Integrin engagement, the actin cytoskeleton, and c-Src are required for the calcitonin-induced tyrosine phosphorylation of paxillin and HEF1, but not for calcitonin-induced Erk1/2 phosphorylation

Osteoclast Signal Transduction During Bone Metastasis Formation

Osteoclasts are myeloid lineage-derived bone-resorbing cells of hematopoietic origin. They differentiate from myeloid precursors through a complex regulation process where the differentiation of preosteoclasts is followed by intercellular fusion to generate large multinucleated cells. Under physiological conditions, osteoclastogenesis is primarily directed by interactions between CSF-1R and macrophage colony-stimulating factor (M-CSF, CSF-1), receptor activator of nuclear factor NF-κB (RANK) and RANK ligand (RANKL), as well as adhesion receptors (e.g., integrins) and their ligands. Osteoclasts play a central role in physiological and pathological bone resorption and are also required for excessive bone loss during osteoporosis, inflammatory bone and joint diseases (such as rheumatoid arthritis) and cancer cell-induced osteolysis. Due to the major role of osteoclasts in these diseases the better understanding of their intracellular signaling pathways can lead to the identification of potential novel therapeutic targets. Non-receptor tyrosine kinases and lipid kinases play major roles in osteoclasts and small-molecule kinase inhibitors are emerging new therapeutics in diseases with pathological bone loss. During the last few years, we and others have shown that certain lipid (such as phosphoinositide 3-kinases PI3Kβ and PI3Kδ) and tyrosine (Src−family and Syk) kinases play a critical role in osteoclast differentiation and function in humans and mice. Some of these signaling pathways shows similarity to immunoreceptor-like receptor signaling and involves important other enzymes (e.g., PLCγ2) and adapter proteins (such as the ITAM−bearing adapters DAP12 and the Fc-receptor γ-chain). Here, we review recently identified osteoclast signaling pathways and their role in osteoclast differentiation and function as well as pathological bone loss associated with osteolytic tumors of the bone. A better understanding of osteoclast signaling may facilitate the design of novel and more efficient therapies for pathological bone resorption and osteolytic skeletal metastasis formation.

Non-visual arrestins regulate the focal adhesion formation via small GTPases RhoA and Rac1 independently of GPCRs

Arrestins recruit a variety of signaling proteins to active phosphorylated G protein-coupled receptors in the plasma membrane and to the cytoskeleton. Loss of arrestins leads to decreased cell migration, altered cell shape, and an increase in focal adhesions. Small GTPases of the Rho family are molecular switches that regulate actin cytoskeleton and affect a variety of dynamic cellular functions including cell migration and cell morphology. Here we show that non-visual arrestins differentially regulate RhoA and Rac1 activity to promote cell spreading via actin reorganization, and focal adhesion formation via two distinct mechanisms. Arrestins regulate these small GTPases independently of G-protein-coupled receptor activation.

miR-18a counteracts AKT and ERK activation to inhibit the proliferation of pancreatic progenitor cells

Activation of endogenous stem/progenitor cells to repair injured tissues is an ideal option for disease treatment. However, adult pancreatic progenitor cells remain in a quiescent state in vivo. Thus, it is difficult to stimulate proliferation and differentiation in these progenitor cells, and the cause remains elusive. miR-17-92 cluster miRNAs are highly conserved in mammals and are expressed in multiple tissue stem/progenitor cells, but their role in pancreatic progenitor cells are less well known. In the present study, we demonstrate that miR-18a, but not the other members of the miR-17-92 gene cluster, inhibits the proliferation of pancreatic progenitor cells in vitro and ex vivo. miR-18a inhibits proliferation of adult pancreatic progenitor cells through arresting the cell cycle at G1 stage, indicating that miR-18a plays a role in keeping the adult pancreatic progenitor cells in quiescence. miR-18a inhibits pancreatic progenitor proliferation by targeting the gene expressions of connective tissue growth factor (CTGF), neural precursor cell expressed, developmentally down-regulated 9 (Nedd9), and cyclin dependent kinase 19 (CDK19), as well as by suppressing activation of the proliferation-related signaling pathways phosphatidylinositol 3-kinase–protein kinase B (PI3K/AKT) and extracellular signal-regulated kinase (ERK).

Adaptors for disorders of the brain? The cancer signaling proteins NEDD9, CASS4, and PTK2B in Alzheimer's disease

No treatment strategies effectively limit the progression of Alzheimer's disease (AD), a common and debilitating neurodegenerative disorder. The absence of viable treatment options reflects the fact that the pathophysiology and genotypic causes of the disease are not well understood. The advent of genome-wide association studies (GWAS) has made it possible to broadly investigate genotypic alterations driving phenotypic occurrences. Recent studies have associated single nucleotide polymorphisms (SNPs) in two paralogous scaffolding proteins, NEDD9 and CASS4, and the kinase PTK2B, with susceptibility to late-onset AD (LOAD). Intriguingly, NEDD9, CASS4, and PTK2B have been much studied as interacting partners regulating oncogenesis and metastasis, and all three are known to be active in the brain during development and in cancer. However, to date, the majority of studies of these proteins have emphasized their roles in the directly cancer relevant processes of migration and survival signaling. We here discuss evidence for roles of NEDD9, CASS4 and PTK2B in additional processes, including hypoxia, vascular changes, inflammation, microtubule stabilization and calcium signaling, as potentially relevant to the pathogenesis of LOAD. Reciprocally, these functions can better inform our understanding of the action of NEDD9, CASS4 and PTK2B in cancer.

Calcitonin: physiology or fantasy?

Calcitonin, a potent hypocalcemic hormone produced by the C-cells of the thyroid, was first discovered by Harold Copp in 1962. The physiological significance of calcitonin has been questioned, but recent studies using genetically modified mouse models have uncovered additional actions of calcitonin acting through its receptor (CTR) that are of particular significance to the regulation of bone and calcium homeostasis. Mice in which the CTR is deleted in osteoclasts are more susceptible to induced hypercalcemia and exogenous calcitonin is able to lower serum calcium in younger animals. These data are consistent with the hypothesis that calcitonin can regulate serum calcium by inhibiting the efflux of calcium from bone, and that this action is most important when bone turnover is high. Calcitonin has also been implicated in protecting the skeleton from excessive loss of bone mineral during times of high calcium demand, such as lactation. This action may be linked to an intriguing and as yet unexplained observation that calcitonin inhibits bone formation, because deletion of the CTR leads to increased bone formation. We propose several mechanisms by which calcitonin could protect the skeleton by regulating bone turnover, acting within the bone and/or centrally. A new more holistic notion of the physiological role of calcitonin in bone and calcium homeostasis is required and we have highlighted some important knowledge gaps so that future calcitonin research will help to achieve such an understanding.

Antitumor activity of methyl gallate by inhibition of focal adhesion formation and Akt phosphorylation in glioma cells

BACKGROUND

Methyl gallate (MG) possesses a wide range of biological properties that include anti-oxidant, anti-inflammatory, and anti-microbial activities. However, its anti-tumor activity has not been extensively examined in cancer cells. Thus, we examined the effect of MG in both glutamate-induced rat C6 and human U373 glioma cell proliferation and migration.

METHODS

MG was isolated from the stem bark of Acer barbinerve. Cell viability and migration were analyzed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and scratch wound-healing assay, respectively. Focal adhesion formation was detected with immunofluorescence.

RESULTS

Treatment of C6 and U373 glioma cells with MG significantly reduced cell viability, migration, and Akt phosphorylation level. Glutamate stimulation markedly increased the level of ERK1/2 phosphorylation. However, cells treated with MG displayed decreased ERK1/2 phosphorylation. Inhibition of ERK1/2 by MG or MEK1/2 inhibitor significantly inhibited paxillin phosphorylation at Ser(83) and focal adhesion turn-over produced inefficient glioma cell migration. In addition, activation of Akt and ERK1/2 upon glutamate stimulation was independently regulated by Ca(2+) and protein kinase C activity, respectively, via the α-amino-3-hydroxy-5-methy-4-isoxazolepropionate acid glutamate receptor and metabotropic glutamate receptor.

GENERAL SIGNIFICANCE

Our results clearly indicate that MG has a strong anti-tumor effect through the down-regulation of the Akt and ERK1/2 signaling pathways. Thus, methyl gallate is a potent anti-tumor and novel therapeutic agent for glioma.

β-arrestin 2-dependent activation of ERK1/2 is required for ADP-induced paxillin phosphorylation at Ser(83) and microglia chemotaxis

Microglia play crucial roles in increased inflammation in the central nervous system upon brain injuries and diseases. Extracellular ADP has been reported to induce microglia chemotaxis and membrane ruffle formation through P2Y(12) receptor. In this study, we examined the role of ERK1/2 activation in ADP-induced microglia chemotaxis. ADP stimulation increases the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and paxillin phosphorylation at Tyr(31) and Ser(83) . Inhibition of ERK1/2 significantly inhibited paxillin phosphorylation at Ser(83) and the retraction of membrane ruffles, causing inefficient chemotaxis. Close examination of dynamics of focal adhesion (FA) formation with green fluorescent protein-paxillin revealed that the disassembly of FAs in U0126-treated cells was significantly impaired. Depletion of β-Arrestin 2 (β-Arr2) with short hairpin RNA markedly reduced the phosphorylation of ERK1/2 and Pax/Ser(83) , indicating that β-Arr2 is required for ERK1/2 activation upon ADP stimulation. A large fraction of phosphorylated ERK1/2 and β-Arr2 were translocated and co-localized at focal contacts in the newly forming lamellipodia. Examination of kinetics and rate constant of paxillin formation and disassembly revealed that the phosphorylation of paxillin at Tyr(31) by c-Src appears to be involved in adhesion formation upon ADP stimulation while Ser(83) required for adhesion disassembly.

Calmodulin activation of Aurora-A kinase (AURKA) is required during ciliary disassembly and in mitosis

This study demonstrates for the first time that binding of calcium-activated calmodulin to a minimal interaction site within the disordered N-terminal domain is required for the essential Aurora-A activity in mitosis and in regulation of ciliary disassembly.

The centrosomal Aurora-A kinase (AURKA) regulates mitotic progression, and overexpression and hyperactivation of AURKA commonly promotes genomic instability in many tumors. Although most studies of AURKA focus on its role in mitosis, some recent work identified unexpected nonmitotic activities of AURKA. Among these, a role for basal body–localized AURKA in regulating ciliary disassembly in interphase cells has highlighted a role in regulating cellular responsiveness to growth factors and mechanical cues. The mechanism of AURKA activation involves interactions with multiple partner proteins and is not well understood, particularly in interphase cells. We show here that AURKA activation at the basal body in ciliary disassembly requires interactions with Ca²⁺ and calmodulin (CaM) and that Ca²⁺/CaM are important mediators of the ciliary disassembly process. We also show that Ca²⁺/CaM binding is required for AURKA activation in mitosis and that inhibition of CaM activity reduces interaction between AURKA and its activator, NEDD9. Finally, mutated derivatives of AURKA impaired for CaM binding and/or CaM-dependent activation cause defects in mitotic progression, cytokinesis, and ciliary resorption. These results define Ca²⁺/CaM as important regulators of AURKA activation in mitotic and nonmitotic signaling.

Serum starvation: caveat emptor

Serum starvation is one of the most frequently performed procedures in molecular biology and there are literally thousands of research papers reporting its use. In fact, this method has become so ingrained in certain areas of research that reports often simply state that cells were serum starved without providing any factual details as to how the procedure was carried out. Even so, we quite obviously lack unequivocal terminology, standard protocols, and perhaps most surprisingly, a common conceptual basis when performing serum starvation. Such inconsistencies not only hinder interstudy comparability but can lead to opposing and inconsistent experimental results. Although it is frequently assumed that serum starvation reduces basal activity of cells, available experimental data do not entirely support this notion. To address this important issue, we studied primary human myotubes, rat L6 myotubes and human embryonic kidney (HEK)293 cells under different serum starvation conditions and followed time-dependent changes in important signaling pathways such as the extracellular signal-regulated kinase 1/2, the AMP-activated protein kinase, and the mammalian target of rapamycin. Serum starvation induced a swift and dynamic response, which displayed obvious qualitative and quantitative differences across different cell types and experimental conditions despite certain unifying features. There was no uniform reduction in basal signaling activity. Serum starvation clearly represents a major event that triggers a plethora of divergent responses and has therefore great potential to interfere with the experimental results and affect subsequent conclusions.

Integrins and bone metastasis: integrating tumor cell and stromal cell interactions

Integrins on both tumor cells and the supporting host stromal cells in bone (osteoclasts, new blood vessels, inflammatory cells, platelets and bone marrow stromal cells) play key roles in enhancing bone metastasis. Tumor cells localize to specific tissues through integrin-mediated contacts with extracellular matrix and stromal cells. Integrin expression and signaling are perturbed in cancer cells, allowing them to "escape" from cell-cell and cell-matrix tethers, invade, migrate and colonize within new tissues and matrices. Integrin signaling through αvβ3 and VLA-4 on tumor cells can promote tumor metastasis to and proliferation in the bone microenvironment. Osteoclast (OC) mediated bone resorption is a critical component of bone metastasis and can promote tumor growth in bone and αvβ3 integrins are critical to OC function and development. Tumors in the bone microenvironment can recruit new blood vessel formation, platelets, pro-tumor immune cells and bone marrow stromal cells that promote tumor growth and invasion in bone. Integrins and their ligands play critical roles in platelet aggregation (αvβ3 and αIIbβ3), hematopoietic cell mobilization (VLA-4 and osteopontin), neoangiogenesis (αvβ3, αvβ5, α6β4, and β1 integrin) and stromal function (osteopontin and VLA-4). Integrins are involved in the pathogenesis of bone metastasis at many levels and further study to define integrin dysregulation by cancer will yield new therapeutic targets for the prevention and treatment of bone metastasis.

CAS proteins in normal and pathological cell growth control

Proteins of the CAS (Crk-associated substrate) family (BCAR1/p130Cas, NEDD9/HEF1/Cas-L, EFS/SIN and CASS4/HEPL) are integral players in normal and pathological cell biology. CAS proteins act as scaffolds to regulate protein complexes controlling migration and chemotaxis, apoptosis, cell cycle, and differentiation, and have more recently been linked to a role in progenitor cell function. Reflecting these complex functions, over-expression of CAS proteins has now been strongly linked to poor prognosis and increased metastasis in cancer, as well as resistance to first-line chemotherapeutics in multiple tumor types including breast and lung cancers, glioblastoma, and melanoma. Further, CAS proteins have also been linked to additional pathological conditions including inflammatory disorders, Alzheimer's and Parkinson's disease, as well as developmental defects. This review will explore the roles of the CAS proteins in normal and pathological states in the context of the many mechanistic insights into CAS protein function that have emerged in the past decade.

Phosphorylation of human enhancer of filamentation (HEF1) on serine 369 induces its proteasomal degradation

Human enhancer of filamentation 1 (HEF1) is a multi-domain docking protein of the p130 Cas family. HEF1 is present at focal adhesions and is involved in integrin signalling mediating cytoskeleton reorganization associated with cell migration, adhesion or apoptosis. HEF1 functions are regulated in part by phosphorylation on tyrosine residues. HEF1 is also phosphorylated on serines/threonines leading to two isoforms refered to as p105 and p115. In most cases, the serine/threonine kinase(s) responsible for HEF1 phosphorylation have not been identified. In the present study, we have investigated HEF1 ser/thr phosphorylation. In the HCT-116 cell line transiently overexpressing Flag-HEF1 we showed that Hesperadin, a synthetic indolinone displaying antiproliferative effect and described as an inhibitor of various kinases including Aurora-B, prevented HEF1 phosphorylation induced by the ser/thr phosphatase PP2A inhibitor: okadaic acid (OA). In addition we showed that conversion of endogenous HEF1 p105 to p115 in HaCaT cells was prevented upon treatment with Hesperadin, resulting in accumulation of p105HEF1. We also identified serine 369 as the target site of phosphorylation by this Hesperadin-inhibited kinase in HCT-116. Finally, we provide evidence that phosphorylation on serine 369 but not phosphorylation on serine 296, triggers HEF1 degradation by the proteasomal machinery. These data suggest that conversion of p105 to p115 results from a ser-369-dependent phosphorylation mediated by an Hesperadin-sensitive kinase and regulates the half-life of HEF1.

Targeted transgenic expression of an osteoclastic transmembrane protein-tyrosine phosphatase in cells of osteoclastic lineage increases bone resorption and bone loss in male young adult mice

This study evaluated whether transgenic expression of PTP-oc (osteoclastic transmembrane protein-tyrosine phosphatase) in cells of the osteoclast lineage would affect bone resorption and bone density in young adult mice. Transgenic mice were generated with a transgenic construct using a tartrate-resistant acid phosphatase exon 1C promoter to drive expression of rabbit PTP-oc in osteoclastic cells. pQCT evaluation of femurs of young adult male progeny of three lines showed that transgenic mice had reduced bone volume and area, cortical and trabecular bone mineral content, and density. Histomorphometric analyses at secondary spongiosa of the femur and at metaphysis of the L4 vertebra confirmed that male transgenic mice had decreased trabecular surface, reduced percentage of trabecular area, decreased trabecular number, increased trabecular separation, and increased osteoclast number per bone surface length. Consistent with an increase in bone resorption, the serum C-telopeptide level was 25% higher in transgenic mice than in wild-type littermates. However, the bone phenotype was not readily observed in female young adult transgenic mice. This could in part be due to potential interactions between estrogen and PTP-oc signaling, since the bone loss phenotype was seen in young adult ovariectomized transgenic mice by microcomputed tomography analysis. In vitro, the average pit area per resorption pit created by marrow-derived transgenic osteoclasts was approximately 50% greater than that created by wild-type osteoclasts. Transgenic osteoclasts showed a lower c-Src phosphotyrosine 527 level, greater c-Src kinase activity, and increased tyrosine phosphorylation of paxillin. In summary, this study provides compelling in vivo evidence that PTP-oc is a positive regulator of osteoclasts.

Osteoclast receptors and signaling

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. Besides the well known Receptor Activator of Nuclear factor-kappaB (RANK), RANK ligand and osteoprotegerin axis, a variety of factors tightly regulate osteoclast formation, adhesion, polarization, motility, resorbing activity and life span, maintaining bone resorption within physiological ranges. Receptor-mediated osteoclast regulation is rather complex. Nuclear receptors, cell surface receptors, integrin receptors and cell death receptors work together to control osteoclast activity and prevent both reduced or increased bone resorption. Here we will discuss the signal transduction pathways activated by the main osteoclast receptors, integrating their function and mechanisms of action.

Calcitonin receptor-stimulated migration of prostate cancer cells is mediated by urokinase receptor-integrin signaling

Abundance of calcitonin (CT) and calcitonin receptor (CTR) mRNA in primary prostate tumors positively correlates with tumor grade, and exogenously added CT increases the invasion of prostate cancer cell lines. We examined acute and chronic actions of CT on migration of highly metastatic PC-3M cells and poorly invasive LNCaP cells on several extracellular matrices in a spheroid disaggregation/migration assay. While PC-3M spheroids displayed maximum disaggregation/migration on vitronectin (VN), LNCaP spheroids preferred collagen but also migrated significantly on VN. Up-regulation of CT significantly enhanced disaggregation/migration of PC-3M spheroids on VN, but not on fibronectin. In contrast, down-regulation of CT, CTR, protein kinase A or urokinase-type plasminogen activator receptor (uPAR) led to amelioration of PC-3M spheroid disaggregation/migration. CT selectively increased surface activity of alpha v beta 3 or alpha 6 beta 5 integrins in PC-3M and LNCaP cell lines, respectively, and uPAR-integrin association. Finally, either CT or urokinase could completely restore migration of CT-knock-down PC-3M spheroids. But, only forced expression of urokinase receptor coupled with exogenous addition of urokinase restored migration of CTR-knock-down spheroids. These results support our hypothesis that up-regulation of CT biosynthesis and activation of CT-CTR axis in primary prostate tumors may have direct relevance in their progression to the metastatic phenotype.

Crk-associated substrate lymphocyte type regulates transforming growth factor-beta signaling by inhibiting Smad6 and Smad7

Crk-associated substrate lymphocyte type (Cas-L) is a 105 kDa docking protein with diverse functional properties, including regulation of cell division, proliferation, migration and adhesion. Cas-L is also involved in beta1 integrin- or antigen receptor-mediated signaling in B and T cells. In the present study, we demonstrate that Cas-L potentiates transforming growth factor-beta (TGF-beta) signaling pathway by interacting with Smad6 and Smad7. Immunoprecipitation experiments reveal that single domain deletion of full-length Cas-L completely abolishes its docking function with Smad6 and Smad7, suggesting that the natural structure of Cas-L is necessary for its association with Smad6 and Smad7. On the other hand, both N-terminal and C-terminal deletion mutants of Smad6 and Smad7 still retain their docking ability to Cas-L, suggesting that Smad6 and Smad7 possess several binding motifs to Cas-L. Moreover, Cas-L interaction with Mad-homology (MH)2 domain, but not with MH1 domain of Smad6 or Smad7, ameliorates TGF-beta-induced signaling pathway. Finally, depletion of Cas-L by small-interfering RNA oligo attenuates TGF-beta-induced growth inhibition of Huh-7 cells, with a concomitant reduction in phosphorylation of Smad2 and Smad3. These results strongly suggest that Cas-L is a potential regulator of TGF-beta signaling pathway.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Molecular regulation of osteoclast activity

Osteoclasts are multinucleated cells derived from hematopoietic precursors that are primarily responsible for the degradation of mineralized bone during bone development, homeostasis and repair. In various skeletal disorders such as osteoporosis, hypercalcemia of malignancy, tumor metastases and Paget's disease, bone resorption by osteoclasts exceeds bone formation by osteoblasts leading to decreased bone mass, skeletal fragility and bone fracture. The overall rate of osteoclastic bone resorption is regulated either at the level of differentiation of osteoclasts from their monocytic/macrophage precursor pool or through the regulation of key functional proteins whose specific activities in the mature osteoclast control its attachment, migration and resorption. Thus, reducing osteoclast numbers and/or decreasing the bone resorbing activity of osteoclasts are two common therapeutic approaches for the treatment of hyper-resorptive skeletal diseases. In this review, several of the key functional players involved in the regulation of osteoclast activity will be discussed.

Cell adhesion regulates Ser/Thr phosphorylation and proteasomal degradation of HEF1

Human enhancer of filamentation 1 (HEF1), a multifunctional docking protein of the Cas family, participates in integrin and growth factor signaling pathways that regulate global cellular processes including growth, motility and apoptosis. HEF1 consists of two isoforms, p105 and p115, the larger molecular weight form resulting from Ser/Thr phosphorylation of p105HEF1. The molecular mechanisms that regulate the interconversion of the two HEF1 species as well as the function of HEF1 Ser/Thr phosphorylation are unknown. Our study reveals that cell adhesion and detachment regulate the interconversion of the two HEF1 isoforms. Experiments using various inhibitors of cytoskeletal organization indicated that disruption of actin microfilaments but not intermediate filaments or microtubules resulted in a complete conversion of p115HEF1 to p105HEF1. The conversion of p115HEF1 to p105HEF1 was prevented by inhibition of protein phosphatase 2A (PP2A), suggesting that cytoskeletal regulation of PP2A activity controlled the dephosphorylation of p115HEF1. Degradation of endogenous HEF1 was dependent on proteasomes with the p115 species of HEF1 being preferentially targeted for turnover. Dephosphorylation of HEF1 by suspending cells or disrupting actin filaments protected HEF1 from degradation. These results suggest that the adhesion-dependent actin organization regulates proteasomal turnover of HEF1 through the activity of PP2A.

Wnt proteins prevent apoptosis of both uncommitted osteoblast progenitors and differentiated osteoblasts by beta-catenin-dependent and -independent signaling cascades involving Src/ERK and phosphatidylinositol 3-kinase/AKT

Genetic studies in humans and mice have revealed an important role of the Wnt signaling pathway in the regulation of bone mass, resulting from potent effects on the control of osteoblast progenitor proliferation, commitment, differentiation, and perhaps osteoblast apoptosis. To establish the linkage between Wnts and osteoblast survival and to elucidate the molecular pathways that link the two, we have utilized three cell models: the uncommitted bipotential C2C12 cells, the pre-osteoblastic cell line MC3T3-E1, and bone marrow-derived OB-6 osteoblasts. Serum withdrawal-induced apoptosis was prevented by the canonical Wnts (Wnt3a and Wnt1) and the noncanonical Wnt5a in all cell types. Wnt3a induced LRP5-independent transient phosphorylation and nuclear accumulation of ERKs and phosphorylation of Src and Akt. The anti-apoptotic effect of Wnt3a was abrogated by inhibitors of canonical Wnt signaling, as well as by inhibitors of MEK, Src, phosphatidylinositol 3-kinase (PI3K), or Akt kinases, or by the addition of cycloheximide to the culture medium. Wnt3a-induced phosphorylation of GSK-3beta and downstream activation of beta-catenin-mediated transcription required ERK, PI3K, and Akt signaling. Wnt3a increased the expression of the anti-apoptotic protein Bcl-2 in an ERK-dependent manner. Beta-catenin-mediated transcription was permissive for the anti-apoptotic actions of Wnt1 and Wnt3a but was dispensable for the anti-apoptotic action of Wnt5a. However, Src, ERKs, PI3K, and Akt kinases were required for the anti-apoptotic effects of Wnt5a. These results demonstrate for the first time that Wnt proteins, irrespective of their ability to stimulate canonical Wnt signaling, prolong the survival of osteoblasts and uncommitted osteoblast progenitors via activation of the Src/ERK and PI3K/Akt signaling cascades.

Mechanical stimulation prevents osteocyte apoptosis: requirement of integrins, Src kinases, and ERKs

Osteocytes, former osteoblasts entombed in the bone matrix, form an extensive cell communication network that is thought to detect microdamage and mechanical strains and to transmit signals leading to repair and compensatory bone augmentation or reduction. Bone active hormones and drugs control the integrity of this network by regulating osteocyte apoptosis, which might be a determinant of bone strength. Herein we demonstrate that mechanical stimulation by stretching activates the ERKs, which in turn are responsible for the attenuation of osteocyte apoptosis. The effect of osteocyte stretching is transmitted by integrins and cytoskeletal and catalytic molecules, such as Src kinases. Stretch-induced antiapoptosis also requires nuclear translocation of ERKs and new gene transcription. The evidence linking mechanical stimulation, activation of an integrin/cytoskeleton/Src/ERK signaling pathway, and osteocyte survival provides a mechanistic basis for the profound role of mechanical forces, or lack thereof, on skeletal health and disease.

The Kinetics of Thermal Injury in Human Renal Carcinoma Cells

In this study, the thermal injury behavior of both suspended and attached SN12 human renal carcinoma cells (RCC) under thermal therapy conditions (i.e., heating cells to elevated temperature for seconds to minutes) was investigated using a non-isothermal method. This non-isothermal method entailed heating the cells using a programmable heating stage from room temperature at 130 degrees C min(-1) to various peak temperatures from 45 to 70 degrees C, held for 0-10 min, and then cooling down to room temperature at 65 degrees C min(-1). It was found that the suspended SN12 cells are more heat susceptible than attached ones. The non-isothermal portions (i.e., the heat-up and cool-down portions) of the thermal histories were found to be able to cause significant injury (> 10%) in both suspended and attached SN12 cells when the peak temperature is above 60 degrees C. Therefore, a non-isothermal method, which accounts for both the isothermal and non-isothermal portions of the thermal histories, was used to extract the kinetic parameters (i.e., the activation energy and frequency factor) in the Arrhenius injury model for SN12 cells. Furthermore, these results suggest that this non-isothermal method can be used to extract kinetic parameters from in vivo heating studies using minimally invasive surgical probes, where it is very difficult to get a thermal history in tissue with a dominant isothermal portion.

Use of polyethyleneimine polymer in cell culture as attachment factor and lipofection enhancer

Background

Several cell lines and primary cultures benefit from the use of positively charged extracellular matrix proteins or polymers that enhance their ability to attach to culture plates. Polyethyleneimine is a positively charged polymer that has gained recent attention as a transfection reagent. A less known use of this cationic polymer as an attachment factor was explored with several cell lines.

Results

Polyethyleneimine compared favorably to traditional attachment factors such as collagen and polylysine. PC-12 and HEK-293 cells plated on dishes coated with polyethyleneimine showed a homogeneous distribution of cells in the plate, demonstrating strong cell adhesion that survived washing procedures. The polymer could also be used to enhance the adherence and allow axonal outgrowth from zebrafish retinal explants. The effects of this coating agent on the transfection of loosely attaching cell lines were studied. Pre-coating with polyethyleneimine had the effect of enhancing the transfection yield in procedures using lipofection reagents.

Conclusion

Polyethyleneimine is an effective attachment factor for weakly anchoring cell lines and primary cells. Its use in lipofection protocols makes the procedures more reliable and increases the yield of expressed products with commonly used cell lines such as PC-12 and HEK-293 cells.

Cardiomyocyte apoptosis triggered by RAFTK/pyk2 via Src kinase is antagonized by paxillin

Altered cellular adhesion and apoptotic signaling in cardiac remodeling requires coordinated regulation of multiple constituent proteins that comprise cytoskeletal focal adhesions. One such protein activated by cardiac remodeling is related adhesion focal tyrosine kinase (RAFTK, also known as pyk2). Adenoviral-mediated expression of RAFTK in neonatal rat cardiomyocytes involves concurrent increases in phosphorylation of Src, c-Jun N-terminal kinase, and p38 leading to characteristic apoptotic changes including cleavage of poly(ADP-ribose) polymerase, caspase-3 activation, and increased DNA laddering. DNA laddering was decreased by mutation of the Tyr(402) Src-binding site in RAFTK, suggesting a central role for Src activity in apoptotic cell death that was confirmed by adenoviral-mediated Src expression. Multiple apoptotic signaling cascades are recruited by RAFTK as demonstrated by prevention of apoptosis using caspase-3 inhibitor IV (caspase-3 specific inhibitor), PP2 (Src-specific kinase inhibitor), or Csk (cellular negative regulator for Src), as well as dominant negative constructs for p38beta or MKP-1. These RAFTK-mediated phenotypic characteristics are prevented by concurrent expression of wild-type or a phosphorylation-deficient paxillin mutated at Tyr(31) and Tyr(118). Wild-type or mutant paxillin protein accumulation in the cytoplasm has no overt effect upon cell structure, but paxillin accumulation prevents losses of myofibril organization as well as focal adhesion kinase, vinculin, and paxillin protein levels mediated by RAFTK. Apoptotic signaling cascade inhibition by paxillin indicates interruption of signaling proximal to but downstream of RAFTK activity. Chronic RAFTK activation in cardiac remodeling may represent a maladaptive reactive response that can be modulated by paxillin, opening up novel possibilities for inhibition of cardiomyocyte apoptosis and structural degeneration in heart failure.

Kinase-mediated regulation of common transcription factors accounts for the bone-protective effects of sex steroids

It has been found that 4-estren-3alpha,17beta-diol, a synthetic ligand for the estrogen receptor (ER) or androgen receptor (AR), which does not affect classical transcription, reverses bone loss in ovariectomized females or orchidectomized males without affecting the uterus or seminal vesicles, demonstrating that the classical genotropic actions of sex steroid receptors are dispensable for their bone-protective effects, but indispensable for their effects on reproductive organs. We have now investigated the mechanism of action of this compound. We report that, identically to 17beta-estradiol or dihydrotestosterone, but differently from raloxifene, estren alters the activity of Elk-1, CCAAT enhancer binding protein-beta (C/EBPbeta), and cyclic adenosine monophosphate-response element binding protein (CREB), or c-Jun/c-Fos by an extranuclear action of the ER or AR, resulting in activation of the Src/Shc/ERK pathway or downregulation of JNK, respectively. All of these effects are non-sex specific, require only the ligand-binding domain of the receptor, and are indispensable for the antiapoptotic action of these ligands on osteoblastic and HeLa cells. Moreover, administration of 17beta-estradiol or 4-estren-3alpha,17beta-diol to ovariectomized mice induces phosphorylation of ERKs, Elk-1, and C/EBPbeta, downregulates c-Jun, and upregulates the expression of egr-1, an ERK/SRE target gene. Kinase-initiated regulation of commonly used transcription factors offers a molecular explanation for the profound skeletal effects of sex steroid receptor ligands, including synthetic ones that are devoid of classical transcriptional activity.

Transmodulation between phospholipase D and c-Src enhances cell proliferation

Phospholipase D (PLD) has been implicated in the signal transduction pathways initiated by several mitogenic protein tyrosine kinases. We demonstrate for the first time that most notably PLD2 and to a lesser extent the PLD1 isoform are tyrosine phosphorylated by c-Src tyrosine kinase via direct association. Moreover, epidermal growth factor induced tyrosine phosphorylation of PLD2 and its interaction with c-Src in A431 cells. Interaction between these proteins is via the pleckstrin homology domain of PLD2 and the catalytic domain of c-Src. Coexpression of PLD1 or PLD2 with c-Src synergistically enhances cellular proliferation compared with expression of either molecule. While PLD activity as a lipid-hydrolyzing enzyme is not affected by c-Src, wild-type PLDs but not catalytically inactive PLD mutants significantly increase c-Src kinase activity, up-regulating c-Src-mediated paxillin phosphorylation and extracellular signal-regulated kinase activity. These results demonstrate the critical role of PLD catalytic activity in the stimulation of Src signaling. In conclusion, we provide the first evidence that c-Src acts as a kinase of PLD and PLD acts as an activator of c-Src. This transmodulation between c-Src and PLD may contribute to the promotion of cellular proliferation via amplification of mitogenic signaling pathways.

Regulation of HEF1 expression and phosphorylation by TGF-beta 1 and cell adhesion

Transforming growth factor-beta1 (TGF-beta1) is a multipotential cytokine, which regulates remodeling of tissue extracellular matrix during early tumorigenesis and wound healing. Human enhancer of filamentation-1 (HEF1), a multifunctional docking protein, is involved in integrin-based signaling, which affects cell motility, growth, and apoptosis. Our studies reveal that TGF-beta1 is a potent inducer of HEF1 gene transcription in human dermal fibroblasts. TGF-beta1 promoted HEF1 expression in a dose-dependent manner and resulted in a 16-fold increase in HEF1 protein level. TGF-beta1 had no effect on the stability of either HEF1 protein or mRNA. The TGF-beta1-induced HEF1 expression was independent of cell adhesion and resistant to cytoskeleton disruption. TGF-beta1 increased levels of both p105 and p115 HEF1 in adherent fibroblasts. Digestion with specific phosphatases indicated that the p115HEF1 resulted from serine/threonine phosphorylation of p105HEF1. The appearance of the p115HEF1 as well as tyrosine phosphorylation of p105HEF1 required cell adhesion and/or an organized cytoskeleton. An in vitro kinase assay indicated that p105HEF1 was a substrate for Src. PP1, a specific Src kinase inhibitor, was able to block adhesion-dependent tyrosine phosphorylation of p105HEF1. These findings suggest that TGF-beta1 regulates HEF1 gene expression and that HEF1 phosphorylation is dependent on cell adhesion and Src kinase activity.

pp60(c-src) is a negative regulator of laminin-1-mediated neurite outgrowth in chick sensory neurons

Multiple protein tyrosine kinases regulate neurite outgrowth in the developing nervous system. To begin to unravel the complexity of this regulation, we addressed the role of one specific kinase, pp60(c-src), in chick dorsal root ganglion (DRG) neurons grown on laminin-1, a well-characterized system to study neurite outgrowth. Pharmacological inhibition of all tyrosine kinases by genestein treatment of chick DRG neurons significantly increased neurite number and length by approximately 50%. Similar increases in these parameters occurred when src-family kinases were inhibited using PP2. To implicate pp60(c-src) directly in neurite outgrowth, we inactivated it in DRG neuronal growth cones using Chromophore-Assisted Laser Inactivation (CALI). CALI of pp60(c-src) resulted in an 85% inactivation of its kinase activity and a 63% reduction in phosphotyrosine immunofluorescence in neurons. Microscale CALI of pp60(c-src) in DRG growth cones caused a significant and acute two-fold increase in neurite extension rate during irradiation. These findings demonstrate that pp60(c-src) is a negative regulator of laminin-1-mediated neurite outgrowth in chick sensory neurons.

Calcitonin: the other thyroid hormone

Calcitonin was originally discovered as a hypocalcemic factor synthesized by thyroid parafollicular C cells. Early experiments demonstrated that calcitonin inhibited bone resorption and decreased calcium efflux from isolated cat tibiae and subsequent histologic and culture studies confirmed the osteoclast as its major site of action. Its potent antiresorptive effect and analgesic action have led to its clinical use in treatment of Paget's bone disease, osteoporosis, and hypercalcemia of malignancy. This review surveys the cellular and molecular basis of these physiologic and clinical actions.

ACK family tyrosine kinase activity is a component of Dcdc42 signaling during dorsal closure in Drosophila melanogaster

We have characterized Drosophila melanogaster ACK (DACK), one of two members of the ACK family of nonreceptor tyrosine kinases in Drosophila. The ACKs are likely effectors for the small GTPase Cdc42, but signaling by these proteins remains poorly defined. ACK family tyrosine kinase activity functions downstream of Drosophila Cdc42 during dorsal closure of the embryo, as overexpression of DACK can rescue the dorsal closure defects caused by dominant-negative Dcdc42. Similar to known participants in dorsal closure, DACK is enriched in the leading edge cells of the advancing epidermis, but it does not signal through activation of the Jun amino-terminal kinase cascade operating in these cells. Transcription of DACK is responsive to changes in Dcdc42 signaling specifically at the leading edge and in the amnioserosa, two tissues involved in dorsal closure. Unlike other members of the ACK family, DACK does not contain a conserved Cdc42-binding motif, and transcriptional regulation may be one route by which Dcdc42 can affect DACK function. Expression of wild-type and kinase-dead DACK transgenes in embryos, and in the developing wing and eye, reveals that ACK family tyrosine kinase activity is involved in a range of developmental events similar to that of Dcdc42.

Forty years of calcitonin--where are we now? A tribute to the work of Iain Macintyre, FRS

Calcitonin was discovered as a hypocalcemic principal that was initially thought to originate from the parathyroid gland. This view was corrected subsequently, and an origin from the thyroid C cells was documented. The purification and sequencing of various calcitonins soon followed. Calcitonin is a 32-amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The peptide was shown to potently inhibit bone resorption; however, a direct osteoclastic action of the peptide was confirmed only in the early 1980s. Several osteoclast calcitonin receptors have subsequently been cloned and sequenced. Specific regions of the receptor necessary for ligand binding and intracellular signaling through cyclic AMP and calcium have been identified through systematic deletion mutagenesis and chimeric receptor studies. Calcitonin's potent antiresorptive effect has led to its use in treating Paget's disease of bone, osteoporosis, and hypercalcemia. This review retraces key aspects of the synthesis and structure of calcitonin, its cellular and molecular actions, and its therapeutic uses as they have emerged over the 40 years since its discovery. The review also examines the implications of these findings for future clinical applications as a tribute to early workers to whom credit must be given for creation of an important and expanding field. Notable are the new approaches currently being used to enhance calcitonin action, including novel allosteric activators of the calcitonin receptor, modulation of the release of endogenous calcitonin by calcimimetic agents, as well as the development of oral calcitonins.

Differential involvement of Src family kinases in pervanadate-mediated responses in rat myometrial cells

We previously described that pervanadate, a potent tyrosine phosphatase inhibitor, induced contraction of rat myometrium via phospholipase (PL) C-gamma1 activation [Biol Reprod 54 (1996) 1383]. In this study, we found that pervanadate induced tyrosine phosphorylation of the platelet-derived growth factor (PDGF)-beta receptor, interaction of the phosphorylated PDGF receptor with the phosphorylated PLC-gamma1, production of inositol phosphates (InsPs), extracellular signal-regulated kinase (ERK) activation and DNA synthesis. All these responses were insensitive to PDGF receptor kinase inhibition or PDGF receptor down-regulation. We showed that Src family kinases were activated by pervanadate, and that InsPs production and phosphorylation of both PLC-gamma1 and the PDGF receptor were blocked by PP1, an Src inhibitor. In contrast, the stimulation of ERK by pervanadate was totally refractory to PP1. These results demonstrated that the activation of Src by pervanadate is involved in PLC-gamma1/InsPs signalling but does not play a major role in ERK activation.

Transduction of cell survival signals by connexin-43 hemichannels

Bisphosphonates, drugs used widely in the treatment of bone diseases, prevent osteoblast and osteocyte apoptosis by a mechanism involving extracellular signal-regulated kinase (ERK) activation. We report herein that hexameric connexin (Cx)-43 hemichannels, but not gap junctions, are the essential transducers of the ERK-activating/anti-apoptotic effects of bisphosphonates. Transfection of Cx-43, but not other Cxs, into Cx-43 naive cells confers de novo responsiveness to the drugs. The signal-transducing property of Cx-43 requires the pore forming as well as the C-terminal domains of the protein, the activation of both Src and ERK kinases, and the SH2 and SH3 domains of Src. This evidence adds Cx-43 to the list of transmembrane proteins capable of transducing survival signals in response to extracellular cues and raises the possibility that it may serve in this capacity for endogenously produced molecules or even other drugs.