Meprin β Modulates Cellular Proliferation Through The IL‐6 Signaling Pathway In IRInduced Kidney Injury

Inflammation plays a central role in the progression of kidney injury induced by ischemia/reperfusion (IR). Interleukin‐6 (IL‐6) trans‐signaling has been shown to play a protective role by promoting repair processes in IR‐induced renal injury. Meprin metalloproteases have been implicated in inflammation‐induced kidney injury. Meprins proteolytically process the proinflammatory cytokine, IL‐6, causing IL‐6 inactivation. It was recently reported that meprins also cleave the IL‐6 receptor (IL‐6R). IL‐6 binds to its receptor (IL‐6R) in two ways, membrane bound (mbIL‐6R), activating the classic IL‐6 signaling pathway, or the soluble form (sIL‐6R), activating the IL‐6 trans‐signaling pathway. IL‐6 trans‐signaling induces proliferation through either MAPK/ERK or PI3K/AKT pathway or in crosstalk with AKT/ERK. We previously showed that meprin β modulates cellular survival (BCL‐2) through IL‐6/JAK/STAT signaling pathway in IRinduced kidney injury. However, it’s not known how meprin β modulation of the IL‐6 signaling pathway impacts the cellular proliferation in IR‐induced acute kidney injury. Previous studies have shown that IL‐6 binds to its receptor forming IL‐6/IL6R complex, which binds to the membranebound gp130 dimer. This leads to activation phosphorylation of ERK and AKT pathways, which in turn leads to induction of cellular proliferation. PCNA is a cellular proliferation marker that is induced through activation of the IL‐6 signaling pathway. The goal of the current study was to determine how meprin β modulation of the IL‐6 signaling pathway impacts downstream cellular proliferation in IR‐induced kidney injury. We used the unilateral IR as a model of renal inflammation in wild‐type (WT) and meprin β knockout (βKO) male mice, with the contralateral kidneys serving as controls. The mice were sacrificed at 96 h post‐IR, and kidney tissue processed for evaluation by RT‐PCR and immunohistochemistry (IHC). To determine staining intensity, the tissue sections were evaluated for IL‐6 and PCNA levels using light microscopy and imaged using Image J analysis Software. Statistical analysis of the optical density (OD) data utilized two‐way ANOVA. Our PCR data showed a significant increase in mRNA levels for IL‐6 and PCNA in WT and βKO mice (P ≤ 0.01) at 96 h‐post IR when compared to WT control kidneys. However, the baseline mRNA levels for PCNA were significantly higher in βKO (P ≤ 0.01) when compared to WT kidneys. Immunohistochemical data showed significant increases (P<0.05) in IL‐6 and PCNA in select tubules in both genotypes at 96 h post‐IR when compared to control kidneys for each genotype. Data from immunofluorescence counterstaining of kidney tissues showed that the levels of IL‐6, and PCNA were higher in meprin β‐expressing proximal tubules (PTs), at 96 h post‐IR when compared to the distal kidney tubules (DTs), which lack meprins. High levels of IL‐6 were also present in the lumen of PTs and DTs from WT and βKO kidneys at 96 h post‐IR, suggesting increased release into filtrate and subsequently into urine. However, high levels of PCNA were present in the lumen of PTs only from both genotypes at 96 h post‐IR. In conclusion, our data shows that meprin β expression modulates cellular proliferation through the IL‐6 signaling pathway in IR‐induced kidney injury.