The role of osteopontin as a candidate biomarker of renal involvement in systemic lupus erythematosus.

OBJECTIVES: Kidney biopsy is the gold standard for the diagnosis of lupus nephritis (LN). Conventional biomarkers of disease activity or renal function, such as complement levels, anti-dsDNA, serum creatinine, urinary sediment and proteinuria, do not have a sensitive diagnostic and prognostic value, therefore new biomarkers are needed to help predict or monitor LN. Osteopontin (OPN) is a pro-inflammatory molecule detectable in serum and renal tissue. The aim of this study was to evaluate OPN as a biomarker of renal involvement in patients with systemic lupus erythematosus (SLE) and correlate its levels with disease activity and laboratory features. METHODS: OPN was measured in the serum and urine of SLE patients with active LN (n=14), LN in remission (n=20), SLE without kidney involvement (n=22) and age- and sex-matched healthy controls (HC, n=20). RESULTS: OPN levels were significantly higher in urine than in serum in both groups of patients and controls (p<0.001). Serum OPN levels were higher in the LN patients than in HC and in SLE patients without renal involvement (p<0.0001 and 0.0032, respectively), regardless of the phase of renal activity. SLE patients without renal involvement and controls showed similar serum levels. We detected a direct correlation between low complement levels and OPN serum levels in patients with LN (p=0.014; R=0.438). Moreover, a higher percentage of patients with LN, compared to SLE without LN and HC, showed abnormal serum OPN. CONCLUSIONS: Our data suggest that serum OPN could be considered a biomarker of renal involvement, without differentiating between active and remission LN.