Conserved amino acids in the ligand-binding and tau(i) domains of the peroxisome proliferator-activated receptor alpha are necessary for heterodimerization with RXR.

The peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. These ligand-activated transcription factors are implicated in the regulation of lipid metabolism and adipocyte differentiation and in the regulation of anti-inflammatory processes. In order to bind to DNA and activate transcription PPAR requires the formation of heterodimers with the retinoid X receptor (RXR). We have previously reported that replacement of a single leucine by an arginine at position 433 of hPPAR alpha (L433R), located in a highly conserved region of the ninth heptad repeat of a leucine-zipper-like motif in the ligand binding domain, abolished heterodimerization of PPAR with RXR and hence its trans-activating capacity. The aim of our present work was to investigate if other conserved amino acids of the ligand binding domain are important for heterodimerization of PPAR with RXR. We found that conserved leucines, L370 and L391, in a leucine-zipper-like motif of hPPAR alpha, as well as a highly conserved aspartic acid (D304) in the tau(i) domain are necessary for heterodimerization with RXR. In contrast, mutations of non-conserved amino acids within the leucine-zipper-like motif do not affect PPAR:RXR heterodimerization. Surprisingly, we found that some mutants deficient in heterodimerization with RXR (hPPAR alpha-L370R and -L391R) were still functional on specific peroxisome proliferator-activator response elements (PPREs). Both mutants could trans-activate on a PPRE from the P450 cytochrome promoter CYP4A1, whereas only the hPPAR alpha-L391R mutant could trans-activate from the acyl-CoA oxidase PPRE (ACOA) and, when stimulated with the peroxisome proliferator Wy14643, also from the bifunctional enzyme PPRE. We therefore hypothesize either that: (i) these mutants might be able to heterodimerize with a protein other than RXR and the affinity for this novel partner may depend on the nature of the PPRE and to some degree on the choice of the activator, or alternatively; (ii) that additional nuclear proteins might compensate in vivo for the decreased binding of RXR to these mutant PPARs observed in vitro.

Regulation of the transcriptional activity of the peroxisome proliferator-activated receptor alpha by phosphorylation of a ligand-independent trans-activating domain.

The peroxisome proliferator-activated receptors (PPARs) are a subgroup of nuclear receptors activated by fatty acids and eicosanoids. In addition, they are subject to phosphorylation by insulin, resulting in the activation of PPARalpha, while inhibiting PPARgamma under certain conditions. However, it was hitherto unclear whether the stimulatory effect of insulin on PPARalpha was direct and by which mechanism it occurs. We now demonstrate that amino acids 1-92 of hPPARalpha contain an activation function (AF)-1-like domain, which is further activated by insulin through a pathway involving the mitogen-activated protein kinases p42 and p44. Further analysis of the amino-terminal region of PPARalpha revealed that the insulin-induced trans-activation occurs through the phosphorylation of two mitogen-activated protein kinase sites at positions 12 and 21, both of which are conserved across evolution. The characterization of a strong AF-1 region in PPARalpha, stimulating transcription one-fourth as strongly as the viral protein VP16, is compatible with the marked basal transcriptional activity of this isoform in transfection experiments. However, it is intriguing that the activity of this AF-1 region is modulated by the phosphorylation of two serine residues, both of which must be phosphorylated in order to activate transcription. This is in contrast to PPARgamma2, which was previously shown to be phosphorylated at a single site in a motif that is not homologous to the sites now described in PPARalpha. Although the molecular details involved in the phosphorylation-dependent enhancement of the transcriptional activity of PPARalpha remain to be elucidated, we demonstrate that the effect of insulin on the AF-1 region of PPARalpha can be mimicked by the addition of triiodothyronine receptor beta1, a strong binder of corepressor proteins. In addition, a triiodothyronine receptor beta1 mutant deficient in interacting with corepressors is unable to activate PPARalpha. These observations suggest that the AF-1 region of PPARalpha is partially silenced by corepressor proteins, which might interact in a phosphorylation-dependent manner.

DNA binding properties of peroxisome proliferator-activated receptor subtypes on various natural peroxisome proliferator response elements. Importance of the 5'-flanking region.

The three subtypes of the peroxisome proliferator-activated receptors (PPARalpha, beta/delta, and gamma) form heterodimers with the 9-cis-retinoic acid receptor (RXR) and bind to a common consensus response element, which consists of a direct repeat of two hexanucleotides spaced by one nucleotide (DR1). As a first step toward understanding the molecular mechanisms determining PPAR subtype specificity, we evaluated by electrophoretic mobility shift assays the binding properties of the three PPAR subtypes, in association with either RXRalpha or RXRgamma, on 16 natural PPAR response elements (PPREs). The main results are as follows. (i) PPARgamma in combination with either RXRalpha or RXRgamma binds more strongly than PPARalpha or PPARbeta to all natural PPREs tested. (ii) The binding of PPAR to strong elements is reinforced if the heterodimerization partner is RXRgamma. In contrast, weak elements favor RXRalpha as heterodimerization partner. (iii) The ordering of the 16 natural PPREs from strong to weak elements does not depend on the core DR1 sequence, which has a relatively uniform degree of conservation, but correlates with the number of identities of the 5'-flanking nucleotides with respect to a consensus element. This 5'-flanking sequence is essential for PPARalpha binding and thus contributes to subtype specificity. As a demonstration of this, the PPARgamma-specific element ARE6 PPRE is able to bind PPARalpha only if its 5'-flanking region is exchanged with that of the more promiscuous HMG PPRE.

Lack of loss of heterozygosity at the c-erbA beta locus in gastrointestinal tumors.

The cellular homologues of the viral oncogene c-erbA encode for the nuclear thyroid hormone receptor c-erbA (TR). The gene for the human TR beta subtype is located on chromosome 3p24, and a loss of heterozygosity around this region has been reported in breast and small cell lung cancers, suggesting that TR beta might act as a tumor suppressor gene. In the present study, we used PCR-based restriction fragment length polymorphisms to examine the 3p24 region of 19 patients with gastrointestinal tumors for loss of heterozygosity (LOH). Interestingly, only 1 of the patients had an LOH at this locus, while 4 patients had a microdeletion of both alleles in the 3p24 region. These results suggest that, in contrast to previous reports on lung and breast cancers, a loss of heterozygosity of the TR locus at 3p24 is a rare event. A critical review of the literature, however, suggests, that some of the earlier studies have used markers whose location is only imprecisely mapped and may hence point to a tumor suppressor gene candidate other than TR beta. However, a selective microdeletion of both alleles was detected in the tumors of 4 of the 19 patients, indicating that this region on chromosome 3p may be genetically unstable in gastrointestinal tumors.

Direct effects of leptin on brown and white adipose tissue.

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

Quantitation of beta 1 triiodothyronine receptor mRNA in human tissues by competitive reverse transcription polymerase chain reaction.

Thyroid hormones act by binding to nuclear receptor proteins, the thyroid hormone receptors (TR) alpha and beta. Data from cell culture and animal studies indicate that TR expression may be regulated to modulate target organ responsiveness to thyroid hormone. To investigate whether such adaptive changes in TR expression occur in humans, we determined the mRNA levels of the hTR beta 1 in various thyroid states. Patients with overt hypo- or hyperthyroidism were enrolled in the study. Total RNA was isolated from peripheral blood mononuclear cells and hTR beta 1 mRNA levels determined by quantitative competitive reverse transcription PCR. For comparison, hTR beta 1 mRNA levels were determined in lymphocytes and normal thyroid tissue of euthyroid patients. Human TR beta 1 mRNA levels in lymphocytes were 1.8 +/- 0.4, 1.9 +/- 0.5, 1.1 +/- 0.4 10(-18) mol/microgram RNA in hypo-, eu- and hyperthyroid patients, respectively, corresponding to an estimated 0.5 - 2 molecules per cell. Although the mean hTR beta 1 mRNA levels were 40% lower in hyperthyroid than in euthyroid subjects, this difference did not reach statistical significance. Similar levels of hTR beta 1 mRNA levels were detected in thyroid gland from euthyroid patients. In summary, we developed an assay for the quantitative determination of hTR beta 1 mRNA levels in small human tissue samples, containing as little as 50 ng of total RNA. Absolute hTR beta 1 mRNA levels are very low with an estimated one molecule of mRNA being present in a mononuclear blood cell or thyrocyte. No up-regulation of hTR beta 1 was seen in hypothyroid relative to euthyroid patients. However, there is a non-significant trend towards a down-regulation of hTR beta 1 mRNA levels in hyperthyroid patients.

Peroxisome proliferator-activated receptor mediates cross-talk with thyroid hormone receptor by competition for retinoid X receptor. Possible role of a leucine zipper-like heptad repeat.

The peroxisome proliferator-activated receptors (PPAR) and thyroid hormone receptors (TR) are members of the nuclear receptor superfamily, which regulate lipid metabolism and tissue differentiation. In order to bind to DNA and activate transcription, PPAR requires the formation of heterodimers with the retinoid X receptor (RXR). In addition to activating transcription through its own response elements, PPAR is able to selectively down-regulate the transcriptional activity of TR, but not vitamin D receptor. The molecular basis of this functional interaction has not been fully elucidated. By means of site-directed mutagenesis of hPPAR alpha we mapped its inhibitory action on TR to a leucine zipper-like motif in the ligand binding domain of PPAR, which is highly conserved among all subtypes of this receptor and mediates heterodimerization with RXR. Replacement of a single leucine by arginine at position 433 of hPPAR alpha (L433R) abolished heterodimerization of PPAR with RXR and consequently its trans-activating capacity. However, a similar mutation of a leucine residue to arginine at position 422 showed no alteration of heterodimerization, DNA binding, or transcriptional activation. The dimerization deficient mutant L433R was no longer able to inhibit TR action, demonstrating that the selective inhibitory effect of PPAR results from the competition for RXR as well as possibly for other TR-auxiliary proteins. In contrast, abolition of DNA binding by a mutation in the P-box of PPAR (C122S) did not eliminate the inhibition of TR trans-activation, indicating that competition for DNA binding is not involved. Additionally, no evidence for the formation of PPAR:TR heterodimers was found in co-immunoprecipitation experiments. In summary, we have demonstrated that PPAR selectively inhibits the transcriptional activity of TRs by competition for RXR and possibly non-RXR TR-auxiliary proteins. In contrast, this functional interaction is independent of the formation of PPAR:TR heterodimers or competition for DNA binding.

Long-lasting effects of Triac and thyroxine on the control of thyrotropin and hepatic deiodinase type I.

The purpose of this study was to investigate the relation between the serum levels of thyroid hormones and their biological effects. For this purpose, hypothyroid rats were studied after stopping treatment with a long-acting thyroid hormone, thyroxine (T4) and a short-acting one, triiodothyroacetic acid (Triac). Based on preliminary experiments with different doses of T4 and Triac, hypothyroid rats (N = 84) received over 6 days' injections of 10 nmol Triac or 2 nmol T4/100 g body wt per day. Biological effects of Triac and T4 were measured in the pituitary, liver and kidney up to 8 days after stopping treatment. With Triac, serum thyrotropin (TSH) levels were inhibited completely 6 h after injection, yet after 24 h they were 4.9 +/- 1.8 micrograms/l (hypothyroid 14.5 +/- 0.8 micrograms/l). The rapid changes in serum TSH levels were followed by a more gradual increase in serum TSH levels were followed by a more gradual increase in serum TSH, which was similar to that after T4 injection. Even 8 days after Triac treatment, serum TSH levels did not reach the hypothyroid control levels. Changes in beta-TSH mRNA levels also showed a prolonged inhibition after both treatments and a slow return to hypothyroid values, which was not complete 8 days after stopping treatment. A second parameter was hepatic 5'-deiodinase type I (5'D-I). The 6-day treatment with Triac had a markedly stronger effect on 5'D-I enzyme activity and mRNA levels than treatment with T4.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of thyroid hormone action by mutant thyroid hormone receptors, c-erbA alpha 2 and peroxisome proliferator-activated receptor: evidence for different mechanisms of inhibition.

Thyroid hormone action is not only determined by hormone availability, but also by target organ sensitivity. A dominant negative interaction is known to occur between thyroid hormone receptors (TRs) and the non-ligand binding splicing variant c-erbA alpha 2 as well as mutant TR beta 1 from kindreds with resistance to thyroid hormone. We compared the inhibitory effect of naturally occurring mutant hTR beta 1, artificially created hTR alpha 1 mutants, c-erbA alpha 2 and the human peroxisome proliferator-activated receptor (hPPAR) on three prototypic T3-response elements (TREs), TRE-PAL, DR + 4 and TRE-LAP. The inhibitory effect of mutant hTR alpha 1 and beta 1 occurred only on TRE-LAP and to a minor degree on DR + 4 when equimolar ratios of mutant/wildtype receptor were present. In contrast, the c-erbA alpha 2 splicing variant and the hPPAR inhibited TR action on all three TREs. Gel mobility shift experiments in the presence of T3 showed increased binding of mutant hTR alpha 1 and beta 1 only to TRE-LAP compared to the binding of wildtype hTRs, thereby explaining their TRE-selective dominant negative potency. Contrarily, equal amounts of c-erbA alpha 2 or hPPAR protein did not bind to either of the three response elements even in the presence of RXR. Since the TR:RXR heterodimers were only partially displaced from DNA in the presence of excess amounts of c-erbA alpha 2, it is likely that the TRE-unspecific dominant negative action of c-erbA alpha 2 is due in part to competition for DNA-binding and for TR-auxiliary proteins. In contrast, equimolar amounts of hPPAR completely inhibited the DNA-binding of hTR beta 1:RXR heterodimers, but not of TR:TR homodimers, suggesting that hPPAR has a higher RXR-binding affinity and is therefore a potent competitor for intranuclear RXR. Since thyroid hormones and peroxisome proliferators regulate in part a similar subset of target genes involved in fatty acid metabolism, these results suggest the possibility of cross-talk among the thyroid hormone and peroxisome proliferator signalling pathways. In summary, the results suggest that thyroid hormone action can be modulated by at least three different mechanisms: (i) increased binding of mutant hTRs to specific TREs; (ii) efficient competition for limiting amounts of RXR through the preferential formation of hPPAR:RXR, rather than TR:RXR heterodimers; and (iii) competition for binding to DNA and to auxiliary proteins other than RXR in the case of c-erbA alpha 2.