title: Thyrotropin-releasing hormone stimulates rapid breakdown of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate in GH3 pituitary tumor cells.
abstract: Thyrotropin-releasing hormone (TRH) induced a rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5P2) and phosphatidylinositol 4-phosphate (PtdIns 4P) in GH3 cells labeled to isotopic equilibrium with [3H]inositol. Within 10 sec of the addition of TRH (1 microM), there was a maximal 60% decrease in PtdIns 4,5P2 and 40% decrease in PtdIns 4P. Breakdown of phosphatidylinositol (PtdIns) occurred only after a lag of 30 sec. While the reduced levels of the polyphosphoinositides had almost returned to control values by 5 min, the GH3 cell PtdIns content remained at around 85% of controls for at least 2 hr. Both phosphatidic acid (PA) and 1,2-diacylglycerol levels increased in response to TRH in [32P]PO4- and [3H]glycerol-labeled GH3 cells. 1,2-Diacylglycerol accumulated in a biphasic manner with an early peak 10 sec after addition of the peptide. This early rise in 1,2-diacylglycerol levels coincided in time and was equivalent in lipid mass with the decrease in the polyphosphoinositide content, suggesting the involvement of a phospholipase C-type enzyme. 1,2-Diacylglycerol levels subsequently fell toward control values and, after 3 min of treatment with TRH, rose again to levels 50% above normal. PA levels reached a peak value approximately 2-fold above normal 1 min after the addition of TRH. At all times after TRH addition, the bulk of the inositol phospholipid lost was recovered as 1,2-diacylglycerol. These results suggest that TRH stimulates a cycle of events in which the breakdown of the polyphosphoinositides, PtdIns 4,5P2 and, perhaps, PtdIns 4P by a phospholipase C enzyme could be the initiating event.
mesh_terms: D000818:Animals; D002448:Cell Adhesion; D002460:Cell Line; D004075:Diglycerides / Q000378:metabolism; D008563:Membrane Lipids / Q000378:metabolism; D010712:Phosphatidic Acids / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism*; D010902:Pituitary Gland / Q000378:metabolism*; D051381:Rats; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D014280:Triglycerides / Q000378:metabolism; D010738:Type C Phospholipases / Q000378:metabolism
chemical_list: D004075:Diglycerides; D008563:Membrane Lipids; D010712:Phosphatidic Acids; D010716:Phosphatidylinositols; D014280:Triglycerides; D013973:Thyrotropin-Releasing Hormone; D010738:Type C Phospholipases