MDM2 Antagonists Attenuate Harmful Signaling from Senescent Cells

A fair number of the scientists working towards therapies to address cellular senescence, one of the causes of aging, are more interested in suppressing signaling from these cells than in destroying them. Cynically, a treatment one has to keep using consistently is much more interesting to pharmaceutical companies than a treatment that only has to be applied once every few years at most. Until researchers encounter a population of senescent cells that cannot be safely removed, destruction continues to look like the far better option. Senescent cells are harmful because of the mix of signals they generate, a mix that is still comparatively poorly mapped and understood. Suppressing it may well prove to be a lengthy and difficult process of progress by small degrees, while destruction can be achieved in the near future and removes all of the harmful signaling whether or not it is understood.

Astrocytes are one potential candidate for a population of senescent cells that might be challenging to remove. It isn't completely clear that all of the astrocytes showing markers of senescence are actually senescent, but if so it represents a large portion of all astrocytes in the aging brain. Abrupt clearance of these cells would probably not be healthy, regardless of the incremental harms they are causing. With this sort of thing in mind, it is prudent to have a backup strategy under development, whether that is some form of careful incremental winnowing and replacement of these cells over time, or a form of suppression of their bad behavior while allowing them to live.

One of the common features of aging is low-level chronic inflammation, termed sterile inflammation or inflammaging. Even though all the sources of inflammaging are unclear, it likely derives at least partly from senescent cells. Mammalian cells undergo senescence in response to stressful stimuli. An important feature of senescent cells is the secretion of a myriad of biologically active factors, termed the senescence-associated secretory phenotype (SASP).

The SASP is similar between mice and humans, and comprises inflammatory cytokines such as IL-6 and IL-8. The SASP can disrupt the surrounding microenvironment and normal cell functions, and stimulate malignant phenotypes in nearby cells. Senescent cells can also promote tumor growth in mice. Because senescent cells increase with age and are frequently found within hyperplastic and degenerative tissues, the SASP may be a major cause of inflammaging. Compounds that modulate the SASP hold promise for ameliorating a number of diseases of aging, including cancer.

Nutlins were originally identified as potent small molecules that inhibit the interaction between p53 and MDM2, which promote p53 degradation. Nutlin therefore stabilizes p53, thereby promoting the apoptotic death of cancer cells. Importantly, in cancer cells, nutlin-3a inhibits the activity of NF-κB, a potent transcriptional stimulator of genes encoding inflammatory cytokines, in a p53-dependent manner. The clinical importance of small-molecule MDM2 inhibitors like nutlin-3a spurred the discovery of similar compounds, such as MI-63, which are more efficient inhibitors of the MDM2-p53 interaction.

We investigated the effects of small-molecule MDM2-p53 interaction antagonists on senescent phenotypes, including the SASP, of primary human fibroblasts and epithelial cells. We used nutlin-3a, as well as the non-peptide small molecule inhibitor of MDM2, MI-63. We compared these compounds for their ability to induce a growth-arrested state, whether quiescence or senescence, in human cells, and evaluated their ability to modulate the SASP. We found that both compounds trigger selected markers of a senescent-like state, but the growth arrest was reversible, and both significantly suppressed the SASP, suggesting potential utility as therapeutic agents.



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