Sigma-1 Receptor as a Novel Therapeutic Target in Diabetic Kidney Disease

Abstract

Diabetic kidney disease (DKD) is the leading cause of chronic kidney disease. Current treatments for DKD do not halt renal injury progression, highlighting an urgent need for therapies targeting key disease mechanisms. Our previous studies demonstrated that activating the Sigma-1 receptor (S1R) with fluvoxamine (FLU) protects against acute kidney injury by inhibiting inflammation and ameliorating the effect of hypoxia. Based on these, we hypothesized that FLU might exert a similar protective effect in DKD. Diabetes was induced in male Wistar rats using streptozotocin, followed by a seven-week FLU treatment. Metabolic and renal parameters were assessed along with a histological analysis of glomerular damage and fibrosis. The effects of FLU on inflammation, hypoxia, and fibrosis were tested in human proximal tubular cells and normal rat kidney fibroblasts. FLU improved renal function and reduced glomerular damage and tubulointerstitial fibrosis. It also mitigated inflammation by reducing TLR4, IL6, and NFKB1 expressions and moderated the cellular response to tubular hypoxia. Additionally, FLU suppressed TGF-β1-induced fibrotic processes and fibroblast transformation. These findings suggest that S1R activation can slow DKD progression and protect renal function by modulating critical inflammatory, hypoxic, and fibrotic pathways; therefore, it might serve as a promising novel drug target for preventing DKD.

Keywords: Sigma-1 receptor; diabetic kidney disease; fibrosis.

Figure 1

Fluvoxamine mitigated tubular damage. Control, diabetic (D), and fluvoxamine-treated diabetic (D + FLU) rats. (A,B) Urinary concentrations of kidney injury molecule-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL). (C,D) KIM-1 (Havcr1) and NGAL (Lcn2) mRNA expressions. mRNA expressions were normalized to Rn18S. Values are represented as means ± SD. * p < 0.05 vs. Control, ** p < 0.01 vs. Control, *** p < 0.001 vs. Control, # p < 0.05 vs. D (n = 6/group).

Figure 2

Fluvoxamine mitigated mesangial matrix expansion in diabetic kidneys. (A) Representative PAS-stained kidney sections of control, diabetic (D), and fluvoxamine-treated diabetic (D + FLU) rats. (B) Mesangial fractional volume values (Vv) were defined by the ratio of the PAS-stained mesangial area to the glomerular tuft area. Original magnification = 200×. Scale bar = 100 µm. Values are represented as means ± SD. *** p < 0.001 vs. Control, ## p < 0.01 vs. D (n = 6/group).

Figure 3

Fluvoxamine diminished tubulointerstitial fibrosis. (A) Representative Masson’s trichrome-stained kidney sections of control, diabetic (D), and fluvoxamine-treated diabetic (D + FLU) rats. (B) Renal tubulointerstitial fibrosis was quantified by the ratio of Masson-positive (blue), glomerulus-free areas (px) to the total area in the kidney cortex. Original magnification = 200×. Scale bar = 100 µm. Values are represented as means ± SD. *** p < 0.001 vs. Control, ### p < 0.001 vs. D (n = 6/group).

Figure 4

Fluvoxamine reduced inflammatory responses in HK-2 cells. Control, lipopolysaccharide (LPS), and LPS + fluvoxamine-treated (LPS + FLU) HK-2 cells. (A–F) toll-like receptor-2 (TLR2), toll-like receptor-4 (TLR4), interleukin-6 (IL6), interleukin-1β (IL1B), nuclear factor κB (NFKB1), and tumor necrosis factor-α (TNF) mRNA expression. mRNA expressions were normalized to RN18S. Values are represented as means ± SD. * p < 0.05 vs. Control, ** p < 0.01 vs. Control, *** p < 0.001 vs. Control, # p < 0.05 vs. LPS, ## p < 0.01 vs. LPS (n = 4–5/group).

Figure 5

Fluvoxamine abolished the activation of the HIF-1α pathway and decreased pro-fibrotic response. Control, hypoxia (H), and hypoxia + fluvoxamine-treated (H + FLU) HK-2 cells. (A) Representative immunocytochemistry staining of HIF-1α (green: HIF-1α; blue: nucleus; original magnification 200×. Scale bar = 100 μm). (B–G) mRNA expression of hypoxia-inducible factor-1α (HIF1A), hypoxia-inducible factor-2α (EPAS1), erythropoietin (EPO), vascular endothelial growth factor A (VEGFA), glucose transporter-1 (SLC2A1), and transforming growth factor-β1 (TGFB1) normalized to RN18S. Values are represented as means ± SD. ** p < 0.01 vs. Control, *** p < 0.001 vs. Control, # p < 0.05 vs. Hypoxia, ## p < 0.01 vs. Hypoxia, ### p < 0.001 vs. Hypoxia (n = 4–5/group).

Figure 6

Fluvoxamine mitigated ECM deposition and cellular morphology changes in NRK-49F cells. Control, transforming growth factor-β1 (TGF-β1), and TGF-β1 + fluvoxamine-treated (TGF-β1 + FLU) NRK-49F cells. (A) Representative immunocytochemistry staining of phalloidin and vimentin (magenta: phalloidin; cyan: vimentin; blue: nucleus. Original magnification 600×. Scale bar = 50 μm). (B–D) collagen type I α1 (Col1a1), collagen type III α1 (Col3a1), fibronectin (Fn) mRNA expression. mRNA expressions were normalized to Rn18S. Values are represented as means ± SD. *** p < 0.001 vs. Control, # p < 0.05 vs. TGF-β1, ## p < 0.01 vs. TGF-β1, ### p < 0.001 vs. TGF-β1 (n = 4–6/group).