Kidney injury molecule‐1 (KIM‐1)‐mediated anti‐inflammatory activity is preserved by Mucin 1 (MUC1) induction in the proximal tubule during ischemia‐reperfusion injury

Abstract

Background . Cell-associated KIM-1 exerts an anti-inflammatory role following kidney injury by mediating apoptotic cell phagocytosis and downregulating the NF-kB pathway. KIM-1 cleavage blunts its anti-inflammatory activities. We reported that MUC1 is protective in a mouse model of ischemia-reperfusion injury (IRI). As both KIM-1 and MUC1 are induced in the proximal tubule (PT) during IRI and are ADAM17 substrates, we designed experiments to understand if MUC1 affects KIM-1 expression and activity. Methods . Muc1 KO mice and wild-type (WT) littermates were subjected to IRI. KIM-1 levels in kidneys and urine were compared and signaling pathways assessed by immunoblotting. Cellular localization was assessed by confocal microscopyand in situ proximity ligation assay. Findings were further studied using human kidneys, patient urine and mouse primary cultured PTCs where KIM-1-mediated efferocytosis was evaluated. Results . In response to tubular injury in mouse and human kidneys, we observed induction and co-expression of KIM-1 and MUC1 in the PT. Compared to WT mice, Muc1 KO mice had more severe kidney injury during IRI, higher urinary KIM-1 and lower kidney KIM-1. KIM-1 was apical in PT of WT kidneys, but predominately with luminal debris in Muc1 KO mice. Primary cultured PTCs generated from Muc1 KO mouse kidney exhibited a significant decrease in efferocytosis when compared to WT PTCs. Immunoblotting kidney extracts revealed increased inflammation in Muc1 KO mice, compared to WT mice. MUC1 is an ADAM17 substrate as indicated by confocal microscopy, in situ proximity ligation and shedding assays. Analysis of KIM-1 and MUC1 levels in urine of patients with kidney injury, and in medium from transfected MDCK cells, supported the hypothesis that MUC1 competes with KIM-1 as a substrate for ADAM17. Conclusions . We conclude that KIM-1-mediated efferocytosis and thus anti-inflammatory activity during IRI is preserved in the injured kidney by MUC1 inhibition of KIM-1 shedding.