James Kirkland, MD, PhD, on senescence research and early senolytics (Part 1)

Dr. James Kirkland’s research explores how cellular aging impacts a range of age-related and chronic diseases. In a recent interview, Dr. Kirkland explains what senescent cells are, how they impact aging processes, and how his lab’s research is pushing toward treatments to remove these cells. This research holds the promise of preventing, delaying, and alleviating multiple diseases, including cancers, dementias, diabetes, arthritis, and more.

In the first of a two-part Ask the Expert series, Dr. Kirkland speaks to the foundation of senescence research and its translation into senolytic therapeutics.

What are senescent cells and how do they change with age?

Senescence is a cell fate, much like cell proliferation, differentiation or cell death through apoptosis or necrosis. Senescent cells are cells that were formerly capable of dividing, but lose their ability to divide. They resist dying and are normally cleared by the immune system. Senescent cells accumulate with aging, especially in individuals with frailty or dysfunction. They also can appear at any point during life, if there's a chronic disease or disorder of some sort. For example, you find them in children who've been treated with certain cancer drugs or radiation for leukemias, lymphomas, or other tumors. They also occur in fat tissue of younger individuals who are obese and diabetic, in their pancreas and elsewhere. For the most part, senescent cells are caused by cell stresses, such as mechanical stress (as in osteoarthritis), shear stress (like in damaged arteries or veins), metabolic insults, repeated cell divisions, or damage-related signals arising from injured tissue or due to pathogens (such as viral or bacterial infections). These factors can make a cell become senescent, which takes anywhere from 10 days to six weeks. It takes longer for cellular senescence to become established than is the case for other cell fates.

Are senescent cells good or bad?

Some senescent cells develop a damaging pro-inflammatory senescence-associated secretary phenotype (SASP), in which they produce factors that kill the cells and damage the tissues around them. They also have effects at a great distance and can spread senescence from cell to cell. Anywhere from 30 to 70% of senescent cells develop a SASP, others don’t. Senescent cells can also have good functions. They help to clear damaged tissue, so they're important in wound healing. They're involved in remodeling during development and growth. They're also involved in pregnancy. They accumulate in the placenta and they produce the factors that propel the baby through the birth canal, for example. So there are some beneficial effects of senescent cells. Cells with cancer mutations can become senescent, arresting their growth. At the same time, this means that many senescent cells can harbor cancerous mutations and can develop into cancers if they escape senescence and begin to divide.

Tell us a bit about the path to developing therapies that target senescent cells.

Senescent cells were first described by Hayflick and Morehead in 1961. There were some important papers that came out of the National Institutes of Health (NIH) and elsewhere in the mid 1970s, suggesting they may accumulate with aging and frailty in people. We noted that senescent cells accumulate with aging in fat tissue of rats that are Caesarian born and double barrier-reared and treated the same way throughout their lives (in other words, free of environmental influences). In the early nineties, we found that cells with limited replicative potential accumulated in adipose tissue of such animals and that the senescent cells release the tissue-damaging, pro-apoptotic factor, TNF alpha.

In 2004 a very important paper came out by Norm Sharpless, who's now Director of the National Cancer Institute, in the Journal of Clinical Investigation. He found that senescent cell burden in mice was inversely related to healthspan and health. He found that senescent cells accumulate with aging in mice, then used food restriction to extend their lifespan and health-span. He also introduced a mutation called the Ames dwarf mutation, which makes mice live longer and have an increase in health span. He found that these two interventions (food restriction and the Ames dwarf mutation) delayed the appearance of senescent cells.

This association between cell senescence and health span and disease onset led me to ask: What would happen if we got rid of senescent cells? So we started trying to create drugs and strategies to clear senescent cells. First, we tried to make fusion proteins that carried a cargo that was damaging and would bind to the senescent cell membrane. We worked on that for a long time, got nowhere. Next, we resorted to high throughput screens, and couldn't really get a screen working properly to detect drugs that would selectively kill senescent cells. Since then, Paul Robbins at University of Minnesota developed such a high throughput screen that's being used to discover new senolytic drugs.

In May, 2013, we remembered that Eugenia Wang had showed that senescent cells are resistant to dying. We also knew from our own work and AFAR grantee Judy Campisi's work that senescent cells produce factors that kill cells around them. So we wondered: Why do senescent cells survive despite the fact they kill the cells around them and destroy the tissue around them? This led us to use bioinformatics approaches based on proteomics and transcriptomic databases to ask if there are survival networks that senescent cells rely on to defend themselves against death from the things that they're using to kill the cells around them. In other words, why don't senescent cells commit suicide? To us, that meant there must be pro-survival networks that they use as defenses. We noted that senescent cells were very much like cancer cells and cancer cells have those sorts of networks. We used proteomic and transcriptomic databases to look for such pathways, and we found five that we called senescent cell anti-apoptotic pathways (SCAPs). Since then, three more have been discovered. These pathways work in a network to defend senescent cells against their own death-inducing secretions.

How did you develop the first senolytic drugs?

We used RNA interference approaches to selectively knock down nodes on this SCAP network to ask if we could selectively allow senescent cells to die (while not affecting normal human cells in culture) by transiently disabling some of the components of the network. We found when using RNA interference approaches with the key SCAP network nodes that senescent cells died but normal cells didn’t. The next thing we did was look for drugs or compounds that would hit those nodes or combinations of those nodes. We noticed that for some kinds of senescent cells, hitting one node is not enough; you need to hit several nodes. And different kinds of senescent cells depended on different SCAP pathways to defend themselves against dying. In some cases, these networks were redundant; if you knock down one pathway, the senescent cells would use another one to defend themselves against their SASP.

Knowing all that, we went to the computer to look for agents with strategic targets across the network, not just one target. We came up with a long list of drugs, which turned out to be the first senolytics. The drugs we decided to focus on earliest were the ones that have known safety records in humans, have been on the market for a while, or are natural products that are in our food. We also purposely picked agents that have a short elimination half-life, which would be gone from the body quickly because we wanted to use what we call a “hit-and-run” approach to kill senescent cells. Because it takes time for more senescent cells to appear, we wouldn’t have to give drugs continuously like you do with normal drugs. It takes senescent cells 10 days to six weeks to re-appear, so we reasoned that we could give these drugs 1-2 days out of every two weeks or every month. That's how we came up with the first senolytic drugs. Since then, many more have been developed using that same approach. Now there are high throughput screens and other ways of killing senescent cells under development, including what we call CAR-T approaches, vaccines, and a variety of other approaches.

Read part 2 of Dr. Kirkland's Ask the Expert interview here.