SPP1 and chronic kidney disease: However, a large number of studies have shown increased OPN mRNA and OPN protein levels in several forms of CKD including DKD, LN, IgAN, tubulointerstitial nephritis, glomerulonephritis, acute ischemic renal injury, interstitial inflammation, fibrosis, and hydronephrosis [13,64,65,66,67,68] (Table 2).