Background: Oxalate is a metabolic end product excreted primarily by the kidney and associated with several pathologic conditions. The most common pathologic condition involving oxalate is the formation of calcium oxalate stones in the kidney. Several stimuli have been implicated in the development of glomerular and tubular injury in various forms of immune-mediated renal diseases. The elevated level of interleukin-6 (IL-6) has been reported in the urine of kidney stone-forming patients. In the present study, we investigated the role of oxalate, a major constituent of calcium oxalate kidney stone disease, in the production of IL-6 in normal human HK-2 kidney cells.
Methods: Confluent cultures of HK-2 cells (a renal epithelial cell line of human origin) were exposed to various concentrations of oxalate (0.2 to 2.0 mmol/L) and lipopolysaccharide (LPS) (0.1 and 10 mug/mL) for various time points (4-24 h) under serum-free conditions. The conditioned mediums were collected, and an IL-6 protein level was measured by enzyme-linked immunosorbent assay (ELISA). The total cellular RNA was isolated from the cells and subjected to relative quantitative reverse transcription-polymerase chain reaction (RT-PCR) to determine the expression of IL-6 mRNA. The statistical analysis of the results was carried out using the Student t test.
Results: HK-2 cells express IL-6 mRNA and protein. Oxalate increased the secretion of IL-6 protein in HK-2 cells in a concentration-dependent fashion. Oxalate exposure to HK-2 cells also induced transcriptional up-regulation of the IL-6 gene, as determined by the increased level of IL-6 mRNA expression following treatment with oxalate. Moreover, the effects of oxalate on IL-6 expression were time- and concentration-dependent. This is the first report demonstrating the regulation of IL-6 by oxalate.
Conclusion: This study provides the first direct evidence that oxalate up-regulates the expression and secretion of IL-6 in renal epithelial cells. The increased IL-6 expression and secretion by renal epithelial cells may play a critical role in the progression of urolithiasis in hyperoxaluric conditions.