Polyoxometalates Function as Indirect Activators of a G Protein-Coupled Receptor
The luteinizing hormone receptor (LHR), a G protein-coupled receptor (GPCRs), can initiate signaling in the presence of some vanadium-containing compounds as a result of vanadium compounds interactions with the membrane lipids and/or the cell membrane lipid interface. The ability of LHR expressed in CHO cells to initiate signaling in the presence of highly charged and water-soluble polyoxovanadates (POV) including Na3[H3V10O28] (V10) and two mixed-valence heteropolyoxovanadates, K(NH4)4[H6V14O38(PO4)]·11H2O (V14) and [(CH3)4N]6[V15O36(Cl)] (V15), was investigated here. Interactions of the vanadium compounds with CHO cells decreased the packing of membrane lipids, drove aggregation of LHR and increased signal transduction by LHR. Cell responses were comparable to, or in the case of V14 and V15, greater than those seen for cells treated with human chorionic gonadotropin (hCG), a naturally-occurring LHR ligand produced in early pregnancy in humans. POV effects were observed for CHO cells where LHR was expressed at 10,000 or 32,000 LHR per cell but not when LHR was overexpressed with receptor numbers >100,000 LHR/cell. To determine which POV species was present in the cell medium during cell studies, the speciation of vanadate (V1), V10, V14 or V15 in cell medium was monitored using 51V NMR and EPR spectroscopies. We found that all the POVs initiated signaling, but V15 and V10 had the greatest effects on cell function, while V1 was significantly less active. However, because of the complex nature of vanadium compounds speciation, the effects on cell function may be due to vanadium species formed in the cell medium over time.