Thyroxine signal transduction in liver cells involves phospholipase C and phospholipase D activation. Genomic independent action of thyroid hormone

Background: Numerous investigations demonstrate a novel role of thyroid hormone as a modulator of signal transduction. Protein kinase C ( PKC) is critical to the mechanism by which thyroid hormones potentiate both the antiviral and immunomodulatory actions of IFN in different cells and regulate the exchange of signalling phospholipids in hepatocytes. Because nothing is known about accumulation of PKC modulator-diacylglycerol in cells treated with T-4, we examined the nongenomic effect of thyroid hormones on DAG formation and phospholipase activation in liver cells. Results: The results obtained provide the first demonstration of phospholipase C, phospholipase D and protein kinase C nongenomic activation and diacylglycerol ( DAG) accumulation by L-T-4 in liver cells. The experiments were performed in either the [C-14]CH3COOH-labeled rat liver slices or isolated hepatocytes pre-labeled by [C-14]oleic acid. L-T-4 activates the DAG production in a concentration- and time-dependent manner. DAG formation in stimulated cells is biphasic and short-lived event: there is an initial, rapid rise in DAG concentration and then a slower accumulation that can be sustained for a few minutes. The early phase of L-T-4 generated DAG only is accompanied by phosphatidylinositol 4,5-bisphosphate level decrease and inositol 1,4,5-trisphosphate formation while the second phase is abolished by PKC inhibitor 1,( 5-isoquinolinesulphonyl) 2methylpiperasine dihydrochloride (H7) and propranolol. The second phase of DAG production is accompanied by free choline release, phosphatidylcholine content drop and phosphatidylethanol (Peth) formation. Inhibitor of phospholipase-C-dependent phosphoinositide hydrolysis, neomycin sulfate, reduced the Peth as well as the DAG response to L-T-4. Conclusions: The present data have indicated the DAG signaling in thyroid hormone-stimulated liver cells. L-thyroxine activates a dual phospholipase pathway in a sequential and synchronized manner: phospholipase C initiates the DAG formation, and PKC mediates the integration of phospholipase D into the signaling response during the sustained phase of agonist stimulation.