Perceived Hyperoxia: Oxygen-Regulated Signal Transduction Pathways in the Heart

An Injectable Oxygen Release System to Augment Cell Survival and Promote Cardiac Repair Following Myocardial Infarction

Oxygen deficiency after myocardial infarction (MI) leads to massive cardiac cell death. Protection of cardiac cells and promotion of cardiac repair are key therapeutic goals. These goals may be achieved by re-introducing oxygen into the infarcted area. Yet current systemic oxygen delivery approaches cannot efficiently diffuse oxygen into the infarcted area that has extremely low blood flow. In this work, we developed a new oxygen delivery system that can be delivered specifically to the infarcted tissue, and continuously release oxygen to protect the cardiac cells. The system was based on a thermosensitive, injectable and fast gelation hydrogel, and oxygen releasing microspheres. The fast gelation hydrogel was used to increase microsphere retention in the heart tissue. The system was able to continuously release oxygen for 4 weeks. The released oxygen significantly increased survival of cardiac cells under the hypoxic condition (1% O₂) mimicking that of the infarcted hearts. It also reduced myofibroblast formation under hypoxic condition (1% O₂). After implanting into infarcted hearts for 4 weeks, the released oxygen significantly augmented cell survival, decreased macrophage density, reduced collagen deposition and myofibroblast density, and stimulated tissue angiogenesis, leading to a significant increase in cardiac function.

Hyperoxia sensing: from molecular mechanisms to significance in disease

Oxygen therapy using mechanical ventilation with hyperoxia is necessary to treat patients with respiratory failure and distress. However, prolonged exposure to hyperoxia leads to the generation of excessive reactive oxygen species (ROS), causing cellular damage and multiple organ dysfunctions. As the lungs are directly exposed, hyperoxia can cause both acute and chronic inflammatory lung injury and compromise innate immunity. ROS may contribute to pulmonary oxygen toxicity by overwhelming redox homeostasis, altering signaling cascades that affect cell fate, ultimately leading to hyperoxia-induced acute lung injury (HALI). HALI is characterized by pronounced inflammatory responses with leukocyte infiltration, injury, and death of pulmonary cells, including epithelia, endothelia, and macrophages. Under hyperoxic conditions, ROS mediate both direct and indirect modulation of signaling molecules such as protein kinases, transcription factors, receptors, and pro- and anti-apoptotic factors. The focus of this review is to elaborate on hyperoxia-activated key sensing molecules and current understanding of their signaling mechanisms in HALI. A better understanding of the signaling pathways leading to HALI may provide valuable insights on its pathogenesis and may help in designing more effective therapeutic approaches.

Characterization of the structural and functional changes in the myocardium following focal ischemia-reperfusion injury

High-resolution (11.7 T) cardiac magnetic resonance imaging (MRI) and histological approaches have been employed in tandem to characterize the secondary damage suffered by the murine myocardium following the initial insult caused by ischemia-reperfusion (I/R). I/R-induced changes in the myocardium were examined in five separate groups at the following time points after I/R: 1 h, day 1, day 3, day 7, and day 14. The infarct volume increased from 1 h to day 1 post-I/R. Over time, the loss of myocardial function was observed to be associated with increased infarct volume and worsened regional wall motion. In the infarct region, I/R caused a decrease in end-systolic thickness coupled with small changes in end-diastolic thickness, leading to massive wall thickening abnormalities. In addition, compromised wall thickening was also observed in left ventricular regions adjacent to the infarct region. A tight correlation (r2 = 0.85) between measured MRI and triphenyltetrazolium chloride (TTC) infarct volumes was noted. Our observation that until day 3 post-I/R the infarct size as measured by TTC staining and MRI was much larger than that of the myocyte-silent regions in trichrome- or hematoxylin-eosin-stained sections is consistent with the literature and leads to the conclusion that at such an early phase, the infarct site contains structurally intact myocytes that are functionally compromised. Over time, such affected myocytes were noted to structurally disappear, resulting in consistent infarct sizes obtained from MRI and TTC as well as trichrome and hematoxylin-eosin analyses on day 7 following I/R. Myocardial remodeling following I/R includes secondary myocyte death followed by the loss of cardiac function over time.

Role of oxygen in postischemic myocardial injury

Myocardial function is dependent on a constant supply of oxygen from the coronary circulation. A reduction of oxygen supply due to coronary obstruction results in myocardial ischemia, which leads to cardiac dysfunction. Reperfusion of the ischemic myocardium is required for tissue survival. Thrombolytic therapy, coronary artery bypass surgery and coronary angioplasty are some of the treatments available for the restoration of blood flow to the ischemic myocardium. However, the restoration of blood flow may also lead to reperfusion injury, resulting in myocyte death. Thus, any imbalance between oxygen supply and metabolic demand leads to functional, metabolic, morphologic, and electrophysiologic alterations, causing cell death. Myocardial ischemia reperfusion (IR) injury is a multifactorial process that is mediated by oxygen free radicals, neutrophil activation and infiltration, calcium overload, and apoptosis. Controlled reperfusion of the ischemic myocardium has been advocated to prevent the IR injury. Studies have shown that reperfusion injury and postischemic cardiac function are related to the quantity and delivery of oxygen during reperfusion. Substantial evidence suggests that controlled reoxygenation may ameliorate postischemic organ dysfunction. In this review, we discuss the role of oxygenation during reperfusion and subsequent biochemical and pathologic alterations in reperfused myocardium and recovery of heart function.

Cell passage-associated transient high oxygenation causes a transient decrease in cellular glutathione and affects T cell responses to apoptotic and mitogenic stimuli

Routine cell line maintenance involves removal of waste products and replenishment of nutrients via replacement of cell culture media. Here, we report that routine maintenance of three discrete cell lines (HSB-CCRF-2 and Jurkat T cells, and phaeo-chromocytoma PC12 cells) decreases the principal cellular antioxidant, glutathione, by up to 42% in HSB-CCRF-2 cells between 60 and 120min after media replenishment. However, cellular glutathione levels returned to baseline within 5h after passage. The decrease in glutathione was associated with modulation of the response of Jurkat T cells to apoptotic and mitogenic signals. Methotrexate-induced apoptosis over 16h, measured as accumulation of apoptotic nucleoids, was decreased from 22 to 17% if cells were exposed to cytotoxic agent 30min after passage compared with cells exposed to MTX in the absence of passage. In contrast, interleukin-2 (IL-2) production over 24h in response to the toxin phytohaemagglutinin (PHA), was increased by 34% if cells were challenged 2h after passage compared with PHA treatment in the absence of passage. This research highlights the presence of a window of time after cell passage of non-adherent cells that may lead to over- or under-estimation of subsequent cell responses to toxins, which is dependent on cellular antioxidant capacity or redox state.

Laser microdissection and capture of pure cardiomyocytes and fibroblasts from infarcted heart regions: perceived hyperoxia induces p21 in peri-infarct myocytes

Myocardial infarction caused by ischemia-reperfusion in the coronary vasculature is a focal event characterized by an infarct-core, bordering peri-infarct zone and remote noninfarct zone. Recently, we have reported the first technique, based on laser microdissection pressure catapulting (LMPC), enabling the dissection of infarction-induced biological responses in multicellular regions of the heart. Molecular mechanisms in play at the peri-infarct zone are central to myocardial healing. At the infarct site, myocytes are more sensitive to insult than robust fibroblasts. Understanding of cell-specific responses in the said zones is therefore critical. In this work, we describe the first technique to collect the myocardial tissue with a single-cell resolution. The infarcted myocardium was identified by using a truncated hematoxylin-eosin stain. Cell elements from the infarct, peri-infarct, and noninfarct zones were collected in a chaotropic RNA lysis solution with micron-level surgical precision. Isolated RNA was analyzed for quality by employing microfluidics technology and reverse transcribed to generate cDNA. Purity of the collected specimen was established by real-time PCR analyses of cell-specific genes. Previously, we have reported that the oxygen-sensitive induction of p21/Cip1/Waf1/Sdi1 in cardiac fibroblasts in the peri-infarct zone plays a vital role in myocardial remodeling. Using the novel LMPC technique developed herein, we confirmed that finding and report for the first time that the induction of p21 in the peri-infarct zone is not limited to fibroblasts but is also evident in myocytes. This work presents the first account of an analytical technique that applies the LMPC technology to study myocardial remodeling with a cell-type specific resolution.

Oxygen-sensitive reset of hypoxia-inducible factor transactivation response: prolyl hydroxylases tune the biological normoxic set point

Cellular O(2) sensing enables physiological adjustments to variations in tissue pO(2). Under basal conditions, cells are adjusted to an O(2) environment biologically read as normoxia. Any sharp departure from that state of normoxia triggers O(2)-sensitive biological responses. The stabilization of hypoxia-inducible factor (HIF) signifies a robust biological readout of hypoxia. In the presence of sufficient O(2), HIF is hydroxylated and degraded. HIF prolyl hydroxylation is catalyzed by prolyl hydroxylase isoenzymes PHD1, 2, and 3. Using HT22 neurons stably transfected with a HIF reporter construct, we tested a novel hypothesis postulating that biological cells are capable of resetting their normoxic set point by O(2)-sensitive changes in PHD expression. Results of this study show that the pO(2) of the mouse brain cortex was 35 mm Hg or 5% O(2). Exposure of HT22, adjusted to growing in 20% O(2), to 5% O(2) resulted in HIF-driven transcription. However, cells adjusted to growing in 5% O(2) did not report hypoxia. Cells adjusted to growing in 30% O(2) reported hypoxia when acutely exposed to room air culture conditions. When grown under high O(2) conditions, cells reset their normoxic set point upward by down-regulating the expression of PHD1-3. When grown under low O(2) conditions, cells reset their normoxic set point downward by inducing the expression of PHD1-3. Exposure of mice in vivo to a hypoxic 10% O(2) environment lowered blood as well as brain pO(2). Such hypoxic exposure induced PHD1-3. Exposure of mice to a hyperoxic 50% O(2) ambience repressed the expression of PHD1-3, indicating that O(2)-sensitive regulation of PHD expression is effective in the brain in vivo. siRNA dependent knockdown of PHD expression revealed that O(2)-sensitive regulation of PHD may contribute to tuning the normoxic set point in biological cells.

Transcriptome analysis of the ischemia-reperfused remodeling myocardium: temporal changes in inflammation and extracellular matrix

cDNA microarray analysis was performed to screen 15,000 genes and expressed sequence tags (ESTs) to identify changes in the ischemia-reperfused (I-R) rat myocardial transcriptome in the early ( day 2) and late ( day 7) inflammatory phases of acute myocardial infarction. Lists of candidate genes that were affected by I-R transiently (2 or 7 days only) or on a more sustained basis (2 and 7 days) were derived. The candidate genes represented three major functional categories: extracellular matrix, apoptosis, and inflammation. To expand on the findings from microarray studies that dealt with the two above-mentioned time points, tissues collected from days 0, 0.25, 2, 3, 5, and 7 after reperfusion were examined. Acute myocardial infarction resulted in upregulation of IL-6 and IL-18. Genes encoding extracellular matrix proteins such as types I and III collagen were upregulated in day 2, and that response progressively grew stronger until day 7 after I-R. Comparable response kinetics was exhibited by the candidate genes of the apoptosis category. Caspases-2, -3, and -8 were induced in response to acute infarction. Compared with the myocardial tissue from the sham-operated rats, tissue collected from the infarct region stained heavily positive for the presence of active caspase-3. Laser microdissection and pressure catapulting technology was applied to harvest infarct and adjacent noninfarct control tissue from a microscopically defined region in the rat myocardium. Taken together, this work presents the first evidence gained from the use of DNA microarrays to understand the molecular mechanisms implicated in the early and late inflammatory phases of the I-R heart.

Laser microdissection and pressure-catapulting technique to study gene expression in the reoxygenated myocardium

For focal events such as myocardial infarction, it is important to dissect infarction-induced biological responses as a function of space with respect to the infarct core. Laser microdissection pressure catapulting (LMPC) represents a recent variant of laser capture microdissection that enables robot-assisted rapid capture of catapulted tissue without direct user contact. This work represents the maiden effort to apply laser capture microdissection to study spatially resolved biological responses in myocardial infarction. Infarcted areas of the surviving ischemic-reperfused murine heart were identified using a standardized hematoxylin QS staining procedure. Standard staining techniques fail to preserve tissue RNA. Exposure of the tissue to an aqueous medium (typically used during standard immunohistochemical staining), with or without RNase inhibitors, resulted in a rapid degradation of genes, with approximately 80% loss in the 1st h. Tissue elements (1 x 10(4)-4 x 10(6) microm(2)) captured from infarcted and noninfarcted sites with micrometer-level surgical precision were collected in a chaotropic RNA lysis solution. Isolated RNA was analyzed for quality by microfluidics technology and reverse transcribed to generate high-quality cDNA. Real-time PCR analysis of the cDNA showed marked (200- and 400-fold, respectively) induction of collagen Ia and IIIa at the infarcted site compared with the noninfarcted site. This work reports a sophisticated yet rapid approach to measurement of relative gene expressions from tissue elements captured from spatially resolved microscopic regions in the heart with micrometer-level precision.

Neuroprotective properties of the natural vitamin E alpha-tocotrienol

BACKGROUND AND PURPOSE

The current work is based on our previous finding that in neuronal cells, nmol/L concentrations of alpha-tocotrienol (TCT), but not alpha-tocopherol (TCP), blocked glutamate-induced death by suppressing early activation of c-Src kinase and 12-lipoxygenase.

METHODS

The single neuron microinjection technique was used to compare the neuroprotective effects of TCT with that of the more widely known TCP. Stroke-dependent brain tissue damage was studied in 12-Lox-deficient mice and spontaneously hypertensive rats orally supplemented with TCT.

RESULTS

Subattomole quantity of TCT, but not TCP, protected neurons from glutamate challenge. Pharmacological as well as genetic approaches revealed that 12-Lox is rapidly tyrosine phosphorylated in the glutamate-challenged neuron and that this phosphorylation is catalyzed by c-Src. 12-Lox-deficient mice were more resistant to stroke-induced brain injury than their wild-type controls. Oral supplementation of TCT to spontaneously hypertensive rats led to increased TCT levels in the brain. TCT-supplemented rats showed more protection against stroke-induced injury compared with matched controls. Such protection was associated with lower c-Src activation and 12-Lox phosphorylation at the stroke site.

CONCLUSIONS

The natural vitamin E, TCT, acts on key molecular checkpoints to protect against glutamate- and stroke-induced neurodegeneration.