Emerging targets: molecular mechanisms of cell contact-mediated growth control

Fabrication of micropatterned PCL-collagen nanofibrous scaffold for cellular confinement induced early osteogenesis

The intricate interaction of the scaffold's architecture/geometry and with the cells is essential for tissue engineering and regenerative medicine. Cells sense their surrounding dynamic cues such as biophysical, biomechanical, and biochemical, and respond to them differently. Numerous studies have recently explored and reported the effect of contact guidance by culturing various types of cells on different types of micropatterned substrates such as microgrooves, geometric (square and triangle) micropattern, microstrips, micropatterned nanofibers. Amongst all of these micropatterned polymeric substrates; electrospun nanofibers have been regarded as a suitable substrate as it mimics the native ECM architectures. Therefore, in the present study; stencil-assisted electrospun Grid-lined micropatterned PCL-Collagen nanofibers (GLMPCnfs) were fabricated and its influence on the alignment and differentiation of pre-osteoblast cells (MC3T3-E1) was investigated. The randomly orientated Non-patterned PCL-Collagen nanofibers (NPPCnfs) were used as control. The patterns were characterized for their geometrical features such as area and thickness of deposition using surface profiler and scanning electron microscopy. A 61 % decrease in the overall area of GLMPCnfs as compared to the stencil area demonstrated the potential of electrofocusing phenomenon in the process of patterning electrospun nanofibers into various micron-scale structures. The MC3T3-E1 cells were confined and aligned in the direction of GLMPCnfs as confirmed by a high cellular aspect ratio (AR = 5.41), lower cellular shape index (CSI = 0.243), and cytoskeletal reorganization assessed through the F-actin filament immunocytochemistry (ICC) imaging. The aligned cells along the GLMPCnfs exhibited elevated alkaline phosphatase activity and enhanced mineralization. Furthermore, the gene expression profiling revealed upregulation of key osteogenic markers, such as ALP, OCN, OPN, COL1A1, and osteocyte markers DMP1, and SOST. Consequently, the research highlights the impact of GLMPCnfs on the cellular behaviour that results to the pre-osteoblast differentiation and the potential for stimulant-free early osteogenesis. These results offer an extensive understanding and mechanistic insight into how scaffold topography can be modified to influence cellular responses for effective bone regeneration strategies.

Human umbilical cord mesenchymal stem cells conditioned medium exerts anti-tumor effects on KGN cells in a cell density-dependent manner through activation of the Hippo pathway

BACKGROUND

The conditioned medium from human umbilical cord mesenchymal stem cells (UCMSCs-CM) provides a new cell-free therapy for tumors due to its unique secretome. However, there are many contradictory reports about the effect of UCMSCs-CM on tumor cells. The loss of contact inhibition is a common characteristic of tumor cells. A relationship between the effect of UCMSCs-CM on tumor cells and contact inhibition in tumor cells is rarely concerned. Whether the effect of UCMSCs-CM on tumor cells is affected by cell density? Here, we explored the effect of UCMSCs-CM on granulosa tumor cell line (KGN) cells at low or high density.

METHODS

Growth curve and CCK8 assay were used to assess cell proliferation and viability. Scratch wound and matrigel invasion assay were implicated to detect cell motility of KGN cells. UCMSCs-CM effects on cell cycle, apoptosis and pathway-related proteins were investigated by flow cytometry, TUNEL assay, western blot and immunofluorescence analysis respectively.

RESULTS

In growth curve analysis, before KGN cells proliferated into confluence, UCMSCs-CM had no effect on cell proliferation. However, once the cells proliferate to contact each other, UCMSCs-CM significantly inhibited proliferation. Meanwhile, when KGN cells were implanted at high density, UCMSCs-CM could induce cell cycle arrest at G1 phase, inhibit cell migration, invasion and promote apoptosis. While it had no similar effect on KGN cells implanted at low density. In mechanism, the UCMSCs-CM treatment activated the Hippo pathway when KGN cells were implanted at high density. Consistently, the MST1/2 inhibitor, XMU-MP-1, inhibited the activation of the Hippo pathway induced by UCMSCs-CM treatment and accordingly declined the anti-tumor effect of UCMSCs-CM on KGN cells.

CONCLUSIONS

The effect of UCMSCs-CM on tumor cells is affected by cell density. UCMSCs-CM exerted anti-tumor effect on KGN cells by activating Hippo pathway to restore contact inhibition. Our results suggest that UCMSCs-CM is a promising therapeutic candidate for GCT treatment.

R-PTP-κ Inhibits Contact-Dependent Cell Growth by Suppressing E2F Activity

Density-dependent regulation of cell growth is presumed to be caused by cell-cell contact, but the underlying molecular mechanism is not yet clearly defined. Here, we report that receptor-type protein tyrosine phosphatase-kappa (R-PTP-κ) is an important regulator of cell contact-dependent growth inhibition. R-PTP-κ expression increased in proportion to cell density. siRNA-mediated R-PTP-κ downregulation led to the loss of cell contact-mediated growth inhibition, whereas its upregulation reduced anchorage-independent cell growth in soft agar as well as tumor growth in nude mice. Expression profiling and luciferase reporter system-mediated signaling pathway analysis revealed that R-PTP-κ induced under cell contact conditions distinctly suppressed E2F activity. Among the structural domains of R-PTP-κ, the cytoplasmic domain containing the tandemly repeated PTP motif acts as a potent downregulator of the E2F pathway. Specifically, R-PTP-κ suppressed CDK2 activity through the induction of p21Cip1/WAF-1 and p27Kip1, resulting in cell cycle arrest at the G1 phase. In transcriptome-based public datasets generated from four different tumor types, R-PTP-κ expression was negatively correlated with the expression pattern and prognostic value of two known E2F1 target genes (CCNE1 and CDC25A). Therefore, our results indicate that the R-PTP-κ-E2F axis plays a crucial role in cell growth-inhibitory signaling arising from cell-cell contact conditions.

Together we stand, apart we fall: how cell-to-cell contact/interplay provides resistance to ferroptosis

Contextualisation of the new type of cell death called “ferroptosis” opened a completely new avenue for the development of anti-cancer therapies. Cumulative fundamental research dating back to the mid-20th century, crowned by the extraordinary work of the group led by Dr. Stockwell from Columbia University in 2012, finally got its candidature to be applied in the clinical settings. Although the potential for clinical importance is undoubtedly growing every day, as showed by the increasing number of papers dealing with ferroptosis and its applications, long experience of cancer research and treatment taught us that caution is still necessary. The plasticity of the tumour cells, particularly acute, along with its involvement in the resistance mechanisms, that have been seen, to greater or lesser extent, for almost all currently used therapies, represents the biggest fascinations in biomedical research field and also the biggest challenge to achieving cures in cancer patients. Accordingly, the main features of fundamental research have to be vigilance and anticipation. In this review, we tried to summarize the literature data, accumulated in the past couple of years, which point out the pitfalls in which “ferroptosis inducers” can fall if used prematurely in the clinical settings, but at the same time can provide a great advantage in the exhausting battle with cancer resistance. This is the first comprehensive review focusing on the effects of the cell-to-cell contact/interplay in the development of resistance to ferroptosis, while the contribution of cell-born factors has been summarized previously so here we just listed them.

Boolean model of anchorage dependence and contact inhibition points to coordinated inhibition but semi-independent induction of proliferation and migration

Epithelial cells respond to their physical neighborhood with mechano-sensitive behaviors required for development and tissue maintenance. These include anchorage dependence, matrix stiffness-dependent proliferation, contact inhibition of proliferation and migration, and collective migration that balances cell crawling with the maintenance of cell junctions. While required for development and tissue repair, these coordinated responses to the microenvironment also contribute to cancer metastasis. Predictive models of the signaling networks that coordinate these behaviors are critical in controlling cell behavior to halt disease. Here we propose a Boolean regulatory network model that synthesizes mechanosensitive signaling that links anchorage to a matrix of varying stiffness and cell density sensing to contact inhibition, proliferation, migration, and apoptosis. Our model can reproduce anchorage dependence and anoikis, detachment-induced cytokinesis errors, the effect of matrix stiffness on proliferation, and contact inhibition of proliferation and migration by two mechanisms that converge on the YAP transcription factor. In addition, we offer testable predictions related to cell cycle-dependent anoikis sensitivity, the molecular requirements for abolishing contact inhibition, and substrate stiffness dependent expression of the catalytic subunit of PI3K. Moreover, our model predicts heterogeneity in migratory vs. non-migratory phenotypes in sub-confluent monolayers, and co-inhibition but semi-independent induction of proliferation vs. migration as a function of cell density and mitogenic stimulation. Our model serves as a stepping-stone towards modeling mechanosensitive routes to the epithelial to mesenchymal transition, capturing the effects of the mesenchymal state on anoikis resistance, and understanding the balance between migration versus proliferation at each stage of the epithelial to mesenchymal transition.

The essential role of tumor suppressor gene ING4 in various human cancers and non-neoplastic disorders

Inhibitor of growth 4 (ING4), a member of the ING family discovered in 2003, has been shown to act as a tumor suppressor and is frequently down-regulated in various human cancers. Numerous published in vivo and in vitro studies have shown that ING4 is responsible for important cancer hallmarks such as pathologic cell cycle arrest, apoptosis, autophagy, contact inhibition, and hypoxic adaptation, and also affects tumor angiogenesis, invasion, and metastasis. These characteristics are typically associated with regulation through chromatin acetylation by binding histone H3 trimethylated at lysine 4 (H3K4me3) and through transcriptional activity of transcription factor P53 and NF-κB. In addition, emerging evidence has indicated that abnormalities in ING4 expression and function play key roles in non-neoplastic disorders. Here, we provide an overview of ING4-modulated chromosome remodeling and transcriptional function, as well as the functional consequences of different genetic variants. We also present the current understanding concerning the role of ING4 in the development of neoplastic and non-neoplastic diseases. These studies offer inspiration for pursuing novel therapeutics for various cancers.

Density-dependent ERK MAPK expression regulates MMP-9 and influences growth

Previous work has shown that expression of the extracellular signal-regulated kinase (ERK) is decreased by high density in normal fibroblast cells, and this was correlated with increased expression of mitogen-activated protein kinase phosphatases. Because of these differences in ERK regulation upon contact inhibition, it is likely that other cellular responses may be influenced by the attainment of a contact-inhibited state. Expression of matrix metalloproteinase-9 and cadherin cleavage were both found to be decreased upon reaching high culture density. Inhibition of ERK activity with the MEK inhibitor PD98059 resulted in increased expression of cadherins, while constitutive activation of ERK through the use of expression of an ERK construct with a D319N sevenmaker mutation resulted in decreased expression of cadherins and enhanced colony formation of HT-1080 fibrosarcoma cells. Taken together, these results corroborate a role for the regulation of ERK upon the attainment of a contact-inhibited state with increased expression of cadherins.

GDNF enhances human blood-nerve barrier function in vitro via MAPK signaling pathways

The human blood-nerve barrier (BNB) formed by endoneurial microvascular endothelial cells, serves to maintain the internal microenvironment in peripheral nerves required for normal axonal signal transduction to and from the central nervous system. The mechanisms of human BNB formation in health and disease are not fully elucidated. Prior work established a sufficient role for glial-derived neurotrophic factor (GDNF) in enhancing human BNB biophysical properties following serum withdrawal in vitro via RET-tyrosine kinase-dependent cytoskeletal remodeling. The objective of the study was to ascertain the downstream signaling pathway involved in this process and more comprehensively determine the molecular changes that may occur at human BNB intercellular junctions under the influence of GDNF. Proteomic studies suggested expression of several mitogen-activated protein kinases (MAPKs) in confluent GDNF-treated endoneurial endothelial cells following serum withdrawal. Using electric cell-substrate impedance sensing to continuously measure transendothelial electrical resistance and static transwell solute permeability assays with fluoresceinated small and large molecules to evaluate BNB biophysical function, we determined MAPK signaling was essential for GDNF-mediated BNB TEER increase following serum withdrawal downstream of RET-tyrosine kinase signaling that persisted for up to 48 hours in vitro. This increase was associated with reduced solute permeability to fluoresceinated sodium and high molecular weight dextran. Specific GDNF-mediated alterations were detected in cytoskeletal and intercellular junctional complex molecular transcripts and proteins relative to basal conditions without exogenous GDNF. This work provides novel insights into the molecular determinants and mechanisms responsible for specialized restrictive human BNB formation in health and disease.

Dexamethasone-Mediated Activation of Fibronectin Matrix Assembly Reduces Dispersal of Primary Human Glioblastoma Cells

Despite resection and adjuvant therapy, the 5-year survival for patients with Glioblastoma multiforme (GBM) is less than 10%. This poor outcome is largely attributed to rapid tumor growth and early dispersal of cells, factors that contribute to a high recurrence rate and poor prognosis. An understanding of the cellular and molecular machinery that drive growth and dispersal is essential if we are to impact long-term survival. Our previous studies utilizing a series of immortalized GBM cell lines established a functional causation between activation of fibronectin matrix assembly (FNMA), increased tumor cohesion, and decreased dispersal. Activation of FNMA was accomplished by treatment with Dexamethasone (Dex), a drug routinely used to treat brain tumor related edema. Here, we utilize a broad range of qualitative and quantitative assays and the use of a human GBM tissue microarray and freshly-isolated primary human GBM cells grown both as conventional 2D cultures and as 3D spheroids to explore the role of Dex and FNMA in modulating various parameters that can significantly influence tumor cell dispersal. We show that the expression and processing of fibronectin in a human GBM tissue-microarray is variable, with 90% of tumors displaying some abnormality or lack in capacity to secrete fibronectin or assemble it into a matrix. We also show that low-passage primary GBM cells vary in their capacity for FNMA and that Dex treatment reactivates this process. Activation of FNMA effectively “glues” cells together and prevents cells from detaching from the primary mass. Dex treatment also significantly increases the strength of cell-ECM adhesion and decreases motility. The combination of increased cohesion and decreased motility discourages in vitro and ex vivo dispersal. By increasing cell-cell cohesion, Dex also decreases growth rate of 3D spheroids. These effects could all be reversed by an inhibitor of FNMA and by the glucocorticoid receptor antagonist, RU-486. Our results describe a new role for Dex as a suppressor of GBM dispersal and growth.

Stem cells, progenitor cells, and lineage decisions in the ovary

Exploring stem cells in the mammalian ovary has unleashed a Pandora's box of new insights and questions. Recent evidence supports the existence of stem cells of a number of the different cell types within the ovary. The evidence for a stem cell model producing mural granulosa cells and cumulus cells is strong, despite a limited number of reports. The recent identification of a precursor granulosa cell, the gonadal ridge epithelial-like cell, is exciting and novel. The identification of female germline (oogonial) stem cells is still very new and is currently limited to just a few species. Their origins and physiological roles, if any, are unknown, and their potential to produce oocytes and contribute to follicle formation in vivo lacks robust evidence. The precursor of thecal cells remains elusive, and more compelling data are needed. Similarly, claims of very small embryonic-like cells are also preliminary. Surface epithelial cells originating from gonadal ridge epithelial-like cells and from the mesonephric epithelium at the hilum of the ovary have also been proposed. Another important issue is the role of the stroma in guiding the formation of the ovary, ovigerous cords, follicles, and surface epithelium. Immune cells may also play key roles in developmental patterning, given their critical roles in corpora lutea formation and regression. Thus, while the cellular biology of the ovary is extremely important for its major endocrine and fertility roles, there is much still to be discovered. This review draws together the current evidence and perspectives on this topic.

Sprouty2 expression controls endothelial monolayer integrity and quiescence

Vascular integrity is fundamental to the formation of mature blood vessels and depends on a functional, quiescent endothelial monolayer. However, how endothelial cells enter and maintain quiescence in the presence of angiogenic factors is still poorly understood. Here we identify the fibroblast growth factor (FGF) antagonist Sprouty2 (Spry2) as a key player in mediating endothelial quiescence and barrier integrity in mouse aortic endothelial cells (MAECs): Spry2 knockout MAECs show spindle-like shapes and are incapable of forming a functional, impermeable endothelial monolayer in the presence of FGF2. Whereas dense wild type cells exhibit contact inhibition and stop to proliferate, Spry2 knockout MAECs remain responsive to FGF2 and continue to proliferate even at high cell densities. Importantly, the anti-proliferative effect of Spry2 is absent in sparsely plated cells. This cell density-dependent Spry2 function correlates with highly increased Spry2 expression in confluent wild type MAECs. Spry2 protein expression is barely detectable in single cells but steadily increases in cells growing to high cell densities, with hypoxia being one contributing factor. At confluence, Spry2 expression correlates with intact cell-cell contacts, whereas disruption of cell-cell contacts by EGTA, TNFα and thrombin decreases Spry2 protein expression. In confluent cells, high Spry2 levels correlate with decreased extracellular signal-regulated kinase 1/2 (Erk1/2) phosphorylation. In contrast, dense Spry2 knockout MAECs exhibit enhanced signaling by Erk1/2. Moreover, inhibiting Erk1/2 activity in Spry2 knockout cells restores wild type cobblestone monolayer morphology. This study thus reveals a novel Spry2 function, which mediates endothelial contact inhibition and barrier integrity.

Involvement of the transcription factor FoxM1 in contact inhibition

Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Although it is generally accepted that contact inhibition plays a pivotal role in maintaining tissue homeostasis, the molecular mechanisms of contact inhibition are still not fully understood. FoxM1 is known as a proliferation-associated transcription factor and is upregulated in many cancer types. Vice versa, anti-proliferative signals, such as TGF-β and differentiation signals decrease FoxM1 expression. Here we investigated the role of FoxM1 in contact inhibition in fibroblasts. We show that protein expression of FoxM1 is severely and rapidly downregulated upon contact inhibition, probably by inhibition of ERK activity, which then leads to decreased expression of cyclin A and polo-like kinase 1. Vice versa, ectopic expression of FoxM1 prevents the decrease in cyclin A and polo-like kinase 1 and causes a two-fold increase in saturation density indicating loss of contact inhibition. Hence, we show that downregulation of FoxM1 is required for contact inhibition by regulating expression of cyclin A and polo-like kinase 1.

Adipose tissue-organotypic culture system as a promising model for studying adipose tissue biology and regeneration

Adipose tissue consists of mature adipocytes, preadipocytes and mesenchymal stem cells (MSCs), but a culture system for analyzing their cell types within the tissue has not been established. We have recently developed "adipose tissue-organotypic culture system" that maintains unilocular structure, proliferative ability and functions of mature adipocytes for a long term, using three-dimensional collagen gel culture of the tissue fragments. In this system, both preadipocytes and MSCs regenerate actively at the peripheral zone of the fragments. Our method will open up a new way for studying both multiple cell types within adipose tissue and the cell-based mechanisms of obesity and metabolic syndrome. Thus, it seems to be a promising model for investigating adipose tissue biology and regeneration. In this article, we introduce adipose tissue-organotypic culture, and propose two theories regarding the mechanism of tissue regeneration that occurs specifically at peripheral zone of tissue fragments in vitro.

SSeCKS sequesters cyclin D1 in glomerular parietal epithelial cells and influences proliferative injury in the glomerulus

Glomerular parietal epithelial cells (PECs) are precursors to podocytes in mature glomeruli; however, as progenitors, the distinct intrinsic mechanisms that allow for repeated periods of cell-cycle arrest and re-entry of PECs after glomerulogenesis are unknown. Here, we show that the Src-suppressed protein kinase C substrate (SSeCKS), a multivalent scaffolding A kinase anchoring protein, sequesters cyclin D1 in the cytoplasm of quiescent PECs. SSeCKS expression is induced in embryonic PECs, but not in embryonic podocytes, starting at the S phase of glomerulogenesis, and is constitutively expressed postnatally by PECs, but not podocytes, in normal glomeruli. Cyclin D1 was immunoprecipitated with SSeCKS from capsulated glomeruli containing PECs, whereas decapsulated glomeruli without PECs lacked SSeCKS and cyclin D1. Cell-cell contact inhibition of proliferation in cultured PECs induced SSeCKS expression and binding of cyclin D1 by SSeCKS in the cytoplasm, whereas phosphorylation of SSeCKS by activated protein kinase C disrupted binding, resulting in nuclear translocation of cyclin D1. SSeCKS(-/-) mice showed hyperplasia of PECs in otherwise normal glomeruli and developed significantly worse proteinuric glomerular disease, marked by increased PEC proliferation and expression of nuclear cyclin D1, from nephrotoxic nephritis. These results suggest that SSeCKS controls the localization and activity of cyclin D1 in PECs and influences proliferative injury in the glomerulus.

Cell density-dependent reduction of dihydroceramide desaturase activity in neuroblastoma cells

We applied a metabolic approach to investigate the role of sphingolipids in cell density-induced growth arrest in neuroblastoma cells. Our data revealed that sphingolipid metabolism in neuroblastoma cells significantly differs depending on the cells' population context. At high cell density, cells exhibited G0/G1 cell-cycle arrest and reduced ceramide, monohexosylceramide, and sphingomyelin, whereas dihydroceramide was significantly increased. In addition, our metabolic-labeling experiments showed that neuroblastoma cells at high cell density preferentially synthesized very long chain (VLC) sphingolipids and dramatically decreased synthesis of sphingosine-1-phosphate (S1P). Moreover, densely populated neuroblastoma cells showed increased message levels of both anabolic and catabolic enzymes of the sphingolipid pathway. Notably, our metabolic-labeling experiments indicated reduced dihydroceramide desaturase activity at confluence, which was confirmed by direct measurement of dihydroceramide desaturase activity in situ and in vitro. Importantly, we could reduce dihydroceramide desaturase activity in low-density cells by applying conditional media from high-density cells, as well as by adding reducing agents, such as DTT and L-cysteine to the media. In conclusion, our data suggest a role of the sphingolipid pathway, dihydroceramides desaturase in particular, in confluence-induced growth arrest in neuroblastoma cells.

Free-form-fabricated commercially pure Ti and Ti6Al4V porous scaffolds support the growth of human embryonic stem cell-derived mesodermal progenitors

Commercially-pure titanium (cp-Ti) and the titanium-aluminum-vanadium alloy (Ti6Al4V) are widely used as reconstructive implants for skeletal engineering applications, due to their good mechanical properties, biocompatibility and ability to integrate with the surrounding bone. Electron beam melting technology (EBM) allows the fabrication of customized implants with tailored mechanical properties and high potential in the clinical practice. In order to augment the interaction with the biological tissue, stem cells have recently been combined with metallic scaffolds for skeletal engineering applications. We previously demonstrated that human embryonic stem cell-derived mesodermal progenitors (hES-MPs) hold a great potential to provide a homogeneous and unlimited supply of cells for bone engineering applications. This study demonstrates the effect of EBM-fabricated cp-Ti and Ti6Al4V porous scaffolds on hES-MPs behavior, in terms of cell attachment, growth and osteogenic differentiation. Displaying different chemical composition but similar surface properties, EBM-fabricated cp-Ti and Ti6Al4V scaffolds supported cell attachment and growth, and did not seem to alter the expression of genes involved in osteogenic differentiation and affect the alkaline phosphatase activity. In conclusion, interfacing hES-MPs to EBM-fabricated scaffolds may represent an interesting strategy for design of third-generation biomaterials, with the potential to promote implant integration in clinical conditions characterized by poor bone quality.

Pentoxifylline decreases serum levels of tumor necrosis factor alpha, interleukin 6 and C-reactive protein in hemodialysis patients: results of a randomized double-blind, controlled clinical trial

AIM

The aim of this study was to compare the effect of pentoxifylline versus placebo on serum concentrations of tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6) and C-reactive protein (CRP) of hemodialysis (HD) patients.

METHODS

This is a randomized double-blind, controlled clinical trial. HD patients without infection or drugs with anti-inflammatory effect were randomly allocated to a study (n = 18, pentoxifylline 400 mg/day) or control (n = 18, placebo) group; all patients had arteriovenous fistula. Besides clinical and laboratory monthly assessments, serum TNF-α and IL-6 (ELISA) and CRP (nephelometry) were measured at 0, 2 and 4 months.

RESULTS

All the inflammation markers significantly (P < 0.05) decreased in the pentoxifylline group: TNF-α [baseline 0.4 (0-2) versus final 0 (0-0) pg/mL], IL-6 [baseline 9.4 (5-14) versus final 2.9 (2-5) pg/mL] and CRP [baseline 7.1 (3-20) versus final 2.6 (1-8) mg/L], whereas no significant changes were observed in the placebo group: TNF-α [baseline 0 (0-0) versus final 1.2 (0-4) pg/mL], IL-6 [baseline 8.0 (5-11) versus final 8.7 (4-11) pg/mL] and CRP [baseline 4.5 (2-9) versus final 3.8 (3-23) mg/L].

CONCLUSIONS

Pentoxifylline significantly decreased serum concentrations of TNF-α, IL-6 and CRP compared to placebo. Pentoxifylline could be a promising and useful strategy to reduce the systemic inflammation frequently observed in patients on HD.

Cell Contact Accelerates Replicative Senescence of Human Mesenchymal Stem Cells Independent of Telomere Shortening and p53 Activation: Roles of Ras and Oxidative Stress

Mesenchymal stem cells (MSCs) are of great therapeutic potentials due to their multilineage differentiation capabilities. Before transplantation, in vitro culture expansion of MSCs is necessary to get desired cell number. We observed that cell contact accelerated replicative senescence during such process. To confirm the finding as well as to investigate the underlying mechanisms, we cultured both human bone marrow- and umbilical cord blood-derived MSCs under noncontact culture (subculture performed at 60–70% of confluence), or contact culture (cell passage performed at 100% of confluence). It was found that MSCs reached cellular senescence earlier in contact culture, and the doubling time was significantly prolonged. Marked increase of senescence-associated β-galactosidase-positive staining was also observed as a result of cell contact. Cell cycle analysis revealed increased frequency of cell cycle arrest after contact culture. It was noted, however, that the telomere length was not altered during contact-induced acceleration of senescence. Moreover, cell cycle checkpoint regulator P53 expression was not affected by cell contact. Marked increase in intracellular reactive oxygen species (ROS) and a concomitant decrease in the activities of antioxidative enzymes were also observed during contact-induced senescence. Importantly, increased p16INK4a following Ras upregulation was found after contact culture. Taken together, cell contact induced accelerated senescence of MSCs, which is telomere shortening and p53 independent. ROS accumulation due to defective ROS clearance function together with Ras and p16INK4a upregulation play an important role in contact-induced senescence of MSCs. Overconfluence should therefore be avoided during in vitro culture expansion of MSCs in order to maintain their qualities for clinical application purposes. The contact-induced senescence model reported in this study will serve as a useful model system that allows further study of the molecular mechanisms of senescence in MSCs.

RNAi-mediated knock-down of arylamine N-acetyltransferase-1 expression induces E-cadherin up-regulation and cell-cell contact growth inhibition

Arylamine N-acetyltransferase-1 (NAT1) is an enzyme that catalyzes the biotransformation of arylamine and hydrazine substrates. It also has a role in the catabolism of the folate metabolite p-aminobenzoyl glutamate. Recent bioinformatics studies have correlated NAT1 expression with various cancer subtypes. However, a direct role for NAT1 in cell biology has not been established. In this study, we have knocked down NAT1 in the colon adenocarcinoma cell-line HT-29 and found a marked change in cell morphology that was accompanied by an increase in cell-cell contact growth inhibition and a loss of cell viability at confluence. NAT1 knock-down also led to attenuation in anchorage independent growth in soft agar. Loss of NAT1 led to the up-regulation of E-cadherin mRNA and protein levels. This change in E-cadherin was not attributed to RNAi off-target effects and was also observed in the prostate cancer cell-line 22Rv1. In vivo, NAT1 knock-down cells grew with a longer doubling time compared to cells stably transfected with a scrambled RNAi or to parental HT-29 cells. This study has shown that NAT1 affects cell growth and morphology. In addition, it suggests that NAT1 may be a novel drug target for cancer therapeutics.

Possible involvement of NEDD4 in keloid formation; its critical role in fibroblast proliferation and collagen production

Keloid represents overgrowth of granulation tissue, which is characterized by collection of atypical fibroblasts with excessive deposition of extracellular matrix components, after skin injury, but its etiology is still largely unknown. We recently performed genome-wide association study (GWAS) of keloid and identified NEDD4 to be one of candidate molecules associated with keloid susceptibility. Here we demonstrate a possible mechanism of NEDD4 involvement in keloid formation through enhancement of the proliferation and invasiveness of fibroblasts as well as upregulation of type 1 collagen expression. Activation of NEDD4 affected subcellular localization and protein stability of p27 which was implied its critical role in contact inhibition. It also induced accumulation of β-catenin in the cytoplasm and activated the TCF/β-catenin transcriptional activity. Furthermore, NEDD4 upregulated expressions of fibronectin and type 1 collagen and contributed to the excessive accumulation of extracellular matrix. Our findings provide new insights into mechanism developing keloid and can be applied for development of a novel treatment for keloid.

Elevated C-reactive protein level in hemodialysis patients with moderate/severe uremic pruritus: a potential mediator of high overall mortality

BACKGROUND

Dialysis patients with uremic pruritus have worse outcomes. However, the pathophysiology of the high mortality in these patients remains inconclusive except for links with calcium/phosphate imbalance and sleep disturbance. Whether inflammation, an outcome predictor in dialysis patients, plays a role is unknown.

METHODS

This prospective study included 321 chronic hemodialysis (HD) patients (>3 months) for survival analysis. A visual analog scale (VAS) was used to measure the severity of itching, and the patients were divided into four groups: no pruritus (VAS = 0, N = 118), mild (VAS 1-3, N = 76), moderate (VAS 4-7, N = 89) and severe pruritus (VAS 8-10, N = 38). The Pittsburgh Sleep Quality Index (PSQI) was used to define sleep disturbance, while high-sensitive C-reactive protein (hs-CRP) and tumor necrosis factor α (TNF-α) were used to evaluate inflammation. The patients were followed-up for 30 months.

RESULTS

Patients with moderate/severe pruritus had higher hs-CRP, but similar TNF-α levels; they also had a worse survival rate (P = 0.0197, log rank test). By stratifying hs-CRP levels, those with higher hs-CRP had worse survival regardless of the severity of uremic pruritus. In a Cox proportional hazard model, hs-CRP levels and moderate/severe uremic pruritus were independent predictors of mortality after adjusting for age, poor sleeper (PSQI > 5), diabetes, albumin, phosphate, hemoglobin and parathyroid hormone levels and (hs-CRP) × (moderate/severe uremic pruritus) (all P < 0.05).

CONCLUSION

In moderate/severe pruritic HD patients, those with higher hs-CRP suffer from worse overall mortality. Inflammation may bridge uremic pruritus to high mortality, and elevated hs-CRP predicts a worse outcome in this population.

The aryl hydrocarbon receptor (AhR) in the regulation of cell-cell contact and tumor growth

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, which is activated by a large group of environmental pollutants including polycyclic aromatic hydrocarbons, dioxins and planar polychlorinated biphenyls. Ligand binding leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes, such as cytochrome P4501A1 and glutathione-S-transferase, respectively. Since phase I enzymes convert inert carcinogens to active genotoxins, the AhR plays a key role in tumor initiation. Besides this classical route, the AhR mediates tumor promotion and recent evidence suggests that the AhR also plays a role in tumor progression. To date, no mechanistic link could be established between the canonical pathway involving xenobiotic metabolism and AhR-dependent tumor promotion and progression. A hallmark of tumor promotion is unbalanced proliferation, whereas tumor progression is characterized by dedifferentiation, increased motility and metastasis of tumor cells. Tumor progression and presumably also tumor promotion are triggered by loss of cell-cell contact. Cell-cell contact is known to be a critical regulator of proliferation, differentiation and cell motility in vitro and in vivo. Increasing evidence suggests that activation of the AhR may lead to deregulation of cell-cell contact, thereby inducing unbalanced proliferation, dedifferentiation and enhanced motility. In line with this is the finding of increased AhR expression and malignancy in some animal and human cancers. Here, we summarize our current knowledge on non-canonical AhR-driven pathways being involved in deregulation of cell-cell contact and discuss the data with respect to tumor initiation, promotion and progression.

Restoration of contact inhibition in human glioblastoma cell lines after MIF knockdown

BACKGROUND

Studies of the role of the cytokine macrophage-migration-inhibitory-factor (MIF) in malignant tumors have revealed its stimulating influence on cell-cycle progression, angiogenesis and anti-apoptosis.

RESULTS

Here we show that in vitro targeting MIF in cultures of human malignant glioblastoma cells by either antisense plasmid introduction or anti-MIF antibody treatment reduced the growth rates of tumor cells. Of note is the marked decrease of proliferation under confluent and over-confluent conditions, implying a role of MIF in overcoming contact inhibition. Several proteins involved in contact inhibition including p27, p21, p53 and CEBPalpha are upregulated in the MIF antisense clones indicating a restoration of contact inhibition in the tumor cells. Correspondingly, we observed a marked increase in MIF mRNA and protein content under higher cell densities in LN18 cells. Furthermore, we showed the relevance of the enzymatic active site of MIF for the proliferation of glioblastoma cells by using the MIF-tautomerase inhibitor ISO-1.

CONCLUSION

Our study adds another puzzle stone to the role of MIF in tumor growth and progression by showing the importance of MIF for overcoming contact inhibition.

Hypersensitivity to contact inhibition provides a clue to cancer resistance of naked mole-rat

The naked mole-rat is the longest living rodent with a maximum lifespan exceeding 28 years. In addition to its longevity, naked mole-rats have an extraordinary resistance to cancer as tumors have never been observed in these rodents. Furthermore, we show that a combination of activated Ras and SV40 LT fails to induce robust anchorage-independent growth in naked mole-rat cells, while it readily transforms mouse fibroblasts. The mechanisms responsible for the cancer resistance of naked mole-rats were unknown. Here we show that naked mole-rat fibroblasts display hypersensitivity to contact inhibition, a phenomenon we termed "early contact inhibition." Contact inhibition is a key anticancer mechanism that arrests cell division when cells reach a high density. In cell culture, naked mole-rat fibroblasts arrest at a much lower density than those from a mouse. We demonstrate that early contact inhibition requires the activity of p53 and pRb tumor suppressor pathways. Inactivation of both p53 and pRb attenuates early contact inhibition. Contact inhibition in human and mouse is triggered by the induction of p27(Kip1). In contrast, early contact inhibition in naked mole-rat is associated with the induction of p16(Ink4a). Furthermore, we show that the roles of p16(Ink4a) and p27(Kip1) in the control of contact inhibition became temporally separated in this species: the early contact inhibition is controlled by p16(Ink4a), and regular contact inhibition is controlled by p27(Kip1). We propose that the additional layer of protection conferred by two-tiered contact inhibition contributes to the remarkable tumor resistance of the naked mole-rat.

Tumor suppressor density-enhanced phosphatase-1 (DEP-1) inhibits the RAS pathway by direct dephosphorylation of ERK1/2 kinases

Density-enhanced phosphatase-1 (DEP-1) is a trans-membrane receptor protein-tyrosine phosphatase that plays a recognized prominent role as a tumor suppressor. However, the mechanistic details underlying its function are poorly understood because its primary physiological substrate(s) have not been firmly established. To shed light on the mechanisms underlying the anti-proliferative role of this phosphatase, we set out to identify new DEP-1 substrates by a novel approach based on screening of high density peptide arrays. The results of the array experiment were combined with a bioinformatics filter to identify eight potential DEP-1 targets among the proteins annotated in the MAPK pathway. In this study we show that one of these potential targets, the ERK1/2, is indeed a direct DEP-1 substrate in vivo. Pulldown and in vitro dephosphorylation assays confirmed our prediction and demonstrated an overall specificity of DEP-1 in targeting the phosphorylated tyrosine 204 of ERK1/2. After epidermal growth factor stimulation, the phosphorylation of the activation loop of ERK1/2 can be modulated by changing the concentration of DEP-1, without affecting the activity of the upstream kinase MEK. In addition, we show that DEP-1 contains a KIM-like motif to recruit ERK1/2 proteins by a docking mechanism mediated by the common docking domain in ERK1/2. ERK proteins that are mutated in the conserved docking domain become insensitive to DEP-1 de-phosphorylation. Overall this study provides novel insights into the anti-proliferative role of this phosphatase and proposes a new mechanism that may also be relevant for the regulation of density-dependent growth inhibition.

Cell cycle quiescence can suppress transcription from an ecdysone receptor–based inducible promoter in mammalian cells

Inducible gene expression is a powerful tool for basic research, gene therapy and biotechnology, whose utility depends in part on consistent levels of induction regardless of metabolic status or physiological context. Here we examined the inducibility of the ecdysone receptor–based RheoSwitch mammalian inducible expression system in proliferating cells and in cell cycle–arrested cells. We found that both contact inhibition and growth arrest subsequent to serum deprivation dramatically reduced the levels of induction of reporter genes that could be achieved in 3T3 fibroblasts but in not NMuMG mammary epithelial cells. These data have implications for the use of the RheoSwitch system in inducible gene expression applications.

Production of cloned pigs by nuclear transfer of preadipocytes following cell cycle synchronization by differentiation induction

Four methods of cell cycle synchronization of porcine preadipocytes for use as nuclear donors in somatic cell cloning were compared: serum starvation, differentiation induction, contact inhibition and roscovitine treatment. After three days of differentiation induction, the percentage of nuclear donor cells synchronized at the G0/G1 phase reached a peak value of 91.8%, which was significantly higher (P<0.05) than the percentage attained by serum starvation (84.9-89.8%), contact inhibition (78.3-83.7%) or roscovitine treatment (67.8-80.3%). Cell cycle synchronization by serum starvation, contact inhibition and roscovitine treatment all increased the percentage of apoptotic cells, while no increase was observed when the donor-cell cycle was synchronized by differentiation induction (Annexin V-positive: 15.7% to 19.3% vs. 7.7%, P<0.05; TUNEL-positive: 12.8% to 14.0% vs. 8.3%, P<0.05). Additionally, comparison of the in vitro development of nuclear transfer (NT) embryos formed from the nuclei of differentiation-induced or serum-starved preadipocytes revealed that, in both cases, a high proportion of embryos developed to the blastocyst stage (39.0 and 33.7%, respectively). In this study, NT embryos reconstructed with preadipocytes synchronized by differentiation induction were transferred to four recipient pigs, three of which gave birth to a total of 17 piglets (4.2%, 17/403). These results demonstrate that donor-cell cycle synchronization by differentiation induction enables effective production of cloned pigs. The findings also indicate that differentiation induction of multipotent cells is an excellent method of cell cycle synchronization that permits highly efficient synchronization of cells at the G0/G1 phase.

PIASy represses CCAAT/enhancer-binding protein delta (C/EBPdelta) transcriptional activity by sequestering C/EBPdelta to the nuclear periphery

CCAAT/enhancer binding proteindelta (C/EBPdelta) plays a key role in mammary epithelial cell G(0) growth arrest, and "loss of function" alterations in C/EBPdelta have been reported in breast cancer and acute myeloid leukemia. C/EBPdelta is regulated at the transcriptional, post-transcriptional, and post-translational levels, suggesting tight control of C/EBPdelta content and function. Protein inhibitors of activated STATs (PIASs) regulate a growing number of transcription factors, including C/EBPs. HC11 nontransformed mammary epithelial cells express PIAS3, PIASxbeta, and PIASy, and all three PIAS family members repress C/EBPdelta transcriptional activity. PIASy is the most potent, however, repressing C/EBPdelta transcriptional activity by >80%. PIASy repression of C/EBPdelta transcriptional activity is dependent upon interaction between the highly conserved PIASy N-terminal nuclear matrix binding domain (SAPD) and the C/EBPdelta transactivation domain (TAD). PIASy repression of C/EBPdelta transcriptional activity is independent of histone deacetylase activity, PIASy E3 SUMO ligase activity, and C/EBPdelta sumoylation status. PIASy expression is associated with C/EBPdelta translocation from nuclear foci, where C/EBPdelta co-localizes with p300, to the nuclear periphery. PIASy-mediated translocation of C/EBPdelta is dependent upon the PIASy SAPD and C/EBPdelta TAD. PIASy reduces the expression of C/EBPdelta adhesion-related target genes and enhances repopulation of open areas within a cell monolayer in the in vitro "scratch" assay. These results demonstrate that PIASy represses C/EBPdelta by a mechanism that requires interaction between the PIASy SAPD and C/EBPdelta TAD and does not require PIASy SUMO ligase activity or C/EBPdelta sumoylation. PIASy alters C/EBPdelta nuclear localization, reduces C/EBPdelta transcriptional activity, and enhances cell proliferation/migration.

Cell density dependent increase of constitutive signal transducers and activators of transcription 3 activity in melanoma cells is mediated by Janus kinases

Signal transducers and activators of transcriptions (STAT) are key mediators of cytokine signaling. Moreover, these transcription factors play a crucial role in oncogenic signaling where inappropriate and sustained activation of STATs, especially STAT3, is a trait of many different cancers and their derived cell lines. Constitutively active STAT3 has been reported to prevent programmed cell death and enhance cell proliferation, whereas the disruption of STAT3 signaling can inhibit tumor growth. The physiologic activation of STAT3 by cytokines has been well established; however, little is known about altered, stimulation-independent STAT3 activation. Here, we show that, in most but not all melanoma cell lines, STAT3 phosphorylation increased substantially with cell density and that this STAT3 was able to bind to DNA and to activate transcription. Inhibitor studies showed that the cell density-dependent STAT3 activation relies on Janus kinases (JAK) rather than Src kinases. Using a specific JAK inhibitor, sustained STAT3 activation was completely abrogated in all tested melanoma lines, whereas inhibition of Src or mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 had no effect on constitutively tyrosine-phosphorylated STAT3 levels. Although STAT3 activation was completely blocked with JAK inhibitor I and to a lesser extent with the common JAK inhibitor AG490, only the latter compound markedly decreased proliferation and induced apoptosis. Taken together, variations in cell density can profoundly modify the extent of JAK-mediated persistent STAT3 phosphorylation; however, STAT3 activation was not sufficient to provide critical growth and survival signals in melanoma cell lines.

Mouse EP3 alpha, beta, and gamma receptor variants reduce tumor cell proliferation and tumorigenesis in vivo

Prostaglandin E(2), which exerts its functions by binding to four G protein-coupled receptors (EP1-4), is implicated in tumorigenesis. Among the four E-prostanoid (EP) receptors, EP3 is unique in that it exists as alternatively spliced variants, characterized by differences in the cytoplasmic C-terminal tail. Although three EP3 variants, alpha, beta, and gamma, have been described in mice, their functional significance in regulating tumorigenesis is unknown. In this study we provide evidence that expressing murine EP3 alpha, beta, and gamma receptor variants in tumor cells reduces to the same degree their tumorigenic potential in vivo. In addition, activation of each of the three mEP3 variants induces enhanced cell-cell contact and reduces cell proliferation in vitro in a Rho-dependent manner. Finally, we demonstrate that EP3-mediated RhoA activation requires the engagement of the heterotrimeric G protein G(12). Thus, our study provides strong evidence that selective activation of each of the three variants of the EP3 receptor suppresses tumor cell function by activating a G(12)-RhoA pathway.

Ultraviolet radiation (UVR) activates p38 MAP kinase and induces post-transcriptional stabilization of the C/EBPδ mRNA in G0 growth arrested mammary epithelial cells

The G(0) growth arrest (quiescent) state is highly conserved in evolution to promote survival under adverse environmental conditions. To maintain viability, G(0) growth arrested cells limit gene expression to essential growth control and pro-survival genes. CCAAT enhancer binding protein delta (C/EBPdelta), a member of the C/EBP family of nuclear proteins, is highly expressed in G(0) growth arrested mammary epithelial cells (MECs). Although C/EBPdelta gene transcription is elevated during G(0) growth arrest, C/EBPdelta mRNA and protein are relatively short lived, suggesting tight control of the cellular C/EBPdelta content in unstressed, quiescent cells. Treatment of G(0) growth arrested MECs with ultraviolet radiation (UVR) dramatically increases the C/EBPdelta mRNA half-life (approximately 4-fold) and protein content (approximately 3-fold). The mRNA stabilizing effects of UVR treatment are mediated by the C/EBPdelta mRNA 3'untranslated region, which contains an AU rich element. UVR increased p38 MAP kinase (MAPK) activation and SB203580, a p38 MAPK inhibitor, blocked UVR-induced C/EBPdelta mRNA stabilization. UVR increased the nuclear to cytoplasmic translocation of HuR, an ARE-binding protein that functions in mRNA stabilization. Finally, HuR siRNA treatment blocked UVR-induced stabilization of the C/EBPdelta and C/EBPbeta mRNAs but had no effect on C/EBPzeta (CHOP) mRNA stability. In summary, G(0) growth arrested MECs respond to UVR treatment by activating p38 MAPK, increasing HuR translocation and HuR/C/EBPdelta mRNA binding and stabilizing the C/EBPdelta mRNA. These results identify post-transcriptional stabilization of the C/EBPdelta mRNA as a mechanism to increase C/EBPdelta levels in the stress response of quiescent cells to UVR.

The role of connexins in controlling cell growth and gene expression

The purpose of this paper is to provide a brief overview of current thinking on the role of connexins, in particular Cx43, in growth regulation, and a more detailed discussion as to potential mechanisms involved with an emphasis on gene expression. While the precise molecular mechanism by which connexins can affect the growth of normal or tumor cells remains elusive, a number of exciting reports have expanded our understanding and are presented in some detail. Thus, we will discuss (Section 2): the role of protein-protein interactions in integrating connexins into multiple signal transduction pathways; phosphorylation at specific sites and reversal of growth inhibition; the role of the carboxy-terminal regulatory domain as a signaling molecule. Some of our latest work on the potential functions of endogenously produced carboxy-terminal fragments of Cx43 are also presented (Section 3). Finally, Section 4 will pay tribute to the rapidly emerging realization that connexins such as Cx43 and Cx32 exert important and extensive effects on gene expression, particularly those genes linked to growth regulation.

In vivo time-lapse imaging shows dynamic oligodendrocyte progenitor behavior during zebrafish development

Myelinating oligodendrocytes arise from migratory and proliferative oligodendrocyte progenitor cells (OPCs). Complete myelination requires that oligodendrocytes be uniformly distributed and form numerous, periodically spaced membrane sheaths along the entire length of target axons. Mechanisms that determine spacing of oligodendrocytes and their myelinating processes are not known. Using in vivo time-lapse confocal microscopy, we show that zebrafish OPCs continuously extend and retract numerous filopodium-like processes as they migrate and settle into their final positions. Process remodeling and migration paths are highly variable and seem to be influenced by contact with neighboring OPCs. After laser ablation of oligodendrocyte-lineage cells, nearby OPCs divide more frequently, orient processes toward the ablated cells and migrate to fill the unoccupied space. Thus, process activity before axon wrapping might serve as a surveillance mechanism by which OPCs determine the presence or absence of nearby oligodendrocyte-lineage cells, facilitating uniform spacing of oligodendrocytes and complete myelination.

Notch controls proliferation and differentiation of stem cells in a dose-dependent manner

Self-renewal and differentiation of CNS stem cells are regulated by still poorly understood cell-cell interactions. Notch is a well-known cell surface protein that can promote both cell cycle progression and mitotic arrest but the molecular mechanism controlling these opposite effects is unknown. Here we demonstrate that, in CNS stem cells, the level of active Notch1 determines the cellular response. Specifically, low levels of the active form of Notch1 promote proliferation whereas high levels lead to growth arrest. Here we provide the first evidence that Notch effects on proliferation and differentiation are a function of dose, and propose a hypothesis on how oncogenes may also act as tumor suppressors.

Isolation and characterization of the TIGA genes, whose transcripts are induced by growth arrest

We report here the isolation of 44 genes that are upregulated after serum starvation and/or contact inhibition. These genes have been termed TIGA, after Transcript Induced by Growth Arrest. We found that there are two kinds of G0 phases caused by serum starvation, namely, the shallow G0 (or G0/G1) and the deep G0 phases. The shallow G0 is induced by only a few hours of serum starvation, while deep G0 is generated after 3 days of serum starvation. We propose that mammalian cells enter deep G0 through a G0 gate, which is only opened on the third day of serum starvation. TIGA1, one of the uncharacterized TIGA genes, encodes a homolog of cyanate permease of bacteria and localizes in mitochondria. This suggests that Tiga1 is involved in the inorganic ion transport and metabolism needed to maintain the deep G0 phase. Ectopic expression of TIGA1 inhibited not only tumor cell proliferation but also anchorage-independent growth of cancer cell lines. A microsatellite marker, ENDL-1, allowed us to detect loss of heterozygosity around the TIGA1 gene region (5q21-22). Further analysis of the TIGA genes we have identified here may help us to better understand the mechanisms that regulate the G0 phase.

Mutation of Cys105 inhibits dimerization of p12CDK2-AP1 and its growth suppressor effect

p12(CDK2-AP1) (p12) is a CDK2-associated protein that negatively regulates its kinase activity. Growth arrest of normal diploid cells by contact inhibition resulted in an induction of p27(kip1) and reduction of CDK2 levels. Interestingly, we observed concomitantly in growth-arrested cells, there was a reduction of nuclear p12 and the appearance of a nuclear 25-kDa molecule (p25) recognized by anti-p12 polyclonal antibody. Biochemical analysis showed that bacterial His-tagged p12 could be converted into a dimeric p25 in a reducing agent-dependent manner, and mutating the only cysteine residue of p12 (Cys(105) --> Ala(105)) abolished the dimerization. Transient transfection of wild type p12 into U2OS cells showed a reducing agent-sensitive dimerization that was also abolished by the C105A mutation. Furthermore, reduction of p12 expression by a short interfering RNA resulted in a parallel reduction of p25. These data supports the possibility that p25 is a homodimeric form of p12 through the cysteine residue. More interestingly, transient transfection of p12 (C105A) into the normal diploid lung fibroblast CCD18LU cells resulted in a reduction of the growth-inhibitory effect of p12 and abolished the inhibitory effect of p12 on CDK2 kinase activity. In addition, we found that the C105A mutation did not alter nuclear localization of p12, but it prevented association with CDK2. Taken together, our data suggest that p12 forms a nuclear homodimers in contact inhibited normal diploid cells and dimerization of p12 is a necessary process for the growth inhibition effect by p12.

Cell-to-cell adhesion modulates Stat3 activity in normal and breast carcinoma cells

Stat3 (signal transducer and activator of transcription-3) activity is required for transformation by a number of oncogenes, while a constitutively active form of Stat3 alone is sufficient to induce neoplastic transformation. Although in most instances Stat3 is growth-promoting, the impact of cell density on Stat3 activation status and the biological importance of Stat3 during growth arrest have not been characterized. Previous results indicated that cell density alters tyrosine phosphorylation levels of cultured cells. Since signalling through Stat3 is determined by a key phosphorylation at tyr705, we examined the effects of cell density upon Stat3 activity in normal breast epithelial cells, breast carcinoma lines and normal mouse fibroblasts. Intriguingly, the results revealed a dramatic increase in Stat3, tyr705 phosphorylation and activity with cell density, which gradually declined at later stages. This activation was dependent upon cell-cell contact, since it was eliminated if cell adhesion was disrupted through calcium chelation, while it was reinstated through cell aggregation. Furthermore, this activation was suppressed following inhibition of JAKs (Janus kinases) but not inhibition of Fer, IGF1-R, or kinases of the c-Src family. On the other hand, constitutively active Stat3 in carcinoma lines, known to harbor activated Src, was blocked by pharmacological inhibitors of Src as well as JAKs. These results point to the existence of two distinct pathways of Stat3 activation in breast carcinomas, based on Src dependence. More importantly, our results suggest that Stat3 activity is upregulated during the confluence-mediated growth arrest by a signalling mechanism that requires JAKs.

RUNX1 transformation of primary embryonic fibroblasts is revealed in the absence of p53

The mammalian Runx gene family (Runx1-3) are transcription factors that play essential, lineage-specific roles in development. A growing body of evidence implicates these genes as mutational targets in cancer where, in different contexts, individual family members have been reported to act as tumour suppressors, dominant oncogenes or mediators of metastasis. We are exploring these paradoxical observations by ectopic expression of RUNX genes in primary murine embryonic fibroblasts where, in common with a number of other dominant oncogenes, RUNX1 induces senescence-like growth arrest in the presence of an intact p19(ARF)-p53 pathway. We now report that, in MEFs lacking functional p53, RUNX1 has apparently pro-oncogenic effects on cell growth that include cytoskeletal reorganization, reduced contact inhibition at confluence and accelerated tumour expansion in vivo. On the other hand, RUNX1 conferred no obvious growth advantage at low cell density and actually delayed entry of primary MEFs into S phase. We also found that ectopic RUNX1 interferes with the morphological and growth responses of p53-null MEFs to TGFbeta indicating that these effects are mediated by overlapping pathways. These observations help to elucidate the context-dependent consequences of loss and gain of Runx activity.

E-cadherin-mediated adhesion inhibits ligand-dependent activation of diverse receptor tyrosine kinases

E-cadherin is an essential adhesion protein as well as a tumor suppressor that is silenced in many cancers. Its adhesion-dependent regulation of signaling has not been elucidated. We report that E-cadherin can negatively regulate, in an adhesion-dependent manner, the ligand-dependent activation of divergent classes of receptor tyrosine kinases (RTKs), by inhibiting their ligand-dependent activation in association with decreases in receptor mobility and in ligand-binding affinity. E-cadherin did not regulate a constitutively active mutant RTK (Neu*) or the ligand-dependent activation of LPA receptors or muscarinic receptors, which are two classes of G protein-coupled receptors. EGFR regulation by E-cadherin was associated with complex formation between EGFR and E-cadherin that depended on the extracellular domain of E-cadherin but was independent of beta-catenin binding or p120-catenin binding. Transfection of E-cadherin conferred negative RTK regulation to human melanoma and breast cancer lines with downregulated endogenous E-cadherin. Abrogation of E-cadherin regulation may contribute to the frequent ligand-dependent activation of RTK in tumors.

p53 mediates density-dependent growth arrest

While the stress-response-associated importance of the p53 tumor suppressor is well established, recent studies have also linked p53 with several basic parameters in the normal behavior of cells. Here, we present evidence that basal p53 expression in WI38 human embryonic lung fibroblasts restricts growth rate and mediates density-dependent inhibition of growth and the associated G1 phase arrest of the cell cycle by affecting the density-dependent regulation of p16/INK4a. Additionally, we show that prolonged culturing of hTert-immortalized WI38 cells leads to a loss of density-dependent growth inhibition that correlates with p27/KIP deregulation as well as the previously shown INK4a locus silencing, and to an onset of contact-induced, p53-dependent cell death.

HIV-1 expression induces cyclin D1 expression and pRb phosphorylation in infected podocytes: cell-cycle mechanisms contributing to the proliferative phenotype in HIV-associated nephropathy

Background

The aberrant cell-cycle progression of HIV-1-infected kidney cells plays a major role in the pathogenesis of HIV-associated nephropathy, however the mechanisms whereby HIV-1 induces infected glomerular podocytes or infected tubular epithelium to exit quiescence are largely unknown. Here, we ask whether the expression of HIV-1 genes in infected podocytes induces cyclin D₁ and phospho-pRb (Ser780) expression, hallmarks of cyclin D1-mediated G₁ → S phase progression.

Results

We assessed cyclin D₁ and phospho-pRb (Ser780) expression in two well-characterized models of HIV-associated nephropathy pathogenesis: HIV-1 infection of cultured podocytes and HIV-1 transgenic mice (Tg26). Compared to controls, cultured podocytes expressing HIV-1 genes, and podocytes and tubular epithelium from hyperplastic nephrons in Tg26 kidneys, had increased levels of phospho-pRb (Ser780), a target of active cyclin D₁/cyclin-dependent kinase-4/6 known to promote G₁ → S phase progression. HIV-1-infected podocytes showed markedly elevated cyclin D₁ mRNA and cyclin D₁ protein, the latter of which did not down-regulate during cell-cell contact or differentiation, suggesting post-transcriptional stabilization of cyclin D₁ protein levels by HIV-1. The selective suppression of HIV-1 transcription by the cyclin-dependent kinase inhibitor, flavopiridol, abrogated cyclin D₁ expression, underlying the requirement for HIV-1 encoded products to induce cyclin D₁. Indeed, HIV-1 virus deleted of nef failed to induce cyclin D₁ mRNA to the level of other single gene mutant viruses.

Conclusions

HIV-1 expression induces cyclin D₁ and phospho-pRb (Ser780) expression in infected podocytes, suggesting that HIV-1 activates cyclin D1-dependent cell-cycle mechanisms to promote proliferation of infected renal epithelium.