Andrea Knau

FOXO-dependent expression of the proapoptotic protein Bim: pivotal role for apoptosis signaling in endothelial progenitor cells

Endothelial progenitor cells (EPCs) contribute to postnatal neovascularization. Risk factors for coronary artery disease reduce the number of EPCs in humans. Since EPC apoptosis might be a potential mechanism to regulate the number of EPCs, we investigated the effects of oxidative stress and HMG‐CoA‐reductase inhibitors (statins) on EPC apoptosis. Atorvastatin, mevastatin, or VEGF prevented EPC apoptosis induced by H2O2. The antiapoptotic effect was reversed by inhibition of the PI3K/Akt pathway. Forkhead transcription factors (FOXO1, FOXO3a, FOXO4) exert proapoptotic effects and are phosphorylated and, thereby, inactivated by Akt. Therefore, we elucidated the involvement of forkhead transcription factors. Atorvastatin induced the phosphorylation of the predominant forkhead factor FOXO4 in EPCs. In addition, atorvastatin reduced the expression of the proapoptotic forkhead‐regulated protein Bim in a PI3K‐dependent manner. Consistently, overexpression of FOXO4 activated the Bim promoter as determined by reporter gene expression and stimulated the expression of Bim, resulting in an increased EPC apoptosis. Statins failed to prevent EPC apoptosis induced by overexpression of Bim or nonphosphorylatable FOXO4, suggesting that the protective effects of statins depend on this pathway. In summary, our results show that FOXO‐dependent expression of Bim plays a pivotal role for EPC apoptosis. Statins reduce oxidative stress‐induced EPC apoptosis, inactivate FOXO4, and down‐regulate Bim.

Characterization of Levels and Cellular Transfer of Circulating Lipoprotein-Bound MicroRNAs

Objective—MicroRNAs are important intracellular regulators of gene expression, but also circulate in the blood being protected by extracellular vesicles, proteins, or high-density lipoprotein (HDL). Here, we evaluate the regulation and potential function of HDL- and low-density lipoprotein–bound miRs isolated from healthy subjects and patients with coronary artery disease. Approach and Results—HDL-bound miRs with known effects in the cardiovascular system were analyzed in HDL isolated from healthy subjects (n=10), patients with stable coronary artery disease (n=10), and patients with an acute coronary syndrome (n=10). In HDL from healthy subjects, miR-223 was detected at concentrations >10 000 copies/µg HDL, and miR-126 and miR-92a at about 3000 copies/µg HDL. Concentrations of most miRs were substantially higher in HDL as compared with low-density lipoprotein. However, HDL-bound miR-223 contributed to only 8% of the total circulating miRs. The signatures of miRs varied only slightly in HDL derived from patients with coronary artery disease. We did not observe a significant uptake of HDL-bound miRs into endothelial cells, smooth muscle cells, or peripheral blood mononuclear cells. However, patient-derived HDL transiently reduced miR expression particularly when incubated with smooth muscle and peripheral blood mononuclear cells. Conclusions—Circulating miRs are detected in HDL and to a lesser extent in low-density lipoprotein, and the miR-signatures are only slightly altered in patients with coronary artery disease. Lipoprotein-bound miRs were not efficiently delivered to endothelial, smooth muscle, and peripheral blood mononuclear cells suggesting that the lipoprotein-associated pool of miRs is not regulating the function of the studied cells in vitro.

HDAC5 is a repressor of angiogenesis and determines the angiogenic gene expression pattern of endothelial cells

Class IIa histone deacetylases (HDACs) are signal-responsive regulators of gene expression involved in vascular homeostasis. To investigate the differential role of class IIa HDACs for the regulation of angiogenesis, we used siRNA to specifically suppress the individual HDAC isoenzymes. Silencing of HDAC5 exhibited a unique pro-angiogenic effect evidenced by increased endothelial cell migration, sprouting, and tube formation. Consistently, overexpression of HDAC5 decreased sprout formation, indicating that HDAC5 is a negative regulator of angiogenesis. The antiangiogenic activity of HDAC5 was independent of myocyte enhancer factor-2 binding and its deacetylase activity but required a nuclear localization indicating that HDAC5 might affect the transcriptional regulation of gene expression. To identify putative HDAC5 targets, we performed microarray expression analysis. Silencing of HDAC5 increased the expression of fibroblast growth factor 2 (FGF2) and angiogenic guidance factors, including Slit2. Antagonization of FGF2 or Slit2 reduced sprout induction in response to HDAC5 siRNA. Chromatin immunoprecipitation assays demonstrate that HDAC5 binds to the promoter of FGF2 and Slit2. In summary, HDAC5 represses angiogenic genes, such as FGF2 and Slit2, which causally contribute to capillary-like sprouting of endothelial cells. The derepression of angiogenic genes by HDAC5 inactivation may provide a useful therapeutic target for induction of angiogenesis.

Synergistic effects of chemotherapeutic drugs in lymphoma cells are associated with down-regulation of inhibitor of apoptosis proteins (IAPs), prostate-apoptosis-response-gene 4 (Par-4), death-associated protein (Daxx) and with enforced caspase activation

Cytotoxic drugs mediate apoptotic tumor cell death by influencing key regulator proteins of programmed cell death. In clinical practice cytotoxic drug combinations are desired to potentiate tumor cell kill and to minimize side effects. Nevertheless, the molecular mechanisms underlying synergistic and antagonistic effects on tumor cells are still poorly understood. In order to elucidate these molecular mechanisms we established models of synergistic and antagonistic drug combinations within the same lymphoma cell lines. By combination index method we demonstrated that bendamustine in combination with either doxorubicin or mitoxantrone caused antagonistic effects on disruption of mitochondrial membrane potential as well as on the rate of apoptosis. In contrast the combination of bendamustine with cladribine acted synergistically on these parameters. By using the IC(50) (dosages causing 50% rate of apoptosis) the synergistic effect of the combination of bendamustine and cladribine was associated with an enhanced mitochondrial release of cytochrome c and Smac/DIABLO, by down-regulation of x-linked inhibitor of apoptosis (XIAP), cIAP1, Par-4 and Daxx as well as by a significantly increased activation of caspases-3, -6, -7, -8 and -9. At the same rate of apoptosis (IC(50)), the antagonistic combinations did not increase the release of cytochrome c or Smac/DIABLO, nor down-regulate the expression of XIAP, cIAP1, Par-4 and Daxx, nor increase the activation of caspases. The role of down-regulation of IAPs and of enforced caspase activation for synergism in this model was supported by the observation, that broad spectrum inhibition of caspases re-established expression of XIAP. Our study is the first to outline the molecular alterations caused by synergistic and antagonistic drug combinations within the same lymphoma cell model. The above described mechanisms were already assessable at a point where the effects of synergistic or antagonistic combinations could not yet be discriminated quantitatively by the level of apoptosis rate of the lymphoma cells.

Inhibiting effects on the induction of cytotoxic T lymphocytes by dendritic cells pulsed with lysates from acute myeloid leukemia blasts

Dendritic cells (DCs) were established from 25 patients in complete remission of acute myeloid leukemia (AML). In patients during hematopoietic regeneration following chemotherapy the yield of DC was comparable to that of healthy donors. In patients, more than 2 months after chemotherapy, significantly less DC were generated. Comparison of the antigen-presenting capacity using tetanus toxoid of six AML patients and six healthy volunteers did not show significant differences. In six AML patients, lymphocytes stimulated with blast cell lysate pulsed DC were analyzed for cytotoxic activity against autologous blast cells. 8.4-35.6% of autologous blast cells were lysed by DC stimulated lymphocytes. In three of the six patients maximum lysis of target cells was achieved by unpulsed DC. Thus, it seems that in some patients blast cell lysates mediate inhibitory effects, which may explain to some extend immune escape mechanisms in AML.

In AML Cell Lines Ara-C Combined with Purine Analogues is Able to Exert Synergistic as Well as Antagonistic Effects on Proliferation, Apoptosis and Disruption of Mitochondrial Membrane Potential

The pyrimidine analogue Ara-C and the purine analogues fludarabine and cladribine (2-CdA) are essential compounds in the treatment of acute myeloid leukemia (AML). Inhibition of cell proliferation and induction of apoptosis are the major mechanisms of cytotoxic agents to cause tumor cell death. Therefore, we studied whether Ara-C in combination with the purine analogues exerts synergistic or antagonistic effects on cell proliferation, phosphatidylserine exposure and disruption of mitochondrial membrane potential (MMP) in the AML cell lines HL60 and HEL. Furthermore, effects of the combination of Ara-C with bendamustine, a new bifunctional agent with alkylating activity and a purine nucleus, was investigated. Assessment by combination index analysis showed that Ara-C combined with fludarabine or bendamustine exhibited additive to antagonistic effects on inhibition of cell proliferation, induction of apoptosis as well as on disruption of mitochondrial membrane potential, independent of a simultaneous or consecutive (purine analogues before Ara-C) incubation schedule. In contrast, the combination of Ara-C with 2-CdA exclusively yielded synergistic effects. While inducing IC50 levels of apoptosis neither the antagonistic nor the synergistic drug combinations caused a specific expression pattern of apoptosis-associated proteins such as the pro- or antiapoptotic Bcl-2 family members, executioner caspases, IAPs (inhibitor of apoptosis proteins), proapoptotic Par-4, PARP, or p53. In conclusion, we here demonstrate that the in vitro efficacy of drug combinations containing Ara-C and purine analogues depends on the purine analogue applied, whereas incubation schedules or escalating dosages do not contribute to the synergistic effects.

The lncRNA GATA6-AS epigenetically regulates endothelial gene expression via interaction with LOXL2

Impaired or excessive growth of endothelial cells contributes to several diseases. However, the functional involvement of regulatory long non-coding RNAs in these processes is not well defined. Here, we show that the long non-coding antisense transcript of GATA6 (GATA6-AS) interacts with the epigenetic regulator LOXL2 to regulate endothelial gene expression via changes in histone methylation. Using RNA deep sequencing, we find that GATA6-AS is upregulated in endothelial cells during hypoxia. Silencing of GATA6-AS diminishes TGF-β2-induced endothelial–mesenchymal transition in vitro and promotes formation of blood vessels in mice. We identify LOXL2, known to remove activating H3K4me3 chromatin marks, as a GATA6-AS-associated protein, and reveal a set of angiogenesis-related genes that are inversely regulated by LOXL2 and GATA6-AS silencing. As GATA6-AS silencing reduces H3K4me3 methylation of two of these genes, periostin and cyclooxygenase-2, we conclude that GATA6-AS acts as negative regulator of nuclear LOXL2 function.LncRNAs influence endothelial cell function via a number of mechanisms. Here the authors show that the lncRNA GATA6-AS regulates endothelial gene expression through interaction with the nuclear deaminase LOXL2, with functional consequences on endothelial-mesenchymal transition and angiogenesis.