Heme oxygenase-2 mRNA: developmental expression in the rat liver and response to cobalt chloride

Altered Expression of Heme Oxygenase 2 in Heme Oxygenase 1-deficient Mouse Embryos

Heme oxygenases (Hmoxs) are enzymes that catalyze the first and rate-limiting step in the degradation of heme to carbon monoxide, iron, and biliverdin. The two main isozymes, namely Hmox1 and Hmox2, are encoded by two different genes. Mutation of the Hmox1 gene in mice is known to cause extensive prenatal lethality, and limited information is available about the expression of Hmox proteins in developing mouse embryos. In this study, immunohistochemistry was used to perform a detailed investigation comparing Hmox proteins in Hmox1 wild-type and knockout (KO) mouse embryos collected from wild-type and heterozygous timed-matings. Western analysis for Hmoxs was also done in the organs of late-gestation embryos. The results demonstrated cytoplasmic and nuclear localization of Hmoxs in all the organs examined in wild-type embryos. Interestingly, Hmox2 immunoreactive protein signals were significantly low in most of the organs of mid- and late-gestation Hmox1-KO embryos. Furthermore, relative levels of Hmox2 were revealed to be significantly lower in the lung and kidney of late-gestation Hmox1-KO embryos by western analysis, which complemented the immunohistochemistry findings in these two organs. The current study provides detailed immunoexpression patterns of Hmox proteins in wild-type and Hmox1-KO mouse embryos in mid- and late-gestation.

Developmental expression of heme oxygenase in the rat lung

Heme oxygenase (HO), the rate-limiting enzyme in the formation of bilirubin, is expressed in the lung and may serve as an antioxidant. This enzyme results in the formation of antioxidant bile pigments and the degradation of pro-oxidant heme. We wanted to evaluate the differences in expression of HO-1, the inducible form, and HO-2, the constitutive isoenzyme, during lung maturation and document whether lung HO expression was similar to that of other antioxidant enzymes. Lung total HO activity and HO-1 and HO-2 proteins as well as HO-1 and HO-2 mRNA were evaluated in animals from 16 d of gestation (e(16.5)) to 2 mo of age. Heme content was also evaluated because heme is the substrate of the reaction. HO-1 mRNA was maximal at e(19.5) and e(20.5), whereas HO-2 mRNA was not changed throughout maturation. Lung HO-1 protein was highest on the first days of life and lowest in adults, whereas HO-2 protein was maximally expressed at postnatal d 5 and then declined to reach adult values. As to HO activity, there was a prenatal peak at e(20.5), a second lesser peak at d 5, and thereafter a decline to adult values. Lung heme content was inversely correlated with HO activity or protein as the highest heme values were seen in adults with the lowest HO activity. In response to hyperoxia, HO-1 mRNA was induced only in the adult lungs. A better understanding of the maturational regulation of lung HO will define a role for HO in newborns at risk for oxygen toxicity.

Induction of heme oxygenase-1 during endotoxemia is downregulated by transforming growth factor-beta1

Heme oxygenase (HO)-1 generates CO, a gas with vasodilatory properties, during heme metabolism. HO-1 is expressed highly in vascular tissue after endotoxin stimulation, and generation of CO through the HO-1 pathway contributes to the hemodynamic compromise of endotoxic shock. Shock related to endotoxemia is an immune-mediated process that involves the generation of proinflammatory cytokines such as interleukin (IL)-1beta. Because transforming growth factor (TGF)-beta1 is a modulator of immune-mediated inflammatory responses and it blocks the hypotension of endotoxic shock, we determined whether TGF-beta1 could be used to reduce expression of HO-1 in vascular tissue and smooth muscle cells. In a rat model of endotoxic shock, lipopolysaccharide-induced HO-1 mRNA and protein expression was reduced by TGF-beta1 in highly vascularized tissue, such as heart and lung, by Northern and Western analysis. Furthermore, TGF-beta1 downregulated HO-1 mRNA after its induction by IL-1beta in vascular smooth muscle cells in culture. TGF-beta1 also decreased HO-1 but not HO-2 protein expression in these cells. TGF-beta1 decreased HO enzyme activity induced in IL-1beta treated vascular smooth muscle cells to a level not different from that in vehicle-treated cells. These studies suggest that this downregulation of HO-1 mRNA and protein expression and decrease in IL-1beta-induced HO enzyme activity may contribute to the beneficial effect of TGF-beta1 on endotoxic shock.

The heme oxygenase system: a regulator of second messenger gases

The heme oxygenase (HO) system consists of two forms identified to date: the oxidative stress-inducible protein HO-1 (HSP32) and the constitutive isozyme HO-2. These proteins, which are different gene products, have little in common in primary structure, regulation, or tissue distribution. Both, however, catalyze oxidation of heme to biologically active molecules: iron, a gene regulator; biliverdin, an antioxidant; and carbon monoxide, a heme ligand. Finding the impressive heme-degrading activity of brain led to the suggestion that "HO in brain has functions aside from heme degradation" and to subsequent exploration of carbon monoxide as a promising and potentially significant messenger molecule. There is much parallelism between the biological actions and functions of the CO- and NO-generating systems; and their regulation is intimately linked. This review highlights the current information on molecular and biochemical properties of HO-1 and HO-2 and addresses the possible mechanisms for mutual regulatory interactions between the CO- and NO-generating systems.

Hepatic heme oxygenase is inducible in neonatal rats during the early postnatal period

Heme oxygenase (HO) is the rate-limiting enzyme in the catabolism of heme to bilirubin. Cobalt chloride (CoCl2) and many other agents that generate oxidant stresses induce the HO-1 isoform. Furthermore, HO-1 has been shown to protect against oxidant stress in vitro and in vivo by mechanisms involving increased ferritin synthesis. However, little is known about the inducibility of hepatic HO-1 during the very early postnatal period, and whether HO-1 induction is associated with increased ferritin synthesis in neonates. Therefore, we studied hepatic HO-1 mRNA, HO-1 protein concentration, total HO activity, and ferritin protein levels in neonatal rats. Neonatal rats 0-5 d of age were injected with 250 mumol/kg body weight of CoCl2. 6H2O in saline or with an equal volume of saline in age-matched controls. Liver samples were collected 4 h after injection for HO-1 mRNA analysis and 20 h after injection for analysis of HO-1 protein concentration, total HO activity, and ferritin protein levels. In CoCl2-treated rats, hepatic HO-1 mRNA was 3-10 times the levels in control rats (p < 0.05), HO-1 protein concentration was 2-5 times the levels in control rats (p < 0.05), and total HO activity was higher by 20-80% than in control rats (p < 0.05). There were no differences in hepatic ferritin protein levels between CoCl2-treated neonatal rats and controls; however, in CoCl2-treated adult rats, hepatic ferritin protein levels were 1.6 times the levels in controls (p < 0.05). Thus, neonatal rats can up-regulate hepatic HO-1 mRNA, HO-1 protein concentration, and total HO activity in response to CoCl2; however, no upregulation of hepatic ferritin protein levels was observed in neonatal rats after CoCl2 administration or subsequent HO-1 induction. We speculate that neonatal rats induce hepatic HO-1 and up-regulate ferritin by different mechanisms than do adult rats.

Immunohistochemical localization of biliverdin reductase in rat brain: age related expression of protein and transcript

Biliverdin reductase regulates heme oxygenase activity by removing the inhibitory product of the oxygenase activity, biliverdin; and reducing it to bilirubin. The other products of the oxygenase are carbon monoxide and Fe. To date, biliverdin reductase remains unique among all enzymes described by using 2 different cofactors (NADPH and NADH) at different pH ranges. The present study reports on the developmentally regulated changes in the pattern of protein expression and the level of biliverdin reductase transcript in rat brain. Biliverdin reductase activity of the brain cytosol with both NADPH (pH 8.7) and NADH (pH 6.7) exhibited developmental changes with the activity increasing after birth, reaching an adult level by day 28 postpartum. When analyzed by Western blotting the immunoreactive protein detected increased as the animal matured (day 1 to 28 postparturition). Northern blot hybridization of RNA isolated from rat brain revealed the presence of approximately 1.5 kb biliverdin reductase transcript at all stages of development ranging from 1 day post partum to 20 months. The level of the transcript was developmentally regulated and a gradual increase ( approximately 4-fold) was observed from day 1 after birth to adulthood and was maintained in 20 month old animals. Cellular localization, using immunohistochemical technique, revealed age-related pattern of expression of the reductase in select regions such as the cortex, substantia nigra, hippocampus and in the cerebellum; the changes, however, did not follow the same pattern. To elaborate, in the cortex, the reductase expression increased when 7-day-old animals were compared with young adults (2 months old) and then declined in the 20-month-old animals. In the substantia nigra the level of reductase expression progressively declined with age when 7-day-old neonate, 2- and 20-month-old animals were compared. In the hippocampus, a distinct reductase-expressing cell population residing between CA1 and the dentate gyrus was observed in the 7-day-old animals; these cells were not detected in the adults (2 or 20 months old). In the cerebellum, the expression of the reductase reflected the developmental organization of this region. We postulate that age-dependent increase of the brain reductase at the transcript and protein levels in the course of maturation serves to control heme oxygenase activity which also displays a developmental pattern in the organ. As such, the reductase modulates generation of biologically active heme degradation products; bilirubin, carbon monoxide and Fe.

Mitochondrial biogenesis in striated muscle

Mitochondrial biogenesis (synthesis) has been observed to occur in skeletal muscle in response to chronic use. It also occurs in cardiac muscle during growth and hypertrophy, and it may be impaired during the aging process. This review summarizes the literature on the processes of mitochondrial biogenesis at the biochemical and molecular levels, with particular reference to striated muscles. Mitochondrial biogenesis involves the expression of nuclear and mitochondrial genes and the coordination of these two genomes, the synthesis of proteins and phospholipids and their import into the organelle, and the incorporation of these lipids and proteins into their appropriate locations within the matrix, inner or outer membranes. The emphasis is on the regulation of these events, with information derived in part from other cellular systems. Although descriptions of mitochondrial content changes in heart and skeletal muscle during altered physiological states are plentiful, much work is needed at the molecular level to investigate the regulatory processes involved. A knowledge of biochemical and molecular biology techniques is essential for continued progress in the field. This is a promising area, and potential new avenues for future research are suggested.

Differential regulation of heme oxygenase isozymes by Sn- and Zn-protoporphyrins: possible relevance to suppression of hyperbilirubinemia

Synthetic metalloporphyrins decrease heme oxygenase (HO)-dependent bilirubin formation. Presently, the effects in vivo and in vitro of Sn- and Zn-protoporphyrins on HO-1 (HSP-32) and HO-2 at the protein and transcript levels were examined. Western blot analysis of HO-2 in testes microsomes of Sn-protoporphyrin-treated rats revealed a dramatic disruption of the integrity of the HO-2 protein. Similar observations were made with the liver and adrenal HO-2 and the NADPH-cytochrome P-450 reductase of treated rats. Northern blot analysis, however, suggested unaltered tissue levels of HO-2 transcripts (approximately 1.9 and approximately 1.3 kb). The HO-1 protein integrity in organs of treated rats was less dramatically affected by the metalloporphyrin and an increase in its 1.8 kb mRNA level in the testes was detected. Zn-protoporphyrin also increased HO-1 mRNA level in the testes, but did not affect HO-2 protein integrity. In in vitro studies with purified HO-1 and HO-2, both Sn- and Zn-protoporphyrins were equally inhibitory to HO-1 activity; Sn-protoporphyrin, however, was by far more inhibitory to HO-2-dependent activity than to that of HO-1. Together, these findings and the fact that HO-2 under normal conditions is the predominant form of the enzyme in most organs suggest that loss of HO-2 protein integrity may to a significant degree account for suppression of bilirubin formation by Sn-protoporphyrin. These in turn may reflect differences between HO-1 and HO-2, both at the transcriptional level with HO-2 being noninducible, and in structure/composition of the isozymes, with HO-2 being more labile.

Human heme oxygenase-2: characterization and expression of a full-length cDNA and evidence suggesting that the two HO-2 transcripts may differ by choice of polyadenylation signal

We show by Northern blot analysis that human HO-2 is encoded by two transcripts (1.3 and 1.7 kb) and is a single-copy gene as judged by Southern blot analysis. We further provide evidence based on Northern blot and sequence analysis of a cDNA representing the larger transcript that the transcripts differ in the 3' untranslated region. A 274-base-pair DNA fragment from the rat heme oxygenase-2 gene (I. Cruse and M.D. Maines, 1988, J. Biol. Chem. 263, 3348-3353) was used to isolate a human HO-2 cDNA from a fetal kidney library in lambda gt11. The clone, designated hK-1, was sequenced and the cDNA insert was determined to be 1625 base pairs in length, encoding a protein of 313 amino acids. Two consensus polyadenylation signals separated by 440 nucleotides were identified in the 3' untranslated region. The size of the cDNA insert closely approximated the larger of two mRNAs. The nucleotide sequence was 88% identical to the rat HO-2 gene within the predicted coding region and the putative translation product was also estimated to be 88% identical to the rat gene product (M. O. Rotenberg and D. Maines, 1990, J. Biol. Chem. 265, 7501). The predicted size, 36 kDa, corresponded well with HO-2 detected in human testis microsomes by Western blot analysis. Further, the fusion protein expressed in Escherichia coli displayed significant heme oxygenase activity, which was inhibited by Zn- and Sn-protoporphyrins, known inhibitors of eukaryotic heme oxygenase, but not by sulfhydryl reagents.

Characterization of a cDNA-encoding rabbit brain heme oxygenase-2 and identification of a conserved domain among mammalian heme oxygenase isozymes: possible heme-binding site?

A 1.3-kb rat testis cDNA clone for heme oxygenase-2 (HO-2) was used as a Northern blot hybridization probe, and a single homologous mRNA species, of approximately 1.3 kb in rabbit brain and testis was detected. This contrasted with the observation made with rat brain in which two HO-2 transcripts of approximately 1.3 and 1.9 kb were detected. Use of the same rat HO-2 probe to screen a rabbit brain cDNA library in lambda gt11 resulted in the recovery of a single 1.2-kb cDNA clone. This cDNA exhibits 84% overall nucleotide sequence homology with rat HO-2 and encodes a protein of 35,352 Da, displaying 88% amino acid sequence homology with rat testis HO-2. Furthermore, when expressed in Escherichia coli, the rabbit cDNA-encoded protein displays heme oxygenase activity and cross-reactivity with antibody to rat HO-2. Based on findings obtained through Western immunoblot analysis of partially purified HO-2 protein prepared from rabbit testis and brain, the 35- to 36-kDa molecular form appears to be the major HO-2 form detected in the brain, whereas a 42-kDa species is the predominant form observed in rabbit testis. Having deduced the amino acid sequence of rabbit brain HO-2, we provide a comparison of this sequence with those of rat, mouse, and human HO-1 and rat HO-2, and thereby identify a 24-amino-acid-long peptide region which, except for one residue, is identical in all five species of HO-1 and HO-2 compared (96% similarity), and exhibits 100% similarity in predicted secondary structure (for this region) in all five proteins. We propose that this peptide may be important to the heme binding and isomer-specific tetrapyrrole cleavage activities of the heme oxygenase isozymes.

Rapid induction of heme oxygenase 1 mRNA and protein by hyperthermia in rat brain: heme oxygenase 2 is not a heat shock protein

Catalytic activity of heme oxygenase (heme, hydrogen-donor:oxygen oxidoreductase, EC 1.14.99.3) isozymes, HO-1 and HO-2, permits production of physiologic isomers of bile pigments. In turn, bile pigments biliverdin and bilirubin are effective antioxidants in biological systems. In the rat brain we have identified only the HO-1 isozyme of heme oxygenase as a heat shock protein and defined hyperthermia as a stimulus that causes an increase in brain HO-1 protein. Exposure of male rats to 42 degrees C for 20 min caused a rapid and marked increase in brain 1.8-kilobase HO-1 mRNA. Specifically, a 33-fold increase in brain HO-1 mRNA was observed within 1 h and sustained for at least 6 h posttreatment. In contrast, the two HO-2 homologous transcripts (1.3 and 1.9 kilobases) did not respond to heat shock; neither the ratio nor the level of the two messages differed from that of the control when measured either at 1, 6, or 24 h after hyperthermia. The induction of a 1.8-kilobase HO-1 mRNA resulted in a pronounced increase in HO-1 protein 6 h after hyperthermia, as detected by both Western immunoblot and RIA. Immunocytochemistry of rat brain showed discrete localization of HO-1-like protein only in neurons of select brain regions. Six hours after heat shock, an intense increase in HO-1-like protein was observed in both Purkinje cells of the cerebellum and epithelial cells lining the cerebral aqueduct of the brain. We suggest that the increase in HO-1 protein, hence increased capacity to form bile pigments, represents a neuronal defense mechanism against heat shock stress.