Protein phosphatase 2A promotes endothelial survival via stabilization of translational inhibitor 4E-BP1 following exposure to tumor necrosis factor-α

Collagen XVII inhibits breast cancer cell proliferation and growth through deactivation of the AKT/mTOR signaling pathway

Collagen XVII (COL17), a cell-matrix adhesion protein, has been found to be suppressed in breast cancer. Our previous data demonstrated a preventive role of COL17 in breast cancer invasiveness. The present study used the stable COL17-overexpressing MCF7 and MDA-MB-231 cells to reveal an anti-proliferative effect of COL17 on breast cancer cell through mTOR deactivation. Cell proliferation was negatively correlated with the expression level of COL17 in a concentration-dependent manner in both conventional and three-dimensional (3D) culture systems. The correlation was confirmed by decreased expression of the proliferative marker Ki67 in COL17-expressing cells. In addition, overexpression of COL17 reduced the clonogenicity and growth of the cells. We demonstrated that COL17 affects the AKT/mTOR signaling pathway by deactivation of AKT, mTOR and downstream effectors, particularly 4EBP1. Moreover, mice xenografted with high COL17-expressing cells exhibited delayed tumor progression and prolonged survival time. The high expression of COL17A1 gene encoding COL17 is associated with low-proliferation tumors, extended tumor-free period, and overall survival of breast cancer patients. In conclusion, our results revealed the novel function of COL17 using in vitro and in vivo models and elucidated the related pathway in breast cancer cell growth and proliferation.

GSK3β suppression inhibits MCL1 protein synthesis in human acute myeloid leukemia cells

Previous studies have shown that glycogen synthase kinase 3β (GSK3β) suppression is a potential strategy for human acute myeloid leukemia (AML) therapy. However, the cytotoxic mechanism associated with GSK3β suppression remains unresolved. Thus, the underlying mechanism of N-(4-methoxybenzyl)-N'-(5-nitro-1,3-thiazol-2-yl)urea (AR-A014418)-elicited GSK3β suppression in the induction of AML U937 and HL-60 cell death was investigated in this study. Our study revealed that AR-A014418-induced MCL1 downregulation remarkably elicited apoptosis of U937 cells. Furthermore, the AR-A014418 treatment increased p38 MAPK phosphorylation and decreased the phosphorylated Akt and ERK levels. Activation of p38 MAPK subsequently evoked autophagic degradation of 4EBP1, while Akt inactivation suppressed mTOR-mediated 4EBP1 phosphorylation. Furthermore, AR-A014418-elicited ERK inactivation inhibited Mnk1-mediated eIF4E phosphorylation, which inhibited MCL1 mRNA translation in U937 cells. In contrast to GSK3α, GSK3β downregulation recapitulated the effect of AR-A014418 in U937 cells. Transfection of constitutively active GSK3β or cotransfection of constitutively activated MEK1 and Akt suppressed AR-A014418-induced MCL1 downregulation. Moreover, AR-A014418 sensitized U937 cells to ABT-263 (BCL2/BCL2L1 inhibitor) cytotoxicity owing to MCL1 suppression. Collectively, these results indicate that AR-A014418-induced GSK3β suppression inhibits ERK-Mnk1-eIF4E axis-modulated de novo MCL1 protein synthesis and thereby results in U937 cell apoptosis. Our findings also indicate a similar pathway underlying AR-A014418-induced death in human AML HL-60 cells.

Serine-threonine protein phosphatases: Lost in translation

Protein synthesis is one of the most complex and energy-consuming processes in eukaryotic cells and therefore is tightly regulated. One of the main mechanisms of translational control is post-translational modifications of the components of translational apparatus. Phosphorylation status of translation factors depends on the balanced action of kinases and phosphatases. While many kinase-dependent events are well defined, phosphatases that counteract phosphorylation are rarely determined. This mini-review focuses on the regulation of activity of translational initiation factors by serine/threonine phosphatases.

Transient PP2A inhibition alleviates normal tissue stem cell susceptibility to cell death during radiotherapy

Unintended outcomes of cancer therapy include ionizing radiation (IR)-induced stem cell depletion, diminished regenerative capacity, and accelerated aging. Stem cells exhibit attenuated DNA damage response (DDR) and are hypersensitive to IR, as compared to differentiated non-stem cells. We performed genomic discovery research to compare stem cells to differentiated cells, which revealed Phosphoprotein phosphatase 2A (PP2A) as a potential contributor to susceptibility in stem cells. PP2A dephosphorylates pATM, γH2AX, pAkt etc. and is believed to play dual role in regulating DDR and apoptosis. Although studied widely in cancer cells, the role of PP2A in normal stem cell radiosensitivity is unknown. Here we demonstrate that constitutively high expression and radiation induction of PP2A in stem cells plays a role in promoting susceptibility to irradiation. Transient inhibition of PP2A markedly restores DNA repair, inhibits apoptosis, and enhances survival of stem cells, without affecting differentiated non-stem and cancer cells. PP2Ai-mediated stem cell radioprotection was demonstrated in murine embryonic, adult neural, intestinal, and hematopoietic stem cells.

The Role of Epithelial to Mesenchymal Transition in Human Amniotic Membrane Rupture

CONTEXT

Biochemical weakening of the amnion is a major factor preceding preterm premature rupture of membranes (PPROMs), leading to preterm birth. Activation of matrix metalloproteinases (MMPs) is known to play a key role in collagen degradation of the amnion; however, epithelial to mesenchymal transition (EMT) that is also induced by MMP activation has not been investigated as a mechanism for amnion weakening.

OBJECTIVE

To measure amniotic EMT associated with vaginal delivery (VD) compared with unlabored cesarean sections (CSs), and to assess changes in amniotic mechanical strength with pharmacologic inhibitors and inducers of EMT, thus testing the hypothesis that EMT is a key biochemical event that promotes amniotic rupture.

FINDINGS

(1) Amnions taken from VD contained a significantly increased number of mesenchymal cells relative to epithelial cells compared with unlabored CS by fluorescence-activated cell sorting analysis (60% vs 10%); (2) tumor necrosis factor (TNF)-α stimulation of amniotic epithelial cells increased expression of the mesenchymal marker vimentin after 2 days; (3) EMT inhibitor, etodolac, significantly increased the time and mechanical pressure required to rupture the amnion; and (4) TNF-α and another pharmacologic EMT inducer, ethacridine, decreased the time and mechanical pressure required for amnion rupture, further confirming that the mesenchymal phenotype significantly weakens the amnion.

CONCLUSIONS

This work demonstrated amniotic cell EMT was associated with labor and EMT decreased the tensile strength of the amnion. These findings suggest a role for EMT in the pathophysiology of PPROM and may provide a basis for development of therapies to prevent preterm labor.

4E-BP1, a multifactor regulated multifunctional protein

Eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) is a member of a family of translation repressor proteins, and a well-known substrate of mechanistic target of rapamycin (mTOR) signaling pathway. Phosphorylation of 4E-BP1 causes its release from eIF4E to allow cap-dependent translation to proceed. Recently, 4E-BP1 was shown to be phosphorylated by other kinases besides mTOR, and overexpression of 4E-BP1 was found in different human carcinomas. In this review, we summarize the novel findings on mTOR independent 4E-BP1 phosphorylation in carcinomas. The implications of overexpression and possible multi-function of 4E-BP1 are also discussed.

γ-Aminobutyric Acid Is Synthesized and Released by the Endothelium: Potential Implications

RATIONALE

Gamma aminobutyric acid (GABA), a neurotransmitter of the central nervous system, is found in the systemic circulation of humans at a concentration between 0.5 and 3 μmol/L. However, the potential source of circulating GABA and its significance on the vascular system remains unknown. We hypothesized that endothelial cells (ECs) may synthesize and release GABA to modulate some functions in the EC and after its release into the circulation.

OBJECTIVE

To assess whether GABA is synthesized and released by the EC and its potential functions.

METHODS AND RESULTS

Utilizing the human umbilical vein ECs and aortic ECs, we demonstrated for the first time that ECs synthesize and release GABA from [1-(14)C]glutamate. Localization of GABA and the presence of the GABA-synthesizing enzyme, glutamic acid decarboxylase in EC were confirmed by immunostaining and immunoblot analysis, respectively. The presence of GABA was further confirmed by immunohistochemistry in the EC lining the human coronary vessel. EC-derived GABA regulated the key mechanisms of ATP synthesis, fatty acid, and pyruvate oxidation in EC. GABA protected EC by inhibiting the reactive oxygen species generation and prevented monocyte adhesion by attenuating vascular cell adhesion molecule -1 and monocyte chemoattractant protein-1 expressions. GABA had no relaxing effect on rat aortic rings. GABA exhibited a dose-dependent fall in blood pressure. However, the fall in BP was abolished after pretreatment with pentolinium.

CONCLUSIONS

Our findings indicate novel potential functions of endothelium-derived GABA.

Translational control of PML contributes to TNFα-induced apoptosis of MCF7 breast cancer cells and decreased angiogenesis in HUVECs

The tumor suppressor protein promyelocytic leukemia (PML) is a key regulator of inflammatory responses and tumorigenesis and functions through the assembly of subnuclear structures known as PML nuclear bodies (NBs). The inflammation-related cytokine tumor necrosis factor-α (TNFα) is known to induce PML protein accumulation and PML NB formation that mediate TNFα-induced cell death in cancer cells and inhibition of migration and capillary tube formation in endothelial cells (ECs). In this study, we uncover a novel mechanism of PML gene regulation in which the p38 MAPK and its downstream kinase MAP kinase-activated protein kinase 1 (MNK1) mediate TNFα-induced PML protein accumulation and PML NB formation. The mechanism includes the presence of an internal ribosome entry site (IRES) found within the well-conserved 100 nucleotides upstream of the PML initiation codon. The activity of the PML IRES is induced by TNFα in a manner that involves MNK1 activation. It is proposed that the p38–MNK1–PML network regulates TNFα-induced apoptosis in breast cancer cells and TNFα-mediated inhibition of migration and capillary tube formation in ECs.

Knockdown of phosphotyrosyl phosphatase activator induces apoptosis via mitochondrial pathway and the attenuation by simultaneous tau hyperphosphorylation

Phosphotyrosyl phosphatase activator (PTPA) is decreased in the brains of Alzheimer's disease (AD) and the AD transgenic mouse models. Here, we investigated whether down-regulation of PTPA affects cell viability and the underlying mechanisms. We found that PTPA was located in the integral membrane of mitochondria, and knockdown of PTPA induced cell apoptosis in HEK293 and N2a cell lines. PTPA knockdown decreased mitochondrial membrane potential and induced Bax translocation into the mitochondria with a simultaneous release of Cyt C, activation of caspase-3, cleavage of poly (DNA ribose) polymerase (PARP), and decrease in Bcl-xl and Bcl-2 protein levels. Over-expression of Protein phosphatase 2A (PP2A) catalytic subunit (PP2AC ) did not rescue the apoptosis induced by PTPA knockdown, and PTPA knockdown did not affect the level of and their phosphorylation of mitogen-activated protein kinases (MAPKs), indicating that PP2A and MAPKs were not involved in the apoptosis induced by PTPA knockdown. In the cells with over-expression of tau, PTPA knockdown induced PP2A inhibition and tau hyperphosphorylation but did not cause significant cell death. These data suggest that PTPA deficit causes apoptotic cell death through mitochondrial pathway and simultaneous tau hyperphosphorylation attenuates the PTPA-induced cell death. Phosphotyrosyl phosphatase activator (PTPA) is decreased in the brains of Alzheimer's disease (AD) and AD transgenic mouse models. Here, we investigated whether down-regulation of PTPA affects cell viability. We found that PTPA located in the integral membrane of mitochondria, and knockdown of PTPA induced cell apoptosis in HEK293 and N2a cell lines by decreasing mitochondrial membrane potential, which leads to translocation of Bax and a simultaneous release of Cyt C. In the cells with tau over-expression, PTPA knockdown inactivated PP2A to phosphorylate tau to avoid cell apoptosis which induced by PTPA knockdown.

Gestational diabetes, preeclampsia and cytokine release: similarities and differences in endothelial cell function

Gestational diabetes, pre-eclampsia as well as intra-uterine infection during pregnancy affects the function of the endothelium both in the mother and the fetus leading to endothelial dysfunction. Gestational diabetes is also associated with an increased incidence of pre-eclampsia and it is likely that both the hyperglycemia as well as the release of cytokines especially TNFα during hyperglycemia may play an important role in the pathogenesis of endothelial dysfunction leading to preeclampsia. Similarly, some but not all studies have suggested that infection of the mother under certain circumstances can also lead to preeclampsia as women with either a bacterial or viral infection were at a higher risk of developing preeclampsia, compared to women without infection and infection also leads to a release in TNFα. Endothelial cells exposed to either high glucose or TNFα leads to an increase in the production of H2O2 and to a decrease in endothelial cell proliferation. The cellular and molecular mechanisms involved in this phenomenon are discussed.Gestational diabetes, pre-eclampsia as well as intra-uterine infection during pregnancy has profound effects on the fetus and long term effects on the neonate. All three conditions affect the function of the endothelium both in the mother and the fetus leading to endothelial dysfunction. Gestational diabetes is also associated with an increased incidence of pre-eclampsia and it is likely that both the hyperglycemia as well as the release of cytokines especially TNFα during hyperglycemia may play an important role in the pathogenesis of endothelial dysfunction leading to preeclampsia. It has also been suggested although not universally accepted that under certain circumstances maternal infection may also predispose to pre-eclampsia. Pre-eclampsia is also associated with the release of TNFα and endothelial dysfunction. However, the cellular and molecular mechanism(s) leading to the endothelial dysfunction by either hyperglycemia or by the cytokine TNFα appear to be different. In this chapter, we explore some of the similarities and differences leading to endothelial dysfunction by both hyperglycemia and by the inflammatory cytokine TNFα and the cellular and molecular mechanism(s) involved.