Effects of eicosatetraynoic acid (ETYA) on cultured pig thyroid cells. Relationships between the inhibition of the phosphatidate-phosphatidyl inositol cycle, the iodination and the cyclic AMP responsiveness to thyrotropin

Decrease in cAMP-dependent protein kinase activity in suspension cultures of porcine thyroid cells exposed to TSH or forskolin

Suspension cultures of porcine thyroid cells were used to study the action of TSH and forskolin (Fk) on cAMP-dependent (PKa) and Ca2+-phospholipid-dependent (PKc) protein kinase--enzymes which represent the key step in the transduction of extracellular signals. The PKa activity in cells cultured for 2 days in the presence of TSH was decreased to about 50% of control level with a TSH dose of 0.1 mU/ml. This decrease is dose dependent; only traces of PKa activity remained at very high doses of TSH (50 mU/ml). Similar results were obtained with Fk (10(-5) M), the adenylate cyclase activator. It decreased the PKa activity to the level obtained with 0.1-1.0 mU/ml TSH. The loss of the PKa activity was parallel in cytosol and particulate fractions, suggesting that there is no translocation of enzymes under the action of either TSH or Fk. Neither TSH nor Fk had any effect on PKc, which became the predominant activity in cells exposed to either of the regulators. The cAMP-dependent phosphorylation of endogenous proteins was lower in TSH- or Fk-treated cells than in controls, and was dependent, like the PKa activity, on the dose of TSH. Polyacrylamide gel electrophoresis (PAGE) revealed the specific substrates of PKa in cultured thyroid cells. Proteins of 28, 30 and 33 kDa were regularly found, while 58 kDa protein was not present in all experiments. PAGE patterns showed that the decrease in endogenous phosphorylation in TSH- and Fk-treated cells was due to decreased labelling of PKa-specific substrates. The observed down-regulation of PKa activity could have an influence on the expression of thyroid cell differentiation.

The role of calcium in the induction of refractoriness to cyclic AMP stimulation by TSH

An initial exposure of beef thyroid slices to 25 mU/mL thyroid-stimulating hormone (TSH) for two hours induces a diminished stimulation of cyclic adenosine monophosphate (AMP) production upon subsequent readdition of TSH but does not modify the effect of prostaglandin E1 (PGE1). Incubation of thyroid slices in calcium-free buffer with or without 2 mmol/L ethylene glycol bis (beta-aminoethyl ether)--N,N' = tetracetic acid (EGTA) prevented desensitization induced by TSH and PGE1, to the subsequent stimulation by TSH and PGE1, respectively, despite the presence of calcium in subsequent incubations. TSH-induced desensitization was not modified by increasing the calcium concentration up to 50 mmol/L in the initial incubation. However, the stimulatory effect of TSH upon cyclic AMP levels was decreased as the calcium concentration in the first incubation was increased. In the presence of at least 1 mmol/L calcium, an initial incubation of thyroid slices with 20 mumol/L ionophore A-23187 decreased the stimulation of cyclic AMP by 25 mU/mL TSH added to the slices for the first time during a subsequent incubation. Under these conditions, A-23187 had no effect on PGE1 stimulation of cyclic AMP. These results indicate that calcium may play a role in the TSH-induced, but not PGE1, desensitization of cyclic AMP formation.

Tetradecanoyl phorbol-13-acetate counteracts the responsiveness of cultured thyroid cells to thyrotropin

We have studied the effects of TPA on the metabolism of porcine thyroid cells cultured for 1-4 days in the absence (control cells) and in the presence of 0.1 mU/ml TSH (TSH cells). The phospholipid turnover, evaluated after a 2 hr incorporation of 32P-phosphate into phospholipids, is markedly modified by the presence of TPA (1.5 microM, 2 hr) in the incubation medium of control and TSH treated cells. The total incorporation is 3-4 times higher than untreated cells, the labelling of phosphatidylinositol (PI) is slightly decreased or unchanged whereas that of phosphatidylcholine (PC) is strongly increased. The increased labelling of PI, promoted by an acute TSH treatment is counteracted by TPA. This TPA effect is not observed when prelabelled cells are challenged for 5 min with the drug. A similar effect is observed when 10 nM TPA is added in the culture medium for 20 hr. The addition of TPA does not affect significantly the protein iodine content in 3 or 4 days control cells incubated for 45 min or 2 hr with 125I-iodine, but dramatically decreases the very high iodination rate of TSH cells. We have tested the TPA effect on the cyclic AMP accumulation for the last 5 min of a 2 hr incubation. TPA inhibits by about 50-80% the stimulation evoked by TSH and only by 10% that evoked by forskolin (0.1 mM). These results suggest a possible link between the PC turnover and the adenylate cyclase responsiveness to TSH and the iodination rate.

Interactions between thyreostimulin and PAF-acether on thyroid cell metabolism

1-O-Alkyl, 2-acetyl sn-glycerylphosphorylcholine (platelet activating factor (PAF)-acether) originally described as a platelet activating factor, is effective on various parameters in different cells. It seemed interesting to us to test it on porcine thyroid cells cultured for 1 to 5 days in the absence (control cells) or in the presence of 0.1 mU/ml thyreostimulin (TSH cells). At concentrations ranging from 0.5 to 2.5 microM, PAF-acether inhibited significantly the accumulation of cyclic AMP resulting from a 5 min incubation of the cells with TSH (40 mU/ml) or forskolin (0.1 mM). PAF-acether alone did not affect basal cyclic AMP accumulation. The maximal inhibition was obtained on a 3 day culture and amounted to 40-50%. The inhibition was transient and vanished after a 30 min incubation. The effects of PAF-acether (0.5 microM) on phospholipid metabolism depended closely upon the physiological state of the cells and upon the age of the culture. When PAF-acether was incubated for 2 h with [32P]phosphate, it mimicked the effects of TSH, i.e. it increased phosphatidylinositol (PI) labelling on 1 day control cells (expected effect) and decreased it with 1 day TSH cells (reverse effects). The PAF-acether effect was rapid in onset. After cell prelabelling for 2 h in the presence of TSH, PAF-acether added for 15 min completely counteracted the hormone effects on PI and phosphatidylcholine (PC) but increased the phosphatidic acid (PA) labelling. The effect of PAF- acether on PI labelling was partially antagonized by forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)