Absolute oxygen tension (pO(2)) in murine fatty and muscle tissue as determined by EPR

Surface dielectric resonator for in vivo EPR measurements

This research report describes a novel surface dielectric resonator (SDR) with a flexible connector for in vivo electron paramagnetic resonance (EPR) spectroscopy. Contrary to the conventional cavity or surface loop-gap resonators, the newly developed SDR is constructed from a ceramic dielectric material, and it is tuned to operate at the L-band frequency band (1.15 GHz) in continuous-wave mode. The SDR is designed to be critically coupled and capable of working with both very lossy samples, such as biological tissues, and non-lossy materials. The SDR was characterized using electromagnetic field simulations, assessed for sensitivity with a B1 field-perturbation method, and validated with tissue phantoms using EPR measurements. The results showed remarkably higher sensitivity in lossy tissue phantoms than the previously reported multisegment surface-loop resonators. The new SDR can provide potential new insights for advancements in the application of in vivo EPR spectroscopy for biological measurements, including clinical oximetry.

Hypoxia and Hypoxia Mimetic Agents As Potential Priming Approaches to Empower Mesenchymal Stem Cells

Mesenchymal stem cells (MSC) exhibit self-renewal capacity and multilineage differentiation potential, making them attractive for research and clinical application. The properties of MSC can vary depending on specific micro-environmental factors. MSC resides in specific niches with low oxygen concentrations, where oxygen functions as a metabolic substrate and a signaling molecule. Conventional physical incubators or chemically hypoxia mimetic agents are applied in cultures to mimic the original low oxygen tension settings where MSC originated. This review aims to focus on the current knowledge of the effects of various physical hypoxic conditions and widely used hypoxia-mimetic agents-PHD inhibitors on mesenchymal stem cells at a cellular and molecular level, including proliferation, stemness, differentiation, viability, apoptosis, senescence, migration, immunomodulation behaviors, as well as epigenetic changes.

Effects of selenium deficiency on biological results of X-ray and carbon-ion beam irradiation in mice

The impact of radiation-induced hydrogen peroxide (H₂O₂) on the biological effects of X-rays and carbon-ion beams was investigated using a selenium-deficient (SeD) mouse model. Selenium is the active center of glutathione peroxidase (GSH-Px), and SeD mice lack the ability to degrade H₂O₂. Male and female SeD mice were prepared by feeding a torula yeast-based SeD diet and ultrapure water. Thirty-day survival rates after whole-body irradiation, radiation-induced leg contracture, and MRI-based redox imaging of the brain were assessed and compared between SeD and normal mice. Thirty-day lethality after whole-body 5.6 Gy irradiation with X-rays or carbon-ion beams was higher in the SeD mice than in the normal mice, while SeD did not give the notable difference between X-rays and carbon-ion beams. SeD also did not affect the maximum leg contracture level after irradiation with carbon-ion beams, but delayed the leg contraction rate. In addition, no marked effects of SeD were observed on variations in the redox status of the brain after irradiation. Collectively, the present results indicate that SeD slightly altered the biological effects of X-rays and/or carbon-ion beams. GSH-Px processes endogenous H₂O₂ generated through mitochondrial respiration, but does not have the capacity to degrade H₂O₂ produced by irradiation.

In Vivo Partial Oxygen Pressure Assessment in Subcutaneous and Intraperitoneal Sites Using Imaging of Solid Oxygen Probe

The purpose of this study was to assess the natural partial oxygen pressure (pO₂) of subcutaneous (SC) and intraperitoneal (IP) sites in mice to determine their relative suitability as sites for placement of implants. The pO₂ measurements were performed using oxygen imaging of solid probes using lithium phthalocyanine (LiPc) as the oxygen sensitive material. LiPc is a water-insoluble crystalline probe whose spin-lattice and spin-spin relaxation rates (R₁ and R₂) are sensitive to the local oxygen concentration. To facilitate direct in vivo oxygen imaging, we prepared a solid probe containing encapsulated LiPc crystals in polydimethylsiloxane (PDMS), an oxygen-permeable and bioinert polymer. Although LiPc-PDMS or similar probes have been used in repeated spectroscopic or average oxygen measurements using continuous wave electron paramagnetic resonance (EPR) since the late 1990s and now have advanced to clinical applications, they have not been used for pulse EPR oxygen imaging. One LiPc-PDMS probe of 2 mm diameter and 10 mm length was implanted in SC or IP sites (left or right side) in each animal. The pO₂ imaging of implanted LiPc-PDMS probes was performed weekly for 6 weeks using O2M preclinical 25 mT oxygen imager, JIVA-25™, using the pulse inversion recovery electron spin echo method. At week 6, the probes were recovered, and histological examinations were performed. We report in this study, first-ever solid probe oxygen imaging of implanted devices and pO₂ assessment of SC and IP sites.

Estimation of the Local Concentration of the Markedly Dense Hydroxyl Radical Generation Induced by X-rays in Water

The linear-density (number of molecules on an arbitrary distance) of X-ray-induced markedly dense hydroxyl radicals (•OH) in water was estimated based on EPR spin-trapping measurement. A lower (0.13 mM-2.3 M) concentration series of DMPO water solutions and higher (1.7-6.0 M) concentration series of DMPO water solutions plus neat DMPO liquid (8.8 M as DMPO) were irradiated with 32 Gy of X-rays. Then, the yield of DMPO-OH in DMPO water solutions and the total spin-adduct of DMPO in neat DMPO were quantified. For the higher concentration DMPO series, the EPR peak area was estimated by double integration, and the baseline correction of the integral spectrum is necessary for accurate estimation of the peak area. The preparation of a suitable standard sample corresponding to the electric permittivity according to DMPO concentration was quite important for quantification of DMPO-OH, especially in DMPO concentration beyond 2 M. The linear-density of •OH generation in water by X-ray irradiation was estimated from the inflection point on the plot of the DMPO-OH yield versus DMPO linear-density. The linear-density of X-ray-induced markedly dense •OH was estimated as 1168 μm^-1, which was converted to 0.86 nm as the intermolecular distance and 2.6 M as the local concentration.

Effects of LET on oxygen-dependent and-independent generation of hydrogen peroxide in water irradiated by carbon-ion beams

Linear energy transfer (LET) dependence of yields of O₂-dependent and O₂-independent hydrogen peroxide (H₂O₂) in water irradiated by ionizing radiation was investigated. The radiation-induced hydroxyl radical (•OH) generation in an aqueous solution was reported to occur in two different localization densities, the milli-molar (relatively sparse) and/or molar (markedly-dense) levels. In the milli-molar-level •OH generation atmosphere, •OH generated at a molecular distance of ∼7 nm are likely unable to interact. However, in the molar-level •OH generation atmosphere, several •OH were generated with a molecular distance of 1 nm or less, and two •OH can react to directly make H₂O₂. An aliquot of ultra-pure water was irradiated by 290-MeV/nucleon carbon-ion beams at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, NIRS/QST, Chiba, Japan). Irradiation experiments were performed under aerobic or hypoxic (<0.5% oxygen) conditions, and several LET conditions (13, 20, 40, 60, 80, or >100 keV/μm). H₂O₂ generation in irradiated samples was estimated by three methods. The amount of H₂O₂ generated per dose was estimated and compared. O₂-independent H₂O₂ generation, i.e. H₂O₂ generation under hypoxic conditions, increased with increasing LET. On the other hand, the O₂-dependent H₂O₂ generation, i.e. subtraction of H₂O₂ generation under hypoxic conditions from H₂O₂ generation under aerobic conditions, decreased with increasing LET. This suggests that the markedly-dense •OH generation is positively correlated with LET. High-LET beams generate H₂O₂ in an oxygen-independent manner.

A Multichamber Gas System to Examine the Effect of Multiple Oxygen Conditions on Cell Culture

The classic bone tissue engineering model for bone regeneration combines three elements: scaffolds, biomaterials, and mesenchymal stem cells (MSCs). Incorporation of MSCs and growth factors into a scaffold implanted into the area of bone injury is a proven strategy to achieve successful bone regeneration as demonstrated in the literature. However, a major limitation of using bone grafts or scaffolds is oxygen (O₂) deprivation in the inner sections of the construct, due to lack of adequate vascularization. To address this limitation, we proposed two treatment strategies for MSC-seeded constructs or adipose tissue scaffolds before implantation: (1) O₂ enrichment and (2) acclimation to hypoxia. Based on previous studies, the significance of the different O₂ concentrations on MSC biological characteristics remains controversial. Therefore, the optimal O₂ condition for engineered bone tissues should be determined. Thus, we designed an innovative multichamber gas system aimed to simultaneously assess the effects of different O₂ levels on cell culture. This system was assembled using three isolated chambers integrated into a single incubator. To explore the efficacy of our method, we investigated the effect of hyperoxia, normoxia, and hypoxia, (50-60%, 21%, and 5-7.5% O₂, respectively) on the biological characteristics of human adipose-derived MSCs: immunophenotyping, adhesion, proliferation, and osteogenic, and angiogenic differentiation. Our findings demonstrated that hypoxic adipose-derived mesenchymal stem cells (ASCs) conditions exhibited significantly lower levels of CD34 (p = 0.014), with significantly higher osteogenic and angiogenic differentiation capacities (p = 0.023 and p = 0.0042, respectively) than normoxia. Conversely, hyperoxia-cultured ASCs demonstrated significantly higher levels of CD73 and CD90 expression than both normoxic ASCs (p = 0.006 and p = 0.025, respectively) and hypoxic ASCs (p = 0.003 and p = 0.003, respectively). In addition, hyperoxic ASCs showed significantly reduced proliferation capacity by day 11 (p = 0.032) and significantly enhanced migration rates after 48 h (p = 0.044). The newly developed controllable multichamber gas system was cost-effective and easy to use. Different assays can be performed concurrently while preserving all other conditions identical, and the use of other ranges of O₂ concentrations is feasible and also necessary to determine the ideal O₂ concentration. Furthermore, the multichamber gas system has the potential for wide application, including other cell cultures, grafts, or scaffolds for in vitro and in vivo experimentation. This study was approved by the Galilee Medical Center Helsinki Committee (No. 0009-19-NHR). Impact statement The introduced multichamber gas system provides a custom-made setup for simultaneous control of three oxygen (O₂) levels in a single incubator. The use of our innovative multichamber gas system is essential to determine the ideal O₂ levels for engineered tissues by examining multiple O₂ concentrations on cells in vitro. The determined ideal O₂ concentration will then be used through this system to investigate the engrafted cell survival ex vivo, to ensure successful integration of the engineered tissues and tissue regeneration in vivo. Use of this method may promote a therapeutic tool for a major limitation in tissue engineering due to the problematic O₂ insufficiency in tissue scaffolds.

Effects of oxygen challenging to tissue redox and pO2 status

Nitroxide free radicals can serve as redox-sensitive MRI contrast agents useful to image the redox status of tissue of interest. In this study, the effect of oxygen content in the inspired gas on the kinetics of metabolism of three nitroxides has been evaluated in the muscle and tumor in mice. SCC tumors (approximate size of 1.0 cm³) on the right hind leg of female C3H/Hen MTV^- mice were prepared. Three nitroxides, 3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CxP), 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), and 4-hydroxy-tetramethylpiperidine-N-oxyl (TEMPOL), having different lipophilicities were compared using MR redox imaging. T₁-mapping of the tissues was obtained using a multi-slice multi-echo (MSME) sequence with several TRs. The three nitroxides showed differences in accumulation and metabolism/clearance in muscle and tumor. The cell impermeable nitroxide CxP displayed kinetic patterns of slow enhancement followed by a slow decline typical of clearance rather than metabolism. The cell permeable CmP on the other hand showed a relatively faster uptake and metabolism with a modestly higher rate of metabolism in the tumor than muscle. The TEMPOL on the other hand displayed a rapid uptake and reduction with a trend of significantly rapid decay rate in tumor tissue, while slightly higher maximum signal intensity and slower decay rate was observed in normal muscle. The reduction rate of TEMPOL in the tumor was significantly enhanced when the breathing gas had 100%-oxygen while it was not significantly different in the muscle. EPR oximetry studies monitoring the oxygen dependent linewidth of TEMPOL showed that the pO₂ in the healthy tissue during carbogen breathing significantly increased normal tissue pO₂ compared to air breathing whereas breathing 100%-oxygen made normal tissue slight hypoxic. Since TEMPOL is a radioprotector, our studies show that a combination of 100%-oxygen breathing and TEMPOL has a potential to enhance radioprotective effects to normal tissue.

Muscle hypertrophy following blood flow-restricted, low-force isometric electrical stimulation in rat tibialis anterior: role for muscle hypoxia

Low-force exercise training with blood flow restriction (BFR) elicits muscle hypertrophy as seen typically after higher-force exercise. We investigated the effects of microvascular hypoxia [i.e., low microvascular O2 partial pressures (P mvO2)] during contractions on muscle hypertrophic signaling, growth response, and key muscle adaptations for increasing exercise capacity. Wistar rats were fitted with a cuff placed around the upper thigh and inflated to restrict limb blood flow. Low-force isometric contractions (30 Hz) were evoked via electrical stimulation of the tibialis anterior (TA) muscle. The P mvO2 was determined by phosphorescence quenching. Rats underwent acute and chronic stimulation protocols. Whereas P mvO2 decreased transiently with 30 Hz contractions, simultaneous BFR induced severe hypoxia, reducing P mvO2 lower than present for maximal (100 Hz) contractions. Low-force electrical stimulation (EXER) induced muscle hypertrophy (6.2%, P < 0.01), whereas control group conditions or BFR alone did not. EXER+BFR also induced an increase in muscle mass (11.0%, P < 0.01) and, unique among conditions studied, significantly increased fiber cross-sectional area in the superficial TA ( P < 0.05). Phosphorylation of ribosomal protein S6 was enhanced by EXER+BFR, as were peroxisome proliferator-activated receptor gamma coactivator-1α and glucose transporter 4 protein levels. Fibronectin type III domain-containing protein 5, cytochrome c oxidase subunit 4, monocarboxylate transporter 1 (MCT1), and cluster of differentiation 147 increased with EXER alone. EXER+BFR significantly increased MCT1 expression more than EXER alone. These data demonstrate that microvascular hypoxia during contractions is not essential for hypertrophy. However, hypoxia induced via BFR may potentiate the muscle hypertrophic response (as evidenced by the increased superficial fiber cross-sectional area) with increased glucose transporter and mitochondrial biogenesis, which contributes to the pleiotropic effects of exercise training with BFR that culminate in an improved capacity for sustained exercise.

NEW & NOTEWORTHY We investigated the effects of low microvascular O2 partial pressures (P mvO2) during contractions on muscle hypertrophic signaling and key elements in the muscle adaptation for increasing exercise capacity. Although demonstrating that muscle hypoxia is not obligatory for the hypertrophic response to low-force, electrically induced muscle contractions, the reduced P mvO2 enhanced ribosomal protein S6 phosphorylation and potentiated the hypertrophic response. Furthermore, contractions with blood flow restriction increased oxidative capacity, glucose transporter, and mitochondrial biogenesis, which are key determinants of the pleiotropic effects of exercise training.

Hypoxia Signaling Pathway in Stem Cell Regulation: Good and Evil

Purpose of Review

This review summarizes the role of hypoxia and hypoxia-inducible factors (HIFs) in the regulation of stem cell biology, specifically focusing on maintenance, differentiation, and stress responses in the context of several stem cell systems. Stem cells for different lineages/tissues reside in distinct niches, and are exposed to diverse oxygen concentrations. Recent studies have revealed the importance of the hypoxia signaling pathway for stem cell functions.

Recent Findings

Hypoxia and HIFs contribute to maintenance of embryonic stem cells, generation of induced pluripotent stem cells, functionality of hematopoietic stem cells, and survival of leukemia stem cells. Harvest and collection of mouse bone marrow and human cord blood cells in ambient air results in fewer hematopoietic stem cells recovered due to the phenomenon of Extra PHysiologic Oxygen Shock/Stress (EPHOSS).

Summary

Oxygen is an important factor in the stem cell microenvironment. Hypoxia signaling and HIFs play important roles in modeling cellular metabolism in both stem cells and niches to regulate stem cell biology, and represent an additional dimension that allows stem cells to maintain an undifferentiated status and multilineage differentiation potential.

EPR-based oximetric imaging: a combination of single point-based spatial encoding and T1 weighting

PURPOSE

Spin-lattice relaxation rate (R₁ )-based time-domain EPR oximetry is reported for in vivo applications using a paramagnetic probe, a trityl-based Oxo71.

METHODS

The R₁ dependence of the trityl probe Oxo71 on partial oxygen pressure (pO₂ ) was assessed using single-point imaging mode of spatial encoding combined with rapid repetition, similar to T₁ -weighted MRI, for which R₁ was determined from 22 repetition times ranging from 2.1 to 40.0 μs at 300 MHz. The pO₂ maps of a phantom with 3 tubes containing 2 mM Oxo71 solutions equilibrated at 0%, 2%, and 5% oxygen were determined by R₁ and apparent spin-spin relaxation rate ( R2*) simultaneously.

RESULTS

The pO₂ maps derived from R₁ and R2* agreed with the known pO₂ levels in the tubes of Oxo71. However, the histograms of pO₂ revealed that R₁ offers better pO₂ resolution than R2* in low pO₂ regions. The SDs of pixels at 2% pO₂ (15.2 mmHg) were about 5 times lower in R₁ -based estimation than R2*-based estimation (mean ± SD: 13.9 ± 1.77 mmHg and 18.3 ± 8.70 mmHg, respectively). The in vivo pO₂ map obtained from R₁ -based assessment displayed a homogeneous profile in low pO₂ regions in tumor xenografts, consistent with previous reports on R2*-based oximetric imaging. The scan time to obtain the R₁ map can be significantly reduced using 3 repetition times ranging from 4.0 to 12.0 μs.

CONCLUSION

Using the single-point imaging modality, R₁ -based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated.

Comparative studies with EPR and MRI on the in vivo tissue redox status estimation using redox-sensitive nitroxyl probes: influence of the choice of the region of interest

In vivo decay rates of a nitroxyl contrast agent were estimated by a MR redox imaging (MRRI) technique and compared with the decay rates obtained by the electron paramagnetic resonance spectroscopy (EPRS) and imaging (EPRI). MRRI is a dynamic imaging technique employing T₁-weighted pulse sequence, which can visualise a nitroxyl-induced enhancement of signal intensity by T₁-weighted contrast. EPR techniques can directly measure the paramagnetic nitroxyl radical. Both the squamous cell carcinoma (SCC) tumour-bearing and normal legs of a female C3H mouse were scanned by T₁-weighted SPGR sequence at 4.7 T with the nitroxyl radical, carbamoyl-proxyl (CmP), as the contrast agent. Similarly, the time course of CmP in normal muscle and tumour tissues was obtained using a 700-MHz EPR spectrometer with a surface coil. The time course imaging of CmP was also performed by 300 MHz CW EPR imager. EPRS and EPRI gave slower decay rates of CmP compared to the MRRI. Relatively slow decay rate at peripheral region of the tumour tissues, which was found in the image obtained by MRRI, may contribute to the slower decay rates observed by EPRS and/or the EPRI measurements. To reliably determine the tissue redox status from the reduction rates of nitroxyls such as CmP, heterogenic structure in the tumour tissue must be considered. The high spatial and temporal resolution of T₁-weighted MRI and the T₁-enhancing capabilities of nitroxyls support the use of this method to map tissue redox status which can be a useful biomarker to guide appropriate treatments based on the tumour microenvironment.

Effect of body temperature on the pharmacokinetics of a triarylmethyl-type paramagnetic contrast agent used in EPR oximetry

PURPOSE

Pharmacokinetics of the tri[8-carboxy-2,2,6,6-tetrakis(2-hydroxymethyl)benzo[1,2-d:4,5-d']bis(1,3)dithio-4-yl]methyl radical (Oxo63) after a single bolus and/or continuous intravenous infusion was investigated in tumor-bearing C3H mice with or without body temperature control while under anesthesia.

METHOD

The in vivo time course of Oxo63 in blood was measured using X-band electron paramagnetic resonance spectroscopy. Distribution of Oxo63 in normal muscle and tumor tissues was obtained using a surface coil resonator and a 700-MHz electron paramagnetic resonance spectrometer. The whole-body distribution of Oxo63 was obtained by 300-MHz continuous-wave electron paramagnetic resonance imaging. The high-resolution 300-MHz time-domain electron paramagnetic resonance imaging was also carried out to probe the distribution of Oxo63.

RESULTS

Urination of mice was retarded at low body temperature, causing the concentration of Oxo63 in blood to attain high levels. However, the concentration of Oxo63 in tumor tissue was lower with no control of body temperature than active body temperature control. The nonsystemized blood flow in the tumor tissues may pool Oxo63 at lower body temperature.

CONCLUSIONS

Pharmacokinetics of the contrast agent were found to be significantly affected by body temperature of the experimental animal, and can influence the probe distribution and the image patterns. Magn Reson Med 79:1212-1218, 2018. © Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

The importance of controlling in vitro oxygen tension to accurately model in vivo neurophysiology

The majority of in vitro studies modeling in vivo conditions are performed on the lab bench in atmospheric air. However, the oxygen tension (pO₂) present in atmospheric air (160mm Hg, ∼21% O₂) is in great excess to the pO₂ that permeates tissues within the brain (5-45mm Hg, ∼1-6% O₂). This review will discuss the differentiation between pO₂ in the in vivo environment and the pO₂ commonly used during in vitro experiments, and how this could affect assay outcomes. Also highlighted are studies linking changes in pO₂ to changes in cellular function, particularly the role of pO₂ in mitochondrial function, reactive oxygen species production, and cellular growth and differentiation. The role of hypoxia inducible factor 1 and oxygen sensing is also presented. Finally, emerging literature exploring sex differences in tissue oxygenation is discussed.

In vivo electron paramagnetic resonance oximetry and applications in the brain

Molecular oxygen (O₂) is essential to brain function and mechanisms necessary to regulate variations in delivery or utilization of O₂ are crucial to support normal brain homeostasis, physiology and energy metabolism. Any imbalance in cerebral tissue partial pressure of O₂ (pO₂) levels may lead to pathophysiological complications including increased reactive O₂ species generation leading to oxidative stress when tissue O₂ level is too high or too low. Accordingly, the need for oximetry methods, which assess cerebral pO₂in vivo and in real time, is imperative to understand the role of O₂ in various metabolic and disease states, including the effects of treatment and therapy options. In this review, we provide a brief overview of the common in vivo oximetry methodologies for measuring cerebral pO₂. We discuss the advantages and limitations of oximetry methodologies to measure cerebral pO₂in vivo followed by a more in-depth review of electron paramagnetic resonance oximetry spectroscopy and imaging using several examples of current electron paramagnetic resonance oximetry applications in the brain.

Effect of amifostine, a radiation-protecting drug, on oxygen concentration in tissue measured by EPR oximetry and imaging

Effect of amifostine, a radiation-protecting drug, on muscle tissue partial pressure of oxygen was investigated by electron paramagnetic resonance spectroscopy and imaging. When amifostine was administered intraperitoneally or intravenously to mice, the linewidth of the electron paramagnetic resonance spectra of the lithium octa-n-butoxy-substituted naphthalocyanine implanted in the mouse leg muscle decreased. Electron paramagnetic resonance oximetry using a lithium octa-n-butoxy-substituted naphthalocyanine probe and electron paramagnetic resonance oxygen mapping using a triarylmethyl radical paramagnetic probe was useful to quantify pressure of oxygen in the tissues of living mice. The result of electron paramagnetic resonance oximetric imaging showed that administration of amifostine could decrease pressure of oxygen in the muscle and also tumor tissues. This finding suggests that lowering pressure of oxygen in tissues might contribute in part to the radioprotection of amifostine.

Hypoxia enhances the viability, growth and chondrogenic potential of cryopreserved human adipose-derived stem cells

Cryopreservation is the only existing method of storage of human adipose-derived stem cells (ASCs) for clinical use. However, cryopreservation has been shown to be detrimental to ASCs, particularly in term of cell viability. To restore the viability of cryopreserved ASCs, it is proposed to culture the cells in a hypoxic condition. To this end, we aim to investigate the effect of hypoxia on the cryopreserved human ASCs in terms of not only cell viability, but also their growth and stemness properties, which have not been explored yet. In this study, human ASCs were cultured under four different conditions: fresh (non-cryopreserved) cells cultured in 1) normoxia (21% O₂) and 2) hypoxia (2% O₂) and cryopreserved cells cultured in 3) normoxia and 4) hypoxia. ASCs at passage 3 were subjected to assessment of viability, proliferation, differentiation, and expression of stemness markers and hypoxia-inducible factor-1 alpha (HIF-1α). We found that hypoxia enhances the viability and the proliferation rate of cryopreserved ASCs. Further, hypoxia upregulates HIF-1α in cryopreserved ASCs, which in turn activates chondrogenic genes to promote chondrogenic differentiation. In conclusion, hypoxic-preconditioned cryopreserved ASCs could be an ideal cell source for cartilage repair and regeneration.

Hypoxic culture conditions induce increased metabolic rate and collagen gene expression in ACL-derived cells

There has been substantial effort directed toward the application of bone marrow and adipose-derived mesenchymal stromal cells (MSCs) in the regeneration of musculoskeletal tissue. Recently, resident tissue-specific stem cells have been described in a variety of mesenchymal structures including ligament, tendon, muscle, cartilage, and bone. In the current study, we systematically characterize three novel anterior cruciate ligament (ACL)-derived cell populations with the potential for ligament regeneration: ligament-forming fibroblasts (LFF: CD146(neg) , CD34(neg) CD44(pos) , CD31(neg) , CD45(neg) ), ligament perivascular cells (LPC: CD146(pos) CD34(neg) CD44(pos) , CD31(neg) , CD45(neg) ) and ligament interstitial cells (LIC: CD34(pos) CD146(neg) , CD44(pos) , CD31(neg) , CD45(neg) )-and describe their proliferative and differentiation potential, collagen gene expression and metabolism in both normoxic and hypoxic environments, and their trophic potential in vitro. All three groups of cells (LIC, LPC, and LFF) isolated from adult human ACL exhibited progenitor cell characteristics with regard to proliferation and differentiation potential in vitro. Culture in low oxygen tension enhanced the collagen I and III gene expression in LICs (by 2.8- and 3.3-fold, respectively) and LFFs (by 3- and 3.5-fold, respectively) and increased oxygen consumption rate and extracellular acidification rate in LICs (by 4- and 3.5-fold, respectively), LFFs (by 5.5- and 3-fold, respectively), LPCs (by 10- and 4.5-fold, respectively) as compared to normal oxygen concentration. In summary, this study demonstrates for the first time the presence of three novel progenitor cell populations in the adult ACL that demonstrate robust proliferative and matrix synthetic capacity; these cells may play a role in local ligament regeneration, and consequently represent a potential cell source for ligament engineering applications. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:985-994, 2016.

Direct Measurements of Oxygen Gradients in Spheroid Culture System Using Electron Parametric Resonance Oximetry

Advanced in vitro culture from tissues of different origin includes three-dimensional (3D) organoid micro structures that may mimic conditions in vivo. One example of simple 3D culture is spheroids; ball shaped structures typically used as liver and tumour models. Oxygen is critically important in physiological processes, but is difficult to quantify in 3D culture: and the question arises, how small does a spheroid have to be to have minimal micro-environment formation? This question is of particular importance in the growing field of 3D based models for toxicological assessment. Here, we describe a simple non-invasive approach modified for the quantitative measurement and subsequent evaluation of oxygen gradients in spheroids developed from a non-malignant fish cell line (i.e. RTG-2 cells) using Electron Paramagnetic Resonance (EPR) oximetry. Sonication of the paramagnetic probe Lithium phthalocyanine (LiPc) allows for incorporation of probe particulates into spheroid during its formation. Spectra signal strength after incorporation of probe into spheroid indicated that a volume of 20 μl of probe (stock solution: 0.10 mg/mL) is sufficient to provide a strong spectra across a range of spheroid sizes. The addition of non-toxic probes (that do not produce or consume oxygen) report on oxygen diffusion throughout the spheroid as a function of size. We provide evidence supporting the use of this model over a range of initial cell seeding densities and spheroid sizes with the production of oxygen distribution as a function of these parameters. In our spheroid model, lower cell seeding densities (∼2,500 cells/spheroid) and absolute size (118±32 μm) allow control of factors such as pre-existing stresses (e.g. ∼ 2% normoxic/hypoxic interface) for more accurate measurement of treatment response. The applied methodology provides an elegant, widely applicable approach to directly characterize spheroid (and other organoid) cultures in biomedical and toxicological research.

Repetitive restriction of muscle blood flow enhances mTOR signaling pathways in a rat model

Skeletal muscle is a plastic organ that adapts its mass to various stresses by affecting pathways that regulate protein synthesis and degradation. This study investigated the effects of repetitive restriction of muscle blood flow (RRMBF) on microvascular oxygen pressure (PmvO2), mammalian target of rapamycin (mTOR) signaling pathways, and transcripts associated with proteolysis in rat skeletal muscle. Eleven-week-old male Wistar rats under anesthesia underwent six RRMBF consisting of an external compressive force of 100 mmHg for 5 min applied to the proximal portion of the right thigh, each followed by 3 min rest. During RRMBF, PmvO2 was measured by phosphorescence quenching techniques. The total RNA and protein of the tibialis anterior muscle were obtained from control rats, and rats treated with RRMBF 0-6 h after the stimuli. The protein expression and phosphorylation of various signaling proteins were determined by western blotting. The mRNA expression level was measured by real-time RT-PCR analysis. The total muscle weight increased in rats 0 h after RRMBF, but not in rats 1-6 h. During RRMBF, PmvO2 significantly decreased (36.1 ± 5.7 to 5.9 ± 1.7 torr), and recovered at rest period. RRMBF significantly increased phosphorylation of p70 S6-kinase (p70S6k), a downstream target of mTOR, and ribosomal protein S6 1 h after the stimuli. The protein level of REDD1 and phosphorylation of AMPK and MAPKs did not change. The mRNA expression levels of FOXO3a, MuRF-1, and myostatin were not significantly altered. These results suggested that RRMBF significantly decreased PmvO2, and enhanced mTOR signaling pathways in skeletal muscle using a rat model, which may play a role in diminishing muscle atrophy under various conditions in human studies.

The effects of hypoxia and serum-free conditions on the stemness properties of human adipose-derived stem cells

The need to have a better and safer culture condition for expansion of human mesenchymal stem cells (MSCs) is crucial particularly to prevent infection and immune rejection. This is normally associated with the use of animal-based serum in the culture media for cell expansion. The aim of this study is to investigate alternative culture conditions which may provide better and safer environment for cell growth. In the present study, human adipose-derived stem cells (ASCs) at passage 3 were subjected to treatment in 4 conditions: (1) 21 % O2 with fetal bovine serum (FBS), (2) 21 % O2 without FBS, (3) 2 % O2 with FBS and (4) 2 % O2 without FBS followed by subsequent analysis of their phenotype, viability and functionality. We observed that ASCs cultured in all conditions present no significant phenotypic changes. It was found that ASCs cultured in 2 % O2 without serum showed an increase in viability and growth to a certain extent when compared to those cultured in 21 % O2 without serum. However, ASCs cultured in 2 % O2 without serum displayed a relatively low adipogenic and osteogenic potential. On the other hand, interestingly, there was a positive enhancement in chondrogenic differentiation of ASCs cultured in 21 % O2 without serum. Our findings suggest that different culture conditions may be suitable for different indications. In summary, ASCs cultured in serum-free condition can still survive, proliferate and undergo subsequent adipogenic, osteogenic and chondrogenic differentiation. Therefore, FBS is feasible to be excluded for culture of ASCs, which avoids clinical complications.

Application of Electron Paramagnetic Resonance (EPR) Oximetry to Monitor Oxygen in Wounds in Diabetic Models

A lack of oxygen is classically described as a major cause of impaired wound healing in diabetic patients. Even if the role of oxygen in the wound healing process is well recognized, measurement of oxygen levels in a wound remains challenging. The purpose of the present study was to assess the value of electron paramagnetic resonance (EPR) oximetry to monitor pO2 in wounds during the healing process in diabetic mouse models. Kinetics of wound closure were carried out in streptozotocin (STZ)-treated and db/db mice. The pO2 was followed repeatedly during the healing process by 1 GHz EPR spectroscopy with lithium phthalocyanine (LiPc) crystals used as oxygen sensor in two different wound models: a full-thickness excisional skin wound and a pedicled skin flap. Wound closure kinetics were dramatically slower in 12-week-old db/db compared to control (db/+) mice, whereas kinetics were not statistically different in STZ-treated compared to control mice. At the center of excisional wounds, measurements were highly influenced by atmospheric oxygen early in the healing process. In pedicled flaps, hypoxia was observed early after wounding. While reoxygenation occurred over time in db/+ mice, hypoxia was prolonged in the diabetic db/db model. This observation was consistent with impaired healing and microangiopathies observed using intravital microscopy. In conclusion, EPR oximetry using LiPc crystals as the oxygen sensor is an appropriate technique to follow wound oxygenation in acute and chronic wounds, in normal and diabetic animals. Nevertheless, the technique is limited for measurements in pedicled skin flaps and cannot be applied to excisional wounds in which diffusion of atmospheric oxygen significantly affects the measurements.

In situ normoxia enhances survival and proliferation rate of human adipose tissue-derived stromal cells without increasing the risk of tumourigenesis

Adipose tissue-derived stromal cells (ASCs) natively reside in a relatively low-oxygen tension (i.e., hypoxic) microenvironment in human body. Low oxygen tension (i.e., in situ normoxia), has been known to enhance the growth and survival rate of ASCs, which, however, may lead to the risk of tumourigenesis. Here, we investigated the tumourigenic potential of ASCs under their physiological condition to ensure their safe use in regenerative therapy. Human ASCs isolated from subcutaneous fat were cultured in atmospheric O₂ concentration (21% O₂) or in situ normoxia (2% O₂). We found that ASCs retained their surface markers, tri-lineage differentiation potential, and self-renewal properties under in situ normoxia without altering their morphology. In situ normoxia displayed a higher proliferation and viability of ASCs with less DNA damage as compared to atmospheric O₂ concentration. Moreover, low oxygen tension significantly up-regulated VEGF and bFGF mRNA expression and protein secretion while reducing the expression level of tumour suppressor genes p16, p21, p53, and pRb. However, there were no significant differences in ASCs telomere length and their relative telomerase activity when cultured at different oxygen concentrations. Collectively, even with high proliferation and survival rate, ASCs have a low tendency of developing tumour under in situ normoxia. These results suggest 2% O₂ as an ideal culture condition for expanding ASCs efficiently while maintaining their characteristics.

Activation of p38, p21, and NRF-2 mediates decreased proliferation of human dental pulp stem cells cultured under 21% O2

High rates of stem cell proliferation are important in regenerative medicine and in stem cell banking for clinical use. Ambient oxygen tensions (21% O₂) are normally used for in vitro culture, but physiological levels in vivo range between 3% and 6% O₂. We compared proliferation of human dental pulp stem cells (hDPSCs) cultured under 21% versus 3% O₂. The rate of hDPSC proliferation is significantly lower at 21% O₂ compared to physiological oxygen levels due to enhanced oxidative stress. Under 21% O_2, increased p38 phosphorylation led to activation of p21. Increased generation of reactive oxygen species and p21 led to activation of the NRF-2 signaling pathway. The upregulation of NRF-2 antioxidant defense genes under 21% O₂ may interact with cell-cycle-related proteins involved in regulating cell proliferation. Activation of p38/p21/NRF-2 in hDPSCs cultured under ambient oxygen tension inhibits stem cell proliferation and upregulates NRF-2 antioxidant defenses.

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hDPSC proliferation rate is significantly lower at 21% O₂ than at 3% O₂

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This is due to increased levels oxidative stress at 21% O₂

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Oxidative stress activates the cell signaling pathway p38 →p21 → NRF-2

Scavenging of reactive oxygen species induced by hyperthermia in biological fluid

The scavenging activity of rat plasma against hyperthermia-induced reactive oxygen species was tested. The glutathione-dependent reduction of a nitroxyl radical, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, which was restricted by adding superoxide dismutase or by deoxygenating the reaction mixture, was applied to an index of superoxide (O₂^•−) generation. A reaction mixture containing 0.1 mM 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl and 1 mM glutathione was prepared using 100 mM phosphate buffer containing 0.05 mM diethylenetriaminepentaacetic acid. The reaction mixture was kept in a screw-top vial and incubated in a water bath at 37 or 44°C. The time course of the electron paramagnetic resonance signal of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl in the reaction mixture was measured by an X-band EPR spectrometer (JEOL, Tokyo, Japan). When the same experiment was performed using rat plasma instead of 100 mM PB, the glutathione-dependent reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, i.e., generation of O₂^•−, was not obtained. Only the first-order decay reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, which indicates direct reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, was obtained in rat plasma. Adding 0.5% albumin to the phosphate buffer reaction mixture could almost completely inhibit O₂^•− generation at 37°C. However, addition of 0.5% albumin could not inhibit O₂^•− generation at 44°C, i.e., hyperthermic temperature. Ascorbic acid also showed inhibition of O₂^•− generation by 0.01 mM at 37°C, but 0.02 mM or more could inhibit O₂^•− generation at 44°C. A higher concentration of ascorbic acid showed first-order reduction, i.e., direct one-electron reduction, of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl. Hyperthermia-induced O₂^•− generation in rat plasma can be mostly inhibited by albumin and ascorbic acid in the plasma.

Emerging evidence of the physiological role of hypoxia in mammary development and lactation

Hypoxia is a physiological or pathological condition of a deficiency of oxygen supply in the body as a whole or within a tissue. During hypoxia, tissues undergo a series of physiological responses to defend themselves against a low oxygen supply, including increased angiogenesis, erythropoiesis, and glucose uptake. The effects of hypoxia are mainly mediated by hypoxia-inducible factor 1 (HIF-1), which is a heterodimeric transcription factor consisting of α and β subunits. HIF-1β is constantly expressed, whereas HIF-1α is degraded under normal oxygen conditions. Hypoxia stabilizes HIF-1α and the HIF complex, and HIF then translocates into the nucleus to initiate the expression of target genes. Hypoxia has been extensively studied for its role in promoting tumor progression, and emerging evidence also indicates that hypoxia may play important roles in physiological processes, including mammary development and lactation. The mammary gland exhibits an increasing metabolic rate from pregnancy to lactation to support mammary growth, lactogenesis, and lactation. This process requires increasing amounts of oxygen consumption and results in localized chronic hypoxia as confirmed by the binding of the hypoxia marker pimonidazole HCl in mouse mammary gland. We hypothesized that this hypoxic condition promotes mammary development and lactation, a hypothesis that is supported by the following several lines of evidence: i) Mice with an HIF-1α deletion selective for the mammary gland have impaired mammary differentiation and lipid secretion, resulting in lactation failure and striking changes in milk compositions; ii) We recently observed that hypoxia significantly induces HIF-1α-dependent glucose uptake and GLUT1 expression in mammary epithelial cells, which may be responsible for the dramatic increases in glucose uptake and GLUT1 expression in the mammary gland during the transition period from late pregnancy to early lactation; and iii) Hypoxia and HIF-1α increase the phosphorylation of signal transducers and activators of transcription 5a (STAT5a) in mammary epithelial cells, whereas STAT5 phosphorylation plays important roles in the regulation of milk protein gene expression and mammary development. Based on these observations, hypoxia effects emerge as a new frontier for studying the regulation of mammary development and lactation.

Hypoxia induces osteogenesis in rabbit adipose-derived stem cells overexpressing bone morphogenic protein-2

OBJECTIVE

Hypoxic culture potentiates mesenchymal stem cells (MSCs) to survive and secrete various growth factors. Genetically modified stem cells overexpressing bone morphogenic protein-2 (BMP-2) demonstrate strong osteogenic ability. Hence, we investigated the coeffect of hypoxic culture conditions and BMP-2 overexpression on the osteogenic ability of rabbit adipose-derived stem cells (rASCs) in vitro.

MATERIALS AND METHODS

Rabbit adipose-derived stem cells with or without adenoviral-BMP-2 transduction were cultured in hypoxic (1%) and normoxic (21%) conditions. Cell viability, attachment, and proliferation were compared. Real-time PCR amplification of osteogenic and angiogenic genes including alkaline phosphatase (ALP), osteocalcin (OCN), HIF-1α, and vascular endothelial growth factor (VEGF) was performed. Moreover, ALP activity, immunofluorescent staining of OCN, and mineralization assay by alizarin red S quantification and von Kossa staining were conducted.

RESULTS

Cells under hypoxic conditions attached better within 12 h and proliferated faster. While BMP-2 overexpression and hypoxic condition separately elevated the transcription of key osteogenic and angiogenic genes, a cooperative effect was observed to enhance the upregulation of osteogenic as well as angiogenic genes. Identical changes were observed in ALP activity, immunofluorescent staining of OCN, and mineralization assay.

CONCLUSIONS

Hypoxic culture can enhance the osteogenic ability of BMP-2 gene-modified rASCs, which provides a strategy to improve the osteogenesis of rASCs for in vivo bone regeneration.

Low oxygen tension enhances proliferation and maintains stemness of adipose tissue-derived stromal cells

Recent evidence indicates that low oxygen tension or hypoxia alters the characteristics of stem cells. The actions of hypoxia are mediated through the hypoxia-inducible factor, a critical mediator of the cellular response to hypoxia. Adipose tissue–derived stromal cells (ASCs) are one of the most promising cell sources for tissue engineering applications. This study investigated the effect of hypoxia on ASCs in terms of the ability to proliferate and differentiate. ASCs were extracted from mice and maintained under hypoxic atmosphere (2% O₂) for up to eight in vitro passages. The proliferation rate was examined as a growth curve, and the potency of differentiation was evaluated. To investigate the cell characteristics, we checked several stem-cell markers and growth factors. Compared with the normoxic state (20% O₂), hypoxia enhances proliferation with an approximately six- to sevenfold higher ASC expansion over 6 weeks. The expression of Oct3/4 and Nanog (stem-cell marker) and the amount of secreted growth factors were increased under the hypoxic condition. These results suggest that low oxygen tension enhances proliferation and maintains stemness of ASCs. Thus, this study emphasizes the profitability of hypoxic culture for expansion of ASCs and maintenance of their undifferentiated state for further therapeutic use.

Low ATP level is sufficient to maintain the uncommitted state of multipotent mesenchymal stem cells

BACKGROUND

Multipotent mesenchymal stromal cells (MMSCs) are minimally differentiated precursors with great potential to transdifferentiate. These cells are quite resistant to oxygen limitation, suggesting that a hypoxic milieu can be physiological for MMSCs.

METHODS

Human MMSCs isolated from adipose tissue were grown at various oxygen concentrations. Alteration in cell immunophenotype was determined by flow cytometry after staining with specific antibodies. Concentrations of glucose and lactate were determined using the Biocon colorimetric test. Cellular respiration was assessed using oxygen electrode. The modes of cell death were analyzed by flow cytometry after staining with Annexin V and propidium iodide.

RESULTS

We found that permanent oxygen deprivation attenuated cellular ATP levels in these cells, diminishing mitochondrial ATP production but stimulating glycolytic ATP production. At the same time, permanent hypoxia did not affect MMSCs' viability, stimulated their proliferation and reduced their capacity to differentiate. Further, permanent hypoxia decreased spontaneous cell death by MMSCs.

CONCLUSIONS

Under hypoxic conditions glycolysis provides sufficient energy to maintain MMSCs in an uncommitted state.

GENERAL SIGNIFICANCE

These findings are of interest not only for scientific reasons, but also in practical terms. Oxygen concentration makes an essential contribution to MMSC physiology and should be taken into account in the setting of protocols for cellular therapy.

Hypoxia and visualization of the stem cell niche

It is widely accepted that mammalian stem cells reside in a specialized cellular and a cellular microenvironment called the niche. The niche contrary to other tissues is characterized by a low partial Oxygen pressure (ppO2). This microenvironment protects stem cells from deleterious effects of O2 on proteins and DNA, through the production of reactive oxygen species (ROS). In addition there is now solid evidence that this physiological hypoxia helps stem cells maintaining their major characteristics: multipotency and ability to differentiate and migrate from the niche to specialized tissues in order to fulfill the needs of the organism. Immuno Histological techniques can stain stem cells in situ by specific Abs (such as against CD34 and CD45 for Hematopoietic Stem Cells HSC). However, a universal marker of hypoxia is Hypoxia-Inducible Factor-1, HIF-1, which is stabilized by low ppO2 and acts as a transcription factor to regulate a vast array of genes downstream. HIF-1, together with pimonidazole, a chemical compound interacting with proteins that are reduced in a hypoxic environment, are bona fide markers of the stem cell niche.

Where it's at really matters: in situ in vivo vascular endothelial growth factor spatially correlates with electron paramagnetic resonance pO2 images in tumors of living mice

Purpose

Tumor microenvironments show remarkable tumor pO₂ heterogeneity, as seen in prior EPR pO₂ images (EPROI). pO₂ correlation with hypoxia response proteins is frustrated by large rapid pO₂ changes with position.

Procedures

To overcome this limitation, biopsies stereotactically located in the EPROI were used to explore the relationship between vascular endothelial growth factor A (VEGF) concentrations in living mouse tumors and the local EPROI pO₂.

Results

Quantitative ELISA VEGF concentrations correlated (p = 0.0068 to 0.019) with mean pO₂, median pO₂, and the fraction of voxels in the biopsy volume with pO₂ less than 3, 6, and 10 Torr.

Conclusions

This validates EPROI hypoxic fractions at the molecular level and provides a new paradigm for the assessment of the relationship, in vivo, between hypoxia and hypoxia response proteins. When translated to human subjects, this will enhance understanding of human tumor pathophysiology and cancer response to therapy.

Genome integrity, stem cells and hyaluronan

Faithful preservation of genome integrity is the critical mission of stem cells as well as of germ cells. Reviewed are the following mechanisms involved in protecting DNA in these cells: (a) The efflux machinery that can pump out variety of genotoxins in ATP-dependent manner; (b) the mechanisms maintaining minimal metabolic activity which reduces generation of reactive oxidants, by-products of aerobic respiration; (c) the role of hypoxic niche of stem cells providing a gradient of variable oxygen tension; (d)(e) the presence of hyaluronan (HA) and HA receptors on stem cells and in the niche; (f) the role of role of HA in protecting DNA from oxidative damage; (g) the specific role of HA that may play a role protecting DNA in stem cells; (h) the interactions of HA with sperm cells and oocytes that also may shield their DNA from oxidative damage, and (e) mechanisms by which HA exerts the anti-oxidant activity. While HA has multitude of functions its anti-oxidant capabilities are often overlooked but may be of significance in preservation of integrity of stem and germ cells genome.

Intracellular hypoxia of tumor tissue estimated by noninvasive electron paramagnetic resonance oximetry technique using paramagnetic probes

Electron paramagnetic resonance (EPR) oximetry at 700 MHz operating frequency employing a surface coil resonator is used to assess tissue partial pressure of oxygen (pO(2)) using paramagnetic media whose linewidth and decay constant are related to oxygen concentration. Differences in extracellular and intracellular pO(2) in squamous cell carcinoma (SCC) tumor tissue were tested using several types of water-soluble paramagnetic media, which localize extracellularly or permeate through the cell membrane. The nitroxide carboxy-PROXYL (CxP) can only be distributed in blood plasma and extracellular fluids whereas the nitroxides carbamoyl-PROXYL (CmP) and TEMPOL (TPL) can permeate cell membranes and localize intracellularly. EPR signal decay constant and the linewidth of the intravenously administered nitroxides in SCC tumor tissues implanted in mouse thigh and the contralateral normal muscle of healthy mice breathing gases with different pO(2) were compared. The pO(2) in the blood can depend on the oxygen content in the breathing gas while tissue pO(2) was not directly influenced by pO(2) in the breathing gas. The decay constants of CmP and TPL in tumor tissue were significantly larger than in the normal muscles, and lower linewidths of CmP and TPL in tumor tissue was observed. The SCC tumor showed intracellular hypoxia even though the extracellular pO(2) is similar to normal tissue in the peripheral region.

The pivotal role of reactive oxygen species generation in the hypoxia-induced stimulation of adipose-derived stem cells

Adipose-derived stem cells (ASCs) offer a potential alternative for tissue repair and regeneration. We have recently shown that hypoxia stimulates ASCs and enhances the regenerative potential of ASCs, which is beneficial for ASC therapy. In the present study, we further investigated a key mediator and a signal pathway involved in the stimulation of ASC during hypoxia. Culturing ASC in a hypoxic incubator (2% oxygen tension) increased the proliferation and migration, and this was mediated by Akt and ERK pathways. To determine the generation of reactive oxygen species (ROS), 2',7'-dichlorofluorescin diacetate intensity was detected by fluorescence-activated cell sorting. Hypoxia significantly increased the dichlorofluorescin diacetate intensity, which was greatly reduced by N-acetyl-cysteine and diphenyleneiodonium treatment. Likewise, the hypoxia-induced proliferation and migration of ASCs were reversed by N-acetyl-cysteine and diphenyleneiodonium treatment, suggesting the involvement of ROS generation in ASC stimulation. Further, we examined the activation of receptor tyrosine kinases and observed that hypoxia stimulated the phosphorylation of platelet-derived growth factor receptor-β. In summary, the ROS produced by ASCs in response to hypoxia was mostly likely due to NADPH oxidase activity. The increased cellular ROS was accompanied by the phosphorylation of platelet-derived growth factor receptor-β as well as by the activation of ERK and Akt signal pathways. Our results suggest a pivotal role for ROS generation in the stimulation of ASCs by hypoxia.

Oxygen in stem cell biology: a critical component of the stem cell niche

The defining hallmark of stem cells is their ability to self-renew and maintain multipotency. This capacity depends on the balance of complex signals in their microenvironment. Low oxygen tensions (hypoxia) maintain undifferentiated states of embryonic, hematopoietic, mesenchymal, and neural stem cell phenotypes and also influence proliferation and cell-fate commitment. Recent evidence has identified a broader spectrum of stem cells influenced by hypoxia that includes cancer stem cells and induced pluripotent stem cells. These findings have important implications on our understanding of development, disease, and tissue-engineering practices and furthermore elucidate an added dimension of stem cell control within the niche.

Comparison of intensity-modulated continuous-wave lasers with a chirped modulation frequency to pulsed lasers for photoacoustic imaging applications

Using a Green's function solution to the photoacoustic wave equation, we compare intensity-modulated continuous-wave (CW) lasers with a chirped modulation frequency to pulsed lasers for photoacoustic imaging applications. Assuming the same transducer is used in both cases, we show that the axial resolution is identical and is determined by the transducer and material properties of the object. We derive a simple formula relating the signal-to-noise ratios (SNRs) of the two imaging systems that only depends on the fluence of each pulse and the time-bandwidth product of the chirp pulse. We also compare the SNR of the two systems assuming the fluence is limited by the American National Standards Institute (ANSI) laser safety guidelines for skin. We find that the SNR is about 20 dB to 30 dB larger for pulsed laser systems for reasonable values of the parameters. However, CW diode lasers have the advantage of being compact and relatively inexpensive, which may outweigh the lower SNR in many applications.

Hemodynamic and metabolic diffuse optical monitoring in a mouse model of hindlimb ischemia

Murine hindlimb ischemia is a useful model for investigation of the mechanisms of peripheral arterial disease and for understanding the role of endothelial cells and generic factors affecting vascular regeneration or angiogenesis. To date, important research with these models has explored tissue reperfusion following ischemia with Laser Doppler methods, methods which provide information about superficial (~mm) vascular regeneration. In this work, we employ diffuse correlation spectroscopy (DCS) and diffuse optical spectroscopy (DOS) in mice after hindlimb ischemia. We hypothesize that vascular re-growth is not uniform in tissue, and therefore, since diffuse optical methods are capable of probing deep tissues, that the diffuse optics approach will provide a more complete picture of the angiogenesis process throughout the whole depth profile of the limb. Besides increased depth penetration, the combined measurements of DCS and DOS enable all-optical, noninvasive, longitudinal monitoring of tissue perfusion and oxygenation that reveals the interplay between these hemodynamic parameters during angiogenesis. Control mice were found to reestablish 90% of perfusion and oxygen consumption during this period, but oxygen saturation in the limb only partially recovered to about 30% of its initial value. The vascular recovery of mice with endothelial cell-specific deletion of HIF-2α was found to be significantly impaired relative to control mice, indicating that HIF-2α is important for endothelial cell functions in angiogenesis. Comparison of DOS/DCS measurements to parallel measurements in the murine models using Laser Doppler Flowmetry reveal differences in the reperfusion achieved by superficial versus deep tissue during neoangiogenesis; findings from histological analysis of blood vessel development were further correlated with these differences. In general, the combination of DCS and DOS enables experimenters to obtain useful information about oxygenation, metabolism, and perfusion throughout the limb. The results establish diffuse optics as a practical noninvasive method to evaluate the role of transcription factors, such as the endothelial cell-specific HIF-2α, in genetic ally modified mice.

Potential application of adipose-derived stem cells and their secretory factors to skin: discussion from both clinical and industrial viewpoints

IMPORTANCE OF THE FIELD

Adipose tissue is one of the richest sources of mesenchymal stem cells. Even more interesting is the fact that adipose-derived stem cells (ASCs) show an outstanding ability to regenerate damaged skin. Thus, ASCs are a popular and feasible treatment in clinical dermatology.

AREAS COVERED IN THIS REVIEW

This review discusses the potential applications of ASCs and conditioned medium of ASC (ASC-CM) to skin, and briefly touches on the mechanisms by which ASCs promote skin regeneration.

WHAT THE READER WILL GAIN

Clinically, processed lipo-aspirated (PLA) cells are commonly used for treatment of aged skin; however, the use of PLA cells for cosmetic purposes is not convenient, because PLA cells are prepared from patients. Alternatively, cosmetics that contain ASC-CM can be pre-made from healthy volunteers such that they are immediately available for clinical treatment of aged skin. Cell-based therapies are adequate for improvement of wrinkles or for soft tissue augmentation, whereas ASC-CM has merit for amelioration of skin tone. When culturing ASCs for the production of cosmetic raw materials, hypoxic culture conditions and transduction of specific genes into ASCs may increase the regenerative protein content of the conditioned medium.

TAKE HOME MESSAGE

Application of ASCs and ASC-CM to dermatology shows promising results for skin regeneration.

Responses of adipose-derived stem cells during hypoxia: enhanced skin-regenerative potential

Mesenchymal stem cells within the stromal-vascular fraction of subcutaneous adipose tissue (i.e., adipose-derived stem cells (ASCs)), have been used for tissue engineering. In addition to serving a building-block function, ASCs are reported to secrete growth factors that are essential for their function. Increasing evidence indicates that ASCs play a significant role in skin regeneration, a function that is enhanced by hypoxia through upregulating secretion of growth factors. Although the anatomical sites of ASCs in the body are relatively oxygen-deficient, ASCs are usually cultured under normoxic conditions (i.e., atmospheric oxygen levels). Culturing ASCs under physiologically relevant low-oxygen-tension conditions may uniquely benefit the expansion, differentiation, adhesion, growth factor secretion and regenerative potential of ASCs. Therefore, understanding the response and adaptation of ASCs to hypoxia may be invaluable for developing novel cell- and cyto-therapy strategies. This review highlights our current understanding of cellular and molecular responses of ASCs to hypoxia, focusing on the enhancement of ASC function and secretory activity by hypoxic culture conditions.

In vivo measurement of tissue oxygen using electron paramagnetic resonance spectroscopy with oxygen-sensitive paramagnetic particle, lithium phthalocyanine

The partial pressure of oxygen (pO(2)) plays a determining role in the energy metabolism of aerobic cells. However, low pO(2) level induces pathophysiological conditions such as tumor hypoxia, ischemia or reperfusion injury, and delayed/altered wound healing. Especially, pO(2) level in the tumor is known to be related to tumor progression and effectiveness of radiotherapy. To monitor the pO(2) levels in vivo, continuous wave (CW) and time-domain (TD) electron paramagnetic resonance (EPR) spectroscopy method was used, in which surface coil resonator and Lithium phthalocyanine (LiPc) as oxygen sensor were crucial. Once LiPc particles are embedded in a desired location of organ/tissue, the pO(2) level can be monitored repeatedly and non-invasively. This method is based on the effect of oxygen concentration on the EPR spectra of LiPc which offers several advantages as follows: (1) high sensitivity, (2) minimum invasiveness, (3) repeated measurements, (4) absence of toxicity (non-toxic), and (5) measurement in a local region of the tissue with embedded LiPc. Therefore, in this chapter, we describe the method using CW and TD EPR spectroscopy with oxygen-sensitive particle, LiPc, for in vivo monitoring of oxygen.

The role of hypoxia in stem cell differentiation and therapeutics

Stem cells differentiate into a variety of cell lines, making them attractive for tissue engineering and regenerative medicine. Specific microenvironmental cues regulate self-renewal and differentiation capabilities. Oxygen is an important component of the cellular microenvironment, serving as both metabolic substrate and signaling molecule. Oxygen has been shown to have a variety of effects on embryonic and adult stem cells. This review examines the role of hypoxia in regulating stem cell biology, specifically focusing on growth, maintenance of pluripotency, differentiation, and production of growth factors. Particular attention is paid to hypoxia and stem cells in relation to therapeutic angiogenesis. We conclude that further study is needed to optimize the use of hypoxia as a stimulus for various stem cell functions, including its potential role in therapeutic angiogenesis.

Oxygen sensitivity of mitochondrial reactive oxygen species generation depends on metabolic conditions

The mitochondrial generation of reactive oxygen species (ROS) plays a central role in many cell signaling pathways, but debate still surrounds its regulation by factors, such as substrate availability, [O2] and metabolic state. Previously, we showed that in isolated mitochondria respiring on succinate, ROS generation was a hyperbolic function of [O2]. In the current study, we used a wide variety of substrates and inhibitors to probe the O2 sensitivity of mitochondrial ROS generation under different metabolic conditions. From such data, the apparent Km for O2 of putative ROS-generating sites within mitochondria was estimated as follows: 0.2, 0.9, 2.0, and 5.0 microM O2 for the complex I flavin site, complex I electron backflow, complex III QO site, and electron transfer flavoprotein quinone oxidoreductase of beta-oxidation, respectively. Differential effects of respiratory inhibitors on ROS generation were also observed at varying [O2]. Based on these data, we hypothesize that at physiological [O2], complex I is a significant source of ROS, whereas the electron transfer flavoprotein quinone oxidoreductase may only contribute to ROS generation at very high [O2]. Furthermore, we suggest that previous discrepancies in the assignment of effects of inhibitors on ROS may be due to differences in experimental [O2]. Finally, the data set (see supplemental material) may be useful in the mathematical modeling of mitochondrial metabolism.

Hypoxia and stem cell-based engineering of mesenchymal tissues

Stem cells have the ability for prolonged self-renewal and differentiation into mature cells of various lineages, which makes them important cell sources for tissue engineering applications. Their remarkable ability to replenish and differentiate in vivo is regulated by both intrinsic and extrinsic cellular mechanisms. The anatomical location where the stem cells reside, known as the "stem cell niche or microenvironment," provides signals conducive to the maintenance of definitive stem cell properties. Physiological condition including oxygen tension is an important component of the stem cell microenvironment and has been shown to play a role in regulating both embryonic and adult stem cells. This review focuses on oxygen as a signaling molecule and the way it regulates the stem cells' development into mesenchymal tissues in vitro. The physiological relevance of low oxygen tension as an environmental parameter that uniquely benefits stem cells' expansion and maintenance is described along with recent findings on the regulatory effects of oxygen on embryonic stem cells and adult mesenchymal stem cells. The relevance to tissue engineering is discussed in the context of the need to specifically regulate the oxygen content in the cellular microenvironment in order to optimize in vitro tissue development.

The bioenergetics of cancer: is glycolysis the main ATP supplier in all tumor cells?

The molecular mechanisms by which tumor cells achieve an enhanced glycolytic flux and, presumably, a decreased oxidative phosphorylation are analyzed. As the O(2) concentration in hypoxic regions of tumors seems not limiting for oxidative phosphorylation, the role of this mitochondrial pathway in the ATP supply is re-evaluated. Drugs that inhibit glycoysis and oxidative phosphorylation are analyzed for their specificity toward tumor cells and effect on proliferation. The energy metabolism mechanisms involved in the use of positron emission tomography are revised and updated. It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.