Activation of phospholipase C/Ca^<2+> system in thyroid cells and its physiological and pathological roles.

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We have recently found that adenosine inhibits TSH-induced activation of adenylyl cyclase/cAMP system and enhances TSH-induced activation of phospholipase C/Ca^<2+> system in rat thyroid cells. In thyroid cells, in addition to norepinephrine, acetylcholine and ATP,lysosphingolipids such as sphingosine 1-phosphate (SlP) and sphingosylsphosphorylcholine (SPC) has also been suggested to be a potent Ca^<2+> mobilizer. In this study, we planned to clarify the roles of phospholipase C/Ca^<2+> system in thyroid cells. We obtained the following novel findings. (1) In human thyroid cells as well, it was demonstrated that phospholipase C/Ca^<2+> system was regulated through the cross-talk of TSH and adenosine. (2) To clarify the cross-talk mechanism by which TSH and adenosine regulate phospholipase C activity, we used in vivo Cos-7 cell reconstitution system, in which cDNAs encoding receptors, each wild type G-protein subunit and mutant subunit. We found that adenosine-induced modification of two TSH-induced responses were mediated by a single type of Gi protein, Gi2 or Gi3 ; alpha subunit mediates inhibition of AC and betagamma subunits activation of phospholipase C.(3) Grave's IgG in the presence of adenosine induced activation of phospholipase C/Ca^<2+> system, and hence it was suggested that the measurement of phospholipase C/Ca^<2+> system is applied for a novel diagnosis of Grave's disease. (4) SlP and SPC regulated phospholipase C/Ca^<2+> system, H_2O_2 generation, cell proliferation and so on in thyroid cells. Thus, these lipid mediators were suggested to be novel agonists in these cells.