lithium-chloride and Pituitary-Neoplasms

We report that there are distinct thyrotropin-releasing hormone (TRH)-responsive and -unresponsive pools of inositol (Ins) lipids in rat pituitary tumour (GH3) cells, and present evidence that the size of the responsive pool is determined by the number of activated TRH-receptor complexes. By use of an experimental protocol in which cycling of [3H]Ins is inhibited and resynthesis occurs with unlabelled Ins only, we were able to measure specifically the effects of TRH on the hydrolysis of the Ins lipids present before stimulation. A maximally effective dose of TRH (1 microM) caused a time-dependent decrease in 3H-labelled Ins lipids that attained a steady-state value of 42 +/- 1% of the initial level between 1.5 and 2 h. After 2 h, even though there was no further decrease in 3H-labelled Ins lipids, and no increase in [3H]Ins or [3H]Ins phosphates, turnover of Ins lipids, as assessed as incorporation of [32P]Pi into PtdIns, continued at a rate similar to that in cells incubated without LiCl or unlabelled Ins. These data indicate that Ins lipid turnover was not desensitized during prolonged TRH stimulation. Depletion of lipid 3H radioactivity by TRH occurred at higher TRH doses on addition of the competitive antagonist chlordiazepoxide. Addition of 1 microM-TRH after 3 h of stimulation by a sub-maximal (0.3 nM) TRH dose caused a further decrease in 3H radioactivity to the minimum level (40% of initial value). We propose that the TRH-responsive pool of Ins lipids in GH3 cells is composed of the complement of Ins lipids that are within functional proximity of activated TRH-receptor complexes.

Topics: Animals; Chlorides; Inositol; Inositol Phosphates; Kinetics; Lithium; Lithium Chloride; Phosphatidylinositols; Pituitary Neoplasms; Rats; Receptors, Neurotransmitter; Receptors, Thyrotropin-Releasing Hormone; Thyrotropin-Releasing Hormone; Tumor Cells, Cultured

Previous studies have shown that phorbol esters and lithium each stimulate the secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary tumor cell line AtT20/D16-16. Pretreatment with either lithium or phorbol ester desensitizes the cells to subsequent stimulation by phorbol ester. An early consequence of phorbol ester action in other systems is the translocation of protein kinase C from cytosol to membranes. We have assayed protein kinase C activity in cytosol and membranes of AtT20 cells after treatment with phorbol dibutyrate, lithium, or other agents that stimulate secretion of ACTH in these cells. Phorbol dibutyrate clearly induced translocation of protein kinase C, but lithium treatment did not cause translocation itself, nor did pretreatment with lithium affect the translocation induced by phorbol dibutyrate. These results are consistent with a role for translocation of protein kinase C in the stimulatory and desensitizing effects of phorbol esters but fail to implicate translocation in the actions of lithium on AtT20 cells.

Topics: Adrenocorticotropic Hormone; Animals; Cell Line; Cell Membrane; Chlorides; Cytoplasm; Drug Interactions; Lithium; Lithium Chloride; Mice; Phorbol 12,13-Dibutyrate; Phorbol Esters; Pituitary Gland, Anterior; Pituitary Neoplasms; Protein Kinase C

The conversion of T4 to T3 in the brain and anterior pituitary gland contributes significantly to the T3 content of these tissues and appears to be an important modulator of thyroid hormone action. In the present study, the antimanic agent lithium was demonstrated in cultured neural and pituitary tissue to have a significant inhibitory effect on the activity of low Km (type II) iodothyronine 5'-deiodinase (I5'D), the enzyme mediating T3 formation. At medium lithium concentrations of 3.3-5 mM, 15'D activity was decreased 44 +/- 3% (P less than 0.001) in the NB41A3 mouse neuroblastoma cell line and 48 +/- 2% (P less than 0.001) in the GH3 rat pituitary tumor cell line. This inhibitory effect was only observed in intact cells. Significant inhibition of this enzymatic process was also noted in the anterior pituitary gland of thyroidectomized rats injected 3-24 h earlier with either 4 or 10 mmol/kg BW LiCl. This decrease in low Km I5'D activity was accompanied by significant decreases in the serum T3 concentration and the pituitary nuclear T3 content. Renal high Km (type I) I5'D activity was unaffected by lithium administration. These studies demonstrate that lithium, an agent of proven therapeutic benefit in patients with manic-depressive illness, can affect changes in T4 metabolism and cellular T3 content in neural and anterior pituitary tissue. Given the prominent mood changes that occur in patients with disordered thyroid function, this finding suggests that the therapeutic benefits of lithium in affective illness may be derived in part from alterations in thyroid hormone economy in the brain.

Topics: Animals; Cell Line; Cerebral Cortex; Chlorides; Iodide Peroxidase; Kidney; Kinetics; Lithium; Lithium Chloride; Male; Mice; Neuroblastoma; Pituitary Gland, Anterior; Pituitary Neoplasms; Rats; Rats, Inbred Strains; Thyroidectomy; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

Chlordiazepoxide (CDE) has been shown to antagonize the effects of TRH to stimulate the hydrolysis of phosphoinositides and elevate cytoplasmic free calcium in rat pituitary tumor (GH3) cells. Herein, we show that CDE inhibits TRH stimulation of PRL secretion and that the effect of CDE to antagonize TRH action is caused by its ability to compete with TRH for binding to receptors on GH3 cells. We also use CDE to explore whether continued receptor occupancy is required for prolonged stimulation of cellular responses. CDE had no effect on basal PRL secretion, but caused a dose-dependent inhibition of TRH-induced PRL secretion. CDE decreased the affinity of TRH binding to intact GH3 cells without affecting the maximum binding capacity. As shown previously, CDE had no effect on phosphoinositide metabolism, which was monitored because it appears to be a mechanism for signal transduction by TRH, and when added simultaneously with TRH, caused a dose-dependent inhibition of TRH-induced phosphoinositide metabolism. When CDE was added to cells 2.5 or 5 min after TRH, CDE rapidly terminated the stimulation by TRH of phosphoinositide hydrolysis, shown as inhibition of the continued formation of inositol phosphates and inositol, and of the decrease in phosphoinositides. Lastly, when cells were stimulated with 50 nM TRH, then exposed to 100 microM CDE, and finally to 1000 nM TRH, inositol phosphate formation was stimulated, then inhibited, and then restimulated. These data demonstrate that CDE acts as a competitive antagonist of TRH action on GH3 cells by competing with TRH for binding to its receptor and that continued stimulation by TRH of phospholipase C-mediated hydrolysis of phosphoinositides is tightly coupled to receptor occupancy.

Topics: Animals; Binding, Competitive; Cell Line; Chlordiazepoxide; Chlorides; Hydrolysis; Lithium; Lithium Chloride; Phosphatidylinositols; Pituitary Neoplasms; Prolactin; Rats; Receptors, Cell Surface; Receptors, Thyrotropin-Releasing Hormone; Thyrotropin-Releasing Hormone; Type C Phospholipases

Thyrotropin releasing hormone (TRH) accelerates the turnover of phosphatidylinositol in GH3 cells ('phospholipid response'). From the analysis of inositol phosphates in the presence of Li+ which inhibits their dephosphorylation, it can be concluded that the hydrolysis of phosphatidylinositol 4,5-biphosphate, and possibly of phosphatidylinositol 4-phosphate by phospholipase C is markedly accelerated by TRH. It appears that this reaction initiates the acceleration of phosphatidylinositol turnover. The specificity of hormonally regulated phospholipase C reaction for polyphosphoinositides has important implications for the potential role of the phospholipid response as a mechanism of membrane signal transduction.

Topics: Animals; Cell Line; Chlorides; Hydrolysis; Kinetics; Lithium; Lithium Chloride; Phosphatidylinositols; Phospholipases; Pituitary Neoplasms; Rats; Thyrotropin-Releasing Hormone; Type C Phospholipases