Exercise and Stem Cells - Part I, Introduction and Haematopoietic Stem and Progenitor Cells
This introductory post is lengthy, but many of the items are important for the future posts in this mini-series, "Exercise and Stem Cells" as well as referring back to topics from the "Obesity and Stem Cells" mini-series. I would also like to thank a physician that helped me shape this particular blog - Dr Joseph Ruane (founder and lead physician of McConnell Spine, Sport & Joint Physicians; Medical Director of the Columbus Blue Jackets; co-founder of OhioHealth's Sports Medicine Institute. He is a Clinical Assistant Professor of Sports Medicine at The Ohio University College of Osteopathic Medicine.) Also, I would like to thank Dylan Davies for his excellent photography of sprint racing on the velodrome; his unique perspective may be due to the fact that he also races on the velodrome!
The benefits of exercise are so well known that it begs the question: why discuss it in a blog mini-series?
This mini-series blog is important because too few dialogs have occurred in the practice of regenerative medicine that focus upon potential beneficial or negative effects of exercise. We will review original studies and scientific reviews that can help the regenerative medicine practitioner think about adding exercises as a potentially beneficial adjuvant to cellular therapies used in the practice of regenerative medicine. We will discuss the exercise-induced molecular adaptations (e.g., growth factor, cytokine/chemokine, and hormone response) that may have therapeutic effect to improve tissue regeneration. It will also help industry as they work on tools and products that clinicians can use to benefit the health of their patients. Industry active in tissue regeneration products have largely ignored focus on exercise and this may be because little discussion has occurred within.
In 2010, the World Health Organization (WHO) published recommendations on physical activity and demonstrated that physical inactivity is the fourth leading risk for global mortality.(1) “On the other side, in the last 15 years, it has been shown that the practice of regular physical activity reduces the risks of cardiovascular disease, diabetes, colon cancer, breast cancer, and depression. Moreover, physical exercise has been demonstrated to be fundamental in weight control and energy expenditure to contrast the increase in obesity.”(2) Given the negative effects of obesity that we saw in Obesity and Stem Cells, assuming no negative effects in tissue regeneration due to exercise, the negative effects of obesity should be sufficient reason to include exercise discussions and therapies with patients. As we will see, there are indeed positive effects rather than just avoiding negative effects.
There is no doubt that exercise has health benefits (e.g., cardiovascular, cognitive function, muscle); recently, it was shown that exercise can be an effective adjuvant for 26 chronic diseases.(3) “In fact physical exercise induces molecular adaptations like release of growth factors, cytokines and hormones that have therapeutic effects, improving both regeneration and function of many organs (i.e., skeletal muscles, bone, heart, lung, brain).” (2, p. 144, emphasis mine) However, we need to ask if these benefits of exercise applies to (and if it does, how it impacts) stem and progenitor cells? If there is a positive effect on cells through exercise, and given that exercise is recommended for prevention and treatment of various diseases, we should then ask and explore the question of whether exercise regimens be incorporated into pre- and/or post-cellular treatment in the practice of regenerative medicine? If exercise is beneficial to stem/progenitor cells, are there specific exercises that are better than others? Are there intensities of exercise that are more effective than others? We will work to address all these questions, and more, in this mini-series of Exercise and Stem Cells.
Moreover, we will also begin exploring the impact exercise may have by investigating it’s effects on protection, quantity, and function of haematopoietic stem and progenitor cells (HSPCs) by looking at a published review by De Lisio and Praise.(4) Previous work studying the impact of exercise on the haematopoietic system was typically applied to immune system activity (yes, exercise boosts immune function) and oxygen delivery [exercise increases red blood cells (RBCs)]; additionally, it was known that exercise also benefits differentiated cells of haematopoietic lineage but it was unknown as to whether exercise could benefit primitive haematopoietic cells. As we have seen in the Obesity and Stem Cells mini-series, HSPCs are drivers in tissue regeneration and the body’s response to trauma rather than the often assumed MSCs.(5) Table 1 provides a short summary of papers demonstrating the importance of HSPCs in tissue regeneration. As such, how exercise impacts HSPCs may be of tremendous importance for disease treatment and prevention, including tissue regeneration and the tools associated with cellular therapies. For example, if exercise can benefit engraftment or protect HSCPs from damage and apoptosis (cell death), it may be extremely important to incorporate exercise in patients receiving cellular therapies.
While this particular blog post is focused on HSPCs (we will cover additional stem cells in future posts), it is crucial to remember that having HSPCs, MSCs, and “other cells in the stromal and hematopoietic lineages…provide an optimized physiological and cellular milieu.”(8) Additionally, Yasuhara et al (2010) demonstrated that while HSPCs are drivers of vasculogenesis and have more osteogenesis for bone repair than non-HSPCs (including MSCs), having all the cells combined yielded the greatest potential for tissue regeneration.(9) Too many research projects and misinformation has resulted in thinking that only one particular cell is necessary for tissue regeneration. However, the body naturally utilizes many cell types and many signals from these cells in the tissue regeneration pathway and we are only beginning to unravel the details. As such, focus on optimized milieu through native cells seems most appropriate.
De Lisio and Parise’s review (2013) demonstrated that at least 13 publications relating to the effects of exercise on haematopoietic stem cells and progenitor cells (HSPCs) had been published prior to their manuscript. However, most studies on the effects of exercise have been focused on immunity and oxygen delivery. As such, the focus to HSPCs is crucial. In the review, the authors focus upon: (a) the protective effects of exercise training on haematopoietic cells, (b) focus on primitive haematopoietic stem cells (HSCs) - how exercise impacts quantity and function, and (c) the effects of exercise on the haematopoietic stem cell niche (microenvironment).
As we saw in the Obesity and Stem Cells mini-series, obesity and other metabolic diseases negatively impact the quantity and function of HSPCs and other stem cells. We now look at the positive impact of exercise and training and this is hugely critical on the haematopoietic system. As a reminder, approximately 10 billion blood cells must be produced daily, just to keep up with the homeostatic demands of the body. There is certainly a hierarchy of HSCs (i.e., there are long-term and short-term haematopoietic stem cells) based upon their capacity to regenerate the haematopoietic system. However, as we have seen, HSCs are also drivers of tissue regeneration rather than just blood cell production.(5-7, 9-14) As such, we begin the blog mini-series Exercise and Stem Cells with a brief focus on HSCs.
A. Protective Effects of Exercise Training on Haematopoietic Cells
Moreover, the authors concluded that the concept of exercise hormesis is “applicable to primitive hematopoietic cells in the bone marrow and that exercise increases HSC quantity with the effects of exercise primarily mediated via alterations in the HSC niche” (p. 116). Exercise hormesis states that each exercise bout in a training program brings about mild oxidative stress that triggers an adaptive response to reduce the stress of subsequent exercise bouts. “This theory is exemplified in peripheral blood cells, as acute exercise causes increased damage to cellular macromolecules, inflammatory markers, and apoptosis, whereas exercise training has the opposite effects, mediated by an increase in antioxidant and DNA repair capacity” (p. 117). That is to say, that a single time exercising has the opposite effect on the body as routinely exercising. While the beneficial effects of exercise training on mature cells in the circulation has been described in the literature, the protective effects of exercising on primitive haematopoeitic cell in the bone marrow had not been studied. De Lisio and Parise analyzed bone marrow of mice that did acute exercise and exercise training to compare the differences. The conclusion was that, “acute exercise is an oxidative stress for hematopoietic cells while training induces adaptations that protect hematopoietic cells from damage” (p. 117). Following the theory of exercise hormesis, training provides for adaptive responses in haematopoietic cells that are protective against future oxidative stress.
B. Exercise and Haematopoietic Stem Cell Quantity and Function
I. Stem Cell Quantity
Tremendous variability in subject characteristics, exercise intensity, phenotypic markers used to try to identify HSCs, as well as timing of analysis have made many conclusions difficult to make. Acute exercise seems to increase the peripheral blood HSC concentration - these analyses have covered acute exercise in the forms of (a) half/full marathon, (b) 1000 meter rowing sprint, (c) treadmill test to exhaustion, (d) 4 hour cycling at 70% of anaerobic threshold, (e) marathon/1500 meter run, and (f) incremental cycling test; additionally, within the studies, there were variations on the types of subjects - sedentary, active, exercise trained. All of these variations create differences in the studies on acute exercise and make firm conclusions on stem cell quantity difficult.
There are only a few studies published covering of exercise training and HSC quantity and they cover: (a) half/full marathon training, (b) habitually active, (c) 8 week cycling training, (d) treadmill walking of 5 days/week for 4 weeks, (e) 10 week treadmill running, and (f) 8 week treadmill running. In these studies, one element that becomes clear is the difference between intensities in various training. Studies of self-described habitual exercisers, treadmill walkers and mild training, and non-weight-bearing exercise have demonstrated little to no benefit of these training programs for boosting HSC quantity. However, studies that have demonstrated benefit from exercise training generally involved higher intensity exercises. “These data suggest that the ideal training stimulus for increased HSC quantity involves moderate to high training intensity and frequency using weight-bearing exercise modalities” (p. 118). Another tremendous finding is that “the effects of exercise training on HSC quantity are maintained with age” (p. 118). Therefore, elderly patients can reach sufficient intensity that provided increased HSC quantity. While most studies have investigated HSCs in circulation rather than in the bone marrow niche, these authors have investigated specifically bone marrow HSCPs through colony-forming unit (CFU) assays. “We determined that exercise training significantly increased the quantity of HSC by 20% in the vascular niche, with no effects on HSC quantity in the endosteal niche…. [n]or was there a shift in the ratio of vascular to endosteal HSC with exercise” (p. 119). That is to say, the increase in the more mature HSCs did not occur because of a decrease in the more primitive HSC population.
II. Stem Cell Function
While knowing changes in the quantity of HSCs after exercise is helpful, it is crucial to study HSC function as a result of exercise. One study investigated acute exercise and found that acute exercise diminished the functionality of HSCs with in vitro analyses, presumably because of the increased oxidative stress of acute exercise. Whereas, the effects of exercise training has been investigated with bone marrow transplantation (BMT) to investigate functionality in vivo. The result was that unlike acute exercise, exercise training did not induce negative changes in HSC functioning.
C. Exercise and the Haematopoietic Stem Cell Niche
Most acute exercise studies have focused upon the HSC niche by investigating the regulation of HSCs by systemic factors in response to exercise. That is, they have focused upon growth factors, cytokines and chemokines, as well as inflammatory response. Vascular endothelial growth factor (VEGF) increased in response to acute exercise and this suggests that hypoxia (low oxygen tension) in the tissues results in the increase in HSPCs in circulation. The inflammatory response may also regulate the HSPCs in the periphery. For example, levels of interleukin 6 (IL-6) increased in blood following half and full marathons as well as acute treadmill tests; however, following maximal exertion in a 1000 meter rowing test, no increase in IL-6 was found. These findings are in agreement with prior findings on the differences in HSC quantity and function due to differences in intensity. That said, acute changes in IL-6 were not found to relate to changes in circulating HSC quantity. One study found that the peak serum IL-6 occurred after the HSC quantity peaked. As such, IL-6 could represent either (a) a signal to move peripheral HSCs back into the bone marrow or enter damaged tissue, or (b) IL-6 levels may be an indicator of systemic inflammation that regulates HSC quantity. A study found that the proinflammatory cytokine tumor necrosis factor-α (TNF-α) and circulating HSC quantity had an inverse relationship. “These data suggest that the inflammatory environment induced by an acute bout of exercise may promote removal of HSC from circulation via either migration of HSC from circulation to various tissues to facilitate repair or by promoting HSC apoptosis" (p. 120).
Fewer studies have investigated the various growth factors for exercise training. Nevertheless, unlike acute exercise, exercise training is considered to be anti-inflammatory and data on exercise training growth factors found a reduction in proinflammatory cytokines (e.g., interferon-γ, TNF-α, and IL-6). The anti-inflammatory effects of training “may be important for maintaining or improving HSC function over time” (p. 120) and these changes are not an acute response to exercise, existing long after exercise bouts are complete.
The authors also investigated the HSC niche indirectly by investigated bone marrow transplantation (BMT) survival and engraftment of the transplanted cells. Exercise training recipients resulted in a threefold increase in survivability. This is partly explained by the anti-inflammatory properties of exercise training as well as “acute inhibition of native bone marrow cell loss by apoptosis” (p. 121). These results agree with the several observational studies in BMT recipients that found improved hematological outcomes in patients that participated in exercise training prior to BMT and “[t]hese effects of exercise immediately after BMT resulted in increased hematopoietic regeneration in exercise-trained recipients 1 and 3.5 months after BMT” (p. 121); BMT recipients that began exercising only AFTER had no hematological benefit from the exercise.
While more studies must occur in this area, we do have helpful data that demonstrate exercise has positive impacts upon HSPCs and other cells. Additionally, data on cellular therapy for tissue regeneration demonstrate paracrine activity and effects (cells acting upon touching/nearby cells) are the overwhelming majority of activity rather than autocrine effects and activity (cells acting upon themselves; i.e., transplantation of a stem cell that engrafts, proliferates, and differentiates into new tissue). Moreover, we have discussed in previous blog posts about the importance of the microenvironment. In fact, basic and clinical sciences are at a point wherein we now state that the environment is more important than the cell. Exercise training positively impacts cells and also their environment and positive changes in the environment lead to positive changes in cells.
Therefore, we should begin looking at clinical outcomes of cellular therapies in which some subjects have been placed upon an exercise training program for 8-12 weeks prior to receiving a cellular therapy and continued training after cellular therapy, based upon currently available data. Until such time as additional data exist, we must rely upon what is known from several original studies and reviews regarding exercise and stem cells with both in vitro and in vivo data. It is undeniable that exercise is beneficial to health. It is unknown whether or not an exercise program would be a clinically effective adjuvant therapy to be used with cellular therapies.
Based upon these data, physicians should explain the benefits of exercise training to patients; however, they also should explain that the data are limited and more studies are necessary. Nevertheless, there is consistency upon the differences between acute exercise and exercise training and that acute exercise can be harmful (inflammatory) while exercise training is anti-inflammatory.
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