Altered cell adhesion and cell viability in a p38alpha mitogen-activated protein kinase-deficient mouse embryonic stem cell line

Role of l-amino acid oxidase isolated from Calloselasma rhodostoma venom on neutrophil NADPH oxidase complex activation

The action of Cr-LAAO, an l-amino acid oxidase isolated from Calloselasma rhodosthoma snake venom, on NADPH oxidase activation in isolated human neutrophil function was investigated. This enzyme has an intrinsic activity of hydrogen peroxide production. Cr-LAAO, in its native form, induces the ROS production in neutrophil and migration of cytosolic NADPH oxidase components p40^phox, p47^phox and p67^phox to the membrane, and Rac, a GTPase protein member, with the involvement of intracellular signaling mediated by phospho PKC-α. In its inactive form, iCr-LAAO does not induce NADPH oxidase activation in neutrophil showing that the intrinsic enzymatic activity does not have a role in this process, suggesting that its primary structure is essential for the cell's stimulation. Accordingly, the data showed for the first time that the Cr-LAAO has a role in NADPH oxidase complex activation triggering relevant proinflammatory events in human neutrophils.

C3G knock-down enhances migration and invasion by increasing Rap1-mediated p38α activation, while it impairs tumor growth through p38α-independent mechanisms

C3G, a Guanine nucleotide Exchange Factor (GEF) for Rap1 and R-Ras, has been shown to play important roles in development and cancer. Previous studies determined that C3G regulates cell death through down-regulation of p38α MAPK activity. Here, we found that C3G knock-down in MEFs and HCT116 cells promotes migration and invasion through Rap1-mediated p38α hyper-activation. These effects of C3G were inhibited by Rap1 knock-down or inactivation. The enhanced migration observed in C3G depleted HCT116 cells was associated with reduction in E-cadherin expression, internalization of ZO-1, actin cytoskeleton reorganization and decreased adhesion. We also found that matrix metalloproteases MMP2 and MMP9 are involved in the pro-invasive effect of C3G down-regulation. Additionally, our studies revealed that both C3G and p38α collaborate to promote growth of HCT116 cells in vitro and in vivo, possibly by enhancing cell survival. In fact, knocking-down C3G or p38α individually or together promoted cell death in vitro, although only the double C3G-p38α silencing was able to increase cell death within tumors. Notably, we found that the pro-tumorigenic function of C3G does not depend on p38α or Rap1 activation. Altogether, our studies uncover novel mechanisms by which C3G controls key aspects of tumorigenesis.

Sox9 modulates cell survival and adipogenic differentiation of multipotent adult rat mesenchymal stem cells

Sox9 is a key transcription factor in early chondrogenesis with distinct roles in differentiation processes and during embryonic development. Here, we report that Sox9 modulates cell survival and contributes to the commitment of mesenchymal stem cells (MSC) to adipogenic or osteogenic differentiation lineages. We found that the Sox9 activity level affects the expression of the key transcription factor in adipogenic differentiation, C/EBPβ, and that cyclin D1 mediates the expression of the osteogenic marker osteocalcin in undifferentiated adult bone-marrow-derived rat MSC. Introducing a stable Sox9 knockdown into undifferentiated rat MSC resulted in a marked decrease in proliferation rate and an increase in apoptotic activity. This was linked to a profound upregulation of p21 and cyclin D1 gene and protein expression accompanied by an induction of caspase 3/7 activity and an inhibition of Bcl-2. We observed that Sox9 silencing provoked a delayed S-phase progression and an increased nuclear localization of p21. The protein stability of cyclin D1 was induced in the absence of Sox9 presumably as a function of altered p38 signalling. In addition, the major transcription factor for adipogenic differentiation, C/EBPβ, was repressed after silencing Sox9. The nearly complete absence of C/EBPβ protein as a result of increased destabilization of the C/EBPβ mRNA and the impact on osteocalcin gene expression and protein synthesis, suggests that a delicate balance of Sox9 level is not only imperative for proper chondrogenic differentiation of progenitor cells, but also affects the adipogenic and probably osteogenic differentiation pathways of MSC. Our results identified Sox9 as an important link between differentiation, proliferation and apoptosis in undifferentiated adult rat mesenchymal stem cells, emphasizing the importance of the delicate balance of a precisely regulated Sox9 activity in MSC not only for proper skeletal development during embryogenesis but probably also for successful repair and regeneration of tissues and organs in adults.

Mouse embryonic stem cells are deficient in type I interferon expression in response to viral infections and double-stranded RNA

Embryonic stem cells (ESCs) are considered to be a promising cell source for regenerative medicine because of their unlimited capacity for self-renewal and differentiation. However, little is known about the innate immunity in ESCs and ESC-derived cells. We investigated the responses of mouse (m)ESCs to three types of live viruses as follows: La Crosse virus, West Nile virus, and Sendai virus. Our results demonstrated mESCs were susceptible to viral infection, but they were unable to express type I interferons (IFNα and IFNβ, IFNα/β), which differ from fibroblasts (10T1/2 cells) that robustly express IFNα/β upon viral infections. The failure of mESCs to express IFNα/β was further demonstrated by treatment with polyIC, a synthetic viral dsRNA analog that strongly induced IFNα/β in 10T1/2 cells. Although polyIC transiently inhibited the transcription of pluripotency markers, the stem cell morphology was not significantly affected. However, polyIC can induce dsRNA-activated protein kinase in mESCs, and this activation resulted in a strong inhibition of cell proliferation. We conclude that the cytosolic receptor dsRNA-activated protein kinase is functional, but the mechanisms that mediate type I IFN expression are deficient in mESCs. This conclusion is further supported by the findings that the major viral RNA receptors are either expressed at very low levels (TLR3 and MDA5) or may not be active (retinoic acid-inducible gene I) in mESCs.

C3G forms complexes with Bcr-Abl and p38α MAPK at the focal adhesions in chronic myeloid leukemia cells: implication in the regulation of leukemic cell adhesion

Background

Previous studies by our group and others have shown that C3G interacts with Bcr-Abl through its SH3-b domain.

Results

In this work we show that C3G and Bcr-Abl form complexes with the focal adhesion (FA) proteins CrkL, p130Cas, Cbl and Abi1 through SH3/SH3-b interactions. The association between C3G and Bcr-Abl decreased upon Abi1 or p130Cas knock-down in K562 cells, which suggests that Abi1 and p130Cas are essential partners in this interaction. On the other hand, C3G, Abi1 or Cbl knock-down impaired adhesion to fibronectin, while p130Cas silencing enhanced it. C3G, Cbl and p130Cas-SH3-b domains interact directly with common proteins involved in the regulation of cell adhesion and migration. Immunoprecipitation and immunofluorescence studies revealed that C3G form complexes with the FA proteins paxillin and FAK and their phosphorylated forms. Additionally, C3G, Abi1, Cbl and p130Cas regulate the expression and phosphorylation of paxillin and FAK. p38α MAPK also participates in the regulation of adhesion in chronic myeloid leukemia cells. It interacts with C3G, CrkL, FAK and paxillin and regulates the expression of paxillin, CrkL and α5 integrin, as well as paxillin phosphorylation. Moreover, double knock-down of C3G/p38α decreased adhesion to fibronectin, similarly to the single silencing of one of these genes, either C3G or p38α. These suggest that C3G and p38α MAPK are acting through a common pathway to regulate cell adhesion in K562 cells, as previously described for the regulation of apoptosis.

Conclusions

Our results indicate that C3G-p38αMAPK pathway regulates K562 cell adhesion through the interaction with FA proteins and Bcr-Abl, modulating the formation of different protein complexes at FA.

The many faces of p38 mitogen-activated protein kinase in progenitor/stem cell differentiation

Regulation of stem cells is essential for development and adult tissue homoeostasis. The proper control of stem cell self-renewal and differentiation maintains organ physiology, and disruption of such a balance results in disease. There are many mechanisms that have been established as stem cell regulators, such as Wnt or Notch signals. However, the intracellular mechanisms that mediate and integrate these signals are not well understood. A new intracellular pathway that has been reported to be involved in the regulation of many stem cell types is that of p38 MAPK (mitogen-activated protein kinase). In particular, p38α is essential for the proper differentiation of many haematopoietic, mesenchymal and epithelial stem/progenitor cells. Many reports have shown that disruption of this kinase pathway has pathological consequences in many organs. Understanding the extracellular cues and downstream targets of p38α in stem cell regulation may help to tackle some of the pathologies associated with improper differentiation and regulation of stem cell function. In the present review we present a vision of the current knowledge on the roles of the p38α signal as a regulator of stem/progenitor cells in different tissues in physiology and disease.

Transient inhibition of cell proliferation does not compromise self-renewal of mouse embryonic stem cells

Embryonic stem cells (ESCs) have unlimited capacity for self-renewal and can differentiate into various cell types when induced. They also have an unusual cell cycle control mechanism driven by constitutively active cyclin dependent kinases (Cdks). In mouse ESCs (mESCs). It is proposed that the rapid cell proliferation could be a necessary part of mechanisms that maintain mESC self-renewal and pluripotency, but this hypothesis is not in line with the finding in human ESCs (hESCs) that the length of the cell cycle is similar to differentiated cells. Therefore, whether rapid cell proliferation is essential for the maintenance of mESC state remains unclear. We provide insight into this uncertainty through chemical intervention of mESC cell cycle. We report here that inhibition of Cdks with olomoucine II can dramatically slow down cell proliferation of mESCs with concurrent down-regulation of cyclin A, B and E, and the activation of the Rb pathway. However, mESCs display can recover upon the removal of olomoucine II and are able to resume normal cell proliferation without losing self-renewal and pluripotency, as demonstrated by the expression of ESC markers, colony formation, embryoid body formation, and induced differentiation. We provide a mechanistic explanation for these observations by demonstrating that Oct4 and Nanog, two major transcription factors that play critical roles in the maintenance of ESC properties, are up-regulated via de novo protein synthesis when the cells are exposed to olomoucine II. Together, our data suggest that short-term inhibition of cell proliferation does not compromise the basic properties of mESCs.

Effects of oxidative stress on mouse embryonic stem cell proliferation, apoptosis, senescence, and self-renewal

Oxidative stress, associated with either normal metabolism or disease conditions, affects many cellular activities. Most of our knowledge in this field is derived from fully differentiated cells. Embryonic stem cells (ESCs) have attracted enormous attention for their potential applications in cell therapy, but little is known about how the unique properties of ESCs are affected by oxidative stress. We have investigated the effects of oxidative stress induced by H(2)O(2) on several cellular activities of mouse ESCs. Like differentiated cells, ESCs are sensitive to H(2)O(2)-induced apoptosis when continuously exposed to H(2)O(2) at the concentrations above 150 microM. However, unlike differentiated cells, ESCs are resistant to oxidative stress induced senescence. This is demonstrated by the results that when subjected to a short-term sublethal concentration and duration of H(2)O(2) treatment, fibroblasts enter the senescent state with enlarged flattened cell morphology concurrent with increased expression of senescence marker p21. On the contrary, ESCs neither show any sign of senescence nor express p21. Instead, ESCs enter a transient cell cycle arrest state, but they have remarkable recovery capacity to resume the normal cell proliferation rate without losing the ability of self-renewal and pluripotency. Our results further revealed that H(2)O(2) inhibits cell adhesion and the expression of cyclin D1, which are early events proceeding apoptosis and cell cycle arrest. In conclusion, our data suggest that ESCs are sensitive to H(2)O(2) toxicity, but may have unique mechanisms that prevent H(2)O(2)-induced senescence and protect self-renewal capacity.

Biocompatibility of synthetic poly(ester urethane)/polyhedral oligomeric silsesquioxane matrices with embryonic stem cell proliferation and differentiation

Incorporation of polyhedral oligomeric silsesquioxanes (POSS) into poly(ester urethanes) (PEU) as a building block results in a PEU/POSS hybrid polymer with increased mechanical strength and thermostability. An attractive feature of the new polymer is that it forms a porous matrix when cast in the form of a thin film, making it potentially useful in tissue engineering. In this study, we present detailed microscopic analysis of the PEU/POSS matrix and demonstrate its biocompatibility with cell culture. The PEU/POSS polymer forms a continuous porous matrix with open pores and interconnected grooves. From SEM image analysis, it is calculated that there are about 950 pores/mm(2) of the matrix area with pore diameter size in the range 1-15 µm. The area occupied by the pores represents approximately 7.6% of the matrix area. Using mouse embryonic stem cells (ESCs), we demonstrate that the PEU/POSS matrix provides excellent support for cell proliferation and differentiation. Under the cell culture condition optimized to maintain self-renewal, ESCs grown on a PEU/POSS matrix exhibit undifferentiated morphology, express pluripotency markers and have a similar growth rate to cells grown on gelatin. When induced for differentiation, ESCs underwent dramatic morphological change, characterized by the loss of clonogenecity and increased cell size, with well-expanded cytoskeleton networks. Differentiated cells are able to form a continuous monolayer that is closely embedded in the matrix. The excellent compatibility between the PEU/POSS matrix and ESC proliferation/differentiation demonstrates the potential of using PEU/POSS polymers in future ESC-based tissue engineering.

Mouse embryonic stem cells lacking p38alpha and p38delta can differentiate to endothelial cells, smooth muscle cells, and epithelial cells

The p38 mitogen-activated protein (MAP) kinases (p38) are important signaling molecules that regulate various cellular processes. Four isoforms of p38 family, p38alpha, p38beta, p38gamma, and p38delta, have been identified in mammalian cells. Previous studies have shown that p38alpha knockout is embryonic lethal in mice. At the cellular level, p38alpha is abundantly expressed in mouse embryonic stem cells (ESCs), but p38alpha knockout (p38alpha-/-) ESCs can differentiate to endothelial cells (ECs), smooth muscle cells (SMCs), and neurons. We speculate that the lost function of p38alpha in p38alpha-/- ESCs may be compensated for by the redundant function of other isoforms. To test this hypothesis, we used siRNA approach to knock down the expression of p38delta, the second abundant isoform in ESCs. ESCs stably expressing p38delta siRNA were established from p38alpha-/- ESCs, resulting in 80% reduction of p38delta mRNA expression. However, these ESCs, deficient of both p38alpha and p38delta, could still differentiate into ECs and SMCs. We extended our investigation to test if these cells can differentiate into epithelial cells in which p38delta has been shown to regulate epidermis differentiation. Our results demonstrate again that ESC differentiation to epithelial cells is independent of p38alpha and p38delta. We conclude that p38alpha and p38delta are not essential for ESC differentiating into ECs, SMCs, or epithelial cells although numerous studies have shown that the two kinases regulate various cellular activities in aforementioned cells. Our results highlight the possibility that p38 MAP kinases may play less significant roles in ESC differentiation than in the regulation of cellular activities of fully differentiated somatic cells.

p38alpha MAP kinase-deficient mouse embryonic stem cells can differentiate to endothelial cells, smooth muscle cells, and neurons

p38 MAP kinase alpha (p38alpha) regulates various cellular processes in adult cells, but little is known about its function in stem cells. We investigated the potential of wild type and p38alpha deficient mouse embryonic stem cells (ESCs) to differentiate into endothelial cells (ECs), smooth muscle cells (SMCs), and neurons. Our differentiation methods allowed simultaneous development of all these cell types. ECs formed monolayers similar to mature ECs and could assemble into vessel-like structures. SMCs had well-organized actin filaments with morphology similar to adult SMCs. Neurons exhibited well-developed cell bodies and elongated axons. Deletion of the p38alpha gene did not significantly compromise ESC differentiation since p38alpha-/- cells could express cell-specific markers and displayed similar overall morphology to the cells differentiated from p38alpha+/+ ESCs. Although p38alpha regulates various cellular activities of adult SMCs, ECs, and neurons, our data demonstrate that p38alpha is not essential for ESC differentiation to these cell types.

p38alpha MAPK can positively or negatively regulate Rac-1 activity depending on the presence of serum

The small GTP-ase Rac-1 can trigger p38 MAPK activation and, in turn, p38alpha can regulate signalling pathways that potentially impinge on Rac-1 activity. We have investigated the cross-talk between p38alpha and Rac-1 and found that p38alpha regulates the association between Rac-1 and caveolin-1 in serum-deprived cardiomyocytes. This interaction depends on cell attachment and correlates with higher levels of active Rac-1. Actin organization might regulate the formation of Rac-1-caveolin-1 complexes. In contrast, the Rac-1-caveolin-1 interaction is almost undetectable in the presence of serum, where Rac-1 activity is negatively regulated by p38alpha. Our results indicate that p38alpha can differentially contribute to Rac-1 activation depending on the presence of serum.