinical implications remain unclear. Limitations include the use of a proteoliposome system, which may not fully replicate in vivo conditions.
Key findings
- Outwardly directed bicarbonate gradient identified as a thermodynamic driving force for concentrative substrate uptake.
- No evidence found for the role of reduced glutathione efflux in transport.
- His115 residue is needed to couple bicarbonate gradient to substrate uptake.
Limitations
- Not reported in abstract.