Regulation of activity of chromatin receptors for thyroid hormone: possible involvement of histone-like proteins

Osteoarthritic chondrocyte-secreted morphogens induce chondrogenic differentiation of human mesenchymal stem cells

OBJECTIVE

The potential of stem cells to repair compromised cartilage tissue, such as in osteoarthritis (OA), depends strongly on how transplanted cells respond to factors secreted from the residing OA chondrocytes. This study was undertaken to determine the effect of morphogenetic signals from OA chondrocytes on chondrogenic differentiation of human mesenchymal stem cells (MSCs).

METHODS

The effect of OA chondrocyte-secreted morphogens on chondrogenic differentiation of human MSCs was evaluated using a coculture system involving both primary and passaged OA chondrocytes. The findings were compared against findings for human MSCs cultured in OA chondrocyte-conditioned medium. Gene expression analysis, biochemical assays, and immunofluorescence staining were used to characterize the chondrogenic differentiation of human MSCs. Mass spectrometry analysis was used to identify the soluble factors. Numerical analysis was carried out to model the concentration profile of soluble factors within the human MSC-laden hydrogels.

RESULTS

The human MSCs cocultured with primary OA chondrocytes underwent chondrogenic differentiation even in the absence of growth factors; however, the same effect could not be mimicked using OA chondrocyte-conditioned medium or expanded cells. Additionally, the cocultured environment down-regulated hypertrophic differentiation of human MSCs. Mass spectrometry analysis demonstrated cell-cell communication and chondrocyte phenotype-dependent effects on cell-secreted morphogens.

CONCLUSION

The experimental findings, along with the results of the numerical analysis, suggest a crucial role of soluble morphogens and their local concentrations in the differentiation pattern of human MSCs in a 3-dimensional environment. The concept of using a small number of chondrocytes to promote chondrogenic differentiation of human MSCs while preventing their hypertrophic differentiation could be of great importance in formulating effective stem cell-based cartilage repair.

Thyroid hormone receptor expression in rat placenta

The expression of c- erbAalpha and -beta encoded thyroid hormone receptors (TR) was investigated in rat placenta between 16 and 21 days of gestation (dg), and in fetal liver and brain at 16 dg, using semi-quantitative RT-PCR and nuclear 3,5,3'-triiodothyronine (T(3)) binding. TRalpha1, TRbeta1, c- erbAalpha 2 and c- erbAalpha 3 mRNA abundance was unchanged in placenta between 16 and 21 dg, as was the dissociation constant (K(d)) of T(3) binding. The maximal T(3) binding capacity (B(max)) in placenta doubled over this period, suggesting placental TR binding activity is post-transcriptionally regulated. Transcript abundance in tissues at 16 dg can be summarized: TRalpha1, placenta=fetal liver<fetal brain; TRbeta1, placenta=fetal liver>fetal brain; c- erbAalpha 2 and alpha3, placenta=fetal liver<fetal brain; TRbeta2; none detected. T(3)binding in fetal liver and brain exhibited equivalent K(d) and B(max), the K(d) being less than 50 per cent of that in placenta, though B(max) was unchanged. The higher K(d)in placenta may reflect tissue-specific patterns of TR modification. In conclusion, rat placenta expresses significant levels of c- erbAalpha and -beta transcripts and protein, providing a possible mechanism of action for T(3) of maternal and fetal origin in this tissue.

Histone H1 suppresses tumor growth of leukemia cells in vitro, ex vivo and in an animal model suggesting extracellular functions of histones

Purified histone H1 exerts growth inhibition of leukemia cells independent of lineage, stage, and maturation. At 200 micrograms/ml, H1 proved cytotoxic in 19 of 21 of the tested leukemia-derived cell lines and for 11 of 16 of the fresh tumor samples from leukemia patients. In all cases, normal peripheral blood mononuclear cells and bone marrow cells remained unaffected. Multicellular spheroids from the Burkitt's lymphoma cell line IM-9 were growth arrested at 500 micrograms H1/ml. The clonogenic growth of the Burkitt's lymphoma cell line Daudi was arrested at 160 micrograms H1/ml. Synthetic H1-peptides as well as peptides and proteins with biochemical properties similar to H1 had no inhibitory growth effect at equimolar concentrations. Furthermore, 250 micrograms H1 injected into a Burkitt's lymphoma (Daudi), xenotransplanted into nude mice, arrested tumor growth. As shown by electron microscopy and flow cytometry, incubation of leukemia cells with H1 resulted in severe plasma membrane damage and ultimately cytolysis. This report characterizes a 33-kd protein that binds H1 and is responsible for the cell death via destruction of the cell membrane integrity. New extranuclear functions of histones are presented.

Cellular mechanism of action of thyroid hormones

It has emerged in the last decade that the molecular mechanism of action of thyroid hormones resembles that of steroids; thyroid hormones indeed exert their effects mainly by directly regulating gene expression, on association with specific chromatin-bound receptors. Of the two thyroid hormones, thyroxine (T4) appears to be a sort of prohormone, whereas triiodothyronine (T3) seems to be the active form; in this respect, T4-deiodination, which occurs at the level of the target tissues, may be crucial in the local homeostasis of T3. Moreover, many cellular compartments, other than the nucleus, can bind thyroid hormone, and at least some of these further sites might play some role in modulating T3 supply to the nucleus. The binding of the T3-receptor complex to chromatin is likely to regulate the structural organization of specific genes and, in some instances, of the chromatin as a whole.

Effect of DNA on thyroid-hormone binding by specific receptor proteins from rat-liver nuclei

Influence of double-stranded native DNA on the binding of thyroid hormone, 3,5,3'-triiodo-L-thyronine, by the isolated nuclear receptors was studied and the following results were obtained. (1) The receptor-triiodothyronine complexes bound to DNA with moderate affinities. (2) DNA enhanced the hormone binding of the receptors. (3) The stimulatory DNA effect on triiodothyronine binding of the receptors was dependent on DNA concentration, showing its maximum at 30 microgram/ml. (4) The increase in triiodothyronine binding was observed not only in the initial velocity but also in the plateau level which was attained after sufficient incubation time. (5) There were two types of specific receptors in the rat liver nuclear extract. The dissociation constants and the maximal binding capacities for triiodothyronine, which were determined by Scatchard plot analysis in the presence and absence of DNA, suggested that DNA exerted its effect through increasing binding capacity on one class of the receptors and through enhancing affinity for the hormone on the other class of the receptors. (6) Among various polynucleotides examined, the double-stranded eukaryotic DNA was most effective in enhancing the hormone binding by the receptors. These results indicate that the nuclear thyroid hormone receptors interact with double-stranded DNA in a specific manner and are induced to bind more thyroid hormone. We interpret these results as suggesting that a ternary complex of triiodothyronine, the receptor and DNA is formed in the cell nucleus in vivo, probably representing an intrinsic step in the hormone action. Possible physiological significance of this effect of DNA on the receptors is discussed.

Affinity labeling of rat liver thyroid hormone nuclear receptor

The thyroid hormone receptor from rat liver nuclei has been covalently labeled with the N-bromoacetyl derivatives of L-thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). Displacement binding studies showed that, in the presence of 100-fold molar excess of unlabeled N-bromoacetyl-T3 or T4, binding of [125I]T3 or [125I]T4 was nearly totally inhibited. Heat inactivation of the receptor (55 degrees C for 15 min) resulted in parallel losses in the binding of T3 (95%) and N-bromoacetyl-T3 (93%). These results indicated that T3 and T4 and their bromoacetyl derivatives compete for the same binding site. The nuclear receptor showed identical behavior in high-pressure liquid chromatography (HPLC) whether bound to T3 or T4 or covalently labeled with their bromoacetyl derivatives. HPLC provided a single-step 100-fold purification of the nuclear receptor. Na-DodSO4 gel electrophoresis of the nuclear receptor labeled with N-bromoacetyl derivatives of [125I]T3 or [125I]T4 showed one major radioactive component with a molecular weight of 56,000. Furthermore, in the absence of denaturant, the nuclear receptor either bound to [125I]T3 or covalently labeled with N-bromoacetyl-[125I]T3 showed identical mobility. These results suggested that the nuclear receptor is a single polypeptide chain and binds either T3 or T4. Nuclear receptors covalently linked with N-bromoacetyl derivatives of [125I]T3 or [125I]T4 may be useful as a marker for the preparative purification of receptor.

Acetylation of nucleosomal histones in vitro

A new histone-specific acetyltransferase, which is closely associated with nucleosomes prepared from lymphocyte nuclei by treatment with micrococcal nuclease, is described. The acetylating enzyme transfers [3H]acetyl groups from [3H]acetyl-coenzyme A to the endogenous histones H2A, H2B, H3 and H4 in nucleosomes as well as to free histones added to the reaction mixture. Histone H1 is not acetylated by this enzyme. The acetyltransferase was partially purified by DEAE-Sephadex and DNA-cellulose chromatography. The nucleosome-associated enzyme binds to DNA cellulose at low salt concentrations (DNA-binding acetyltransferase), while the previously described histone-specific acetyltransferases have no affinity to DNA under these conditions. This high affinity for DNA may explain the association of DNA-binding acetyltransferase with nucleosomes.