Aging at the cellular level is called “cell senescence”, and it contributes profoundly to whole-body aging. The most promising near-term prospects for a leap in human life expectancy come from drugs that eliminate senescent cells. Programs in universities and pharmaceutical labs around the world are racing to develop “senolytic” drugs, defined as agents that can kill senescent cells with minimal harm to normal cells.
Apoptosis is cell suicide, and (from the perspective of the full organism) it’s the best thing that can happen to senescent cells. The authors of this newest Dutch study ask how it is that senescent cells escape apoptosis.
FOXO is a protein that controls gene expression, a master transcription factor associated with aging and development. (It is the homolog in mammals of the pivotal life extension protein first identified in worms as DAF16 in the 1990s.) FOXO4 activiation in a cell can block apoptosis. P53 is the most common trigger of apoptosis, the first protein biochemists usually think of in connection with apoptosis. P53 has multiple functions in the cell nucleus, but as a trigger for apoptosis, it works through the mitochondria. FOXO4 binds to p53 and blocks its induction of apoptosis.
The treatment studied in this paper is an artificially modified FOXO4, a dummy that binds to p53 in place of regular FOXO4, but doesn’t block senescence. It has been named FOXO4-DRI, and it works by crowding out the native FOXO4.
The authors note, with caution, that mice with no FOXO4 at all appear normal; but apoptosis is an important cell function throughout the lifespan. A cell must have “good judgment” about when to eliminate itself, and that works in both directions; in older animals and people, we not only see failure of apoptosis to eliminate senescent cells, but we also see healthy muscle and nerve cells undergoing apoptosis prematurely, and we lose muscle and brain mass as a result. Other functions of FOXO4 include DNA repair, and mice that lack FOXO4 are subject to a high burden of DNA damage.
By analogy with chemotherapy for cancer, the value of a senolytic treatment is measured by its ability to kill senescent cells without doing harm to normal cells. The index called SI50 (SI for “selectivity index – 50%”) is defined by analogy to LD50 = the “lethal dose” of a toxin, the dose at which half of all cells die. SI50 is defined as the ratio of LD50’s for normal and senescent cells. It is the concentration of the agent at which half the normal cells die, divided by the concentration at which half the senescent cells die. Authors of the current paper report SI50 about 12. My guess is that 12 is an encouraging beginning, but it is not high enough to support a useful therapy. After a standard dose is injected in humans, the cellular concentrations vary from person to person and from tissue to tissue.
The encouraging fact is that, at the optimal dose, more than 80% of the senescent cells have succumbed to apoptosis, while the number of eliminated normal cells is still below detection:
In other words, the vertical distance between the black and red curves is encouraging, but the horizontal distance is cause for concern. Senolytic agent studied previously, including dasatinib, quercetin and ATTAC, did not include measurements of SI50 that we might use for comparison.
How does FOXO4-DRI perform in live mice?
Authors of this study were excited in a rush to publish. They used a fast-aging strain of mice, and even for these, they did not wait to see survival curves. The indicatators of rejuvenation that they do report look positive: increased activity levels, regrowth of lost fur, and improvement of kidney function lost with age.
Comparison to Last Year’s Senolytic
Here’s how authors of the current study characterize FOXO-DRI compared to two previously reported senolytic agents:
Two classes of anti-senescence compounds have been reported so far: Quercetin/Dasatinib, either alone or in combination [ref], and the pan-BCL inhibitors ABT-263/737 [ref, ref]. Quercetin and Dasatinib have been reported to be non-specific. We found no selectivity toward senescent IMR90 and therefore this cocktail was not explored further. ABT-263 and ABT-737 target the BCL-2/W/ XL family of anti-apoptotic guardians. Indeed, ABT-737 showed selectivity for senescent IMR90. However, already at low doses, it appeared to influence control cells as well. Also in a treatment regimen where both compounds were added in consecutive rounds of lower concentrations, FOXO4-DRI proved to be selective against senescence yet safe to normal cells.
I reviewed the Quercetin/Dasanatib paper two years ago. It was an early proof-of-principle, using medications that are already known (and FDA approved). But the 1-2 punch is not sufficiently selective–it is toxic to normal cells.
I missed the two papers about ABT-263 [ref] and ABT-737 [ref]. BCL-2 is the founding member of another family of proteins that signal a cell to resist apoptosis. Both ABT-263 and ABT-737 were identifed in screens for agents that block BCL-2. These two studies published in Nature last year, one from University of Arkansas, the other from the Weizmann Institute, both use radiation exposure to create a large population of senescent cells, and then show that the senescent cells are selectively eliminted by ABT. The ABT-263 paper included some in vivo results, indicating enhanced growth of blood stem cells after senescent cells have been removed. In vivo testing of ABT-737 was limited. Neither group reports the selectivity index (SI50) as calculated by Keizer in the latest study; but from graphs that they do present, it is clear that ABT-263 is more selective than ABT-737, and that neither is as selective as FOXO-DRI.
The original marker used to identify and target senescent cells by the Mayo Clinic’s 2011 study was p16Ink4a. The selective elimination technique they used (in 2011) was limited to genetically modified mice, but a year ago, a new paper from Mayo Clinic demonstrated a similar procedure for ordinary, non-GMO mice. Twice weekly injections of an antibody that induced apoptosis in cells that expressed p16Ink4a extended lifespan of the mice by 25% – 30% compared to controls, comparable to the results in the 2011 paper. Caveat: the control mice received sham injections that shortened their lifespans.
The Bottom Line
The idea of removing senescent cells has a lot of appeal. Not only does it enjoy broad empirical support in mammals; it also pulls together several ideas about the origin of aging:
- Parabiosis experiments and their follow-ons have convinced us that circulating chemical signals form the basis of an epigenetic clock. Some of these circulating molecules are known to come from senescent cells.
- Aging commonly accelerates exponentially with age, as though it were driven by a positive feedback loop. Senescent cells secrete cytokines that make more senescent cells–there’s your feedback.
- Short telomeres initiate senescent cells. At any given time, there is a bell-shaped curve of telomere length among the body’s cells. The tail of the telomere distribution contains a few cells that are driven to senescenceby having very short telomeres.
There is now a world-wide effort, making rapid progress toward specifity in senolytic treatments. In other words, FOXO-DRI is the newest agent, and it shows the best ratio yet for killing senescent cells while avoiding collateral damage to healthy cells. (It cannot be taken orally and must be injected, but perhaps this is not such a great drawback for a treatment that is needed only intermittently, every few years.)
How will such promising mouse results translate into human health and life extension? We have as yet no data, not even anecdotes. But perhaps we are near the point where hope and courage will motivate the first self-experimenting volunteers. Caloric restriction and its mimetics produce much greater percent increases in lifespan in mice (2 year lifespan) compared to dogs (10 yrs) or monkeys or humans. Senolytics work via a completely independent pathway; we can hope that percent benefits in humans will be closer to the mice. Since this is about upregulating elimination of cells via apoptosis, the strongest benefits are likely to be against cancer, and mice are more vulnerable to cancer than humans.
This is a fast-moving field in which researchers are in a rush to publish and (presumably) pharmaceutical companies are taking pains to keep their results hushed up. Sharing of information and resources could push this research over the top and give us the first full decade of human life extension.