GH/IGF-I axis and matrix adaptation of the musculotendinous tissue to exercise in humans

Rapamycin insensitive regulation of engineered ligament structure and function by IGF-1

Following rupture, the anterior cruciate ligament (ACL) will not heal and therefore more than 400,000 surgical repairs are performed annually. Ligament engineering is one way to meet the increasing need for donor tissue to replace the native ligament; however, currently these tissues are too weak for this purpose. Treating engineered human ligaments with insulin-like growth factor-1 (IGF-1) improves the structure and function of these grafts. Since the anabolic effects of IGF-1 are largely mediated by rapamycin complex I (mTORC1), we used rapamycin to determine whether mTORC1 was necessary for the improvement in collagen content and mechanics of engineered ligaments. The effect of IGF-1 and rapamycin was determined independently and interactions between the two treatments were tested. Grafts were treated for 6 days before mechanical testing and analysis of collagen content. Following 8 days of treatment, mechanical properties increased 34% with IGF-1 and decreased 24.5% with rapamycin. Similarly, collagen content increased 63% with IGF-1 and decreased 36% with rapamycin. Interestingly, there was no interaction between IGF-1 and rapamycin, suggesting that IGF-1 was working in a largely mTORC1-independent manner. Acute treatment with IGF-1 did not alter procollagen synthesis in growth media, even though rapamycin decreased procollagen 55%. IGF-1 decreased collagen degradation 15%, whereas rapamycin increased collagen degradation 10%. Once again, there was no interaction between IGF-1 and rapamycin on collagen degradation. Together, these data suggest that growth factor-dependent increases in collagen synthesis are dependent on mTORC1 activity; however, IGF-1 improves human-engineered ligament mechanics and collagen content by decreasing collagen degradation in a rapamycin-independent manner. How the anticatabolic effects of IGF-1 are regulated have yet to be determined.NEW & NOTEWORTHY IGF-1 increases and rapamycin decreases mechanical and material properties of engineered human ligaments by regulating collagen content and concentration. There was no interaction between IGF-1 and rapamycin, suggesting that IGF-1 and rapamycin work independently. We found that IGF-1 improves collagen content by decreasing collagen degradation in a rapamycin-independent manner, whereas growth factor-dependent increases in collagen synthesis are blocked by rapamycin. These data may explain why interventions to increase IGF-1 have not helped rehabilitation.

Biological and Mechanical Factors and Epigenetic Regulation Involved in Tendon Healing

Tendons are an important part of the musculoskeletal system. Connecting muscles to bones, tendons convert force into movement. Tendon injury can be acute or chronic. Noticeably, tendon healing requires a long time span and includes inflammation, proliferation, and remodeling processes. The mismatch between endogenous and exogenous healing may lead to adhesion causing further negative effects. Management of tendon injuries and complications such as subsequent adhesion formation are still challenges for clinicians. Due to numerous factors, tendon healing is a complex process. This review introduces the role of various biological and mechanical factors and epigenetic regulation processes involved in tendon healing.

Growth Factor Roles in Soft Tissue Physiology and Pathophysiology

Repair and healing of injured and diseased tendons has been traditionally fraught with apprehension and difficulties, and often led to rather unsatisfactory results. The burgeoning research field of growth factors has opened new venues for treatment of tendon disorders and injuries, and possibly for treatment of disorders of the aorta and major arteries as well. Several chapters in this volume elucidate the role of transforming growth factor β (TGFß) in pathogenesis of several heritable disorders affecting soft tissues, such as aorta, cardiac valves, and tendons and ligaments. Several members of the bone morphogenetic group either have been approved by the FDA for treatment of non-healing fractures or have been undergoing intensive clinical and experimental testing for use of healing bone fractures and tendon injuries. Because fibroblast growth factors (FGFs) are involved in embryonic development of tendons and muscles among other tissues and organs, the hope is that applied research on FGF biological effects will lead to the development of some new treatment strategies providing that we can control angiogenicity of these growth factors. The problem, or rather question, regarding practical use of imsulin-like growth factor I (IGF-I) in tendon repair is whether IGF-I acts independently or under the guidance of growth hormone. FGF2 or platelet-derived growth factor (PDGF) alone or in combination with IGF-I stimulates regeneration of periodontal ligament: a matter of importance in Marfan patients with periodontitis. In contrast, vascular endothelial growth factor (VEGF) appears to have rather deleterious effects on experimental tendon healing, perhaps because of its angiogenic activity and stimulation of matrix metalloproteinases-proteases whose increased expression has been documented in a variety of ruptured tendons. Other modalities, such as local administration of platelet-rich plasma (PRP) and/or of mesenchymal stem cells have been explored extensively in tendon healing. Though treatment with PRP and mesenchymal stem cells has met with some success in horses (who experience a lot of tendon injuries and other tendon problems), the use of PRP and mesenchymal stem cells in people has been more problematic and requires more studies before PRP and mesenchymal stem cells can become reliable tools in management of soft tissue injuries and disorders.

Characterization of Tendon-Derived Stem Cells and Rescue Tendon Injury

The natural healing ability of tendon is limited, and it cannot restore the native structure and function of tendon injuries. Tendon-derived stem cells (TDSCs) are a new type of pluripotent stem cells with multi-directional differentiation potential and are expected to become a promising cell-seed for the treatment of tendon injuries in the future. In this review, we outline the latest advances in the culture and identification of TDSCs. In addition, the influencing factors on the differentiation of TDSCs are discussed. Moreover, we aim to discuss recent studies to enhance TDSCs treatment of injured tendons. Finally, we identify the limitations of the current understanding of TDSCs biology, the main challenges of using their use, and potential therapeutic strategies to inform cell-based tendon repair.

Effect of amino acids on IGF1 gene expression in human myotubes and skeletal muscle

OBJECTIVE

Insulin-like growth factor I (IGF1) is an important regulator of collagen and extracellular matrix protein expression. We aimed to evaluate the effect of amino acids (AAs) on expression of IGF1 and IGF1-dependent genes in human myotubes and skeletal muscle and supposed that AAs administration increases IGF1 levels in blood and expression of IGF1 and IGF1-dependent genes in trained skeletal muscle, thereby reducing training-induced muscle damage.

DESIGN

Human myotubes were incubated with Arg and Leu for 24 h. Then, the effects of long-term branched chain AAs administration (10 weeks, 0.1 g/kg body mass/day) to volunteers (six subjects per AAs and placebo groups) performing large training volumes regularly (cross country skiers, training twice a day) were examined.

RESULTS

Incubating the myotubes with AAs increases expression of IGF1 mRNA isoforms and IGF1 secretion by 2-3 times. In athletes, long-term AAs administration increased basal blood levels of IGF1 (~50%) and expression of IGF1Ea mRNA slightly in skeletal muscle. There is no marked increase in expression of COL1A1, COL3A1, COL5A1, and LOX genes in skeletal muscle after AAs administration. However, expression of these genes in the combined group (placebo + AAs; n = 12) significantly correlated with the expression of IGF1Ea mRNA in muscle and did not correlate with IGF1 levels in the blood.

CONCLUSIONS

AAs administration increases IGF1 expression in vitro and in vivo. To obtain more pronounced changes in expression of IGF1 and IGF1-dependent genes in skeletal muscle, it may be necessary to increase the dose and/or duration of AAs administration.

The Use of Recombinant Human Growth Hormone to Protect Against Muscle Weakness in Patients Undergoing Anterior Cruciate Ligament Reconstruction: A Pilot, Randomized Placebo-Controlled Trial

BACKGROUND

Anterior cruciate ligament (ACL) tears are common knee injuries. Despite undergoing extensive rehabilitation after ACL reconstruction (ACLR), many patients have persistent quadriceps muscle weakness that limits their successful return to play and are also at an increased risk of developing knee osteoarthritis (OA). Human growth hormone (HGH) has been shown to prevent muscle atrophy and weakness in various models of disuse and disease but has not been evaluated in patients undergoing ACLR.

HYPOTHESIS

Compared with placebo treatment, a 6-week perioperative treatment course of HGH would protect against muscle atrophy and weakness in patients undergoing ACLR.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 2.

METHODS

A total of 19 male patients (aged 18-35 years) scheduled to undergo ACLR were randomly assigned to the placebo (n = 9) or HGH (n = 10) group. Patients began placebo or HGH treatment twice daily 1 week before surgery and continued through 5 weeks after surgery. Knee muscle strength and volume, patient-reported outcome scores, and circulating biomarkers were measured at several time points through 6 months after surgery. Mixed-effects models were used to evaluate differences between treatment groups and time points, and as this was a pilot study, significance was set at P < .10. The Cohen d was calculated to determine the effect size.

RESULTS

HGH was well-tolerated, and no differences in adverse events between the groups were observed. The HGH group had a 2.1-fold increase in circulating insulin-like growth factor 1 over the course of the treatment period (P < .05; d = 2.93). The primary outcome measure was knee extension strength, and HGH treatment increased normalized peak isokinetic knee extension torque by 29% compared with the placebo group (P = .05; d = 0.80). Matrix metalloproteinase-3 (MMP3), which was used as an indirect biomarker of cartilage degradation, was 36% lower in the HGH group (P = .05; d = -1.34). HGH did not appear to be associated with changes in muscle volume or patient-reported outcome scores.

CONCLUSION

HGH improved quadriceps strength and reduced MMP3 levels in patients undergoing ACLR. On the basis of this pilot study, further trials to more comprehensively evaluate the ability of HGH to improve muscle function and potentially protect against OA in patients undergoing ACLR are warranted.

REGISTRATION

NCT02420353 ( ClinicalTrials.gov identifier).

Tendon stem/progenitor cell ageing: Modulation and rejuvenation

Tendon ageing is a complicated process caused by multifaceted pathways and ageing plays a critical role in the occurrence and severity of tendon injury. The role of tendon stem/progenitor cells (TSPCs) in tendon maintenance and regeneration has received increasing attention in recent years. The decreased capacity of TSPCs in seniors contributes to impaired tendon functions and raises questions as to what extent these cells either affect, or cause ageing, and whether these age-related cellular alterations are caused by intrinsic factors or the cellular environment. In this review, recent discoveries concerning the biological characteristics of TSPCs and age-related changes in TSPCs, including the effects of cellular epigenetic alterations and the mechanisms involved in the ageing process, are analyzed. During the ageing process, TSPCs ageing might occur as a natural part of the tendon ageing, but could also result from decreased levels of growth factor, hormone deficits and changes in other related factors. Here, we discuss methods that might induce the rejuvenation of TSPC functions that are impaired during ageing, including moderate exercise, cell extracellular matrix condition, growth factors and hormones; these methods aim to rejuvenate the features of youthfulness with the ultimate goal of improving human health during ageing.

Insulin-like growth factor 1 signaling in tenocytes is required for adult tendon growth

Tenocytes serve to synthesize and maintain collagen fibrils and other extracellular matrix proteins in tendon. Despite the high prevalence of tendon injury, the underlying biologic mechanisms of postnatal tendon growth and repair are not well understood. IGF1 plays an important role in the growth and remodeling of numerous tissues but less is known about IGF1 in tendon. We hypothesized that IGF1 signaling is required for proper tendon growth in response to mechanical loading through regulation of collagen synthesis and cell proliferation. To test this hypothesis, we conditionally deleted the IGF1 receptor (IGF1R) in scleraxis (Scx)-expressing tenocytes using a tamoxifen-inducible Cre-recombinase system and caused tendon growth in adult mice via mechanical overload of the plantaris tendon. Compared with control Scx-expressing IGF1R-positive (Scx:IGF1R⁺) mice, in which IGF1R is present in tenocytes, mice that lacked IGF1R in their tenocytes [Scx-expressing IGF1R-negative (Scx:IGF1R^Δ) mice] demonstrated reduced cell proliferation and smaller tendons in response to mechanical loading. Additionally, we identified that both the PI3K/protein kinase B and ERK pathways are activated downstream of IGF1 and interact in a coordinated manner to regulate cell proliferation and protein synthesis. These studies indicate that IGF1 signaling is required for proper postnatal tendon growth and support the potential use of IGF1 in the treatment of tendon disorders.-Disser, N. P., Sugg, K. B., Talarek, J. R., Sarver, D. C., Rourke, B. J., Mendias, C. L. Insulin-like growth factor 1 signaling in tenocytes is required for adult tendon growth.

Influence of Oral Contraceptive Use on Adaptations to Resistance Training

Introduction: The majority of young women use oral contraceptives (OCs). Use of OCs has been associated with lower myofibrillar protein and tendon collagen synthesis rates, but it is unknown whether OCs will limit the adaptive response of myotendinous tissue to resistance training.

Design and Methods: Fourteen healthy untrained young regular OC users (24 ± 1 years, fat% 32 ± 1, 35 ± 2 ml⋅min^-1⋅kg^-1) and 14 NOC users (non-OC, controls) (24 ± 1 years, fat% 32 ± 2, 34 ± 2 ml⋅min^-1⋅kg^-1) performed a 10-week supervised lower extremity progressive resistance training program. Before and after the intervention biopsies from the vastus lateralis muscle and the patellar tendon were obtained. Muscle (quadriceps) and tendon cross-sectional area (CSA) was determined by magnetic resonance imaging (MRI) scans, and muscle fiber CSA was determined by histochemistry. Maximal isometric knee extension strength was assessed by dynamometry while 1 repetition maximum (RM) was determined during knee extension.

Results: Training enhanced CSA in both muscle (p < 0.001) and tendon (p < 0.01). A trend toward a greater increase in muscle CSA was observed for OC (11%) compared to NOC (8%) (interaction p = 0.06). Analysis of mean muscle fiber type CSA showed a trend toward an increase in type II muscle fiber area in both groups (p = 0.11, interaction p = 0.98), whereas type I muscle fiber CSA increased in the OC group (n = 9, 3821 ± 197 to 4490 ± 313 mm², p < 0.05), but not in NOC (n = 7, 4020 ± 348 to 3777 ± 354 mm², p = 0.40) (interaction p < 0.05). Post hoc analyses indicated that the effect of OCs on muscle mass increase was induced by the OC-users (n = 7), who used OCs containing 30 μg ethinyl estradiol (EE), whereas the response in users taking OCs with 20 μg EE (n = 7) did not differ from NOC. Both the OC and NOC group experienced an increase in maximal knee strength (p < 0.001) and 1RM leg extension (p < 0.001) after the training period with no difference between groups.

Conclusion: Use of OCs during a 10-week supervised progressive resistance training program was associated with a trend toward a greater increase in muscle mass and a significantly greater increase in type I muscle fiber area compared to controls. Yet, use of OCs did not influence the overall increase in muscle strength related to training.

Tendon healing in the context of complex fractures

Tendons connect muscle to bone and play an integral role in bone and joint alignment and loading. Tendons act as pulleys that provide anchorage of muscle forces for joint motion and stability, as well as for fracture reduction and realignment. Patients that experience complex fractures also have concomitant soft tissue injuries, such as tendon damage or rupture. Tendon injuries that occur at the time of bone fracture have long-term ramifications on musculoskeletal health, yet these injuries are often disregarded in clinical treatment and diagnosis for patients with bone fractures as well as in basic science approaches for understanding bone repair processes. Delayed assessment of soft tissue injuries during evaluation of trauma can lead to chronic pain, dysfunction, and delayed bone healing even following successful fracture repair, highlighting the importance of identifying and treating damaged tendons early. Treatment strategies for bone repair, such as mechanical stabilization and biological therapeutics, can impact tendon healing and function. Because poor tendon healing following complex fracture can significantly impact the function of tendon during bone fracture healing, a need exists to understand the healing process of complex fractures more broadly, beyond the healing of bone. In this review, we explored the mechanical and biological interaction of bone and tendon in the context of complex fracture, as well as the relevance and potential ramifications of tendon damage following bone fracture, which has particular impact on patients that experience complex fractures, such as from combat, automobile accidents, and other trauma.

The Growth Hormone Receptor: Mechanism of Receptor Activation, Cell Signaling, and Physiological Aspects

The growth hormone receptor (GHR), although most well known for regulating growth, has many other important biological functions including regulating metabolism and controlling physiological processes related to the hepatobiliary, cardiovascular, renal, gastrointestinal, and reproductive systems. In addition, growth hormone signaling is an important regulator of aging and plays a significant role in cancer development. Growth hormone activates the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway, and recent studies have provided a new understanding of the mechanism of JAK2 activation by growth hormone binding to its receptor. JAK2 activation is required for growth hormone-mediated activation of STAT1, STAT3, and STAT5, and the negative regulation of JAK-STAT signaling comprises an important step in the control of this signaling pathway. The GHR also activates the Src family kinase signaling pathway independent of JAK2. This review covers the molecular mechanisms of GHR activation and signal transduction as well as the physiological consequences of growth hormone signaling.

Specific titin and myomesin domains stimulate myoblast proliferation

Myofibrillar proteins titin and myomesin stimulated myoblast proliferation as determined by MTT-test and labelled thymidine incorporation in the DNA. Specific Fn type III and Ig-like domains of these proteins were able to exert mitogenic effects as well. Proliferative effect of Fn type III domains was highly sensitive to inhibition of Ca²⁺/calmodulin dependent protein kinase, whereas the effect of Ig-like domains showed greater sensitivity to the inhibition of adenylyl cyclase – cAMP – PKA pathway. IGF-1 autocrine signalling inhibition partially suppressed mitogenic effects revealed by both domain types.

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Titin and myomesin as well as their domains stimulate myoblast proliferation.

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Ig-like and Fn type III domains activate proliferation via different pathways.

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IGF-1 autocrine signalling is partially involved in observed proliferative effects.

Recovery responses of testosterone, growth hormone, and IGF-1 after resistance exercise

The complexity and redundancy of the endocrine pathways during recovery related to anabolic function in the body belie an oversimplistic approach to its study. The purpose of this review is to examine the role of resistance exercise (RE) on the recovery responses of three major anabolic hormones, testosterone, growth hormone(s), and insulin-like growth factor 1. Each hormone has a complexity related to differential pathways of action as well as interactions with binding proteins and receptor interactions. Testosterone is the primary anabolic hormone, and its concentration changes during the recovery period depending on the upregulation or downregulation of the androgen receptor. Multiple tissues beyond skeletal muscle are targeted under hormonal control and play critical roles in metabolism and physiological function. Growth hormone (GH) demonstrates differential increases in recovery with RE based on the type of GH being assayed and workout being used. IGF-1 shows variable increases in recovery with RE and is intimately linked to a host of binding proteins that are essential to its integrative actions and mediating targeting effects. The RE stress is related to recruitment of muscle tissue with the glandular release of hormones as signals to target tissues to support homeostatic mechanisms for metabolism and tissue repair during the recovery process. Anabolic hormones play a crucial role in the body’s response to metabolism, repair, and adaptive capabilities especially in response to anabolic-type RE. Changes of these hormones following RE during recovery in the circulatory biocompartment of blood are reflective of the many mechanisms of action that are in play in the repair and recovery process.

Satellite Cells Contribution to Exercise Mediated Muscle Hypertrophy and Repair

Satellite cells (SCs) are the most abundant skeletal muscle stem cells. They are widely recognized for their contributions to maintenance of muscle mass, regeneration and hypertrophy during the human life span. These cells are good candidates for cell therapy due to their self-renewal capabilities and presence in an undifferentiated form. Presently, a significant gap exists between our knowledge of SCs behavior and their application as a means for human skeletal muscle tissue repair and regeneration. Both physiological and pathological stimuli potentially affect SCs activation, proliferation, and terminal differentiation the former category being the focus of this article. Activation of SCs occurs following exercise, post-training micro-injuries, and electrical stimulation. Exercise, as a potent and natural stimulus, is at the center of numerous studies on SC activation and relevant fields. According to research, different exercise modalities end with various effects. This review article attempts to picture the state of the art of the SCs life span and their engagement in muscle regeneration and hypertrophy in exercise.

Therapeutic use of hormones on tendinopathies: a narrative review

BACKGROUND

Hormones can modify tendon homeostasis, some of them leading to tendon damage, while others are essentials for healing. This narrative review summarizes the current knowledge on the topic, focusing on the hormones normally secreted by endocrine glands.

METHODS

A search in PubMed, Web of Knowledge and EMBASE, using the terms tendinopathy or tendon, combined with estrogens, testosterone, thyroid and parathyroid hormones, glucocorticoids and growth hormone, independently, was performed. Relevant articles focusing on the correlation between hormones and tendons, and their therapeutic use in tendinopathies, were selected.

RESULTS

Tendon abnormalities observed in subjects with hyperparathyroidism, hypercortisolism and acromegaly are described. At present, experimental studies and preliminary observations in humans suggest that parathyroid and growth hormones, locally administered, are promising therapeutic tools in specific tendon disorders. Local injections of glucocorticoids are useful in several tendinopathies, exploiting their anti-inflammatory and anti-proliferative properties, but carry the risk of further tendon degeneration and ruptures, due to the detrimental direct effect of glucocorticoids on the tendon structure.

CONCLUSION

Because tendons injuries are frequent, often with long lasting sequels, it is important to improve our understanding concerning the therapeutic potential of hormones on healing.

LEVEL OF EVIDENCE

IV.

Can IGF-I polymorphism affect power and endurance athletic performance?

OBJECTIVE

Insulin-like growth factor-I (IGF-I) plays a key role in exercise-associated muscle growth and development. The regulatory region of the promoter of the IGF-I gene is labile, but changes in this region were studied mostly in the elderly and in relation to pathological states. C-1245T (rs35767) is a genetic variation in the promoter region of the IGF-I gene. The minor allele T was found to be associated with higher circulating IGF-I levels, and possibly with increased muscle mass. The aim of the current study was to analyze the frequency distribution of C-1245T SNP in athletic and nonathletic Israeli populations.

DESIGN

One hundred and sixty-five athletes (78 endurance-type athletes, and 87 power-type athletes) and 159 nonathletic healthy individuals participated in the current study. Genomic DNA was extracted from peripheral EDTA treated anti-coagulated blood using a standard protocol. Genotyping of the IGF1 C-1245T polymorphism was performed using polymerase chain reaction (PCR).

RESULTS

We found that the endurance and power athletes' allele and genotype frequencies were significantly different from those of the control group. Only 4.8% of the athletes were TT carriers, but none of the controls carried this genotype. The T allele was found to be more frequent in the top-level power athletes (international and Olympic level) compared to national level athletes, but such a difference was not found in endurance athletes.

CONCLUSION

Our findings suggest a possible contribution for the relatively rare IGF-I TT genotype to endurance performance, and in particular to power sport excellence in Israeli athletes.

Does an NSAID a day keep satellite cells at bay?

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely consumed by athletes worldwide, despite growing evidence for a negative influence on the adaptation of skeletal muscle to exercise, at least in young healthy individuals. This review focuses on the potential of NSAIDs to alter the activity of satellite cells, the muscle stem cell responsible for repair and maintenance of skeletal muscle. The signaling pathways that are potentially modified by NSAID exposure are also considered. Growth factors as well as inflammatory cells and connective tissue appear to be key factors in the response of muscle under conditions where cyclooxygenase and prostaglandin activity are blocked through NSAID ingestion or infusion. Discrepancies in the literature regarding the response of young and old individuals are addressed, where it appears that the elderly may benefit from NSAID ingestion, although this clearly requires further study. The long-term implications for the muscle of the apparent inhibitory effect of NSAIDs on satellite cells in younger individuals are not clear, and it is possible these may first become apparent with chronic use in athletes training at a high level or with advancing age. Reports of the potential for NSAIDs to alter prostaglandin and growth factor signaling provide a basis for further study of the mechanism of NSAID action on satellite cells.