KIM-1–mediated phagocytosis reduces acute injury to the kidney

摘要

Kidney injury molecule 1 (KIM-1, also known as TIM-1) is markedly upregulated in the proximal tubule after injury and is maladaptive when chronically expressed. Here, we determined that early in the injury process, however, KIM-1 expression is antiinflammatory due to its mediation of phagocytic processes in tubule cells. Using various models of acute kidney injury (AKI) and mice expressing mutant forms of KIM-1, we demonstrated a mucin domain–dependent protective effect of epithelial KIM-1 expression that involves downregulation of innate immunity. Deletion of the mucin domain markedly impaired KIM-1–mediated phagocytic function, resulting in increased proinflammatory cytokine production, decreased antiinflammatory growth factor secretion by proximal epithelial cells, and a subsequent increase in tissue macrophages. Mice expressing KIM-1~(Δmucin) had greater functional impairment, inflammatory responses, and mortality in response to ischemia- and cisplatin-induced AKI. Compared with primary renal proximal tubule cells isolated from KIM-1~(Δmucin) mice, those from WT mice had reduced proinflammatory cytokine secretion and impaired macrophage activation. The antiinflammatory effect of KIM-1 expression was due to the interaction of KIM-1 with p85 and subsequent PI3K-dependent downmodulation of NF-κB. Hence, KIM-1–mediated epithelial cell phagocytosis of apoptotic cells protects the kidney after acute injury by downregulating innate immunity and inflammation. Keywords: Cell Biology, Inflammation, NephrologyIntroductionAcute kidney injury (AKI) is a very common clinical condition. The incidence of hospital-acquired AKI is increasing, and many patients require renal replacement therapy (1, 2). Severe AKI is associated with in-hospital mortality rates of greater than 60% (2). There are also chronic consequences, even if the patients survive their acute illness. Whether or not patients have preexisting chronic kidney disease (CKD), those who have had AKI have a high risk of developing progressive CKD and end-stage renal disease (ESRD) over time (3–5).In contrast to the heart or brain, the kidney can effectively recover from an ischemic or toxic insult that results in cell death, since the tubular epithelium has a potent ability to proliferate and replace lost cells (6–8). This repair, however, can also be maladaptive, leading to CKD or its progression if the injury is severe or repeated or occurs in the setting of CKD (9, 10). The balance between injury and repair is the key determinant of the fate of the injured kidney, and one might expect that there are potent factors induced by injury to facilitate adaptive repair. AKI results in in?ammation, which exacerbates tubular injury (11). Control of the inflammation is important to minimize injury and facilitate recovery from kidney injury. The focus of proposed therapies has, to date, been primarily directed at proin?ammatory factors, and less attention has been paid to the understanding of the endogenous factors normally involved in the downregulation of in?ammation.Many studies have demonstrated that the tubular epithelium is not merely a passive victim of injury. Tubular epithelial cells generate important proinflammatory cytokines and chemokines and express TLRs, making them an important component of the innate immune response. Kidney injury molecule 1/T cell Ig and mucin domain 1 (KIM-1/TIM-1) is upregulated more than any other protein in the proximal tubule of the kidney, with various forms of injury (12, 13). KIM-1 is a phosphatidylserine receptor that mediates phagocytosis of apoptotic bodies and oxidized lipids (14). When KIM-1 is expressed chronically, it results in progressive kidney fibrosis and chronic kidney failure (15), which is perhaps related to its phagocytic function to take up noxious compounds, such as oxidized lipids. In addition to its role in phagocytosis, KIM-1 can activate signaling through the PI3 kinase pathway (16). The role of KIM-1 signaling in proximal tubular cells (PTCs), as well as the connection between KIM-1 phagocytosis and phosphorylation, has yet to be determined. Here, we characterize a KIM-1–mutant mouse that is deficient in phagocytosis, phosphorylation, and interaction with p85. We demonstrate for the first time, to our knowledge, that KIM-1–mediated phagocytic function downregulates inflammation and the innate immune response in acute ischemic and toxic injury.ResultsDecreased phagocytic function of KIM-1~(Δmucin) in PTCs.A KIM-1–mutant mouse was created by replacing exon 3 of the KIM-1 gene with a PGK promoter–driven neomycin-resistance cassette on a C57BL/6 genetic background. This mutant mouse generated KIM-1 proteins with the loss of the mucin domain, encoded by exon 3 (KIM-1~(Δmucin)) (17). KIM-1~(Δmucin) mice were found to have similar mRNA expression levels of the closest of the TIM genes in the KIM-1 locus (17). We confirmed that other TIMs, TIM-3 and TIM-4, are not differentially regulated in KIM-1~(Δmucin) tubul