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The physiological significance of heme oxygenase

Potentially toxic dinoflagellate species Prorocentrum minimum (Pavillard) Schiller, 1933 colonized the brackish-water Baltic Sea in the early 1980s, where thereafter it has been forming regular harmful blooms. Heat shock proteins (HSP) play an important role in their ability to adapt to environmental stress. In this study, we evaluated how the synthesis of heme oxygenase-1 (HO-1/HSP32), an important part of cellular machinery for cell protection, changes in P. minimum under the influence of salinity stress during the day-night cycle. We identified sequences of the HO-1 homologs in the unannotated P. minimum transcriptomes, represented in the Marine Microbial Eukaryote Transcriptome Sequencing Project (MMETSP) database. Phylogenetic analysis showed that this protein clustered in a distinct clade and demonstrated an evolutionary relationship with the HO-1 homologs of other taxonomic groups. To identify P. minimum cells that express the HO-1 protein, we used fluorescent flow cytometry. In control conditions (17‰), the number of cells expressing this protein was 2.2 times higher during the day than at night, with the same average fluorescence intensity (1.7 units). During the daytime, the critical salinity conditions of 8‰ did not lead to significant changes in the number of cells expressing HO-1, while in the night culture this salinity shift caused a 2.7-fold increase in the number of cells expressing HO-1, and led to an increase in the intensity of fluorescence up to 2.0 units. The overall DNA and RNA synthesis rates were estimated by the incorporation of ³H-thymidine and ³H-uridine, respectively. Transcription and replication levels were significantly higher under critical salinity conditions (8‰) compared to the control (17‰) and typical sea salinity conditions (35‰). The greatest increase in the DNA synthesis rates (up to 2.7 times) was observed during the daytime in cells exposed to salinity stress at 8‰. In contrast, the greatest increase in the RNA synthesis rates (2.7 times) was registered in cells exposed to salinity of 8‰ at night. Thus, the replication activity of cells in response to salinity stress increased significantly during the day, while transcription levels reached their maximum values at night. These results indicate that a significant increase in the synthesis of both nucleic acids and the HO-1 stress protein can serve as a biomarker of the impact of environmental stress factors, such as salinity. This allowed concluding that the studied parameters demonstrate a high potential for future modeling of harmful algae blooms, particularly those formed by the invasive potentially toxic species under horohaline conditions of the Baltic Sea.

Summary: Acute kidney injury is a common complication after cardiac surgery that is associated with high postoperative morbidity and mortality. Levels of hemolysis are associated closely with the incidence and severity of kidney injury after cardiac surgery. Hemolysis is caused by prolonged surgical procedures and blood transfusions from cell-saver devices and is associated with the use of cardiopulmonary bypass. Plasma oxyhemoglobin is released into the circulation by damaged red blood cells that, via a dioxygenation reaction, depletes vascular nitric oxide (NO), a potent vasodilator molecule responsible for modulating organ perfusion and vascular homeostasis. Depleted plasma NO and increased levels of plasma oxyhemoglobin in the bloodstream lead to impairment of organ perfusion, inflammation, oxidative stress, and direct tubular injury, which, together, contribute to the development of renal injury after cardiac surgery. The administration of NO, a gas originally approved to treat pulmonary hypertension, maintains organ perfusion by preventing vascular NO depletion. In addition, this treatment improves cardiac output by reducing pulmonary vascular resistance and right heart workload. The clinical evidence of renal protection of NO gas therapy is supported by preclinical animal studies exploring the extrapulmonary protective effects of NO. Recent clinical trials showed a significant reduction of postoperative acute kidney injury when NO gas was administered during and after cardiac surgery.

The omnipresent colorless and odorless carbon monoxide (CO), generated by combustion, is the leading cause of unintentional death worldwide. CO binds to hemoglobin with affinity more than 200-times greater than oxygen and forms carboxyhemoglobin (COHb). The source of the endogenous CO is the breakdown of heme by heme oxygenase (HO), yielding CO, biliverdin, and iron; endogenous CO is similar to another endogenous gas, nitric oxide (NO), and both activate guanylyl cyclase and increase cGMP, which mediates many of their physiological effects. The toxicity of CO is primarily attributable to formation of COHb and resultant hypoxia, which in turn leads to rebound hyperperfusion and generation of oxidant radicals; these oxidants can produce their own toxicity and add to that of CO. The toxicity of CO ranges from mild headache at COHb less than 10% to death at COHb more than 70%. CO is more toxic in subjects with cardiovascular diseases than in healthy individuals, and it increases maternal and fetal morbidity and mortality. Nonfatal exposure to CO can result in delayed neurological, myocardial, and other toxicities. At the same time, there is some adaptation to effects of CO on low level chronic exposure. Hyperbaric oxygen is the main therapy against CO poisoning. Determined public health measures are needed to decrease atmospheric CO, which is already quite high in places such as Mexico City and Los Angeles.

The inducible form of Heme Oxygenase-1 (HO-1), a major endoplasmic reticulum (ER) associated heme protein, is known to play important roles in protection against oxidative and chemical stress by degrading free heme released from degradation of heme proteins. In this study we show that induced expression of HO-1 by subjecting macrophage RAW-264.7 cells to chemical or physiological hypoxia resulted in significant translocation of HO-1 protein to mitochondria. Transient transfection of COS-7 cells with cloned cDNA also resulted in mitochondrial translocation of HO-1. Deletion of N-terminal ER targeting domain increased mitochondrial translocation under the transient transfection conditions. Mitochondrial localization of both intact HO-1 and N-terminal truncated HO-1 caused loss of heme aa-3 and cytochrome c oxidase (CcO) activity in COS-7 cells. The truncated protein, which localizes to mitochondria at higher levels, induced substantially steeper loss of CcO activity and reduced heme aa3 content. Furthermore, cells expressing mitochondria targeted HO-1 also induced higher ROS production. Consistent with dysfunctional state of mitochondria induced by HO-1, the mitochondrial recruitment of autophagy markers LC-3 and Drp-1 was also increased in these cells. Chronic ethanol feeding in rats also caused an increase in mitochondrial HO-1 and decrease in CcO activity. These results show that as opposed to the protective effect of the ER associated HO-1, mitochondria targeted HO-1 under normoxic conditions induces mitochondrial dysfunction.