#036 Judith Campisi, Ph.D. on Cellular Senescence, Mitochondrial Dysfunction, Cancer & Aging

Cellular Senescence: How To

Welcome back to another Scienterific episode of the Found My Fitness podcast. Today's guest is Dr.

"Welcome back to another Scienterific episode of the Found My Fitness podcast. Today's guest is Dr. Judith Campisi, a professor of biogerontology at the Buck Institute for Research on Aging and a co-editor-in-chief of the Aging Journal."

Cellular Senescence: Benefits

Some people have argued that with age, the barrier in your gut breaks down slightly, not enough to cause bacteria to invade your tissues, but enough to enable bacterial products to leak into your bloodstream.

"And now you can imagine that those cells, as they build up, they start to drive the pathologies that we associate with aging. Probably also start to cause more cellular senescence."

Cellular Senescence: Benefits

That's pretty funny. I kind of, I've always been curious about what tissues, if certain tissues accumulate more senescent cells than other tissues, including in the brain.

"And he said, I plan to die at 110 from a bullet wound from a jealous husband. That's pretty funny. But talking about the brain, you just mentioned the brain. I kind of, I've always been curious about what tissues, if certain tissues accumulate more senescent cells than other tissues, including in the brain. Yeah. So we and others have looked at senescence in the brain. Based on the markers we have, so I should also preface this by saying we don't have perfect markers for senescent cells. There are many markers, and so we tend to have some confidence that we say a cell is senescent if we look at multiple markers and we say, well, there's a good chance this is a senescent cell. So the best markers that we have have been used in the brain by a number of laboratories. And it seems that the cells that are more likely to become senescent in the brain are astrocytes. Sporting cells. Exactly. And so that's interesting from two points of view. The first is there are probably more astrocytes in your brain than neurons. There are, absolutely. Yeah. People don't realize that, but there are lots of astrocytes. The second is it's the astrocytes that give rise to brain cancer. So again, consistent with the idea that the stress response protects us from cancer, at least for a while. We've studied human astrocytes, and I can tell you they amount to classic senescence response, including producing all those pro-inflammatory cytokines and growth factors and proteases. And we even have new evidence that astrocytes, as you know, help protect the neurons from certain types of toxicity, like neurotransmitter toxicity. And we can show that when astrocytes become senescent, they become less effective in that protective response. Yeah. Yeah. So again, you know, being able to get rid of those senescent astrocytes could be beneficial for preserving brain function. Now, once neurons die, that may be too late. So we have to distinguish between the ability to prevent degeneration in the brain versus reversing it. I think reversal is going to be much harder. It's always much harder, for sure. Yeah, yeah. Is the senescent cells that occur in the astrocytes, do you think that also, you know, because as we age, brain inflammation also increases. That's correct. Do you think it's a contributing factor? Yeah, we definitely think that's a contributing factor. It may not be the only factor, but definitely. Well, there's now, you know, lots of evidence showing that, you know, a lot of cytokines in the periphery can cross the blood-brain barrier through a variety of mechanisms, including this newly discovered lymphatic system that's connected to the brain, the meninges or something. That's right. So that's interesting. So you don't necessarily even have to have astrocytes in essence to help contribute to neurodegeneration. It could be in your liver or your skin. Exactly. Exactly. So it's sort of like just sort of all connected. Maybe we can talk a little bit about the mechanisms that lead to cellular senescence. I mean, we mentioned inflammation sort of...you mentioned it's a stress response. It's just distress. But, you know, specifically... What are those stresses? What are the main... Like, I know the main mechanism I always think of when I think of cellular senescence. Just DNA damage? DNA damage. Yes, yes. For sure. Anything that causes severe or persistent damage to the genome will drive cells into senescence. It makes sense because that puts you at risk for mutations. Mutation puts you at risk for cancer. So the cells want to shut that damaged cell down. But there are other stresses now that we know. We recently showed, for example, that having bad mitochondria in the absence of DNA damage. So this is just mitochondrial dysfunction, if you will. And we could show that you could cause mitochondrial dysfunction by any number of means, five or six different ways of causing the mitochondria to fail to produce the energy that they need and to produce more free radicals, even in the absence of those free radicals getting to the nucleus, cause the cells to senesce. So they will senesce in response to bad mitochondria. What's interesting is the cells senesce, they stop dividing. They do start secreting molecules, but it's a different complement of secreted molecules. So we can now pretty much determine whether a cell has become senescent due to DNA damage versus bad mitochondria based on their secretory profile. There's overlap. Don't get me wrong. There's overlap. But there's also distinct features that the mitochondria are responsible for, bad mitochondria are responsible for versus damaged DNA. What's like the main, what would you say the main distinctive feature? So one of the main distinguishing features is with DNA damage, there's a pathway that increases cytokines like IL-6, IL-8. These are very prominent pro-inflammatory cytokines. That doesn't happen with bad mitochondria. So that loop is pretty much not activated. But then other molecules are activated that can also be pro-inflammatory, but through a different pathway. Wow. So this kind of is very extremely interesting. I wonder why that is. And I wonder if there's different functions of these senescent cells. But the secretion of some of these pro-inflammatory cytokines and the attraction of immune cells to that area, one would think would then cause that senescent cell to be cleared away. Yes, yes. And it probably does happen. But it probably doesn't happen efficiently enough. Or with age, maybe we make senescent cells at a higher rate. So several labs, not our lab, but several other labs have shown that senescent cells express molecules on their surface that can target them for being killed by the immune system. And the immune system can do that, and it does do that. Nonetheless, we still see this increase with age. But the immune system declines with age, right? Function of the immune system, do you think that may contribute? Well, that's a big unanswered question. So what happens with so-called immune senescence is primarily the adaptive immune system. Oh, that's actually the good part of the immune system. Exactly, exactly. The quieter part. And this is why you become more susceptible to certain types of infections with age. So the adaptive immune system tends to decline with age. The innate immune system, if anything, increases in activity with age. And it's the innate immune system that clearly targets senescent cells for clearance. So for example, senescent cells express on their surface ligands for natural killer cells. And natural killer cells will then attack those senescent cells and kill them. Nonetheless, senescent cells still accumulate with age. So we're considering several possibilities. So one is maybe you're just making them too fast. The immune system can't keep up. The other is that although the innate immune system doesn't decline with age per se, it does change. And it could change in a way that it becomes less efficient at clearing senescent cells. So that's a big open question. We don't know the answer to that. And the third possibility is that some senescent cells may develop mechanisms to protect themselves from immune clearance. And we're sort of still studying that now. And we think maybe all possibilities are still open. Geez, I have so many things that popped in my head that I want to talk about, which order order because I'm going to forget. So talking about these changes in the adaptive versus the innate immune system sort of reminded me of Dr. Valter Longo's research, who I interviewed a few months ago. And he was talking about how this prolonged fasting in mice, which is about 48 hours or translates to like four days in humans, which is quite a long fast, but was able to just very robustly clear away damaged cells, presumably senescent cells as well, also caused cellular death, but followed by a massive and robust increase in stem cell proliferation, sort of replenishing the population. But what was so interesting was that it seemed to, at least in aging mice, if you did this in aged mice, it normalized the difference between the innate and the adaptive. So like you were mentioning, the adaptive immune system declines with age. But this fasting sort of like replenished somehow, I guess, the adaptive, maybe some of the stem cells... In the bone marrow, yeah. Yeah. And so you maybe have more of a 50-50 ratio like you do when you're younger. Right. So what would be interesting is if you, like, somehow did that experiment where you caused, you know, did some fasting and, you know, sort of regenerated that adaptive immune part arm, I guess, of the immune system, and then, like, gave some chemical to cause cellular senescence and see if there's any change in... Well, you know, a lot of Valter's work also has to do with the side effects of chemotherapy, right? So he was able to show that with this intermittent short-term fasting, even in humans, he could not only improve the efficacy of chemotherapy, but prevent some of the side effects. So we've shown very recently using mice, a transgenic mouse model, that some of the so-called genotoxic chemotherapies, the chemotherapies that damage DNA, definitely causes senescence. And if we eliminate those senescent cells in our transgenic mouse model, we can eliminate several of the side effects, several of the bad side effects of chemotherapy. So one possibility is that what the fasting does is it might eliminate senescent cells. More likely, what it might do, we think, is dampen mTOR activity. So just to remind you, mTOR is a kinase, which is highly conserved from yeast all the way to humans. And it's a nutrient and growth factor sensing kinase. So high nutrients, high TOR activity. And what has been shown in yeast and worms and flies and mice is that if you dampen, you can't get rid of TOR activity, you need it for life, but if you dampen TOR activity, either genetically or with the drug rapamycin, which is known to target one arm of the tore pathway, you can extend lifespan. And what we showed recently is that what rapamycin does, or dampening tore activity does, is it also suppresses primarily the inflammatory arm of the secretory phenotype of senescent cells. So it could be that fasting and rapamycin and the secretory phenotype of senescent cells all come together around the TOR pathway and that it's really TOR activity that's driving both the aging phenotypes, the side effects of chemotherapy, and may explain, partly explain, the benefits of the short-term fasting that Valter is a proponent of. Interesting. So if it's dampening the secretion of these cytokines from the senescent cells, but it's not actually getting rid of the senescent cells? It's not. So far as, well, we've shown that either genetically or pharmacologically with drugs like rapamycin suppresses the secretion of senescent cells, but it doesn't kill them. So unlike some of these other drugs, these so-called senolytic drugs that actually kill senescent cells, the mTOR drugs, the mTOR dampening drugs suppress the ability of senescent cells to secrete. And the effects last longer than the application of the drug in the sense that we know that part of that secretory, pro-inflammatory secretory phenotype is due to a feedback loop. And what dampening mTOR does is it breaks the loop. And the loop takes time to reestablish. So even after you withdraw the drug, you still get suppression until eventually the loop reestablishes and then the cell starts secreting again. So you might think that if you were to fast for four days, you know, every few weeks, you might have more benefit, certainly more benefit than taking a drug like rapamycin, which has side effects. Right. Because you're not only, you know, when you're fasting, you're also clearing away the damaged cells, in theory. I mean, we don't know how much of that's occurring in humans yet. Yes. But we do know in mice that happens. And then the other thing, when this kind of goes back to the mitochondria-induced senescence that we talked about a minute ago, is that we know that fasting NAD levels. And so the NAD plus NADH ratio. And that sort of is very interesting because this mitochondrial induced damage, I think, has something to do with declining NADs. It does, actually. So that's what we've shown is that this mitochondrial dysfunction-induced senescence, we call it mitochondrial dysfunction-associated senescence, or MIDAS. So we call it the MIDAS phenotype. It really has to do with this altered NAD-NADH ratio, and that's one of the drivers. Interestingly, so when you change that ratio, you activate a kinase called AMP kinase. AMP kinase is a major regulator of P53. P53 is a major regulator of both senescence and apoptosis. So that could be the link of why in Volta's paradigm you get reduced inflammation. That could be due to suppressing the secretory phenotype, but also increased apoptosis because you now have activated P53. Right, exactly. Yeah. So I just got sort of lost in the question I was going to ask you, but the... So, okay. No, no, one other thing I was going to... Yeah, I don't know if you're going to ask this question, but it's worth discussing, is the stem cell, the regenerative process that Vulture has seen. So we also know that the secretions of senescent cells can have very profound effects on stem cell proliferation and function. So it could also be that by dampening the secretory phenotype of senescent cells, you now release those stem cells from the suppression that was due to those secretory phenotypes and therefore allow them now to do what they do best, which is to proliferate and regenerate a tissue. So all of these things really probably tie in to each other. I mean, they're all interrelated. So the growth factors that are actually secreted by the senescent cells do help with stem cell growth? They can. I always thought of senescent cells like if this happens in a stem cell, you're depleting the stem cell pool and it's contributing to stem cell aging. Both are probably true. Okay. Both are probably true. So we have shown in the skin, for example, that with age, senescent cells do accumulate. But if you clear those cells, you don't get much benefit. And that's because by old age, you've depleted the stem cells. So, you know, you can't, once you've depleted that stem cell pool, you can't go backwards, or at least you can very easily go backwards. But two labs have now shown that senescent cells can also produce growth factors or factors that help neighboring cells reprogram to stimulate regeneration. And they do it, again, by their secretory phenotype. So it's, again, this double-edged sword. Some of the secretions of senescent cells dampen stem cell activity, and others promote stem cell activity. What about the mitochondrial-induced senescent cells? What's their function? I know the DNA-damaged-induced ones, you know, obviously are protecting from cancer. What's the purpose? What is the evolutionary purpose? Well, if you have a cell with bad mitochondria, you probably want to clear that cell, prevent that cell from propagating, because then you're going to have clones of cells with bad mitochondria. And we know that that causes all sorts of degenerative diseases, neurodegeneration, as well as muscle degeneration. And so it probably is also protective, but not so much against cancer, but against accumulating degeneration within a tissue. We know, for example, that people who are born with mitochondrial DNA defects, eventually the bad mitochondria expand. And so that's not good for an organism. So there probably is a protective mechanism to prevent the propagation of cells with bad mitochondria. Okay. Well, that makes more sense. Yeah. I guess, you know, at a certain point, your mitochondria, if you don't have, you know, a really bad defect in mitochondrial DNA, your mitochondria will repair themselves through mitochondrial fusion. Yeah. I mean, right? Isn't fusion also part of how they sort of exchange all their mitochondrial content and the damaged one sort of fixes itself to some degree? Although I guess that gets diluted. Or the bad ones also get eaten up by the lysosomes. Right. Yeah. So I guess there's multiple mechanisms."

Cellular Senescence: Benefits

Do the mitochondrial-induced senescent cells, they produce the growth factors that affect stem cell growth? They produce some growth factors, yes.

"Do the mitochondrial-induced senescent cells, they produce the growth factors that affect stem cell growth? They produce some growth factors, yes. They produce, for example, amfiregulin, which is an EGF-like growth factor. So they do. They do."

Resveratrol: Benefits

The error bars get larger and larger and larger. We don't know whether it's malleable, meaning we don't know whether you can make things less variable or more variable.

"And we don't know. We know that exercise can have some effects on senescent cells in vivo, but we don't know how it works. We don't know precisely what it does. What about the stress response pathways it activates? I mean, exercise is a type of hormetic stress. That's right. Right. So it's a little bit stressful and activating all these. Yeah. So that's one idea is that it's hormetic stress, meaning it's low level stress that then primes everybody else, all your other stress responses to be hypervigilant. And so then when a bad telomere comes along or an insult from radiation or high sugar, because you just couldn't resist that last brownie, you're better able to deal with that stress. It's one hypothesis. I think there's still an ocean of ignorance around how exercise seems to be so beneficial for so many indications of aging. Again, aging, not necessarily maximum lifespan, healthspan. Yeah, it does seem to affect many different diseases of age as well, in addition to sarcopenia, cardiovascular health. I mean, cardiorespiratory fitness is also very tightly correlated with... And there are some groups now that are studying the effects of exercise on side effects of chemotherapy and showing benefits of, again, mitigating some of those side effects simply by an exercise regimen. And we're not talking running marathons. We're talking sort of moderate but persistent exercise. I think that's... It's fascinating. I've read a couple of the studies, at least the animal studies, where they can sort of force them to run a little bit more on this little treadmill. Running wheel, yeah. And it was like, there was a very robust response in terms of, at least in combination, I think, with the standard of care treatment. That's right. Where I want to say something like 50%. It was something very like, really? You know, like, that's very...you know, I've always thought about it as sort of when you're exercising, you're forcing your mitochondria to work harder, and you're producing more reactive oxygen species. And cancer cells don't like that. I mean, they're, you know...so who knows? Who knows? Yeah. I think, yeah, mechanism, so far unknown, but lots of possibilities. Right. What about the hope for a clinical assay for measuring things like cellular senescence or at the very least DNA damage in people? Yeah. So that can be done. I mean, I know of at least a couple of companies that are doing this. Usually they use peripheral blood. and there are very good antibodies that will detect persistent DNA damage. So you can stain these blood cells and get a sense of what your DNA damage load is. There are good markers for senescent cells. Now we always recommend that you use two or three, but we can probably assess the load of senescent cells. The difficulty lies in tissue specificity. So peripheral blood is easy. You know, buccal swabs are easy because they're accessible. But, you know, you really probably want to know how many senescent astrocytes you have in your brain. And, you know, brain biopsies are not going to be approved very soon. So that's the difficulty is we can get a general idea from those easy-to-access tissues. But there are tissues we might want to know about that are not going to be easy to biopsy. Is there a correlation between, let's say, if you were to look at cellular senescence in white blood cells or leukocytes, between that and the heart or the brain? Yes, yes, there is. And of course, the work that really exemplifies that the most is the work on telomere length, where you take peripheral blood. That's mostly what's being used now to assess telomere length. But you look at the data, and of course there's enormous scatter in the data. There's always a young person who's down with a length that's equivalent to a 90-year-old, and a 90-year-old who's up there with the equivalent of a 16-year-old. And part of that, I'm guessing, is due to the fact that you're measuring one tissue and you don't know what the history of that, you know, when was the last time you had a cold? That's going to affect, you know, how many T cells you have with bad telomeres or not. And so human data tends to be messy. It's very messy. Yeah. I've actually seen this quite a bit because I've done a lot of work with Dr. Bruce Ames. And I've measured DNA damage in people, in lean people, obese people, after a certain, you know, giving them a certain intervention. And I do this by measuring phosphorylated H2AX, gamma H2AX, right? Yes, yes, yes. But I've seen, even looking at different age groups, like, you know, sometimes, like, the 20-year-olds will, like, look like a 70-year-old, and you're like, what happened, you know, here? Sometimes the obese people look really great, but most of the time obese have higher levels than lean. But there's certainly, there's a lot of variation. There's a lot of variation. And I think that variation is twofold. The first is, you know, we're people, we're not genetically identical the way our mice are. So there's going to be individual to individual variation because we're not genetically identical. But the other interesting aspect is that we've taken, in our mice, we have transgenic mice in which senescent cells activate a protein, a luciferase, that we can then measure by luminescence in the whole animal. So we can follow the appearance of senescent cells in living animals by looking at this luminescent signal. So we start with, say, 12-month-old mice. So that's a 35- or so-year-old person. Very low signal. And then as these animals age, the luciferase signal goes up and up and up. Then you look at the aero bars, genetically identical animals, sometimes in the same cage, and the error bars get larger and larger and larger. So that says there is stochastic variation that's not due to genetic differences that causes identical animals to have some with a high burden of senescent cells, some with a low burden of senescent cells. And this is true for virtually every aging marker that has been looked at. The error bars get larger and larger and larger. So we call this stochastic variation. We don't know whether it's malleable, meaning we don't know whether you can make things less variable or more variable. We don't know whether it correlates with health of the animals. But it is a very common feature of aging is that things that go wrong do it almost randomly. And there's no idea of what's causing it? Well, I mean, you know that if you take a bunch of cells, identical cells, genetically identical cells, and you apply some toxin and then you look at damage, you get a Gaussian curve, meaning there are some cells that don't respond very well. Most of the cells respond a certain way, and then some cells that are super responders. And so it probably is just the messy nature of biology. We have all these pathways that are intertwined, and by chance, one configuration makes the cell super responsive and another configuration makes the cell less responsive and if that's true of cells then it's definitely going to be true of something as complicated as a mouse much less a person right so i have just a couple more questions um a couple more wacky but just to circle back real since I have you here, I know we've been talking for a while, but back to the NAD levels, that reminded me of this whole new field that's sort of semi-new, I guess, on different precursors of NAD that you can give, like nicotinamide riboside and nicotinamide monoglutide. I know nicotinamide riboside, at least in humans, has been shown to increase NAD levels, at least at very high levels. What do you think...do you think that any of those...and I know that there's been some animal studies showing it increased health spans so that the tissues were aging better and certain organs were aging better. Do you think that any of those effects of the increased NAD had to do with lower cellular senescence? That's a good question. As I mentioned, we know that this mitochondrially driven senescence is definitely driven by this alteration in the NAD-NADH ratio. So it's possible. We haven't really studied those precursors directly on senescent cells. Yeah, it'd be interesting. Yeah. It sounds like a grant, I should. Yeah. Right. There you go. And then one more question about...before I get to the other wacky question was, what do you think about some of these, like, people call them, quote, unquote, fasting mimetics? I don't like that. I think there's too many things going on with fasting. But, you know, that have been shown to clear away damaged cells like spermidine or the hydroxycitrate or resveratrol. Yeah. Do you think that's something? I mean, people are taking these to, like, clear away senescent cells. Yeah. We've explored some of those. some of them have no effect on senescent cells which doesn't mean that they might not have health benefits but it just doesn't act through senescence resveratrol for example we we don't see any effects on say pro-inflammatory cytokine secretion or anything that we think might be important but that doesn't mean that it's not doing other things. I much prefer red wine. Cells don't like it. Yeah. Cells don't like it. The other wacky question that just came to my mind was, so I'm thinking about cellular senescence, not the mitochondrial induced senescence, but, you know, the DNA damage to induce cellular senescence as a protective effect against cancer. And that's why it's evolved. I mean, we have that because we don't want to die of cancer young. What about animals like elephants that don't get cancer? They have a relatively long... I don't think it's that they don't get cancer. I think what's amazing is that they're so big, right? And they have so many cells. Yes. And so you would think that there should be ways, there should be super ways they have of protecting against cancer because it's a hell of a lot of cell division to go from a single elephant, egg, and sperm, you know, a zygote, you know, to an elephant, which is so big. There have been some studies on looking at, for example, tumor suppressor mechanisms in some of these animals. Like P53, right? They have extra copies of P53. And so... But do they have any cellular senescence? I was just wondering if anyone's looked at that. You know, I don't know that anyone has looked, but I would be surprised if they don't. I mean, we've seen cellular senescence in, or we meaning the field, has looked in a number of vertebrate species, and it seems to be common amongst all vertebrate species. Does it occur in lower organisms? They only live... Well, it does. Based on the markers we have, some people have looked in C. elegans, and they don't seem to find it there. But then C. elegans is unusual in that the only dividing cells in the worm is the germline. But in Drosophila, you know, there is a small fraction of cells that undergo division in the gut. And there is some hints that there may be senescence that occurs in the gut of the fly."