Role of thyroid hormone in de novo synthesis of cholinephosphotransferase in guinea pig lung mitochondria and microsomes

Drug-induced and postnatal hypothyroidism impairs the accumulation of diacylglycerol in liver and liver cell plasma membranes

BACKGROUND

Thyroid hormones are well known modulators of signal transduction. The effect of hyper- and hypo-thyroidism on diacylglycerol/protein kinase C (DAG/PKC) signaling in cardiomiocytes has been determined. Triiodothyronine (T3) has been shown to prevent the alpha1-adrenoreceptor-mediated activation of PKC but does not alter the stimulation of enzyme and hepatic metabolism by phorbol ethers. It has been suggested that the elevation of endogenous DAG in senescent or hypothyroid cells changes the PKC-dependent response of cells to phorbol esters and hormones. In the present study, was examined the formation of DAG and activation of PKC in liver cells from rats of different thyroid status.

RESULTS

The results obtained provide the first demonstration of DAG accumulation in liver and cell plasma membranes at age- and drug-dependent thyroid gland malfunction. The experiments were performed in either the [14C]CH3COOH-labeled rat liver, liver slices or hepatocytes labeled by [14C] oleic acid and [3H]arachidonic acid or [14C]palmitic acid as well as in the isolated liver cell plasma membranes of 90- and 720-day-old rats of different thyroid status. The decrease of T4 and T3 levels in blood serum of 720-day-old rats and mercazolil-treated animals was associated with increases of both the DAG mass in liver and liver cell plasma membranes and newly synthesized [14C]DAG level in liver and isolated hepatocytes. Hypothyroidism decreased PKC activity in both membrane and cytosol as well as phospholipid and triacylglycerol synthesis in liver. These hypothyroidism effects were restored in liver by injection of T4. T4 administration to the intact animals of different ages decreased the DAG level in liver and isolated plasma membranes and the content of newly synthesized DAG in liver. The reduction of DAG level in liver was not associated with increasing free fatty acid level. DAG labeling ratio 14C/3H in liver slices of rats of different thyroid state sharply differed from PL. DAG was relatively enriched in [14C]oleic acid whereas PL were enriched in [3H]arachidonic acid.

CONCLUSIONS

The above data have indicated that thyroid hormones are important physiological modulators of DAG level in rat liver and cell plasma membranes. Age- and drug-induced malfunction of thyroid gland resulted in a prominent decrease of glycerolipid synthesis which may promote DAG accumulation in liver.

Regulation of cholinephosphotransferase by thyroid hormone

We have demonstrated earlier that thyroid hormone (T3) regulates the activity of cholinephosphotransferase (CPT) in guinea pig lung. This effect of T3 is not organ specific because we found T3 also regulates CPT activity in the guinea pig liver. Northern blot analysis using two oligonucleotide probes, one synthesized on the basis of the yeast CPT gene sequence and another on the basis of partial cDNA clone from guinea pig CPT clone, revealed that T3 stimulates the expression of new CPT mRNA. Studies with transcriptional and translational inhibitors indicated that T3 enhanced the translation of the CPT mRNA as well as translocation of preformed CPT enzyme protein from cytosol to mitochondria. Furthermore, it strengthens our previous finding that yeast CPT and guinea pig CPT have high homology in their sequence as both the oligonucleotide probes gave the similar type of Northern blot in the present study.