Triiodothyronine regulation of multiple rat hepatic genes: requirement for ongoing protein synthesis

"Spot 14" protein: a metabolic integrator in normal and neoplastic cells

"Spot 14" (S14) was originally identified as a mRNA from rat liver that responded rapidly to thyroid hormone, and has now been shown to play a key role in the tissue-specific regulation of lipid metabolism. In addition to its responsiveness to thyroid hormone, S14 gene transcription is controlled by dietary substrates, such as glucose and polyunsaturated fatty acids, and by fuel-related hormones including insulin and glucagon. The S14 protein forms homodimers via a carboxyl-terminal "zipper" domain. The protein is located primarily in the cell nucleus, and its expression in liver is limited to the perivenous portion of the hepatic lobule, the site of fatty acid synthesis. S14 protein is critical for the induction of key enzymes involved in the switching of hepatic metabolism from the fasted to the fed state. S14 antisense oligonucleotides inhibit both the intracellular production of lipids and their export as very low-density lipoprotein (VLDL) particles. S14 acts at the level of transcription to regulate expression of genes encoding key metabolic enzymes, including those required for long-chain fatty acid synthesis. The human S14 gene is located at 11q13.5, a region that is amplified in a subset of aggressive breast cancers. S14 mRNA is expressed in most breast cancer-derived cell lines, and the protein is found in the nuclei of two thirds of human breast cancer specimens, but not in normal nonlactating mammary glands. S14 expression in breast tumors is highly concordant with overabundance of a key lipogenic enzyme. This indicates the association of S14 with enhanced tumor lipogenesis, an established marker of poor prognosis. In addition to the utility of S14 as a model system for elucidation of the mechanism of thyroid hormone action, studies of its regulation and function have provided insights into tissue-specific metabolic control by hormones and dietary substrates in both normal and neoplastic tissues.

Thyroid hormone inhibits fatty acid synthase gene transcription in chicken liver

The effect of triiodothyronine (T3) on regulation of fatty acid synthase in chicken liver was investigated. In hypothyroid animals, enzyme activity was about one half of that in euthyroid animals. T3 treatment increased the enzyme activity in hypothyroid animals. There is little difference in both the mRNA concentration and the transcription rate between euthyroid and hypothyroid animals. T3 treatment markedly decreased both the mRNA concentration and the transcription rate in euthyroid and hypothyroid animals. These results suggested that T3 maintained the normal level of enzyme expression primarily by stimulating the post-transcriptional step, while the transcription of the gene was inhibited by hyperthyroidism.

Thyroid hormone receptor induction by triiodothyronine in tadpole erythrocytes in vivo and in vitro and the effect of cycloheximide and actinomycin-D

Tadpole erythrocyte nuclei contain specific T3 binding sites which increase in number during spontaneous or T3-induced metamorphosis. In the present studies the increase in the number of T3 binding sites after a T3 injections appeared to be completely prevented by cycloheximide and actinomycin D, inhibitors of protein synthesis and RNA synthesis, respectively. However, in some experiments the effect was not statistically significant. The increase in sites was prevented only if the inhibitors were administered at 0 or 24 hr after T3 injection, but not at 48 or 72 hr after T3. When tadpole erythrocytes were incubated with T3 in vitro in M199 culture medium, the number of nuclear T3 binding sites increased within 48 hr. This increase was highly sensitive to inhibition by cycloheximide (maximal at 1 x 10(-6) M) or actinomycin D (maximal at 0.02 micrograms/ml). These inhibitor concentrations only slightly reduced the incorporation of labeled precursors. The T3 concentration required to induce a half-maximal increase in binding sites in vitro was about the same as the T3 concentration at which half the binding sites were occupied. The T3 binding sites had a high affinity for thyroid hormone analogs which stimulate metamorphosis. These results support the designation of the T3 binding sites as T3 receptors. They show that the tadpole erythrocytes respond to T3 with an increase in the number of T3 binding sites without the involvement of other tissues. It is proposed that this is a receptor induction involving synthesis of RNA and protein.

Sex differences of the influence of T3 on the topical distribution of phosphoenolpyruvate carboxykinase activity in the liver acinus

Phosphoenolpyruvate carboxykinase (PEPCK) activity was investigated in livers of triiodothyronine (T3) treated male and female rats with special regard to its intraacinar localization. In untreated controls of both, male and female rats, the activity was heterotopically distributed within the acinus with highest values in the periportal zone, and with lowest values in the perivenous zone. This periportal to perivenous activity gradient revealed to be under the influence of T3. Application of T3 resulted in a relative increase of PEPCK activity which was much greater in the livers of females than in males. The extent of T3-induced augmentation of PEPCK activity was dependent on the intraacinar position. In both sexes greatest relative activation was found in the perivenous zone. In female animals, the perivenous activity of T3 treated livers was comparable to that observed in the periportal zone of controls.

Thyroid hormone nuclear receptors and their role in the metabolic action of the hormone

Thyroid hormone nuclear receptor molecules have been characterized as proteins of approximately 49,000 molecular weight existing in cells attached to chromatin and with 4000-8000 copies per nucleus. They bind T3 with Ka of 0.2 X 10(10) l/mol and show microheterogeneity on isoelectric focusing. Hormone responsiveness varies with receptor content in the nucleus and occupancy of receptor by T3. Recent investigations have shown that the receptors are part of the v-erbA related super family of nuclear hormone receptors. At least two types of T3 receptors (TR) exist, one coded by a gene on chromosome 3 (TR beta) and a second coded on chromosome 17 (hTR alpha). Receptors are low in the fetus and, in the adult, are dramatically reduced by starvation, illness and glucagon. Receptors function through binding of T3 or other hormone analogs to a domain in the carboxyl portion of the protein, and binding of the receptor-T3 complex through 'DNA-fingers' to specific response elements as enhancers and located in the 5'-flanking DNA of thyroid hormone responsive genes. Extensive studies on regulation of rat growth hormone have suggested binding of receptor or associated factors to several positions in the 5'-flanking DNA, and recent studies suggest that a crucial area may be a 15 bp segment between bases -179 and -164. Abnormal receptors are believed to be responsible for the syndrome of generalized resistance to thyroid hormone action, but it is yet unclear as to which form (or forms) of the receptor is abnormal in this syndrome.

Ongoing protein synthesis needed for 1,25-(OH)2D3-mediated rapid increase of cyclic GMP in human skin fibroblasts

Recently we reported that 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) through interaction with its specific receptor rapidly (within 1 min) stimulated intracellular cGMP production in cultured human skin fibroblasts. Here we show that this effect of 100 nM 1,25-(OH)2D3 is prevented by brief (30 min) inhibition of RNA synthesis (with actinomycin D or alpha-amanitin) or by brief inhibition of protein synthesis (with cycloheximide or diphtheria toxin). The protein synthesis inhibitors also blocked stimulation of cGMP by other steroids (testosterone or dexamethasone at 100 nM) but did not block cGMP stimulation by sodium nitroprusside. Since the time for the 1,25-(OH)2D3 receptor to increase cGMP seems too short to require de novo protein synthesis, we conclude that the 1,25-(OH)2D3 receptor acts together with rapidly turning over protein(s) to stimulate cGMP synthesis.