The heme oxygenases, which consist of constitutive and inducible isozymes (HO-1, HO-2), catalyze the rate-limiting step in the metabolic conversion of heme to the bile pigments (i.e., biliverdin and bilirubin) and thus constitute a major intracellular source of iron and carbon monoxide (CO). In recent years, endogenously produced CO has been shown to possess intriguing signaling properties affecting numerous critical cellular functions including but not limited to inflammation, cellular proliferation, and apoptotic cell death. The era of gaseous molecules in biomedical research and human diseases initiated with the discovery that the endothelial cell-derived relaxing factor was identical to the gaseous molecule nitric oxide (NO). The discovery that endogenously produced gaseous molecules such as NO and now CO can impart potent physiological and biological effector functions truly represented a paradigm shift and unraveled new avenues of intense investigations. This review covers the molecular and biochemical characterization of HOs, with a discussion on the mechanisms of signal transduction and gene regulation that mediate the induction of HO-1 by environmental stress. Furthermore, the current understanding of the functional significance of HO shall be discussed from the perspective of each of the metabolic by-products, with a special emphasis on CO. Finally, this presentation aspires to lay a foundation for potential future clinical applications of these systems.

/pdf/heme-oxygenase-1-carbon-monoxide-from-basic-science-to-2rhjm6458g.pdf

Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications.

Reactive oxygen intermediates are produced in all aerobic organisms during respiration and exist in the cell in a balance with biochemical antioxidants. Excess reactive oxygen resulting from exposure to environmental oxidants, toxicants, and heavy metals perturbs cellular redox balance and disrupts normal biological functions. The resulting imbalance may be detrimental to the organism and contribute to the pathogenesis of disease and aging. To counteract the oxidant effects and to restore a state of redox balance, cells must reset critical homeostatic parameters. Changes associated with oxidative damage and with restoration of cellular homeostasis often lead to activation or silencing of genes encoding regulatory transcription factors, antioxidant defense enzymes, and structural proteins. In this review, we examine the sources and generation of free radicals and oxidative stress in biological systems and the mechanisms used by reactive oxygen to modulate signal transduction cascades and redirect gene expression.

Regulation of gene expression by reactive oxygen

This review is intended to stimulate interest in the effect of increased expression of heme oxygenase-1 (HO-1) protein and increased levels of HO activity on normal and pathological states. The HO system includes the heme catabolic pathway, comprising HO and biliverdin reductase, and the products of heme degradation, carbon monoxide (CO), iron, and biliverdin/bilirubin. The role of the HO system in diabetes, inflammation, heart disease, hypertension, neurological disorders, transplantation, endotoxemia and other pathologies is a burgeoning area of research. This review focuses on the clinical potential of increased levels of HO-1 protein and HO activity to ameliorate tissue injury. The use of pharmacological and genetic probes to manipulate HO, leading to new insights into the complex relationship of the HO system with biological and pathological phenomena under investigation, is reviewed. This information is critical in both drug development and the implementation of clinical approaches to moderate and to alleviate the numerous chronic disorders in humans affected by perturbations in the HO system.

Pharmacological and Clinical Aspects of Heme Oxygenase

How a cell responds to stress is a central problem in cardiovascular biology. Diverse physiological stresses (eg, heat, hemodynamics, mutant proteins, and oxidative injury) produce multiple changes in a cell that ultimately affect protein structures and function. Cells from different phyla initiate a cascade of events that engage essential proteins, the molecular chaperones, in decisions to repair or degrade damaged proteins as a defense strategy to ensure survival. Accumulative evidence indicates that molecular chaperones such as the heat shock family of stress proteins (HSPs) actively participate in an array of cellular processes, including cytoprotection. The versatility of the ubiquitous HSP family is further enhanced by stress-inducible regulatory networks, both at the transcriptional and posttranscriptional levels. In the present review, we discuss the regulation and function of HSP chaperones and their clinical significance in conditions such as cardiac hypertrophy, vascular wall injury, cardiac surgery, ischemic preconditioning, aging, and, conceivably, mutations in genes encoding contractile proteins and ion channels.

Stress (Heat Shock) Proteins: Molecular Chaperones in Cardiovascular Biology and Disease

The discovery of the gaseous molecule nitric oxide in 1987 unraveled investigations on its functional role in the pathogenesis of a wide spectrum of biological and pathological processes. At that time, the novel concept that an endogenous production of a gaseous substance such as nitric oxide can impart such diverse and potent cellular effects proved to be very fruitful in enhancing our understanding of many disease processes including lung disorders. Interestingly, we have known for a longer period of time that there exists another gaseous molecule that is also generated endogenously; the heme oxygenase (HO) enzyme system generates the majority if not all of the endogenously produced carbon monoxide. This enzyme system also liberates two other by-products, bilirubin and ferritin, each possessing important biological functions and helping to define the uniqueness of the HO enzyme system. In recent years, interest in HO has emerged in numerous disciplines including the central nervous system, cardiovascular physiology, renal and hepatic systems, and transplantation. We review the functional role of HO in lung biology and its real potential application to lung diseases.

https://www.physiology.org/doi/pdf/10.1152/ajplung.2000.279.6.L1029

Heme oxygenase: colors of defense against cellular stress

Two isozymes of heme oxygenase (HO), HO-1 or HSP32 and the constitutive form HO-2, have been characterized to date. We report the discovery of a third protein species and refer to it as HO-3. HO-3 is the product of a single transcript of approximately 2.4 kb and can encode a protein of approximately 33 kDa. The HO-3 transcript is found in the spleen, liver, thymus, prostate, heart, kidney, brain and testis and is the product of a single-copy gene. The predicted amino acid structure of HO-3 differs from both HO-1 (HSP32) and HO-2 but is closely related to HO-2 (approximately 90%). Escherichia coli expressed and purified HO-3 protein does not cross react with polyclonal antibodies to either rat HO-1 or HO-2, is a poor heme catalyst, and displays hemoprotein spectral characteristics. The predicted protein has two heme regulatory motifs that may be involved in heme binding. These motifs and the hemoprotein nature of HO-3 suggest a potential regulatory role for the protein in cellular processes which are heme-dependent.

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1432-1033.1997.00725.x

Isolation and Characterization of a cDNA from the Rat Brain that Encodes Hemoprotein Heme Oxygenase-3

Heme Oxygenase-2 Is a Hemoprotein and Binds Heme through Heme Regulatory Motifs That Are Not Involved in Heme Catalysis

Biliverdin reductase catalyzes the reduction of biliverdin, the product of heme oxygenase (HO) activity, to bilirubin. The reductase is unique among all enzymes characterized to date in being dual pH/cofactor-dependent. Until now the enzyme was assumed to be a noninducible cytosolic protein. This report, for the first time, demonstrates induction and nuclear localization of reductase in rat kidney in response to HO-1 inducers: bacterial lipopolysaccharide (LPS) and bromobenzene. The study also demonstrates that nuclear localization requires an intact nuclear localization signal and is responsive to cGMP. Specifically 16 h after treatment of rats (i.p.) with LPS (5 mg/kg), there was an increase in nuclear biliverdin reductase as determined by immunostaining, Western blotting, and activity analysis. Induction and nuclear localization of the reductase in kidney was also observed in bromobenzene-treated rats (2 mmol/kg, s.c., 24 h). The reductase message levels, however, were not increased in response to either treatment, suggesting post-transcriptional activation of the reductase by LPS and bromobenzene. The mechanism of nuclear transport of the reductase was examined using HeLa cells transfected with the hemagglutinin-tagged reductase construct. When cells were treated with 8-Br-cGMP the protein translocated into the nucleus. Mutation of the putative nuclear localization signal domain of the reductase blocked nuclear transport of the protein. We suggest the significance of nuclear localization of the reductase may relate to: 1) chain-breaking antioxidant activity of bilirubin; 2) inhibition of superoxide formation by bilirubin; and 3) modulation of the signal transduction pathways.

Nuclear localization of biliverdin reductase in the rat kidney: response to nephrotoxins that induce heme oxygenase-1.

Heme oxygenase-2 (HO-2) degrades heme [Fe-protoporphyrin IX (Fe-PP)] to CO and bilirubin. The enzyme is a hemoprotein and interacts with nitric oxide. HO-2 has two copies of heme regulatory motif (...

Tian J. Huang

Papers

Isolation and Characterization of a cDNA from the Rat Brain that Encodes Hemoprotein Heme Oxygenase-3

Heme Oxygenase-2 Is a Hemoprotein and Binds Heme through Heme Regulatory Motifs That Are Not Involved in Heme Catalysis

Nuclear localization of biliverdin reductase in the rat kidney: response to nephrotoxins that induce heme oxygenase-1.

Heme oxygenase-2 interaction with metalloporphyrins: function of heme regulatory motifs.

THE OXIDOREDUCTASE, BILIVERDIN REDUCTASE, IS INDUCED IN HUMAN RENAL CARCINOMA - pH AND COFACTOR-SPECIFIC INCREASE IN ACTIVITY