Fatty acids and their metabolism critically regulate podocyte survival

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease in industrialized countries, and most affected patients have type 2 diabetes. Podocyte injury and loss are considered critical in the development, and progression of DN. Several factors of the diabetic milieu are well known to impair function and survival of podocytes. However, the role of free fatty acids (FFAs), which are elevated in type 2 diabetes, and the role of their metabolism are just emerging in the pathogenesis of DN. FFAs were reported to regulate podocyte survival. Saturated FFAs, i.e. palmitic acid, were found to induce endoplasmic reticulum (ER) stress and podocyte death, whereas monounsaturated FFAs, i.e. palmitoleic acid or oleic acid, were protective.
The aims of the present study were to investigate whether FFA metabolism is regulated in glomeruli of type 2 diabetic patients with DN and whether regulation of FFA metabolism affects the susceptibility of podocytes towards palmitic acid. Particularly, I aimed to investigate whether regulation of fatty acid oxidation (FAO) modifies palmitic acid-induced podocyte death. As genome wide association studies suggest that acetyl CoA carboxylase (ACC) 2, an important enzyme in the regulation of FAO, is involved in the pathogenesis of DN, I performed detailed studies investigating the role of ACCs in podocytes. Furthermore, I explored the effect of palmitic acid on podocytes in combination with well-known proapoptotic stimuli of the diabetic milieu.
The present study uncovered that palmitic acid can aggravate the toxicity of other factors which are known to be important in the pathogenesis of DN and which are considered to cause podocyte loss. In particular the toxicity of high glucose concentrations and transforming growth factor (TGF)-β are substantially increased by palmitic acid, whereas the effect of palmitic acid on tumor necrosis factor (TNF)-α induced podocyte death is discret.
In the main part of this study FFA metabolism and its effect on palmitic acid induced podocyte death was investigated. The study finds that in glomeruli of type 2 diabetic patients mRNA expression levels of several key enzymes involved in fatty acid metabolism are altered. Of particular relevance for my detailed studies on FAO, a significant upregulation of all three isoforms of carnitine palmitoyltransferase (CPT)-1, the rate-limiting enzyme for FAO, and a downregulation of ACC-2, which catalyzes the formation of the CPT-1 inhibitor malonyl-CoA, are found which suggest a disposition for increased FAO. In vitro, stimulation of FAO by aminoimidazole-4-carboxamide-1β-D-ribofuranoside (Aicar) or by adiponectin, activators of the low-energy sensor AMP-activated protein kinase (AMPK), protect from palmitic acid induced podocyte death. Conversely, inhibition of CPT-1, a downstream target of AMPK, by etomoxir augments palmitic acid toxicity and impedes the protective Aicar effect. Etomoxir blocked the Aicar induced FAO measured with tritium labeled palmitic acid. Of note, only double knockdown of ACC1 and ACC2 has a protective effect on palmitic acid induced cell death, which indicates that both isoforms contribute to the regulation of FAO in podocytes. Furthermore, the effect of Aicar is associated with a reduction of ER-stress as indicated by a significant attenuation of the palmitic acid induced upregulation of immunoglobulin heavy chain binding protein (BiP), an ER chaperone, and of the proapoptotic transcription factor C/EBP homologous protein (CHOP).
In conclusion, palmitic acid increases the toxicity of other factors known to contribute to podocyte loss, which underlines the potentially important contribution of elevated saturated FFAs in the pathogenesis of DN. An important role of FFAs and of their metabolism in the pathogenesis of DN is further suggested by profound changes in gene expression levels of key enzymes of FFA metabolism in glomerular extracts of type 2 diabetic patients. The changed expression profile indicates a compensatory, protective response. Moreover, the results of this study uncover that stimulation of FAO by modulating the AMPK-ACC-CPT-1 pathway protects from palmitic acid induced podocyte death. The results of this study should encourage further investigations to evaluate the therapeutic potential of interfering with FFA metabolism specifically with stimulating FAO for the prevention and therapy of DN.