KIM-1-mediated anti-inflammatory activity is preserved by MUC1 induction in the proximal tubule during ischemia-reperfusion injury

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Abstract
Cell-associated kidney injury molecule-1 (KIM-1) exerts an anti-inflammatory role following kidney injury by mediating efferocytosis and downregulating the NF-κB pathway. KIM-1 cleavage blunts its anti-inflammatory activities. We reported that Mucin 1 (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 tested the hypothesis that MUC1 protects KIM-1 activity. Muc1 KO mice and wild-type (WT) littermates were subjected to IRI. KIM-1, MUC1 and ADAM17 levels (and signaling pathways) were assessed by immunoblotting. PT localization was assessed by confocal microscopy and in situ proximity ligation assay. Findings were extended using human kidneys and urine, and KIM-1-mediated efferocytosis assays in mouse PT cultures. 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, Muc1 KO mice had 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. Efferocytosis was reduced in Muc1 KO PT cultures when compared to WT cells, while inflammation was increased in Muc1 KO kidneys when compared to WT mice. MUC1 was cleaved by ADAM17 in PT cultures, and blocked KIM-1 shedding in MDCK cells. 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. NEW & NOTEWORTHY: KIM-1 plays a key role in the recovery of the tubule epithelium during renal IRI by mediating efferocytosis and associated signaling that suppresses inflammation. Excessive cleavage of KIM-1 by ADAM17 provides decoy receptor that aggravates efferocytosis and subsequent signaling. Our data from studies in mice, patients and cultured cells show that MUC1 is also induced during IRI and competes with KIM-1 for cleavage by ADAM17. Consequently, MUC1 protects KIM-1 anti-inflammatory activity in the damaged kidney.

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