Low molecular weight phosphotyrosine-protein phosphatase (LMW-PTP) shares no general sequence homology with other PTPs, although it has an active site sequence motif CXXXXXR and a reaction mechanism identical to those of all PTPs. The main function of this enzyme is the down-regulation of platelet-derived growth factor and insulin receptors. Both human LMW-PTP isoenzymes are inactivated by H 2 O 2 . The enzymes are protected from inactivation by P i , a competitive inhibitor, suggesting that the H 2 O 2 reaction is directed to active site. Analysis of free thiols performed on the inactivated enzymes demonstrates that only two out of the eight LMW-PTP cysteines are modified. Time-course high performance liquid chromatography-electrospray mass spectrometry, together with specific radiolabeling and tryptic fingerprint analyses, enables us to demonstrate that H 2 O 2 causes the oxidation of Cys-12 and Cys-17 to form a disulfide bond. Because both residues are localized into the active site region, this modification inactivates the enzyme. Fluorescence spectroscopy experiments suggest that the fold of the enzyme is modified during oxidation by H 2 O 2 . Because a physiological concentration of H 2 O 2 produces enzyme inactivation and considering that the activity is restored by reduction with low molecular weight thiols, we suggest that oxidative stress conditions and other processes producing hydrogen peroxide regulate the LMW-PTP in the cell.Protein tyrosine phosphorylation in eucaryotes is a key mechanism for cellular control, because it is involved in several processes, such as cellular metabolism, proliferation, differentiation, and oncogenic transformation (1). A fine balancing of cellular protein tyrosine phosphorylation levels is determined by regulating the activities of protein-tyrosine kinases and/or protein-tyrosine phosphatases (PTPs).1 Receptor protein-tyrosine kinases are considered to be the major enzymes regulating mitogenic protein phosphorylation cascades; nevertheless, the presence of SH2 domains in particular PTPs and the receptorlike structure of some membrane PTPs clearly indicate that PTPs are also regulated in the cell. The PTP superfamily consists of four main families: the tyrosine-specific phosphatases, the VH1-like dual specificity phosphatases, the cdc25 phosphatases, and the low molecular weight phosphatases (LMW-PTPs). Despite extremely limited sequence similarity, all share an active site motif consisting of a cysteine and an arginine separated by five residues (CXXXXXR, where X is any amino acid). All PTPs have identical catalytic mechanism, which involves the formation of a cysteinyl-phosphate intermediate (2).Recent papers from our laboratory have demonstrated that LMW-PTP is involved in the regulation of cellular signaling started by the activation of PDGF and insulin receptors (3-5). In fact, the overexpression of the wild type enzyme in NIH/3T3 cells causes decrease of cellular growth rate and of phosphorylation level of the PDGF receptor (3). Furthermore, the overexpression in ...
It is common knowledge that platelet-derived growth factor (PDGF) is a critical regulator of mesenchymal cell migration and proliferation. Nevertheless, these two cellular responses are mutually exclusive. To solve this apparent contradiction, we studied the behavior of NIH3T3 fibroblasts in response to increasing concentrations of PDGF. We found that there is strong cell proliferation induction only with PDGF concentrations >5 ng/ml, whereas the cell migration response arises starting from 1 ng/ml and is negligible at higher PDGF concentrations. According to these phenotypic evidences, our data indicate that cells display a differential activation of the main signaling pathways in response to PDGF as a function of the stimulation dose. At low PDGF concentrations, there is maximal activation of signaling pathways linked to cytoskeleton rearrangement needed for cell motility, whereas high PDGF concentrations activate pathways linked to mitogenesis induction. Our results suggest a mechanism by which cells switch from a migrating to a proliferating phenotype sensing the increasing gradient of PDGF. In addition, we propose that the cell decision to proliferate or migrate relies on different endocytotic routes of the PDGF receptor in response to different PDGF concentrations.
Morin is a natural polyphenol, originally isolated from members of the Moraceae family that can be extracted from leaves, fruits, stems and branches of numerous plants. Several evidence have demonstrated that Morin could have a beneficial effect on several human diseases. In fact, Morin exerts antioxidant, antidiabetic, anti-inflammatory, antitumoral, antihypertensive, antibacterial, hypouricemic, and neuroprotective effects, by modulating the activity of many enzymes. In some cases, Morin shows a systemic protective action, reducing negative side effects of several drugs, without interfering with their functions. In addition, in vitro and in vivo studies demonstrated that Morin exhibits very low toxicity levels and its chronic administration is well tolerated. All these findings suggest that Morin could be used, either alone or in combination with other drugs, to prevent many human pathologies.
Despite marked tumor shrinkage after 5-FU treatment, the frequency of colon cancer relapse indicates that a fraction of tumor cells survives treatment causing tumor recurrence. The majority of cancer cells divert metabolites into anabolic pathways through Warburg behavior giving an advantage in terms of tumor growth. Here, we report that treatment of colon cancer cell with 5-FU selects for cells with mesenchymal stem-like properties that undergo a metabolic reprogramming resulting in addiction to OXPHOS to meet energy demands. 5-FU treatment-resistant cells show a de novo expression of pyruvate kinase M1 (PKM1) and repression of PKM2, correlating with repression of the pentose phosphate pathway, decrease in NADPH level and in antioxidant defenses, promoting PKM2 oxidation and acquisition of stem-like phenotype. Response to 5-FU in a xenotransplantation model of human colon cancer confirms activation of mitochondrial function. Combined treatment with 5-FU and a pharmacological inhibitor of OXPHOS abolished the spherogenic potential of colon cancer cells and diminished the expression of stem-like markers. These findings suggest that inhibition of OXPHOS in combination with 5-FU is a rational combination strategy to achieve durable treatment response in colon cancer.
Fibroblasts are the most abundant cells in connective tissue and, with fibrillar extracellular matrix, form the structural scaffolding of organs. In solid tumors, interaction with cancer cells induces fibroblasts transdifferentiation into an activated form, which become a fundamental part of the tumor stroma. Within tumor microenvironment stromal and cancer cells engage a crosstalk that is mediated by soluble factors, cellcell contacts and extracellular vesicles trafficlking. Here we report that fibroblasts have the ability to transfer a remarkable amount of proteins and lipids to neighboring cells, in an ectosome-dependent fashion, identifying a novel and native property of these cells. Cancer-associated fibroblasts show an enhanced production and delivering of ectc:Jsomes to cancer cells compared to normal fibroblasts. As a consequence of this phenomenon, tumor cells increase their proliferation rate, indicating that ectosome-mediated trafficking could be a relevant mechanism mediating the trophic function of activated connective tissue on tumor cells.
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