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Acute kidney injury (AKI) continues to be a clinical challenge. At present, AKI has no effective treatments, and severe or repeated episodes of AKI can lead to chronic kidney disease (CKD) and irreversible kidney scarring. In fact, individuals with a history of AKI are at a higher risk of sustaining a second or third injury, compounding the short- and long-term consequences. CKD also is limited in its treatment — most individuals are treated with angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARB), but these are only partially effective and come with significant side effects. Much research and progress is underway to better understand the cellular and molecular mechanisms driving AKI/CKD, their various phenotypes, targets for therapeutic intervention, and early detection modalities.
Roderick Tan, MD, PhD, is a physician-scientist who is working to unravel the molecular mechanisms underlying the development of AKI, CKD, and fibrosis utilizing both in vivo and in vitro approaches. Dr. Tan is assessing novel ways in which the glomerular and tubular compartment cross-talk in disease, and how the Keap1/Nrf2 pathway is involved in AKI/CKD. Dr. Tan has also studied the Wnt/ß-catenin pathway and how matrix metalloproteinases affect renal injury.
More recently, Dr. Tan has begun to study the renal microvasculature using novel ultrasound imaging technology in collaboration with Kang Kim, PhD, from the University of Pittsburgh.
Keap1 and the Nrf2 Pathway in AKI and CKD
The Keap1 and Nrf2 proteins work in tandem in a pathway that modulates oxidative stress, inflammation, and cytoprotective mechanisms in the kidney. Dr. Tan’s research has analyzed the roles of Keap1 and Nrf2 in murine models of ischemia-reperfusion injury (IRI) and unilateral ureteral obstruction (UUO), and how this leads to fibrosis and ultimately CKD. Nrf2, or nuclear factor erythroid 2, is a transcription factor that enhances antioxidant responses and is also associated with an anti-inflammatory response via effects on NFκB. The Keap1 (Kelch-like ECH associated protein-1) protein functions as an inhibitor of Nrf2 function.
Dr. Tan utilizes Keap1 hypomorph murine models from the lab of Thomas W. Kensler, PhD, formerly with the University of Pittsburgh Department of Pharmacology and Chemical Biology, but now at the University of Washington, who also studies the Keap1/Nrf2 pathway. These models have been genetically engineered to underproduce or under-express the Keap1 protein.
A decrease in expression of Keap1 leads to increased Nrf2 activity. In models of IRI
and UUO, Dr. Tan and his study team have shown that Nrf2 hyperactivity leads to a protective effect in the kidney that can mitigate the injury and stop the progression to fibrosis and CKD.
“By hyperactivating the Nrf2 transcription factor through reduction of Keap1 levels, we were able to show that after IRI there is protection against the development of scarring or fibrosis and CKD. This may prove to be a viable therapeutic target in the future — activating the Nrf2 pathway in the setting of IRI,” says Dr. Tan.
His studies also investigated this protective effect in the setting of UUO. While the mechanism of injury is different, the protective effect was confirmed as fibrosis and progression to CKD were inhibited by hyperactivation of Nrf2.
Keap1, Nrf2, and Proteinuria
Dr. Tan’s research adds to a large body of literature suggesting that pharmacologic activation of the Keap1/Nrf2 pathway can be protective in AKI. However, there are many different pathways leading to kidney injury or damage. One such pathway is glomerular disease leading to proteinuria.
In early clinical trials in patients with diabetic kidney disease and proteinuria, the enhancement of the Nrf2 pathway showed increases in kidney filtration function, but the studies also showed an increase in proteinuria, which would be expected to worsen disease in the long run. Dr. Tan’s research group is interested in this conundrum.
“We exposed our Keap1 hypomorphs to a number of proteinuric injuries. Instead of being protected, these mice actually exhibited worse proteinuria. So, while Nrf2 activity appears to be protective in AKI, it appears that for proteinuric kidney disease the opposite is true. We have presented our research at an international kidney conference and have submitted our manuscript for publication,” says Dr. Tan.
Studying the Microvasculature of the Kidney
In 2018, Dr. Tan was awarded the prestigious Edith H. Blattner Young Investigator Grant from the National Kidney Foundation (NKF), one of several grants awarded every year through the NKF Young Investigator Research Grant Program. Dr. Tan’s award will support new research he is conducting that will examine the microvasculature of the kidney before and after injury using high-resolution ultrasound technology coupled with the use of a microbubble contrast agent.
Dr. Tan’s collaborator on the project is Kang Kim, PhD, associate professor of medicine and bioengineering at the University of Pittsburgh and the UPMC Heart and Vascular Institute. Dr. Kang is an expert in the use of this high-resolution ultra- sound technology and microbubble contrast agent, and he is involved in numerous studies across the University and UPMC that is employing this imaging modality to gain new insights.
Past studies in animal models have shown that the density of the renal microvasculature (the small blood vessels of the kidney) becomes decreased after an AKI caused by IRI. This decrease in the vascularity of the kidney leads to a decrease in perfusion and oxygenation, making the organ more prone to future IRI or the development of CKD.
Studying this in humans has always been difficult and confined mainly to postmortem biopsy and analysis. However, that may be changing with the advent of high-resolution ultrasound combined with microbubble contrast agent. This technique is routinely used clinically in the field of cardiology.
“We currently are working on these studies in animal models with Dr. Kim, and that work is funded through a pilot award from the Pittsburgh Center for Kidney Research P30 grant. Our new NKF grant will allow us to pilot a study in human subjects whom we hope to soon begin enrolling,” says Dr. Tan.
Dr. Tan’s goal is to eventually study AKI patients in the hospital, examine their renal microvasculature, and use the findings as a prognostic tool to determine who will likely recover their kidney function and who will progress to CKD.
“As clinicians we see the spectrum of AKI and have a sense for which patients may end up on a particular trajectory, but we do not have good objective measures to accurately tell us who will get better or worse. I am hopeful that this research will eventually lead to a more quantitative measure of an AKI patient’s kidney prognosis,” says Dr. Tan.
1. Tan RJ, Chartoumpekis DV, Rush BM, Zhou D, Fu H, Kensler TW, Liu Y. Keap1 Hypomorphism Protects Against Ischemic and Obstructive Kidney Disease. Sci Rep. 2016 Nov 2; 6: 36185. doi: 10.1038/srep36185.
Tan RJ, Zhou D, Liu Y. Signaling Crosstalk Between Tubular Epithelial Cells and Interstitial Fibroblasts After Kidney Injury. Kidney Dis (Basel). 2016 Oct; 2(3): 136-144.
Jobbagy S, Tan RJ. Nitrolipids in Kidney Physiology and Disease. Nitric Oxide. 2018 Mar 29. pii: S1089-8603(18)30006-5. doi: 10.1016/j.niox.2018.03.021.
Tan RJ, Zhou D, Xiao L, Zhou L, Li Y, Bastacky SI, Oury TD, Liu Y. Extracellular Superoxide Dismutase Protects Against Proteinuric Kidney Disease. J Am Soc Nephrol. 2015 Oct; 26(10): 2447-59.
Xiao L, Zhou D, Tan RJ, Fu H, Zhou L, Hou FF, Liu Y. Sustained Activation of Wnt/ß-Catenin Signaling Drives AKI to CKD Progression. J Am Soc Nephrol. 2016 Jun; 27(6): 1727-40.
Tan RJ, Zhou D, Zhou L, Liu Y. Wnt/ß-Catenin Signaling and Kidney Fibrosis. Kidney Int Suppl (2011). 2014 Nov; 4(1): 84-90. Review.