Abstract MP32: Renal Tubular Epithelial Cell-derived IL-1β Triggers The Inflammatory Response That Induces Salt Sensitivity In Diabetes

Renal inflammation, and specifically interleukin (IL)-6, induces salt-sensitivity in diabetic mice by upregulating the epithelial sodium channel (ENaC). However, the origin of this inflammatory response is still unknown. We hypothesize that, during diabetes, tubular epithelial cells initiate renal inflammation by releasing IL-1β that further activates renal macrophages to produce IL-6 and impair kidney function. A primary culture of renal tubular epithelial cells from wild-type (WT) mice was exposed to either low (LG, 5mM) or high (HG, 15mM) glucose media. HG-treated cells released higher levels of IL-1β (28 ± 12 vs. 8 ± 4 pg/ml, P<0.01) and have higher expression of NLRP3 (4 ± 1 fold increase, P<0.001) and Caspase-1 (2.1 ± 0.2 fold increase, P<0.001) compared to LG-treated cells. No effect was observed using mannitol as an osmotic control. Tubular cells were also co-cultured with bone marrow-derived macrophages. Flow cytometry analysis revealed that HG-treated tubular cells induced an M1-type polarization of macrophages. This effect was blunted when macrophages were obtained from IL-1 receptor KO mice (IL1RKO) confirming a role of IL-1β. Here, the percentage of M1 macrophages (CD45 + F4/80 + CD80 + ) of total macrophages (CD45 + F4/80 + ) decreased from 31 ± 4% to 10 ± 3% (P<0.01). To evaluate whether IL-1β-mediated macrophage activation plays a role in vivo, 2-mo-old diabetic db/db mice (n=6) were transplanted with bone marrow from IL1RKO mice. In these mice, named db/db-IL1RKO, immune cells cannot respond to IL-1β. Db/db receiving wild-type (WT) bone marrow (db/db-WT) were used as control. No differences in body weight and hyperglycemia were observed after transplantation. At 7 months, mice were exposed to a high salt diet (NaCl 4% w/w) for 4 weeks. Db/db-IL1RKO did not develop salt-sensitivity (blood pressure was 110 ± 7 vs. 122 ± 9 mmHg, P<0.05), have less renal IL-6 (138 ± 35 vs. 263 ± 28 pg/mg kidney, P<0.05) and lower ENaC activity (0.4-fold decrease by amiloride test, P<0.05) compared to db/db-WT. In conclusion, our data show that tubular epithelial cells are a major source of IL-1β under high glucose conditions. This cytokine promotes M1 macrophage polarization resulting in higher renal IL-6 levels which increases ENaC activity leading to salt sensitivity.