Working Out: The Molecular Biology of Exercise

Abstract

The many health benefits of exercise are well-known. Conversely, the pathologies associated with a sedentary lifestyle are also well-documented. However, science and medicine have only recently begun to explain how exercise does what it does. Here, I discuss recent insight into the biochemical mechanisms underlying the benefits of exercise and the pathologies of inactivity.

“[W]hile exercise is an effective prescription to promote health, there is no minimum dose, no optimal dose, and no dose without risks or negative consequences.”

-Daniel E. Lieberman

Introduction

The benefits of exercise are many. Exercise expands peripheral blood circulation, increases cardiac output and improves vascular elasticity. Load-bearing exercise stimulates bone deposition and counteracts osteoporosis. Exercise helps prevent clinical depression and appears to slow some forms of dementia.

While the benefits of an active lifestyle have long been known, the mechanisms underlying these benefits have only recently been defined. These mechanisms not only drive healthy adaptations but also antagonize the inflammation and metabolic consequences of atrophy and aging. Exercise increases the expression of key adaptive genes and promotes the secretion of molecules that signal adaptive responses in various tissues and slows age-related decline.

In this essay, I summarize some of the key mechanisms by which exercise promotes health and the biochemical pathways that underlie and serve these mechanisms, both in health and in disease. By harnessing this understanding, we can maximize the benefits of exercise and develop therapies and medications that provide the same benefits.

Evolved to Exercise

Anthropological and archeological evidence points to the conclusion that humans evolved anatomically and metabolically to sustain long periods of endurance exercise.¹ Our nearest living relatives—chimps, bonobos, orangutans, and gorillas—spend very little time and effort obtaining food and most of their time resting.² Based on the fossil record, so did our last common ancestor. But as hominins (chimpanzees and humans) speciated, the branches leading to modern humans became increasingly adapted to living on the ground and to a hunter-gatherer lifestyle. As our species evolved from its last common ancestor with the other primates, we evolved longer hind limbs and a more efficient walking gait.¹ Indeed, it is believed that when the first hominins became hunters, before they developed projectile weapons they would run their quarry to exhaustion.³

Studies on modern hunter-gatherers in Africa like the Hadza hunter-gatherers of northern Tanzania show that this lifestyle, most like our ancestors, is extremely active.⁴ Hadza men walk an average of 12–14 km/day and women 6–8 km/day, which is more than the average American does in an entire week! Even at age 65, men and women walk 11 km/day and 5 km/day, respectively. Curiously, the Hadza don’t burn significantly more calories than sedentary Americans when adjusted for nonfat body mass.⁵

Skeletal Muscle and Exercise

There are many ways to exercise. The physiological response to exercise varies depending on the type and duration. At the extremes, we can distinguish two types of exercise:⁶

Low-Load Endurance Exercise

Mechanical stress is low, but depending on the duration, metabolic disturbances can be severe and protracted. The outcome is characterized by an increase in structures supporting oxygen delivery (capillaries) and consumption (mitochondria). Examples include running, cycling, rowing, swimming, and cross-country skiing.

High-Load Strength Exercise

Here, mechanical stress predominates, and growth of muscle fibers occurs primarily through an increase in the amount of contractile proteins. Weight lifting and other forms of resistance training are examples.

Either type of exercise (and exercises that blend both) entails major physiological stressors of skeletal muscle such as mechanical load, hormonal, and metabolic changes and neuronal activation. The response to these stressors provokes molecular changes that adapt the muscle to future stress.

Mitochondria, Mitohormesis, and the Yin/Yang of Exercise

Mitochondria are the cellular organelles that provide much of the energy in our cells. While they are present in most cells, they are particularly important in the energy-intensive tissues of the central nervous system and muscles. Skeletal muscle cells are rich in mitochondria. A ca. 30-fold increase in intramuscular oxygen consumption accompanies strenuous exercise. Aerobic exercise training increases the number and volume of mitochondria, with an increase in muscle mitochondrial density of ca. 50–100% after six weeks of exercise training. This is accompanied by reduced carbohydrate consumption and smaller declines in ATP, the energy currency of the cell, with the same absolute power output in muscle.⁷ The net effect of training is to increase endurance by enhancing fatigue resistance owing to a reduction in muscle glycogen depletion, tighter regulation of ATP consumption, and reduction in metabolic by-products that interfere with performance.

The benefits of exercise stress come at a price, though. Increased mitochondrial metabolism results in increased intracellular levels of free radicals that, if unchecked, can damage proteins, DNA, and lipids. How can exercise be beneficial if a direct cellular byproduct is toxic? The answer lies in a phenomenon called “hormesis,” the adaptive response of cells to intermittent stress. Specifically in the case of exercise, “mitohormesis” refers to the mitigating response to the oxidative stress resulting from increased muscle mitochondrial activity that may temporarily impair or damage mitochondria. It ultimately stimulates a robust cellular response that confers stress resistance and improves mitochondrial and cellular function.⁸ Repeated cycles of acute stress occasioned by aerobic exercise is a clear example of mitohormesis.

What is the source of cellular stress caused by exercise? Aberrant by-products of the natural energy-generating chemistry of the mitochondria include reactive oxygen species—superoxide, hydrogen peroxide, and hydroxyl radicals—that damage DNA, proteins, and lipids. The mechanism of mitohormesis senses the accumulation of these damaging reactive oxygen species and reduces them to harmless water.

A central molecule in the mechanism of mitohormesis is Peroxisome proliferator-activated receptor Gamma Coactivator-1alpha (PGC-1α). PGC-1α regulates key genes in skeletal muscles. Endurance training induces elevated levels of PGC-1α. The significance of PGC-1α expression to muscle physiology is demonstrated by genetic manipulation in rodents: overexpression in skeletal muscle increases mitochondrial density, respiratory capacity, and improved exercise performance, while inactivation causes impaired mitochondrial function, and reduced capacity for, and slower recovery from, exercise.⁷

Another key player in mitohormesis is Nuclear factor erythroid 2-related factor 2 (Nrf2). This protein activates various genes that encode antioxidant enzymes. The oxidative stress generated by increased mitochondrial activity during exercise liberates Nrf2 from an inactivating complex in the cell cytoplasm, allowing it to enter the nucleus where it can dial up levels of antioxidant enzymes such as superoxide dismutase, glutathione synthetase, and heme oxygenase. Thus, Nrf2 activation counterbalances the elevated reactive oxygen species, resulting in net improved overall cellular function during exercise.⁸

Exercise and Angiogenesis

Another concomitant of exercise is increased angiogenesis. Mitochondrial content and capillary content are highly correlated in skeletal muscle. Increased blood flow induces shear stress that elicits the angiogenic response. Key to the mechanism by which increased skeletal muscle use drives angiogenesis is Vascular Endothelial Growth Factor (VEGF). Muscle contraction triggers the release of VEGF stored in vesicles into the extracellular space, where it acts through specific cell surface receptors to trigger capillary growth. Exercise induces VEGF expression in skeletal muscle via PGC-1α.⁶

The partial pressure of inspired oxygen (PiO2) in resting skeletal muscle is ca. 1/5th of the oxygen pressure of inhaled air. Acute exercise reduces PiO2 in contracting muscle to ca. 1/40th of that of inhaled air, creating a situation of transient hypoxia. Hypoxia-inducible factor-1 (HIF-1) is the master transcriptional regulator of cellular oxygen balance. HIF-1 is a transcription factor composed of two subunits, HIF-1alpha and HIF-1beta. Under normoxic conditions, HIF-1 alpha subunit is targeted for degradation. During hypoxia or reduced O2 tension, HIF-1alpha is stabilized and translocates to the nucleus to form an active complex with HIF-1beta. HIF-1 activation induces transcription of target genes involved in erythropoiesis, angiogenesis, glycolysis and energy metabolism in a manner analogous to exercise.⁶

Myokines and Exercise

Skeletal muscle is an endocrine organ. It produces signaling molecules in response to contraction that influence its own metabolism and the metabolisms of other tissues and organs. Acting through cell surface receptors, these signaling molecules, called “myokines,” have anti-inflammatory effects (Table 1). The founding member of the myokine family is Interleukin-6 (IL-6). In addition to opposing inflammation by inhibiting Tumor Necrosis Factor α (TNF-α), IL-6 stimulates glucose uptake.⁹ Muscle-derived IL-6 levels in circulation are elevated by up to 100- fold during exercise, and are correlated with exercise intensity and duration.^10,11

Table 1.

Myokines and their metabolic effects (modified from ref. 9)

Myokine	Metabolic Effects on Muscle	Metabolic Effect on Other Tissues
IL-6	Induce muscle growth, glucose uptake, glycogen breakdown, lipolysis	Increase lipolysis and free fatty acid oxidation in adipocytes, induce adipocyte browning
IL-15	Stimulate muscle growth and glucose uptake, enhance mitochondrial activity, antioxidant effects	Inhibit lipid accumulation in adipose tissue through adiponectin stimulation
Irisin/FNDC5	Stimulate glucose uptake and lipid metabolism, involved in muscle growth	Induce adipocyte browning and lipolysis, stimulate glycogenesis and reduce gluconeogenesis/lipogenesis in liver
myostatin	Inhibit muscle hypertrophy	Myostatin inhibition results in adipocyte lipolysis and mitochondrial lipid oxidation, accelerates osteoclast formation
BDNF	Enhance fatty acid oxidation and glucose utilization	Induce adipocyte browning indirectly through FNDC5
BAIBA	Increase mitochondrial free fatty acid oxidation, ameliorate insulin signaling, anti-inflammatory	Increase mitochondrial free fatty acid oxidation and browning in adipocytes, reduce hepatic de novo lipogenesis and hepatic endoplasmic reticulum stress
LIF	Induce muscle hypertrophy and glucose uptake	Stimulate osteoblast differentiation, inhibit adipocyte differentiation
SPARC	Regulate muscle tissue remodeling, enhance glucose metabolism	Inhibit adipogenesis

IL-6 is induced by muscle contraction that stimulates both glucose and lipid metabolism. IL-6 levels are increased by both concentric (e.g. biking or leg-kicking) and eccentric (e.g. strength training or running) contractions. Induction of IL-6 in skeletal muscle occurs via a calcium-dependent pathway. IL-6 promotes hypertrophy by stimulating proliferation of muscle stem cells (satellite cells) and by augmenting the rate of protein synthesis in muscle sarcomeres.¹²

IL-6 has long been recognized as a cytokine secreted by immune cells and adipocytes to promote inflammation. Tociluzimab, a therapeutic antibody against the receptor for IL-6, is used clinically to treat cytokine release syndrome in, e.g., patients with severe COVID-19. The paradoxical benefits of IL-6 during exercise contrasts with the systemic pro-inflammatory effects of IL-6 during infections, and points to cell-specific roles for this cytokine.¹³ More research is required to define the mechanisms underlying these antipodal effects.

An adaptive response to chronic exercise is an increase in muscle mass through the increase in muscle protein biosynthesis. Mammalian target of rapamycin complex 1 (mTORC1) is the key regulator of protein synthesis in skeletal muscle. mTORC1 can be activated or depressed depending on upstream signaling events whereby muscle mass is regulated mostly through modification of protein synthesis rather than through changes in protein degradation. mTORC1 is activated through membrane-bound receptors activated by insulin and growth factors.⁶

Muscle Satellite Cells and Exercise

Satellite cells are a heterogeneous population of cells. The majority of cells are committed myogenic cells, which, upon stimulation, undergo symmetric division, and differentiation. A smaller number of satellite cells (satellite stem cells) undergo asymmetric division, repopulate the satellite cell niche and maintain long-term muscle regenerative potential. Strength training increases skeletal muscle fiber cross-sectional area and satellite cell number. Androgens are steroids, and thus activate transcription of specific genes through an intracellular receptor that binds the enhancers of target genes. Androgens promote satellite cell activation and proliferation and further support skeletal muscle hypertrophy by elevating IGF-1 levels. Myostatin is a myokine, a protein produced and released by myocytes that is a negative regulator of muscle cell growth and differentiation. It is a potent player in muscle mass homoeostasis, inhibiting protein synthesis, and activating catabolic pathways. It acts through a cell-surface receptor to turn on gene transcription. Androgens also suppress myostatin to promote muscle growth.⁶

The Price of Inactivity

Muscle tissue responds to demand by growing. Conversely, it responds to disuse—due to injury, sedentary lifestyle, and age—by atrophy. The absence of physical activity not only means foregoing the benefits of regular exercise, it has negative consequences. Within a couple of weeks, muscle wasting and increased plasma triglycerides and insulin insensitivity are observed. Muscle wasting is accompanied by degradation of myofibrils by the ubiquitin-proteosome pathway (reducing contractile force) and mitochondrial degradation (reducing endurance).⁶ In the elderly, this exacerbates the natural progression of age-related sarcopenia.

Skeletal muscle wastage is an important consequence of continuously elevated glucocorticoid levels. Glucocorticoids suppress mTORC1, thereby inhibiting muscle protein synthesis. They also increase myostatin levels by increasing both myostatin expression and action, inhibiting muscle growth. Skeletal muscle wasting is also associated with elevated levels of pro-inflammatory cytokines such as Tumor Necrosis Factor α, IL-1, and IL-6.⁶

Extreme Sports: Too Much of a Good Thing?

If some exercise is good, more is better, right? When it comes to extreme endurance sports, the data suggest a shift in the risk-benefit ratio towards increased risk. The “extreme exercise hypothesis” holds that there is a U-shaped curve for the health risk of exercise training, where risks reach a low point at some intensity of training, but then increase when that optimum is exceeded (Figure. 1). Where, exactly, this exercise optimum occurs is unclear, although some data suggest that it may be at three to five times the current recommendations.¹⁴ A study of 284 male amateur athletes found that the most active athletes had significantly higher levels of coronary artery calcification than the least active.¹⁵ Importantly, the most active athletes had mostly calcified plaques, which show a high association with future cardiovascular events, while the least active athletes had a higher prevalence of mixed plaques.¹⁶ Similar results were found in comparing 152 male veteran amateur athletes and 92 age- and risk factor-matched sedentary controls.¹⁷ Other pathologies that have been associated with extreme sports include cardiac fibrosis and arrythmias.¹⁸

Figure 1.

The Extreme Exercise Hypothesis posits that the benefits of exercise may be partially reversed when exercise training volume exceeds an optimal level.¹⁴

How does this translate into longevity? Does extreme exercise increase the chance of premature death compared to the general population? A meta-analysis of ten studies involving 42,807 elite athletes found 67% of the expected number of deaths based on the comparison population.¹⁹ Importantly, deaths from cardiovascular disease alone were 73% of expected. A separate study looking at 57 studies including 465,575 elite athletes confirmed that the overwhelming majority of these athletes enjoyed superior life-span compared to their age- and sex-matched peers in the general population.²⁰ It must be acknowledged, however, that the elite athlete cohort likely consists of people who are already among the healthiest and fittest in the population, so the quantitative contribution of extreme exercise specifically to superior longevity outcome is unclear.²⁰ It seems likely, then, that other factors benefit those who engage in intensive exercise that can offset the risks attached to arterial plaque formation alone.

Is Exercise Medicine?

The long and familiar list of benefits associated with regular moderate exercise, and the many pathologies associated with inactivity, testifies that exercise is potent medicine (Table 2). In aging adults, resistance exercise can increase healthspan by slowing age-associated sarcopenia, as well as improving balance to prevent falls. As with any medicine, the dose makes the poison, and evidence suggests that sustained extreme exercise may be toxic.

Table 2.

Benefits of Physical Activity and Exercise²³

Improvements in Exercise Capacity Maximal Oxygen Consumption (Peak VO2) Estimated METs

Improvements in Lipids Total cholesterol HDL-C LDL-C Triglycerides Total cholesterol/HDL-C LDL-C/HDL-C

Reduction in Obesity Indices Weight Percentage Body fat Body Mass index

Improvements in Blood Rheology Reduction in Homocysteine Levels Improvement in Viscosity

Improvement in Psychosocial Factors Depression Score Anxiety Score Hostility Score

Major Morbidity and Mortality Reduction in Overall Mortality Reduction in Hospital Costs Reduction in Non-Fatal Myocardial Infarction

Unlike our hunter-gatherer ancestors, we are no longer obliged to walk and run long distances to obtain food. But there is a wide range of age- and lifestyle-appropriate exercise to be had in modern society that can trigger all the genes and signaling pathways to improve and sustain skeletal muscle, cardiovascular and CNS health. The challenge is to find rewarding and sustainable exercise matched to the abilities of each person, and to create work, school and home environments that encourage movement and physical activity.²¹

Conclusion

The benefits of regular moderate exercise are well-documented. Now, the molecular mechanisms underlying these benefits are being identified. With this new understanding comes the hope that these mechanisms can be harnessed to maximize benefits to healthy individuals and to develop pharmacological substitutes that provide similar benefits to those who, because of illness, disability or age, cannot engage in exercise.

Whether or not exercise is judged to be medicine, its benefits can be analogized to a drug. As Schwartz et al.²² noted in these pages:

“Exercise might be best understood as a drug with powerful benefits, especially for cardiovascular health. As with any potent drug, establishing the safe and effective dose range is critically important—an inadequately low dose may not confer full benefits, whereas an excessive dose may produce adverse effects that outweigh its benefits.”