Osteosarcoma paraosteal (juxtacortical): revisión de un caso clínico patológico en un perro / Paraosteal (juxtacortical) osteosarcoma: report of clinical-pathological case in a dog

El osteosarcoma paraosteal constituye una neoplasia primaria de hueso, caracterizada por desarrollarse en la superficie ósea; está compuesta por tejido fibroso, óseo y cartilaginoso bien diferenciado pero con características de malignidad. El caso se presentó en un perro Pastor Alemán, macho, de 8 años de edad, diagnosticado por punción con aguja delgada y estudio radiográfico, y confirmado por estudio histológico

A direct protein-protein interaction is involved in the cooperation between thyroid hormone and retinoic acid receptors and the transcription factor GHF-1.

The nuclear receptors for thyroid hormone (TRs) and retinoic acid (RARs and RXRs) cooperate with the pituitary-specific transcription factor GHF-1 to activate the rat growth hormone (GH) gene. The GH promoter contains a hormone response element (HRE), which binds TR/RXR and RAR/RXR heterodimers, located close to two binding sites for GHF-1. GHF-1 inhibits binding of TR/RXR and RAR/RXR heterodimers to an isolated HRE. Similarly, the receptors inhibit binding of GHF-1 to its cognate site. These results suggest the existence of direct protein to protein interactions between the receptors and the pituitary transcription factor. This was confirmed by in vitro binding studies with GST fusion proteins, which demonstrated a strong association of GHF-1 with RXR and a weaker interaction with RAR and TR. GHF-1 and the receptor heterodimers form a ternary complex with a fragment of the rat GH promoter, which contains binding sites for both, and GHF-1 increases receptor binding to the promoter when present in limiting conditions. These results suggest that the synergistic activation of the rat GH gene involves protein-DNA interactions as well as a physical association between the nuclear receptors and the pituitary-specific transcription factor GHF-1.

The transcription factor GHF-1, but not the splice variant GHF-2, cooperates with thyroid hormone and retinoic acid receptors to stimulate rat growth hormone gene expression.

The rat growth hormone (GH) promoter was significantly activated in non-pituitary cells by the expression of unliganded trioodothyronine (T3) and retinoic acid (RA) receptors. Furthermore, a strong ligand-dependent activation was found in the presence of the pituitary-specific transcription factor GHF-1. When compared with GHF-1, the splice variant GHF-2 showed a decreased ability to bind the cognate site in the GH promoter. As a consequence, expression of GHF-2 had little stimulatory effect on the GH promoter and did not show cooperation with T3 or RA receptors even in the presence of ligands. Furthermore, over-expression of GHF-2 inhibited the response to T3 and RA in pituitary cells. These results show that alternative splicing of the GHF-1 gene gives rise to two isoforms that differ in their transactivating properties and in their ability to synergize with the nuclear thyroid hormone and retinoic acid receptors on GH gene expression.

Retinoic acid induces expression of the transcription factor GHF-1/Pit-1 in pituitary prolactin- and growth hormone-producing cell lines.

PRL and GH gene expression depend on the presence of the pituitary-specific transcription factor GHF-1/Pit-1. We have examined the effects of retinoic acid (RA) on the expression of the GHF-1/Pit-1 gene. RA induced a time- and dose-dependent increase in GHF-1/Pit-1 messenger RNA in the PRL-producing cell line 235-1. A maximal effect (a 2- to 3-fold increase) was obtained after 12-24 h of incubation with 1 microM RA. The level of the transcription factor determined by both Western blotting and gel retardation analysis with a GHF-1/Pit-1-binding site was increased in RA-treated cells compared to that in control cells. Sequences located between -400 and -90 bp mediated a 2- to 3-fold activation of the GHF-1/Pit-1 promoter by RA. The retinoid also increased the response to cAMP and phorbol esters that is mediated by two cAMP-responsive elements (CREs) located in the same promoter fragment. Both CREs are required for RA induction, as deletion of either CRE abolished the response to the retinoid RA also induced GHF-1/Pit-1 gene expression in GH4C1 cells, which produce both PRL and GH. T3 did not affect expression of the GHF-1/Pit-1 gene in 235-1 cells, but decreased basal GHF-1/Pit-1 messenger RNA and promoter activity in GH4C1 cells and blocked the stimulatory effect of RA.

Negative regulation of expression of the pituitary-specific transcription factor GHF-1/Pit-1 by thyroid hormones through interference with promoter enhancer elements.

Expression of the growth hormone gene is due to the presence of the pituitary-specific transcription factor GHF-1/Pit-1. The action of the thyroid hormone T3 is mediated by nuclear receptors that regulate transcription by interaction with DNA elements located near promoters of the regulated genes. In this study, we show that T3 inhibits expression of the GHF-1/Pit-1 gene in rat pituitary GH4C1 cells by a novel mechanism that involves transcriptional interference with other regulatory elements of the promoter. Sequences between bp -90 and -200 of the rat GHF-1/Pit-1 gene which do not contain a hormone response element but contain two cyclic AMP-responsive elements mediate most of the repressive effect of T3. The hormone reduces basal levels of GHF-1/Pit-1 promoter activity and antagonizes its response to cyclic AMP and the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). A similar repression is found with a heterologous promoter that contains four copies of the cyclic AMP-responsive element motif. This regulation provides a novel example of the cross-talk between the thyroid hormone receptor and the signal transduction pathways used by different hormones and growth factors. Additionally, T3 interferes with in vitro binding of GHF-1/Pit-1 to a positive autoregulatory element located at bp -45 to -63 and has a detectable inhibitory effect on the activity of a promoter construct which extends to bp -90 of 5'-flanking DNA. The regulation of the transcription factor provides a novel example of negative transcriptional regulation by thyroid hormones.

Nicotinamide analogs and DNA-damaging agents deplete thyroid hormone receptor and c-erbA mRNA levels in pituitary GH1 cells.

Incubation of pituitary GH1 cells with N'-methylnicotinamide, nicotinamide and 3-acetylpyridine which inhibit nuclear ADP-ribosylation and/or the cellular concentration of its substrate NAD+ reduced the amount of nuclear thyroid hormone receptors in a time- and dose-dependent manner without altering the affinity of the receptors for the hormone. A transient activation of poly(ADP-ribose)polymerase by methyl methanesulfonate, ultraviolet irradiation or spermine caused a rapid depletion of cellular NAD+ content and was followed by a strong inhibition of ADP-ribosylation. These agents also produced a very rapid and marked reduction of receptor numbers. The decrease of receptors caused by the different compounds is not secondary to a generalized inhibition of protein synthesis or to an alteration in hormone availability. The abundance of c-erbA alpha and beta mRNAs, which encode thyroid hormone receptors, was reduced in cells treated with the compounds that decrease receptor number, thus suggesting that this effect is caused by a decrease in the expression of c-erbA genes.

The thyroid hormone response element is required for activation of the growth hormone gene promoter by nicotinamide analogs.

N'-Methylnicotinamide and nicotinamide, which decreased in vitro ADP-ribosylation of nuclear proteins and/or cellular NAD+ content, selectively increased the basal expression of the rat growth hormone (GH) gene promoter and its response to triiodothyronine (T3). This increase was not found when the thyroid hormone response element (TRE) was deleted from the promoter. Transfection with an expression vector for the T3 receptor inhibited basal activity of the TRE-containing promoter and repressed the stimulatory effect of N'-methylnicotinamide. The addition of hormone relieved this inhibition and enhanced transcription above levels found in the absence of the transfected receptors. These results suggest a modulatory role of ADP-ribosylation in hormonal regulation of gene expression.

Retinoic acid decreases thyroid hormone receptor expression in pituitary GH1 cells.

Retinoic acid produced a time and dose-dependent depletion of thyroid hormone receptors in GH1 cells without modifying their affinity for triiodothyronine (T3). A maximal decrease (50-70%) was obtained after 24-48 h incubation with 5-10 microM retinoic acid. Treatment with 0.8 nM T3 for 24 h caused a similar reduction and did not potentiate the decrease produced by these concentrations of retinoic acid. However, the combination of sub-maximally effective doses of both ligands had an additive effect on receptor levels. The reduction of receptor caused by retinoic acid is accompanied by a decreased expression of c-erbA alpha 1 and alpha 2 mRNAs, but the retinoid did not reduce the abundance of c-erbA beta mRNA. In contrast, T3 decreased the levels of both alpha and beta transcripts.