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.
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|