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Gerhard Fritz

Publications and source records attributed to Gerhard Fritz.

27 records · Page 2Linked to original sources
PubMed2003

Lovastatin inhibits Rho-regulated expression of E-selectin by TNFalpha and attenuates tumor cell adhesion.

E-selectin mediated cell-cell adhesion plays an important role in inflammatory processes and extravasation of tumor cells. Tumor necrosis factor-alpha (TNF-alpha) induces E-selectin gene and protein expression in primary human endothelial cells (HUVEC) and in an endothelial cell line (EA.hy-926). As shown by ELISA and FACS analyses, HMG-CoA reductase inhibitors (e.g., lovastatin) impair the TNF-alpha stimulated increase in E-selectin protein expression. Similar results were obtained for E-selectin mRNA expression and promoter activity, indicating that the effect of lovastatin is based on inhibition of gene expression. The effective inhibitory concentration of lovastatin was in a physiologically relevant range (IC50<0.1 microM). Lovastatin-mediated block of TNF-alpha induced E-selectin expression is due to inhibition of protein geranylgeranylation rather than farnesylation. Down-regulation of Rho signaling by coexpression of dominant-negative Rho mutants (i.e RhoA, RhoB and Rac) impaired TNF-alpha driven E-selectin gene expression, indicating Rho signaling to be essential for transcriptional activation of the E-selectin gene. Inhibition of E-selectin expression by lovastatin gives rise to a significant reduction in TNF-alpha stimulated adhesion of colon carcinoma cells to HUVEC. Furthermore, low concentration of lovastatin (i.e., < or =1 microM) attenuated TNF-alpha induced tumor cell invasion in vitro. The data support the view that statins might be clinically useful in protection against E-selectin mediated metastasis.

Antineoplastic Agents↗
PubMed2003

APE/Ref-1 and the mammalian response to genotoxic stress.

Human apurinic/apyrimidinic endonuclease/redox factor-1 (hAPE/Ref-1) is a multifunctional protein involved in the repair of DNA damaged by oxidative or alkylating compounds as well as in the regulation of stress inducible transcription factors such as AP-1, NF-kappaB, HIF-1 and p53. With respect to transcriptional regulation, both redox dependent and independent mechanisms have been described. APE/Ref-1 also acts as a transcriptional repressor. Recent data indicate that APE/Ref-1 negatively regulates the activity of the Ras-related GTPase Rac1. How these different physiological activities of APE/Ref-1 are coordinated is poorly understood. So far, convincing evidence is available that the expression of the APE/Ref-1 gene is inducible by oxidative stress and that overexpressed APE/Ref-1 protein protects cells against the genotoxic and cell killing effects of reactive oxygen species (ROS), whereas down-regulation sensitizes cells. Therefore, APE/Ref-1 can be considered to be part of an adaptive cellular response mechanism to oxidative genotoxic stress. The physiological relevance of increase of either the repair or redox activity of APE/Ref-1 for this adaptive response is unclear. Data will be shown that transfection of the truncated protein exhibiting either one of the activities provoked increase of resistance. Since APE/Ref-1 expression level and intracellular localization is variable in different types of tumors and frequently found to be different in non-malignant compared to the corresponding malignant human tissue, the protein is thought to be a diagnostic and prognostic tumor marker. Because of its involvement in DNA repair and apoptosis-related signaling mechanisms, APE/Ref-1 is also being discussed as a novel target for tumor-therapeutic approaches.

Amino Acid Sequence↗
PubMed2003

Ultraviolet light-induced apoptotic death is impaired by the HMG-CoA reductase inhibitor lovastatin.

HMG-CoA reductase inhibitors (i.e., statins) attenuate C-terminal isoprenylation of Rho GTPases, thereby inhibiting UV-C-induced activation of c-Jun-N-terminal kinases/stress-activated protein kinases (JNKs/SAPKs). Inhibition of UV-C-triggered JNK/SAPK activation by lovastatin is due to inhibition of Rac-SEK1/MKK4-mediated phosphorylation of JNKs/SAPKs at Thr183/Tyr185. UV-C-stimulated phosphorylation of p38 kinase (Thr180/Tyr182) is also impaired by lovastatin. Cell killing provoked by UV-C irradiation was significantly inhibited by lovastatin. This was paralleled by a reduced frequency of chromosomal aberrations, accelerated recovery from UV-C-induced transient replication blockage, inhibition of Chk1 kinase activation and impaired cyclinB1 expression. Furthermore, UV-C-induced activation of caspases and apoptotic death was largely reduced by lovastatin. Inhibition of JNK/SAPK by transient overexpression of dominant-negative JNK1/SAPK1 also conferred resistance to UV-C light and attenuated activation of caspase 3. Based on the data, we suggest that lovastatin-provoked resistance to UV-C light is due to the inhibition of UV-C-inducible Rac-SEK1/MKK4-JNK/SAPK-dependent signal mechanisms regulating cell cycle progression and activation of caspases and apoptotic death.

Animals↗
PubMed2003

Resistance of p53 knockout cells to doxorubicin is related to reduced formation of DNA strand breaks rather than impaired apoptotic signaling.

The anthracycline doxorubicin (adriamycin) is an important chemotherapeutic agent used in the treatment of solid epithelial and mesenchymal tumors as well as leukemias. A variety of mechanisms has been proposed to be involved in doxorubicin-induced cytotoxicity such as DNA intercalation, oxidative stress, DNA strand breakage by inhibition of topoisomerase II, activation of death receptors, and altered p53 expression. Concerning doxorubicin resistance and p53 status data reported are contradictory. Here, we show that mouse fibroblasts deficient in p53 (p53(-/-)) are more resistant to doxorubicin than p53 wild-type (p53 wt) cells. This is in contrast to other genotoxic agents (UV-light, alkylating drugs) for which p53(-/-) fibroblasts proved to be more sensitive. Resistance of p53(-/-) cells to doxorubicin is related to reduced induction of apoptosis. This is not likely to be due to altered apoptotic signaling since the expression of Bax and Bcl-2 was unchanged and the induction of Fas/CD95/APO-1 receptor and caspase-8 was the same in p53(-/-) and p53 wt cells on treatment with doxorubicin. However, we observed a clearly lower level of doxorubicin-induced DNA strand breaks in p53(-/-) cells compared to the wt. P170 glycoprotein was equally expressed and the accumulation and elimination of the drug occurred with identical kinetics in both cell types. p53 deficient cells were cross-resistant to another topoisomerase II inhibitor etoposide, which also provoked increased DNA strand breakage in p53 wt cells. Based on the data we conclude that the p53 status significantly impacts the generation of DNA strand breaks because of drug-induced topoisomerase inhibition rather than death receptor signaling. Since human tumors are frequently mutated in p53 the findings bear clinical implications.

Animals↗
PubMed2003

Small angle neutron scattering study of sodium dodecyl sulfate micellar growth driven by addition of a hydrotropic salt.

The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.

Journal Article↗
PubMed2003

CD28-dependent Rac1 activation is the molecular target of azathioprine in primary human CD4+ T lymphocytes.

Azathioprine and its metabolite 6-mercaptopurine (6-MP) are immunosuppressive drugs that are used in organ transplantation and autoimmune and chronic inflammatory diseases such as Crohn disease. However, their molecular mechanism of action is unknown. In the present study, we have identified a unique and unexpected role for azathioprine and its metabolites in the control of T cell apoptosis by modulation of Rac1 activation upon CD28 costimulation. We found that azathioprine and its metabolites induced apoptosis of T cells from patients with Crohn disease and control patients. Apoptosis induction required costimulation with CD28 and was mediated by specific blockade of Rac1 activation through binding of azathioprine-generated 6-thioguanine triphosphate (6-Thio-GTP) to Rac1 instead of GTP. The activation of Rac1 target genes such as mitogen-activated protein kinase kinase (MEK), NF-kappaB, and bcl-x(L) was suppressed by azathioprine, leading to a mitochondrial pathway of apoptosis. Azathioprine thus converts a costimulatory signal into an apoptotic signal by modulating Rac1 activity. These findings explain the immunosuppressive effects of azathioprine and suggest that 6-Thio-GTP derivates may be useful as potent immunosuppressive agents in autoimmune diseases and organ transplantation.

Adult↗
PubMed2002

Proteasomal degradation of cytotoxic necrotizing factor 1-activated rac.

The cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli has been shown to activate members of the Rho family by deamidation of glutamine 63. This amino acid is essential for hydrolysis of GTP, and any substitution results in a constitutively active Rho. Activation of Rho induces the formation of stress fibers, filopodia, and membrane ruffles due to activation of RhoA, Cdc42, and Rac, respectively. Here we show that the level of endogenous Rac decreased in CNF1-treated HEK293 and HeLa cells. The amount of mRNA remained unaffected, leaving the possibility that Rac is subject to proteolytic degradation. Treatment of cells with lactacystin, an inhibitor of the 26S proteasome, protected Rac from degradation. We have previously shown that CNF1 activates the c-Jun N-terminal kinase (JNK) only transiently in HeLa cells (M. Lerm, J. Selzer, A. Hoffmeyer, U. R. Rapp, K. Aktories, and G. Schmidt, Infect. Immun. 67:496-503, 1998). Here we show that CNF1-induced JNK activation is stabilized in the presence of lactacystin. The data indicate that Rac is degraded by a proteasome-dependent pathway in CNF1-treated cells.

Acetylcysteine↗
PubMed2002

The HMG-CoA reductase inhibitor lovastatin protects cells from the antineoplastic drugs doxorubicin and etoposide.

Ras-homologous GTPases are involved in the regulation of genotoxic stress-induced gene expression and cell death. Since they need C-terminal isoprenylation for correct intracellular localization and function, we investigated whether depletion of cells from isopren precursor moieties using the HMG-CoA reductase inhibitor lovastatin affects cellular sensitivity to DNA damaging drugs. Here we show that lovastatin renders cells highly resistant to the tumor-therapeutic compound doxorubicin. Desensitization by lovastatin was reverted by co-treatment with GGPP indicating that inhibition of protein geranylgeranylation is involved in acquired doxorubicin resistance. Lovastatin does not influence cellular sensitivity to DNA damaging compounds such as cisplatin, methyl methanesulfonate and ionizing radiation. The frequency of apoptotic cell death induced by doxorubicin was not affected by lovastatin as shown by both annexin V and DNA fragmentation assay. However, lovastatin releases cells from doxorubicin induced G2 blockage. Furthermore, lovastatin protects cells from doxorubicin-induced DNA strand breakage without affecting drug uptake or the expression of multidrug resistance protein (mdr-1). Since lovastatin confers cross-resistance to the topoisomerase II specific inhibitor etoposide, we suggest desensitization by the statin to be related to topoisomerase II function. The finding that lovastatin renders cells resistant to doxorubicin and etoposide by reducing their genotoxic and cytotoxic effects might have clinical implications for cancer therapy.

Animals↗