17-500 MilliporeCatch and Release® v2.0 Reversible Immunoprecipitation System

Galectin-1 is a lectin recognized by galactoside-containing glycoproteins, and is involved in cancer progression and metastasis. The role of galectin-1 in radiosensitivity has not previously been investigated. Therefore, this study tests whether galectin-1 is involved in the radiosensitivity mediated by the H-Ras signaling pathway using cervical carcinoma cell lines. A knockdown of galectin-1 expression in HeLa cells decreased clonogenic survival following irradiation. The clonogenic survival increased in both HeLa and C33A cells with galectin-1 overexpression. The overexpression or knockdown of galectin-1 did not alter radiosensitivity, whereas H-Ras was silenced in both cell lines. Whereas K-Ras was knocked down, galectin-1 restored the radiosensitivity in HeLa cells and C33A cells. The knockdown of galectin-1 increased the high-dose radiation-induced cell death of HeLa cells transfected by constitutively active H-Ras. The knockdown of galectin-1 inhibited the radiation-induced phosphorylation of Raf-1 and ERK in HeLa cells. Overexpression of galectin-1 enhanced the phosphorylation of Raf-1 and ERK in C33A cells following irradiation. Galectin-1 decreased the DNA damage detected using comet assay and γ-H2AX in both cells following irradiation. These findings suggest that galectin-1 mediates radioresistance through the H-Ras-dependent pathway involved in DNA damage repair.

The Forkhead Box O (FOXO) transcription factors regulate diverse cellular processes, and in skeletal muscle are both necessary and sufficient for muscle atrophy. Although the regulation of FOXO by Akt is well evidenced in skeletal muscle, the current study demonstrates that FOXO is also regulated in muscle via the histone acetyltransferase (HAT) activities of p300/CREB-binding protein (CBP). Transfection of rat soleus muscle with a dominant-negative p300, which lacks HAT activity and inhibits endogenous p300 HAT activity, increased FOXO reporter activity and induced transcription from the promoter of a bona fide FOXO target gene, atrogin-1. Conversely, increased HAT activity via transfection of either wild-type (WT) p300 or WT CBP repressed FOXO activation in vivo in response to muscle disuse, and in C2C12 cells in response to dexamethasone and acute starvation. Importantly, manipulation of HAT activity differentially regulated the expression of various FOXO target genes. Cotransfection of FOXO1, FOXO3a, or FOXO4 with the p300 constructs further identified p300 HAT activity to also differentially regulate the activity of the FOXO homologues. Markedly, decreased HAT activity strongly increased FOXO3a transcriptional activity, while increased HAT activity repressed FOXO3a activity and prevented its nuclear localization in response to nutrient deprivation. In contrast, p300 increased FOXO1 nuclear localization. In summary, this study provides the first evidence to support the acetyltransferase activities of p300/CBP in regulating FOXO signaling in skeletal muscle and suggests that acetylation may be an important mechanism to differentially regulate the FOXO homologues and dictate which FOXO target genes are activated in response to varying atrophic stimuli.

A three amino acid sequence, Ser/Thr-Pro-Ser/Thr, was recently identified and characterized as a novel nuclear localization signal (Chuderland et al., 2008). The immediate-early gene product, early growth response-1 is a three zinc finger containing transcription factor implicated in a wide variety of pathologies, and has a bipartite nuclear localization domain identified two decades ago. Efficient nuclear localization of Egr-1 is vital to its function as a transcription factor. Interestingly, Egr-1 also contains a C-terminal SPS domain (residues 482-484 in murine Egr-1). We hypothesized that (482)SPS(484) may also serve as a novel nuclear localization signal in Egr-1. We found that this sequence directs Egr-1 to the nucleus in transfected Chinese hamster ovary cells and show by co-immunoprecipitation analysis that Egr-1 forms a complex with importin-7. (482)SPS(484) is required for Egr-1's interaction with importin-7. Moreover, importin-7 knockdown with RNAi showed that Egr-1 nuclear translocation is importin-7-dependent. This study demonstrates that the nuclear translocation of Egr-1 is partially dependent on (482)SPS(484) and involves importin-7, and sheds light on the molecular mechanisms regulating the cellular localization of this pathophysiologically important transcription factor.

Cells exposed to high glucose may undergo hypertrophy, proliferation, and apoptosis, but the role of hexosamine flux in mediating these effects has not been fully elucidated. Accordingly, we studied the effects of glucose and glucosamine on rat glomerular mesangial cells (MC) turnover. In comparison with physiologic glucose (5.6mM), treatment with high glucose (25mM) for 24 hours stimulated MC proliferation, an effect that was mimicked by exposure to low concentrations of glucosamine (0.05mM). The percentage of cells in G(0)/G(1) phase of the cell cycle was reduced with a concomitant increase of the number of cells in G2/M phase. Proliferating cell nuclear antigen (PCNA), phosphorylated mammalian target of rapamycin (phospho-mTOR [ser2448]) and total regulatory associated protein of mTOR (Raptor) were increased by high glucose and glucosamine treatment. Inhibition of glutamine:fructose-6-phosphate amidotransferase (GFAT), the rate-limiting enzyme for hexosamine flux, with 6-diazo-5-oxonorleucine (DON, 10microM) and of mTOR with rapamycin both attenuated glucose-mediated MC proliferation. Higher glucosamine concentrations (0.25mM to 10mM) caused MC apoptosis after 48 hours and, in addition, GFAT overexpression also increased MC apoptosis (TUNEL-positive cells: 3.8+/-0.3 % versus 1.1+/-0.2% for empty vector; p<0.001). Hence, hexosamine flux is an important determinant of MC proliferation and apoptosis. The proliferative response to high glucose and hexosamine flux is rapamycin-sensitive, suggesting that this effect is associated with signaling through rapamycin-sensitive mTOR complex 1 (mTORC1).

The intracellular signaling events controlling human mesenchymal stem cells (hMSC) differentiation into osteoblasts are not entirely understood. We recently demonstrated that contact with extracellular matrix (ECM) proteins is sufficient to induce osteogenic differentiation of hMSC through an ERK-dependent pathway. We hypothesized that FAK signaling pathways provide a link between activation of ERK1/2 by ECM, and stimulate subsequent phosphorylation of the Runx2/Cbfa-1 transcription factor that controls osteogenic gene expression. We plated hMSC on purified collagen I (COLL-I) and vitronectin (VN) in the presence or absence of FAK-specific siRNA, and assayed for phosphorylation of Runx2/Cbfa-1 as well as expression of established osteogenic differentiation markers (bone sialoprotein-2, osteocalcin, alkaline phosphatase, calcium deposition, and spectroscopically determined mineral:matrix ratio). We found that siRNA treatment reduced FAK mRNA levels by >40% and decreased ECM-mediated phosphorylation of FAK Y397 and ERK1/2. Serine phosphorylation of Runx2/Cbfa-1 was significantly reduced after 8 days in treated cells. Finally, FAK inhibition blocked osterix transcriptional activity and the osteogenic differentiation of hMSC, as assessed by lowered expression of osteogenic genes (RT-PCR), decreased alkaline phosphatase activity, greatly reduced calcium deposition, and a lower mineral:matrix ratio after 28 days in culture. These results suggest that FAK signaling plays an important role in regulating ECM-induced osteogenic differentiation of hMSC.

Human mesenchymal stem cell (hMSC) differentiation into osteoblasts and the signaling events involved are poorly understood. We recently established that contact with specific extracellular matrix (ECM) proteins, in particular laminin-5, is sufficient to induce an osteogenic phenotype in hMSC through an extracellular signal-related kinase (ERK)-dependent pathway. Activation of ERK 1/2 by laminin-5 induces phosphorylation of the runx2/cbfa-1 transcription factor that controls osteogenic gene expression. We hypothesized that focal adhesion kinase (FAK) mediated signaling pathways supply a link between cell surface integrin-ECM binding and activation of ERK 1/2, and that laminin-5 promotes its osteogenic effects through this pathway. To test this hypothesis, we plated hMSC on a laminin-5 matrix in the presence or absence of FAK-specific small inhibitory RNAs (siRNA), and assayed for phosphorylation of runx2/cbfa-1 as well as expression of established osteogenic differentiation markers (bone sialoprotein, osteocalcin, alkaline phosphatase, calcium deposition, and mineral:matrix ratio). We found that siRNA treatment reduced total endogenous FAK protein by approximately 40%, and reduced FAK phosphorylation on Y397 by approximately 33% in cells plated on laminin-5 for 30 min. SiRNA treated cells exhibited a decrease in ERK 1/2 phosphorylation after 1 h, and reduced serine/threonine phosphorylation of Runx2/Cbfa-1 after 8 days. Finally, FAK inhibition blocked osteogenic differentiation of hMSC, as assessed by lowered expression of osteogenic genes (RT-PCR), decreased alkaline phosphatase activity, greatly reduced calcium deposition, and a lower mineral:matrix ratio after 28 days in culture. These results establish FAK as an important mediator of laminin-5-induced osteogenic differentiation of hMSC.

The genetic variability and covalent modifications associated with the amino terminus of the protein kinase A (PKA) catalytic (C) subunit suggest that it may contribute to protein-protein interactions and/or localization. By using a yeast two-hybrid screen, we identified a PKA-interacting protein (AKIP1) that binds to the amino terminus (residues 1-39) of the C subunit of PKA. The interaction was localized to the A helix (residues 14-39) of the C subunit and to the carboxyl terminus of AKIP1. AKIP1 thus defines the amino-terminal A helix of PKA as a protein interaction motif. In normal breast (Hs 578 Bst) and HeLa cells, AKIP1 is present in the nucleus as speckles. A nuclear localization signal (Arg-14 and Arg-15) was identified. Upon stimulation with forskolin, HeLa cells expressing AKIP1 accumulated higher levels of the endogenous C subunit in the nucleus. Deletion of the carboxyl terminus of AKIP1 or overexpression of residues 1-39 of the C subunit abolished nuclear localization of the activated endogenous C subunit. Thus, AKIP1 describes a PKA-interacting protein that can contribute to localization by a mechanism that is distinct from A-kinase anchoring proteins that interact with the regulatory subunits.