Mapk7 deletion in chondrocytes causes vertebral defects by reducing MEF2C/PTEN/AKT signaling

Mutation of the MAPK7 gene was related to human scoliosis. Mapk7 regulated the development of limb bones and skulls in mice. However, the role of MAPK7 in vertebral development is still unclear. In this study, we constructed Col2a1-cre; Mapk7f/f transgenic mouse model to delete Mapk7 in cartilage, which displayed kyphosis and osteopenia. Mechanistically, Mapk7 loss decreased MEF2C expression and thus activated PTEN to oppose PI3K/AKT signaling in vertebral growth plate chondrocytes, which impaired chondrocyte hypertrophy and attenuated vertebral ossification. In vivo, systemic pharmacological activation of AKT rescued impaired chondrocyte hypertrophy and alleviated mouse vertebral defects caused by Mapk7 deficiency. Our study firstly clarified the mechanism by which MAPK7 was involved in vertebral development, which might contribute to understanding the pathology of spinal deformity and provide a basis for the treatment of developmental disorders of the spine.

Pten knockout in mouse preosteoblasts leads to changes in bone turnover and strength

Bone development and remodeling are controlled by the phosphoinositide-3-kinase (Pi3k) signaling pathway. We investigated the effects of downregulation of phosphatase and tensin homolog (Pten), a negative regulator of Pi3k signaling, in a mouse model of Pten deficiency in preosteoblasts. We aimed to identify mechanisms that are involved in the regulation of bone turnover and are linked to bone disorders. Femora, tibiae, and bone marrow stromal cells (BMSCs) isolated from mice with a conditional deletion of Pten (Pten cKO) in Osterix/Sp7-expressing osteoprogenitor cells were compared to Cre-negative controls. Bone phenotyping was performed by μCT measurements, bone histomorphometry, quantification of bone turnover markers CTX and procollagen type 1 N propeptide (P1NP), and three-point bending test. Proliferation of BMSCs was measured by counting nuclei and Ki-67-stained cells. In vitro, osteogenic differentiation capacity was determined by ALP staining, as well as by detecting gene expression of osteogenic markers. BMSCs from Pten cKO mice were functionally different from control BMSCs. Osteogenic markers were increased in BMSCs derived from Pten cKO mice, while Pten protein expression was lower and Akt phosphorylation was increased. We detected a higher trabecular bone volume and an altered cortical bone morphology in Pten cKO bones with a progressive decrease in bone and tissue mineral density. Pten cKO bones displayed fewer osteoclasts and more osteoblasts (P = .00095) per trabecular bone surface and a higher trabecular bone formation rate. Biomechanical analysis revealed a significantly higher bone strength (P = .00012 for males) and elasticity of Pten cKO femora. On the cellular level, both proliferation and osteogenic differentiation capacity of Pten cKO BMSCs were significantly increased compared to controls. Our findings suggest that Pten knockout in osteoprogenitor cells increases bone stability and elasticity by increasing trabecular bone mass and leads to increased proliferation and osteogenic differentiation of BMSCs.

HRD1 in human malignant neoplasms: Molecular mechanisms and novel therapeutic strategy for cancer

In tumor cells, the endoplasmic reticulum (ER) plays an essential role in maintaining cellular proteostasis by stimulating unfolded protein response (UPR) underlying stress conditions. ER-associated degradation (ERAD) is a critical pathway of the UPR to protect cells from ER stress-induced apoptosis and the elimination of unfolded or misfolded proteins by the ubiquitin-proteasome system (UPS). 3-Hydroxy-3-methylglutaryl reductase degradation (HRD1) as an E3 ubiquitin ligase plays an essential role in the ubiquitination and dislocation of misfolded protein in ERAD. In addition, HRD1 can target other normal folded proteins. In various types of cancer, the expression of HRD1 is dysregulated, and it targets different molecules to develop cancer hallmarks or suppress the progression of the disease. Recent investigations have defined the role of HRD1 in drug resistance in types of cancer. This review focuses on the molecular mechanisms of HRD1 and its roles in cancer pathogenesis and discusses the worthiness of targeting HRD1 as a novel therapeutic strategy in cancer.

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

Enchondromatosis and Growth Plate Development

PURPOSE OF REVIEW

Enchondroma is a common cartilage benign tumor that develops from dysregulation of chondrocyte terminal differentiation during growth plate development. Here we provide an overview of recent progress in understanding causative mutations for enchondroma, dysregulated signaling and metabolic pathways in enchondroma, and the progression from enchondroma to malignant chondrosarcoma.

RECENT FINDINGS

Several signaling pathways that regulate chondrocyte differentiation are dysregulated in enchondromas. Somatic mutations in the metabolic enzymes isocitrate dehydrogenase 1 and 2 (IDH1/2) are the most common findings in enchondromas. Mechanisms including metabolic regulation, epigenetic regulation, and altered signaling pathways play a role in enchondroma formation and progression. Multiple pathways regulate growth plate development in a coordinated manner. Deregulation of the process can result in chondrocytes failing to undergo differentiation and the development of enchondroma.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Conditional ablation of MAPK7 expression in chondrocytes impairs endochondral bone formation in limbs and adaptation of chondrocytes to hypoxia

Background

Long bones of limbs are formed through endochondral bone formation, which depends on the coordinated development of growth plates. Our previous studies have demonstrated that dysfunction of mitogen-activated protein kinase 7 (MAPK7) can cause skeletal dysplasia. However, little is known about the role of MAPK7 in the regulation of proliferation and differentiation of chondrocytes during growth plate development.

Results

Ablation of MAPK7 expression in chondrocytes led to growth restriction, short limbs and bone mass loss in postnatal mice. Histological studies revealed that MAPK7 deficiency increased the apoptosis and decreased the proliferation of chondrocytes in the center of the proliferative layer, where the most highly hypoxic chondrocytes are located. Accordingly, hypertrophic differentiation markers were downregulated in the central hypertrophic layer, beneath the site where abnormal apoptosis was observed. Simultaneously, we demonstrated that hypoxic adaptation and hypoxia-induced activation of hypoxia-inducible factor 1 subunit α (HIF1α) were impaired when MAPK7 could not be activated normally in primary chondrocytes. Concomitantly, vascular invasion into epiphyseal cartilage was inhibited when Mapk7 was deleted.

Conclusions

We demonstrated that MAPK7 is necessary for maintaining proliferation, survival, and differentiation of chondrocytes during postnatal growth plate development, possibly through modulating HIF1α signaling for adaptation to hypoxia. These results indicate that MAPK7 signaling might be a target for treatment of chondrodysplasia.

Bone tissue and mineral metabolism in hereditary endocrine tumors: clinical manifestations and genetic bases

Inherited endocrine tumors are neoplasms of endocrine cells, transmitted via autosomal dominant germinal mutations. They present in two different forms: non-syndromic (patient has a single affected endocrine organ during his/her lifetime) or syndromic forms (multiple tumors in endocrine and non-endocrine organs during his/her lifetime).In addition to their common tumoral manifestations, many of these diseases present clinical affection of bone tissues and/or mineral metabolism, both as secondary complications of primary tumors and as primary defects due to genetic mutation. To date, few studies have documented these bone complications, and there are no systematic reviews in this area.We present a revision of medical literature about skeletal and mineral metabolism affections in inherited endocrine tumor syndromes, and studies, in cells and animal models, investigating the direct role of some genes, whose mutations are responsible for the development of endocrine tumors, in the regulation of bone and mineral metabolism.

Sustained Akt signaling in articular chondrocytes causes osteoarthritis via oxidative stress-induced senescence in mice

Osteoarthritis (OA) is an age-related disorder that is strongly associated with chondrocyte senescence. The causal link between disruptive PTEN/Akt signaling and chondrocyte senescence and the underlying mechanism are unclear. In this study, we found activated Akt signaling in human OA cartilage as well as in a mouse OA model with surgical destabilization of the medial meniscus. Genetic mouse models mimicking sustained Akt signaling in articular chondrocytes via PTEN deficiency driven by either Col2a1-Cre or Col2a1-Cre^ERT2 developed OA, whereas restriction of Akt signaling reversed the OA phenotypes in PTEN-deficient mice. Mechanistically, prolonged activation of Akt signaling caused an accumulation of reactive oxygen species and triggered chondrocyte senescence as well as a senescence-associated secretory phenotype, whereas chronic administration of the antioxidant N-acetylcysteine suppressed chondrocyte senescence and mitigated OA progression in PTEN-deficient mice. Therefore, inhibition of Akt signaling by PTEN is required for the maintenance of articular cartilage. Disrupted Akt signaling in articular chondrocytes triggers oxidative stress-induced chondrocyte senescence and causes OA.

Inhibition of microRNA-103 attenuates inflammation and endoplasmic reticulum stress in atherosclerosis through disrupting the PTEN-mediated MAPK signaling

Atherosclerosis (AS), a chronic disorder of large arteries, is the underlying pathological process of heart disease and stroke. Former researchers have found that microRNAs (miRs) are involved in the several key processes of AS. Apolipoprotein E knockout (ApoE^-/- ) mice fed a high-fat-diet (HFD) to establish AS model. The expression of miR-103 was characterized in the mice model. The effects of miR-103 on inflammation and endoplasmic reticulum stress (ERS) were analyzed when the expression of miR-103 was inhibited in ApoE ^-/- mice fed an HFD and human aortic endothelial cells (HAECs) exposed to oxidized low-density lipoprotein (ox-LDL). The relationship between miR-103 and phosphatase and tensin homolog (PTEN) was identified by luciferase activity detection and real-time quantitative polymerase chain reaction (RT-qPCR). Gain- and loss-function approaches were further applied for investigating the regulatory effects of miR-103 and PTEN on ERS. Role of MAPK signaling was then analyzed using PD98059 to block this pathway. miR-103 was highly expressed in the ApoEApoE ^-/- mice fed an HFD. Downregulation of miR-103 suppressed inflammation and ERS in endothelial cells isolated from ApoE ^-/- mice fed a HFD and ox-LDL-exposed HAECs. In addition, miR-103 can target PTEN and downregulate its expression. Overexpression of PTEN reversed the miR-103-induced activation of MAPK signaling. Moreover, PTEN upregulation or MAPK signaling inhibition ease miR-103-induced inflammation and ERS in vivo and in vitro. Thus, miR-103 depletion restrains the progression of AS through blocking PTEN-mediated MAPK signaling.

Lin28a overexpression reveals the role of Erk signaling in articular cartilage development

Adult articular cartilage shows limited tissue turnover, and therefore development of the proper structure of articular cartilage is crucial for life-long joint function. However, the mechanism by which the articular cartilage structure is developmentally regulated is poorly understood. In this study, we show evidence that activation of extracellular signal-regulated kinases (Erk1/2) in articular chondrocyte progenitors during developmental stages control articular cartilage thickness. We found that overexpression of Lin28a, an RNA-binding protein that regulates organismal growth and metabolism, in articular chondrocyte progenitor cells upregulated Erk signaling and increased articular cartilage thickness. Overexpression of a constitutively active Kras mimicked Lin28a overexpression, and inhibition of Erk signaling during embryonic stages normalized the cartilage phenotype of both Kras- and Lin28a-overexpressing mice. These results suggest that articular cartilage thickness is mainly determined during the process of embryonic synovial joint development, which is positively regulated by Erk signaling.

Oncostatin M promotes the osteogenic differentiation of mouse MC3T3‑E1osteoblasts through the regulation of monocyte chemotactic protein‑1

The present study investigated the function of oncostatin M (OSM), which may be associated with monocyte chemotactic protein-1 (MCP-1), on mouse MC3T3-E1 osteoblast development and bone remodeling. Levels of MCP-1, macrophage inflammatory protein 1α (MIP1α) and regulated upon activation normal T cell expressed and secreted (RANTES) were measured by ELISA. Cell viability, migration and invasion abilities were detected by MTT, wound healing and Transwell assays, respectively. Western blotting was performed to detect levels of phosphorylated protein kinase B (Akt). Reverse transcription-quantitative polymerase chain reaction and western blotting were performed to detect the levels of matrix metalloproteinases (MMP)-1, −2 and −3. The results demonstrated that OSM treatment significantly increased MCP-1 levels in a dose-dependent manner. Interleukin (IL)-1, also significantly increased MCP-1 levels; however, treatment with other cytokines, including IL-6, IL-11 and leukemia inhibitory factor did not affect MCP-1 levels to the same extent. In addition, OSM did not affect levels of the chemokines MIP1α and RANTES; indeed, only IL-1 significantly increased levels of MIP1α and RANTES. OSM treatment promoted the proliferation, migration and invasion in a dose-dependent manner, which were inhibited by MCP-1 silencing. The expression of phosphorylated-Akt, MMP-1, −2 and −3 were increased by OSM treatment; however, these increases were reversed following MCP-1 silencing. Collectively these data suggest that OSM promotes the differentiation of mouse MC3T3-E1 osteoblasts via regulation of MCP-1 expression. These results may therefore provide novel insights into bone repair and remodeling.

Enhanced insulin receptor, but not PI3K, signalling protects podocytes from ER stress

Disruption of the insulin-PI3K-Akt signalling pathway in kidney podocytes causes endoplasmic reticulum (ER) stress, leading to podocyte apoptosis and proteinuria in diabetic nephropathy. We hypothesised that by improving insulin sensitivity we could protect podocytes from ER stress. Here we use established activating transcription factor 6 (ATF6)- and ER stress element (ERSE)-luciferase assays alongside a novel high throughput imaging-based C/EBP homologous protein (CHOP) assay to examine three models of improved insulin sensitivity. We find that by improving insulin sensitivity at the level of the insulin receptor (IR), either by IR over-expression or by knocking down the negative regulator of IR activity, protein tyrosine-phosphatase 1B (PTP1B), podocytes are protected from ER stress caused by fatty acids or diabetic media containing high glucose, high insulin and inflammatory cytokines TNFα and IL-6. However, contrary to this, knockdown of the negative regulator of PI3K-Akt signalling, phosphatase and tensin homolog deleted from chromosome 10 (PTEN), sensitizes podocytes to ER stress and apoptosis, despite increasing Akt phosphorylation. This indicates that protection from ER stress is conferred through not just the PI3K-Akt pathway, and indeed we find that inhibiting the MEK/ERK signalling pathway rescues PTEN knockdown podocytes from ER stress.

Pten regulates neural crest proliferation and differentiation during mouse craniofacial development

BACKGROUND

The phosphatase and tensin homolog deleted on chromosome TEN (Pten) is implicated in a broad range of developmental events and diseases. However, its role in neural crest and craniofacial development has not been well illustrated.

RESULTS

Using genetically engineered mouse models, we showed that inactivating Pten specifically in neural crest cells causes malformation of craniofacial structures. Pten conditional knockout mice exhibit perinatal lethality with overgrowth of craniofacial structures. At the cellular level, Pten deficiency increases cell proliferation rate and enhances osteoblast differentiation. Our data further revealed that inactivating Pten elevates PI3K/Akt signaling activity in neural crest derivatives, and confirmed that attenuation of PI3K/Akt activity led to decreased neural crest cell proliferation and differentiation both in vitro and in vivo.

CONCLUSIONS

Our study revealed that Pten is essential for craniofacial morphogenesis in mice. Inactivating Pten in neural crest cells increases proliferation rate and promotes their differentiation toward osteoblasts. Our data further indicate that Pten acts via modulating PI3K/Akt activity during these processes. Developmental Dynamics 247:304-314, 2018. © 2017 Wiley Periodicals, Inc.

Conditional Deletion of the Phd2 Gene in Articular Chondrocytes Accelerates Differentiation and Reduces Articular Cartilage Thickness

Based on our findings that PHD2 is a negative regulator of chondrocyte differentiation and that hypoxia signaling is implicated in the pathogenesis of osteoarthritis, we investigated the consequence of disruption of the Phd2 gene in chondrocytes on the articular cartilage phenotype in mice. Immunohistochemistry detected high expression of PHD2 in the superficial zone (SZ), while PHD3 and HIF-1α (target of PHD2) are mainly expressed in the middle-deep zone (MDZ). Conditional deletion of the Phd2 gene (cKO) in chondrocytes accelerated the transition of progenitors to hypertrophic (differentiating) chondrocytes as revealed by reduced SZ thickness, and increased MDZ thickness, as well as increased chondrocyte hypertrophy. Immunohistochemistry further revealed decreased levels of progenitor markers but increased levels of hypertrophy markers in the articular cartilage of the cKO mice. Treatment of primary articular chondrocytes, in vitro, with IOX2, a specific inhibitor of PHD2, promoted articular chondrocyte differentiation. Knockdown of Hif-1α expression in primary articular chondrocytes using lentiviral vectors containing Hif-1α shRNA resulted in reduced expression levels of Vegf, Glut1, Pgk1, and Col10 compared to control shRNA. We conclude that Phd2 is a key regulator of articular cartilage development that acts by inhibiting the differentiation of articular cartilage progenitors via modulating HIF-1α signaling.

Forkhead box O transcription factors in chondrocytes regulate endochondral bone formation

The differentiation of embryonic mesenchymal cells into chondrocytes and the subsequent formation of a cartilaginous scaffold that enables the formation of long bones are hallmarks of endochondral ossification. During this process, chondrocytes undergo a remarkable sequence of events involving proliferation, differentiation, hypertrophy and eventually apoptosis. Forkhead Box O (FoxO) transcription factors (TFs) are well-known regulators of such cellular processes. Although FoxO3a was previously shown to be regulated by 1,25-dihydroxyvitamin D₃ in osteoblasts, a possible role for this family of TFs in chondrocytes during endochondral ossification remains largely unstudied. By crossing Collagen2-Cre mice with FoxO1^lox/lox;FoxO3a^lox/lox;FoxO4^lox/lox mice, we generated mice in which the three main FoxO isoforms were deleted in growth plate chondrocytes (chondrocyte triple knock-out; CTKO). Intriguingly, CTKO neonates showed a distinct elongation of the hypertrophic zone of the growth plate. CTKO mice had increased overall body and tail length at eight weeks of age and suffered from severe skeletal deformities at older ages. CTKO chondrocytes displayed decreased expression of genes involved in redox homeostasis. These observations illustrate the importance of FoxO signaling in chondrocytes during endochondral ossification.

Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Regulates Chondrocyte Differentiation and Secondary Ossification in Mice

Endochondral ossification plays an important role in the formation of the primary ossification centers (POCs) and secondary ossification centers (SOCs) of mammalian long bones. However, the molecular mechanisms that regulate POC and SOC formation are different. We recently demonstrated that Prolyl Hydroxylase Domain-containing Protein 2 (Phd2) is a key mediator of vitamin C effects on bone. We investigated the role of Phd2 on endochondral ossification of the epiphyses by conditionally deleting the Phd2 gene in osteoblasts and chondrocytes. We found that the deletion of Phd2 in osteoblasts did not cause changes in bone parameters in the proximal tibial epiphyses in 5 week old mice. In contrast, deletion of Phd2 in chondrocytes resulted in increased bone mass and bone formation rate (normalized to tissue volume) in long bone epiphyses, indicating that Phd2 expressed in chondrocytes, but not osteoblasts, negatively regulates secondary ossification of epiphyses. Phd2 deletion in chondrocytes elevated mRNA expression of hypoxia-inducible factor (HIF) signaling molecules including Hif-1α, Hif-2α, Vegfa, Vegfb, and Epo, as well as markers for chondrocyte hypertrophy and mineralization such as Col10, osterix, alkaline phosphatase, and bone sialoprotein. These data suggest that Phd2 expressed in chondrocytes inhibits endochondral ossification at the epiphysis by suppressing HIF signaling pathways.

Conditional Deletion of Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Gene Reveals Its Essential Role in Chondrocyte Function and Endochondral Bone Formation

The hypoxic growth plate cartilage requires hypoxia-inducible factor (HIF)-mediated pathways to maintain chondrocyte survival and differentiation. HIF proteins are tightly regulated by prolyl hydroxylase domain-containing protein 2 (Phd2)-mediated proteosomal degradation. We conditionally disrupted the Phd2 gene in chondrocytes by crossing Phd2 floxed mice with type 2 collagen-α1-Cre transgenic mice and found massive increases (>50%) in the trabecular bone mass of long bones and lumbar vertebra of the Phd2 conditional knockout (cKO) mice caused by significant increases in trabecular number and thickness and reductions in trabecular separation. Cortical thickness and tissue mineral density at the femoral middiaphysis of the cKO mice were also significantly increased. Dynamic histomorphometric analyses revealed increased longitudinal length and osteoid surface per bone surface in the primary spongiosa of the cKO mice, suggesting elevated conversion rate from hypertrophic chondrocytes to mineralized bone matrix as well as increased bone formation in the primary spongiosa. In the secondary spongiosa, bone formation measured by mineralizing surface per bone surface and mineral apposition rate were not changed, but resorption was slightly reduced. Increases in the mRNA levels of SRY (sex determining region Y)-box 9, osterix (Osx), type 2 collagen, aggrecan, alkaline phosphatase, bone sialoprotein, vascular endothelial growth factor, erythropoietin, and glycolytic enzymes in the growth plate of cKO mice were detected by quantitative RT-PCR. Immunohistochemistry revealed an increased HIF-1α protein level in the hypertrophic chondrocytes of cKO mice. Infection of chondrocytes isolated from Phd2 floxed mice with adenoviral Cre resulted in similar gene expression patterns as observed in the cKO growth plate chondrocytes. Our findings indicate that Phd2 suppresses endochondral bone formation, in part, via HIF-dependent mechanisms in mice.

Rb1 and Pten Co-Deletion in Osteoblast Precursor Cells Causes Rapid Lipoma Formation in Mice

The Rb and Pten tumor suppressor genes are important regulators of bone development and both are frequently mutated in the bone cancer osteosarcoma (OS). To determine if Rb1 and Pten synergize as tumor suppressor genes for osteosarcoma, we co-deleted them in osteoprogenitor cells. Surprisingly, we observed rapid development of adipogenic but not osteosarcoma tumors in the ΔRb1/Pten mice. ΔPten solo deleted mice also developed lipoma tumors but at a much reduced frequency and later onset than those co-deleted for Rb1. Pten deletion also led to a marked increase in adipocytes in the bone marrow. To better understand the function of Pten in bone development in vivo, we conditionally deleted Pten in OSX⁺ osteoprogenitor cells using OSX-Cre mice. μCT analysis revealed a significant thickening of the calvaria and an increase in trabeculae volume and number in the femur, consistent with increased bone formation in these mice. To determine if Pten and Rb1 deletion actively promotes adipogenic differentiation, we isolated calvarial cells from Pten^fl/fl and Pten^fl/fl; Rb1^fl/fl mice, infected them with CRE or GFP expressing adenovirus, treated with differentiation media. We observed slightly increased adipogenic, and osteogenic differentiation in the ΔPten cells. Both phenotypes were greatly increased upon Rb1/Pten co-deletion. This was accompanied by an increase in expression of genes required for adipogenesis. These data indicate that Pten deletion in osteoblast precursors is sufficient to promote frequent adipogenic, but only rare osteogenic tumors. Rb1 hetero- or homo-zygous co-deletion greatly increases the incidence and the rapidity of onset of adipogenic tumors, again, with only rare osteosarcoma tumors.

FGFR3 Deficiency Causes Multiple Chondroma-like Lesions by Upregulating Hedgehog Signaling

Most cartilaginous tumors are formed during skeletal development in locations adjacent to growth plates, suggesting that they arise from disordered endochondral bone growth. Fibroblast growth factor receptor (FGFR)3 signaling plays essential roles in this process; however, the role of FGFR3 in cartilaginous tumorigenesis is not known. In this study, we found that postnatal chondrocyte-specific Fgfr3 deletion induced multiple chondroma-like lesions, including enchondromas and osteochondromas, adjacent to disordered growth plates. The lesions showed decreased extracellular signal-regulated kinase (ERK) activity and increased Indian hedgehog (IHH) expression. The same was observed in Fgfr3-deficient primary chondrocytes, in which treatment with a mitogen-activated protein kinase (MEK) inhibitor increased Ihh expression. Importantly, treatment with an inhibitor of IHH signaling reduced the occurrence of chondroma-like lesions in Fgfr3-deficient mice. This is the first study reporting that the loss of Fgfr3 function leads to the formation of chondroma-like lesions via downregulation of MEK/ERK signaling and upregulation of IHH, suggesting that FGFR3 has a tumor suppressor-like function in chondrogenesis.

TGFβ signaling in cartilage development and maintenance

Members of the transforming growth factor beta (TGFβ) superfamily of secreted factors play essential roles in nearly every aspect of cartilage formation and maintenance. However, the mechanisms by which TGFβs transduce their effects in cartilage in vivo remain poorly understood. Mutations in several TGFβ family members, their receptors, extracellular modulators, and intracellular transducers have been described, and these usually impact the development of the cartilaginous skeleton. Furthermore, genome-wide association studies have linked components of the (TGFβ) superfamily to susceptibility to osteoarthritis. This review focuses on recent discoveries from genetic studies in the mouse regarding the regulation of TGFβ signaling in developing growth plate and articular cartilage, as well as the different modes of crosstalk between canonical and noncanonical TGFβ signaling. These new insights into TGFβ signaling in cartilage may open new prospects for therapies that maintain healthy articular cartilage.

Advances in Skeletal Dysplasia Genetics

Skeletal dysplasias result from disruptions in normal skeletal growth and development and are a major contributor to severe short stature. They occur in approximately 1/5,000 births, and some are lethal. Since the most recent publication of the Nosology and Classification of Genetic Skeletal Disorders, genetic causes of 56 skeletal disorders have been uncovered. This remarkable rate of discovery is largely due to the expanded use of high-throughput genomic technologies. In this review, we discuss these recent discoveries and our understanding of the molecular mechanisms behind these skeletal dysplasia phenotypes. We also cover potential therapies, unusual genetic mechanisms, and novel skeletal syndromes both with and without known genetic causes. The acceleration of skeletal dysplasia genetics is truly spectacular, and these advances hold great promise for diagnostics, risk prediction, and therapeutic design.

Zbtb20 regulates the terminal differentiation of hypertrophic chondrocytes via repression of Sox9

The terminal differentiation of hypertrophic chondrocytes is a tightly regulated process that plays a pivotal role in endochondral ossification. As a negative regulator, Sox9 is essentially downregulated in terminally differentiated hypertrophic chondrocytes. However, the underlying mechanism of Sox9 silencing is undefined. Here we show that the zinc finger protein Zbtb20 regulates the terminal differentiation of hypertrophic chondrocytes by repressing Sox9. In the developing skeleton of the mouse, Zbtb20 protein is highly expressed by hypertrophic chondrocytes from late embryonic stages. To determine its physiological role in endochondral ossification, we have generated chondrocyte-specific Zbtb20 knockout mice and demonstrate that disruption of Zbtb20 in chondrocytes results in delayed endochondral ossification and postnatal growth retardation. Zbtb20 deficiency caused a delay in cartilage vascularization and an expansion of the hypertrophic zone owing to reduced expression of Vegfa in the hypertrophic zone. Interestingly, Sox9, a direct suppressor of Vegfa expression, was ectopically upregulated at both mRNA and protein levels in the late Zbtb20-deficient hypertrophic zone. Furthermore, knockdown of Sox9 greatly increased Vegfa expression in Zbtb20-deficient hypertrophic chondrocytes. Our findings point to Zbtb20 as a crucial regulator governing the terminal differentiation of hypertrophic chondrocytes at least partially through repression of Sox9.

The impact of the endoplasmic reticulum protein-folding environment on cancer development

The endoplasmic reticulum (ER) is an essential organelle in eukaryotic cells for the storage and regulated release of calcium and as the entrance to the secretory pathway. Protein misfolding in the ER causes accumulation of misfolded proteins (ER stress) and activation of the unfolded protein response (UPR), which has evolved to maintain a productive ER protein-folding environment. Both ER stress and UPR activation are documented in many different human cancers. In this Review, we summarize the impact of ER stress and UPR activation on every aspect of cancer and discuss outstanding questions for which answers will pave the way for therapeutics.

PTEN regulates matrix synthesis in adult human chondrocytes under oxidative stress

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) was identified as an important tumor suppressor gene. PTEN functions as a negative regulator of phosphoinositol-3-kinase (PI3K)-Akt and MEK/ERK signaling. The PI3K-Akt pathway is critical for cell survival, differentiation, and matrix synthesis. Oxidative stress is considered a critical factor in the onset and progression of osteoarthritis (OA). Therefore, we investigated the function of PTEN in OA chondrocytes under oxidative stress. Chondrocytes were treated with insulin-like growth factor-1 (IGF-1) and/or tert-butyl hydroperoxide (tBHP), which causes oxidative stress. The expression levels of type2 collagen (Col2a1) and aggrecan were analyzed by real-time PCR, and phosphorylation of Akt and ERK1/2 was analyzed by Western blotting. Chondrocytes were treated with PTEN-specific small interfering RNA (siRNA), as well as IGF-1 and/or tBHP. PTEN and IGF-1 expressions in OA chondrocytes were increased. The downregulation of PTEN expression increased the expression levels of Col2a1 and aggrecan, and increased proteoglycan synthesis under oxidative stress. Oxidative stress decreased the phosphorylation of Akt and increased that of ERK1/2. The downregulation of PTEN expression increased Akt phosphorylation, but did not increase that of ERK 1/2. Our results suggest that PTEN regulates matrix synthesis via the PI3K-Akt pathway under oxidative stress.

Overview of skeletal development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on the well-coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Smad7 regulates terminal maturation of chondrocytes in the growth plate

Members of the bone morphogenetic protein (BMP) superfamily, including transforming growth factor-betas (TGFβ), regulate multiple aspects of chondrogenesis. Smad7 is an intracellular inhibitor of BMP and TGFβ signaling. Studies in which Smad7 was overexpressed in chondrocytes demonstrated that Smad7 can impact chondrogenesis by inhibiting BMP signaling. However, whether Smad7 is actually required for endochondral ossification in vivo is unclear. Moreover, whether Smad7 regulates TGFβ in addition to BMP signaling in developing cartilage is unknown. In this study, we found that Smad7 is required for both axial and appendicular skeletal development. Loss of Smad7 led to impairment of the cell cycle in chondrocytes and to defects in terminal maturation. This phenotype was attributed to upregulation of both BMP and TGFβ signaling in Smad7 mutant growth plates. Moreover, Smad7-/- mice develop hypocellular cores in the medial growth plates, associated with elevated HIF1α levels, cell death, and intracellular retention of types II and X collagen. Thus, Smad7 may be required to mediate cell stress responses in the growth plate during development.

Frequent mutation of the major cartilage collagen gene COL2A1 in chondrosarcoma

Chondrosarcoma is a heterogeneous collection of malignant bone tumors and is the second most common primary malignancy of bone after osteosarcoma. Recent work has identified frequent, recurrent mutations in IDH1 or IDH2 in nearly half of central chondrosarcomas. However, there has been little systematic genomic analysis of this tumor type, and, thus, the contribution of other genes is unclear. Here we report comprehensive genomic analyses of 49 individuals with chondrosarcoma (cases). We identified hypermutability of the major cartilage collagen gene COL2A1, with insertions, deletions and rearrangements identified in 37% of cases. The patterns of mutation were consistent with selection for variants likely to impair normal collagen biosynthesis. In addition, we identified mutations in IDH1 or IDH2 (59%), TP53 (20%), the RB1 pathway (33%) and Hedgehog signaling (18%).

Endoplasmic reticulum stress plays a pivotal role in cell death mediated by the pan-deacetylase inhibitor panobinostat in human hepatocellular cancer cells

Panobinostat, a pan-deacetylase inhibitor, represents a novel therapeutic option for cancer diseases. Besides its ability to block histone deacetylases (HDACs) by promoting histone hyperacetylation, panobinostat interferes with several cell death pathways providing a potential efficacy against tumors. We have previously demonstrated that panobinostat has a potent apoptotic activity in vitro and causes a significant growth delay of hepatocellular carcinoma (HCC) tumor xenografts in nude mice models. Here, we show that treatment with panobinostat is able to induce noncanonical apoptotic cell death in HepG2 and in Hep3B cells, involving the endoplasmic reticulum (ER) stress by up-regulation of the molecular chaperone binding immunoglobulin protein/glucose-regulated protein 78, activation of eukaryotic initiation factor 2α-activating transcription factor 4 (tax-responsive enhancer element B67) and inositol requiring 1α-X-box binding protein 1 factors, strong increase and nuclear translocation of the transcription factor C/EBP homologous protein/growth arrest and DNA damage-inducible gene 153, and involvement of c-Jun N-terminal kinase. These signaling cascades culminate into the activation of the ER-located caspase-4/12 and of executioner caspases, which finally lead to cell demise. Our results clearly show that panobinostat induces an alternative ER stress-mediated cell death pathway in HCC cells, independent of the p53 status.

Disruption of Wnt/β-catenin signaling in odontoblasts and cementoblasts arrests tooth root development in postnatal mouse teeth

Tooth development undergoes a series of complex reciprocal interactions between dental epithelium and the underlying mesenchymal cells. Compared with the study in tooth crown formation, little is known about the molecular mechanism underlying the development of tooth roots. In the present study, we conditionally knock out β-catenin gene (Ctnnb1) within developing odontoblasts and cementoblasts during the development of tooth roots, and observed rootless molars as well as incomplete incisors. Histological analyses revealed intact structure of molar crown and labial side of incisor, however, as for the molar roots and the lingual portion of incisor, the formation of dentin and periodontal tissues were greatly hampered. In situ hybridization experiments using probes of odontoblastic marker genes collagen type I, alpha 1 (Col1a1), osteocalcin (OC) and dentin sialophosphoprotein (Dspp) manifested striking undifferentiation of root odontoblasts in which Ctnnb1 was eliminated. Bromodeoxyuridine (BrdU) labeling and proliferating cell nuclear antigen (PCNA) immunohistochemical experiments also showed retarded proliferation of pre-odontoblasts in mutant mice. However, cell apoptosis was not affected. Additionally, a disrupted formation of cementoblasts, suggested by the absence of transcripts of bone sialoprotein (Bsp) in follicle mesenchyme, was also evident in mutant mice. Our study provides strong in vivo evidence to confirm that Wnt/β-catenin signaling is functionally significant to root odontogenesis and cementogenesis during the tooth root development.

Cross-talk between FGF and other cytokine signalling pathways during endochondral bone development

FGF (fibroblast growth factor)/FGFR (FGF receptor) signalling plays an essential role in both endochondral and intramembranous bone development. FGF signalling pathways are important for the earliest stages of limb development and throughout skeletal development. The activity and the outcome of this signalling pathway during bone development are also influenced by many other intracellular and extracellular signals. In this review, we focus on the interplay between FGF signalling and other pathways, which is tightly regulated both spatially and temporally during endochondral skeletal development.

HIF1α is a central regulator of collagen hydroxylation and secretion under hypoxia during bone development

Collagen production is fundamental for the ontogeny and the phylogeny of all multicellular organisms. It depends on hydroxylation of proline residues, a reaction that uses molecular oxygen as a substrate. This dependency is expected to limit collagen production to oxygenated cells. However, during embryogenesis, cells in different tissues that develop under low oxygen levels must produce this essential protein. In this study, using the growth plate of developing bones as a model system, we identify the transcription factor hypoxia-inducible factor 1 α (HIF1α) as a central component in a mechanism that underlies collagen hydroxylation and secretion by hypoxic cells. We show that Hif1a loss of function in growth plate chondrocytes arrests the secretion of extracellular matrix proteins, including collagen type II. Reduced collagen hydroxylation and endoplasmic reticulum stress induction in Hif1a-depleted cells suggests that HIF1α regulates collagen secretion by mediating its hydroxylation and consequently its folding. We demonstrate in vivo the ability of Hif1α to drive the transcription of collagen prolyl 4-hydroxylase, which catalyzes collagen hydroxylation. We also show that, concurrently, HIF1α maintains cellular levels of oxygen, most likely by controlling the expression of pyruvate dehydrogenase kinase 1, an inhibitor of the tricarboxylic acid cycle. Through this two-armed mechanism, HIF1α acts as a central regulator of collagen production that allows chondrocytes to maintain their function as professional secretory cells in the hypoxic growth plate. As hypoxic conditions occur also during pathological conditions such as cancer, our findings may promote the understanding not only of embryogenesis, but also of pathological processes.

PTEN hamartoma of soft tissue: a distinctive lesion in PTEN syndromes

PTEN hamartoma tumor syndrome (PHTS) presents in a spectrum that encompasses the eponymous disorders Cowden and Bannayan-Riley-Ruvalcaba. Herein, we delineate the distinctive histopathology of a predominantly intramuscular lesion in PHTS, often called "arteriovenous malformation," because of certain imaging and histopathologic features. Cases were identified by review of lesions resected from patients with PHTS registered at our Vascular Anomalies Center and of unusual intramuscular vascular anomalies in our pathology database from 1985 to 2008. Thirty-four patients with this lesion were identified: 20 had a clinical diagnosis of, or were suspected to have, PHTS (genetically confirmed in 16). In 4 patients without clinical manifestations of PHTS, 2 had PTEN mutations, 1 did not, and in 1 the mutation was intronic. In the remaining 10, there was insufficient clinical information to fully assess whether they had manifestations of PHTS. Lesions manifested by 15 years of age, normally with pain and swelling, and were most often located in the lower extremity. The major mass was usually intramuscular, but often there were fascial and subcutaneous components and not infrequently a cutaneous vascular stain. Magnetic resonance imaging generally showed an infiltrative soft tissue lesion involving the muscle, fascia, and subcutis with frequently enlarged, serpiginous vessels, small arteriovenous fistulae with disproportionately dilated draining veins, and a prominent adipocytic component. Some lesions involved contiguous muscles, and 20% were multifocal. Resected specimens ranged in size from 1.2 to 25 cm; in 1 patient, amputation was necessary. Histopathologically, these unencapsulated masses, often with a nodular appearance at scanning magnification, consisted of: (1) a variable admixture of mature adipocytic and dense and/or myxoid fibrous tissues (50% to 90% of surface area); (2) a vascular component (10% to 50% of surface area) with: (a) clusters of venous channels, some with excessively and irregularly muscularized complex walls and lumens, and others with thin walls resembling pulmonary alveoli, (b) tortuous, thick-walled arteries with concentric muscular hyperplasia and relatively small lumens, (c) numerous small vessels (arteries, veins, and indeterminate channels), and (d) occasional arteriovenous communications; (3) lymphoid follicles (50%); (4) foci of bone (20%); and (5) hypertrophic nerves with "onion bulb" proliferation of periaxonal spindled cells (9%). We designate this disorganized overgrowth of essentially mesenchymal elements as PTEN hamartoma of soft tissue. It differs from other vascular and connective tissue lesions that occur in patients with PHTS. PTEN hamartoma of soft tissue is histopathologically distinctive, and its identification should prompt a thorough investigation for PHTS.

The PTEN and Myotubularin phosphoinositide 3-phosphatases: linking lipid signalling to human disease

Two classes of lipid phosphatases selectively dephosphorylate the 3 position of the inositol ring of phosphoinositide signaling molecules: the PTEN and the Myotubularin families. PTEN dephosphorylates PtdIns(3,4,5)P(3), acting in direct opposition to the Class I PI3K enzymes in the regulation of cell growth, proliferation and polarity and is an important tumor suppressor. Although there are several PTEN-related proteins encoded by the human genome, none of these appear to fulfill the same functions. In contrast, the Myotubularins dephosphorylate both PtdIns(3)P and PtdIns(3,5)P(2), making them antagonists of the Class II and Class III PI 3-kinases and regulators of membrane traffic. Both phosphatase groups were originally identified through their causal mutation in human disease. Mutations in specific myotubularins result in myotubular myopathy and Charcot-Marie-Tooth peripheral neuropathy; and loss of PTEN function through mutation and other mechanisms is evident in as many as a third of all human tumors. This chapter will discuss these two classes of phosphatases, covering what is known about their biochemistry, their functions at the cellular and whole body level and their influence on human health.

The skeleton: a multi-functional complex organ: new insights into osteoblasts and their role in bone formation: the central role of PI3Kinase

Studies on bone development, formation and turnover have grown exponentially over the last decade in part because of the utility of genetic models. One area that has received considerable attention has been the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which has emerged as a major survival network for osteoblasts. Genetic engineering has enabled investigators to study downstream effectors of PI3K by directly overexpressing activated forms of AKT in cells of the skeletal lineage or deleting Pten that leads to a constitutively active AKT. The results from these studies have provided novel insights into bone development and remodeling, critical processes in the lifelong maintenance of skeletal health. This paper reviews those data in relation to recent advances in osteoblast biology and their potential relevance to chronic disorders of the skeleton and their treatment.

Identification of novel synergistic targets for rational drug combinations with PI3 kinase inhibitors using siRNA synthetic lethality screening against GBM

Several small molecules that inhibit the PI3 kinase (PI3K)-Akt signaling pathway are in clinical development. Although many of these molecules have been effective in preclinical models, it remains unclear whether this strategy alone will be sufficient to interrupt the molecular events initiated and maintained by signaling along the pathways because of the activation of other pathways that compensate for the inhibition of the targeted kinase. In this study, we performed a synthetic lethality screen to identify genes or pathways whose inactivation, in combination with the PI3K inhibitors PX-866 and NVPBEZ-235, might result in a lethal phenotype in glioblastoma multiforme (GBM) cells. We screened GBM cells (U87, U251, and T98G) with a large-scale, short hairpin RNA library (GeneNet), which contains 43 800 small interfering RNA sequences targeting 8500 well-characterized human genes. To decrease off-target effects, we selected overlapping genes among the 3 cell lines that synergized with PX-866 to induce cell death. To facilitate the identification of potential targets, we used a GSE4290 dataset and The Cancer Genome Atlas GBM dataset, identifying 15 target genes overexpressed in GBM tissues. We further analyzed the selected genes using Ingenuity Pathway Analysis software and showed that the 15 genes were closely related to cancer-promoting pathways, and a highly interconnected network of aberrations along the MYC, P38MAPK, and ERK signaling pathways were identified. Our findings suggest that inhibition of these pathways might increase tumor sensitivity to PX-866 and therefore represent a potential clinical therapeutic strategy.

Conditional ablation of Pten in osteoprogenitors stimulates FGF signaling

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a direct antagonist of phosphatidylinositol 3 kinase. Pten is a well recognized tumor suppressor and is one of the most commonly mutated genes in human malignancies. More recent studies of development and stem cell behavior have shown that PTEN regulates the growth and differentiation of progenitor cells. Significantly, PTEN is found in osteoprogenitor cells that give rise to bone-forming osteoblasts; however, the role of PTEN in bone development is incompletely understood. To define how PTEN functions in osteoprogenitors during bone development, we conditionally deleted Pten in mice using the cre-deleter strain Dermo1cre, which targets undifferentiated mesenchyme destined to form bone. Deletion of Pten in osteoprogenitor cells led to increased numbers of osteoblasts and expanded bone matrix. Significantly, osteoblast development and synthesis of osteoid in the nascent bone collar was uncoupled from the usual tight linkage to chondrocyte differentiation in the epiphyseal growth plate. The expansion of osteoblasts and osteoprogenitors was found to be due to augmented FGF signaling as evidenced by (1) increased expression of FGF18, a potent osteoblast mitogen, and (2) decreased expression of SPRY2, a repressor of FGF signaling. The differentiation of osteoblasts was autonomous from the growth plate chondrocytes and was correlated with an increase in the protein levels of GLI2, a transcription factor that is a major mediator of hedgehog signaling. We provide evidence that increased GLI2 activity is also a consequence of increased FGF signaling through downstream events requiring mitogen-activated protein kinases. To test whether FGF signaling is required for the effects of Pten deletion, we deleted one allele of fibroblast growth factor receptor 2 (FGFR2). Significantly, deletion of FGFR2 caused a partial rescue of the Pten-null phenotype. This study identifies activated FGF signaling as the major mediator of Pten deletion in osteoprogenitors.

Sox9 sustains chondrocyte survival and hypertrophy in part through Pik3ca-Akt pathways

During endochondral bone formation, Sox9 expression starts in mesenchymal progenitors, continues in the round and flat chondrocyte stages at high levels, and ceases just prior to the hypertrophic chondrocyte stage. Sox9 is important in mesenchymal progenitors for their differentiation into chondrocytes, but its functions post-differentiation have not been determined. To investigate Sox9 function in chondrocytes, we deleted mouse Sox9 at two different steps after chondrocyte differentiation. Sox9 inactivation in round chondrocytes resulted in a loss of Col2a1 expression and in apoptosis. Sox9 inactivation in flat chondrocytes caused immediate terminal maturation without hypertrophy and with excessive apoptosis. Inactivation of Sox9 in the last few cell layers resulted in the absence of Col10a1 expression, suggesting that continued expression of Sox9 just prior to hypertrophy is necessary for chondrocyte hypertrophy. SOX9 knockdown also caused apoptosis of human chondrosarcoma SW1353 cells. These phenotypes were associated with reduced Akt phosphorylation. Forced phosphorylation of Akt by Pten inactivation partially restored Col10a1 expression and cell survival in Sox9(floxdel/floxdel) mouse chondrocytes, suggesting that phosphorylated Akt mediates chondrocyte survival and hypertrophy induced by Sox9. When the molecular mechanism of Sox9-induced Akt phosphorylation was examined, we found that expression of the PI3K subunit Pik3ca (p110α) was decreased in Sox9(floxdel/floxdel) mouse chondrocytes. Sox9 binds to the promoter and enhances the transcriptional activities of Pik3ca. Thus, continued expression of Sox9 in differentiated chondrocytes is essential for subsequent hypertrophy and sustains chondrocyte-specific survival mechanisms by binding to the Pik3ca promoter, inducing Akt phosphorylation.

In vivo cellular adaptation to ER stress: survival strategies with double-edged consequences

Disturbances to the balance of protein synthesis, folding and secretion in the endoplasmic reticulum (ER) induce stress and thereby the ER stress signaling (ERSS) response, which alleviates this stress. In this Commentary, we review the emerging idea that ER stress caused by abnormal physiological conditions and/or mutations in genes that encode client proteins of the ER is a key factor underlying different developmental processes and the pathology of diverse diseases, including diabetes, neurodegeneration and skeletal dysplasias. Recent studies in mouse models indicate that the effect of ERSS in vivo and the nature of the cellular strategies induced to ameliorate pathological ER stress are crucial factors in determining cell fate and clinical disease features. Importantly, ERSS can affect cellular proliferation and the differentiation program; cells that survive the stress can become 'reprogrammed' or dysfunctional. These cell-autonomous adaptation strategies can generate a spectrum of context-dependent cellular consequences, ranging from recovery to death. Secondary effects can include altered cell-extracellular-matrix interactions and non-cell-autonomous alteration of paracrine signaling, which contribute to the final phenotypic outcome. Recent reports showing that ER stress can be alleviated by chemical compounds suggest the potential for novel therapeutic approaches.

Regulation of PTEN stability and activity by Plk3

By studying primary isogenic murine embryonic fibroblasts (MEFs), we have shown that PLK3 null MEFs contain a reduced level of phosphatase and tensin homolog (PTEN) and increased Akt1 activation coupled with decreased GSK3β activation under normoxia and hypoxia. Purified recombinant Plk3, but not a kinase-defective mutant, efficiently phosphorylates PTEN in vitro. Mass spectrometry identifies threonine 366 and serine 370 as two putative residues that are phosphorylated by Plk3. Immunoblotting using a phosphospecific antibody confirms these sites as Plk3 phosphorylation sites. Moreover, treatment of MEFs with LiCl, an inhibitor of GSK3β and CK2, only partially suppresses the phosphorylation, suggesting Plk3 as an additional kinase that phosphorylates these sites in vivo. Plk3-targeting mutants of PTEN are expressed at a reduced level in comparison with the wild-type counterpart, which is associated with an enhanced activity of PDK1, an upstream activator of Akt1. Furthermore, the reduced level of PTEN in PLK3 null MEFs is stabilized by treatment with MG132, a proteosome inhibitor. Combined, our study identifies Plk3 as a new player in the regulation of the PI3K/PDK1/Akt signaling axis by phosphorylation and stabilization of PTEN.

Molecular genetic studies of gene identification for osteoporosis: the 2009 update

Osteoporosis is a complex human disease that results in increased susceptibility to fragility fractures. It can be phenotypically characterized using several traits, including bone mineral density, bone size, bone strength, and bone turnover markers. The identification of gene variants that contribute to osteoporosis phenotypes, or responses to therapy, can eventually help individualize the prognosis, treatment, and prevention of fractures and their adverse outcomes. Our previously published reviews have comprehensively summarized the progress of molecular genetic studies of gene identification for osteoporosis and have covered the data available to the end of September 2007. This review represents our continuing efforts to summarize the important and representative findings published between October 2007 and November 2009. The topics covered include genetic association and linkage studies in humans, transgenic and knockout mouse models, as well as gene-expression microarray and proteomics studies. Major results are tabulated for comparison and ease of reference. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis.

Hypoxia, HIFs and bone development

Oxygen is not only an obviously important substrate, but it is also a regulatory signal that controls expression of a specific genetic program. Crucial mediator of the adaptive response of cells to hypoxia is the family of Hypoxia-Inducible Transcription Factors (HIFs).The fetal growth plate, which is an avascular structure of mesenchymal origin, has a unique out-in gradient of oxygenation. HIF-1alpha is necessary for chondrogenesis in vivo by controlling a complex homeostatic response that allows chondrocytes to survive and differentiate in a hypoxic environment. Moreover, HIFs are also essential in osteogenesis and joint development. This brief Perspective summarizes the critical role of HIFs in endochondral bone development.

Biology and pathology of Rho GTPase, PI-3 kinase-Akt, and MAP kinase signaling pathways in chondrocytes

Chondrocytes provide the framework for the developing skeleton and regulate long-bone growth through the activity of the growth plate. Chondrocytes in the articular cartilage, found at the ends of bones in diarthroidial joints, are responsible for maintenance of the tissue through synthesis and degradation of the extracellular matrix. The processes of growth, differentiation, cell death and matrix remodeling are regulated by a network of cell signaling pathways in response to a variety of extracellular stimuli. These stimuli consist of soluble ligands, including growth factors and cytokines, extracellular matrix proteins, and mechanical factors that act in concert to regulate chondrocyte function through a variety of canonical and non-canonical signaling pathways. Key chondrocyte signaling pathways include, but are not limited to, the p38, JNK and ERK MAP kinases, the PI-3 kinase-Akt pathway, the Jak-STAT pathway, Rho GTPases and Wnt-beta-catenin and Smad pathways. Modulation of the activity of any of these pathways has been associated with various pathological states in cartilage. This review focuses on the Rho GTPases, the PI-3 kinase-Akt pathway, and some selected aspects of MAP kinase signaling. Most studies to date have examined these pathways in isolation but it is becoming clear that there is significant cross-talk among the pathways and that the overall effects on chondrocyte function depend on the balance in activity of multiple signaling proteins.

Osteoblastic molecular scaffold Gab1 is required for maintaining bone homeostasis

The Grb2-associated binder 1 (Gab1), which serves as a scaffolding adaptor protein, plays a crucial role in transmitting key signals that control cell growth, differentiation and function from multiple receptors. However, its biological role in osteoblast activity and postnatal bone metabolism remains unclear. To elucidate the in vivo function of Gab1 in postnatal bone remodeling, we generated osteoblast-specific Gab1 knockout mice. Disruption of Gab1 expression in osteoblasts led to decreased trabecular bone mass with a reduced bone formation rate and a decreased bone resorption. Bones from Gab1 mutants also exhibited inferior mechanical properties. Moreover, primary osteoblasts from Gab1 mutant mice demonstrated markedly suppressed osteoblast mineralization, increased susceptibility to apoptosis and decreased expression of receptor activator of NF-kappaB ligand (RANKL). Activation of serine-threonine Akt kinase and extracellular signal-regulated kinase in response to insulin and insulin-like growth factor 1 was attenuated in Gab1 mutant osteoblasts. Our results show that Gab1-mediated signals in osteoblasts are crucial for normal postnatal bone homeostasis.

The role of hypoxia in development of the Mammalian embryo

Hypoxia inducible factor (HIF) is a transcription factor that acts in low-oxygen conditions. The cellular response to HIF activation is transcriptional upregulation of a large group of genes. Some target genes promote anaerobic metabolism to reduce oxygen consumption, while others "alleviate" hypoxia by acting non-cell-autonomously to extend and modify the surrounding vasculature. Although hypoxia is often thought of as being a pathological phenomenon, the mammalian embryo in fact develops in a low-oxygen environment, and in this context HIF has additional responsibilities. This review describes how low oxygen and HIF affect gene expression, cell behavior, and ultimately morphogenesis of the embryo and placenta.

Disruption of Smad4 in odontoblasts causes multiple keratocystic odontogenic tumors and tooth malformation in mice

Keratocystic odontogenic tumors (KCOTs) are cystic epithelial neoplasias with a high recurrence rate. However, the molecular mechanisms underlying the initiation and progression of KCOTs are still largely unknown. Here, we show that specific ablation of Smad4 in odontoblasts unexpectedly resulted in spontaneous KCOTs in mice. The mutant mice exhibited malformed teeth characterized by fractured incisors and truncated molar roots. These abnormalities were mainly caused by disrupted odontoblast differentiation that led to irregular dentin formation. The cystic tumors arising from the reactivation of epithelial rests of Malassez (ERM), in which Smad4 remained intact, proliferated and formed stratified and differentiated squamous epithelia that exhibited a dramatic upregulation of Hedgehog signaling. Odontoblasts, which are responsive to transforming growth factor beta (TGF-beta)/bone morphogenetic protein (BMP) signals, may produce signal molecules to inhibit the activation of ERM. Indeed, we observed a downregulation of BMP signals from Smad4 mutant odontoblasts to the adjacent Hertwig's epithelial root sheath (HERS). Intriguingly, KCOTs frequently emerged from Smad4-deficient ERM in keratinocyte-specific Smad4 knockout mice, suggesting a novel mechanism in which reciprocal TGF-beta/BMP signaling between odontoblasts and HERS was required for tooth root development and suppression of KCOT formation. These findings provide insight into the genetic basis underlying KCOTs and have important implications for new directions in KCOT treatment.

Pten null prostate tumorigenesis and AKT activation are blocked by targeted knockout of ER chaperone GRP78/BiP in prostate epithelium

GRP78/BiP has recently emerged as a novel biomarker for aggressive prostate cancer. Here, we report that homozygous deletion of Grp78 specifically in mouse prostate epithelium suppresses prostate tumorigenesis without affecting postnatal prostate development and growth. Mouse prostates with double conditional knockout of Grp78 and Pten exhibit normal histology and cytology, in contrast to the invasive adenocarcinoma in mouse prostates with Pten inactivation. AKT activation in Pten null prostate epithelium is inhibited by Grp78 homozygous deletion, corresponding with suppression of AKT phosphorylation by GRP78 knockdown in prostate cancer cell line. Thus, inactivation of GRP78 may represent a previously undescribed approach to stop prostate cancer and potentially other cancers resulting from the loss of PTEN tumor suppression and/or activation of the oncogenic AKT.