Nuclear structure--skeletal gene expression interrelationships

Dexamethasone for Severe COVID-19: How Does It Work at Cellular and Molecular Levels?

The coronavirus disease 2019 (COVID-19) caused by infection of the severe respiratory syndrome coronavirus-2 (SARS-CoV-2) significantly impacted human society. Recently, the synthetic pure glucocorticoid dexamethasone was identified as an effective compound for treatment of severe COVID-19. However, glucocorticoids are generally harmful for infectious diseases, such as bacterial sepsis and severe influenza pneumonia, which can develop respiratory failure and systemic inflammation similar to COVID-19. This apparent inconsistency suggests the presence of pathologic mechanism(s) unique to COVID-19 that renders this steroid effective. We review plausible mechanisms and advance the hypothesis that SARS-CoV-2 infection is accompanied by infected cell-specific glucocorticoid insensitivity as reported for some other viruses. This alteration in local glucocorticoid actions interferes with undesired glucocorticoid to facilitate viral replication but does not affect desired anti-inflammatory properties in non-infected organs/tissues. We postulate that the virus coincidentally causes glucocorticoid insensitivity in the process of modulating host cell activities for promoting its replication in infected cells. We explore this tenet focusing on SARS-CoV-2-encoding proteins and potential molecular mechanisms supporting this hypothetical glucocorticoid insensitivity unique to COVID-19 but not characteristic of other life-threatening viral diseases, probably due to a difference in specific virally-encoded molecules and host cell activities modulated by them.

Expression profiling of Dexamethasone-treated primary chondrocytes identifies targets of glucocorticoid signalling in endochondral bone development

Background

Glucocorticoids (GCs) are widely used anti-inflammatory drugs. While useful in clinical practice, patients taking GCs often suffer from skeletal side effects including growth retardation in children and adolescents, and decreased bone quality in adults. On a physiological level, GCs have been implicated in the regulation of chondrogenesis and osteoblast differentiation, as well as maintaining homeostasis in cartilage and bone. We identified the glucocorticoid receptor (GR) as a potential regulator of chondrocyte hypertrophy in a microarray screen of primary limb bud mesenchyme micromass cultures. Some targets of GC regulation in chondrogenesis are known, but the global effects of pharmacological GC doses on chondrocyte gene expression have not been comprehensively evaluated.

Results

This study systematically identifies a spectrum of GC target genes in embryonic growth plate chondrocytes treated with a synthetic GR agonist, dexamethasone (DEX), at 6 and 24 hrs. Conventional analysis of this data set and gene set enrichment analysis (GSEA) was performed. Transcripts associated with metabolism were enriched in the DEX condition along with extracellular matrix genes. In contrast, a subset of growth factors and cytokines were negatively correlated with DEX treatment. Comparing DEX-induced gene expression data to developmental changes in gene expression in micromass cultures revealed an additional layer of complexity in which DEX maintains the expression of certain chondrocyte marker genes while inhibiting factors that promote vascularization and ultimately ossification of the cartilaginous template.

Conclusion

Together, these results provide insight into the mechanisms and major molecular classes functioning downstream of DEX in primary chondrocytes. In addition, comparison of our data with microarray studies of DEX treatment in other cell types demonstrated that the majority of DEX effects are tissue-specific. This study provides novel insights into the effects of pharmacological GC on chondrocyte gene transcription and establishes the foundation for subsequent functional studies.

Smad3 Interacts with JunB and Cbfa1/Runx2 for Transforming Growth Factor-β1-stimulated Collagenase-3 Expression in Human Breast Cancer Cells

We have previously shown that transforming growth factor (TGF)-beta1, a crucial molecule in metastatic bone cancer, stimulates collagenase-3 expression in the human breast cancer cell line, MDA-MB231. To understand the molecular mechanisms responsible for TGF-beta1 response on collagenase-3 promoter activity, a functional analysis of the promoter region of the collagenase-3 gene was carried out, and we identified the distal runt domain (RD) and proximal RD/activator protein-1 (AP-1) sites as necessary for full TGF-beta1-stimulated collagenase-3 promoter activity. Gel shift, real time reverse transcriptase-PCR, and Western blot analyses showed increased levels of c-Jun, JunB, and Cbfa1/Runx2 upon TGF-beta1 treatment in MDA-MB231 cells. Co-immunoprecipitation in vitro studies identified no physical interaction between JunB and Cbfa1/Runx2, whereas Smad3 interacted with both. Chromatin immunoprecipitation experiments confirmed interaction of Smad3 with JunB and Cbfa1/Runx2. Under basal conditions, Cbfa1/Runx2 bound to both the proximal RD/AP-1 and distal RD sites. In response to TGF-beta1, Cbfa1/Runx2 was seen only at the distal RD site, whereas JunB occupied the proximal RD/AP-1 site. An assemblage of Smad3, JunB, and Cbfa1/Runx2 at the distal RD site of the collagenase-3 promoter occurred in response to TGF-beta1 in MDA-MB231 cells. Co-transfection of Smad3, JunB, and Cbfa1/Runx2 constructs along with a constitutively active TGF-beta type I receptor construct identified functional interaction of these proteins and transcriptional activation of the collagenase-3 gene by TGF-beta1. Taken together, our results suggest that TGF-beta1 stimulated JunB and Cbfa1/Runx2 to bind to their respective DNA consensus sites and that Smad3 is likely to stabilize their interaction to confer functional TGF-beta1-stimulation of collagenase-3 expression in MDA-MB231 cells.

Activation of osteocalcin transcription involves interaction of protein kinase A- and protein kinase C-dependent pathways

Osteocalcin is a major noncollagenous protein component of bone extracellular matrix, synthesized and secreted exclusively by osteoblastic cells in the late stage of maturation, and is considered indicator of osteoblast differentiation. Osteocalcin expression is modulated by parathyroid hormone (PTH) and a variety of other factors. The cAMP-dependent protein kinase pathway has been shown previously to have an essential role in PTH signaling and regulation of osteocalcin expression. To determine the extent to which other pathways may also participate in osteocalcin expression, we used rat and human osteoblast-like cell lines to generate stably transfected clones in which the osteocalcin promoter was fused to a luciferase reporter gene. These clones were examined for their responsiveness to agents known to activate or interfere with protein kinase A (PKA)- and protein kinase C (PKC)-dependent pathways. We have found that forskolin, cAMP, and PTH, as well as insulin-like growth factor I (IGF-I) and basic fibroblast growth factor, all were effective in activating the osteocalcin promoter. Phorbol 12-myristate 13-acetate (PMA) was also a strong inducer of the promoter, indicating that PKC plays a role in expression of osteocalcin. In combination with PTH or forskolin, the effect of PMA was additive to synergistic. Calphostin C, a selective inhibitor of PKC, decreased the PMA-, PTH-, and IGF-I-induced luciferase activity in a dose-dependent manner; a PKA inhibitor, H-89, also blocked the induction by PTH and IGF-I but not by PMA. We conclude that regulation of osteocalcin transcription is mediated by both PKA-dependent and PKC-dependent mechanisms and that the respective kinases reside on a linear or convergent pathway.

Differential expression and activity of tissue-nonspecific alkaline phosphatase (TNAP) in rat odontogenic cells in vivo

Among the four existing isoforms of alkaline phosphatase (AP), the present study is devoted to tissue-nonspecific alkaline phosphatase (TNAP) in mineralized dental tissues. Northern blot analysis and measurements of phosphohydrolase activity on microdissected epithelium and ectomesenchyme, in situ hybridization, and immunolabeling on incisors confirmed that the AP active in rodent teeth is TNAP. Whereas the developmental pattern of TNAP mRNA and protein and the previously described activity were similar in supra-ameloblastic and mesenchymal cells, they differed in enamel-secreting cells, the ameloblasts. As previously shown for other proteins involved in calcium and phosphate handling in ameloblasts, a biphasic pattern of steady-state TNAP mRNA levels was associated with additional variations in ameloblast TNAP protein levels during the cyclic modulation process. Although the association of TNAP upregulation and the initial phase of biomineralization appeared to be a basic feature of all mineralized tissues, ameloblasts (and to a lesser extent, odontoblasts) showed a second selectively prominent upregulation of TNAP mRNA/protein/activity during terminal growth of large enamel crystals only, i.e., the maturation stage. This differential expression/activity for TNAP in teeth vs bone may explain the striking dental phenotype vs bone reported in hypophosphatasia, a hereditary disorder related to TNAP mutation. (J Histochem Cytochem 47:1541-1552, 1999)

Crystal structure of the nuclear matrix targeting signal of the transcription factor acute myelogenous leukemia-1/polyoma enhancer-binding protein 2alphaB/core binding factor alpha2

Transcription factors of the acute myelogenous leukemia (AML)/polyoma enhancer-binding protein (PEBP2alpha)/core-binding factor alpha (CBFA) class are key transactivators of tissue-specific genes of the hematopoietic and bone lineages. AML-1/PEBP2alphaB/CBFA2 proteins participating in transcription are associated with the nuclear matrix. This association is solely dependent on a highly conserved C-terminal protein segment, designated the nuclear matrix targeting signal (NMTS). The NMTS of AML-1 is physically distinct from the nuclear localization signal, operates autonomously, and supports transactivation. Our data indicate that the related AML-3 and AML-2 proteins are also targeted to the nuclear matrix in situ by analogous C-terminal domains. Here we report the first crystal structure of an NMTS in an AML-1 segment fused to glutathione S-transferase. The model of the NMTS consists of two loops connected by a flexible U-shaped peptide chain.