Allosteric modulation of the human 5-HT7A receptor by lipidic amphipathic compounds

Human 5-HT7A receptors positively modulated adenylyl cyclases via G(s) subtypes of G proteins in human embryonic kidney 293 cells, and bound 5-hydroxytryptamine (HT) with high and low affinity (K-1 values of 1.5 +/- 0.3 and 93 +/- 4 nM). More than 60% of 5-HT7A receptors, however, displayed the high-affinity 5-HT binding with no sensitivity to 5'-guanylylimidodiphosphate. In this study, we found that select amphipathic agents affected the high-affinity 5-HT binding to 5-HT7A. Oleic acid at low concentrations (<15 <mu>M), but not palmitic, stearic, and arachidonic acids, increased maximal [H-3]5-HT binding without affecting its K-D value and [H-3]mesulergine (antagonist) binding. Fatty acid-free bovine serum albumin (FF-BSA), a scavenger of fatty acids and lipid metabolites, substantially reduced maximal [H-3]5-HT binding (no change in K-D value and antagonist binding) but lost its action upon treatment with inactive stearic acid. FF-BSA and oleic acid produced no appreciable effects on [H-3]5-HT binding to analogous 5-HT receptors 5-HT1D and 5-H-2C. Among various lysophospholipids, lysophosphatidyl choline (50 muM) decreased maximal [H-3]5-HT binding, and a similar zwitterion, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS; 0.1%), increased it (no change in K-D). Functionally, 5-HT-induced guanosine-5'-O-(3-[S-35]thio)-triphosphate (GTP-gamma S-35) binding was enhanced by oleic acid and CHAPS, but reduced by FF-BSA and lysophosphatidyl choline; the amphipathic agents and FF-BSA did not affect dopamine-induced GTP gamma S-35 binding at D1, a prototypic G(s)-coupled receptor. At 5-HT7A, oleic acid, FF-BSA, CHAPS, and lysophosphatidyl choline also brought about corresponding changes in the half-maximal 5-HT concentration for cAMP production, without affecting the maximal and basal levels. We propose that endogenous, amphipathic lipid metabolites may modulate 5-HT7A receptors allosterically to promote high-affinity 5-HT binding and to enable receptors to couple more efficiently to G(s) subtypes of G proteins.