Nuclear triiodothyronine binding sites in rat adipocytes

Tri-iodothyronine upregulates adiponutrin mRNA expression in rat and human adipocytes

Adiponutrin (PNPLA3) is expressed in adipose tissue. Although its precise function is unknown, some data suggest a dual role in lipid homeostasis. We have investigated the influence of thyroid hormone (TH) on PNPLA3 mRNA, in rat and human cultured white adipocytes and in rat white adipose tissue (WAT). Pnpla3 mRNA increased during differentiation of rat adipocytes in an insulin-dependent manner. Tri-iodothyronine further increased Pnpla3 expression at any day during differentiation and its effects were time and dose-dependent. The Pnpla3 mRNA half-life was stabilized by tri-iodothyronine, but a transcriptional component was also observed. Pnpla3 mRNA decreased in WAT of hypothyroid rats and was partially restored by treatment with TH. Taqman analysis showed that tri-iodothyronine also increased human PNPLA3 expression in cultured subcutaneous adipocytes from obese patients. In conclusion, PNPLA3 mRNA expression is upregulated by tri-iodothyronine in adipocytes in vitro, in humans and rats, and in vivo in rat WAT.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinoic acid and dexamethasone interact to regulate S14 gene transcription in 3T3-F442A adipocytes

We have examined the effects of retinoic acid (RA) on S14 gene expression in 3T3-F442A preadipocytes and adipocytes. RA induced mRNAS14 14-fold in adipocytes, but had no effect on S14 gene expression in preadipocytes. Northern analysis of retinoic acid receptor (RAR) isoforms revealed the presence of RAR alpha, but not RAR beta, in both preadipocytes and adipocytes. These results suggest that adipocyte-specific factors expressed during differentiation may be required for RA control of S14 gene expression in cultured adipocytes. When compared to dexamethasone (DEX), RA was a weak inducer of S14 gene expression. However, when added together, RA and DEX acted synergistically to induce S14 gene expression. Since changes in S14 gene transcription paralleled changes in mRNAS14 levels, the principal target for RA and DEX action on S14 gene expression was at the level of gene transcription. In the presence of DEX (0.1 nM), RA effects on S14 gene expression were dose-dependent (ED50 = 5 nM) and rapid (within 4 h). In contrast to S14, RA inhibited glycerol 3-phosphate dehydrogenase (GPD) gene transcription and mRNAGDP abundance by 75%. The effects of RA on S14 and GPD gene transcription in cultured adipocytes suggest that RA action may extend beyond growth and differentiation to include effects on lipid metabolism.

Nuclear triiodothyronine receptor of human adipose tissue

L-Triiodothyronine induces malic enzyme in explants from human adipose tissue. Consequently, we looked for the presence of receptors for L-triiodothyronine in nuclei isolated from human adipose tissue. The binding of 125I-triiodothyronine by the nuclei was time- and temperature-dependent. At 37 degrees C binding reached a steady state after 60 minutes. Dithiothreitol enhanced total binding and suppressed nonspecific binding. Scatchard analysis showed the presence of a single class of binding sites. The apparent association constant, Ka, was 0.13 +/- 0.03 X 10(10) M-1, the maximal binding capacity 2.20 +/- 0.81 pmol/mg DNA (mean +/- SD, n = 7) and the number of binding sites 8,000/nucleus. L-Triiodothyronine and D-triiodothyronine had equal affinity to the nuclear receptor; triiodothyroacetic acid had three times higher affinity. L- and D-thyroxine had 8% and 12%, respectively, and tetraiodothyroacetic acid had 19% affinity compared to that of L-triiodothyronine. Reverse triiodothyronine was a weak competitor. Digestion of nuclei with micrococcal nuclease abolished specific binding. These results show that nuclei from human white adipose tissue possess high affinity receptors for L-triiodothyronine, which are associated with nuclear chromatin. It is likely that induction of malic enzyme in human adipose tissue by L-triiodothyronine is mediated by the nuclear receptors.