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Max Gassmann

Publications and source records attributed to Max Gassmann.

At least 37 records · Page 2Linked to original sources
PubMed2004

HIF-1alpha expression follows microvascular loss in advanced murine adriamycin nephrosis.

Cellular hypoxia has been proposed as a major factor in the pathogenesis of chronic renal injury, yet to date there has been no direct evidence to support its importance. Therefore, we examined cortical hypoxia in an animal model of chronic renal injury (murine adriamycin nephrosis; AN) by assessing nuclear localization of the oxygen-dependent alpha-subunit of hypoxia-inducible factor-1 (HIF-1alpha) in animals 7, 14, and 28 days after adriamycin. Results were assessed in conjunction with quantitation of the cortical microvasculature (by CD34 immunostaining) and cortical expression of VEGF. Cortical apoptosis was also examined by terminal deoxynucleotidyl transferase dUTP nick-end labeling staining. A dramatic and significant increase in nuclear localization of HIF-1alpha was seen 28 days after adriamycin in the context of severe glomerular and tubulointerstitial damage. Areas of nuclear HIF-1alpha staining did not colocalize with areas of cellular apoptosis. AN was also associated with a significant attenuation of the peritubular capillaries that was significant at 14 and 28 days after adriamycin. Cortical VEGF expression fell in a stepwise manner from day 7 until day 28 after adriamycin. In conclusion, these data are consistent with a significant increase in cellular hypoxia occurring in the advanced stages of murine AN. Increased cortical hypoxia was preceded by significant reductions in both the number of peritubular capillaries (i.e., oxygen supply) and the angiogenic cytokine VEGF. Apart from providing the first direct evidence for cellular hypoxia in a model of chronic renal disease, these results suggest that a primary dysregulation of angiogenesis may be the cause of increased hypoxia in this model.

Animals↗
PubMed2004

Hypoxia enhances human B19 erythrovirus gene expression in primary erythroid cells.

Human B19 erythrovirus replicates in erythroid progenitors present in bone marrow and fetal tissues where partial oxygen tension is low. Here we show that infected human primary erythroid progenitor cells exposed to hypoxia (1% O2) in vitro increase viral capsid protein synthesis, virus replication, and virus production. Hypoxia-inducible factor-1 (HIF-1), the main transcription factor involved in the cellular response to reduced oxygenation, is shown to bind an HIF binding site (HBS) located in the distal part of the B19 promoter region, but the precise mechanism involved in the oxygen-sensitive upregulation of viral gene expression remains to be elucidated.

Base Sequence↗
PubMed2004

Constitutive overexpression of human erythropoietin protects the mouse retina against induced but not inherited retinal degeneration.

Elevation of erythropoietin (Epo) concentrations by hypoxic preconditioning or application of recombinant human Epo (huEpo) protects the mouse retina against light-induced degeneration by inhibiting photoreceptor cell apoptosis. Because photoreceptor apoptosis is also the common path to cell loss in retinal dystrophies such as retinitis pigmentosa (RP), we tested whether high levels of huEpo would reduce apoptotic cell death in two mouse models of human RP. We combined the two respective mutant mouse lines with a transgenic line (tg6) that constitutively overexpresses huEpo mainly in neural tissues. Transgenic expression of huEpo caused constitutively high levels of Epo in the retina and protected photoreceptors against light-induced degeneration; however, the presence of high levels of huEpo did not affect the course or the extent of retinal degeneration in a light-independent (rd1) and a light-accelerated (VPP) mouse model of RP. Similarly, repetitive intraperitoneal injections of recombinant huEpo did not protect the retina in the rd1 and the VPP mouse. Lack of neuroprotection by Epo in the two models of inherited retinal degeneration was not caused by adaptational downregulation of Epo receptor. Our results suggest that apoptotic mechanisms during acute, light-induced photoreceptor cell death differ from those in genetically based retinal degeneration. Therapeutic intervention with cell death in inherited retinal degeneration may therefore require different drugs and treatments.

Animals↗
PubMed2004

Uncontrolled expression of vascular endothelial growth factor and its receptors leads to insufficient skin angiogenesis in patients with systemic sclerosis.

Systemic sclerosis (SSc) skin lesions are characterized by disturbed vessel morphology with enlarged capillaries and an overall reduction in capillary density, suggesting a deregulated, insufficient angiogenic response. It has been postulated that this phenomenon is due to reduced expression of the potent angiogenic factor vascular endothelial growth factor (VEGF). In contrast to this hypothesis, we demonstrate that the expression of both VEGF and its receptors VEGFR-1 and VEGFR-2 is dramatically upregulated in skin specimens of SSc patients throughout different disease stages. Interestingly, upregulation of VEGF was not mediated by hypoxia-inducible transcription factor-1 (HIF-1) as indicated by only a weak expression of the oxygen-sensitive alpha-subunit of HIF-1 in the skin of SSc patients. This was unexpected on measuring low Po2 values in the SSc skin by using a polarographic oxygen microelectrode system. Considering our observation that PDGF and IL-1beta costimulated VEGF expression, we propose that chronic and uncontrolled VEGF upregulation that is mediated by an orchestrated expression of cytokines rather than VEGF downregulation is the cause of the disturbed vessel morphology in the skin of SSc patients. Consequently, for therapeutic approaches aiming to improve tissue perfusion in these patients, a controlled expression and timely termination of VEGF signaling appears to be crucial for success of proangiogenic therapies.

Cell Hypoxia↗
PubMed2004

Rescue of lethal c-KitW/W mice by erythropoietin.

Homozygous natural white-spotted (W) mutations in the gene encoding the receptor tyrosine kinase c-Kit are associated with hypoplastic bone marrow, severe macrocytic anemia, and lethality during early postnatal life. c-Kit(W/W) mice can be rescued by wild-type hematopoietic stem cells (HSCs), but it is not known whether the lethality of c-Kit(W/W) mice is the result of HSC failure or defects specific for erythropoiesis. Here we show that transgenic expression of erythropoietin (EPO) can overcome the lethality caused by the c-Kit(W/W) mutation. In W mutant mice rescued by EPO, termed WEPO, erythrocyte colony-forming units (CFU-Es) are rescued to normal frequencies. Hence, Epo receptor signals can partially bypass the strict requirement for c-Kit signaling in erythropoiesis in the absence of c-Kit in vivo. Using a series of W and rescue mouse strains, we define here the erythropoietic threshold permitting survival in vivo. The lethality of c-Kit(W/W) mice has precluded analysis of this crucial receptor-ligand pair in adult stem/progenitor cells. Our strategy to generate viable c-Kit(W/W) mice will be useful to analyze the role of this important receptor tyrosine kinase in adult life in vivo.

Aging↗
PubMed2004

Oxygenation of spontaneous canine tumors during fractionated radiation therapy.

BACKGROUND AND PURPOSE: Tumor oxygenation predicts treatment outcome, and reoxygenation is considered important in the efficacy of fractionated radiation therapy. Therefore, the purpose of this study was to document the changes of the oxygenation status in spontaneous canine tumors during fractionated radiation therapy using polarographic needle electrodes. MATERIAL AND METHODS: Tumor oxygen partial pressure (pO(2)) measurements were performed with the Eppendorf-pO(2)-Histograph. The measurements were done under general anesthesia, and probe tracks were guided with ultrasound. pO(2) was measured before radiation therapy in all dogs. In patients treated with curative intent, measurements were done sequentially up to eight times (total dose: 45-59.5 Gy). Oxygenation status of the palliative patient group was examined before each fraction of radiation therapy up to five times (total dose: 24-30 Gy). RESULTS: 15/26 tumors had a pretreatment median pO(2) < or = 10 mmHg. The pO(2) values appeared to be quite variable in individual tumors during fractionated radiation therapy. The pO(2) of initially hypoxic tumors (pretreatment median pO(2) < or = 10 mmHg) remained unchanged during fractionated radiotherapy, whereas in initially normoxic tumors the pO(2) decreased. CONCLUSION: Hypoxia is common in spontaneous canine tumors, as 57.7% of the recorded values were < or = 10 mmHg. The data of this study showed that initially hypoxic tumors remained hypoxic, whereas normoxic tumors became more hypoxic.

Animals↗
PubMed2004

High altitude training of dogs results in elevated erythropoietin and endothelin-1 serum levels.

Living at 2300-m altitude combined with intermittent training at 3500 m leads to cardiovascular alterations in dogs, including increase in systemic and pulmonary artery pressure. Despite moderate to marked hypoxemia at these altitudes, erythrocytosis does not develop. To study humoral mechanisms of acclimatisation to high altitude, erythropoietin (EPO), endothelin-1 (ET-1), big endothelin (Big-ET) and vascular endothelial growth factor (VEGF) were measured in dogs living at 2300 m and intermittently ascending to 3500 m, and compared to the values obtained in control dogs living at 700-900 m. While the median EPO and ET-1 level in dogs at 2300 m did not differ from the one measured at 700-900 m, exposure from 2300 to 3500 m resulted in significantly elevated EPO and ET-1 levels. Big-ET levels were significantly higher at 2300 and 3500 m compared to dogs at low altitude, but did not differ between 2300 and 3500 m. VEGF was significantly elevated in dogs at 2300 m compared to dogs at low altitude. The increases in EPO, VEGF, ET-1 and Big-ET are thought to reflect the effect of hypoxia on a cellular level in these dogs. Obviously, the mild elevation of EPO levels observed at 3500 m was not sufficient to cause erythrocytosis. Elevations of the vasoconstrictors Big-ET and ET-1 may play some, but not a central role in hypoxic vasoconstriction in these dogs. Finally, serum VEGF measurement may be a sensitive and useful test to assess hypoxic stress in dogs.

Acclimatization↗
PubMed2004

HIFs and tumors--causes and consequences.

For most organisms oxygen is essential fo life. When oxygen levels drop below those required to maintain the minimum physiological oxygen requirement of an organism or tissue it is termed hypoxia. To counter act possible deleterious effects of such a state, an immediate molecular response is initiated causing adaptation responses aimed at cell survival. This response is mediated by the hypoxia-inducible factor-1 (HIF-1), which is a heterodimer consisting of an alpha- and a beta-subunit. HIF-1 alpha protein is stabilized under hypoxic conditions and therefore confers selectivity to this response. Hypoxia is characteristic of tumors, mainly because of impaired blood supply resulting from abnormal growth. Over the past few years enormous progress has been made in the attempt to understand how the activation of the physiological response to hypoxia influences neoplastic growth. In this review some aspects of HIF-1 pathway activation in tumors and the consequences for pathophysiology and treatment of neoplasia are discussed.

Animals↗
PubMed2003

Constitutive upregulation of hypoxia-inducible factor-1alpha mRNA occurring in highly metastatic lung carcinoma cells leads to vascular endothelial growth factor overexpression upon hypoxic exposure.

Neoangiogenesis is crucial for tumor growth and metastasis and is regulated by various angiogenic factors including vascular endothelial growth factor (VEGF). However, little is known whether highly metastatic cells express higher level of VEGF in response to various stimuli, thereby increasing neoangiogenesis compared to low-metastatic cells. Here we report that hypoxia markedly induced the expression of VEGF mRNA in the cell lines with high-metastatic potential (A11 and D6 cells) compared to the cell lines with low-metastatic potential (P29 and P34 cells) established from Lewis lung carcinoma. A11 cells exhibited higher VEGF gene-promoter activity, produced a larger amount of VEGF and showed higher activity to induce neoangiogenesis than P29 cells. Although the degradation rate of VEGF mRNA under hypoxic conditions was similar in both cell lines, hypoxia-inducible factor-1alpha (HIF-1alpha) mRNA, but not HIF-1beta mRNA, was found to be constitutively upregulated in A11 cells compared to P29 cells. Accordingly, the level of HIF-1alpha protein in response to hypoxia was higher in A11 cells than in P29 cells. Upregulation of HIF-1alpha mRNA was also observed in D6 cells but not in P34 cells. Thus, the high-metastatic cells produced a larger amount of VEGF under hypoxic conditions through constitutive HIF-1alpha mRNA upregulation compared to the low-metastatic cells, thereby leading to extensive neoangiogenesis.

Animals↗
PubMed2003

Differential role of p300 and CBP acetyltransferase during myogenesis: p300 acts upstream of MyoD and Myf5.

Studies in tissue culture cells have implicated p300 and CBP acetyltransferases in myogenic regulatory factor (MRF) mediated transcription and terminal differentiation of skeletal muscle cells. However, in vivo data placing p300 and CBP on myogenic differentiation pathways are not yet available. In this report we provide genetic evidence that p300 but not CBP acetyltransferase (AT) activity is required for myogenesis in the mouse and in embryonic stem (ES) cells. A fraction of embryos carrying a single p300 AT- deficient allele exhibit impaired MRF expression, delayed terminal differentiation and a reduced muscle mass. In mouse embryos lacking p300 protein, Myf-5 induction is severely attenuated. Similarly, ES cells homozygous for a p300 AT or a p300 null mutation fail to activate Myf5 and MyoD transcription efficiently, while Pax3, acting genetically upstream of these MRFs, is expressed. In contrast, ES cells lacking CBP AT activity express MyoD and Myf5 and undergo myogenic differentiation. These data reveal a specific requirement for p300 and its AT activity in the induction of MRF gene expression and myogenic cell fate determination in vivo.

Acetyltransferases↗
PubMed2003

Transgenic mice overexpressing erythropoietin adapt to excessive erythrocytosis by regulating blood viscosity.

Severe elevation of red blood cell number is often associated with hypertension and thromboembolism resulting in severe cardiovascular complications. However, some individuals such as high altitude dwellers cope well with an increased hematocrit level. We analyzed adaptive mechanisms to excessive erythrocytosis in our transgenic (tg) mice that, due to hypoxia-independent erythropoietin (Epo) overexpression, reached hematocrit values of 0.8 to 0.9 without alteration of blood pressure, heart rate, or cardiac output. Extramedullar erythropoiesis occurred in the tg spleen, leading to splenomegaly. Upon splenectomy, hematocrit values in tg mice decreased from 0.89 to 0.62. Tg mice showed doubled reticulocyte counts and an increased mean corpuscular volume. In tg mice, plasma volume was not elevated whereas blood volume was up to 25% of the body weight compared with 8% in wild-type (wt) siblings. Although plasma viscosity did not differ between tg and wt mice, tg whole-blood viscosity increased to a lower degree (4-fold) than expected from corresponding hemoconcentrated wt blood (8-fold). This moderate increase in viscosity is explicable by the up to 3-fold higher elongation of tg erythrocytes at physiologic shear rates. Apart from the nitric oxide-mediated vasodilation we reported earlier, adaptation to high hematocrit levels in tg mice involves regulated elevation of blood viscosity by increasing erythrocyte flexibility.

Adaptation, Physiological↗
PubMed2003

Hemostasis and coagulation at a hematocrit level of 0.85: functional consequences of erythrocytosis.

We have generated a transgenic mouse line that reaches a hematocrit concentration of 0.85 due to constitutive overexpression of human erythropoietin in an oxygen-independent manner. Unexpectedly, this excessive erythrocytosis did not lead to thrombembolic complications in all investigated organs at any age. Thus, we investigated the mechanisms preventing thrombembolism in this mouse model. Blood analysis revealed an age-dependent elevation of reticulocyte numbers and a marked thrombocytopenia that matched the reduced megakaryocyte numbers in the bone marrow. However, platelet counts were not different from wild-type controls, when calculations were based on the distribution (eg, plasma) volume, thereby explaining why thrombopoietin levels did not increase in transgenic mice. Nevertheless, bleeding time was significantly increased in transgenic animals. A longitudinal investigation using computerized thromboelastography revealed that thrombus formation was reduced with increasing age from 1 to 8 months in transgenic animals. We observed that increasing erythrocyte concentrations inhibited profoundly and reversibly thrombus formation and prolonged the time of clot development, most likely due to mechanical interference of red blood cells with clot-forming platelets. Transgenic animals showed increased nitric oxide levels in the blood that could inhibit vasoconstriction and platelet activation. Finally, we observed that plasmatic coagulation activity in transgenic animals was significantly decreased. Taken together, our findings suggest that prevention of thrombembolic disease in these erythrocytotic transgenic mice was due to functional consequences inherent to increased erythrocyte concentrations and a reduction of plasmatic coagulation activity, the cause of which remains to be elucidated.

Age Factors↗
PubMed2003

Regulation of the multidrug resistance transporter P-glycoprotein in multicellular tumor spheroids by hypoxia-inducible factor (HIF-1) and reactive oxygen species.

Hypoxia in tumors is generally associated with chemoresistance and radioresistance. However, the correlation between the heterodimeric hypoxia-inducible factor-1 (HIF-1) and the multidrug resistance transporter P-glycoprotein (P-gp) has not been investigated. Herein, we demonstrate that with increasing size of DU-145 prostate multicellular tumor spheroids the pericellular oxygen pressure and the generation of reactive oxygen species decreased, whereas the alpha-subunit of HIF-1 (HIF-1alpha) and P-gp were up-regulated. Furthermore, P-gp was up-regulated under experimental physiological hypoxia and chemical hypoxia induced by either cobalt chloride or desferrioxamine. The pro-oxidants H2O2 and buthionine sulfoximine down-regulated HIF-1alpha and P-gp, whereas up-regulation was achieved with the radical scavengers dehydroascorbate, N-acetylcysteine, and vitamin E. The correlation of HIF-1alpha and P-gp expression was validated by the use of hepatoma tumor spheroids that were either wild type (Hepa1) or mutant (Hepa1C4) for aryl hydrocarbon receptor nuclear translocator (ARNT), i.e., HIF-1beta. Chemical hypoxia robustly increased HIF-1alpha as well as P-gp expression in Hepa1 tumor spheroids, whereas no changes were observed in Hepa1C4 spheroids. Hence, our data demonstrate that expression of P-gp in multicellular tumor spheroids is under the control of HIF-1.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2003

Non-erythroid functions of erythropoietin.

The oxygen-dependent, renal cytokine eythropoietin (Epo) is well known to increase red cell production. Binding of Epo to the Epo receptor (EpoR) represses apoptosis of erythroid progenitor cells, thereby allowing their final maturation. We and others showed that Epo and its receptor are expressed in many other tissues, including brain, spinal cord, retina and testis. The presence of a blood barrier suggests that Epo plays a local role in these organs. Indeed, therapeutically applied or hypoxically induced Epo has been shown to reduce the infarct volume in various stroke animal models, to prevent retinal degeneration, and to ameliorate spinal cord injury. In a study conducted by Ehrenreich and colleagues, stroke patients treated with Epo showed reduced infarct volume, fast neurological recovery and improved clinical outcome. In analogy to its function on erythroid progenitor cells, this neuroprotective effect of Epo might be explained by repression of programmed cell death. Apart from neuroprotection, there is an assumption that Epo present in breast milk has the potential to protect against mother-to-infant transmission of HIV. When using Epo at high doses for longer time periods; however, care has to be taken to control the resulting chronic polycythemia that most probably caused enlarged cerebral infarct volumes in a transgenic mouse model that due to Epo-overexpression reached hematocrit levels of about 0.8. Overall, these data strongly support the notion that Epo will soon find new applications in the clinic.

Animals↗
PubMed2003

Hypoxia and high altitude. The molecular response.

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

Adaptation, Physiological↗
PubMed2003

Defective brain development in mice lacking the Hif-1alpha gene in neural cells.

Hypoxia-inducible factor 1alpha (HIF-1alpha) is essential for vascular development during embryogenesis and pathogenesis. However, little is known about its role in brain development. To investigate the function of HIF-1alpha in the central nervous system, a conditional knockout mouse was made with the Cre/LoxP system with a nestin promoter-driven Cre. Neural cell-specific HIF-1alpha-deficient mice exhibit hydrocephalus accompanied by a reduction in neural cells and an impairment of spatial memory. Apoptosis of neural cells coincided with vascular regression in the telencephalon of mutant embryos, and these embryonic defects were successfully restored by in vivo gene delivery of HIF-1alpha to the embryos. These results showed that expression of HIF-1alpha in neural cells was essential for normal development of the brain and established a mouse model that would be useful for the evaluation of therapeutic strategies for ischemia, including hypoxia-mediated hydrocephalus.

Animals↗