Reduced stress defense in heme oxygenase 1-deficient cells

Stressed mammalian cells up-regulate heme oxygenase 1 (Hmox1; EC 1.14.99.3), which catabolizes heme to biliverdin, carbon monoxide, and free iron. To assess the potential role of Hmox1 in cellular antioxidant defense, we analyzed the responses of cells from mice lacking functional Hmox1 to oxidative challenges. Cultured Hmox1(-/-) embryonic fibroblasts demonstrated high oxygen free radical production when exposed to hemin, hydrogen peroxide, paraquat, or cadmium chloride, and they were hypersensitive to cytotoxicity caused by hemin and hydrogen peroxide. Furthermore, young adult Hmox1(-/-) mice were vulnerable to mortality and hepatic necrosis when challenged with endotoxin. Our in vitro and in vivo results provide genetic evidence that up-regulation of Hmox1 serves as an adaptive mechanism to protect cells from oxidative damage during stress.

Heme oxygenase 1 is required for mammalian iron reutilization

The majority of iron for essential mammalian biological activities such as erythropoiesis is thought to be reutilized from cellular hemoproteins. Here, we generated mice lacking functional heme oxygenase 1 (Hmox1; EC 1.14.99.3), which catabolizes heme to biliverdin, carbon monoxide, and free iron, to assess its participation in iron homeostasis. Hmox1-deficient adult mice developed an anemia associated with abnormally low serum iron levels, yet accumulated hepatic and renal iron that contributed to macromolecular oxidative damage, tissue injury, and chronic inflammation. Our results indicate that Hmox1 has an important recycling role by facilitating the release of iron from hepatic and renal cells, and describe a mouse model of human iron metabolic disorders.

Expression of heme oxygenase-1 can determine cardiac xenograft survival

The rejection of concordant xenografts, such as mouse-to-rat cardiac xenografts, is very similar to the delayed rejection of porcine-to-primate discordant xenografts. In concordant models, this type of rejection is prevented by brief complement inhibition by cobra venom factor (CVF) and sustained T-cell immunosuppression by cyclosporin A (CyA). Mouse hearts that survive indefinitely in rats treated with CVF plus CyA express the anti-inflammatory gene heme oxygenase-1 (HO-1) in their endothelial cells and smooth muscle cells. The anti-inflammatory properties of HO-1 are thought to rely on the ability of this enzyme to degrade heme and generate bilirubin, free iron and carbon monoxide. Bilirubin is a potent anti-oxidant, free iron upregulates the transcription of the cytoprotective gene, ferritin, and carbon monoxide is thought to be essential in regulating vascular relaxation in a manner similar to nitric oxide. We show here that the expression of the HO-1 gene is functionally associated with xenograft survival, and that rapid expression of HO-1 in cardiac xenografts can be essential to ensure long-term xenograft survival.

Haem oxygenase-1 prevents cell death by regulating cellular iron

Haem oxygenase-1 (HO1) is a heat-shock protein that is induced by stressful stimuli. Here we demonstrate a cytoprotective role for HO1: cell death produced by serum deprivation, staurosporine or etoposide is markedly accentuated in cells from mice with a targeted deletion of the HO1 gene, and greatly reduced in cells that overexpress HO1. Iron efflux from cells is augmented by HO1 transfection and reduced in HO1-deficient fibroblasts. Iron accumulation in HO1-deficient cells explains their death: iron chelators protect HO1-deficient fibroblasts from cell death. Thus, cytoprotection by HO1 is attributable to its augmentation of iron efflux, reflecting a role for HO1 in modulating intracellular iron levels and regulating cell viability.

Roles for Fgf signaling during zebrafish fin regeneration

Following amputation of a urodele limb or teleost fin, the formation of a blastema is a crucial step in facilitating subsequent regeneration. Using the zebrafish caudal fin regeneration model, we have examined the hypothesis that fibroblast growth factors (Fgfs) initiate blastema formation from fin mesenchyme. We find that fibroblast growth factor receptor 1 (fgfr1) is expressed in mesenchymal cells underlying the wound epidermis during blastema formation and in distal blastemal tissue during regenerative outgrowth. fgfr1 transcripts colocalize with those of msxb and msxc, putative markers for undifferentiated, proliferating cells. A zebrafish Fgf member, designated wfgf, is expressed in the regeneration epidermis during outgrowth. Furthermore, we show that a specific inhibitor of Fgfr1 applied immediately following fin amputation blocks blastema formation, without obvious effects on wound healing. This inhibitor blocks the proliferation of blastemal cells and the onset of msx gene transcription. Inhibition of Fgf signaling during ongoing fin regeneration prevents further outgrowth while downregulating the established expression of blastemal msx genes and epidermal sonic hedgehog. Our findings indicate that zebrafish fin blastema formation and regenerative outgrowth require Fgf signaling.

Heme oxygenase-1 gene ablation or expression modulates cisplatin-induced renal tubular apoptosis

Heme oxygenase-1 (HO-1) is a 32-kDa microsomal enzyme that catalyzes the conversion of heme to biliverdin, releasing iron and carbon monoxide. Induction of HO-1 occurs as a protective response in cells/tissues exposed to a wide variety of oxidant stimuli. The chemotherapeutic effects of cis-diamminedichloroplatinum(II) (cisplatin), a commonly used anticancer drug, are limited by significant nephrotoxicity, which is characterized by varying degrees of renal tubular apoptosis and necrosis. The purpose of this study was to evaluate the functional significance of HO-1 expression in cisplatin-induced renal injury. Our studies demonstrate that transgenic mice deficient in HO-1 (-/-), develop more severe renal failure and have significantly greater renal injury compared with wild-type (+/+) mice treated with cisplatin. In vitro studies in human renal proximal tubule cells demonstrate that hemin, an inducer of HO-1, significantly attenuated cisplatin-induced apoptosis and necrosis, whereas inhibition of HO-1 enzyme activity reversed the cytoprotective effect. Overexpression of HO-1 resulted in a significant reduction in cisplatin-induced cytotoxicity. These studies provide a basis for future studies using targeted gene expression of HO-1 as a therapeutic and preventive modality in high-risk settings of acute renal failure.

The indispensability of heme oxygenase-1 in protecting against acute heme protein-induced toxicity in vivo

Heme oxygenase (HO) is the rate limiting enzyme in the degradation of heme, and its isozyme, HO-1, may protect against tissue injury. One posited mechanism is the degradation of heme released from destabilized heme proteins. We demonstrate that HO-1 is a critical protectant against acute heme protein-induced toxicity in vivo. In the glycerol model of heme protein toxicity-one characterized by myolysis, hemolysis, and kidney damage-HO-1 is rapidly induced in the kidney of HO-1 +/+ mice as the latter sustain mild, reversible renal insufficiency without mortality. In stark contrast, after this insult, HO-1 -/- mice exhibit fulminant, irreversible renal failure and 100% mortality; HO-1 -/- mice do not express HO-1, and evince an eightfold increment in kidney heme content as compared to HO-1 +/+ mice. We also demonstrate directly the critical dependency on HO-1 in protecting against a specific heme protein, namely, hemoglobin: doses of hemoglobin which exert no nephrotoxicity or mortality in HO-1 +/+ mice, however, precipitate rapidly developing, acute renal failure and marked mortality in HO-1 -/- mice. We conclude that the induction of HO-1 is an indispensable response in protecting against acute heme protein toxicity in vivo.

Targeted gene deletion of heme oxygenase 2 reveals neural role for carbon monoxide

Neuronal nitric oxide synthase (nNOS) generates NO in neurons, and heme-oxygenase-2 (HO-2) synthesizes carbon monoxide (CO). We have evaluated the roles of NO and CO in intestinal neurotransmission using mice with targeted deletions of nNOS or HO-2. Immunohistochemical analysis demonstrated colocalization of nNOS and HO-2 in myenteric ganglia. Nonadrenergic noncholinergic relaxation and cyclic guanosine 3',5' monophosphate elevations evoked by electrical field stimulation were diminished markedly in both nNOSDelta/Delta and HO-2(Delta)/Delta mice. In wild-type mice, NOS inhibitors and HO inhibitors partially inhibited nonadrenergic noncholinergic relaxation. In nNOSDelta/Delta animals, NOS inhibitors selectively lost their efficacy, and HO inhibitors were inactive in HO-2(Delta)/Delta animals.

Oxygen toxicity and iron accumulation in the lungs of mice lacking heme oxygenase-2

Heme oxygenase (HO) activity leads to accumulation of the antioxidant bilirubin, and degradation of the prooxidant heme. Moderate overexpression of the inducible form, HO-1, is associated with protection against oxidative injury. However, the role of HO-2 in oxidative stress has not been explored. We evaluated survival, indices of oxidative injury, and lung and HO expression in HO-2 null mutant mice exposed to > 95% O2 compared with wild-type controls. Similar basal levels of major lung antioxidants were observed, except that the knockouts had a twofold increase in total glutathione content. Despite increased HO-1 expression from HO-1 induction, knockout animals were sensitized to hyperoxia-induced oxidative injury and mortality, and also had significantly increased markers of oxidative injury before hyperoxic exposure. Furthermore, during hyperoxia, lung hemoproteins and iron content were significantly increased without increased ferritin, suggesting accumulation of available redox-active iron. These results demonstrate that the absence of HO-2 is associated with induction of HO-1 and increased oxygen toxicity in vivo, apparently due to accumulation of lung iron. These results suggest that HO-2 functions to augment the turnover of lung iron during oxidative stress, and that this function does not appear to be compensated for by induction of HO-1 in the knockouts.

Hippocampal long-term potentiation is normal in heme oxygenase-2 mutant mice

We have generated mice deficient in HO-2, the major cerebral isoform of heme oxygenase, in order to assess the potential role of carbon monoxide as a retrograde messenger in hippocampal LTP. Cerebral HO catalytic activity was markedly reduced in the HO-2 mutant mice, yet no differences were found between wild types and mutants in gross neuroanatomical structure, in basal hippocampal synaptic transmission, or in the amount of potentiation produced by various LTP induction protocols. Furthermore, zinc protoporphyrin IX, an inhibitor of HO, had nearly identical inhibitory effects on LTP in wild-type and HO-2 mutant hippocampal slices. Our data indicate that carbon monoxide produced endogenously by HO is unlikely to be a neuromodulator required for LTP in the hippocampus.

Induction of lef1 during zebrafish fin regeneration

Because the transcription factor Lef1 is important for development of several vertebrate organs but has not been investigated for involvement in epimorphic regeneration, we examined its expression during regeneration of amputated adult zebrafish caudal fins. We found that lef1 is markedly up-regulated in the newly formed wound epidermis of the fin regenerate and is maintained in the basal epidermal layer during formation of the regeneration blastema. During regenerative outgrowth, lef1 expression is strongest in epidermal cells adjacent to newly aligned scleroblasts that secrete bone matrix, while it is low or undetectable in epidermis adjacent to mesenchymal areas with either mature bone or proliferative distal blastema cells. This localization is similar to that of the putative fin ray patterning signal Shh. In addition, brief treatments of fin regenerates with retinoic acid or the synthetic Fgfr1 inhibitor SU5402 down-regulate epidermal lef1, similar to their effects on shh. These results suggest a role for Lef1 in scleroblast alignment analogous to that proposed for Shh. Other Wnt signaling pathway members wnt3a, wnt5, and beta-catenin are also expressed in the fin regenerate. Our data suggest that Lef1 has specific roles in inducing and patterning vertebrate regenerating tissue.

Cellular uptake and trafficking of a prototypical magnetic iron oxide label in vitro

RATIONALE AND OBJECTIVES

Target-specific magnetic resonance (MR) contrast agents are being developed to improve the accuracy of MR imaging. The purpose of this study was to determine the mechanism of cell uptake, and modes of intracellular trafficking of a prototypical iron oxide label (RMA) used in the synthesis of some target-specific MR contrast agents.

METHODS

The prototypical agent (RMA) consisted of a dextran-coated monocrystalline iron oxide that was modified with rhodamine (fluorescent label) and opsonized with albumin. Fluorescence microscopy was performed in a phagocytic C6 cell line and in murine bone marrow macrophages. Immunohistochemistry against lysosomal markers was used to confirm the intracellular location of the label.

RESULTS

RMA was identified inside cells after incubation at concentrations as low as 4.0 x 10(-10) M Fe, typically observed with receptor mediated endocytosis and several orders of magnitude lower than that expected with fluid phase pinocytosis. Cell uptake could be inhibited by excess protein but not by dextran. RMA localized initially to tubular and to round intracellular organelles and co-localized with an antibody against a murine lysosomal glycoprotein antibodies (LGP-A) in macrophages. Three days after incubation, RMA was concentrated in perinuclear vesicles, which most likely represent terminal lysosomes where final breakdown appears to occur.

CONCLUSIONS

The mechanism of cellular uptake of a prototypical opsonized iron oxide label is consistent with receptor-mediated endocytosis. Immediately after cell contact, RMA localizes to the lysosomal compartment and at long time points reside in vesicles that by morphology and distribution appear to be terminal lysosomes. Iron oxides therefore demonstrate metabolism via the lysosomal pathway.

Ejaculatory abnormalities in mice with targeted disruption of the gene for heme oxygenase-2

Nitric oxide (NO) is well established as a neurotransmitter in the central and peripheral nervous systems. More recently, another gas, carbon monoxide (CO) has also been implicated in neurotransmission. In the nervous system CO is formed by a subtype of heme oxygenase (HO) designated HO2. HO2 is localized to discrete neuronal populations in the brain resembling localizations of soluble guanylyl cyclase, which is activated by CO. CO may also function in the peripheral autonomic nervous system, in conjunction with NO. The majority of ganglia in the myenteric plexus possess both HO2 and neuronal NO synthase (NOS). Defects in myenteric plexus neurotransmission occur both in mice with targeted deletion of genes for HO2 and neuronal NOS. HO2 also occurs in other autonomic ganglia including the petrosal, superior cervical and nodose ganglia. Neuronal NOS is localized to neurons regulating male reproductive behavior, such as penile erection, and NOS inhibitors prevent erection. Because of the other parallels between NO and CO, we speculated that CO may play a role in male reproductive behavior. In the present study we describe HO2 localization in neuronal structures regulating copulatory reflexes. Reflex activity of the bulbospongiosus muscle, which mediates ejaculation and ejaculatory behavior, is markedly diminished in mice with targeted deletion of the gene for HO2 (HO2-).

MR imaging of phagocytosis in experimental gliomas

PURPOSE

To determine whether phagocytosis can be observed in vivo in glioma cells.

MATERIALS AND METHODS

Rat C6 glioma cells were studied in culture and after intracerebral implantation into 13 rats. Monocrystalline iron oxide nanoparticles (MION), a model marker of phagocytosis, was administered intravenously to tumor-bearing rats at 2-20 mg of iron per kilogram. Magnetic resonance (MR) imaging was performed at multiple time points.

RESULTS

Glioma cells in culture showed uptake of MION in amounts of up to 10 ng of iron per 10(6) cells, corresponding to approximately 50,000 particles per cell. Fluorescently labeled MION was found to be located primarily in tubular lysosomes. Intracerebral gliomas showed characteristic changes in signal intensity at MR imaging that peaked 12 hours after administration of MION and lasted up to 5 days; these changes corresponded to uptake and subsequent biodegradation of MION by tumor cells.

CONCLUSION

Phagocytosis of glioma cells can be detected in vivo with iron oxide-enhanced MR imaging, and this may permit accurate delineation of tumor margins.

MR lymphography with a lymphotropic T1-type MR contrast agent: Gd-DTPA-PGM

A model system of a paramagnetic lymphotropic MR contrast agent (Gd-DTPA labeled polyglucose associated macrocomplex, PGM) for T1-weighted MR imaging of lymph nodes in rats and rabbits was evaluated. Pharmacokinetic (tissue accumulation) and MR imaging data (optimal dose and timing parameters) were obtained in normal rats (n = 88) after subcutaneous (SC) injection of paramagnetic, radiolabeled [111In]Gd-DTPA-PGM. A rabbit model of lymph node metastases (n = 8) was ultimately used to demonstrate the potential of MR imaging with Gd-DTPA-PGM for nodal tumor detection. Maximum concentrations of Gd-DTPA-PGM were found in popliteal and paraaortic lymph nodes within 24 h after SC administration, and highest lymph node SNR values were obtained by MR imaging at this time point. The optimum imaging dose was 6-12 mumol Gd/kg. Tumor-lymph node contrast increased from 0.0 +/- 1.2 precontrast to 19.2 +/- 6.5 (spoiled gradient echo sequence, TR 50/TE 7/flip angle 60 degrees) postcontrast and conspicuity of nodal metastases was improved. Gd-DTPA-PGM accumulates in lymph nodes after SC administration and significantly enhances lymph node signal intensity of normal animals but not metastatic lymph nodes.

Macromolecular intravenous contrast agent for MR lymphography: characterization and efficacy studies

PURPOSE

To determine the pharmacokinetic and magnetic resonance (MR) imaging properties of diethylenetriaminepentaacetic acid (DTPA) conjugated with a polyglucose-associated macrocomplex (PGM), which accumulates in lymph nodes.

MATERIALS AND METHODS

In 124 normal and 20 tumor-bearing rats, Gd-DTPA PGM was administered intravenously in doses of 2, 10, 20 mumol gadolinium per kilogram of tissue.

RESULTS

Mean blood half-life was 2 hours. Maximum accumulation in peripheral (33.0% injected dose [ID]/g +/- 16.2 [standard deviation]) and central lymph nodes (63.2% ID/g +/- 16.5) was observed within 24 hours after administration. The optimum dose range was 10-20 mumol Gd/kg in rats. At 24 hours after administration of 20 mumol Gd/kg, the signal-to-noise ratio increased from 30.9 +/- 0.4 to 83.2 +/- 5.2 in normal lymph nodes (P < .001). Differentiation between normal and metastatic lymph nodes was improved.

CONCLUSION

When labeled with Gd-DTPA, the PGM-based graft copolymer significantly increases signal intensity at MR imaging of normal but not metastatic lymph nodes without causing distortion artifacts.

Magnetically labeled secretin retains receptor affinity to pancreas acinar cells

Previously, we have developed a colloidal dextran-stabilized monocrystalline iron oxide nanocompound (MION-46) as a magnetic label for magnetic resonance imaging (MRI). In an effort to use this magnetic label to visualize pancreatic receptor function by MRI in vivo, we investigated the potential of secretin as a vector molecule. Secretin receptors, abundant on exocrine pancreas cells, recognize secretin through its amidated carboxyl terminal. In order to conjugate secretin to MION, we utilized the specific interaction between biotin and streptavidin, since direct conjugation of human secretin to MION has previously resulted in low yields and low affinity of the conjugate (unpublished results). Initially, we biotinylated the N-terminal primary amino group of secretin (60% yield). In a separate step, streptavidin (SA) was immobilized onto the surface dextran molecules of MION (79% yield) by reductive amination. Each secretin molecule was conjugated to one biotin molecule and each MION particle to an average of two SA molecules. The biotinylated secretin was then conjugated to MION through the biotin-streptavidin interaction (90% yield). The secretin-biotin-streptavidin-MION construct thus contained approximately two secretin molecules per MION. An in vitro competitive binding assay of pancreatic acinar cells demonstrated that the magnetically labeled secretin retained affinity to the secretin receptors. In vivo distribution studies in rats showed a significantly higher pancreatic accumulation of the secretin-biotin-streptavidin-MION construct as compared to the control group that had received unmodified MION. Our data indicate that bioactive peptides can be attached to dextran-coated iron oxide particles through the biotin-streptavidin interaction while retaining receptor affinity. Such target-specific agents have potential use in MR imaging to probe for a variety of receptor systems.

Changes in gravitational force induce alterations in gene expression that can be monitored in the live, developing zebrafish heart

Little is known about the effect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a beta-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart. Zebrafish embryos, at the 18-20 somite-stage, were exposed to simulated-microgravity for 24 hours. The intensity of GFP fluorescence associated with the heart was then determined using fluorescence microscopy. Our measurements indicated that simulated-microgravity induced a 23.9% increase in GFP-associated fluorescence in the heart. In contrast, the caudal notochord showed a 17.5% increase and the embryo as a whole showed only an 8.5% increase in GFP-associated fluorescence. This suggests that there are specific effects on the heart causing the more dramatic increase. These studies indicate that microgravity can influence gene expression and demonstrate the usefulness of this in vivo model of 'reporter-gene' expression for studying the effects of microgravity.

BMS-201620: a selective beta 3 agonist

A series of N-(4-hydroxy-3-methylsulfonanilidoethanol)arylglycinamides were prepared and evaluated for their human beta3 adrenergic receptor agonist activity. SAR studies led to the identification of BMS-201620 (39), a potent beta3 full agonist (Ki = 93 nM, 93% activation). Based on its favorable safety profile, BMS-201620 was chosen for clinical evaluation.

Quantitation of slow drug release from an implantable and degradable gentamicin conjugate by in vivo magnetic resonance imaging

A biodegradable model hydrogel containing a covalently bound aminoglycoside in which drug release can be monitored by magnetic resonance imaging (MRI) in vivo was developed. The hydrogel consists of the bishydroxysuccinimide ester of polyethylene glycol disuccinate cross-linked albumin, to which gentamicin and Gd-diethylenetriaminepentaacetic acid are covalently attached in stochiometric quantities. MRI allowed us to depict the three-dimensional structure of implanted gels, to accurately calculate their volumes, and thus to calculate the concentration of hydrogel-bound gentamicin. The correlation coefficient for the concentration of released gentamicin and the hydrogel volume was 0.965. Free and hydrogel-released gentamicin conjugates had similar antibiotic efficacies when tested in microbiological agar diffusion assays. In vivo, hydrogel-released gentamicin had a longer half-life in plasma than unaltered gentamicin (5.6 versus 0.7 h), presumably because of residual bound polyethylene glycol residues. Hydrogel implants into rats resulted in a prolonged (7 to 10 days) release of gentamicin and a decreased 24-h mortality in mice infected with a lethal dose of Pseudomonas aeruginosa. The results indicate the feasibility of imaging and quantitating therapeutic drug concentrations in vivo by MRI.