Reciprocal Effects of Oxidative Stress on Heme Oxygenase Expression and Activity Contributes to Reno-Vascular Abnormalities in EC-SOD Knockout Mice.

Heme oxygenase (HO) system is one of the key regulators of cellular redox homeostasis which responds to oxidative stress (ROS) via HO-1 induction. However, recent reports have suggested an inhibitory effect of ROS on HO activity. In light of these conflicting reports, this study was designed to evaluate effects of chronic oxidative stress on HO system and its role in contributing towards patho-physiological abnormalities observed in extracellular superoxide dismutase (EC-SOD, SOD3) KO animals. Experiments were performed in WT and EC-SOD((-/-)) mice treated with and without HO inducer, cobalt protoporphyrin (CoPP). EC-SOD((-/-)) mice exhibited oxidative stress, renal histopathological abnormalities, elevated blood pressure, impaired endothelial function, reduced p-eNOS, p-AKT and increased HO-1 expression; although, HO activity was significantly (P < 0.05) attenuated along with attenuation of serum adiponectin and vascular epoxide levels (P < 0.05). CoPP, in EC-SOD((-/-)) mice, enhanced HO activity (P < 0.05) and reversed aforementioned pathophysiological abnormalities along with restoration of vascular EET, p-eNOS, p-AKT and serum adiponectin levels in these animals. Taken together our results implicate a causative role of insufficient activation of heme-HO-adiponectin system in pathophysiological abnormalities observed in animal models of chronic oxidative stress such as EC-SOD((-/-)) mice.

1. Introduction

Oxidative stress induces NRF2-dependent antioxidant enzymes including the heme-HO system [1], whose two isoforms HO-1 (inducible) and HO-2 (constitutive) catabolizes free heme to equimolar concentrations of biliverdin (BV), carbon monoxide (CO), and iron. Excess-free heme, due to its pro-oxidant and proinflammatory properties, contributes to an increase in free radical formation and cellular injury [1, 2], thus necessitating its catabolism by HO. Apart from restricting accumulation of pro-oxidant heme, antioxidant properties of heme-HO system arise from production of BV and bilirubin (BR), which have potent antioxidant properties [3, 4]. In addition, CO has been shown to exhibit antioxidant [5, 6] (reviewed by [1]), antiapoptotic [7], and vasomodulatory properties. These properties of the heme-HO system are pertinent to redox balance and its associated physiological ramifications, especially in the cardiovascular-renal systems [1, 8]. Further evidence of HO-mediated sustenance of renovascular homeostasis is provided by studies demonstrating HO-dependent activation of adiponectin release [9], which has antioxidant and anti-inflammatory properties [10] in addition to its renoprotective effects [11]. An increase in adiponectin has also been shown to lead to increased levels of mitochondrial transport carriers and cytochrome oxidases via an increase in Bcl-XL [12]. The Bcl-2 family of proteins, consisting of anti- and proapoptotic proteins, along with serine-threonine kinase (Akt) (protein kinase B), are critical in cell death/survival pathways [13]. Akt is activated through phosphorylation at either threonine-308 or serine-473 [14]. Activated Akt inhibits ASK-1, a proapoptotic member of the MAP kinase kinase family, and protects against stress induced apoptosis in endothelial cells [15].

Apart from inducible NRF2-dependent genes, constitutive enzymes such as superoxide dismutases (SOD) regulate basal redox and prevent excess-free radical accumulation. SOD enzymes are known to exist in three isoforms: Cu-Zn SOD (SOD1), predominately located in the cytoplasm, Mn-SOD (SOD2) in the mitochondria, and EC-SOD (SOD3) in the extracellular space. Although SOD1 accounts for 60% to 80% of SOD activity in vivo [16, 17], SOD3 is highly expressed in renal and vascular tissues, particularly in the arterial wall, and its activity constitutes almost half of the total SOD activity in the human aorta [18]. Gene deletion of EC-SOD results in chronic oxidative stress, endothelial dysfunction, and increased blood pressure [17, 19, 20], implicating this enzyme in the regulation of redox homeostasis and preservation of cardiovascular and renal function. Where, in an event of increased oxidative stress HO-1 is rapidly induced, recent reports have emerged suggesting an inhibitory effect of the same on HO activity [21]. In this contradictory contexture, the present study was designed to examine the effects of chronic oxidative stress, as observed in EC-SOD(−/−) mice, on HO expression and activity with regards to the pathophysiological abnormalities observed in these animals. The study was performed in WT and EC-SOD(−/−) mice in the absence and in the presence of HO inducer, CoPP. Our results demonstrate that EC-SOD deficiency, although accompanied by oxidative stress and induction of HO-1, is characterized by attenuation of HO activity with the resultant attenuation of vascular epoxide and adiponectin levels. Phenotypic analysis of EC-SOD knockout mice revealed renal microvascular and corticomedullary damage along with elevated blood pressure and vascular endothelial dysfunction. Induction of HO-1 in SOD3-deficient mice not only restored HO activity and redox homeostasis, but also prevented renovascular injury and offset endothelial dysfunction and elevated blood pressure. These events are accompanied by the restoration of vascular epoxide and serum adiponectin levels with a concomitant increase in p-AKT and p-eNOS expression.

2. Research Design and Methods

2.1. Animal Treatment

Three-month-old homozygote, male, EC-SOD(−/−) and C57BL/6 genetic background EC-SOD(+/+) mice were used for this study. EC-SOD(−/−) mice were a gift from University of Pittsburgh, PA, USA. These mice had been backcrossed to C57BL/6 mice and are congenic with this line of mice [22]; as such, C57BL/6, purchased from The Jackson Laboratories, were used as WT controls. All animals were maintained on a standard laboratory diet and water, and every effort was made to minimize animal suffering according to the National Institutes of Health (NIH), Institutional Animal Care and Use Committee guidelines. The studies were approved by the animal use committee at New York Medical College.

Mice, both WT and KO, were divided into two groups (6 mice/group): controls, injected with saline solution, and the treatment group, treated with cobalt protoporphyrin (CoPP) 5 mg/Kg i.p. weekly for 6 weeks. CoPP was dissolved in trisma base as previously described [23]. The last injection of CoPP was made three days before sacrificing the animals. Mice were followed for 6 weeks (duration of the experiments), at which time they were sacrificed with CO2 gas. Blood and tissues were collected and stored, as described, for appropriate experiments.

2.2. Morphology and Proteins Analysis

Renal cross-sections were fixed in formalin, paraffin-embedded, and processed for histology using standard techniques. Tissue sections were cut at 3-4 micron thickness and stained with H&E and periodic acid Schiff (PAS). A portion of the tissues was snap-frozen and kept at −80° for proteins determinations and Western blot analysis.

2.3. Tissue Preparation for Western Blot, HO Activity, and O2 − Levels

Sections of femoral arteries and whole kidneys were homogenized in homogenization buffer (255 mM sucrose, 20 mM tris-hcl, 1 mM EDTA, 0.1 mM PMSF, and 0.5% Nonidet P-40 at pH 7.4) containing a cocktail of protease inhibitors (Roche, Indiana, IN) and Halt, a phosphatase inhibitor cocktail (Pierce Biotechnology, Rockford, IL). The homogenates were centrifuged at 14000 g for 10 min at 4°C, supernatant was isolated and protein levels assayed using a Bio-Rad kit on the basis of the Bradford dye binding procedure.

2.4. Measurement of O2 − Levels

Employing previously described methods [24], kidney and arterial samples were placed in glass scintillation mini vials containing 5 μM lucigenin, for the detection of O2 − in a final volume of 1 mL of air-equilibrated Krebs solution buffered with 10 mM HEPES-NaOH (pH 7.4). Lucigenin chemiluminescence was measured in a liquid scintillation counter (LS6000IC, Beckman Instruments, and San Diego, CA), and data are reported as counts/minute/mg protein after background subtraction.

2.5. Measurement of HO Activity

Tissue HO activity was assayed as described previously [25] using a technique in which bilirubin, the end product of heme degradation, was extracted with chloroform, and its concentration was determined spectrophotometrically (dual UV/VIS beam spectrophotometer lambda 25; PerkinElmer Life and Analytical Sciences, Wellesley, MA, USA) using the difference in absorbance at a wavelength from λ460 to λ530 nm with an absorption coefficient of 40 mM−1 cm−1. Under these conditions, HO activity was linear with protein concentration, time-dependent, and substrate-dependent [26].

2.6. RT-PCR Analysis

Frozen arterial and renal segments were pulverized under liquid nitrogen and RNA extracted and used for RT/PCR. Reverse transcription (RT) was carried out using the Advantage RT-for-PCR Kit (Clontech). Poly-d (T) n was used as the reverse transcription primer. Specific primers for amplifying EC-SOD and 18 s cDNA were employed. PCR was performed using a Taq PCR kit (Roche, Indianapolis, IN, USA). For each RT-PCR, a sample without reverse transcriptase was processed in parallel and served as a negative control. Cycling parameters for amplifying RT products were as follows: 95°C, 1′, 60°C, 1′, and 72°C, 1–3′, for 30 cycles, and then extended at 72°C for another 5′. After amplification, PCR products were electrophoresed on 1.2% agarose gel, stained with ethylene bromide, and visualized under UV light.

2.7. Assessment of Vascular Reactivity

The femoral artery was removed and placed in cold oxygenated Krebs-bicarbonate solution, cleaned of fat and loose connective tissue, and sectioned into rings of approximately 3 mm length. Two rings per artery were obtained, and each ring was mounted on stainless steel hooks and suspended in a 5-mL tissue DMT myograph bath (DMT, Atlanta, GA) filled with Krebs solution, pH 7.4, gassed with 95% O2/5% CO2, and maintained at 37°C. The rings were incubated under a passive tension of 0.2 g for 1 hour. The Krebs buffer solution was replaced every 15 minutes and the tension readjusted each time. Force was recorded from force displacement transducers via AD Instrument's Powerlab system, running Chart 5 software. At the end of the equilibration period, the maximal force generated by the addition of a depolarizing solution of KCl (60 mM) was determined. To evaluate acetylcholine-induced vasorelaxation, the rings were preconstricted with phenylephrine to obtain maximal contraction followed by cumulative dose-response curves to acetylcholine.

2.8. Blood Pressure Measurements

Blood pressure was measured by the tail cuff method before the start and at the completion of the experiment (6 wks). Prior to the experiment, mice were all acclimated to the tail cuff method. Mice were placed in a heat controlled box (36°C–38°C) for approximately 10 min. before applying the tail cuff. The mean of a minimum of 5 measurements was obtained from each mouse. All measurements were determined at the same time of day, between 9:00 and 13:00 hr. Systolic blood pressure is reported in mm of Hg.

2.9. Adiponectin Measurements

Adiponectin was determined using an ELISA assay (Pierce Biotechnology, Inc., Woburn, MA, USA).

2.10. Western Blot Analysis

The supernatant from tissue homogenates was used for evaluation of protein expression via Western blot analysis. Primary antibodies used are listed as following: HO-1 and HO-2 (Assay Designs, Inc.), ASK1 (Abcam), eNOS, Bcl-Xl and AKT (Cell Signaling Technologies, Inc.), p-eNOS-Thr495 (Cell Signaling Technologies, Inc.), and p-Akt-Ser473 (Cell Signaling Technologies, Inc.). B-actin (Sigma-Aldrich) was used as housekeeping gene for normalization. Antibodies were prepared in the following dilutions: HO-1 and HO-2 (1 : 1000); eNOS, p-eNOS, Bcl-xl, ASK1, and p-AKT antibodies (1 : 5000); β-actin (1 : 10000). Experimental protocol in brief: 20 μg of lysate supernatant was separated by 12% SDS-polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane (Amersham Biosciences, Uppsala, Sweden) with a semidry transfer apparatus (Bio-Rad, Hercules, CA, USA). The membranes were incubated with 10% milk in tris-buffered saline with tween 20 (10 mM Tris-HCl, pH 7.4, 150 mM NaCl, and 0.05% Tween 20) at 4°C, overnight. After incubation, the membranes were washed with tris-buffered saline with tween 20, and the membranes were then incubated with secondary antibodies for 1 h at room temperature with constant shaking. The filters were washed and subsequently probed with horseradish peroxidase-conjugated donkey anti-rabbit or anti-mouse IgG (Amersham Biosciences). Chemiluminescence detection was performed with the Amersham Biosciences enhanced chemiluminescence detection kit, according to the manufacturer's instructions.

2.11. Measurement of EETs

Femoral arterial segments (two segments per tube) were incubated in 1 mL of oxygenated Krebs' buffer containing 1 mM NADPH at 37°C for 1 h. Thereafter, internal standards were added to each sample followed by acidification to pH ≈ 4.0 with glacial acetic acid. Eicosanoids were extracted with twice the volume of ethyl acetate (performed three times), dried under gentle stream of nitrogen, and stored at −80°C until further analysis. Identification and quantification of EETs and DiHETs was performed with a Q-trap 3200 linear ion trap quadruple liquid chromatography-tandem mass spectrometry system equipped with a turbo V ion source operated in negative electro spray mode (Applied Biosystems, Foster City, CA, USA). Extracted samples were suspended in 10 μL of methanol and injected into the high-performance liquid chromatography via an Agilent 1200 standard series auto sampler equipped with a thermostat set at 4°C (Agilent Technologies). The high-performance liquid chromatographic component consisted of an Agilent 1100 series binary gradient pump equipped with an Eclipse plus C18 column (50 × 4.6 mm; 1.8 mm) (Agilent Technologies). The column was eluted at a flow rate of 0.5 mL/min with 100% mobile phase A [methanol/water/acetic acid (60 : 40 : 0.01, v/v/v)] from 0 to 2 min and a gradient increasing to 100% B (100% methanol) at 13 min. Multiple reaction monitoring was used with a dwell time of 25 or 50 ms for each compound, with the following source parameters: ion spray voltage, −4500 V, curtain gas, 40 U, ion source gas flow rate 1, 65 U, ion source gas flow 2, 50 U, and temperature, 600°C. Synthetic standards were used to obtain standard curves (5–500 pg) for each compound. These standard curves were used and extrapolated to calculate the final EET concentrations, which are presented as nanograms per milligram of protein per hour [27].

2.12. Statistical Analysis

The data are presented as mean ± SE for the number of experiments. Statistical significance (P ≤ 0.05) was determined by the Fisher method of multiple comparisons. For comparison between treatment groups, the Null hypothesis was tested by a single-factor analysis of variance (ANOVA) for multiple groups or unpaired t-test for two groups.

3. Results

3.1. Effect of SOD3 Gene Deletion on HO Expression and Activity

The effectiveness of EC-SOD gene knockdown was analyzed in renal homogenates using R.T-PCR. SOD3 KO animals demonstrated significant (P < 0.05) reduction in EC-SOD mRNA as compared to their WT counterparts (WT −2.55 ± 0.5 versus KO −0.36 ± 0.04). The administration of CoPP had no effect on SOD3 mRNA levels in both WT and KO animals (Figure 1(a)). Oxidative stress was confirmed in renal homogenates from SOD3 KO animals using lucigenin detectable chemiluminescence (WT −0.78 ± 0.1 versus KO −1.85 ± 0.14 cpm × 1000/mg, P < 0.05). Renal homogenates from EC-SOD(−/−) animals demonstrate significant (P < 0.05) induction of HO-1 which was further accentuated in animals undergoing CoPP treatment (Figure 1(b)). However, this HO-1 induction was accompanied by attenuation (P < 0.05) of HO activity in SOD3 KO animals, as measured by total bilirubin generation (WT −0.53 ± 0.12 versus KO −0.28 ± 0.09 nmol/mg protein). EC-SOD(−/−) mice concurrently exposed to CoPP demonstrated increased HO-1 expression which was accompanied by a corollary increase in (P < 0.05) HO activity in these animals (Figure 1(c)). That chronic oxidative stress induces HO expression but suppresses HO activity, is substantiated by these results which also suggest that porphyrin-induced overexpression of HO overwhelms this inhibitory effect and restores HO activity.

Figure 1

(a) Detection of EC-SOD mRNA transcripts by RT-PCR analysis in renal tissues. EC-SOD(−/−) animals demonstrated significant (P < 0.05) reduction in EC-SOD mRNA as compared to their WT counterparts, and the administration of CoPP had no significant effect on SOD3 mRNA levels in both WT and KO animals. Results are expressed as mean ± SE, *P < 0.01 versus WT mice. (b) Western blot analysis of HO-1 and HO-2 proteins in kidney homogenates of WT and EC-SOD(−/−) mice. Immunoblots were performed using antibodies against mice HO-1 and HO-2 proteins. Blots are representative of six separate experiments. Results are expressed as mean ± SE, *P < 0.01 versus WT, + P < 0.05 versus WT + CoPP, ‡ P < 0.05 versus EC-SOD(−/−). (c) Bilirubin production in WT, EC-SOD(−/−) mice and EC-SOD(−/−) mice treated with CoPP. Values are expressed as the mean ± SD of 4 experiments, *P < 0.05 versus WT mice, ‡ P < 0.01 versus EC-SOD(−/−).

3.2. Effects of HO Induction on Renal Histopathology in EC-SOD(−/−) Animals

HO deficiency, as seen in HO-2(−/−) animals, has already been shown to exacerbate renal histopathological abnormalities in streptozotocin- (STZ-) induced model of oxidative stress [28]. SOD3(−/−) mice also exhibit chronic oxidative stress with insufficient stimulation of the heme-HO system; as such, we examined renal morphology in these mice (and WT controls) with and without pharmacological induction of HO. Histological examination of renal sections revealed no significant abnormalities in WT mice treated or not with CoPP, showing no dilatation of glomeruli, proximal and distal tubules, and absence of inflammatory infiltration (Figures 2(a), 2(b), 2(e), and 2(f)). In contrast, EC-SOD(−/−) mice exhibited corticotubular damage, characterized by dilatation and loss of epithelial cells in the tubular structures with the presence of cellular infiltrate (Figure 2(c)). A considerable number of Bowman's capsules showed dilatation with hyperproliferation of capillary tufts and cellular infiltration (Figure 2(c)). Inner and outer medulla showed breakage of vasa recta (Figure 2(g)). CoPP treatment abrogated this glomerular-tubulointerstitial and microvascular pathology in EC-SOD(−/−) mice (Figures 2(d), 2(h)).

Figure 2

Representative images of renal parenchyma (haematoxylin-eosin staining, 200X) in EC-SOD(−/−) and WT mice. WT mice and WT mice treated with CoPP showed no significant morphologic alterations both in cortex (arrow's head for proximal tubules and black point for distal tubules) and medulla (asterisk for vasa recta) ((a), (b), (e), (f)). EC-SOD(−/−) mice displayed tubular damage, hyperproliferation in the glomerulii (red arrows) with Bowman's capsule dilatation (red point), infiltrates (black arrows) and breakage of vasa recta in outer and inner medulla (double asterisks). ((c), (g)). Tubular interstitial and microvascular pathology are abrogated in CoPP-treated EC-SOD(−/−) mice ((d), (h)). Western blot analysis of (i) ASK1 (j) BCL-xl protein expression in kidney of WTmic, EC-SOD(−/−) mice, and mice treated with CoPP. Quantitative densitometry evaluation of Bcl-xl, ASK-1-to-β-actin ratio was determined. Blots are representative of four separate experiments. Results are expressed as mean ± SE, *P < 0.05 versus WT, + P < 0.01 versus WT + CoPP, ‡ P < 0.05 versus EC-SOD(−/−).

Renal morphological abnormalities, in EC-SOD(−/−), were accompanied by alterations in apoptotic and antiapoptotic signals. ASK1 was significantly (P < 0.05) elevated in renal homogenates of SOD3 KO animals as compared to WT animals. Similarly, antiapoptotic protein Bcl-xl was attenuated (P < 0.05) in renal homogenates from SOD3 KO as compared to their WT counterparts. The expression levels of ASK1 and Bcl-xl proteins were restored in EC-SOD (−/−) mice treated with CoPP (Figures 2(i), 2(j)).

3.3. Effects of HO Induction, in SOD3 KO Animals, on Systolic Blood Pressure and Vascular Endothelial Function

EC-SOD(−/−) mice have been documented to have elevated blood pressure and exhibit vascular endothelial dysfunction [17]. In this regard, we examined the effect of HO induction on vascular endothelial function, and factors regulating it, in EC-SOD(−/−) mice. First, vascular redox status was assessed using lucigenin detectable chemiluminescence which showed that SOD3 gene deletion was associated with increased (P < 0.05) vascular O2 − levels; attenuated by concurrent exposure to CoPP (P < 0.05) (Figure 3(a)). EC-SOD(−/−) mice demonstrated a significantly (P < 0.05) attenuated vasorelaxation response to increasing concentrations of acetylcholine. CoPP treatment enhanced acetylcholine-induced vasodilation (P < 0.05) in these mice (Figure 3(b)) and in complimentary experiments, CoPP rescued the attenuative effects (P < 0.05) of SOD3 gene knockdown on the expression of p-eNOS (P < 0.05) (Figure 3(d)). Blood pressure measurements, performed in WT and EC-SOD-deficient mice, revealed elevated baseline blood pressure in EC-SOD(−/−) mice (WT −112.4 ± 1.8 versus KO −120.4 ± 0.9 mm Hg, P < 0.05). This elevated blood pressure was successfully normalized in mice exposed to CoPP (KO + CoPP −109.8 ± 2.4) (Figure 3(c)).

Figure 3

(a) Superoxide levels in femoral arteries of WT mice, EC-SOD(−/−) mice, and mice injected with CoPP. Superoxide levels were determined as described in methods. Results are mean ± SE, n = 4; *P < 0.01 versus WT, ‡ P < 0.01 versus EC-SOD(−/−); + P < 0.05 versus WT + CoPP. (b) Dose-response curves for acetylcholine-induced vascular relaxation after preconstriction with phenylephrine. Results are mean ± SE, n = 4; *P < 0.05 versus WT, ‡ P < 0.05 versus EC-SOD(−/−), + P < 0.05 versus WT + CoPP, and (c) systolic blood pressure measurements at the completion of the study (n = 4). Mice were injected with CoPP once a week as described in Methods. Blots are representative of four separate experiments. Results are expressed as mean ± SE, *P < 0.05 versus WT, ‡ P < 0.01 versus EC-SOD(−/−), + P < 0.05 versus WT + CoPP mice, and Western blot and densitometry analysis of (d) eNOS and p-eNOS.

3.4. Effects of HO Induction, in SOD3 KO Animals, on Epoxide and Serum Adiponectin Levels

In addition to endothelial dysfunction, vascular levels of epoxides were significantly reduced in EC-SOD(−/−) mice (WT −2.15 ± 0.2 versus KO −0.89 ± 0.3 ng/mg protein/hr, P < 0.05). This effect of SOD3 KO on EET levels was reversed in CoPP treated animals (2.35 ± 0.2 ng/mg/protein/hr, P < 0.05 versus KO) (Figure 4(a)).

Figure 4

(a) EETs levels in WT and EC-SOD(−/−) mice and mice treated with CoPP. EETs levels were determined as described in Methods. Results are mean ± SE, n = 4; *P < 0.05 versus WT mice, ‡ P < 0.05 versus EC-SOD(−/−) and + P < 0.05 versus WT + CoPP mice. (b) Adiponectin levels in WT and EC-SOD(−/−) mice and mice treated with CoPP. Adiponectin levels were determined as described in methods. Results are mean ± SE, n = 4; *P < 0.05 versus WT mice, ‡ P < 0.05 versus EC-SOD(−/−) mice, and + P < 0.05 versus WT + CoPP mice. (c) AKT and pAKT proteins in arteries of WT and EC-SOD(−/−) mice. Mice were injected with CoPP once a week as described in Methods. Blots are representative of four separate experiments. Results are expressed as mean ± SE, *P < 0.05 versus WT, ‡ P < 0.01 versus EC-SOD(−/−), + P < 0.05 versus WT + CoPP mice.

Stimulation of the heme-HO system, along with enhancement of epoxides, has been shown to be associated with increased serum and tissue levels of adiponectin. Attenuation of HO activity, in EC-SOD(−/−) mice, was accompanied by significant (P < 0.05) attenuation of serum adiponectin levels (WT −6.10 ± 0.3 versus KO −0.93 ± 0.45 μg/mL). WT mice treated with CoPP exhibited serum adiponectin levels similar to those in controls; however, CoPP treatment restored serum adiponectin levels in SOD3 deficient mice (KO + CoPP −8.05 ± 0.9 μg/mL, P < 0.05) (Figure 4(b)), indicative of its association with HO activity. In addition, expression of p-AKT, an adiponectin-dependent regulatory pathway, was also attenuated (P < 0.05) in vascular tissues from EC-SOD(−/−) mice and was restored in animals concurrently exposed to CoPP (P < 0.05) (Figure 4(c)).

4. Discussion

The present study demonstrates that chronic oxidative stress, as seen in EC-SOD(−/−) mice, has reciprocal effects on HO expression and activity and that pharmacological induction of HO restores redox homeostasis and reverts pathophysiological abnormalities observed in these animals. These effects of HO induction are associated with concomitant increases in epoxide and adiponectin levels, which may contribute towards the HO-induced reversal of renovascular abnormalities in EC-SOD(−/−) mice.

The first key finding presented in this study is the observation that mice expressing reduced levels EC-SOD have increased renovascular oxidative stress which, although accompanied by induction of HO-1 expression, concurrently results in a significant (P < 0.05) attenuation in HO activity. This discordance in HO-1 expression and activity is in line with recent reports showing that oxidative stress, in in vitro systems, inhibits HO activity by a yet unidentified post-translational modification [21, 29, 30]. In addition, investigators have also shown an insufficient induction of HO-1 in conditions of severe prolonged oxidative stress, where levels of HO are unable to provide adequate cellular antioxidant protection [31]. Accordingly, irrespective of the underlying mechanisms involved, accumulating evidence now suggests attenuation of HO activity by chronic redox imbalances in face of an increased HO-1 expression. Such an effect of prolonged oxidative stress, though counterintuitive, may explain the need and protective role of therapeutic HO induction in various models of chronic oxidative stress [32], including the current model of SOD3 KO animals. How pharmacological HO-1 induction does overcome the inhibitory effect of chronic oxidative stress and restores HO activity cannot be fully explained at this time but may entail an excess of HO and HO-derived products overwhelming the suppressive effect of cellular redox.

The second key finding presented in this study is the modulatory effect of heme-HO system on vascular epoxide and serum adiponectin levels in EC-SOD(−/−) mice. In these mice, a chronic oxidative state is characterized by insufficient HO activity and where both epoxide and adiponectin levels are attenuated. Recovery of vascular epoxide and serum adiponectin levels with CoPP-induced HO stimulation implicates the role of HO in modulating these pathways. These results are in line with our earlier report [27] showing interplay between HO and epoxides in animal models of metabolic syndrome. Recent reports have also shown that induction of HO-1 expression and activity leads to enhancement of adiponectin levels along with activation of adiponectin directed signaling pathways [31, 33, 34]. HO-mediated stimulation of adiponectin release from adipose tissues [31, 35] appears to involve the HO-induced restoration of adipocyte function including reduced oxidative stress, inflammation, and increased release of protective adipokines [9]. In addition, epoxides, whose synthesis was enhanced by HO-induction in EC-SOD(−/−) mice, have been shown in the past to attenuate dysfunctional adipogenesis and enhance adiponectin release from adipocytes [36], in an HO-dependent manner. Thus, our results presented here support the existence of an interdependent physiological axis formed by the heme-HO, epoxide, and adiponectin systems, which regulates cardiovascular-renal function, and where redox-induced attenuation of HO activity negatively affects the other two components.

Physiological implications of attenuation of HO-adiponectin axis are evident as renal corticomedullary lesions in EC-SOD(−/−) mice, amenable to reversal with CoPP. Abrogation of these renal histopathological changes, in mice treated with CoPP, was accompanied by attenuation of apoptotic and enhancement of antiapoptotic pathways, including ASK1 and Bcl-Xl. ASK1 is a stress-response protein negatively regulated by cellular redox [37], where reduced thioredoxin binds and inhibits ASK1. This inhibitory effect is overcome under conditions of oxidative stress [38], thus increasing ASK1 expression. Increased cellular expression of ASK1 has been linked to recruitment of inflammatory components, tumorogenesis, and endothelial dysfunction [39]. Reduced HO activity, in EC-SOD(−/−) mice, could facilitate oxidative stress in turn increasing ASK1 expression, and contributing towards pathological alterations observed in these animals. Induction of HO-1 expression and activity, via CoPP administration, restores cellular redox, attenuates ASK1 expression and prevents renal damage. In addition, EC-SOD(−/−) animals also demonstrate reduced tissue expression of Bcl-XL and p-AKT/AKT proteins. AKT interacts with HSP-90 and is involved in inhibition of ASK1 and associated pathways [15]. Increased HO-1 and adiponectin levels enhance p-AKT/AKT expression with resultant cardio-reno-protective effects [36, 40] including suppression of ASK1. These results are also in line with earlier reports [11, 41] demonstrating renoprotective effects of adiponectin in patients with chronic kidney disease.

Finally, significant improvement of endothelial function and attenuation of elevated blood pressure, in EC-SOD(−/−) mice, by induction of heme-HO system underscores the role of the HO-epoxide-adiponectin system in regulating vascular homeostasis. Induction of HO via CoPP provides excess CO and BV, where BV acts as an endogenous chain breaking antioxidant [1] and increases NO bioavailability and CO has been shown to induce vasodilation via activation of vascular Kca channels [1, 42]. Another aspect of the HO-1 induction-mediated amelioration of vascular dysfunction may involve upregulation of eNOS and stimulation of epoxide synthesis. Epoxides are one of the candidates for endothelium derived hyperpolarizing factor (EDRF) and induce vasodilation along with inhibition of inflammatory response and stimulation of epithelial cell growth [43, 44]. In addition, HO stimulation is accompanied by increased adiponectin release and adiponectin has been shown to induce generation of NO in vascular endothelial cells [45] and has also been shown to mediate AKT-dependent vasodilation in the rat aorta [46]. Thus, current evidence, in light of previous reports, implicates the interaction between the HO, epoxide and adiponectin systems in governing vascular endothelial function. The antioxidant and vasomodulatory properties of HO products are complimented by upregulation of vascular p-eNOS, increase in vasodilatory epoxides and stimulation of adiponectin-AKT pathway. These vascular and hemodynamic effects EC-SOD gene deletion may also contribute towards renal histopathological alteration observed in these mice and hemodynamic improvement, observed in CoPP-treated SOD3 KO mice, could thus contribute towards attenuation of renal pathology in these animals.

In conclusion (Figure 5), this study puts in perspective two emerging concepts in cardiovascular pathophysiology. First, it demonstrates a reciprocal effect of chronic oxidative stress on HO-1 expression and activity and secondly, the present study suggests interdependence amongst three physiological systems, that is, the heme-HO, adiponectin and epoxide systems. Where chronic oxidative stress conditions, such as SOD3 deficiency potentiate HO-1 expression, a concurrent inhibitory effect on HO activity is observed. This effect is accompanied by renovascular abnormalities and suppression of epoxide and adiponectin synthesis. Restoration of HO activity, via pharmacological modulators of the heme-HO system, not only restores cellular redox, but also recovers vascular epoxide and serum adiponectin levels along with abrogation of renal pathology, improvement of vascular function, and attenuation of blood pressure.

Figure 5

The schematic outlines the salient points discussed in the paper. Chronic oxidative stress upregulates HO-1 expression whose products, CO & BV have antioxidant properties and help restore redox homeostasis. In addition, HO induction stimulates epoxide synthesis and these pathways combined enhance adiponectin release. Antioxidant, antiapoptotic, vasomodulatory, and renoprotective properties of HO-EET-adiponectin axis restore cardiovascular-renal homeostasis often disrupted in conditions of oxidative stress. HO induction overcomes a concurrent inhibition of HO activity by redox-dependent mechanisms which offset the protective effects of enhanced HO expression thus contributing towards pathophysiological abnormalities observed in conditions of chronic oxidative stress.