ERK pathway mediates the activation of Cdk2 in IGF-1-induced proliferation of human osteosarcoma MG-63 cells

Surface modification of additive manufactured Ti6Al4V scaffolds with gelatin/alginate- IGF-1 carrier: An effective approach for healing bone defects

The study investigates the potential of porous scaffolds with Gel/Alg-IGF-1 coatings as a viable candidate for orthopaedic implants. The scaffolds are composed of additively manufactured Ti6Al4V lattices, which were treated in an alkali solution to obtain the anatase and rutile phases. The treated surface exhibited hydrophilicity of <11.5°. A biopolymer carrier containing Insulin-like growth factor 1 was coated on the samples using immersion treatment. This study showed that the surface-modified porous Ti6Al4V scaffolds increased cell viability and proliferation, indicating potential for bone regeneration. The results demonstrate that surface modifications can enhance the osteoconduction and osteoinduction of Ti6Al4V implants, leading to improved bone regeneration and faster recovery. The porous Ti6Al4V scaffolds modified with surface coating of Gel/Alg-IGF-1 exhibited a noteworthy increase in cell viability (from 80.7 to 104.1%viability) and proliferation. These results suggest that the surface modified scaffolds have potential for use in treating bone defects.

RNA 5-methylcytosine status is associated with DNMT2/TRDMT1 nuclear localization in osteosarcoma cell lines

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Selected phenotypic features of three osteosarcoma (OS) cell lines were evaluated.

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Redox disequilibrium promoted sustained AKT and ERK1/2 activation.

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Redox imbalance modulated cell death pathways in OS cells.

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Nuclear levels of TRDMT1 methyltransferase were associated with RNA methylation.

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A novel marker for predicting therapy response in OS patients is proposed.

Osteosarcoma (OS) is a pediatric malignant bone tumor with unsatisfying improvements in survival rates due to limited understanding of OS biology and potentially druggable targets. The present study aims to better characterize osteosarcoma U-2 OS, SaOS-2, and MG-63 cell lines that are commonly used as in vitro models of OS. We focused on evaluating the differences in cell death pathways, redox equilibrium, the activity of proliferation-related signaling pathways, DNA damage response, telomere maintenance, DNMT2/TRDMT1-based responses and RNA 5-methylcytosine status. SaOS-2 cells were characterized by higher levels of superoxide and nitric oxide that promoted AKT and ERK1/2 activation thus modulating cell death pathways. OS cell lines also differed in the levels and localization of DNA repair regulator DNMT2/TRDMT1. SaOS-2 cells possessed the lowest levels of total, cytoplasmic and nuclear DNMT2/TRDMT1, whereas in MG-63 cells, the highest levels of nuclear DNMT2/TRDMT1 were associated with the most pronounced status of RNA 5-methylcytosine. In silico analysis revealed potential phosphorylation sites at DNMT2/TRDMT1 that may be related to the regulation of DNMT2/TRDMT1 localization. We postulate that redox homeostasis, proliferation-related pathways and DNMT2/TRDMT1-based effects can be modulated as a part of anti-osteosarcoma strategy reflecting diverse phenotypic features of OS cells.

IGF-1R Transported to the Cell Nuclei to Regulate the Proliferation of Breast Cancer Cells

Under normal physiological conditions, IGF-1 (insulin-like growth factor-1) has important biological effects. However, many studies have found that IGF-1 is closely related to the occurrence and development of breast cancer. But up to now, the cellular properties of IGF-1 have not been systematically explored in breast cancer cell. It is well-known that the cellular properties and behaviors of IGF-1/IGF-1R are closely related to its biological functions. In the current study, we used the breast cancer cell line as a model to explore the biological characteristics of IGF-1/IGF-1R, and found that IGF-1/IGF-1R can be internalized into the cytoplasm. In addition, we also found that IGF-1R can also enter cell nuclei under the mediation of IGF-1. Further research found that the nuclear-localized IGF-1R has important potential biological effects, which is closely associated to the proliferation of breast cancer cell, this may be achieved by regulating IGF-1R-mediated intracellular signaling. The current research has laid the foundation for investigating the relationship between IGF-1/IGF-1R system and the occurrence and development of breast cancer.

LncRNA HCP5 promotes cell proliferation and inhibits apoptosis via miR-27a-3p/IGF-1 axis in human granulosa-like tumor cell line KGN

This study aimed to reveal the potential roles of long non-coding RNA HCP5 (lncRNA HCP5) and its potential molecular mechanism in polycystic ovarian syndrome (PCOS). The human granulosa-like tumor cell line KGN was used for assessing the effects of HCP5 in the proliferation and apoptosis of granulosa cells (GCs). The results showed that downregulation of HCP5 suppressed cell proliferation through arresting cell cycle progression at G1 phase, and induced the apoptosis via activating mitochondrial pathway, while overexpression of HCP5 played the opposite effects in KGN cells. We predicted and confirmed miR-27a-3p was a directly target to HCP5 and it could directly bind with insulin-like growth factor-1 (IGF-1). Next, we performed gain- and loss-of-functions approaches by transfecting miR-27a-3p inhibitor into HCP5 knocking down cells and transfecting miR-27a-3p mimics into HCP5 overexpressing cells. The results demonstrated that downregulation and upregulation of miR-27a-3p could block the effects on the proliferation and apoptosis mediated by silencing and overexpressing HCP5 in KGN cells. Additionally, miR-27a-3p inhibitor remarkably reversed the IGF-1 decrease regulated by knocking down HCP5 and miR-27a-3p mimics inhibited the IGF-1 increase modulated by overexpressing HCP5 in KGN cells. Furthermore, we observed that the promoted cell vitality and reduced apoptosis mediated by enforced expression of HCP5 could be alleviated when the KGN cells transfected with IGF-1 siRNA. Our findings indicate that HCP5 might be a potential regulatory factor for development of PCOS through regulating the miR-27a-3p/IGF-1 axis.

Ras-ERK1/2 Signaling Promotes The Development Of Osteosarcoma By Regulating H2BK12ac Through CBP

Background

H2BK12ac is an important histone acetylation pattern of H2B, which has been reported in several cancers. However, whether H2BK12ac joins in Ras-ERK1/2 activation-induced osteosarcoma (OS) cell behaviors remain unclear. The study explored this peradventure and revealed the underlying mechanism.

Methods

MG-63 cells were transfected with pEGFP-N1, pEGFP-Ras^WT and pEGFP-K-Ras^G12V/T35S, H2BK12ac and ERK1/2 expression levels were analyzed by Western blot. Effects of H2BK12ac on cell viability, migration, colony formation and cell cycle were investigated by MTT, Transwell, soft-agar colony formation and flow cytometry assays. RT-qPCR and ChIP were performed to study the effect of H2BK12ac and CBP on ERK1/2-downstream gene transcriptions.

Results

H2BK12ac was specifically down-regulated by Ras-ERK1/2 activation in MG-63 cells. Down-regulated H2BK12ac participated in regulating cell proliferation and migration of MG-63 cells, meanwhile, affected the transcription of ERK1/2-downstream genes. Additionally, silence of HDAC1 up-regulated H2BK12ac expression, and inhibited the promoting effect of Ras-ERK1/2 on MG-63 cells' proliferation, migration and RNA expression levels of ERK1/2-downstream genes. Further, the degradation of CBP mediated by MDM2 was discovered to be linked to Ras-ERK1/2 activation-induced H2BK12ac down-regulation.

Conclusion

These findings from the study demonstrated that Ras-ERK1/2 signaling could promote the development of OS via regulating H2BK12ac through MDM2-mediated CBP degradation.

The lack of Raf-1 kinase feedback regulation enhances antiapoptosis in cancer cells

Raf-1 has an important role in cellular antiapoptosis. So far, there is no solid evidence that shows that Raf-1 mutation is associated with cancer development. In the course of further study of Raf-1 signaling, we have reported that Raf-1 hyperphosphorylation inhibits its kinase activity toward its downstream mitogen-activated protein kinase kinase 1/2 (MEK1/2) and proposed a model for negative feedback regulation of Raf-1. Here, we show that there is no hyperphosphorylation in some cancer cells, which results in increased kinase activity and enhances the antiapoptotic ability. Inhibition of either Raf-1 or ALG-2 (apoptosis-linked gene 2) expression results in apoptosis signal-regulating kinase 1/c-Jun N-terminal kinase (ASK1/JNK) signaling activation, and cell sensitivity to chemotherapeutic reagents, indicating that inhibition of ASK1/JNK apoptotic signaling by Raf-1 is mediated by ALG-2. A previous report indicated that extracellular signal-regulated kinase 1/2 (ERK1/2) were responsible for Raf-1 hyperphosphorylation. However, our evidence shows that when ERK1/2 are activated and the Raf-1 gene is not mutated, Raf-1 is not hyperphosphorylated in these cells, indicating that ERK1/2 are not responsible for the Raf-1 hyperphosphorylation in these cancer cell lines. Surprisingly, we also found that Raf-1 is not a necessary kinase for MEK1/2 activation under normal tissue culture conditions, but is required for MEK1/2 activation under apoptosis-inducing conditions. Our research demonstrates that although Raf-1 gene is not mutated, an abnormality of Raf-1 kinase feedback regulation enhances its antiapoptotic function, and Raf-1 can still be a pharmaceutical target to increase chemotherapy or radiotherapy sensitivity in these cancer cells.

Applied Nanotechnology and Nanoscience in Orthopedic Oncology

Nanomedicine is based on the fact that biological molecules behave similarly to nanomolecules, which have a size of less than 100 nm, and is now affecting most areas of orthopedics. In orthopedic oncology, most of the in vitro and in vivo studies have used osteosarcoma or Ewing sarcoma cell lineages. In this article, tumor imaging and treatment nanotechnology applications, including nanostructure delivery of chemotherapeutic agents, gene therapy, and the role of nano-selenium-coated implants, are outlined. Finally, the potential role of nanotechnology in addressing the challenges of drug and radiotherapy resistance is discussed. [Orthopedics. 2016; 39(5):280-286.].

Impact of RANK signalling on survival and chemotherapy response in osteosarcoma

The receptor activator of NF-κB (RANK) signalling pathway represents a promising target for the therapy of bone-related tumours. In the present study we evaluated the impact of the expression of RANK and its ligand (RANKL) on survival and response to chemotherapy in osteosarcoma patients.Expression of RANK and RANKL was examined in 91 human osteosarcomas by immunohistochemistry using formalin fixed, paraffin embedded (FFPE) tumour samples. Results of the stainings were correlated with clinicopathological parameters and patient survival.Sixty-three osteosarcomas (69.2%) expressed RANK, whereas only eight cases (8.8%) showed expression of RANKL. Expression of RANK was significantly associated with shorter disease-free survival by Kaplan-Meier analysis (p=0.031). We further observed worse response to chemotherapy in RANK expressing tumours, which was statistically not significant (p=0.099). RANKL expression was significantly more frequent in osteosarcoma of the lower extremity than in any other location. Analysis of RANKL expression did not reveal any statistically significant correlation with disease-free or osteosarcoma-specific survival.In our study, we identified RANK expression as a negative prognostic factor regarding disease-free survival in osteosarcoma. Moreover, RANK might modulate response of human osteosarcoma to chemotherapy. Therefore, RANK signalling cascade is likely to provide a novel alternative to targeted therapy of osteosarcoma and deserves further investigation.

Network-based identification of key proteins involved in apoptosis and cell cycle regulation

OBJECTIVES

Cancer cells differ from normal body cells in their ability to divide indefinitely and to evade programmed cell death. Crosstalk between apoptosis and cell cycle processes promotes balance between proliferation and death, and limits population growth and survival of cells. However, intricate relationships between them and how they are able to manipulate the fate of cancer cells still remain to be clarified. Identification of key factors involved in both apoptosis and cell cycle regulation may help to address this problem.

MATERIALS AND METHODS

Identification of such key proteins was carried out, using a series of bioinformatics methods, such as network construction and key protein identification.

RESULTS

In this study, we computationally constructed human apoptotic/cell cycle-related protein-protein interactions (PPIs) networks from five experimentally supported protein interaction databases, and further integrated these high-throughput data sets into a Naïve Bayesian model to predict protein functional connections. On the basis of modified apoptotic/cell cycle related PPI networks, we calculated and ranked all protein members involved in apoptosis and cell cycle regulation. Our results not only identified some already known key proteins such as p53, Rb, Myc and Src but also found that the proteasome, Cullin family members, kinases and transcriptional repressors play important roles in regulating apoptosis and the cell cycle. Furthermore, we found that the top 100 proteins ranked by PeC were enriched in some pathways such as those of cancer, the proteasome, the cell cycle and Wnt signalling.

CONCLUSIONS

We constructed the global human apoptotic/cell cycle related PPI network based on five online databases, and a Naïve Bayesian model. In addition, we systematically identified apoptotic/cell cycle related key proteins in cancer cells. These findings may uncover intricate relationships between apoptosis and cell cycle processes and thus provide further new clues towards future anticancer drug discovery.

A new approach for prediction of tumor sensitivity to targeted drugs based on functional data

Background

The success of targeted anti-cancer drugs are frequently hindered by the lack of knowledge of the individual pathway of the patient and the extreme data requirements on the estimation of the personalized genetic network of the patient’s tumor. The prediction of tumor sensitivity to targeted drugs remains a major challenge in the design of optimal therapeutic strategies. The current sensitivity prediction approaches are primarily based on genetic characterizations of the tumor sample. We propose a novel sensitivity prediction approach based on functional perturbation data that incorporates the drug protein interaction information and sensitivities to a training set of drugs with known targets.

Results

We illustrate the high prediction accuracy of our framework on synthetic data generated from the Kyoto Encyclopedia of Genes and Genomes (KEGG) and an experimental dataset of four canine osteosarcoma tumor cultures following application of 60 targeted small-molecule drugs. We achieve a low leave one out cross validation error of <10% for the canine osteosarcoma tumor cultures using a drug screen consisting of 60 targeted drugs.

Conclusions

The proposed framework provides a unique input-output based methodology to model a cancer pathway and predict the effectiveness of targeted anti-cancer drugs. This framework can be developed as a viable approach for personalized cancer therapy.

H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide) induces autophagy independently of protein kinase A inhibition

Autophagy is a degradation pathway for cytoplasmic proteins and organelles in eukaryotes. Although the mechanisms of autophagy regulation are not completely understood, the target of rapamycin (TOR) signaling pathway plays a major role in controlling the induction of autophagy. Cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase A (PKA) has also been shown to regulate autophagy in yeast and mammalian cells. In an effort to elucidate the role of the cAMP/PKA pathway in autophagy, we used the PKA inhibitor N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide (H89) to treat mammalian cells. Our data demonstrated that H89 induced autophagy at 10 μM, which is a commonly used concentration for PKA inhibition, but PKA inhibition was not involved in the induction of autophagy. The effects of cAMP on autophagy seemed to be dependent on the cell type and the culture conditions. In addition, we investigated which protein kinase was involved in H89-induced autophagy because several kinases other than PKA have been shown to be inhibited by 10 μM of H89. There was no protein kinase largely responsible for autophagy induction, although the inhibition of Akt, which is a downstream effector protein kinase of phosphatidylinositol-3-kinase, appeared to be partially associated. Furthermore, H89-induced autophagy was independent of TOR. H89 is a widely used PKA inhibitor, but PKA-independent effects have been reported. Therefore, it is suggested that autophagy induction is a nonspecific effect of H89, and H89-induced autophagy is independent of the cAMP-PKA and the TOR pathways.

Insulin-like growth factor-I enhances proliferation and differentiation of human mesenchymal stromal cells in vitro

Human mesenchymal stromal cells (hMSCs) offer great potential for bone tissue engineering applications, but their in vivo performance remains limited. Preconditioning of these cells with small molecules to improve their differentiation before implantation, or incorporation of growth factors are possible solutions. Insulin-like growth factor-1 (IGF-1) is one of the most abundant growth factors in bone, involved in growth, development, and metabolism, but its effects on hMSCs are still subject of debate. Here we examined the effects of IGF-1 on proliferation and differentiation of hMSCs in vitro and we found that serum abolished the effects of IGF-1. Only in the absence of serum, IGF-1 increased proliferation, alkaline phosphatase expression, and osteogenic gene expression of hMSCs. Furthermore, we examined synergistic effects of bone morphogenetic protein-2 (BMP-2) and IGF-1 and, although IGF-1 enhanced BMP-2-induced mineralization, IGF-1 only slightly affected in vivo bone formation.

Insulin-like growth factor I regulates G2/M progression through mammalian target of rapamycin signaling in oligodendrocyte progenitors

Extrinsic factors including growth factors influence decisions of oligodendrocyte progenitor cells (OPCs) to continue cell cycle progression or exit the cell cycle and terminally differentiate into oligodendrocytes capable of producing myelin. Multiple studies have elucidated how the G1/S transition is regulated in OPCs; however, little is known about how S phase progression and the G2/M transition are regulated in these cells. Herein, we report that insulin-like growth factor (IGF)-I coordinates with FGF-2 to promote S phase progression but regulates G2/M progression independently. During S phase, IGF-I/FGF-2 enhances protein expression of cyclin A and cdk2, and further increases effective complex formation resulting in enhanced cdk2 activity. Surprisingly, however, OPCs exposed to FGF-2 in the absence of IGF-I fail to traverse through G2/M. Consistent with this observation, OPCs exposed to IGF-I, but not FGF-2, increase cell number over 48 h. IGF-I enhances cdk1 kinase activity during G2/M by promoting nuclear localization of cyclin B/cdk1 as well as of Cdc25C, an activator of cdk1. IGF-I also induces phosphorylation of histone 3 indicating traverse of cells through mitosis. Finally, we demonstrate that IGF-I-mediated G2/M regulation requires mammalian target of rapamycin activity. These data support an important function for IGF-I in G2/M progression in OPCs.

Proliferative and mineralogenic effects of insulin, IGF-1, and vanadate in fish osteoblast-like cells

Fish have recently been recognized as a suitable model and a promising alternative to mammalian systems to study skeletogenesis. In this regard, several fish bone-derived cell lines have been developed and are being used to investigate mechanisms associated with insulin-like action of vanadium on extracellular matrix (ECM) mineralization. Although proliferative and mineralogenic effects of vanadate, insulin-like growth factor 1 (IGF-1), and insulin have recently been evaluated in a fish prechondrocyte cell line, no data are available in fish bone-forming cells, the osteoblasts. Using fish preosteoblast cells, we showed that IGF-1, but not insulin or vanadate, stimulated cell proliferation through the mitogen-activated protein kinase (MAPK) pathway, while both IGF-1 and vanadate inhibited cell differentiation/ECM mineralization through the same mechanism. Our data also indicated that the phosphatidyl inositol-3 kinase (PI-3K) pathway stimulates differentiation/ECM mineralization in osteoblasts and could represent a way to balance MAPK pathway action. The comparison of these new data obtained in fish with those available in mammals clearly evidenced a conservation of regulatory mechanisms among vertebrate bone-derived systems, although different players are involved.

Synergistic effect of low-dose cucurbitacin B and low-dose methotrexate for treatment of human osteosarcoma

We investigated the use of cucurbitacin B, a plant-derived tetracyclic triterpenoid, as a single agent or in combination with methotrexate (MTX) for human osteosarcoma (OS) treatment. Cucurbitacin B showed antiproliferative activity against seven human OS cell lines in vitro accompanying G2/M cell cycle arrest, apoptosis, and inhibition of ERK, Akt, and mTOR proteins. Cucurbitacin B in combination with MTX synergistically inhibited OS cell growth in vitro. Low-dose cucurbitacin B (LD-CuB, 0.5 mg/kg body weight) or low-dose MTX (LD-MTX, 150 mg/kg) failed to decrease the size of human OS xenografts in nude mice. However, combined therapy at identical concentrations inhibited tumor growth by 62% vs. LD-CuB and 81% vs. LD-MTX (p<0.001). Strikingly, the effect persisted even when the dose of MTX was decreased by two thirds (VLD-MTX, 50 mg/kg). In conclusion, cucurbitacin B alone or in combination with MTX shows promising antiproliferative activity against human OS.

STRAP regulates c-Jun ubiquitin-mediated proteolysis and cellular proliferation

STRAP is a ubiquitous WD40 protein that has been implicated in tumorigenesis. Previous studies suggest that STRAP imparts oncogenic characteristics to cells by promoting ERK and pRb phosphorylation. While these findings suggest that STRAP can activate mitogenic signaling pathways, the effects of STRAP on other MAPK pathways have not been investigated. Herein, we report that STRAP regulates the expression of the c-Jun proto-oncogene in mouse embryonic fibroblasts. Loss of STRAP expression results in reduced phospho-c-Jun and total c-Jun but does not significantly reduce the level of two other early response genes, c-Myc and c-Fos. STRAP knockout also decreases expression of the AP-1 target gene, cyclin D1, which is accompanied by a reduction in cell growth. No significant differences in JNK activity or basal c-Jun mRNA levels were observed between wild type and STRAP null fibroblasts. However, proteasomal inhibition markedly increases c-Jun expression in STRAP knockout MEFs and STRAP over-expression decreases the ubiquitylation of c-Jun in 293T cells. Loss of STRAP accelerates c-Jun turnover in fibroblasts and ectopic over-expression of STRAP in STRAP null fibroblasts increases c-Jun expression. Collectively, our findings indicate that STRAP regulates c-Jun stability by decreasing the ubiquitylation and proteosomal degradation of c-Jun.

Functional alterations in mechanical loading of condylar cartilage induces changes in the bony subcondylar region

Bone remodeling is orchestrated by cells of the osteoblast lineage and involves an intricate network of cell-cell and cell-matrix interactions. This dynamic process engages systemic hormones, locally produced cytokines and growth factors, as well as the mechanical environment of the cells. In growing subjects, the mandibular condyle consists of both articular and growth components and the presence of progenitor cells is verified by their anabolic responses to growth hormones. The pathways of chondrocyte and osteoblast differentiation during endochondral bone formation are interconnected and controlled by key transcription factors. The present study was undertaken to explore the possibility and the extent by which the mechano-transduction events in chondrocytes are 'sensed' in the subchondral bony area under altered functional loading. To this end, the involvement of the JNK/ERK-AP-1/Runx2 signaling axe was investigated by immunohistochemistry in temporomandibular joints of young rats subjected to different functional mastication loads. Our results showed that mechanical load triggers differentiation phenomena through the induction of master tissue regulators, namely the expression and/or activation of the JNK-c-Jun signaling pathway components and c-Fos in subchondral osteoblasts, as well as the activation of ERK/MAPK and the cellular expression of the transcription factor Runx2 in subchondral osteoblasts.

Novel agents in development for pediatric sarcomas

PURPOSE OF REVIEW

The survival curves for many pediatric sarcomas have remained flat for the past 2 decades or more and novel therapeutics - those small molecule medicines that selectively inhibit specific signaling molecules - have been slow to enter into pediatric practice. The preclinical basis for their use is reviewed here.

RECENT FINDINGS

Preclinical and phase I studies showing efficacy of antiinsulin-like growth factor receptor 1 therapies for Ewing sarcoma have led to numerous ongoing clinical trials using these agents for Ewing and other sarcomas. Early studies of ERBB signaling as a target in sarcoma therapy have been tantalizing, but progress in this area has been controversial. In-vitro analysis of Src inhibitors suggested that these agents would prevent metastasis in osteosarcoma, whereas in-vivo analysis showed no effect on metastasis, underscoring the need for thorough preclinical investigations of promising new therapies to guide future clinical trials. Antiangiogenic and immunomodulatory therapies are gaining momentum in the pediatric arena and should be tested in combination with traditional cytotoxic agents for recurrent and high-risk primary pediatric sarcomas.

SUMMARY

Pediatric sarcomas have diverse biology and distinct signaling pathways, making detailed preclinical evaluation of small molecule inhibitors essential to guiding the design of further clinical investigations.

Mechanisms of drug combinations: interaction and network perspectives

Understanding the molecular mechanisms underlying synergistic, potentiative and antagonistic effects of drug combinations could facilitate the discovery of novel efficacious combinations and multi-targeted agents. In this article, we describe an extensive investigation of the published literature on drug combinations for which the combination effect has been evaluated by rigorous analysis methods and for which relevant molecular interaction profiles of the drugs involved are available. Analysis of the 117 drug combinations identified reveals general and specific modes of action, and highlights the potential value of molecular interaction profiles in the discovery of novel multicomponent therapies.

Abnormalities of IGF-I signaling in the pathogenesis of diseases of the bone, brain, and fetoplacental unit in humans

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a transmembrane tyrosine kinase that phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the postreceptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero.

IGF-I regulates tight-junction protein claudin-1 during differentiation of osteoblast-like MC3T3-E1 cells via a MAP-kinase pathway

Insulin-like growth factor I (IGF-I) is expressed in many tissues, including bone, and acts on the proliferation and differentiation of osteoblasts as an autocrine/paracrine regulator. Tight-junction proteins have been detected in osteoblasts, and direct cell-to-cell interactions may modulate osteoblast function with respect, for example, to gap junctions. In order to investigate the regulation of expression of tight-junction molecules and of function during bone differentiation, osteoblast-like MC3T3-E1 cells and osteocyte-like MLO-Y4 cells were treated with IGF-I. In both MC3T3-E1 cells and MLO-Y4 cells, the tight-junction molecules occludin, claudin-1, -2, and -6, and the gap-junction molecule connexin 43 (Cx43) were detected by reverse transcription with polymerase chain reaction. In MC3T3-E1 cells but not MLO-Y4 cells, mRNAs of claudin-1, -2, and -6, Cx43, and type I collagen, and proteins of claudin-1 and Cx43 were increased after treatment with IGF-I. Such treatment significantly decreased paracellular permeability in MC3T3-E1 cells. The expression of claudin-1 in MC3T3-E1 cells after IGF-I treatment was mainly upregulated via a mitogen-activated protein (MAP)-kinase pathway and, in part, modulated by a PI3-kinase pathway, whereas Cx43 expression and the mediated gap-junctional intercellular communication protein did not contribute to the upregulation. Furthermore, in MC3T3-E1 cells during wound healing, upregulation of claudin-1 was observed together with an increase of IGF-I and type I collagen. These findings suggest that the induction of tight-junction protein claudin-1 and paracellular permeability during the differentiation of osteoblast-like MC3T3-E1 cells after treatment with IGF-I is regulated via a MAP-kinase pathway, but not with respect to gap junctions.

Change of p16(INK4a) and PCNA protein expression in myocardium after injection of hIGF-1 gene modified skeletal myoblasts into post-infarction rats

This study examined the change of p16(INK4a) and PCNA protein expression in myocardium after injection of hIGF-1 gene modified skeletal myoblasts into post-infarction rats. HIGF-1 gene modified skeletal myoblasts (hIGF-1-myoblasts) were injected into hind limb muscles of 18 post-infraction rats (experimental group). Primary-myoblasts were injected into 18 post-infraction rats (control group) and 12 non-infarction rats (sham group). Expression of p16(INK4a) and PCNA protein in myocardiums were separately detected immunocytochemically 1, 2 and 4 weeks after the injection. The level of hIGF-1 and rIGF-1 protein in serum and myocardium were detected by enzyme-linked immunosorbent assay (ELISA). Compared with the sham group, the percentage of p16(INK4a) and PCNA positive cells reached a peak after 1 week in the control group and the experimental group (P<0.01). Moreover, the percentage of p16(INK4a)-positive cells in the experimental group was lower than in control group whereas the percentage of PCNA-positive cells was lower in the control group than in the experimental group (P<0.01). The percentage of p16(INK4a)-positive cells in the experimental group and the percentage of PCNA-positive cells in the control group were close to that in the sham group from the 2nd week (P>0.05). ELISA analysis disclosed that the myocardium level of rIGF-1 protein increased gradually in the controls and especially in the experimental group (P<0.01). The serum level of rIGF-1 decreased significantly in post-infraction rats, but these conditions were improved in the experimental group (P<0.01). The hIGF-1 protein in serum and myocardium were detected from the 1st week to the 4th week in the experimental group. Statistical analysis revealed significant associations of myocardium level of hIGF-1 protein with expression of p16(INK4a) and PCNA protein (r=-0.323, P<0.05; r=0.647, P<0.01). It is concluded that genetically hIGF-1-myoblast provides a means for constant synthesis and release of hIGF-1. It could not only improve the expression of rIGF-1 and PCNA protein in myocardium, but also suppress the expression of p16(INK4a) protein for 30 days in post-infraction rats. Myoblasts-mediated IGF-1 gene therapy may provide a new alternative for the clinical treatment of heart failure.

Abnormalities of insulin-like growth factor-I signaling and impaired cell proliferation in osteoblasts from subjects with osteoporosis

IGF-I regulates bone acquisition and maintenance, even though the cellular targets and signaling pathways responsible for its action in human bone cells are poorly understood. Whether abnormalities in IGF-I action and signaling occur in human osteoblasts under conditions of net bone loss has not been determined. Herein we carried out a comparative analysis of IGF-I signaling in primary cultures of human osteoblasts from osteoporotic and control donors. In comparison with control cells, osteoporotic osteoblasts showed increased tyrosine phosphorylation of the IGF-I receptor in the basal state and blunted stimulation of receptor phosphorylation by IGF-I. Augmentation of basal IGF-I receptor phosphorylation was associated with coordinate increases in basal tyrosine phosphorylation of insulin receptor substrate (IRS)-2 and activation of Erk, which were also minimally responsive to IGF-I stimulation. By contrast, phosphorylation levels of IRS-1, Akt, and glycogen synthase kinase-3 were similar in the basal state in control and osteoporotic osteoblasts and showed marked increases after IGF-I stimulation in both cell populations, even though these responses were significantly lower in the osteoporotic osteoblasts. The IGF-I signaling abnormalities in osteoporotic osteoblasts were associated with reduced DNA synthesis both under basal conditions and after stimulation with IGF-I. Interestingly, treatment of the osteoporotic osteoblasts with the MAPK kinase inhibitor PD098059 reduced the elevated levels of Erk phosphorylation and increased basal DNA synthesis. Collectively, our data show that altered osteoblast proliferation in human osteoporosis may result from dysregulation of IGF-I receptor signaling, including constitutive activation of the IRS-2/Erk signaling pathway, which becomes unresponsive to IGF-I, and defective induction of the IRS-1/Akt signaling pathway.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

Role of ATP in DNA synthesis of renal proximal tubule cells: involvement of calcium, MAPKs, and CDKs

Although ATP has been shown to act as a modulator in various kidney functions, its effect on renal proximal tubule cell (PTC) proliferation has not been elucidated. This study investigated the effect of ATP on cell proliferation and the effect of its related signal pathways on primary cultured PTCs. Treatment with >10(-5) M ATP for 1 h stimulated incorporation of thymidine and bromodeoxyuridine. ATP (10(-4) M)-induced stimulation of thymidine incorporation was blocked by suramin (a P2X and P2Y receptor antagonist), reactive blue 2 (a P2Y receptor antagonist), MRS-2159 (a P2X1 receptor antagonist), and MRS-2179 (a P2Y1 receptor antagonist). ATP increased intracellular Ca2+ concentration, which was blocked by suramin, methoxyverapamil, and EGTA. ATP-induced stimulation of cell proliferation was also blocked by EGTA (an extracellular Ca2+ chelator), methoxyverapamil (a Ca2+ antagonist), and nifedipine (an L-type Ca2+ channel blocker), suggesting a role for Ca2+ influx. ATP-induced phosphorylation of p38 and p44/42 MAPKs was blocked by nifedipine. ATP increased expression levels of cyclin-dependent kinase (CDK)-2, CDK-4, and cyclin E, which were blocked by suramin, reactive blue 2, MRS-2179, MRS-2159, and nifedipine. However, ATP decreased expression levels of p21WAF1/Cip1 and p27kip1. ATP-induced stimulation of thymidine incorporation and increase of CDK-2 and CDK-4 expression were blocked by SB-203580 (a p38 MAPK inhibitor) and PD-98059 (an MEK inhibitor), but not by SP-600125 (a JNK inhibitor). In conclusion, ATP stimulates proliferation by increasing intracellular Ca2+ concentration and activating p38, p44/42 MAPKs, and CDKs in PTCs.

Prostaglandin-dependent activation of ERK mediates cell proliferation induced by transforming growth factor beta in mouse osteoblastic cells

Transforming growth factor beta (TGF(beta)) is a major coupling factor for bone turnover and is known to stimulate osteoblastic proliferation. Recent information indicates that, in addition to the Smad pathway, TGF(beta) also activates MAP kinases in osteoblastic cells. The role of these signaling cascades in cell proliferation induced by TGF(beta) as well as the cellular and molecular mechanisms of their activation by TGF(beta) has been investigated in this study. In MC3T3-E1 cells, TGF(beta) enhanced cell proliferation by about 2-fold and induced activation of the three MAP kinases, extracellular regulated kinase (ERK), p38, and c-Jun N-terminal kinase (JNK). Surprisingly, however, whereas activation of Smad2 was rapid and maximal after 15-min incubation, activation of MAP kinases was delayed with p38 stimulation detected after 1-h exposure and activation of ERK and JNK after 3 h, suggesting indirect activation of MAP kinases by TGF(beta). Among factors known to be released in response to TGF(beta) in osteoblastic cells and influence their growth, prostaglandins (PGs) were good candidates that were further investigated for mediating TGF(beta)-induced activation of MAP kinases and cell proliferation. Indomethacin, a selective inhibitor of PG synthesis, completely blunted cell proliferation induced by TGF(beta) and markedly reduced activation of MAP kinases without influencing Smad2 phosphorylation. EP4A, a specific PGE2 receptor antagonist, also blunted TGF(beta)-induced osteoblastic proliferation. In addition to these effects, PGE2 rapidly activated MAP kinases in MC3T3-E1 cells and increased cell proliferation by about 2-fold. The role of each MAP kinases in mediating TGF(beta)- and PGE2-induced cell proliferation was investigated using selective inhibitors. U0126, a specific inhibitor of the ERK pathway, completely blocked both TGF(beta)- and PGE2-induced cell proliferation whereas SB203580 and SP600125, which are selective inhibitors of, respectively, p38 and JNK pathways, had no effect. Finally, the effect of PGE2 on activation of ERK was mimicked by phorbol esters and not by forskolin, and was associated with activation of protein kinase C. This latter effect and the stimulation of ERK induced by PGE2 were completely blocked by a specific inhibitor of PKC. In conclusion, data presented in this study strongly suggest that the local release of PGE2 is involved in cell proliferation induced by TGF(beta) in osteoblastic cells. This effect is mediated by the ERK pathway activated by a PKC-dependent mechanism.

Parathyroid hormone uses multiple mechanisms to arrest the cell cycle progression of osteoblastic cells from G1 to S phase

Parathyroid hormone (PTH) plays a major role in bone remodeling and has the ability to increase bone mass if administered daily. In vitro, PTH inhibits the growth of osteoblastic cell lines, arresting them in G(1) phase. Here, we demonstrate that PTH regulates the expression of at least three genes to achieve the following: inducing expression of MAPK phosphatase 1 (MKP-1) and p21(Cip1) and decreasing expression of cyclin D1 at both mRNA and protein levels. The induction of MKP-1 causes the dephosphorylation of extracellular signal-regulated kinase and therefore the decrease in cyclin D1. Overexpression of MKP-1 arrests UMR cells in G(1) phase. The mechanisms involved in PTH regulation of these genes were studied. Most importantly, PTH administration produces similar effects on expression of these genes in rat femoral metaphyseal primary spongiosa. Analyses of p21(Cip1) expression levels in bone indicate that repeated daily PTH injections make the osteoblast more sensitive to successive PTH treatments, and this might be an important feature for the anabolic functions of PTH. In summary, our data suggest that one mechanism for PTH to exert its anabolic effect is to arrest the cell cycle progression of the osteoblast and hence increase its differentiation.

Role of PTEN and Akt in the regulation of growth and apoptosis in human osteoblastic cells

Cancer cells are characterized by either an increased ability to proliferate or a diminished capacity to undergo programmed cell death. PTEN is instrumental in regulating the balance between growth and death in several cell types and has been described as a tumor suppressor. The chromosome arm on which PTEN is located is deleted in a subset of human osteosarcoma tumors. Therefore, we predicted that the loss of PTEN expression was contributing to increased Akt activation and the subsequent growth and survival of osteosarcoma tumor cells. Immunoblot analyses of several human osteosarcoma cell lines and normal osteoblasts revealed relatively abundant levels of PTEN. Furthermore, stimulation of cell growth or induction of apoptosis in osteosarcoma cells failed to affect PTEN expression or activity. Therefore, routine regulation of osteosarcoma cell growth and survival appears to be independent of changes in PTEN. Subsequently, the activation of a downstream target of PTEN activity, the survival factor Akt, was analyzed. Inappropriate activation of Akt could bypass the negative regulation by PTEN. Analyses of Akt expression in several osteosarcoma cell lines and normal osteoblasts revealed uniformly low basal levels of phosphorylated Akt. The levels of phosphorylated Akt did not increase following growth stimulation. In addition, osteosarcoma cell growth was unaffected by inhibitors of phosphatidylinositol-3 kinase, an upstream activator of the Akt signaling pathway. These data further suggest that the Akt pathway is not the predominant signaling cascade required for osteoblastic growth. However, inhibition of PTEN activity resulted in increased levels of Akt phosphorylation and enhanced cell proliferation. These data suggest that while abundant levels of PTEN normally maintain Akt in an inactive form in osteoblastic cells, the Akt signaling pathway is intact and functional.

Suppression of WEE1 and Stimulation of CDC25A Correlates with Endothelin-dependent Proliferation of Rat Aortic Smooth Muscle Cells

Proliferation of vascular smooth muscle cells plays a key role in the pathogenesis of several disorders of the vascular wall. Endothelin (ET), a vasoactive peptide that signals through a G protein-coupled receptor, has been linked to mitogenesis in vascular smooth muscle cells, but the mechanistic details underlying this activity remain incompletely understood. In the present study, we demonstrate that ET-dependent mitogenesis in rat neonatal and adult aortic smooth muscle (RASM) cells is accompanied by an increase (up to 10-fold) in CDK2 activity, but not CDK2 protein levels. This effect is blocked almost entirely by PD98059 and UO126, implying involvement of the MEK/ERK signal transduction cascade in the activation. Extracts of ET-treated cells phosphorylate the N terminus of WEE1, an inhibitory kinase, which negatively regulates CDK2 activity through phosphorylation at Tyr(15), leading to a decrease in WEE1 activity and a reduction in levels of phospho-Tyr(15) in the CDK2 protein. ET also increases expression and activity of CDC25A, the regulatory phosphatase responsible for dephosphorylating Tyr(15). All of these effects are reversible following treatment with the MEK inhibitor PD98059. ET also increases levels of CDC2 activity in these cells in association with a decrease in levels of phospho-Tyr(15) on the CDC2 molecule. Phosphorylation of WEE1 is linked to ERK while phosphorylation of MYT1 (CDC2-selective inhibitory kinase) is tied to the ribosomal S6 kinase (RSK). In summary, ET controls progression through the cell cycle, in part, by increasing CDK2 and CDC2 activity through the MEK/ERK/RSK signal transduction pathway(s). This results from the phosphorylation and subsequent inactivation of two inhibitory kinases (WEE1 and MYT1) that tonically suppress CDK2 and CDC2 activity and activation of a phosphatase (CDC25A) that increases CDK2 activity.

Parallel phosphatidylinositol-3 kinase and p42/44 mitogen-activated protein kinase signaling pathways subserve the mitogenic and antiapoptotic actions of insulin-like growth factor I in osteoblastic cells

IGF-I is an endocrine and paracrine regulator of skeletal homeostasis, principally by virtue of its anabolic effects on osteoblastic cells. In the current study, we examined the intracellular signaling pathways by which IGF-I promotes proliferation and survival in SaOS-2 human osteoblastic cells. Inhibition of each of the phosphatidylinositol-3 kinase (PI-3 kinase), p42/44 MAPK, and p70s6 kinase pathways partially inhibited the ability of IGF-I to stimulate osteoblast proliferation and survival. Because activation of p70s6 kinase is downstream of both PI-3 kinase and p42/44 MAPK activation in osteoblasts treated with IGF-I, this ribosomal kinase represents a convergence point for IGF-I-induced PI-3 kinase and p42/44 MAPK signaling in osteoblastic cells. In addition, abrogation of PI-3 kinase-dependent Akt signaling, which does not inhibit IGF-I-induced p70s6 kinase phosphorylation, also inhibited the antiapoptotic effects of IGF-I in osteoblasts. Finally, interruption of G beta gamma signaling partially abrogated the ability of IGF-I to promote osteoblast survival, without inhibiting signaling through PI-3 kinase/Akt, p42/44 MAPKs, or p70s6 kinase. These data suggest that IGF-I signals osteoblast mitogenesis and survival through parallel, partly overlapping intracellular pathways involving PI-3 kinase, p42/44 MAPKs, and G beta gamma subunits.

Effects of combined treatment of insulin and human parathyroid hormone(1-34) on cancellous bone mass and structure in streptozotocin-induced diabetic rats

The purpose of this study was to test the hypothesis that combined treatment with insulin and human parathyroid hormone (hPTH) is more effective than treatment with insulin or hPTH alone in improving cancellous bone mass, connectivity, and strength in insulin-dependent diabetic rats. Diabetes was induced by intraperitoneal injection of streptozotocin (STZ) in 7-month-old female Wistar rats. The diabetic rats received insulin, hPTH, insulin and hPTH, or hPTH vehicle for 4 weeks, starting 8 weeks after STZ injection. They were compared with baseline controls and normal controls that received STZ alone and STZ vehicle alone, respectively. The rats' proximal right tibias were processed to serve as undecalcified Villanueva-stained bone sections for histomorphometry. Changes in trabecular connectivity were determined through node-strut analysis. The decreased cancellous bone volume (BV/TV) and bone formation in diabetic rats improved in all the drug-treated groups compared with baseline controls. Furthermore, recovery of BV/TV was greater in rats that received the combination of insulin and hPTH than in those that received insulin or hPTH alone. In node-strut analysis, the node-related parameter (N.Nd/TV) significantly increased in rats that received the combination of insulin and hPTH, but did not increase in those that received insulin or hPTH alone. In addition to these results, the combination treatment significantly increased bone mineral density of the femur and bone strength in the femoral metaphysis compared with treatment with insulin or hPTH alone. These results indicate that the doses of insulin and hPTH employed in the combination treatment were more effective in improving not only bone mass but also trabecular connectivity and bone strength than treatment with insulin or hPTH alone in insulin-dependent diabetic rats.

Role of endogenous regucalcin in nuclear regulation of regenerating rat liver: suppression of the enhanced ribonucleic acid synthesis activity

The role of endogenous regucalcin in the regulation of ribonucleic acid (RNA) synthesis activity in the nucleus of normal and regenerating rat livers was investigated. Nuclear RNA synthesis was measured by the incorporation of [(3)H]-uridine 5'-triphosphate into the nuclear RNA in vitro. The presence of regucalcin (0.25 or 0.5 microM) in the reaction mixture caused a significant decrease in nuclear RNA synthesis of normal rat liver. alpha-Amanitin (10(-8)-10(-6) M), an inhibitor of RNA polymerase II and III, decreased significantly nuclear RNA synthesis activity. The effect of regucalcin (0.25 microM) in decreasing nuclear RNA synthesis activity was not seen in the presence of alpha-amanitin (10(-6) M). The calcium chloride (10 microM)-increased nuclear RNA synthesis activity was significantly suppressed by the addition of regucalcin (0.25 microM). RNA synthesis activity was significantly enhanced in the nuclei of regenating rat liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly inhibited in the presence of PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M). Western analysis of the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy showed a significant increase in regucalcin protein as compared with that of sham-operated rats. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) in the reaction mixture caused a significant increase in nuclear RNA synthesis activity of normal rat liver. This increase was completely blocked by the addition of regucalcin (1.0 microM). The effect of anti-regucalcin monoclonal antibody (50 ng/ml) in increasing nuclear RNA synthesis activity was significantly enhanced in the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly suppressed by the addition of alpha-amanitin (10(-6) M), PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M) in the reaction mixture. The present study demonstrates that endogenous regucalcin has a suppressive effect on the enhancement of RNA synthesis activity in the nucleus of regenerating rat liver with proliferative cells.

Immature human osteoblastic MG63 cells predominantly express a subtype 1-like CGRP receptor that inactivates extracellular signal response kinase by a cAMP-dependent mechanism

Although accumulated data suggest that calcitonin gene-related peptide (CGRP) produces anabolic effects in skeletal tissue by directly acting on osteogenic cells, neither the distribution of CGRP receptor subtypes nor the associated cellular signaling pathways are well understood. In this study, we have pharmacologically and biochemically characterized CGRP-binding sites in immature human osteoblastic MG63 cells. In a [125I]CGRP whole-cell-binding assay, nonlinear regression curve-fitting analysis demonstrated a single binding site (K(D)=405+/-29 pM; 13,100+/-223 sites per cell). Immunocytochemical and Western blot analyses demonstrated that 48-, 52-, and 120-kDa forms of the calcitonin receptor-like receptor (CRLR) and a 15-kDa form of the receptor-activity-modifying protein-1 (RAMP-1) was expressed on the plasma membrane. CGRP strongly stimulated cellular cAMP production and this effect was antagonized not only by an antagonist of the subtype-1 CGRP (CGRP(1)) receptor, CGRP-(8-37), but by an agonist of the putative subtype-2 CGRP (CGRP(2)) receptor, [Cys(Acm)(2,7)]-CGRP, that also itself acted as a weak agonist. In contrast to published data, CGRP dose- and time-dependently dephosphorylated and inactivated extracellular signal response kinase (ERK). This action was blocked by CGRP-(8-37), by an inhibitor of cAMP-dependent protein kinase (H-89), or by an inhibitor of protein phosphatases (vanadate). Prolonged CGRP treatments significantly suppressed DNA synthesis at 27 h, but up-regulated type I collagen. Both these actions were blocked by CGRP-(8-37) and mimicked by a specific inhibitor of ERK (PD98059). In summary, our data suggest that the CGRP receptors in MG63 cells meet many, but not all, of the classical criteria used to define CGRP(1) receptors. These receptors that functioned in a pharmacologically distinct manner could inhibit cell proliferation, and were substantially more sensitive to a CGRP(2) receptor agonist than are typical CGRP(1) receptors. These receptor proteins were not exactly matched with the known components of a CGRP(1) receptor that have been reported. Therefore, it is possible that the CGRP receptors expressed in immature osteoblastic human MG63 cells represent a variation of the known CGRP(1) receptor.

Suppressive role of endogenous regucalcin in the enhancement of deoxyribonucleic acid synthesis activity in the nucleus of regenerating rat liver

The role of endogenous regucalcin in the regulation of deoxyribonucleic acid (DNA) synthesis activity in the nuclei of regenerating rat liver after partial hepatectomy was investigated. The addition of regucalcin (0.25 and 0.5 microM) in the reaction mixture caused a significant decrease in the nuclear DNA synthesis activity of normal rat liver. This decrease was also seen in the presence of Ca2+-chelator EGTA (0.4 mM), indicating that the effect of regucalcin is not related to nuclear Ca2+. Nuclear DNA activity was significantly increased in the presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the reaction mixture. The effect was completely abolished by the addition of regucalcin (0.5 microM). Nuclear DNA synthesis activity was significantly increased at 24, 48, and 72 h after partial heptectomy. The effect of anti-regucalcin monoclonal antibody (25 ng/ml) in increasing nuclear DNA synthesis activity was significantly enhanced at 24 and 48 h after partial hepatectomy. The presence of staurospone (10(-6) M), trifluoperazine (2 x 10(-5) M), or PD98059 (10(-5) M) in the reaction mixture caused a significant decrease in DNA synthesis activity in the nuclei obtained at 24 after partial hepateactomy. The effect of these inhibitors in the presence of anti-regucalcin monoclonal antibody (25 ng/ml) was greater than that in the absence of the antibody. The present study suggests that endogenous regucalcin plays a suppressive role in the enhancement of nuclear DNA synthesis activity in regenerating liver with cell proliferation after partial hepatectomy in rats.

MEK and ERK inhibitors enhance the anti-proliferative effect of interferon-alpha2b

Interferon (IFN)-alpha, initially characterized as an antiviral cytokine, affects several cellular functions. It is used in clinical practice for the treatment of several tumors, including hematopoietic malignancies, due to its antiproliferative effects. To better characterize the molecular mechanism(s) underlying this property, we conducted our studies in purified primary CD4+ T cells stimulated with anti-CD3 and interleukin (IL)-2. Upon treatment with IFN-alpha, the cells were blocked in the G0/G1 phase of the cell cycle and exhibited impaired entry into S phase and reduced proliferation. Moreover, we detected short- and long-term inhibition of extracellular signal-regulated kinase (ERK) and mitogen-activated ERK-regulating kinase (MEK) function, known to control cellular proliferation. The activity of the upstream regulators, Ras and Raf-1, was not affected. Analysis of downstream events controlled by the MEK/ERK pathway showed reduced activity of cyclin-dependent kinase (Cdk)-2 and -4, high levels of the mitotic inhibitors, p21Waf1 and p27Kipl, and decreased cyclin D and E expression. When IFN-alpha was used in combination with MEK and ERK inhibitors, we observed a dose-dependent additive effect in reducing cellular proliferation. Our data demonstrate that IFN-alpha may be associated with other molecules to inhibit cellular growth by targeting the MEK/ERK pathway. This may eventually lead to new clinical strategies to strengthen its anticancer effect.

Paracrine activation of extracellular signal-regulated kinase in a simple in vitro model of wounded osteoblasts

The immediate signal-transduction response of osteoblasts to acute trauma is poorly characterized. We have developed a simple in vitro model for osteoblast trauma to investigate aspects of the molecular mechanisms of wound healing in bone. Herein we report the specific, rapid, and transient phosphorylation of extracellular signal-regulated kinase (ERK) 1 and 2 in osteoblasts as a response to disruption ("wounding") of a confluent monolayer. The mitogen-activated protein kinase (MAPK) cascades of p38 and stress-activated protein kinase/c-jun N-terminal kinase (SAPK/JNK) were not activated by this perturbation. The response to wounding was equivalent to the activation of ERK by the addition of exogenous growth factors, and the perturbation-dependent phosphorylation of ERK can be suppressed by an inhibitor of heparin-binding growth factors. Conditioned media from wounded monolayers can induce the phosphorylation of ERK in unperturbed monolayers. Using immunohistochemistry, it was demonstrated that the cells with increased levels of phosphorylated ERK were not localized to the wound edges. These results indicate that ERK activation is the result of an autocrine/paracrine response by osteoblasts to trauma. We speculate that osteoblasts respond to trauma with the release of soluble factors as part of an autocrine/paracrine modulation of the wound-healing process in bone.

Mechanical strain and fluid movement both activate extracellular regulated kinase (ERK) in osteoblast-like cells but via different signaling pathways

Extracellular regulated kinases (ERKs)-1 and -2 are members of the MAPK family of protein kinases involved in the proliferation, differentiation, and apoptosis of bone cells. We have shown previously that ROS 17/2.8 cells show increased activation of ERK-1 or -2, which is sustained for 24 h, when the strips onto which they are seeded are subjected to a 10 min period of cyclic four point bending that produces physiological levels of mechanical strain along with associated fluid movement of the medium. Movement of the strips through the medium without bending causes fluid movement without strain. This also increases ERK-1/2 activation, but in a biphasic manner over the same time period. Our present study investigates the role of components of signaling pathways in the activation of ERK-1/2 in ROS 17/2.8 cells in response to these stimuli. Using a range of inhibitors we show specific differences by which ERK-1 and ERK-2 are activated in response to fluid movement alone, compared with those induced in response to strain plus its associated fluid movement. ERK-1 activation induced by fluid movement was markedly reduced by nifedipine, and therefore appears to involve L-type calcium channels, but was unaffected by either L-NAME or indomethacin. This suggests independence from prostacyclin (PGI(2)) and nitric oxide (NO) production. In contrast, ERK-1 activation induced by application of strain (and its associated fluid disturbance) was abrogated by TMB-8 hydrochloride, L-NAME, and indomethacin. This suggests that strain-induced ERK-1 activation is dependent upon calcium mobilization from intracellular stores and production of NO and PGI(2). ERK-2 activation appears to be mediated by a separate mechanism in these cells. Its activation by fluid movement alone involved both PGI(2) and NO production, but its activation by strain was not affected by any of the inhibitors used. The G protein inhibitor, pertussis toxin, did not cause a reduction in the activation of ERK-1 or -2 in response to either stimulus. These results are consistent with earlier observations of ERK activation in bone cells in response to both strain (with fluid movement) and fluid movement alone, and further demonstrate that these phenomena stimulate distinct signaling pathways.

Transforming growth factor-beta-induced osteoblast elongation regulates osteoclastic bone resorption through a p38 mitogen-activated protein kinase- and matrix metalloproteinase-dependent pathway

Transforming growth factor-beta (TGF-beta) is a powerful modulator of bone metabolism, and both its anabolic and catabolic effects on bone have been described. Here we have tested the hypothesis that TGF-beta-induced changes in osteoblast shape promote bone resorption by increasing the surface area of bone that is accessible to osteoclasts. The addition of TGF-beta1 to MC3T3-E1 cells resulted in cytoskeletal reorganization, augmented expression of focal adhesion kinase, and cell elongation, accompanied by an increase in the area of cell-free substratum. TGF-beta1 also triggered activation of Erk1/2 and p38 mitogen-activated protein (MAP) kinase. The p38 MAP kinase inhibitor PD169316, but not an inhibitor of the Erk1/2 pathway, abrogated the effect of TGF-beta1 on cell shape. The matrix metalloproteinase inhibitor GM6001 also interfered with osteoblast elongation. Treatment of MC3T3-E1 cells seeded at confluence onto bone slices to mimic a bone lining cell layer with TGF-beta1 also induced cell elongation and increased pit formation by subsequently added osteoclasts. These effects were again blocked by PD169316 and GM6001. We propose that this novel pathway regulating osteoblast morphology plays an important role in the catabolic effects of TGF-beta on bone metabolism.

Regulatory role of endogenous regucalcin in the enhancement of nuclear deoxyribonuleic acid synthesis with proliferation of cloned rat hepatoma cells (H4-II-E)

The role of endogenous regucalcin in the regulation of deoxyribonuleic acid (DNA) synthesis in the nuclei of the cloned rat hepatoma cells (H4-II-E) with proliferative cells was investigated. Cells were cultured for 6-96 h in a alpha-minimum essential medium (alpha-MEM) containing fetal bovine serum (FBS; 1 or 10%). Cell number was significantly increased between 24 and 96 h after culture with 10% FBS; cell proliferation was markedly stimulated by culture with 10% FBS as compared with that of 1% FBS. In vitro DNA synthesis activity in the nuclei of cells was significantly elevated 6 h after culture with 10% FBS and its elevation was remarkable at 12 and 24 h after the culture. Nuclear DNA synthesis activity was significantly reduced in the presence of various protein kinase inhibitors (PD98059, staurosprine, or trifluoperazine) in the reaction mixture containing the nuclei of cells cultured for 12 and 24 h with FBS (1 and 10%). The addition of regucalcin (10(-7) and 10(-6)M) in the reaction mixture caused a significant inhibition of nuclear DNA synthesis activity. The presence of anti-regucalcin monoclonal antibody (25-100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS resulted in a significant increase in nuclear DNA synthesis activity. This increase was completely blocked by the addition of regucalcin (10(-6) M). The effect of anti-regucalcin antibody (100 ng/ml) in increasing nuclear DNA synthesis activity was significantly inhibited in the presence of various protein kinase inhibitors. DNA synthesis activity was significantly enhanced in the presence of anti-regucalcin antibody (100 ng/ml) in the reaction mixture containing the nuclei of cells cultured for 24 h with 10% FBS in the presence of Bay K 8644 (2.5 x 10(-6) M). Culture with Bay K 8644 did not cause a significant increase in DNA synthesis activity in the absence of anti-regucalcin antibody. The present study demonstrates that endogenous regucalcin plays a suppressive role in the enhancement of nuclear DNA synthesis with proliferative cells.

Evidence that anabolic effects of PTH on bone require IGF-I in growing mice

Although it has been established that PTH exerts potent anabolic effects on bone in animals and humans, the mechanism of PTH action on bone remains controversial. Based on the previous findings that PTH treatment increased production of IGF-I in bone cells and that PTH effects on bone cells in vitro were blocked by IGF-I-blocking antibodies, we proposed that IGF-I action is required for the stimulatory effects of PTH on bone formation. To test this hypothesis, we evaluated the effects of PTH on bone formation parameters in growing mice lacking functional IGF-I genes. Five-week-old IGF-I(-/-) mice and wild-type littermates were given daily sc injections of 160 microg/kg body weight of PTH (1-34) or vehicle for 10 d. In wild-type animals, PTH caused a significant increase in serum osteocalcin levels (113%), serum alkaline phosphatase activity (48%), and alkaline phosphatase activity in femoral bone extracts (>80%), compared with the vehicle-treated control group. In contrast, in IGF-I(-/-) mice, there was no significant effect of PTH on any bone formation parameters. PTH treatment increased total bone mineral density, as evaluated by peripheral quantitative computer tomography, at the distal metaphysis of the femur by 40% in wild-type mice, but it had no effect on bone mineral density in mice lacking functional IGF-I genes. In vitro studies using osteoblasts derived from control and IGF-I(-/-) mice revealed that PTH treatment increased cell number in osteoblasts derived from IGF-I knockout mice in the presence of exogenously added IGF-I but not without IGF-I. These data to our knowledge provide the first direct evidence that the anabolic effects of PTH on bone formation in vivo require IGF-I action in growing mice.

Osteoprotegerin ameliorates sciatic nerve crush induced bone loss

This study examines the ability of osteoprotegerin (OPG) to prevent the local bone resorption caused by sciatic nerve damage. Sixty-five 18-week-old male mice were assigned to one of six groups (n = 10-11/group). A baseline control group was sacrificed on day zero of the 10-day study. The remaining groups were placebo sham operated, placebo nerve crush (Plac NC) operated, 0.1 mg/kg/day OPG + nerve crush (LOW), 0.3 mg/kg/day OPG + nerve crush (MED), and 1.0 mg/kg/day OPG + nerve crush (HI). Nerve crush or sham operations were performed on the right leg. The left leg served as a contralateral control to the nerve crushed (ipsilateral) leg. The difference in mass between the right and left femur and tibia was examined. Additionally, quantitative histomorphometry was performed on the right and left femur and tibia diaphyses. Nerve crush resulted in a significant loss of bone mass in the ipsilateral side compared to the contralateral side. Bone mass for the ipsilateral bones of the Plac NC group were significantly reduced by 3.8% in the femur and 3.5% in the tibia compared to the contralateral limb. The percent diminution was reduced for OPG treated mice compared to the Plac NC group for both the femur and tibia. In the femur, the percent reduction of ipsilateral bone mass was reduced to 1.0% (LOW), 1.3% (MED) and 1.6% (HI) compared to the contralateral limb. In the tibia, loss of bone mass in the ipsilateral limb was reduced to 1.4% (LOW), 1.4% (MED), and 2.4% (HI) compared to the contralateral. OPG also decreased the amount of tibial endocortical resorption compared to the Plac NC group. In summary, OPG mitigated bone loss caused by damage to the sciatic nerve.

Mitogenic G(i) protein-MAP kinase signaling cascade in MC3T3-E1 osteogenic cells: activation by C-terminal pentapeptide of osteogenic growth peptide [OGP(10-14)] and attenuation of activation by cAMP

In osteogenic and other cells the mitogen-activated protein (MAP) kinases have a key role in regulating proliferation and differentiated functions. The osteogenic growth peptide (OGP) is a 14 mer mitogen of osteogenic and fibroblastic cells that regulates bone turnover, fracture healing, and hematopoiesis, including the engraftment of bone marrow transplants. It is present in the serum and extracellular fluid either free or complexed to OGP-binding proteins (OGPBPs). The free immunoreactive OGP consists of the full length peptide and its C-terminal pentapeptide OGP(10-14). In the present study, designed to probe the signaling pathways triggered by OGP, we demonstrate in osteogenic MC3T3 E1 cells that mitogenic doses of OGP(10-14), but not OGP, enhance MAP kinase activity in a time-dependent manner. The OGP(10-14)-induced stimulation of both MAP kinase activity and DNA synthesis were abrogated by pertusis toxin, a G(i) protein inhibitor. These data offer direct evidence for the occurrence in osteogenic cells of a peptide-activated, mitogenic Gi protein-MAP kinase-signaling cascade. Forskolin and dBu(2)-cAMP abrogated the OGP(10-14)-stimulated proliferation, but induced only 50% inhibition of the OGP(10-14)-mediated MAP kinase activation, suggesting additional MAP kinase-dependent, OGP(10-14)-regulated, cellular functions. Finally, it is demonstrated that OGP(10-14) is the active form of OGP, apparently generated proteolytically in the extracellular milieu upon dissociation of OGP-OGPBP complexes.

Hypoxia induces apoptosis by caspase activation accompanying cytochrome C release from mitochondria in MC3T3E1 osteoblasts. p38 MAPK is related in hypoxia-induced apoptosis

The aim of this study is to elucidate the possible mechanism of apoptosis in response to hypoxia in MC3T3E1 osteoblasts. MC3T3E1 osteoblasts under hypoxic conditions (2% oxygen) resulted in apoptosis in a time-dependent manner estimated by DNA fragmentation assay and nuclear morphologystained with fluorescent dye, Hoechst 33258. Pretreatment with Z-VAD-FMK, a pan-caspase inhibitor, or Z-DEVD-CHO, a specific caspase-3 inhibitor, completely suppressed the DNA ladder in response to hypoxia. An increase in caspase-3-like protease (DEVDase) activity was observed during apoptosis, but no caspase- activity (YVADase) was detected. To confirm what caspases are involved in apoptosis, western blot analysis was performed using anti-caspase-3 or -6 antibody. The 10-kDa protein, corresponding to the active products of caspase-3 and the 10-kDA protein of the active protein of caspase-6 were generated in hypoxia-challenged cells in which processing of the full length form of caspase-3 and -6 was evident. With a time course similar to this caspase-3 and -6 activation was evident, hypoxic stress caused the cleavage of lamin A, typical of caspase-6 activity. In addition, the stress elicited the release of cytochrome c into the cytosol during apoptosis. Furthermore, we have observed that pre-treatment with SB203580, a selective p38 MAP kinase (p38 MAPK) inhibitor, attenuated the hypoxia-induced apoptosis. The addition of SB203580 suppressed caspase-3 and -6-like protease activity by hypoxia up to 50%. In contrast, PD98059 had no effect on the hypoxia-induced apoptosis. To confirm the involvement of MAP kinase, JNK/SAPK, ERK, or p38 kinase assay was performed. Although p38 MAPK was activated in response to hypoxic treatment, the other MAP kinase -JNK/SAPK or ERK- was not or modestly activated. These results suggest that p38 MAPK positively regulates hypoxia-induced apoptosis in MC3T3E1 osteoblasts.