Retinoic acid inhibits chondrogenesis of mesenchymal cells by sustaining expression of N-cadherin and its associated proteins

Effect of a retinoic acid analogue on BMP-driven pluripotent stem cell chondrogenesis

Osteoarthritis is the most common degenerative joint condition, leading to articular cartilage (AC) degradation, chronic pain and immobility. The lack of appropriate therapies that provide tissue restoration combined with the limited lifespan of joint-replacement implants indicate the need for alternative AC regeneration strategies. Differentiation of human pluripotent stem cells (hPSCs) into AC progenitors may provide a long-term regenerative solution but is still limited due to the continued reliance upon growth factors to recapitulate developmental signalling processes. Recently, TTNPB, a small molecule activator of retinoic acid receptors (RARs), has been shown to be sufficient to guide mesodermal specification and early chondrogenesis of hPSCs. Here, we modified our previous differentiation protocol, by supplementing cells with TTNPB and administering BMP2 at specific times to enhance early development (referred to as the RAPID-E protocol). Transcriptomic analyses indicated that activation of RAR signalling significantly upregulated genes related to limb and embryonic skeletal development in the early stages of the protocol and upregulated genes related to AC development in later stages. Chondroprogenitors obtained from RAPID-E could generate cartilaginous pellets that expressed AC-related matrix proteins such as Lubricin, Aggrecan, and Collagen II, but additionally expressed Collagen X, indicative of hypertrophy. This protocol could lay the foundations for cell therapy strategies for osteoarthritis and improve the understanding of AC development in humans.

Optogenetic manipulation of BMP signaling to drive chondrogenic differentiation of hPSCs

Optogenetics is a rapidly advancing technology combining photochemical, optical, and synthetic biology to control cellular behavior. Together, sensitive light-responsive optogenetic tools and human pluripotent stem cell differentiation models have the potential to fine-tune differentiation and unpick the processes by which cell specification and tissue patterning are controlled by morphogens. We used an optogenetic bone morphogenetic protein (BMP) signaling system (optoBMP) to drive chondrogenic differentiation of human embryonic stem cells (hESCs). We engineered light-sensitive hESCs through CRISPR-Cas9-mediated integration of the optoBMP system into the AAVS1 locus. The activation of optoBMP with blue light, in lieu of BMP growth factors, resulted in the activation of BMP signaling mechanisms and upregulation of a chondrogenic phenotype, with significant transcriptional differences compared to cells in the dark. Furthermore, cells differentiated with light could form chondrogenic pellets consisting of a hyaline-like cartilaginous matrix. Our findings indicate the applicability of optogenetics for understanding human development and tissue engineering.

Selective Extracellular Matrix Guided Mesenchymal Stem Cell Self-Aggregate Engineering for Replication of Meniscal Zonal Tissue Gradient in a Porcine Meniscectomy Model

Degenerative meniscus tears (DMTs) are prevalent findings in osteoarthritic knees, yet current treatment is mostly limited to arthroscopic partial meniscectomy rather than regeneration, which further exacerbates arthritic changes. Translational research regarding meniscus regeneration is hindered by the complex, composite nature of the meniscus which exhibit a gradient from inner cartilage-like tissue to outer fibrous tissue, as well as engineering hurdles often requiring growth factors and cross-linking agents. Here, a meniscus zonal tissue gradient is proposed using zone-specific decellularized meniscus extracellular matrix (DMECM) and autologous synovial mesenchymal stem cells (SMSC) via self-aggregation without the use of growth factors or cross-linking agents. Combination with zone-specific DMECM during self-aggregation of MSCs forms zone-specific meniscus tissue that reflects the respective DMECM harvest site. The implantation of these constructs leads to the regeneration of meniscus tissue resembling the native meniscus, demonstrating inner cartilaginous and outer fibrous characteristics as well as recovery of native meniscal microarchitecture in a porcine partial meniscectomy model at 6 months. In all, the findings offer a potential regenerative therapy for DMTs that may improve current partial meniscectomy-based patient care.

Developmental principles informing human pluripotent stem cell differentiation to cartilage and bone

Human pluripotent stem cells can differentiate into any cell type given appropriate signals and hence have been used to research early human development of many tissues and diseases. Here, we review the major biological factors that regulate cartilage and bone development through the three main routes of neural crest, lateral plate mesoderm and paraxial mesoderm. We examine how these routes have been used in differentiation protocols that replicate skeletal development using human pluripotent stem cells and how these methods have been refined and improved over time. Finally, we discuss how pluripotent stem cells can be employed to understand human skeletal genetic diseases with a developmental origin and phenotype, and how developmental protocols have been applied to gain a better understanding of these conditions.

Mechanical Regulation of Limb Bud Formation

Early limb bud development has been of considerable interest for the study of embryological development and especially morphogenesis. The focus has long been on biochemical signalling and less on cell biomechanics and mechanobiology. However, their importance cannot be understated since tissue shape changes are ultimately controlled by active forces and bulk tissue rheological properties that in turn depend on cell-cell interactions as well as extracellular matrix composition. Moreover, the feedback between gene regulation and the biomechanical environment is still poorly understood. In recent years, novel experimental techniques and computational models have reinvigorated research on this biomechanical and mechanobiological side of embryological development. In this review, we consider three stages of early limb development, namely: outgrowth, elongation, and condensation. For each of these stages, we summarize basic biological regulation and examine the role of cellular and tissue mechanics in the morphogenetic process.

Cadherin-mediated cell-cell adhesion and signaling in the skeleton

Direct cell-to-cell interactions via cell adhesion molecules, in particular cadherins, are critical for morphogenesis, tissue architecture, and cell sorting and differentiation. Partially overlapping, yet distinct roles of N-cadherin (cadherin-2) and cadherin-11 in the skeletal system have emerged from mouse genetics and in vitro studies. Both cadherins are important for precursor commitment to the osteogenic lineage, and genetic ablation of Cdh2 and Cdh11 results in skeletal growth defects and impaired bone formation. While Cdh11 defines the osteogenic lineage, persistence of Cdh2 in osteoblasts in vivo actually inhibits their terminal differentiation and impairs bone formation. The action of cadherins involves both cell-cell adhesion and interference with intracellular signaling, and in particular the Wnt/β-catenin pathway. Both cadherin-2 and cadherin-11 bind to β-catenin, thus modulating its cytoplasmic pools and transcriptional activity. Recent data demonstrate that cadherin-2 also interferes with Lrp5/6 signaling by sequestering these receptors in inactive pools via axin binding. These data extend the biologic action of cadherins in bone forming cells, and provide novel mechanisms for development of therapeutic strategies aimed at enhancing bone formation.

Development and characterization of Xl1, a Xenopus laevis chondrocyte-like cell culture

We describe the development and characterization of a new cell line, designated Xl1, derived from vertebra and long bones of Xenopus laevis. These cells can mineralize their extracellular matrix upon addition of an inorganic phosphate donor and vitamin C, as characterized by von Kossa staining. In addition they express genes such as matrix gla protein (mgp), alkaline phosphatase, type II collagen, and retinoic acid receptors, representing a valuable tool to analyze expression and regulation of Xenopus cartilage-associated genes. Continuous treatment with retinoic acid (RA) inhibited mineralization, alkaline phosphatase expression and its activity, suggesting that RA is a potential negative regulator of Xl1 cell differentiation. These cells are receptive to efficient transfer of DNA using conventional methods including calcium phosphate, liposome-mediated transfer or electroporation and were found to express basal levels of mgp at least 50-fold higher than the routinely used Xenopus A6 cell line, as seen by transcription assays with the distal X. laevis mgp promoter. Being the first amphibian cell line derived from bone tissue, the Xl1 culture provides an excellent in vitro tool for functional promoter studies, being suitable, among other uses, for identifying promoter elements mediating cartilage-expressed genes as shown here for mgp.

2-Deoxy-D-glucose regulates dedifferentiation through beta-catenin pathway in rabbit articular chondrocytes

2-deoxy-D-glucose (2DG) is known as a synthetic inhibitor of glucose. 2DG regulates various cellular responses including proliferation, apoptosis and differentiation by regulation of glucose metabolism in cancer cells. However, the effects of 2DG in normal cells, including chondrocytes, are not clear yet. We examined the effects of 2DG on dedifferentiation with a focus on the beta-catenin pathway in rabbit articular chondrocytes. The rabbit articular chondrocytes were treated with 5 mM 2DG for the indicated time periods or with various concentrations of 2DG for 24 h, and the expression of type II collagen, c-jun and beta-catenin was determined by Western blot, RT-PCR, immunofluorescence staining and immunohistochemical staining and reduction of sulfated proteoglycan synthesis detected by Alcain blue staining. Luciferase assay using a TCF (T cell factor)/LEF (lymphoid enhancer factor) reporter construct was used to demonstrate the transcriptional activity of beta-catenin. We found that 2DG treatment caused a decrease of type II collagen expression. 2DG induced dedifferentiation was dependent on activation of beta-catenin, as the 2DG stimulated accumulation of beta-catenin, which is characterized by translocation of beta-catenin into the nucleus determined by immunofluorescence staining and luciferase assay. Inhibition of beta-catenin degradation by inhibition of glycogen synthase kinase 3-beta with lithium chloride (LiCl) or inhibition of proteasome with z-Leu-Leu-Leu-CHO (MG132) accelerated the decrease of type II collagen expression in the chondrocytes. 2DG regulated the post-translational level of beta-catenin whereas the transcriptional level of beta-catenin was not altered. These results collectively showed that 2DG regulates dedifferentiation via beta-catenin pathway in rabbit articular chondrocytes.

N-cadherin and cadherin 11 modulate postnatal bone growth and osteoblast differentiation by distinct mechanisms

We have previously shown that targeted expression of a dominant-negative truncated form of N-cadherin (Cdh2) delays acquisition of peak bone mass in mice and retards osteoblast differentiation; whereas deletion of cadherin 11 (Cdh11), another osteoblast cadherin, leads to only modest osteopenia. To determine the specific roles of these two cadherins in the adult skeleton, we generated mice with an osteoblast/osteocyte specific Cdh2 ablation (cKO) and double Cdh2(+/-);Cdh11(-/-) germline mutant mice. Age-dependent osteopenia and smaller diaphyses with decreased bone strength characterize cKO bones. By contrast, Cdh2(+/-);Cdh11(-/-) exhibit severely reduced trabecular bone mass, decreased in vivo bone formation rate, smaller diaphyses and impaired bone strength relative to single Cdh11 null mice. The number of bone marrow immature precursors and osteoprogenitor cells is reduced in both cKO and Cdh2(+/-);Cdh11(-/-) mice, suggesting that N-cadherin is involved in maintenance of the stromal cell precursor pool via the osteoblast. Although Cdh11 is dispensable for postnatal skeletal growth, it favors osteogenesis over adipogenesis. Deletion of either cadherin reduces β-catenin abundance and β-catenin-dependent gene expression, whereas N-cadherin loss disrupts cell-cell adhesion more severely than loss of cadherin 11. Thus, Cdh2 and Cdh11 are crucial regulators of postnatal skeletal growth and bone mass maintenance, serving overlapping, yet distinct, functions in the osteogenic lineage.

Unique modulation of cadherin expression pattern during posterior frontal cranial suture development and closure

Cranial suture development involves coordinated expression of multiple genes and tissue contribution from neural crest cells and paraxial mesoderm for timely sutural morphogenesis. Transcription factors, growth factors, and neural crest determinant genes play critical roles in calvarial growth ensuring normal development of the underlying brain. In vitro studies have implicated cell-cell adhesion molecules as a driving force behind suture closure. We performed cDNA microarray to study differential expression of adhesion molecules during the timing of suture closure in a mouse model where only the posterior frontal (PF) suture closes. Our results indicate increased expression of E-cadherin during the period of PF suture closure. Quantitative RT-PCR analysis of E- and N-cadherin in PF closing suture revealed a biphasic expression of N-cadherin, the first phase coinciding with cellular condensation preceding chondrogenesis followed by a second phase coinciding with E-cadherin co-expression and suture closure. Furthermore, expression analysis of the N-cadherin and E-cadherin transcriptional repressors Wnt7a and Snail indicate a specific temporal regulation of these genes, suggesting their potential role as regulators of both E- and N-cadherin during the PF suture development and closure. Finally, given the in vitro evidence of fibroblast growth factor (FGF)-2 as a potential regulator of E- and N-cadherin we investigated the expression of E-cadherin during PF suture closure in Fgf-2 deficient mice. In contrast to in vitrodata previously reported, E-cadherin expression is normal in these animals, and PF suture closure occurs properly, probably due to potential redundancy of FGF ligands ensuring normal temporal expression of E-cadherin and PF suture closure.

BMP-2-Modulated Chondrogenic Differentiation In Vitro Involves Down-Regulation of Membrane-Bound Beta-Catenin

Bone morphogenetic proteins (BMPs) are important regulators of cellular differentiation and embryonic development. Beta catenin mediated nuclear signaling has been implicated in BMP-2-modulated chondrogenic differentiation in the pluripotential stem cell line C3H10T1/2. However, there is little information on the functional role of beta catenin in BMP-2-modulated differentiation of primary nontransformed mesenchymal cells. Here, we present evidence to show that BMP-2-induced chondrogenic differentiation in high-density primary mesenchymal culture is associated with a significant decrease in membrane-bound beta catenin by 72 hours compared to controls. Nuclear localization of beta catenin is not detectable by immunofluorescence and the TCF-responsive reporter vector TOPFLASH shows only background activity during chondrogenic differentiation. BMP-2-treated cultures show reduced cell-cell adhesion by 72 hours, which correlates with the changes in levels of membrane-bound beta catenin. Up-regulation of membrane-bound beta catenin blocks the effect of BMP-2 on both chondrogenic differentiation and cell-cell adhesiveness. These findings suggest that BMP-2 can modulate the adhesivity of adherens junctions through regulation of membrane bound beta catenin.

Articular chondrocyte culturing for cell-based cartilage repair: needs and perspectives

Articular cartilage displays a limited capacity of self-regeneration after injury. Thus, the biology of this tissue and its cellular components - the chondrocytes - has become the focus of several investigations, driven by tissue engineering and the basic and clinical research fields, aiming to ameliorate the present clinical approaches to cartilage repair. In this work, we present a brief recapitulation of the events that lead to cartilage development during the skeletal embryonal growth. The intrinsic phenotypic plasticity of the mesenchymal precursors and the adult chondrocytes is evaluated, dependent on the cell source, its physiopathological state, and as a function of the donor's age. The phenotypic changes induced by the basic culturing techniques are also taken into account, thus highlighting the phenotypic plasticity of the chondrocyte as the main property which could couple the differentiation process to the repair process. Chondrocyte proliferation and the contemporary maintenance of the chondrogenic differentiation potential are regarded as the two primary goals to be achieved in order to fulfill the quantitative needs of the clinical applications and the qualitative requirements of a properly repaired tissue. In this light, the effects of several growth factors and medium supplements are investigated. Finally, the latest improvements in culturing conditions and their possible clinical applications are presented as well.

Accentuated ovariectomy-induced bone loss and altered osteogenesis in heterozygous N-cadherin null mice

Ovariectomy-induced bone loss is accentuated in mice with germline Cdh2 haploinsufficiency, the result of a decreased osteoblastogenesis in the face of normal osteoclast number. Reduced N-cadherin abundance in these mice decreases cell-cell adhesion and alters signaling pathways important for osteoblast commitment and differentiation, thus providing in vivo evidence that N-cadherin-mediated cell-cell interactions are involved in homeostatic responses to increased bone remodeling.

INTRODUCTION

We have shown that targeted expression of a dominant negative truncated form of N-cadherin (Cdh2) delays acquisition of peak bone mass in mice and retards osteoblast differentiation. We tested the role of this molecule in the skeletal homeostatic response to ovariectomy in mice with germline Cdh2 haploinsufficiency.

MATERIALS AND METHODS

Heterozygous Cdh2 null (Cdh2+/-) and wildtype mice were ovariectomized and followed up to 13 weeks by in vivo radiodensitometric and ex vivo histologic assessment of bone mass and turnover. Cells isolated from wildtype and Cdh2+/- mice were used to determine the alterations in bone cell function produced by partial loss of N-cadherin.

RESULTS

Bone mass was not significantly different between Cdh2+/- and wildtype littermates, but on ovariectomy, bone loss in Cdh2+/- mice was initially slower, but with time it became significantly greater than in wildtype mice. This accentuated bone loss was associated with lower osteoblast number and serum osteocalcin levels, with no differences in bone resorption. Although development of calcified nodules was faster in calvaria cells isolated from Cdh2+/- mice relative to Cdh2+/+ cells, bone marrow osteogenic precursors were lower in the former than in the latter genotypes. Cdh2 expression was downregulated with differentiation in wildtype calvaria cells, whereas cadherin-11 abundance remained unchanged. Furthermore, cell-cell adhesion (postconfluence) was decreased among heterozygous calvaria cells, as was cell proliferation (preconfluence), relative to wildtype cells. Finally, the abundance and cellular distribution of beta-catenin was minimally decreased in Cdh2+/- cells, whereas mitogen-activated protein kinase (MAPK) signaling was more active in Cdh2 insufficient cells.

CONCLUSIONS

Cdh2 is involved in the homeostatic bone formation response to ovariectomy, presumably by regulating osteoprogenitors number and differentiation through stabilization of cell-cell adhesion and/or signaling modulation.

Identification and characterization of a novel gene, Mcpr1, and its possible function in the proliferation of embryonic palatal mesenchymal cells

We cloned a novel mouse cDNA, Mcpr1 (mouse cleft palate-related gene 1), between retinoic acid (RA)-treated murine embryonic palatal and control shelves by improved subtractive hybridization. Its transcript was identified by Northern blotting. The open reading frame encodes 132 amino acids and shows almost no identity to other genetic products. Mcpr1 expression could be detected extensively in adult mouse tissues and during murine embryonic development. It was identified to be significantly stimulated by RA in murine palatal shelves at embryonic day 12 and in palatal mesenchymal cells in vitro. We demonstrate that MCPR1 protein was localized primarily in the cytoplasm and could be synthesized and secreted by transfected COS-7 cells. Both the secretory and recombinant proteins of Mcpr1 inhibited proliferation of murine embryonic palatal mesenchymal cells and impeded the progression from the G1 to S phase in the cell cycle. The cells were prone to apoptosis after exposure to glutathione S-transferase-MCPR1. Furthermore, knockdown of MCPR1 protein levels by antisense oligodeoxynucleotides promoted progression of cells from the G1 to S phase and completely abolished the RA-induced block of the cell cycle from the G1 to S phase. These findings suggest that Mcpr1 might function as one of the RA-up-regulated genes involved in inhibiting cell proliferation during palatogenesis and RA-induced cleft palate by regulating proliferation and apoptosis of embryonic palatal mesenchymal cells and might even play a role in the development of many other organs.

Novel retinoid targets in the mouse limb during organogenesis

Bioactive retinoids are potent limb teratogens, upregulating apoptosis, decreasing chondrogenesis, and producing limb-reduction defects. To target the origins of these effects, we examined gene expression changes in the developing murine limb after 3 h of culture with teratogenic concentrations of vitamin A. Embryonic day 12 CD-1 limbs were cultured in the absence or presence of vitamin A (retinol acetate) at 1.25 and 62.5muM (n = 5). Total RNA was used to probe Atlas 1.2 cDNA arrays. Eighty-one genes were significantly upregulated by retinol exposure; among these were key limb development signaling molecules, extracellular matrix and adhesion proteins, oncogenes, and a large number of transcriptional regulators, including Eya2, Id3, Snail, and Hes1. To relate these expression changes to teratogenic outcome, the response of these four genes was assessed after culture with vitamin A and retinoid receptor antagonists that are able to rescue retinoid-induced malformations; expression levels were correlated with limb malformations. Lastly, pathways analysis revealed that a large number of the genes significantly affected by retinoid treatment are functionally linked through direct interactions. Several regulatory gene cascades emerged relevant to morphogenesis, cell-fate, and chondrogenesis; moreover, members of these cascades crosstalk with one other. These results indicate that retinoids act in a coordinated fashion to disrupt development at multiple levels. In sum, this work proposes several unifying mechanisms for retinoid-induced limb malformations, identifies novel retinoid targets, and highlights Eya2, Id3, Snail, and Hes1 as potential key teratogenic effectors.

Interactive effects of cadmium and all-trans-retinoic acid on the induction of forelimb ectrodactyly in C57BL/6 mice

BACKGROUND

Most toxicological studies have tested single chemical agents at relatively high doses, and fewer studies have addressed the toxic effects of chemical interactions. It is important to understand the toxicity of chemical mixtures in order to assess the more realistic risks of environmental and occupational exposures. A number of chemicals are known to induce a predominantly postaxial forelimb ectrodactyly in C57BL/6 mice, including acetazolamide, ethanol, cadmium, valproic acid, carbon dioxide, dimethadione, phenytoin, and 13-cis-retinoic acid and all-trans-retinoic acid (RA). In the present study, the interactive effects of coadministration of cadmium and RA on developing limbs were investigated.

METHODS

Pregnant C57BL/6 mice were treated with different intraperitoneal (IP) doses of cadmium chloride (CdCl2) and/or RA on gestational day (GD) 9.5, and fetuses were collected on GD 18 and double stained for examination of skeletal defects.

RESULTS

When RA was given simultaneously with cadmium, a significant increase in the incidence and severity of forelimb ectrodactyly (predominantly postaxial) was observed compared to the results with corresponding doses of cadmium or RA alone. When mice were exposed to subthreshold doses of both cadmium (0.5 mg/kg) and RA (1 mg/kg), the combined treatment exceeded the threshold, resulting in forelimb ectrodactyly in 19% of the fetuses. Moreover, coadministration of cadmium and RA at doses exceeding the respective thresholds showed a synergistic effect, that is, 92% of fetuses were found with the forelimb defect as opposed to 10% if the response were additive.

CONCLUSIONS

The findings demonstrate that concurrent exposure to these teratogens can have a synergistic effect and that subteratogenic doses may combine to exceed a threshold.

All-trans retinoic acid inhibited chondrogenesis of mouse embryonic palate mesenchymal cells by down-regulation of TGF-beta/Smad signaling

Chondrogenesis is a critical step in palatogenesis. All-trans retinoic acid (atRA), a vitamin A derivative, is a known teratogenic effector of cleft palate. Here, we evaluated the effects of atRA on the osteo-/chondrogenic differentiation of mouse embryonic palate mesenchymal (MEPM) cells. MEPM cells, in a high-density micromass environment, undergo active chondrogenesis in a manner analogous to that of limb-derived mesenchymal cells, and served as a valid model system to investigate the mechanisms regulating chondrogenesis during palatogenesis. atRA-treated MEPM micromass expressed relatively higher levels of osteoblastic gene markers (alkaline phosphatase and collagen type I) and lower levels of chondrocytic gene markers (collagen type II and aggrecan). As transforming growth factor-beta3 (TGF-beta3) is an essential growth factor for chondrogenesis of embryonic mesenchymal cells both in in vivo and in vitro conditions, we thereby explored the effects of atRA on TGF-beta3 signaling pathway. atRA led to an increase in mRNA expression of TGF-beta3 and an instantaneous decrease in TGF-beta type II receptor (TbetaRII) as determined by real-time RT-PCR. Further study showed that atRA inhibited phosphorylation of Smad2 and Smad3 and increased Smad7 expression. Activation of the Smad pathways by transfection with Smad7deltaC mutant or constitutively active TbetaRII retroviral vector abolished atRA-induced inhibition of chondrogenesis as indicated by Alcian blue staining, indicating that Smad signaling is essential for this response. Taken together, these data for the first time demonstrated a role for RA-induced hypochondrogenesis through regulation of the TGF-beta3 pathway and suggested a role for TbetaRII /Smad in retinoid-induced cleft palate.

Interaction of nuclear receptors with the Wnt/beta-catenin/Tcf signaling axis: Wnt you like to know?

The cross-regulation of Wnt/beta-catenin/Tcf ligands, kinases, and transcription factors with members of the nuclear receptor (NR) family has emerged as a clinically and developmentally important area of endocrine cell biology. Interactions between these signaling pathways result in a diverse array of cellular effects including altered cellular adhesion, tissue morphogenesis, and oncogenesis. Analyses of NR interactions with canonical Wnt signaling reveal two broad themes: Wnt/beta-catenin modulation of NRs (theme I), and ligand-dependent NR inhibition of the Wnt/beta-catenin/Tcf cascade (theme II). Beta-catenin, a promiscuous Wnt signaling member, has been studied intensively in relation to the androgen receptor (AR). Beta-catenin acts as a coactivator of AR transcription and is also involved in co-trafficking, increasing cell proliferation, and prostate pathogenesis. T cell factor, a transcriptional mediator of beta-catenin and AR, engages in a dynamic reciprocity of nuclear beta-catenin, p300/CREB binding protein, and transcriptional initiation factor 2/GC receptor-interaction protein, thereby facilitating hormone-dependent coactivation and transrepression. Beta-catenin responds in an equally dynamic manner with other NRs, including the retinoic acid (RA) receptor (RAR), vitamin D receptor (VDR), glucocorticoid receptor (GR), progesterone receptor, thyroid receptor (TR), estrogen receptor (ER), and peroxisome proliferator-activated receptor (PPAR). The NR ligands, vitamin D(3), trans/cis RA, glucocorticoids, and thiazolidines, induce dramatic changes in the physiology of cells harboring high Wnt/beta-catenin/Tcf activity. Wnt signaling regulates, directly or indirectly, developmental processes such as ductal branching and adipogenesis, two processes dependent on NR function. Beta-catenin has been intensively studied in colorectal cancer; however, it is now evident that beta-catenin may be important in cancers of the breast, prostate, and thyroid. This review will focus on the cross-regulation of AR and Wnt/beta-catenin/Tcf but will also consider the dynamic manner in which RAR/RXR, GR, TR, VDR, ER, and PPAR modulate canonical Wnt signaling. Although many commonalities exist by which NRs interact with the Wnt/beta-catenin signaling pathway, striking cell line and tissue-specific differences require deciphering and application to endocrine pathology.

Cell-cell interactions in regulating osteogenesis and osteoblast function

Endochondral bone formation requires an elaborate interplay among autocrine, paracrine, and endocrine signals, positional cues, and cell-cell contacts to mediate the complex three-dimensional architecture and function of the skeleton. Embryonic bone development occurs by migration, aggregation, and condensation of immature mesenchymal progenitor cells to form the cartilaginous anlage. Upon vascular invasion, the cartilaginous scaffold is colonized and subsequently mineralized by osteoblasts. Likewise, bone remodeling in the adult skeleton is a dynamic process that requires coordinated cellular activities among osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. This review examines the role of cell-cell interactions mediated by adherens junctions formed by cadherins and communicative gap junctions formed by connexins in regulating bone development and osteogenic function.

Vitamin A up-regulates the expression of thrombospondin-1 and pigment epithelium-derived factor in retinal pigment epithelial cells

Vitamin A is essential for the visual system. It is metabolized in the retina and the resulting product, retinoic acid (RA), greatly affects the structure and functions of retinal pigment epithelial (RPE) cells. RPE cells produce a variety of extracellular matrix (ECM) proteins and angiogenic factors, both of which are expressed at varying levels in the normal RPE layer. In this study, we investigated the effect of all-trans-retinoic acid on the production of an ECM protein, thrombospondin-1 (TSP-1), and two angiogenic factors, pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor (VEGF) by RPE cells. RA increased the release of TSP-1 and PEDF, but not that of VEGF, from human RPE cells in vitro. In vitamin A-deficient mice, the expression of TSP-1 and PEDF in the RPE layer considerably decreased compared with that of normal control mice. The vitamin A deficiency hardly affected the accumulation of VEGF in the RPE layer. These findings suggest that vitamin A modulates the structure and anti-angiogenic functions of the RPE layer partly by up-regulating the expression of the angiogenesis-related ECM protein, TSP-1, and the anti-angiogenic factor, PEDF.