A Molecular Signature of Proteinuria in Glomerulonephritis

Proteinuria is the most important predictor of outcome in glomerulonephritis and experimental data suggest that the tubular cell response to proteinuria is an important determinant of progressive fibrosis in the kidney. However, it is unclear whether proteinuria is a marker of disease severity or has a direct effect on tubular cells in the kidneys of patients with glomerulonephritis. Accordingly we studied an in vitro model of proteinuria, and identified 231 “albumin-regulated genes” differentially expressed by primary human kidney tubular epithelial cells exposed to albumin. We translated these findings to human disease by studying mRNA levels of these genes in the tubulo-interstitial compartment of kidney biopsies from patients with IgA nephropathy using microarrays. Biopsies from patients with IgAN (n = 25) could be distinguished from those of control subjects (n = 6) based solely upon the expression of these 231 “albumin-regulated genes.” The expression of an 11-transcript subset related to the degree of proteinuria, and this 11-mRNA subset was also sufficient to distinguish biopsies of subjects with IgAN from control biopsies. We tested if these findings could be extrapolated to other proteinuric diseases beyond IgAN and found that all forms of primary glomerulonephritis (n = 33) can be distinguished from controls (n = 21) based solely on the expression levels of these 11 genes derived from our in vitro proteinuria model. Pathway analysis suggests common regulatory elements shared by these 11 transcripts. In conclusion, we have identified an albumin-regulated 11-gene signature shared between all forms of primary glomerulonephritis. Our findings support the hypothesis that albuminuria may directly promote injury in the tubulo-interstitial compartment of the kidney in patients with glomerulonephritis

Early and persistent up-regulated expression of renal cortical osteopontin in experimental hydronephrosis.

The mechanical disturbance after unilateral ureteral obstruction (UUO) is a nonimmune stimulus that is capable of eliciting a florid macrophage infiltration of the kidney and subsequent post-inflammatory renal scarring. Osteopontin has potential chemoattractant activity and, for this reason, we delineated the kinetics of its expression in the renal cortex of rats with UUO. Whole body X-irradiation and reversal of UUO were utilized as interventional maneuvers to give additional pathobiological insight into this protein's role in the response of the kidneys to ureteral obstruction. Increased osteopontin mRNA levels in obstructed kidneys versus contralateral unobstructed specimens were evident as early as 4 hours after UUO and steadily increased at 12, 24, 48, and 96 hours after UUO. Both X-irradiation and reversal of UUO failed to significantly modulate renal cortical osteopontin mRNA expression at all of the above time points. Paralleling the increments in renal cortical osteopontin mRNA levels were significant elevations in the cortical renal interstitial macrophage number, which was significantly diminished by previous X-irradiation but not reversal of UUO. Focal labeling of osteopontin was noted in both tubular and Bowman's capsular epithelium in obstructed kidneys as early as 4 hours after UUO, whereas, in the contralateral unobstructed specimens, there was only faint staining in Bowman's capsule. By 96 hours after UUO, obstructed kidneys exhibited intense, diffuse staining for osteopontin in both tubules and Bowman's capsule. Osteopontin's immunolocalization was not modulated by X-irradiation or reversal of UUO. These data support the contention that osteopontin is involved in the accumulation of macrophages within the peritubular and periglomerular interstitium in the obstructed renal cortex