Pathologic alterations in podocytes lead to failure of an essential component of the glomerular filtration barrier and proteinuria in chronic kidney diseases. Elevated levels of saturated free fatty acid (FFA) are harmful to various tissues, implemented in the progression of diabetes and its complications such as proteinuria in diabetic nephropathy. Here, we investigated the molecular mechanism of palmitate cytotoxicity in cultured mouse podocytes. Incubation with palmitate dose-dependently increased cytosolic and mitochondrial reactive oxygen species, depolarized the mitochondrial membrane potential, impaired ATP synthesis and elicited apoptotic cell death. Palmitate not only evoked mitochondrial fragmentation but also caused marked dilation of the endoplasmic reticulum (ER). Consistently, palmitate upregulated ER stress proteins, oligomerized stromal interaction molecule 1 (STIM1) in the subplasmalemmal ER membrane, abolished the cyclopiazonic acid-induced cytosolic Ca2+ increase due to depletion of luminal ER Ca2+. Palmitate-induced ER Ca2+ depletion and cytotoxicity were blocked by a selective inhibitor of the fatty-acid transporter FAT/CD36. Loss of the ER Ca2+ pool induced by palmitate was reverted by the phospholipase C (PLC) inhibitor edelfosine. Palmitate-dependent activation of PLC was further demonstrated by following cytosolic translocation of the pleckstrin homology domain of PLC in palmitate-treated podocytes. An inhibitor of diacylglycerol (DAG) kinase, which elevates cytosolic DAG, strongly promoted ER Ca2+ depletion by low-dose palmitate. GF109203X, a PKC inhibitor, partially prevented palmitate-induced ER Ca2+ loss. Remarkably, the mitochondrial antioxidant mitoTEMPO inhibited palmitate-induced PLC activation, ER Ca2+ depletion and cytotoxicity. Palmitate elicited cytoskeletal changes in podocytes and increased albumin permeability, which was also blocked by mitoTEMPO. These data suggest that oxidative stress caused by saturated FFA leads to mitochondrial dysfunction and ER Ca2+ depletion through FAT/CD36 and PLC signaling, possibly contributing to podocyte injury.
The aim of the study is to characterize the expression pattern of galectin-3 (Gal-3) in renal tissues of patients with systemic lupus erythematosus (SLE) nephritis and to determine whether tissue and serum Gal-3 are associated with SLE nephritis. Gal-3 expressions were examined with immunohistochemistry in renal biopsy specimens of 88 patients with SLE nephritis and in five normal specimens. Activity and chronicity indexes and glomerular Gal-3 expressions were analysed in each specimen. Serum Gal-3 levels were measured using enzyme-linked immunosorbent assays in 20 patients with SLE, including 11 with nephritis, and in 50 healthy controls. Glomerular Gal-3 expression was observed in 81.8% (72/88) of patients with SLE nephritis but not in 5 controls. Gal-3 staining was attributed mainly to its cellular expression rather than its deposition, and Gal-3 expression levels were correlated with histologic activity indexes, anti-dsDNA titers, and complement 3 and 4 levels. Serum Gal-3 levels were higher in patients with SLE, particularly in those with nephritis, than in healthy controls, and correlated with anti-dsDNA titers. In conclusion, glomerular Gal-3 expression in renal tissue and serum Gal-3 levels were elevated in patients with SLE nephritis versus healthy controls; moreover, they reflected disease activity. These findings suggest that Gal-3 might contribute to the inflammatory process in SLE.
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