Role of murine double minute (MDM)-2 in kidney injury and repair

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.