Keywords

1 Introduction

Matrix metalloproteases (MMPs) are a family of highly homologous Zn2+ endopeptidases that collectively cleaves most, if not all, of the constituents of extracellular matrix (ECM). MMPs in the circulation are thought to modulate the activation of growth factors, cytokines [1] and angiogenesis, thereby facilitating physiological adaptations to exercise training [2–4]. The MMP family of enzymes contribute to both normal and pathological tissue remodeling [5, 6]. Each MMP targets a specific substrate, thus the appropriate MMP is released in a time and location specific manner to orchestrate membrane remodeling and adaptation [7, 8].

Regular exercise is of great importance for public health. It has been shown to be effective in the prevention and therapy of many wide spread diseases such as hypertension, coronary heart disease and diabetes [9, 10]. However, exhaustive exercise causing inflammatory responses lead to significant change in the activity of MMPs such as MMP-9 and MMP-2. The heavy activities and deleterious effects that activate MMP-9 and MMP-2 are indicators of inflammatory conditions, which eventually cause degradation of extracellular matrix leading to the incidence of diseases, for instance, arthritis [11].

Many aspects of how the prophylactic effects of exercise on chronic diseases are mediated remain unclear [12, 13]. One important way to improve our understanding of these beneficial effects is to gain insight into the cellular and molecular responses to exercise.

2 Skeletal Muscle and Exercise

Certain stimuli, particularly those that induce high levels of mechanical stress by high impact exercise activate the local production of MMPs in skeletal muscle [14]. Serum concentrations of MMPs are reported to peak within a relatively short time period following a single bout of exercise. MMPs in the circulation are thought to modulate the activation of growth factors and cytokines through degradation of their precursors, binding proteins and inhibitors [15, 16]. Strenuous exercise, especially fast speed running, is known to cause intra- and extra-myofibrils damage [15, 16]. High intensity exercise increased both mRNA and protein levels of MMPs [3, 17]. It is generally accepted that MMPs function in skeletal muscle to process extracellular matrix (ECM) proteins, thereby regulating matrix degradation and repair, while those released into the circulation facilitate angiogenesis [18, 19].

Skeletal muscle fibre possesses a high degree of functional and structural plasticity and is capable of responding rapidly to changes in contractile activity [20]. The diaphragm is a unique skeletal muscle that is considered to be two muscles in one. This fact is based on anatomical and functional differences between the coastal and crural regions [21] that contain different composition in muscle fibre types. The crural diaphragm is dominated by type IIb fast twitch muscle fibres, whereas the coastal diaphragm contains equally type I (slow twitch) and type IIb (fast twitch) fibres [22]. Type IIb muscle fibres are more susceptible to ECM degradation than type I muscle fibres during exercise [12]. Moreover, a degree of muscle tissue change is fibre type specific and appears to be more pronounced in type IIb fibres following exhaustive exercise, which suggests a higher protein degradation in fast twitch (type IIb) than in slow twitch muscle fibres (type I) [12]. Thus, type II muscle fibres are more susceptible to exercise overuse than type I muscle fibres, and fast fibres are more responsive to exercise induced changes in MMP expression [12].

Superoxide dismutase (SOD) level significantly decreased in the crural diaphragm muscle of rat about a month of fast speed running; whereas it remained unchanged in the coastal diaphragm muscle during the period of running [12]. The expression of MMP-2 was found in both fast and slow running groups; however, it was particularly prominent in fast twitch muscle [12, 15]. High intensity endurance exercise increases MMPs such as MMP-2 and MMP-9 expression, whereas low intensity endurance exercise did not alter MMP-9 and MMP-2 activities in the skeletal muscles such as gastrocnemius (back part of lower leg), quadriceps (large muscle group on the front of the thigh) and soleus muscles (closely connected to gastrocnemious) [12, 13, 15]. In contrast, high intensity exercise increases both mRNA and protein levels of MMP-2 in skeletal muscle containing a high percentage of fast type IIb fibres [19]. Thus, high intensity exercise is required to promote the expression of MMP-2 in skeletal muscles and that the influence of exercise on MMP-2 expression has been found to be dominant in the muscle containing a high percentage of fast fibres [12, 15]. Thus, based on the concept that high intensity exercise in untrained animals’ results in significant muscle remodeling [23], Carmeli et al. [12] hypothesized that high intensity exercise would promote markedly the expression of both MMP-2 and MMP-9 in skeletal muscle, whereas low intensity results in limited changes in muscle levels of MMP-2 and MMP-9 [19, 23].

Treadmill exercise can serve a model to demonstrate different damage pattern in the two separated part of diaphragm muscle during exhaustive exercise. The integrity and the composition of ECM were affected considerably through expression of MMPs, for example, MMP-2 and thereby cause change in the collagen synthesis particularly in type IIb muscle fibres [19]. There are potentially three explanations for the differences in MMP-2 expression between the muscle fibre types. First, in rat skeletal muscles, type IIb fibres are at least twice as big as type I fibres, which probably explains a higher volume of collagen; therefore, white fibres (type IIb, fast twitch) requires more MMP-2 to maintain its integrity than red fibres (type I, slow twitch). Secondly, white fibres elicit better muscle plasticity than red fibres and show faster adaptation to exercise training. Under intensified training, fast fibre may undergo transition to intermediate muscle fibres (type Ia) with corresponding changes in ECM compositions [12]. Following the fast and prolong running, the type II muscle fibres are thought to be more susceptible to oxidative stress in order to produce a greater aerobic energy [13]. And, thirdly, the diaphragm differs from locomotor skeletal muscle in adaptation to exercise [24].

The recruitment of coastal fibres increases from rest to low intensity of exercise. During intense exercise, the coastal region reaches a plateau in motor unit recruitment before meeting the ventilatory demand. This suggests that type IIb fibres are not recruited during normal ventilatory behavior. Therefore, in contrast to type I, high intensity performance of type IIa provides a trigger for type IIb intra-and extracellular adaptations [12, 13, 24].

Currently limited information exists regarding the effects of exercise training on MMPs in skeletal muscle. Studies have been made to compare the ways of activity of two types of proteinases: MMP-2 and MMP-9 during a patho-physiological process in the trained subjects (with long term adaptation to exercise) and untrained or non athlete persons (without adaptation to exercise). MMP-2 and MMP-9 activities in athletes have been found to be significantly increased immediately after exhaustive exercise and significantly decreased in the next day; while in non-athletes, MMP-2 and MMP-9 activities were also increased immediately after exhaustive exercise and that was similar to athletes, but did not show any discernible decrease in the activities of the enzymes in the next day after exercise [1].

3 Angiotensin Blockers and Exercise

Exercise training is emerging as an important complementary intervention in heart failure [25, 26]. Exercise has been shown to enhance aerobic capacity, attenuates left ventricle (LV) dilation, regress cellular hypertrophy and improves cardiomyocyte contractility and myofilament function [27, 28]. These beneficial effects could be due to attenuation of renin-angiotensin system (RAs), which is known to improve cardiac remodeling [29, 30]. Indeed, exercise training normalized circulating RA system in patients with heart failure [31].

The impact of exercise training on factors contributing to myocardial fibrosis is not clearly known. It has been suggested that exercise training induced improvement of cardiac function may be due to marked decrease of myocardial fibrosis and remodeling [30]. So, combination of AngII blockade and exercise training on myocardial remodeling and function after myocardial infarction was assessed by many investigators. Studies on both gene and protein levels of MMP-2, MMP-9 and TIMP-1, angiotensin converting enzyme (ACE) and angiotensin receptor-1 (AT-1) after myocardial infarction (MI) suggest that exercise training improves factors involving post-MI fibres, reduce collagen content and thereby attenuates deleterious cardiac remodeling and preserve cardiac function. Such beneficial effects have been found to be further accentuated by angiotensin II type I (AT1) receptor blockers e.g., losartan [30].

MMPs and TIMPs are the key elements involved in matrix degradation and contribute to myocardial remodeling after MI. Increased MMPs expression or decreased TIMPs expression could result in enhanced proteolytic activity and degradation of ECM molecules [32]. Webb et al. [33] demonstrated that TIMP-1 level was high at day 1 post-MI and remain substantially elevated through 6 months in patients with MI. The elevated TIMP-1 level may contribute to the accumulation of collagen content in the infarcted heart, leading to myocardial fibrosis [34]. Interestingly, TIMP-1 has been shown to be increased significantly about 2 months of post MI. Both exercise training and losartan attenuates TIMP-1 expression at both gene and protein levels. Exercise training and losartan treatment in combination after MI has been shown to reduce the TIMP-1 expression, improves the balance between MMPs and TIMPs and enhance the proteolytic activity of post MI, which subsequently decreases collagen accumulation. This leads to decrease in cardiac stiffness, preserve left ventricular systolic pressure and ventricular performance, and reduce left ventricular end diastolic pressure significantly in the late phase of post MI [30]. Although the exact mechanisms of post MI exercise training induced beneficial effects on myocardial remodeling are not fully elucidated, several studies have suggested that the effect of exercise training may be due to increased baroreflex sensitivity, reduced sympathetic activity and enhanced vagal tone [30, 35, 36]. In addition, a reduction in circulating AngII by exercise training may act favorably on baroreflex control of sympathetic activity [30, 37, 38]. Interestingly, early decrease of TIMP-1 in the infracted heart coincides with collagen degradation in the necrotic myocardium, whereas the subsequent increase of TIMP-1 in the infarct heart contributes to collagen accumulation at the late phase of post MI remodeling [39].

Recent research suggest that a decrease in the expression of TIMP-1 and ACE levels, and also a decrease in AT1 receptor numbers are associated with the mechanism by which losartan and exercise could attenuate fibrosis and preserve cardiac function [29]. Exercise training and/or AngII receptor blockage after MI has been suggested to play an important role in cardiac remodeling by attenuating TIMP-1 expression, improving the balance between MMPs and TIMPs, attenuating ACE and AT1 receptor expression, and thereby decreasing collagen content [30]. These improvements, in turn, attenuate myocardial fibrosis and cardiac stiffness and preserve post MI cardiac function [40, 41].

4 Immunity and Exercise

TNF-α level increases within an hour after exhaustive exercise and that has been shown to stimulate activities of MMPs. Cytokines such as TNF-α and interleukins, for example, IL-8 levels in blood of mononuclear cells have been shown to be increased in response to strenuous exercise, which remain elevated, respectively, at 2 h for IL-6 and up to 24 h for IL-1β and TNF-α after exercise [42]. MMP-1, for instance, has been demonstrated to be involved in lymphocyte trafficking [42].

Exercise has been shown to be an important regulator of immune cells and their functions, therefore, white blood cells (WBCs) have been chosen as target cells [43]. In circulating WBC’s, exercise induces upregulation of MMP-9, which breaks down native collagen as well as other extracellular matrix molecules [44]. There are a number of reports on an increase in collagen degrading activity as well as release of MMP-9 in muscle, interstitial fluid and serum especially after strenuous exercise [23]. The role of MMPs in haematopoietic system during exercise involves the mobilization of progenitor cells [45]. Studies in rhesus monkeys suggest involvement of MMP-9 in IL-8 induced mobilization of hematopoietic progenitor cells (HPCs) via cleavage of matrix molecules to which the stem cells adhere [46]. Recently, it has been suggested that activation of MMPs, for instance, MMP-1 in peripheral blood mononuclear cells (PBMCs) during exercise may be involved in lymphocyte trafficking and in the recruitment of progenitor cells from bone marrow [11, 47]. Given that exercise regulates function of immune cells, role of PBMCs in the physiological changes associated with strenuous exercise has been suggested to be an important aspect of exercise physiology [48]. Changes in the expression of a group of genes within the leukocytes may serve as surrogate markers for systemic or local modifications induced by exercise [49, 50]. The response of leukocytes to exercise on the expression of genes is known to some extent [51, 52]; however, complete lists of genes that are differentially expressed have not yet been fully explored. The regulatory mechanisms associated with the expression of the genes are also currently unknown.

Inflammatory responses induced increase in cytokines and subsequently MMPs levels have been shown to be redistributed between the lymphoid and non lymphoid organs, which cause mobilization of HPCs from bone marrow. These processes have been related to exercise induced stress [45, 53]. MMPs, for instance, MMP-1 is involved in the enhanced peripheral invasion and migration to tissues of natural killer cells (NKs). An increase in the production of MMPs from stimulated NK cells plays an important role in the facilitation of lymphocyte trafficking and in the accumulation of lymphocytes in tissues during pathophysiological processes [54]. Cytokines stimulate the expression of multiple MMPs in lymphocytes. MMPs, for example, MMP-1 produced by cytokines, for example, IL-8 stimulated NK cells play a role in the degradation of matrix collagens [55, 56].

Recent reports have suggested that redistribution of leukocytes is a fundamental regulatory mechanism of the haematopoietic system that alters lymphocyte counts during exercise [53]. The ability of immune cells to migrate appears to be closely regulated by molecules such as cytokines and MMPs, for instance, MMP-2, which are mediated through receptors of the integrin family members [57]. Although, the exact mechanism by which lymphocytes invade tissues is not completely understood, this migration seems to be regulated by distinct pathways involving mitogen activated protein kinases (MAPKs). Goda et al. [54] demonstrated that MMP-1, for instance, is produced by the cytokines, for example, IL-8 stimulated NK cells and that is associated with α2β1 integrin, indicating that integrins could play a role in the immobilization of MMP-1 on the cell surface. The binding of MMPs to integrins could be the crucial step for promoting lymphocyte migration given the fact that disruption of their association decreases the migratory activity of cells [58]. Integrins are also known to be involved in signaling pathway(s) of the Rho family of GTPase that has been shown to be associated recently with modulation of the expression of IL-8 and MMP-1, which are induced upon inhibition of the MAPKs pathway [57].

Inflammation and the resulting inflammatory response for athletes indicated that MMP-2 and MMP-9 were not return to normal levels even 24 h after exhaustive activity; while for non athletes, the response was even much weaker [11]. There is evidence that exercise stress works an inflammation like reaction on the immune system with the activation of both pro-inflammatory and anti-inflammatory pathways, which is dependent on exercise intensity and duration [59]. Therefore, acute exhaustive exercise is expected to transiently decrease the individual’s immune competence, while moderate exercise has an anti-inflammatory effect with improved anti-infections capability [60]. There is growing evidence that the immune system may serve as an important physiological indicator for a person’s individual activity to recover from workload stresses [11, 60]. However, the overtraining syndrome, a condition of long term decrement in performance capacity due to continuous training loads, is based on derangement of cellular immune regulation [11, 60].

5 Tendon and Exercise

Tendon has been considered as a model to explore sex differences in mechanical and metabolic properties of connective tissue [23, 61–63]. Tendon is a metabolically active structure that transmits force from muscle to bone for mechanical movement. ECM of tendon mainly consists of collagen and elastin fibres and is surrounded by an aqueous matrix of proteoglycans, glycosaminoglycans and glycoproteins [61, 62]. Mechanical properties and metabolism in tendon have been shown to be different in men and women [63, 64]. The elastic modulus is significantly decreased in women compared with men possibly leading to an inefficient matrix for force transfer between muscle and bone [62, 65]. In addition, cross-sectional structure revealed that tendons of habitually trained women have the same size as those of untrained women, whereas men’s tendon assumed hypertrophy with exhaustive exercise training [65, 66]. Tendons collagen synthesis at rest and after an acute bout of exercise is significantly lower in women than in men [65, 66]. The increased risk for connective tissue injury in women compared with men may be related to structural and regulatory expression differences in tendon [61, 63].

Tendon is mainly composed of collagen and an aqueous matrix of proteoglycans that are regulated by MMPs and TIMPs. It has been demonstrated that collagen type-I, -III and MMP-2 mRNA expressions in patellar tendons were downregulated 24 h after exhaustive exercise. Women had higher mRNA expression of MMP-2 than men after 24 h of exercise [67]. This suggests that sex plays a role on the structural and regulatory mRNA expression of tendon. However, details of the mechanisms by which sex influence tendon metabolism and its mechanical properties is currently unknown.

6 Aging and Exercise

Regular exercise effectively improves heart function in both young and older populations [68, 69]. Exercise training improves maximal cardiovascular work capacity by increasing stroke volume and cardiac output [65, 68, 69]. Conceivably, exercise training in the aging population may reduce accumulation of connective tissue. It has been suggested that exercise training may attenuate collagen content in the aging heart [65, 69]. The ability of exercise training to attenuate diastolic dysfunction and collagen cross-linking has recently been cited [70]. Collagen cross linking (hydroxylysyl pyridinoline) of left ventricle (LV) free wall was demonstrated to be significantly lower in old trained rats compared with their sedentary counterparts [70, 71]. However, potential pathways by which exercise training ameliorates fibrosis in the aging heart are not understood. Collagen fibrosis with aging is progressive and associated with reduced cardiac contractility and risk of heart failure. Elevation of fibrotic connective tissue could lead to decreased cardiac compliance and impaired diastolic function, thereby increasing the risk of heart failure observed with aging [65, 72].

Collagen of the basement membrane is a typical substrate for MMP-2; though fibronectin, a structural component of the sarcolemma, can also be cleaved by MMP-2 and other MMPs that are inhibited by TIMP-1 [73, 74]. Cleavage of the ECM components may cause structural change that allows for adaptive processes such as satellite cell migration and fusion to the myofibre, or it may play a bioactive role in regulating cell proliferation and differentiation [73–75]. The MMPs, for instance, MMP-2; the MMP inhibitor, TIMP-1; and the ECM proteins, for example, fibronectin have been identified as factors whose expressions are altered by aging and exercise [73–75]. The relationship between expression of these factors and strength gain suggests that cleavage of muscle ECM or altered production thereof is an important process during training adaptation [73, 76].

Exercise training reduced fibrosis when visualized with collagen type I positive staining and novel imaging in the hearts of old rats [77]. Exercise training has also alleviated age-related downregulation of active MMPs, which cleaves fibronectin in sarcolemma and connective tissues [74, 75]. Age is known to up regulate TIMP-1 expression [78]. The inhibitory effect of TIMP-1 on MMP activation in aging has been found to be virtually abolished by exercise training [77, 79]. Recent reports have suggested that habitual exercise training attenuates age associated collagen accumulation and fibrosis through signaling pathway(s) that reduces MMP deregulation through TIMP-1 [69, 74, 77, 80].

Lubos et al. [34] reported that human heart failure is associated with a large up regulation of TIMP-1. However, ischemia reperfusion acutely activates MMPs [77, 79]. These observations can be explained by differential regulation between aging and hypertension, where MMP activity decreases with aging and increases with hypertension [77, 79]. Apparently, impaired turnover of ECM proteins could be an important contributory mechanism for accumulation of fibrotic tissue in the hearts due to aging [77].

Exercise training upregulates MMPs such as MMP-1, MMP-2, MMP-3 and MMP-14, thereby mitigating age related reduction in the expressions of MMPs. There are highly novel findings that are consistent with signaling pathways involving MMP regulation eliciting the protective effects of exercise training against remodelling, collagen accumulation and fibrosis. MMP-1, MMP-2, MMP-3 and MMP-14 serve as collagenases and degrade a host of ECM proteins including aggracan (MMP-1, -12 and -3), fibronectin (MMP-2, -3, -14), laminin (MMP-2, -3, -14) and gelatin (MMP-1, -2, -3, -14) [72, 81, 82]. Elevation of TIMP-1 observed with aging appeared to be consistent with upstream suppression of active MMPs such as MMP-1, -2, -3, and -14 [65, 72, 81, 82]. Additionally, exercise training in aged person substantially reduced TIMP-1 along with upregulation of MMP-1, -2, -3 and -14 levels [65]. Given that TIMP-2, TIMP-3 and TIMP-4 are not responsive to exercise training, the TIMP-1/MMP pathway has been suggested to be crucial for exercise-mediated protection against age related fibrosis [78]. Currently, TIMP-1 is considered as a target candidate for the beneficial effects of exercise [65, 78].

7 Endostatin and Exercise

Several members of the MMP family have been shown to be associated with ECM remodeling and these proteases mediate the proteolytic release of the angiostatic factor, endostatin from collagen [83]. Endostatin is a ~20 kDa –COOH terminal fragment of collagens [84]. In a recent study, plasma endostatin concentration has been shown to be increased in response to a single bout of exercise [85]. Regardless of the organ, time and/or the mechanisms responsible for the changed plasma level of endostatin, exercise regulates circulating levels of angiogenesis regulatory factors that could influence angiogenesis-dependent processes in the vascular system, for example, atherosclerosis [86].

Endostatin has been suggested to act as an anti-angiogenic factor by inhibiting vascular endothelial growth factor (VEGF) induced endothelial cell migration and proliferation [87, 88] and by the induction of endothelial cell apoptosis [86, 89]. However, Schmidt et al. [90] demonstrated that both pro-angiogenic and anti-angiogenic effects of endostatin could occur in a dose dependent manner. Interestingly, a recent study demonstrated that endostatin evokes vascular relaxation by increasing the cytosolic NO production [91], which suggests that endostatin regulates the local blood supply during strenuous physical activity.

In the angiogenic process, MMPs play critical roles in regulating endothelial cell adhesion, proliferation and migration and can, therefore, affect neovascularization [83, 92]. MMPs seem to have bilateral functions in angiogenesis. Firstly, in their active form, these enzymes facilitate the degradation of ECM components and neovascularization; and secondly they indirectly inhibit the angiogenic process of endothelial growth by generating anti-angiogenic growth factors and endostatin [87, 93]. In skeletal muscle, angiogenesis occurs as an adaptation to increased work requirements [87]. Exercise studies have focused on stimulatory (angiogenic) and inhibitory (angiostatic) factors in skeletal muscles [86, 94]. Role of angiostatic factors in human skeletal muscle and their change during exercise is an interesting aspect of exercise physiology.

8 Conclusion and Future Direction

Members of the MMP family are present in the skeletal muscles of healthy humans. MMP-9 is induced by a single bout of exercise, presumably by post translational activation and also by an increase in mRNA expression. In contrast, MMP-2 and MMP-14 levels increase after exhaustive exercise training [14]. Further studies are needed to better understand the mechanisms responsible for transcriptional upregulation and activation of the MMP family and to determine the biological significance of MMPs for the adaptation of the skeletal muscle to exercise.

It is currently unknown whether different exercise training programs, to include one that incorporate machine based resistance exercise and one composed of aerobic and body weight exercise such as pull-ups and sit-ups, promote different MMP responses [95–97]. Such information would be valuable in understanding whether the type of exercise and the specific MMP response plays a role in mediating physiological adaptations to exercise training.

mRNA expression of heat shock proteins in muscle has been shown to occur depending on type and intensity of exercise [49]. Mahoney et al. [98] demonstrated that exercise differentially affected the expression of the genes involved in metabolism, cell growth and transcriptional activation as well as apoptosis. Genes that show identical regulatory patterns in leukocytes after exercise are (1) those associated with cell stress management e.g., HSP 90; (2) those associated with proteolysis e.g., MMPs; and (3) those associated with apoptosis e.g., Bcl-2 related anti-apoptotic proteins [11].

Although it is known that vigorous exercise and sex influences tendon metabolism and mechanical properties, it is unknown about the structural and regulatory components that contribute to the responses. It is also currently unknown how sex regulates mRNA expression of collagens, MMPs and TIMPs genes in tendons. Understanding the above will have important implications on why women have differential mRNA expression of MMPs and TIMPs than men.

Microarray technology in exercise physiology is an important tool being used currently for monitoring athletes training process. This is based on the intension of identifying exercise induced gene expression profiles or fingerprints that can be related to exercise intensity(ies) or type(s) [99]. Conceivably, an impact of exercise may be ascertained in the gene expression profiles in blood cells, for example, leukocytes and also exercising muscles and tendons.

The time course of increase in endostatin level may provide evidence of its possible vasoregulatory effect [85] i.e., endostatin could regulate blood supply during physical exercise in addition to its angiogenic effects [90]. MMPs such as MMP-2 and MMP-9 are also elevated after physical performance. Thus, intensified research on endostatin with respect to special physiological and mechanical stimuli that cause an increase in MMPs may lead to better understanding of vascular signaling during exercise in the prevention of a variety of diseases such as coronary heart diseases and diabetes.