BBA - Molecular Cell Research (v.1773, #10)

The morphological features observed by Kerr, Wylie and Currie in 1972 define apoptosis, necrosis and autophagy. An appreciable number of alternative systems do not fall neatly under these categories, warranting a review of alternative proteolytic machinery and its contribution to cell death. This review aims to pinpoint key molecular features of serine protease-mediated pro-apoptotic signalling. The profile created will contribute to a standard set of biochemical criteria that can serve in differentiating within cell death subtypes.
Keywords: Apoptosis; Serine protease; Caspase; Mitochondria;

The mechanism for the transcriptional up-regulation of p27Kip1 due to the formation of the cell–cell contact was investigated in vascular endothelial cells. The induction of the cell–cell contact by adding an extra number of endothelial cells activated Rac1, up-regulated p27Kip1 mRNA and protein, and also facilitated the cell cycle arrest. Transduction of the Rac1 inhibitor protein using the cell-penetrating peptide or treatment with a Rac1 inhibitor NSC23766 inhibited the p27Kip1 up-regulation and delayed the cell cycle arrest. Rac1 was therefore suggested to mediate the contact-induced transcriptional up-regulation of p27Kip1. The role of Rac1 in the regulation of the p27Kip1 promoter activity was next examined with a luciferase reporter assay. The promoter activity was increased by inducing the cell–cell contact, which was significantly inhibited by the Rac1 inhibitory protein and NSC23766. The evaluation of various truncated promoter regions determined region − 620 to − 573 nucleotides from the initiation codon to be responsible for the contact-induced, Rac1-dependent activation of the p27Kip1 promoter. The present study thus demonstrated for the first time that the activation of Rac1 due to the cell–cell contact plays a critical role in the transcriptional up-regulation of p27Kip1 in vascular endothelial cells.
Keywords: Cell cycle; Promoter; Cyclin-dependent kinase inhibitor; Expression; Protein transduction;

Three autocrine feedback loops determine HIF1α expression in chronic hypoxia by Amina A. Qutub; Aleksander S. Popel (1511-1525).
Hypoxia occurs in cancer, prolonged exercise, and long-term ischemia with durations of several hours or more, and the hypoxia-inducible factor 1 (HIF1) pathway response to these conditions differs from responses to transient hypoxia. We used computational modeling, validated by experiments, to gain a quantitative, temporal understanding of the mechanisms driving HIF1 response. To test the hypothesis that HIF1α protein levels during chronic hypoxia are tightly regulated by a series of molecular feedbacks, we took into account protein synthesis and product inhibition, and analyzed HIF1 system changes in response to hypoxic exposures beyond 3 to 4 h. We show how three autocrine feedback loops together regulate HIF1α hydroxylation in different microenvironments. Results demonstrate that prolyl hydroxylase, succinate and HIF1α feedback determine intracellular HIF1α levels over the course of hours to days. The model provides quantitative insight critical for characterizing molecular mechanisms underlying a cell's response to long-term hypoxia.
Keywords: Angiogenesis; Cancer; Chronic hypoxia; Computational modeling; Ischemia; Prolonged exercise;

Drosophila Dynein light chain (DDLC1) binds to gurken mRNA and is required for its localization by Inna Rom; Alice Faicevici; Orna Almog; F. Shira Neuman-Silberberg (1526-1533).
During oogenesis in Drosophila, mRNAs encoding determinants required for the polarization of egg and embryo become localized in the oocyte in a spatially restricted manner. The TGF-alpha like signaling molecule Gurken has a central role in the polarization of both body axes and the corresponding mRNA displays a unique localization pattern, accumulating initially at the posterior and later at the anterior-dorsal of the oocyte. Correct localization of gurken RNA requires a number of cis-acting sequence elements, a complex of trans-acting proteins, of which only several have been identified, and the motor proteins Dynein and Kinesin, traveling along polarized microtubules. Here we report that the cytoplasmic Dynein-light-chain (DDLC1) which is the cargo-binding subunit of the Dynein motor protein, directly bound with high specificity and affinity to a 230-nucleotide region within the 3′UTR of gurken, making it the first Drosophila mRNA-cargo to directly bind to the DLC. Although DDLC1 lacks known RNA-binding motifs, comparison to double-stranded RNA-binding proteins suggested structural resemblance. Phenotypic analysis of ddlc1 mutants supports a role for DDLC1 in gurken RNA localization and anchoring as well as in correct positioning of the oocyte nucleus.
Keywords: gurken; mRNA localization; ddlc1; Drosophila;

Chromatin dynamics during DSB repair by Martin Falk; Emilie Lukasova; Barbora Gabrielova; Vladan Ondrej; Stanislav Kozubek (1534-1545).
We show that double strand breaks (DSBs) induced in chromatin of low as well as high density by exposure of human cells to γ-rays are repaired in low-density chromatin. Extensive chromatin decondensation manifested in the vicinity of DSBs by decreased intensity of chromatin labelling, increased H4K5 acetylation, and decreased H3K9 dimethylation was observed already 15 min after irradiation. Only slight movement of sporadic DSB loci for short distances was noticed in living cells associated with chromatin decondensation around DSBs.This frequently resulted in their protrusion into the low-density chromatin domains. In these regions, the clustering (contact or fusion) of DSB foci was seen in vivo, and in situ after cell fixation. The majority of these clustered foci were repaired within 240 min, but some of them persisted in the nucleus for several days after irradiation, indicating damage that is not easily repaired. We propose that the repair of DSB in clustered foci might lead to misjoining of ends and, consequently, to exchange aberrations. On the other hand, the foci that persist for several days without being repaired could lead instead to cell death.
Keywords: Chromatin structure; DNA damage; Double-strand breaks (DSB); DNA repair; Exchange aberration; Genomic instability;

Caspase-inhibited cells induced to die may exhibit the traits of either apoptosis or necrosis or both, simultaneously. However, mechanisms regulating the commitment to these distinct forms of cell death are barely identified. We found that staurosporine induced both apoptotic and necrotic traits in U937 cells exposed to the caspase inhibitor benzyloxycarbonyl-Val-Ala-dl-Asp(OMe)-fluoromethylketone. Morphology and flow cytometry revealed that individual cells exhibited either apoptotic or necrotic traits, but not the mixed phenotype. Inhibition of cathepsin activity by benzyloxycarbonyl-Phe-Ala-fluoromethylketone rendered caspase-compromised cells resistant to staurosporine-induced apoptosis, but switched the cell death form to necrosis. Inhibition of heat shock protein 90 kDa (Hsp90) chaperon activity by geldanamycin conferred resistance to necrosis in caspase-compromised cells but switched the cell death form to apoptosis. Combination of benzyloxycarbonyl-Phe-Ala-fluoromethylketone and geldanamycin halted the onset of both forms of cell death by saving mitochondrial trans-membrane potential and preventing acidic volume (lysosomes) loss. These effects of benzyloxycarbonyl-Phe-Ala-fluoromethylketone and/or geldanamycin on cell death were restricted to caspase-inhibited cells exposed to staurosporine but influenced neither only the staurosporine-provoked apoptosis nor hydrogen peroxide (H2O2)-generated necrosis. Our results demonstrate that the staurosporine-induced death pathway bifurcates in caspase-compromised cells and commitment to apoptotic or necrotic phenotypes depends on cathepsin protease or Hsp90 chaperon activities.
Keywords: Apoptosis; Caspase-independent; Cathepsin; Heat shock protein 90 kDa; Necrosis; Staurosporine;

Vascularized tumors are exposed to intermittent hypoxia, that is, hypoxia followed by periods of reoxygenation. Abnormal structure and dysfunction of tumor blood vessels are responsible for these conditions. These repeated short periods of hypoxia concern tumor cells as well as endothelial cells. However, the effects of intermittent hypoxia are poorly understood. The aim of this study was to investigate the effects of intermittent hypoxia on endothelial cells and particularly on HIF-1α, a central actor in adaptive response to hypoxia. For that, endothelial cells were exposed to four repeated cycles of 1-h hypoxia followed by 30 min of reoxygenation. We showed that repeated cycles of hypoxia/reoxygenation induced a modification in HIF-lα phosphorylation pattern: a progressive increase in HIF-1α phosphorylated form was observed during the hypoxic periods. Activation of p42/p44, Akt and PKA was observed in parallel. PKA was shown to be involved in the phosphorylation of HIF-lα under intermittent hypoxia, while p42/p44 and Akt were not. As HIF-1 activity is often associated with enhanced cell survival, a better knowledge of the effects of intermittent hypoxia on endothelial cells and the highlight of particular mechanisms induced by intermittent hypoxia are essential to understand the behavior of endothelial cells during neo-angiogenesis.
Keywords: Intermittent hypoxia; HIF-1α; PKA; Neo-angiogenesis; Tumor;

Histone deacetylase 4 is required for TGFβ1-induced myofibroblastic differentiation by Wendy Glenisson; Vincent Castronovo; David Waltregny (1572-1582).
Transforming Growth Factor β1 (TGFβ1) is a crucial cytokine triggering myofibroblastic (MF) differentiation, a process involved in tissue healing as well as in pathologic conditions such as fibrosis and cancer. Together with cell shape modifications, TGFβ1-mediated differentiation of fibroblasts into myofibroblasts is characteristically associated with the neo-expression of smooth muscle α-actin (α-SMA), a cytoskeletal protein that enhances their contractile activity. Several cellular differentiation programs have been linked to epigenetic regulation of gene expression, including gene methylation and histone acetylation. Herein, we sought to investigate the role of histone deacetylases (HDAC) in TGFβ1-induced MF differentiation. We found that TSA, a global inhibitor of class I and class II HDACs, prevented α-SMA transcript and protein expression and morphological changes mediated by TGFβ1 in cultured human skin fibroblasts. In order to identify the HDAC(s) participating in MF differentiation, the impact of specific HDAC silencing (HDAC1 through HDAC8) using RNA interference was investigated in fibroblasts exposed to TGFβ1. Among the eight HDACs tested, silencing of HDAC4, HDAC6, and HDAC8 expression impaired TGFβ1-induced α-SMA expression. HDAC4 silencing most efficiently abrogated α-SMA expression and also prevented TGFß1-mediated morphological changes. Forced down-regulation of HDAC4 stimulated the expression of 5′-TG-3′-Interacting Factor (TGIF) and TGIF2 homeoproteins, two known endogenous repressors of the TGFβ signaling pathway, but not of the inhibitory Smad7. Collectively, these data suggest that HDAC4 is an essential epigenetic regulator of MF differentiation and unveil HDAC4 as a potential target for treating MF-related disorders.
Keywords: Histone deacetylase; TGFβ1; TSA; Myofibroblast; α-SMA; TGIF; Smad7;

Cellular binding, motion, and internalization of synthetic gene delivery polymers by Gaelen T. Hess; William H. Humphries; Nicole C. Fay; Christine K. Payne (1583-1588).
Using fluorescence microscopy we have tracked the cellular binding, surface motion, and internalization of polyarginine and polyethylenimine, cationic ligands used for gene and protein delivery. Each ligand was complexed with a quantum dot to provide a photostable probe. Transfection with exogenous DNA was used to relate the observed motion to gene delivery. Cell surface motion was independent of sulfated proteoglycans, but dependent on cholesterol. Cellular internalization required sulfated proteoglycans and cholesterol. These observations suggest that sulfated proteoglycans act as cellular receptors for the cationic ligands, rather than only passive binding sites. Understanding the interaction of polyarginine and polyethylenimine with the plasma membrane may assist in designing more efficient gene delivery systems.
Keywords: Heparan sulfate proteoglycan; Cholesterol; Gene delivery; Fluorescence microscopy; Single particle tracking; Quantum dot;

Regulation of prostaglandin E synthases: Effects of siRNA-mediated inhibition of microsomal prostaglandin E synthase-1 by Tove Båge; Thomas Modéer; Tomomi Kawakami; Hernan Concha Quezada; Tülay Yucel-Lindberg (1589-1598).
Prostaglandin E2 (PGE2) is a key mediator involved in several inflammatory conditions. In this study, we investigated the expression and regulation of the terminal PGE2 synthesizing enzyme prostaglandin E synthases (mPGES-1, mPGES-2 and cPGES) in gingival fibroblasts stimulated with pro-inflammatory cytokines. We used siRNA knockdown of mPGES-1 to elucidate the impact of mPGES-1 inhibition on mPGES-2 and cPGES expression, as well as on PGE2 production. The cytokines TNFα and IL-1β increased protein expression and activity of mPGES-1, accompanied by increased COX-2 expression and PGE2 production. The isoenzymes mPGES-2 and cPGES, constitutively expressed at mRNA and protein levels, were unaffected by the pro-inflammatory cytokines. We show for the first time that treatment with mPGES-1 siRNA down-regulated the cytokine-induced mPGES-1 protein expression and activity. Interestingly, mPGES-1 siRNA did not affect the cytokine-stimulated PGE2 production, whereas PGF levels were enhanced. Neither mPGES-2 nor cPGES expression was affected by siRNA silencing of mPGES-1. Dexamethasone and MK-886 both inhibited the cytokine-induced mPGES-1 expression while mPGES-2 and cPGES expression remained unaffected. In conclusion, mPGES-1 siRNA down-regulates mPGES-1 expression, and neither mPGES-2 nor cPGES substituted for mPGES-1 in a knockdown setting in gingival fibroblasts. Moreover, mPGES-1 siRNA did not affect PGE2 levels, whereas PGF increased, suggesting a compensatory pathway of PGE2 synthesis when mPGES-1 is knocked down.
Keywords: Prostaglandin E synthase; siRNA; Prostaglandin E2; COX-2; MK-886; Gingival fibroblast;