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Mulay, Shrikant Ramesh (2013): Role of murine double minute (MDM)-2 in kidney injury and repair. Dissertation, LMU München: Medizinische Fakultät



Murine double minute (MDM)-2, an E3 ubiquitin ligase, promotes cancer cell survival and growth, by degrading the cell cycle regulator p53. Antagonism of MDM2 by the small-molecule cis-imidazoline nutlin analogs is currently under study for cancer therapy. We observed that MDM2 is strongly expressed by the epithelial cells in the kidney for example, tubular epithelial cells and podocytes. To test whether MDM2 promotes regenerative cell growth, we studied the effects of MDM2 antagonist, nutlin-3a on tubule cell healing during postischemic acute kidney injury and on podocytes during adriamycin induced chronic renal failure. Consistent with the hypothesis, we observed that treatment with nutlin-3a impaired tubular cell regeneration during postischemic AKI in C57Bl6 wild-type mice in a p53-dependent manner. However, MDM2 blockade also prevented tubular necrosis by suppressing sterile inflammation during the early postischemic phase. This effect also occurred in p53-deficient mice, indicating a second, pro-inflammatory, p53-independent role for MDM2 in AKI. In-vitro experiments confirmed that MDM2 is required to induce mRNA expression and secretion of NF-κB-dependent cytokines upon Toll-like receptor stimulation by enhanced binding of NF-κB to cytokine promoter–binding sites. Thus, MDM2 links inflammation and epithelial healing during AKI. It promotes the inflammatory response after the injury at the same time it drives the regeneration of injured tubular epithelium. Therefore, these additional biological functions need to be regarded when considering MDM2 inhibition therapy in patients with acute renal failure. Since, podocytes strongly express MDM2, we hypothesized that blocking MDM2 during glomerular injury may enhance podocyte apoptosis, proteinuria and glomerulosclerosis. However, unexpectedly MDM2 blockade in early adriamycin nephropathy in Balb/c mice had the opposite effect and reduced intrarenal cytokine and chemokine expression, glomerular macrophage and T cell counts, plasma creatinine and BUN levels. In cultured podocytes exposed to adriamycin, MDM2 blockade did not enhance podocyte apoptosis but rather prevented aberrant nuclear divisions and death of aneuploid podocytes, i.e. mitotic catastrophe. Accordingly, MDM2 blockade induced p21 and prevented podocyte mitosis in-vivo while TUNEL+ apoptotic podocytes were not detected. Thus, mitotic catastrophe is a previously unrecognized variant of podocyte loss where MDM2 promotes podocytes to complete the cell cycle, which in the absence of cytokinesis, leads to podocyte aneuploidy and death. Furthermore, delayed MDM2 blockade also reduced plasma creatinine levels, BUN, tubular atrophy, interstitial leukocyte numbers and cytokine expression as well as interstitial fibrosis. Together, MDM2 blockade with nutlin-3a could be a novel therapeutic strategy to prevent renal inflammation, podocyte loss, glomerulosclerosis, proteinuria, and progressive kidney disease. In conclusion, therapeutic MDM2 blockade may hold the risk of impaired epithelial healing in AKI. On the other hand it may delay or halt the progression of glomerular disorders to CKD by reducing renal inflammation and by directly protecting podocytes from cell death by mitotic catastrophe.