Increasing evidence suggests that the kidney tubule plays an important role in diabetic kidney disease, and we have reported that tubulointerstitial injury may contribute to glomerular changes in diabetes. We have focused on the roles of kidney injury-related molecules, mainly surface receptors of renal tubular epithelia, in both acute and chronic tubular degeneration, inflammation and fibrosis. Kidney injury molecule–1 (KIM-1) was identified originally in our laboratory as an early marker of proximal tubule injury. We reported that KIM-1 acts as a novel epithelial phosphatidylserine and scavenger receptor and that its levels in urine are a very good predictor of the progression of albuminuria in patients with type 1 diabetes. We have also reported that the plasma levels of KIM-1 are predictive of progression of diabetic nephropathy in Type I and Type II diabetic patients. These findings indicate that proximal tubule injury and dysfunction hold a critical role in the progression of diabetic kidney disease (DKD). This is further supported by other recent findings from our laboratory that: 1. injured proximal tubule cells undergo cell cycle arrest in G2M phase and assume a pro-inflammatory and pro-fibrotic secretory phenotype as a consequence of maladaptive response, and 2. repetitive targeted proximal tubule injury alone triggers interstitial fibrosis and secondary glomerulosclerosis in a non-diabetic mouse model.

Proximal tubular KIM-1 expression and urinary KIM-1 levels are increased in an animal model of DKD and correlate with the degree of tubulointerstitial and glomerular pathology. KIM-1 functions as a receptor for endocytosis of advanced glycation endproducts (AGEs) and oxidized low-density lipoproteins (oxLDLs), Free fatty acids (FFAs), all of which are known to be elevated in patients with diabetes, and triggers pro- inflammatory and pro-fibrotic responses. A diabetic mouse with a deletion of the mucin domain of KIM-1 (KIM- 1Δmucin), is protected from proximal tubular uptake of AGEs, oxLDLs, and FFAs, albuminuria, and development of DKD.

In a recent manuscript published in Cell Metabolism we have reported that KIM-1 mediates proximal tubule uptake of free fatty acids (FFAs), particularly the long chain fatty acid, palmitic acid (PA), leads to tubule injury with mitochondrial fragmentation, interstitial inflammation and fibrosis, and glomerulosclerosis in mouse models of diabetes. In proximal tubule cells, KIM-1-dependent internalization of FFAs enhances cell death and DNA damage response (DDR), transforms tubule cells into a pro-fibrotic secretory phenotype, and activates NLRP3 inflammasomes. Mice with a KIM-1 gene mucin domain deletion are protected from DKD and from injury caused by FFA. Inhibition of KIM-1-mediated FFA uptake by our newly identified compound prevented injury both in vitro and in vivo. Thus, in DKD, sustained proximal tubular KIM-1 expression results in uptake of FFAs in a mucin domain-dependent manner and promotes tubule cell death, pro-inflammatory and pro-fibrotic responses leading ultimately to tubule atrophy, interstitial fibrosis and secondary glomerulosclerosis. Our findings support KIM-1 as a new therapeutic target for DKD and introduces a new therapeutic agent (TW-37) which prevents PT from KIM-1-mediated endocytosis of FFA and protects the kidney against injury. In ongoing experiments, we crossed Six-2GFP-Cre transgenic mice (129/B6) with iDTR mice (C57BL/6) to obtain bigenic offspring (DTR), and then crossed with heterozygous C57BL/6-Ins2+/C96Y mice to obtain AkitaDTR mice. The AkitaDTR and control mice were studied with and without unilateral nephrectomy and on a regular diet or HFD, exposed to diphteria toxin injection and followed for 4 months. Kidney disease was evaluated by injury biomarkers, histology, and immunofluorescence staining. The targeted acute tubular injury (ATI) in AkitaDTR mice fed a HFD resulted in long-term albuminuria and azotemia, prolonged DNA damage, tubulointerstitial fibrosis and secondary glomerulosclerosis. In vitro, mechanistic studies have been carried out with LLC-PK1 kidney epithelial cells exposed to normal and high glucose and palmitic acid in the presence or absence of fatty acid oxidation (FAO) modulators and analyzed for phosphorylation of acetyl-CoA carboxylase (p-ACC) and DNA damage. p-ACC was upregulated in high glucose media. Promoting FAO with the ACC inhibitor PF- 05175157 after acute injury ameliorated DDR.

In conclusion diabetes increases susceptibility of the kidney to lipotoxicity. One episode of tubular injury in the setting of reduced renal mass and a high saturated fat diet leads to chronic kidney disease with tubulointerstitial fibrosis. Inhibition of FAO in tubular epithelial cells exposed to injury aggravates DNA damage and maladaptive repair.