Modification of TSH-stimulated adenylate cyclase activity of bovine thyroid by manipulation of membrane phospholipid composition with a nonspecific lipid transfer protein.

The lipid composition of bovine thyroid plasma membranes was modified using the nonspecific lipid transfer protein from bovine liver. Incubation of plasma membranes with transfer protein and phosphatidylinositol-containing liposomes caused a strong, concentration dependent, inhibition of TSH-stimulated adenylate cyclase activity. Other phospholipids such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidic acid were two to four times less effective as inhibitors of TSH-stimulation. The phosphatidylinositol-induced inhibition was not reversed when more than 80% of phosphatidylinositol incorporated was removed using phosphatidylinositol-specific phospholipase C. Incorporation of phosphatidylinositol in plasma membranes provoked no significant change in the fluorescence anisotropies of the fluorophores 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(14-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), indicating that the inhibition was not due to changes in membrane fluidity. At phosphatidylinositol concentrations causing a 66% reduction in TSH-stimulated adenylate cyclase activity cholera toxin- and forskolin-stimulated activity as well as basal activity were decreased by maximally 10%. Since TSH binding to bovine thyroid plasma membranes was not affected it is suggested that phosphatidylinositol can act as a negative modulator of the TSH activation of adenylate cyclase and this probably by interfering with the coupling between the occupied TSH receptor and the stimulatory GTP-binding regulatory protein of the adenylate cyclase complex.