Saturated fatty acids induce insulin resistance in human podocytes: implications for diabetic nephropathy

Background. Cellular insulin resistance is the hallmark of type 2 diabetes and predominantly affects adipose and muscle cells. The saturated free fatty acid palmitate is elevated in insulin-resistant states and may directly contribute to cellular insulin resistance. A spectrum of renal disease is associated with increased markers of insulin resistance, although direct causal mechanisms are not known. In the kidney, glomerular podocytes are novel insulin-sensitive cells that have the ability to rapidly transport glucose. In this study, we tested the hypothesis that palmitate would induce insulin resistance in podocytes. Methods. Conditionally immortalized human podocytes were cultured for up to 24 h with 375-750 mu M palmitate. Functional effects on glucose uptake and ceramide production were measured. Gene expression was investigated using a focused gene array, and protein signalling and trafficking were studied with Western blotting and immunofluorescence. Results. We found that palmitate blocked insulin-stimulated glucose uptake in human podocytes. This was associated with increased ceramide production, and use of the ceramide inhibitors myriocin and fumonisin B I partially recovered the insulin sensitivity. At the level of transcription, palmitate downregulated genes associated with several pathways involved in insulin signalling. At the protein level, phosphorylation of the insulin receptor, IRS 1 and PKB was reduced and there was impaired translocation of GLUT4 to the cell surface. Conclusion. This is the first study to demonstrate a direct effect of saturated fatty acids on podocyte function. These findings may represent a novel link between systemic insulin resistance and the development of nephropathy.