Prior exposure to neurotrophins blocks inhibition of axonal regeneration by MAG and myelin via a cAMP-dependent mechanism.
MAG is a potent inhibitor of axonal regeneration. Here, inhibition by MAG, and myelin in general, is blocked if neurons are exposed to neurotrophins before encountering the inhibitor; priming cerebellar neurons with BDNF or GDNF, but not NGF, or priming DRG neurons with any of these neurotrophins blocks inhibition by MAG/myelin. Dibutyryl cAMP also overcomes inhibition by MAG/myelin, and cAMP is elevated by neurotrophins. A PKA inhibitor present during priming abrogates the block of inhibition. Finally, if neurons are exposed to MAG/myelin and neurotrophins simultaneously, but with the Gi protein inhibitor, inhibition is blocked. We suggest that priming neurons with particular neurotrophins elevates cAMP and activates PKA, which blocks subsequent inhibition of regeneration and that priming is required because MAG/myelin activates a Gi protein, which blocks increases in cAMP. This is important for encouraging axons to regrow in vivo. (+info)
The ras oncogene-mediated sensitization of human cells to topoisomerase II inhibitor-induced apoptosis.
BACKGROUND: Among the inhibitors of the enzyme topoisomerase II (an important target for chemotherapeutic drugs) tested in the National Cancer Institute's In Vitro Antineoplastic Drug Screen, NSC 284682 (3'-hydroxydaunorubicin) and NSC 659687 [9-hydroxy-5,6-dimethyl-1-(N-[2(dimethylamino)ethyl]carbamoyl)-6H-pyrido -(4,3-b)carbazole] were the only compounds that were more cytotoxic to tumor cells harboring an activated ras oncogene than to tumor cells bearing wild-type ras alleles. Expression of the multidrug resistance proteins P-glycoprotein and MRP (multidrug resistance-associated protein) facilitates tumor cell resistance to topoisomerase II inhibitors. We investigated whether tumor cells with activated ras oncogenes showed enhanced sensitivity to other topoisomerase II inhibitors in the absence of the multidrug-resistant phenotype. METHODS: We studied 20 topoisomerase II inhibitors and individual cell lines with or without activated ras oncogenes and with varying degrees of multidrug resistance. RESULTS: In the absence of multidrug resistance, human tumor cell lines with activated ras oncogenes were uniformly more sensitive to most topoisomerase II inhibitors than were cell lines containing wild-type ras alleles. The compounds NSC 284682 and NSC 659687 were especially effective irrespective of the multidrug resistant phenotype. The ras oncogene-mediated sensitization to topoisomerase II inhibitors was far more prominent with the non-DNA-intercalating epipodophyllotoxins than with the DNA-intercalating inhibitors. This difference in sensitization appears to be related to a difference in apoptotic sensitivity, since the level of DNA damage generated by etoposide (an epipodophyllotoxin derivative) in immortalized human kidney epithelial cells expressing an activated ras oncogene was similar to that in the parental cells, but apoptosis was enhanced only in the former cells. CONCLUSIONS: Activated ras oncogenes appear to enhance the sensitivity of human tumor cells to topoisomerase II inhibitors by potentiating an apoptotic response. Epipodophyllotoxin-derived topoisomerase II inhibitors should be more effective than the DNA-intercalating inhibitors against tumor cells with activated ras oncogenes. (+info)
A requirement for protein kinase C inhibition for calcium-triggered apoptosis in acute lymphoblastic leukemia cells.
We have evaluated the cytotoxicities of the combinations of calcium mobilizers and PKC inhibitors against human acute lymphoblastic leukemia (ALL) cells. Here we report that calcium mobilizers alone or PKC inhibitors alone do not induce apoptosis in human ALL cells. However, the combinations of calcium mobilizers with potent inhibitors of PKC cause significant apoptosis in ALL cells. Our results provide experimental evidence that PKC blocks Ca2+-triggered apoptosis in human ALL cells. Thus, PKC inhibitors can be used to enhance the antileukemic activity of chemical or biological agents that trigger an apoptotic calcium signal in ALL cells. The exquisite sensitivity of ALL cells to calcium-dependent apoptosis in the presence of PKC inhibitors could provide the basis for new treatment programs against ALL. (+info)
Stimulation of ultraviolet-induced apoptosis of human fibroblast UVr-1 cells by tyrosine kinase inhibitors.
Damnacanthal is an anthraquinone compound isolated from the root of Morinda citrifolia and was reported to have a potent inhibitory activity towards tyrosine kinases such as Lck, Src, Lyn and EGF receptor. In the present study, we have examined the effects of damnacanthal on ultraviolet ray-induced apoptosis in ultraviolet-resistant human UVr-1 cells. When the cells were treated with damnacanthal prior to ultraviolet irradiation, DNA fragmentation was more pronounced as compared to the case of ultraviolet irradiation alone. The other tyrosine kinase inhibitors, herbimycin A and genistein, also caused similar effects on ultraviolet-induced apoptosis but to a lesser extent. Serine/threonine kinase inhibitors, K252a, staurosporine and GF109203X, rather suppressed the ultraviolet-induced DNA cleavage. Immunoblot analysis showed that pretreatment with damnacanthal followed by ultraviolet irradiation increased the levels of phosphorylated extracellular signal-regulated kinases and stress-activated protein kinases. However, the other tyrosine kinase inhibitors did not increase the phosphorylation of extracellular signal-regulated kinases but stimulated phosphorylation of stress-activated protein kinases. Consequently, the ultraviolet-induced concurrent increase in both phosphorylated extracellular signal-regulated kinases and stress-activated protein kinases after pretreatment with damnacanthal might be characteristically related to the stimulatory effect of damnacanthal on ultraviolet-induced apoptosis. (+info)
Involvement of phosphodiesterase-cGMP-PKG pathway in intracellular Ca2+ oscillations in pituitary GH3 cells.
The present study investigates the potential role of the Ca2+-calmodulin-dependent type I phosphodiesterase (PDE)-cGMP-protein kinase G (PKG) pathway in spontaneous [Ca2+]i oscillations in GH3 cells using fura-2 single cell videoimaging. Vinpocetine (2.5-50 microM), a selective inhibitor of type I PDE, induced a concentration-dependent inhibition of spontaneous [Ca2+]i oscillations in these pituitary cells, and at the same time produced an increase of the intracellular cGMP content. The cell permeable cGMP analog N2,2'-O-dibutyryl-cGMP (dB-cGMP) (1 mM) caused a progressive reduction of the frequency and the amplitude of spontaneous [Ca2+]i oscillations when added to the medium. KT5823 (400 nM), a selective inhibitor of cGMP-dependent protein kinase (PKG), produced an increase of baseline [Ca2+]i and the disappearance of spontaneous [Ca2+]i oscillations. When KT5823 was added before vinpocetine, the PKG inhibitor counteracted the [Ca2+]i lowering effect of the cGMP catabolism inhibitor. Finally, the removal of extracellular Ca2+ or the blockade of L-type voltage-sensitive calcium channels (VSCC) by nimodipine produced a decrease of cytosolic cGMP levels. Collectively, the results of the present study suggest that spontaneous [Ca2+]i oscillations in GH3 cells may be regulated by the activity of type I PDE-cGMP-PKG pathway. (+info)
In vitro and in vivo characterization of intrinsic sympathomimetic activity in normal and heart failure rats.
Clinical studies conducted with carvedilol suggest that beta-adrenoceptor antagonism is an effective therapeutic approach to the treatment of heart failure. However, many beta-adrenoceptor antagonists are weak partial agonists and possess significant intrinsic sympathomimetic activity (ISA), which may be problematic in the treatment of heart failure. In the present study, the ISAs of bucindolol, xamoterol, bisoprolol, and carvedilol were evaluated and compared in normal rats [Sprague-Dawley (SD)], in rats with confirmed heart failure [spontaneously hypertensive heart failure (SHHF)], and in isolated neonatal rat cardiomyocytes. At equieffective beta1-adrenolytic doses, the administration of xamoterol and bucindolol produced a prolonged, equieffective, and dose-related increase in heart rate in both pithed SD rats (ED50 = 5 and 40 microgram/kg, respectively) and SHHF rats (ED50 = 6 and 30 microgram/kg, respectively). The maximum effect of both compounds in SHHF rats was approximately 50% of that observed in SD rats. In contrast, carvedilol and bisoprolol had no significant effect on resting heart rate in the pithed SD or SHHF rat. The maximum increase in heart rate elicited by xamoterol and bucindolol was inhibited by treatment with propranolol, carvedilol, and betaxolol (beta1-adrenoceptor antagonist) but not by ICI 118551 (beta2-adrenoceptor antagonist) in neonatal rat. When the beta-adrenoceptor-mediated cAMP response was examined in cardiomyocytes, an identical partial agonist/antagonist response profile was observed for all compounds, demonstrating a strong correlation with the in vivo results. In contrast, GTP-sensitive ligand binding and tissue adenylate cyclase activity were not sensitive methods for detecting beta-adrenoceptor partial agonist activity in the heart. In summary, xamoterol and bucindolol, but not carvedilol and bisoprolol, exhibited direct beta1-adrenoceptor-mediated ISA in normal and heart failure rats. (+info)
Tolerability and efficacy of carvedilol in patients with New York Heart Association class IV heart failure.
OBJECTIVES: The purpose of this study was to assess the tolerability and efficacy of carvedilol in patients with New York Heart Association (NYHA) functional class IV symptoms. BACKGROUND: Carvedilol, a nonselective beta-adrenergic blocking drug with alpha-adrenergic blocking and antioxidant properties, has been shown to improve left ventricular function and clinical outcome in patients with mild to moderate chronic heart failure. METHODS: We retrospectively analyzed the outcomes of 230 patients with heart failure treated with carvedilol who were stratified according to baseline functional class: 63 patients were NYHA class IV and 167 were NYHA class I, II or III. Carvedilol was commenced at 3.125 mg b.i.d. and titrated to 25 mg b.i.d. as tolerated. Patients with class IV symptoms were older (p = 0.03), had lower left ventricular fractional shortening (p < 0.001), had lower six-min walk distance (p < 0.001) and were receiving more heart failure medications at baseline compared with less symptomatic patients. RESULTS: Nonfatal adverse events while taking carvedilol occurred more frequently in class IV patients (43% vs. 24%, p < 0.0001), and more often resulted in permanent withdrawal of the drug (25% vs. 13%, p < 0.01). Thirty-seven (59%) patients who were NYHA class IV at baseline had improved by one or more functional class at 3 months, 8 (13%) were unchanged and 18 (29%) had deteriorated or died. Among the less symptomatic group, 62 (37%) patients had improved their NYHA status at 3 months, 73 (44%) were unchanged and 32 (19%) had deteriorated or died. The differences in symptomatic outcome at three months between the two groups were statistically significant (p = 0.001, chi-square analysis). Both groups demonstrated similar significant improvements in left ventricular dimensions and systolic function. CONCLUSIONS: Patients with chronic NYHA class IV heart failure are more likely to develop adverse events during initiation and dose titration when compared with less symptomatic patients but are more likely to show symptomatic improvement in the long term. We conclude that carvedilol is a useful adjunctive therapy for patients with NYHA class IV heart failure; however, they require close observation during initiation and titration of the drug. (+info)
Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.
It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion. (+info)