p107-Dependent recruitment of SWI/SNF to the alkaline phosphatase promoter during osteoblast differentiation

Cell cycle regulators and bone: development and regeneration

Cell cycle regulators act as inhibitors or activators to prevent cancerogenesis. It has also been established that they can play an active role in differentiation, apoptosis, senescence, and other cell processes. Emerging evidence has demonstrated a role for cell cycle regulators in bone healing/development cascade. We demonstrated that deletion of p21, a cell cycle regulator acting at the G1/S transition enhanced bone repair capacity after a burr-hole injury in the proximal tibia of mice. Similarly, another study has shown that inhibition of p27 can increase bone mineral density and bone formation. Here, we provide a concise review of cell cycle regulators that influence cells like osteoblasts, osteoclasts, and chondrocytes, during development and/or healing of bone. It is imperative to understand the regulatory processes that govern cell cycle during bone healing and development as this will pave the way to develop novel therapies to improve bone healing after injury in instances of aged or osteoporotic fractures.

Tissue-Nonspecific Alkaline Phosphatase, a Possible Mediator of Cell Maturation: Towards a New Paradigm

Alkaline phosphatase (ALP) is a ubiquitous membrane-bound glycoprotein capable of providing inorganic phosphate by catalyzing the hydrolysis of organic phosphate esters, or removing inorganic pyrophosphate that inhibits calcification. In humans, four forms of ALP cDNA have been cloned, among which tissue-nonspecific ALP (TNSALP) (TNSALP) is widely distributed in the liver, bone, and kidney, making it an important marker in clinical and basic research. Interestingly, TNSALP is highly expressed in juvenile cells, such as pluripotent stem cells (i.e., embryonic stem cells and induced pluripotent stem cells (iPSCs)) and somatic stem cells (i.e., neuronal stem cells and bone marrow mesenchymal stem cells). Hypophosphatasia is a genetic disorder causing defects in bone and tooth development as well as neurogenesis. Mutations in the gene coding for TNSALP are thought to be responsible for the abnormalities, suggesting the essential role of TNSALP in these events. Moreover, a reverse-genetics-based study using mice revealed that TNSALP is important in bone and tooth development as well as neurogenesis. However, little is known about the role of TNSALP in the maintenance and differentiation of juvenile cells. Recently, it was reported that cells enriched with TNSALP are more easily reprogrammed into iPSCs than those with less TNSALP. Furthermore, in bone marrow stem cells, ALP could function as a "signal regulator" deciding the fate of these cells. In this review, we summarize the properties of ALP and the background of ALP gene analysis and its manipulation, with a special focus on the potential role of TNSALP in the generation (and possibly maintenance) of juvenile cells.

RBL1/p107 Expression Levels Are Modulated by Multiple Signaling Pathways

The members of the retinoblastoma (RB) protein family, RB1/p105, retinoblastoma-like (RBL)1/p107 and RBL2/p130 are critical modulators of the cell cycle and their dysregulation has been associated with tumor initiation and progression. The activity of RB proteins is regulated by numerous pathways including oncogenic signaling, but the molecular mechanisms of these functional interactions are not fully defined. We previously demonstrated that RBL2/p130 is a direct target of AKT and it is a key mediator of the apoptotic process induced by AKT inhibition. Here we demonstrated that RBL1/p107 levels are only minorly modulated by the AKT signaling pathway. In contrast, we discovered that RBL1/p107 levels are regulated by multiple pathways linked directly or indirectly to Ca²⁺-dependent signaling. Inhibition of the multifunctional calcium/calmodulin-dependent kinases (CaMKs) significantly reduced RBL1/p107 expression levels and phosphorylation, increased RBL1/p107 nuclear localization and led to cell cycle arrest in G0/G1. Targeting the Ca²⁺-dependent endopeptidase calpain stabilized RBL1/p107 levels and counteracted the reduction of RBL1/p107 levels associated with CaMKs inhibition. Thus, these novel observations suggest a complex regulation of RBL1/p107 expression involving different components of signaling pathways controlled by Ca²⁺ levels, including CaMKs and calpain, pointing out a significant difference with the mechanisms modulating the close family member RBL2/p130.

Phosvitin Derived Phospho-Peptides Show Better Osteogenic Potential than Intact Phosvitin in MC3T3-E1 Osteoblastic Cells

Phosphorylated proteins from food sources have been investigated as regulators of bone formation with potential benefits in treating osteoporosis. Egg, a cheap and nutritious food, is also the source of various proteins and bioactive peptides with applications in human health. Egg yolk is rich in phosvitin, the most phosphorylated protein in nature. Phosvitin has been shown to improve bone health in experimental animals, although the molecular mechanisms and its specific effects on bone-forming osteoblastic cells are incompletely understood. Previous work in our group has identified pancreatin-generated phosvitin phospho-peptides (PPP) as a potential source for bioactive peptides. Given this background, we examined the roles of both phosvitin and PPP in the function of osteoblastic cells. Our results demonstrated their potential to improve bone health by promoting osteoblast differentiation and proliferation, suppressing osteoclast recruitment and the deposition of extracellular matrix, although PPP appeared to demonstrate superior osteogenic functions compared to phosvitin alone.

ER stress regulates alkaline phosphatase gene expression in vascular smooth muscle cells via an ATF4-dependent mechanism

OBJECTIVE

Vascular calcification is the deposition of hydroxyapatite crystals in the blood vessel wall. Osteogenic differentiation of vascular smooth muscle cells (VSMCs) plays a key role in this process. Increased expression of alkaline phosphatase (ALP) occurs in some in vitro models of VSMC calcification and is thought to be crucial for mineralization, however, little is known about the transcriptional regulation of ALP in VSMCs. Recently, ALP upregulation was shown to coincide with endoplasmic reticulum (ER) stress-mediated vascular calcification, specifically with expression of the transcription factor ATF4. As no direct links between ALP expression and ER stress have previously been demonstrated in VSMCs, the aim of this study was to investigate whether ATF4 interacts directly with the ALP promoter.

RESULTS

The present study shows that ALP mRNA and activity were significantly increased by ER stress treatment of human primary VSMCs in vitro and that this was ATF4-dependent. Bioinformatics analysis predicted two ATF4 binding sites in ER-stress responsive regions of the ALP promoter (- 3631 to - 2048 bp from the first intron). However, we found that ATF4 does not bind within this fragment of the ALP promoter region.

Glucocorticoid receptor-mediated cis-repression of osteogenic genes requires BRM-SWI/SNF

Glucocorticoids are an effective therapy for a variety of severe inflammatory and autoimmune disorders; however, the therapeutic use of glucocorticoids is severely limited by their negative side effects, particularly on osteogenesis. Glucocorticoids regulate transcription by binding to the glucocorticoid receptor (GR), which then binds the promoters of target genes to induce either activation or repression. The gene activation effects of nuclear hormone receptors broadly require the cooperation of the chromatin remodeling complex known as SWI/SNF, which is powered by an ATPase core. The well-studied SWI/SNF ATPase, BRG1, is required for gene activation by a spectrum of nuclear hormone receptors including GR. However, glucocorticoid-induced side effects specifically related to impaired osteogenesis are mostly linked with GR-mediated repression. We have considered whether cis-repression of osteogenic genes by GR may be mediated by a distinct subclass of SWI/SNF powered by the alternative ATPase, BRM. BRM does not have an essential role in mammalian development, but plays a repressor role in osteoblast differentiation and favors adipogenic lineage selection over osteoblast commitment, effects that mirror the repressor effects of GR. The studies reported here examine three key GR cis-repression gene targets, and show that GR association with these promoters is sharply reduced in BRM deficient cells. Each of these GR-targeted genes act in a different way. Bglap encodes osteocalcin, which contributes to normal maturation of osteoblasts from committed pre-osteoblasts. The Per3 gene product acts in uncommitted mesenchymal stem cells to influence the osteoblast/adipocyte lineage selection point. Fas ligand, encoded by FasL, is a means by which osteoblasts can modulate bone degradation by osteoclasts. Repression of each of these genes by glucocorticoid favors bone loss. The essential role of BRM in cooperation with GR at each of these control points offers a novel mechanistic understanding of the role of GR in bone loss.

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Therapeutic use of glucocorticoids is limited by their negative effects on osteogenesis.

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The glucocorticoid receptor (GR) can either activate or repress gene expression.

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cis-Repression by GR drives a significant part of the negative osteogenic effects.

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The BRM-containing subclass of SWI/SNF is specifically required for cis-repression by GR.

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BRM depletion reduces negative effects of glucocorticoids on osteogenic gene expression.