Role of M1 Cytokines in the Progression of Chronic Kidney Disease

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Macrophages can have profound effects on the progression of chronic kidney disease (CKD) via the secretion of pro-inflammatory “M1” cytokines including tumor necrosis factor- (TNF) and Interleukin-1 (IL-1). In the previous cycle, we established that the transcription factor Krüppel like factor 4 in macrophages limits kidney damage and fibrosis by suppressing macrophage TNF expression. However, TNF and IL-1 are also produced by renal tubular cells (RTCs), and our preliminary studies have identified 2 factors in RTCs, A20 and Porcupine (PORCN), that together constrain local TNF and IL-1 generation and ameliorate CKD. A20 is a ubiquitin editing protein that downregulates NF-b signaling in myeloid cells and thereby protects against autoimmunity. In our preliminary studies using murine models of inflammation- and toxin-induced renal injury, deletion of A20 selectively from RTCs (A20 KKO) permits enhanced local TNF and IL-1 expression and exacerbations in RTC damage. We therefore hypothesize that A20 in the nephron protects against tubular damage and subsequent CKD by constraining local generation of TNF and IL-1. To test this, we will subject A20 KKO and littermate controls to models of chronic renal tubular injury. We recently reported that TNF in RTCs induces several Wnt ligand isoforms and that the secretion of Wnt ligands permitted by the O-acyl transferase Porcupine exaggerates obstruction-induced renal fibrosis. By contrast, in other models featuring proximal tubular injury Wnt signaling can protect against CKD progression, and in our preliminary studies, inducible nephron-specific deletion of PORCN upregulates renal TNF expression and exacerbates nephrotoxic serum nephritis (NTS). As restoring - catenin signals can protect the proximal tubule, a key site of TNF-induced injury, we posit that PORCN in proximal tubular cells suppresses TNF expression and thereby limits the severity of CKD. To test this hypothesis, we will subject mice lacking PORCN in the proximal tubule (PORCN PTKO) to our tubular injury models. Our preliminary studies indicate that deleting TNF from the proximal tubule (TNF PTKO) mitigates toxin-induced tubular damage and prevents cell cycle arrest. Thus, to directly interrogate whether PORCN-dependent secretion in the proximal tubule limits CKD severity by suppressing TNF-induced cytotoxicity, we will compare the susceptibility to RTC damage of PORCN PTKO mice and mice harboring double PORCN and TNF deletion restricted to the proximal tubule (Dual PTKO). We predict that interrupting TNF’s actions in the proximal tubule will abrogate the augmented susceptibility to CKD accruing from PORCN deficiency in the proximal tubule. With our integrated approach, we will elucidate nephron-specific pathways upstream and downstream of TNF that can be targeted for the amelioration of CKD.