Introduction
Angiogenesis, defined as sprouting of blood vessels from preexisting vascular structures, is a physiological response to ischemia but often fails to produce sufficient collateral flow to rescue ischemic organ [
1]. In animal models of ischemia, increasing evidence indicates that administration of angiogenic growth factors, either through recombinant protein or by gene transfer, can increase nutrients perfusion through neovascularization and reduce ischemia-related organ damage [
2]. However, there is clinical evidence that some patients, unlike healthy experimental animals, fail to develop collateral circulation after tissue ischemia and appear to be refractory to administration of exogenous growth factors [
3]. Patients with diabetes are related to endothelial dysfunction and have diminished ability for collateral vessel formation in response to ischemia in the heart and peripheral tissues [
4‐
6].
Convincing evidence suggests that neovascularization in adults is not solely the result of proliferation of local endothelial cells (angiogenesis), but also involves bone marrow-derived circulating endothelial progenitor cells (EPCs) for the processes of vasculogenesis [
7]. However, patients with cardiovascular risk factors were shown to have decreased numbers and functions of circulating EPCs [
8]. Our recent study also demonstrated that long-term exposure to high glucose may enhance cellular senescence and decrease cell numbers and functional competencies of EPCs via nitric oxide (NO)-related mechanisms [
9]. Recent evidence showed that the circulating EPCs levels and arterial stiffness were closely related to their glycemic control in patients with type 2 diabetes mellitus (DM) [
10]. These findings provide a rationale for potential therapeutic targets on hyperglycemia-related vascular complications in diabetic patients. Far infrared (IFR) radiation is an invisible electromagnetic wave with a characteristic wavelength between 5.6 and 1000 μm that can be perceived as heat by thermo-receptors in the skin [
11,
12]. Recent studies indicate that IFR therapy exerts beneficial effects in the cardiovascular system. IFR radiation improves ventricular arrhythmias and endothelial function in patients with heart disease, and enhances access flow and patency of arteriovenous fistulas in hemodialysis patients [
13‐
15]. In addition, IFR therapy promotes microvascular blood flow and angiogenesis in various animal models [
16,
17]. Although clinical studies have indicated that IFR radiation can exert beneficial effects in the cardiovascular system, the multi-faceted effects of IFR therapy on circulating EPCs and diabetes remain unclear. Therefore, we hypothesized that IFR radiation can improve blood flow recovery after tissue ischemia in diabetic mice and improve functional capacities of EPCs by increasing NO bioavailability.
Materials and methods
Animals
Mice at 6–8 weeks old were purchased from the National Laboratory Animal Center, Taiwan (FVB mice). Experimental diabetes was induced in FVB mice by daily intraperitoneal injections of streptozotocin (STZ) in citrate buffer (40 mg/kg) for 5 days for the type 1 diabetic model [
18]. Mice were considered diabetic only if they developed glycemia >250 mg/dl and overt glycosuria at 14 days after the first STZ injection. Persistence of diabetes was determined at the end of the study. All mice were kept in microisolator cages on a 12-h day/night cycle. The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication 1996). All experimental procedures and protocols involving animals were approved by the institutional animal care committee of National Yang-Ming University (Taipei, Taiwan) and in compliance with the ARRIVE guidelines [
19].
Mouse ischemic hindlimb model
Eight-week-old male wild-type mice and STZ-induced diabetic mice received unilateral hindlimb ischemia surgery by excising the right femoral artery as previously described [
20]. Local IFR therapy was given in STZ-induced diabetic mice for 30 min twice per day for 2 weeks after the surgery, and wild-type and diabetic control mice were placed on a heating plate at 34°C for 30 min twice daily to avoid the thermal effect in this study. Briefly, mice were anaesthetized by an intraperitoneal injection of ketamine (100 mg/kg) and xylazine (10 mg/kg). The depth of anesthesia was checked by ensuring that noxious pinch stimulation (blunt forceps) of the hindpaw, the forepaw, and the ear did not evoke any motor reflexes. The proximal and distal portions of the femoral artery were ligated. Hindlimb blood perfusion was measured with a Laser Doppler perfusion imager system (Moor Instruments Limited, Devon, UK) before and after the surgery and followed weekly. To avoid the influence of ambient light and temperature, the results were expressed as the ratio of perfusion in the right (ischemic) versus left (non-ischemic) limb.
Measurement of capillary density and oxidative stress in the ischemic limb
At 4 weeks after surgery, the mice were euthanized via intravenous ketamine injection. The femora were carefully removed, and the ischemic thigh muscles were embedded in paraffin. Sections (5 μm) were de-paraffinized and incubated with a rat monoclonal antibody against murine CD31 (clone MEC13.1, BD PharMingen, San Diego, CA). Antibody distribution was visualized by using the avidin-biotin-complex technique and Vector Red Chromogenic substrate (Vector Laboratories, Burlingame, CA), followed by counterstaining with hematoxylin. Capillaries were identified by positive staining for CD31. Ten different fields from each tissue preparation were randomly selected, and visible capillaries were counted. Capillary density was expressed as the number of capillaries per square millimeter. To evaluate local oxidative stress levels in ischemic muscles, an antibody against nitrotyrosine (Upstate, Lake Placid, NY, USA) was used.
Flow cytometry
To investigate the effects of IFR therapy on EPC mobilization in response to tissue ischemia, the fluorescence-activated cell sorting (FACS) Caliber flow cytometer (Becton Dickinson, San Jose, CA, USA) was used to assess EPC mobilization [
18]. A volume of 100 μL peripheral blood was incubated with Fluorescein isothiocyanate (FITC) anti-mouse Sca-1 (eBioscience, San Diego, CA, USA), and phycoerythrin (PE) anti-mouse Flk-1 (VEGFR-2, eBioscience) antibodies. Isotype-identical antibodies served as controls (Becton Dickinson, Franklin Lakes, NJ, USA). After incubated for 30 minutes, cells were lysed (PharmLyse; BD Pharmingen), washed with phosphate-buffered saline (PBS), and fixed in 2% paraformaldehyde before analysis. Each analysis included 100,000 events. Circulating EPCs were considered to be from the mononuclear cell population and were gated with double positive for Sca-1 and Flk-1.
Bone marrow transplantation model
Recipient wild-type mice at 8 weeks of age were lethally irradiated with a total dose of 9.0 Gy [
18,
20]. eGFP transgenic mice (FVB background) that ubiquitously expressed enhanced GFP (Level Biotechnology Inc., Taipei, Taiwan) were used as the donors. After being irradiated, a recipient mouse received unfractionated bone marrow cells (5×10
6) from an eGFP mouse by a tail vein injection. Two months after the bone marrow transplantation, the chimeric mice were induced diabetes by daily intravenous injections of STZ as previously described. All mice received a hindlimb ischemic surgery (n = 6 in each group). Repopulation by eGFP-positive bone marrow cells was 95%, as measured by flow cytometry. Two weeks after the induction of hindlimb ischemic surgery in the bone marrow-reconstituted and STZ-induced diabetic mice, tissues were harvested for confocal immunofluorescent and histological analysis. Bone marrow-derived EPCs were stained with antibodies directed against eGFP (Chemicon) and CD31 (BD PharMingen). EPC density was estimated by counting eGFP
+CD31
+ double-positive cells (yellow color) under high power field (HPF, ×100) in at least 6 different cross-sections from different animals.
EPC transplantation in nude mice
Athymic nude mice at 6–8 weeks old were purchased from the National Laboratory Animal Center, Taiwan. Nude mice were then randomly assigned to 5 treatment groups (n = 6 in each group) for intramuscular injection of normal saline, healthy EPC, EPC treated with high glucose for 4 days (EPC-HG), EPC treated with high glucose and IFR for 30 mins (24 hours following by treatment of EPCs in high-glucose conditions for 4 days) (EPC-HG + IFR), and EPC treated with high glucose and transfected with eNOS siRNA (Santa Cruz-Biotechnology Inc., CA, USA) and IFR for 30 mins (EPC-HG + eNOS siRNA + IFR). EPCs were labeled with fluorescent carbocyanine 1,1'-dioctadecyl-1 to 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) dye (Molecular Probes) [
21]. Intramuscular injection was performed 24 hours following unilateral hindlimb ischemia srugery. A total volume of 200 μl normal saline or 5 × 10
6 EPC were injected at 6 sites into the ischemic hind limb distal to the arterial occlusion site. Three ventral injections were placed in the upper limb in proximity to the adductor and semimembranosus muscles. The remaining 3 injections were administered to the ventral lower limb involving the gastrocnemius and flexor digitorum muscles. In order to achieve maximal experimental uniformity, transplanted EPCs were derived from the same donors and used in parallel experiments.
Human EPC isolation and cultivation
Peripheral blood samples (20 ml) were obtained from healthy young adult volunteers, and total mononuclear cells (MNCs) were isolated by density gradient centrifugation with Histopaque-1077 (1.077 g/ml, Sigma, St. Louis, MO, USA) [
20]. Briefly, MNCs (5×10
6) were plated in 2 ml endothelial growth medium (EGM-2 MV Cambrex, East Rutherford, NJ, USA) with supplements on fibronectin-coated 6-well plates. After 4 days of culturing, the medium was changed and non-adherent cells were removed; attached early EPCs appeared to be elongated with spindle shapes. A certain number of early EPCs were allowed to grow into ECFCs (endothelial colonies forming cells), which emerged 2–4 weeks after the start of the MNC culture. The ECFCs exhibited a cobblestone morphology and monolayer growth pattern typical of mature endothelial cells at confluence [
22]. ECFCs were collected and used for all the assays in this study.
ECFC characterization
The ECFCs were characterized as adherent cells that were positive for endothelial cell and haematopoietic stem cell surface makers, as previously described [
20]. The ECFCs were characterized by immunofluorescence staining for the expression of VE-cadherin, platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD-31), and CD34, KDR, AC133 and eNOS (Santa Cruz). The fluorescent images were recorded under a laser scanning confocal microscope.
Measurement of reactive oxygen species (ROS) production and measurement of nitrate levels
The effect of IFR therapy on ROS production in ECFCs was determined by a fluorometric assay using 2',7'-dichlorofluorescein diacetate (DCFH-DA, Molecular Probes) as a probe for the presence of H2O2.21 ECFCs (104 cells/well) in 96-well plates was incubated in high glucose medium for 4 days. After treatment of IFR radiation, cells were incubated with 20 μmol/L DCFH-DA for 45 minutes. The conditioned medium was measured for nitrate level by Griess reagent [1% sulfanilamide and 0.1% N-(1-naphthyl) ethylenediamine in 2% phosphoric acid]. The fluorescence intensity (relative fluorescence units) was assessed at 485-nm excitation and 530-nm emission using a fluorescence microplate reader.
EPC senescence assay
Cellular aging was determined with a Senescence Cell Staining kit (Sigma). Briefly, after washing with PBS, ECFCs were fixed for 6 minutes in 2% formaldehyde and 0.2% glutaraldehyde in PBS, and then incubated for 12 hours at 37°C without CO
2 in fresh X-gal staining solution (1 mg/ml X-gal, 5 mM potassium ferricyanide, and 2 mM MgCl
2; pH6). After staining, green-stained cells and total cells were counted and the percentage of β-galactosidase-positive cells was calculated [
20].
EPC migration assay
The migratory function of ECFCs was evaluated by a modified Boyden chamber assay (Transwell, Costar) [
20]. Briefly, isolated ECFCs were detached as described above with trypsin/EDTA, and then 4×10
4 ECFCs were placed in the upper chambers of 24-well transwell plates with polycarbonate membranes (8-μm pores) that contained serum-free endothelial growth medium. VEGF (50 ng/ml) was added to medium placed in the lower chambers. After incubation for 24 hours, the membrane was washed briefly with PBS and fixed with 4% paraformaldehyde. The upper membrane side was wiped gently with a cotton ball. The membrane was stained using hematoxylin solution and carefully removed. The extent of migration of ECFCs was evaluated by counting the migrated cells in 6 random high-power (×100) microscopic fields.
Western blotting analysis
ECFCs were lysed in buffer (62.5 mM Tris–HCl, 2% SDS, 10% glycerol, 0.5 mM PMSF, 2 μg/ml aprotinin, pepstatin and leupeptin), and the protein lysates were subjected to SDS-PAGE, followed by electroblotting onto a PVDF membrane [
12]. Membranes were probed with monoclonal antibodies against phosphorylated endothelial NO synthase (p-eNOS), eNOS, Akt, p-Akt, VEGF, p-ERK, p-38 MAPK, HO-1 (Cell Signaling) and β-actin (Sigma). Bands were visualized by chemiluminescence detection reagents. Densitometric analysis used ImageQuant (Promega) software.
Statistical analysis
Results are given as means ± standard errors of the mean (SEM). Statistical analysis was done by unpaired Student’s t test or analysis of variance, followed by Scheffe’s multiple-comparison post hoc test. Data were analyzed using SPSS software (version 14; SPSS, Chicago, IL). A p value of < 0.05 was considered statistically significant.
Discussion
This study first to show a favorable effect of IFR radiation on ischemia-induced neovascularization in diabetic mice. Repeated IFR radiation enhanced blood flow recovery and new vessel formation in ischemic hindlimbs, and the beneficial effect may be derived from enhancement of EPC homing process by reduction of oxidative stress in ischemic tissue rather than promotion of EPC mobilization. In addition, direct IFR radiation could ameliorate high glucose-induced oxidative stress, attenuate cellular senescence and improve EPC functions. Mice received EPC treated with IFR radiation showed a significant improvement in blood flow recovery after tissue ischemia in comparison to those received normal saline. Given the evidence mentioned above, our findings indicated the direct beneficial effects of IFR radiation on blood flow recovery after tissue ischemia, and IFR radiation counteracts the detrimental effect of a diabetic environment on improvement of EPC functions, which may provide some novel rationales for its potential clinical impact on vascular protection.
Improved neovascularization in response to tissue ischemia is an important therapeutic strategy to reduce organ damage. Convincing evidence suggests that neovascularization in adults is not solely the result of the proliferation of endothelial cells (angiogenesis) but also involves circulating EPCs in the process of vasculogenesis [
7]. These circulating EPCs are derived from bone marrow and are mobilized endogenously, triggered by tissue ischemia, or exogenously by cytokine stimulation, such as VEGF and stromal cell-derived factor-1 (SDF-1) [
23].
However, patients with diabetes or cardiovascular risk factors were shown to have decreased numbers and function of circulating EPCs [
8,
24,
25]. Recent studies indicated that advanced glycation end products (AGEs) promoted EPC apoptosis [
26], and long-term exposure to high glucose may enhance cellular senescence and reduce cell numbers and functional competencies of EPCs via NO-related mechanisms [
10]. Mobilization and differentiation of EPCs are modified by NO, and bone marrow-expressed eNOS is essential for the mobilization of stem and progenitor cells [
27]. In addition, endogenous NOS inhibitors, such as asymmetric dimethylarginine (ADMA), were shown to suppress EPC differentiation and function, and contributed to impaired endothelial function [
28]. These findings provide a rationale for potential therapeutic targets for hyperglycemia-suppressed EPC functions, and diabetes-related vascular complications.
Diabetic patients frequently suffer from micro- or macrovascular abnormalities, including retinopathy, nephropathy, neuropathy and accelerated atherosclerosis. It is evident that decreased bioavailability of NO produced from eNOS plays a crucial role in the development and progression of atherosclerosis. Under various pathological conditions such as type 2 diabetes eNOS may become dysfunctional or its expression may be decreased. Endothelial dysfunction is associated with childhood obesity and is closely linked to the amount and function of EPCs, and a combined after-school exercise program was shown to increase circulating EPC levels by enhancement of NO bioavailability [
29]. Moreover, enhancement of oxidative stress by tissue ischemia may downregulate NO bioavailability because free radicals can directly inactivate NO [
30]. Inadequate angiogenic response to ischemia in the ischemic limbs or myocardium of diabetic patients could result in poor collateral formation and severe organ damage.
IFR radiation is an invisible electromagnetic wave with a characteristic wavelength between 5.6 and 1000 μm that can be perceived as heat by thermo-receptors in skin [
11,
12]. The technology of IFR has been applied widely in a variety of fields. The thermal effect of IFR results in vasodilation and increasing tissue blood flow. Local IFR therapy may allow multiple energy transfer as deep as 2 to 3 cm into subcutaneous tissue without irritating or overheating the skin like unfiltered heat radiation [
31]. The skin temperature steadily increased to a plateau at approximately 38 to 39°C during the treatment of FIR for 30 to 60 min as long as the distance between the ceramic plate and the skin was >20 cm [
16]. Therefore, infrared therapy can be free of the disadvantages or adverse effects of thermal therapy. In this study, wild-type and diabetic control mice were placed on a heating plate at 34°C for 30 min twice daily to avoid the thermal effect between groups. The rectal temperature was also assessed in these mice, and showed no temperature elevation in mice received IFR therapy compared to those without IFR radiation.
In addition to the thermal effect, increasing evidence suggests nonthermal effects of IFR therapy exert beneficial effects in the cardiovascular system through NO-related pathway [
31,
32]. In animal studies, Akasaki and colleagues demonstrated that repeated IFR therapy could upregulate eNOS expression and augment angiogenesis in an apolipoprotein E–deficient mouse model of unilateral hindlimb ischemia [
16]. In human study, Imamura et al. showed that two weeks of repeated sauna therapy significantly improved vascular endothelial function, resulting in an increase of flow-mediated, endothelium-dependent dilation of the brachial artery from 4 to 5.8% in patients with coronary risk factors [
14]. These findings suggested nonthermal effect of IFR may derive from upregulation of eNOS activity and enhancement of NO bioavailability. In the study, we demonstrated that repeated IFR therapy could activate eNOS and Akt, and upregulate the migration and tube formation capacities of ECFCs. Akt is downstream from PI 3-kinase and is capable of directly phosphorylating eNOS at Ser
1179, resulting in its activation. Recent evidence also indicated that p-38 MAPK plays a key role in downregulating EPCs by hyperglycemia in diabetic patients [
33]. EPCs exposed to IFR upregulated expression of p-38 MAPK and p-ERK. These findings are in line with recent study showing that IFR radiation significantly promoted angiogenesis by a MAP kinase dependent pathway on mature endothelial cells. Mice received EPC treated with IFR radiation in high glucose conditions showed a significant improvement in blood flow recovery after tissue ischemia compared with those received normal saline. These beneficial effects may provide some novel rationale for the vascular protective properties of IFR therapy in diabetic patients with critical limb ischemia or clinical implication to treat dysfunctional EPCs before cell therapy.
Acknowledgements
This study was supported in part by research grants from the NSC 98-2314-B-075-035, and UST-UCSD International Center of Excellence in Advanced Bio-engineering NSC-100-2911-I-009-101-A2 from the National Science Council; VGH-V98B1-003 and VGH-V100E2-002 from Taipei Veterans General Hospital. The funders had no role in study design, data collection, and analysis, decision to publish, or preparation of the manuscript.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Po-Hsun Huang conducted the experiments and contributed to the study implementation, statistical analysis, interpretation, and the preparation of the manuscript. Jaw-Wen Chen contributed to the study conception and design, and the preparation of the manuscript. Chih-Pei Lin, Ying-Hwa Chen, Chao-Hung Wang, and Hsin-Bang Leu helped to conduct the experiments and contributed to the study conception and design, implementation, and interpretation. Jaw-Wen Chen and Shing-Jong Lin supervised the study conduction and contributed to the study conception and design, implementation, statistical interpretation, the preparation and finalization of the manuscript. All authors approved the final manuscript for publication.