A Systems Biology Approach to Manipulating the Biochemistry of Senescent Cells
Cells become senescent in response to reaching the Hayflick limit on replication, or to potentially cancerous mutations, or a toxic environment and consequent cell damage, or signaling from other senescent cells. Senescence is nominally an irreversible state. Replication halts and the cell begins secreting pro-inflammatory signals to attract the attention of the immune system. Senescent cells are normally removed via programmed cell death or the actions of cytotoxic immune cells. With age the rate of creation increases and the rate of removal falls, however, leading to a growing number of senescent cells throughout the body. The signaling of that growing number of senescent cells in aged tissues causes chronic inflammation and disrupts tissue maintenance, leading to age-related disease.
What to do about this? Much of the focus of the research community is on senolytic approaches that force senescent cells into apoptosis and self-destruction, or that provoke the immune system into more efficient clearance of senescent cells. These therapies have achieved impressive results in mice, reversing age-related disease and many measures of aging. Some researchers are interested in the reversal of senescence, however: reprogramming cells in ways that overcome the regulatory processes that normally ensure continuation of the senescent state.
Is reversal of senescence a good idea? It seems likely that at least some senescent cells are senescent for a good reason. That they are damaged, and in some cases that damage is potentially cancerous. Reversing senescence may well produce short term gains that are similar to those of senolytic therapies, since in either case the harmful signaling produced by senescent cells is removed. But a significantly raised risk of cancer may be the cost of that approach.
Systems biology for reverse aging
Although partial reprogramming proved that senescent cells can be reverted, early termination of this reprogramming process is known to cause epigenetic dysregulation, resulting in dedifferentiated dysplastic cells such as renal cancer. Therefore, a novel therapeutic strategy without such critical limitations is highly needed. Cellular senescence is caused by complex interactions among biomolecules that govern cell cycle, DNA damage response, energy metabolism, and cytokine secretion. Recent studies showed that cellular senescence, previously considered as an irreversible biological phenomenon, can be reversed, but due to the nature of such complex interactions governing cellular senescence, the mechanism by which cellular senescence can be reversed has not been revealed.
Researchers reconstructed an ensemble of 5000 Boolean network models that can represent senescence, quiescence, and proliferation phenotypes by integrating information from the literature, network databases and phosphoprotein array data of dermal fibroblasts. In their models, cellular senescence is induced by simultaneous activation of DNA damage signal (doxorubicin) and growth signal (IGF-1 plus serum). They identified 3-phosphoinositide-dependent protein kinase 1 (PDK1) as the optimal protein target that can safely revert senescence to quiescence while avoiding uncontrolled proliferation, through extensive computer simulation analysis of the ensemble model. PDK1 forms a positive feedback structure along with AKT, IKBKB, and PTEN, that simultaneously control both nuclear factor κB, which controls cytokine secretion, and mTOR, which regulates cell growth.
In order to validate the simulation results, researchers conducted in vitro experiments and confirmed that when PDK1 was inhibited, various markers of cellular senescence are returned to normal and proliferation potential is restored. From wound healing assays and 3D reconstructed skin tissue experiments, they also reaffirmed that the reverted cells are able to respond appropriately to external stimuli. In particular, by observing dermal fibroblast within dermis along with keratinocyte within epidermis, 3D reconstructed skin tissue experiments verified that PDK1 inhibition promotes epidermal renewal and restores skin thickness, resulting in reversal of age-related skin degeneration.
Compounds from Natural Sources as Protein Kinase Inhibitors
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7698043/
Fisetin is shown in Figure 1 as an inhibitor of PDK1. A diet rich in plant polyphenols is probably a decent idea anyhow.
https://medicalxpress.com/news/2021-06-cells-contribute-age-related-diseases.html
What does your senolytic research focus on?
There is a specific fatty acid made in small amounts in the body called dihomo-gamma-linoleic acid or DGLA. It's also present in tiny amounts in the diet. When I gave aged mice larger amounts of DGLA, they went from having quite a few senescent cells to having significantly fewer.
This presents a new therapeutic target. I identified a candidate compound using the DGLA metabolic pathway that works at a dose that is over 1,000 times lower than fisetin, so you can imagine we're quite excited by these results.
Like many biomedical discoveries, it was accidental. DGLA makes anti-inflammatory lipids, which help alleviate conditions such as rheumatoid arthritis. I was studying this aspect of DGLA when I was surprised to discover that it killed senescent cells.
My work is in its very early stages, and we've only studied a small number of mice, so it's too early for even tentative conclusions, although I'm obviously pleased that we've seen the elimination of a meaningful number of senescent cells in old mice. We'll be closely monitoring DGLA's positive effects as well as any negative effects on the mice.
How would DGLA be given to people?
We are several years away from that, because everything has to be perfect with mice before we even think about trials with people.
First, we have to figure out how DGLA is killing senescent cells in mice. Again, not all studies with mice yield similar results in humans, so we are very careful about how we convey our findings and possible future actions.
But being at the HNRCA, I have met USDA researchers and nutrition scientists, and discovered that some of those folks were developing DGLA-enriched soybeans. In one scenario, you might go out for sushi and get a little bowl of DGLA-enriched edamame as a side. By the time you're done eating, you've helped reduce the odds of getting some age-related pathology.
I don't know if it will play out that way, but it's an idea we're working toward. I also am working on therapies that elevate the amount of naturally occurring DGLA in senescent cells that I am very excited about, so this would be an alternative approach.
You are also studying ways to test senolytic therapies beyond such measures as improvement in distance walked, right?
Yes, I am developing a quick and easy test to tell if senolytic therapy is working. Testing for senolytic effectiveness is not really being done now-you just look for improvement in symptoms or functioning and essentially conclude that it's due to the therapy.
But we can't say that with full confidence. Currently, researchers obtain skin or fat samples from patients in these trials before and after senolytic treatment to look for senescent cells. But this is an invasive procedure and it's especially challenging for older people to undergo this testing.
One way to solve this dilemma is to identify a biomarker, a measurable compound that consistently and reliably can confirm an intervention's effectiveness. For example, we know that a certain lipid, dihomo-15d-PGJ2, accumulates in large amounts inside of senescent cells.
When we give a senolytic therapy that kills these cells in mice or human cells, this lipid is liberated. Detecting it in blood and urine is far less invasive, so that's what I'm working on now. Our aim is to be able to test people receiving senolytic therapy for the presence of dihomo-15d-PGJ2 in their blood and urine by the end of the summer.