Glomerular Endothelium and its Impact on Glomerular Filtration Barrier in Diabetes: Are the Gaps Still Illusive?

BACKGROUND: Glomerular capillaries are lined with highly specialized fenestrated endothelium which are primarily responsible to regulate high flux filtration of fluid and small solutes. During filtration, plasma passes through the fenestrated endothelium and basement membrane before it reaches the slit diaphragm, a specialized type of intercellular junction that connects neighbouring podocytes. METHODS: A PubMed search was done for recent articles on components of the glomerular filtration barrier such as glomerular endothelial cells, podocytes and glomerular basement membrane, and the effect of diabetes on these structures. RESULTS AND CONCLUSION: Generally, the onset of kidney dysfunction in many diabetic patients is characterized by albuminuria/proteinuria, a pathophysiological event triggered by several factors including; (i) endothelial activation and shading of glycocalyx, (ii) loss of endothelial cell function, (ii) re-uptake of albumin by podocyte through a scavenger receptors and (iv) rearrangement of podocyte cytoskeleton. Howeover, as podocyte effacement does not always lead to proteinuria, the dynamic interplay between all constituents of the glomerular filtration barrier including podocytes, endothelial cells and the basement membrane may be fundamental for the effective filtration in healthy individuals. Thus, a putative cross-talk amongst podocytes, endothelial cells and the basement membrane in the homeostasis of glomerular function is envisaged. Although, the exact nature of this cross-talk remains to be clearly elucidated, it is possible that the interaction between: (i) glomerular endothelial cells and podocytes, (ii) glomerular endothelial cells and glomerular basement membrane, (iii) podocytes and glomerular basement membrane, and (iv) the simultaneous interaction amongst the three components collectively underpin effective filtration in healthy individuals. A comprehensive understanding of these different interactions still remains elusive. The elucidation of these multifaceted interactions will set the stage for greater understanding of the pathophysiology of kidney dysfunction.

Synergistic Interaction Between Heme Oxygenase (HO) and Nuclear-Factor E2- Related Factor-2 (Nrf2) against Oxidative Stress in Cardiovascular Related Diseases.

BACKGROUND: Nuclear factor-erythroid related factor-2 (Nrf2) is a master regulator of transcriptional activation of anti-oxidants in cells. Similarly, heme oxygenase (HO) is a cytoprotective protein with anti-oxidant effects. This review article will shed more light on the interaction between Nrf2 and HO. METHODS AND RESULTS: A PubMed search was done for recent articles on Nrf2 and HO. These studies suggested that under normal physiological conditions, Nrf2 is bound within the cytoplasm to its repressor, Kelch-like ECHassociated protein (Keap1), an oxidative stress sensor. Upon activation, Nrf2 translocates to the nucleus and binds to the antioxidant-response-element located at the promoter region of some anti-oxidants including the cytoprotective protein HO. Since the HO-1 gene harbors binding site for Nrf2, mutual stimulatory and regulatory effects between Nrf2 and HO-1 have been reported. Accordingly, the interaction between Nrf2 and HO-1 has been implicated in the regulation of many physiological anti-oxidants including superoxide dismutases, catalase, glutathione S-transferase, peroxidase, NAD(P)H quinone oxidoreductase, and thioredoxin. CONCLUSION: Although an overwhelming body of evidence has underscored unique anti-oxidant attributes of HO- 1 and Nrf2, emerging evidence suggests that the cytoprotective activities of Nrf2 and HO-1 may be attributed, at least in part, to the potentiation of different anti-oxidants in physiological mileu. Since Nrf2 binds to the antioxidant responsive element of HO-1, the coordinated regulation of Nrf2 and keap1 by the HO-system may constitute the basis of many physiological effects of HO-1 including its effects against oxidative stress and inflammation in a wide spectrum of cardiovascular, cardio-metabolic and other related diseases.

The Different Facets of Dyslipidemia and Hypertension in Atherosclerosis.

Atherosclerosis is the narrowing of arteries due to the accumulation of macrophages overloaded with lipids resulting in foam cell formation, and these events occur preferentially at the branching points of arteries which are particularly susceptible to hyperlipidemic stress-induced inflammation and oxidative stress. The different stages of atherogenesis rely on oxidative stress, endothelial dysfunction, and inflammation, and hypertension or dyslipidemia can independently trigger these stages. Dyslipidemia and hypertension are pathological conditions that damage the endothelium, triggering cell proliferation, vascular remodeling, apoptosis, and increased cellular permeability with increased adhesion molecules that bind monocytes and T lymphocytes to create a vicious cocktail of pathophysiological factors. Correspondingly, the factors are redirected by chemo-attractants and pro-inflammatory cytokines into the intima of the vasculature, where monocytes differentiate into macrophages taking up oxidized LDL uncontrollably to form foam cells and atherosclerotic lesions. Moreover, endothelial damage also causes loss of vasomotor activity, disproportionate vascular contractility, and elevation of blood pressure in dyslipidemic patients, while in hypertensive patients, further elevation of blood pressure occurs, creating a self-perpetuating vicious cycle that aggravates the development and progression of atherosclerotic lesions. This review offers an in-depth analysis of atherosclerosis and the related interplay between dyslipidemia/hypertension and critically appraises the current diagnosis, etiology, and therapeutic options.

Heme oxygenase in cardiac repair and regeneration.

The incidence of cardiac complications such as myocardial infarction and congestive heart failure is increasing. Once congestive heart failure emerges, little can be done to improve long-term cardiac function. The cellular basis of this downward spiral may be the dramatic loss of viable cardiomyocytes following acute ischemia and/or chronic apoptosis/necrosis which are not adequately replaced. Contrary to the old postulates of developmental biology, tissue self-renewal is not limited to blood, intestines and skin, but other organs including the heart have some capabilities of self-renewal. However, the intrinsic ability of self-renewal of adult myocardium or cardiomyogenesis is limited and thus novel paradigms capable of potentiating this process of regenerative organogenesis should be sought. Several strategies and paradigms are being currently explored, and among these is the heme-oxygenase system. Emerging evidence indicate that the heme-oxygenase system could be explored in regenerative medicine given its unique ability to concomitantly suppress apoptosis and necrosis, while facilitating tissue regeneration/repair and the formation of new blood vessels. This review highlights the recent development of heme-oxygenase in myocardial repair and regeneration.

Role of the heme oxygenase-adiponectin-atrial natriuretic peptide axis in renal function.

The incidence of renal complications including kidney failure is on the rise. Moreover, with aging of the population and the high incidence of diabetes, hypertension and obesity, this trend may prevail. An important cytoprotective enzyme that has been shown to improve renal function is heme-oxygenase (HO). HO is known to abate apoptosis and necrosis, and improves cell vitality, which in turn, may enhance tissue regeneration. Consistently, HO has been shown to restore tissue morphology by potentiating potentiate proteins of repair/regeneration and promoting neovascularization. The formation of new tissue may replace damaged or dysfunctional tissue to preserve cellular integrity and function after injury. Emerging evidence indicate that HO-inducers improve kidney function in several models including, (i) streptozotocin-induced diabetic rats, (ii) Zucker-diabetic-fatty rats, (iii) Zucker-fatty rats, (iii) spontaneously hypertensive rats, (iv) uninephrectomized deoxycorticosterone-acetate hypertensive rats, (v) N(ω)-nitro-l-arginine-methyl ester (L-NAME)-induced hypertensive rats, (vi) glycerol induced renal failure, (vii) nephrotoxic nephritis, (viii) sepsis-induced kidney injury, (ix) cystic renal disease, (x) cisplatin-mediated acute kidney injury, and (xi) rhabdomyolysis-induced renal injury. The mechanisms underlying the HO-mediated reno-protection include: (i) the restoration of renal morphology by enhancing proteins of regeneration, (ii) the potentiation of the HO-adiponectin-atrial natriuretic peptide axis, with corresponding suppression of oxidative/inflammatory insults and extracellular matrix/profibrotic factors, and (iii) the potentiation of podocyte cytoskeletal proteins such as nephrin, podocin, podocalyxin and CD2-associated-protein, which are fundamental for forming the glomerular filtration barrier that selectively allows small molecules to pass through but not large protein molecules. Thus, this review highlights the HO-adiponectin-atrial natriuretic peptide axis in renoprotection.

Featured article: induction of heme oxygenase with hemin improves pericardial adipocyte morphology and function in obese Zucker rats by enhancing proteins of regeneration.

Oxidative stress and inflammation are implicated in tissue remodeling, hypertrophy, and organ malfunction. Since heme-oxygenase (HO) is a cytoprotective enzyme with effects against oxidative stress and inflammation, we investigated the effects of upregulating HO with hemin on adipocyte hypertrophy, proteins of repair/regeneration including beta-catenin, Oct3/4 and Pax2 as well as pro-fibrotic/remodeling proteins like osteopontin and transforming growth factor-beta (TGF-ß) in pericardial adipose tissue from obese Zucker rats (ZRs). Treatment with hemin significantly reduced pericardial adipose tissue inflammation/oxidative stress, suppressed osteopontin and TGF-ß, and attenuated pericardial adipocyte hypertrophy in obese ZRs. These were associated with enhanced expression of the stem/progenitor-cell marker cKit; the potentiation of several proteins of regeneration including beta-catenin, Oct3/4, Pax2; and improved pericardial adipocyte morphology. Interestingly, the amelioration of adipocyte hypertrophy in hemin-treated animals was accompanied by improved adipocyte function, evidenced by increased levels of pericardial adipose tissue adiponectin. Furthermore, hemin significantly reduced hypertriglyceridemia and hypercholesteromia in obese ZRs. The protective effects of hemin were accompanied by robust potentiation HO activity and the total antioxidant capacity, whereas the co-administration of hemin with the HO inhibitor, stannous mesoporphyrin abolished the effects of hemin. These data suggest that hemin improves pericardial adipocyte morphology and function by enhancing proteins of repair and regeneration, while concomitantly abating inflammatory/oxidative insults and suppressing extracellular-matrix/profibrotic and remodeling proteins. The reduction of hypertriglyceridemia, hypercholesteromia, pericardial adiposity, and pericardial adipocyte hypertrophy with corresponding improvement of adipocyte morphology/function in hemin-treated animals suggests that HO inducers may be explored for the design of novel remedies against cardiac complications arising from excessive adiposity.

Heme Oxygenase Improves Renal Function by Potentiating Podocyte-Associated Proteins in Nω-Nitro-l-Arginine-Methyl Ester (l-NAME)-Induced Hypertension.

BACKGROUND: Although heme-oxygenase (HO) is cytoprotective, its effects on podocyte regulators like podocalyxin, podocin, CD2-associated protein (CD2AP) in renal dysfunction in N (ω)-nitro-l-arginine-methyl ester (l-NAME) hypertension are largely unclear. METHODS: Hypertension was induced in normotensive Sprague Dawley rats by administering l-NAME for 4 weeks. Enzyme immunoassay, enzyme-linked immunosorbent, histology/morphology, spectrophotometry, and western immunoblotting were used. HO was enhanced with heme-arginate (HA) or inhibited with chromium mesoporphyrin (CrMP). RESULTS: Treatment with heme-arginate reduced several renal histo-pathological lesions including renal arteriolar thickening, glomerular abnormalities, tubular cast, tubular atrophy/fibrosis, and mononuclear cell infiltration in l-NAME-hypertensive rats. Similarly, HA abated the elevated levels of renal extracellular matrix/profibrotic proteins like collagen and fibronectin that deplete nephrin, a fundamental transmembrane protein that forms the scaffoldings of the podocyte slit diaphragm permitting small ions to filter, but not massive excretion of proteins, hence proteinuria. Correspondingly, HA enhanced the aberrant expression of nephrin alongside other important regulators of podocyte like podocalyxin, podocin, and CD2AP, and improved renal function by reducing albuminuria/proteinuria, while increasing creatinine clearance. The renoprotection by HA were accompanied by significant reduction of inflammatory/oxidative mediators including nuclear factor-kappaB, macrophage inflammatory protein-1-alpha, macrophage chemoattractant protein-1, tumor necrosis factor-alpha, interleukin (IL)-6, IL1ß, 8-isoprostane, endothelin-1, and aldosterone. These were associated with increased levels of adiponectin, HO-1, HO activity, cyclic guanosine monophosphate, and atrial natriuretic peptide (ANP), whereas the HO inhibitor, CrMP annulled the renoprotection and exacerbated renal dysfunction. CONCLUSIONS: HA improves renal function by attenuating histopathological lesions, suppressing inflammatory/oxidative mediators, abating profibrotic/extracellular matrix proteins, and reducing albuminuria/proteinuria, while concomitantly potentiating the HO-adiponectin-ANP axis, enhancing nephrin, podocin, podocalyxin, CD2AP and increasing creatinine clearance. Our study underscores the benefit of potentiating the HO-adiponectin-ANP against nephropathy.

Mechanisms by which heme oxygenase rescue renal dysfunction in obesity.

Obesity and excessive inflammation/oxidative stress are pathophysiological forces associated with kidney dysfunction. Although we recently showed that heme-oxygenase (HO) improves renal functions, the mechanisms are largely unclear. Moreover, the effects of the HO-system on podocyte cytoskeletal proteins like podocin, podocalyxin, CD2-associated-protein (CD2AP) and proteins of regeneration/repair like beta-catenin, Oct3/4, WT1 and Pax2 in renal tissue from normoglycemic obese Zucker-fatty rats (ZFs) have not been reported. Treatment with hemin reduced renal histo-pathological lesions including glomerular-hypertrophy, tubular-cast, tubular-atrophy and mononuclear cell-infiltration in ZFs. These were associated with enhanced expression of beta-catenin, Oct3/4, WT1, Pax2 and nephrin, an essential transmembrane protein required for the formation of the scaffoldings of the podocyte slit-diaphragm, permitting the filtration of small ions, but not massive excretion of proteins, hence proteinuria. Besides nephrin, hemin also enhanced other important podocyte-regulators including, podocalyxin, podocin and CD2AP. Correspondingly, important markers of renal dysfunction such as albuminuria and proteinuria were reduced, while creatinine clearance increased, suggesting improved renal function in hemin-treated ZFs. The renoprotection by hemin was accompanied by the reduction of inflammatory/oxidative mediators including, macrophage-inflammatory-protein-1α, macrophage-chemoattractant-protein-1 and 8-isoprostane, whereas HO-1, HO-activity and the total-anti-oxidant-capacity increased. Contrarily, the HO-inhibitor, stannous-mesoporphyrin nullified the reno-protection by hemin. Collectively, these data suggest that hemin ameliorates nephropathy by potentiating the expression of proteins of repair/regeneration, abating oxidative/inflammatory mediators, reducing renal histo-pathological lesions, while enhancing nephrin, podocin, podocalyxin, CD2AP and creatinine clearance, with corresponding reduction of albuminuria/proteinuria suggesting improved renal function in hemin-treated ZFs. Importantly, the concomitant potentiation regeneration proteins and podocyte cytoskeletal proteins are novel mechanisms by which hemin rescue nephropathy in obesity.

Cross-talk between heme oxygenase and peroxisome proliferator-activated receptors in the regulation of physiological functions.

Peroxisome-proliferator-activated-receptors (PPARs) are transcription factors belonging to the superfamily of nuclear receptors. The isoforms of PPAR include PPAR alpha, PPAR gamma and PPAR delta (also known as PPAR beta). Generally, PPARs potentiate insulin sensitivity, improve glucose/lipid metabolism, suppress inflammation/oxidative stress, attenuate excessive immune responses, regulate cell-growth and differentiation. Interestingly, agonists of PPAR gamma and PPAR alpha have been shown to upregulate the heme-oxygenase (HO)-system. Conversely, the HO-system also enhances PPAR alpha, and potentiates the expression and activity of PPAR gamma. Moreover, the HO-system and related products including bilirubin, biliverdin, carbon monoxide and ferritin have been shown to increase insulin sensitivity, improve glucose/lipid metabolism, suppress inflammation/oxidative stress, abate immune response, and modulate cell-growth/differentiation. Therefore, an intimate, reciprocal, stimulatory and synergistic relationship between PPAR-signaling and the HO-system can be envisaged in the regulation of physiological functions. Thus, both the HO-system and PPARs-signaling participate in fine-tuning similar physiological functions, so novel pharmacological agents capable of optimizing this interaction should be sought. The coordinated regulation of PPAR-signaling and the HO-system may constitute the basis for future drug design.

Heme oxygenase suppresses markers of heart failure and ameliorates cardiomyopathy in L-NAME-induced hypertension.

Heart failure and related cardiac complications remains a great health challenge. We investigated the effects of upregulating heme-oxygenase (HO) on myocardial histo-pathological lesions, proinflammatory cytokines/chemokines, oxidative mediators and important markers of heart failure such as osteopontin and osteoprotergerin in N(ω)-nitro-l-arginine methyl ester (L-NAME)-induced hypertension. Treatment with the HO-inducer, heme-arginate improved myocardial morphology in L-NAME hypertensive rats by attenuating subendocardial injury, interstitial fibrosis, mononuclear-cell infiltration and cardiomyocyte hypertrophy. These were associated with the reduction of several inflammatory/oxidative mediators including chemokines/cytokines such as macrophage inflammatory protein-1 alpha (MIP-1α), macrophage chemoattractant protein-1 (MCP-1), tumor necrosis factor alpha (TNF-α), interleukin (IL)-6, IL-1ß, endothelin-1, 8-isoprostane, nitrotyrosine, and aldosterone. Similarly, heme-arginate abated the elevated levels of extracellular matrix/remodeling proteins including transforming-growth factor beta (TGF-ß1) and collagen-IV in the myocardium. These were accompanied by significant reduction of proteins of heart failure such as osteopontin and osteoprotegerin. Interestingly, the cardio-protective effects of heme-arginate were associated with the potentiation of adiponectin, atrial-natriuretic peptide (ANP), HO-1, HO-activity, cyclic gnanosine monophosphate (cGMP) and the total-anti-oxidant capacity, whereas the HO-inhibitor, chromium-mesoporphyrin nullified the effects of heme-arginate, exacerbating inflammatory injury and oxidative insults. We conclude that heme-arginate therapy protects myocardial damage by potentiating the HO-adiponectin-ANP axis, which in turn suppressed the elevated levels of aldosterone, pro-inflammatory chemokines/cytokines, mononuclear-cell infiltration and oxidative stress, with concomitant reduction of extracellular matrix/remodeling proteins and heart failure proteins. These data suggest a cardio-protective role of the HO system against L-NAME-induced hypertension that could be explored in the design of novel strategies against cardiomyopathy.

The heme oxygenase system suppresses perirenal visceral adiposity, abates renal inflammation and ameliorates diabetic nephropathy in Zucker diabetic fatty rats.

The growing incidence of chronic kidney disease remains a global health problem. Obesity is a major risk factor for type-2 diabetes and renal impairment. Perirenal adiposity, by virtue of its anatomical proximity to the kidneys may cause kidney disease through paracrine mechanisms that include increased production of inflammatory cytokines. Although heme-oxygenase (HO) is cytoprotective, its effects on perirenal adiposity and diabetic nephropathy in Zucker-diabetic fatty rats (ZDFs) remains largely unclear. Upregulating the HO-system with hemin normalised glycemia, reduced perirenal adiposity and suppressed several pro-inflammatory/oxidative mediators in perirenal fat including macrophage-inflammatory-protein-1α (MIP-1α), endothelin (ET-1), 8-isoprostane, TNF-α, IL-6 and IL-1ß. Furthermore, hemin reduced ED1, a marker of pro-inflammatory macrophage-M1-phenotype, but interestingly, enhanced markers associated with anti-inflammatory M2-phenotype such as ED2, CD206 and IL-10, suggesting that hemin selectively modulates macrophage polarization towards the anti-inflammatory M2-phenotype. These effects were accompanied by increased adiponectin, HO-1, HO-activity, atrial-natriuretic peptide (ANP), and its surrogate marker, urinary-cGMP. Furthermore, hemin reduced renal histological lesions and abated pro-fibrotic/extracellular-matrix proteins like collagen and fibronectin that deplete nephrin, an important transmembrane protein which forms the scaffolding of the podocyte slit-diaphragm allowing ions to filter but not massive excretion of proteins, hence proteinuria. Correspondingly, hemin increased nephrin expression in ZDFs, reduced markers of renal damage including, albuminuria/proteinuria, but increased creatinine-clearance, suggesting improved renal function. Conversely, the HO-blocker, stannous-mesoporphyrin nullified the hemin effects, aggravating glucose metabolism, and exacerbating renal injury and function. The hemin effects were less-pronounced in Zucker-lean controls with healthy status, suggesting greater selectivity of HO in ZDFs with disease. We conclude that the concomitant reduction of pro-inflammatory/oxidative mediators, macrophage infiltration and profibrotic/extracellular-matrix proteins, coupled to increased nephrin, adiponectin, ANP, cGMP and creatinine clearance may account for improved renal function in hemin-treated ZDFs. These findings suggest that HO-inducers like hemin may be explored against the co-morbidity of perirenal adiposity and diabetic nephropathy.