(Part I) Role Of Arachidonic Acid And Its Metabolites in The Biological And Clinical Manifestations Of Idiopathic Nephrotic Syndrome
Mar 26, 2022
Abstract: Studies concerning the role of arachidonic acid (AA)and its metabolites in kidney disease are scarce, and this applies in particular to idiopathic nephrotic syndrome (INS). is one of the most frequent glomerular diseases in childhood; it is characterized by T-lymphocyte dysfunction, alterations of pro-and anti-coagulant factor levels, and increased platelet count and aggregation, leading to thrombophilia. AA and its metabolites are involved in several biological processes. Herein, we describe the main fields where they may play a significant role, particularly as it pertains to their effects on the kidney and the mechanisms underlying INS.AA and its metabolites influence cell membrane fluidity and permeability, modulate platelet activity and coagulation, regulate lymphocyte activity and inflammation, preserve the permeability of the glomerular barrier, influence podocyte physiology, and play a role in renal fibrosis. We also provide suggestions regarding dietary measures that are able to prevent an imbalance between arachidonic acid and its parental compound linoleic acid, in order to counteract the inflammatory state which characterizes numerous kidney diseases. On this basis, studies of AA in kidney disease appear as an important field to explore, with possible relevant results at the biological, dietary, and pharmacological level, in the final perspective for AA to modulate INS clinical manifestations.
Keywords: kidney; arachidonic acid; nephrotic syndrome; kidney disease; renal fibrosis
CISTANCHE WILL IMPROVE KIDNEY/RENAL DISEASE
1. Introduction Idiopathic nephrotic syndrome (INS) is one of the most frequent glomerular diseases in childhood [1]. It is characterized by proteinuria, caused by podocyte damage, hypoalbuminemia, hyperlipidemia, and edema[1]. While the exact cause of podocyte damage is still not completely understood [1], it is well known that hyperlipidemia is related to urinary loss of transport proteins, which carry free cholesterol, and to the consequent compensatory increase in the synthesis of proteins involved in triglyceride metabolism [1]. Two theories have been proposed to explain the pathogenesis of edema in INS. According to the classical underfill hypothesis, hypoalbuminemia reduces plasma oncotic pressure, which leads to sodium and water retention and water leakage into the interstitium |2]. Meanwhile, the overfill hypothesis postulates proteinuria to be the primary cause of sodium retention, with consequent volume expansion and leakage of excess fluid into the interstitium3. Other biochemical alterations were also described in INS, such as changes in pro-and anti-coagulation factors levels and increased platelet count and aggregation, leading to a hypercoagulable state [4].
Based on their response to corticosteroid therapy, children with INS are classified as steroid-sensitive patients, which includes those with infrequent relapses, frequently relapsing or steroid-dependent patients who present a favorable prognosis, or steroid-resistant patients, who carry an unfavorable prognosis in the majority of cases. Histopathology usually reveals a minimal change of disease, which is characterized by the normal glomerular appearance on light microscopy and evidence of podocyte foot processes' alterations on electron microscopy; focal segmental glomerulosclerosis and interstitial fibrosis may be found in steroid-resistant cases [5,6].
The pathogenesis of INS has not to vet been fully clarified. Excluding genetic causes, the main theory for immune-mediated cases involves dysfunction of T lymphocytes, which would switch to the production of still poorly defined permeability factors that interfere with the expression and/or function of key proteins in the podocyte, thus being the main culprits of proteinuria [7]. Candidates for the circulating factors that affect glomerular permeability include angiopoietin-like 4(ANGPTL4), corticotrophin-like cytokine-1 (CLC-1), and soluble urokinase plasminogen activator receptor(suPAR) [1]. Arachidonic acid (AA) is a long-chain polyunsaturated fatty acid of the omega-6group and represents 7% to 10% of total circulating fatty acids; it is the second most abundant omega-6 fatty acid in the human body[8](Table 1), with linoleic acid (LA)being the first. AA is synthesized endogenously from LA through three steps mediated by two enzymes, desaturase, and elongase, and may also be derived from the diet. In turn, AA is a substrate of elongases for the synthesis of longer fatty acids of the omega-6 series.
AA is metabolized by three types of oxygenases: cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450, leading to the generation of eicosanoids, namely prostaglandins, thromboxane, leukotrienes, and hydroxyeicosatetraenoic acids.
Blood AA levels do not reflect its synthesis and metabolization pathways(Figure 1), as they are maintained as constant, even at the expense of other biological factors, as observed in patients with epidermolysis bullosa 9], where, despite a large number of active AA metabolites, the AA level is comparable to that of healthy controls. This phenomenon has been observed in several other chronic inflammatory disorders, for instance, cystic fibrosis [10], even if the exact mechanism behind it is unclear.
AA is involved in several biological processes, either in health or disease. Herein, we describe its role in nephrotic syndrome from a biological and clinical perspective. AA influences cell membrane fluidity and permeability and modulates platelet function and immune system activation; furthermore, it affects glomerular and tubular function, the physiopathology of podocytes, and the process of renal fibrosis. We also detail the interactions between AA and the common drugs prescribed for INS treatment. Finally, the role of dietary AA balance and its nutritional sources are discussed. For this review, PubMed (www.pubmed.gov, accessed on 28 February 2021) was the only source of the articles. No limit was given regarding the date of publication of the articles, and the following keywords were used: arachidonic acid, arachidonic acid metabolism, cell membrane, immune system, nephrotic syndrome, membrane receptor, coagulation, platelets, arachidonic acid pathway, TXA2, LTB4, PGE2, CNI pharmacogenomics, cyclosporine A, tacrolimus, kidney disease, arachidonic acid and kidney, podocyte, podocyte, and arachidonic acid, 20-HETE,20-HETE metabolism, renal fibrosis, SNI pharmacogenomics, CYP, and all the key words related to the biological mechanism reported in each chapter.
2. Cell Membrane Fluidity and Permeability It was recently described that erythrocyte membranes of patients with INS differ from those of normal subjects, particularly due to reduced membrane fluidity [11].
AA is one of the most abundant fatty acids in the cell membrane, to which it endows mobility and flexibility [12,13]. The fatty acid composition determines the viscosity of the cell lipid bilayer and membrane fluidity, thus directly affecting the function of specific membrane proteins, like, for example, those involved in cellular inflammatory signaling, namely lymphocyte function-associated antigen 1(LFA-1), intercellular adhesion molecule 1 (ICAM-1), and a cluster of differentiation 2 (CD2) [12,13].
With regard to membrane permeability, AA acts on Ca2+ cell load [10] with a double effect: at low micromolar concentrations, it increases Ca2+-ATPase activity, while at higher concentrations it reduces ATPase activity. This may be due to an unspecific and non-physiological inhibitory effect on the hydrolytic activity of P-type ATPase. ATPases are a superfamily of lipid pumps involved, among other functions, in secretion and absorption at the kidney level; these pumps are blocked by protein kinase C inhibitors [14]. AA increases membrane permeability to calcium, which is a key factor for platelet activation |15].
CISTANCHE WILL IMPROVE KIDNEY/RENAL FAILURE
AA may act on ion channels by either binding to or inserting among the membrane molecules, thus modifying the mechanical properties of the cell membrane and modulating channel function [16].AA also has a direct effect on several membrane potassium channels, either by accelerating their inactivation(in particular, the A-type channels and delayed rectifier channels), or by inducing the activation of large-conductance voltage-independent channels. The two-pore domain potassium channels are inactivated by AA as well, in contrast to what usually occurs with classical K channel-blocking drugs 16]. Transient receptor potential channels (TPR) are instead activated directly by AA and its lipoxygenase (LOX)-derived metabolites [16] (namely,12-and 15-(S)-hydroperoxy eicosatetraenoic acids,5-and 15-(S)-hydroxyeicosatetraenoic acids, and leukotriene B4).LOX metabolites can activate the TPR channel by virtue of their structure that mimics the capsaicin structure [17]. Interestingly, AA and its metabolic byproducts effects on calcium and potassium balance at the mem-brane level have been hypothesized to underlie the molecular-related derangements in INS[6].
As it concerns membrane fluidity, albumin is the main fatty acid-binding protein in extracellular fluid, having seven fatty acid-binding sites [18]. Albumin increases AA release from cell membranes in a concentration-dependent manner, by interacting with membrane phospholipids on the extracellular surface; in particular, positively charged arginine residues at or near albumin's binding sites for LCFA interact with AA, determining its release from the phospholipid layer19]. Thus, albumin decreases the cell membrane permeability of endothelial and circulating cells to water and small solutes [19]. In conclusion, the amount of AA in cell membranes regulates several cellular functions and all factors that vary the amount of AA in the membrane may play a significant pathogenetic role in renal disease.
3. Platelet Aggregation and Coagulation Two of the most active compounds related to platelet function are thromboxane and prostacyclin, both metabolites of AA [20]. AA is released from the platelet membrane by phospholipase A2(PLA2), which hydrolyzes the bond between the second fatty acid of phospholipids and the glycerol molecule. The released AA is then metabolized by cyclooxygenase [21], generating prostaglandin G2, and thereafter prostaglandin H. Afterwards, two different pathways can take place: the first one, within the platelets, leads to the synthesis of thromboxane A2 (TXA2) and subsequently B,(TXB); the second one, within the endothelial cells, leads to the synthesis of prostacyclin (PGl2)(Figure 2).
TXA, stimulates platelet activation and aggregation, via platelet fibrinogen-binding αIIbβ3 receptors [4]. Prostacyclins, in contrast, inhibit platelet activation by activating G protein-coupled receptors on platelets and endothelial cells. Upon binding to the prostacyclin receptor, PGI, induces adenyl cyclase cAMP production, which in turn inhibits platelet activation [22].
A higher incidence of increased platelet aggregation and thromboembolism has been reported in nephrotic syndrome, in relation to consistently elevated levels of fibrinogen Moreover, both hyperlipidemia and hypoalbuminemia, which are characteristic findings of nephrotic syndrome, increase thromboxane availability, through the production of TXA, precursors and the removal of TXA, inhibitors [23]. The exact mechanism underlying this process is still unknown, but it probably involves an increase in PLA, activity, related to abnormally high cholesterol levels [24]. Therefore, arachidonic acid, which is the precursor of thromboxane, may be considered a crucial player in the platelet-related coagulation process.
It is worth noting that in a clinical trial that recruited six healthy male volunteers, fed for 50 days with a diet containing 1.7gr/day of AA, and six controls, fed with a diet containing 210 mg/day of AA [20], moderate intakes of foods rich in AA, like those of the first group, had only mild effects on blood coagulation, platelet function, and platelet fatty acid composition compared to controls. The authors attributed the poor efficacy of arachidonic acid supplementation to the moderate amount in which it was supplied. In platelets, PGH2 is metabolized to thromboxane A2, activating coagulation and platelet aggregation, while in endothelial cells, prostaglandin 2 (PGI2), which has an anticoagulant effect, is generated.
4. Immune System The therapeutic efficacy of Rituximab in modifying the course of steroid-dependent nephrotic syndrome suggested that B cells play a key role in the pathogenesis of INS. This was recently confirmed by the evidence of a pathological increase in memory B cells in INS [25]. Moreover, other studies showed a decrease of Treg cells [26], dysregulation of T-cells [27l lower levels of NK and NKT cells, and increased levels of inflammatory markers during proteinuria [28,29]. These studies confirm [1] that the immune system plays a pivotal role in non-genetic INS and specifically in the loss of the glomerular barrier function, by activating the inflammatory process against podocytes.
CISTANCHE WILL IMPROVE KIDNEY/RENAL INFECTION
In immune cells, like lymphocytes, neutrophils, and monocytes, AA constitutes about 20% of total fatty acids, while EPA and DHA constitute 1%and 2.5%, respectively [30]. It was reported that oral administration of omega-3 fatty acids changes the pattern of production of eicosanoids, by increasing resolvins production, thus affecting phagocytosis, T-cell signaling, and antigen presentation capability. These effects seem to be mediated at the membrane level [30].
The distribution of AA within intracellular lipid pools in inflammatory cells has an important role in regulating eicosanoids production. In fact, a pool of AA was identified within the triglycerides of mast cells, eosinophils, monocytes, and platelets [31].
When inflammatory cells are activated, AA is released from membrane phospholipids into the cell and partially incorporated into intracellular triglycerides, ready to supply membrane phospholipids again after cell activation has ended [32].
Thus, AA metabolites can act in several ways on lymphocyte activity, affecting inflammation levels [32-34] (Table 2) and possibly the course of INS. Regarding B, NK, and T cells, the main AA metabolites involved are PGE2, LTB4, and TXA2: PGE is produced by nearly all cells within the body [35]. Secreted PGE, acts in an autocrine or paracrine manner through its four cognate G protein-coupled receptors EP1 to EP4 【36】. It inhibits T-cell and NK cell proliferation， as well as IFN-γ and IL-12 production [37], binding their cell-surface receptors [38]. PGE2 also inhibits B-cell activation secondary to I-4 stimulation in a specific manner and enhances IgE and IgG1 production [39].
LTB exerts pleiotropic effects on lymphocytes and regulates the immune response in a dynamic, cell type- and context-dependent manner: LTB, enhances T-cell recruitment, it inhibits de Moyo iTreg generation, and increases interleukin-17(IL-17) cytokine production during T-cell differentiation. LTB also regulates the migration of various lymphoid-derived cell types in different ways that vary depending on disease and tissue [A0]. TXA,, another product of AA metabolism, inhibits naive T-cell proliferation and exerts several effects on mature T lymphocytes: it inhibits T-cell interaction with dendritic cells, increases T-cell proliferation and activation, and has been shown to topically enhance the cytotoxic activity of immune cells [37]. Moreover, eosinophils, mast cells, macrophages, dendritic cells, and Th2 lymphocytes have surface membrane receptors for arachidonic-derived metabolites, in particular for prostaglandin D2 cysteinyl leukotrienes D4 and E, and lipoxin A4[33], but these findings have not been confirmed so far in patients with INS. Pharmacological modulation of AA metabolites could decrease the inflammatory damage to the podocyte. The pathogenetic role of AA is supported by the fact that medications have been recently administered to target AA metabolism and decrease kidney inflammation [21,34]. They include aspirin, nimesulide, lifelong, baicalein, and others. Some of them are in the early stages of development for kidney diseases like diabetic nephropathy, glomerulonephritis, and idiopathic membranous nephropathy [34].
