Background RGC (Response Gene to Complement)-32 is a cell cycle regulator widely expressed in normal tissues, multiple tumors and in a variety of cell lines. RGC-32 is localized in the cytoplasm and translocates to the nucleus upon upregulation by complement activation, growth factors and cytokines. Depending on the cell type, physiological or pathological conditions, RGC-32 can stimulate cell growth through increased p34CDC2 kinase activity and Akt phosphorylation or suppress it via arrest in mitotic progression. We have shown that RGC-32 is critical for murine and human Th17 cell differentiation. RGC-32 is induced by TGFβ in fibroblasts and human proximal tubular epithelial cells (PTEC) and mediates TGFβ dependent profibrotic pathways that contribute to renal fibrosis. RGC-32 expression has been described in tubules of normal human kidneys and its upregulation was reported in tubules from patients with IgA nephropathy. The expression patterns and function of RGC-32 in lupus nephritis (LN) have not yet been investigated.

Methods In situ expression and localization of RGC-32 was assessed by immunohistochemistry in kidney biopsies from 25 lupus patients with proliferative lupus nephritis and 11 patients with other nephropathies (IgA nephropathy, minimal change disease, ANCA-associated glomerulonephritis, nephrosclerosis, acute tubular necrosis). In vitro, the expression of RGC-32 in human PTEC cells was assessed by Flow cytometry, Western blot and RT-PCR in the presence or absence of cytokines with known nephritogenic potential such as IL-1, TNFα, IFNγ and TGFβ.

Results Consistent with the staining distribution reported in normal kidneys, RGC-32 immunostaining was predominant in proximal and distal tubules and was detected in a focal or diffuse pattern. Tubular mean staining intensity was significantly higher in SLE than in non-SLE specimens (2.0±0.23 vs 1.30±0.49; p=0.04) and was noted both in areas of normal appearing as well as damaged tubules. RGC-32 expression was also detected in glomeruli and in inflammatory cells in the interstitium of LN biopsies and colocalized with CD4+ T cells and CD68+ macrophages, respectively. Staining intensity was significantly higher in glomeruli and interstitium of LN specimens compared to disease controls (2.4±1.4 vs.1.6±0.8 and 1.8± 0.9 vs. 0.96±0.4 respectively) and correlated with the activity (r=0.4), chronicity (r=0.5) and interstitial fibrosis scores (r=0.5). In vitro, RGC- 32 mRNA and protein expression was upregulated in PTEC by nephritogenic cytokines including IL-1 (7.8 fold), TNFα (5 fold), TGFβ (3.1 fold) and to a lesser extent by IFNγ (2.1 fold). TGFβ induced mRNA production of Collagen 1a1 and collagen III by in vitro cultured human PTEC was increased in RGC-32 transfected cells vs. control.

Conclusions RGC-32 expression is increased in glomeruli and tubulointerstitium in kidneys of patients with lupus nephritis. Upregulation of RGC-32 is mediated by proinflammatory cytokines and may play pathogenetic role in organ damage in SLE by promoting manifestations of progressive renal disease such as interstitial fibrosis. Thus RGC-32 is a potential therapeutic target in the treatment of lupus nephritis.

Lay summary RGC-32 expression is increased in glomeruli and tubulointerstitium in kidneys of patients with lupus nephritis. Upregulation of RGC-32 is mediated by proinflammatory cytokines and may play pathogenetic role in organ damage in SLE by promoting manifestations of progressive renal disease such as interstitial fibrosis. Thus RGC-32 is a potential therapeutic target in the treatment of lupus nephritis.