madman
Super Moderator
Abstract
Wound healing is a complex process involving four overlapping phases: hemostasis, inflammation, cell recruitment, and matrix remodeling. In mouse models, surgical, pharmacological, and genetic approaches targeting androgen actions in the skin have shown that androgens increase interleukin-6 and tumor necrosis factor-α production and reduce wound re-epithelization and matrix deposition, retarding cutaneous wound healing. Similarly, clinical studies have shown that cutaneous wound healing is slower in men compared to women. However, in major burn injury, which triggers not only local wound-healing processes but also systemic hypermetabolism, the role of androgens is poorly understood. Recent studies have claimed that a synthetic androgen, oxandrolone, increases protein synthesis, improves lean body mass, and shortens the length of hospital stay. However, the possible mechanisms by which oxandrolone regulates major burn injury have not been reported. In this review, we summarize the current findings on the roles of androgens in cutaneous and major burn wound healing, as well as androgens as a potential therapeutic treatment option for patients with major burn injuries.
Background
Cutaneous injuries and small and major burn injuries trigger a wound-healing process that consists of several highly integrated and overlapping phases, including inflammation, cell recruitment, matrix deposition, epithelization, and tissue remodeling. In addition to local wound repair, large or major burns also stimulate a systemic hypermetabolic catabolic condition and pathophysiological stress response. If not corrected in time, the hypermetabolic response results in a catabolic state, characterized by weight reduction, a negative nitrogen balance, loss of lean body mass, impaired wound healing and sepsis [1]. These syndromes are associated with delayed recovery, prolonged hospital admission, and increased morbidity and mortality.
In men, testosterone is the predominant circulating androgen, which, in target cells, can be rapidly, irreversibly, and almost completely converted to the more potent androgen dihydrotestosterone (DHT) by 5α-reductases, with type 1 and type 2 is highly expressed in the skin [2, 3]. Androgens are known to exacerbate inflammation and the tissue remodeling phases in cutaneous (non-burn) wound healing. In contrast to the retarding effect of androgens in cutaneous wound healing, a few clinical studies reported using the androgen analog oxandrolone in the treatment of major burn injuries in children, increased protein synthesis improved lean body mass, and shortened length of hospital stay. In the present article, we discuss the experimental models used to study androgenic effects in wound healing, if and how the actions of androgens modify the wound-healing process under a pathophysiological stress response and the role of androgens in treating major burn injuries.
Review
Androgens and androgen action
Androgens are a group of 19-carbon steroid hormones produced de novo mainly in testes and, to a much lesser extent, other steroidogenic tissues (ovaries, adrenals, and placenta) by the conversion of pro-androgen precursors in peripheral tissues including the liver, skin, adipose tissue, breast and prostate [4, 5]. Androgens have both virilizing and anabolic effects on their target tissues, mediated by binding to and activating the androgen receptor (AR) [5–7].
Testosterone and dihydrotestosterone
The major circulating androgen in male mammals is testosterone, which is secreted by testicular Leydig cells. After testicular secretion, a small proportion of circulating testosterone (∼5–10%) is irreversibly converted to the more potent androgen DHT by 5α-reductase enzymes [5, 6]. Testosterone also forms the obligate precursor for the synthesis of estradiol by the enzyme aromatase, thereby diversifying the effects of testosterone due to its activation of estrogen receptors (Figure 1). Two isoforms of 5α-reductases, namely type 1 and type 2 5αreductase, convert testosterone to DHT, with type 1 predominantly being expressed in the skin [2, 3] (Figure 1).
Synthetic androgens
Since the discovery of testosterone as the principal mammalian testicular-derived androgen in the mid-1930s, thousands of synthetic steroidal androgens have been produced, and in the late 1990s, the first non-steroidal androgen was synthesized. Testosterone is used exclusively for replacement therapy in men having pathological hypogonadism or disorders of the hypothalamus, pituitary, or testes, which reduce endogenous testosterone production [5, 8– 11]. By contrast, synthetic androgens are used for numerous non-reproductive disorders to exploit their effects on muscle size and strength, and hemoglobin function, as well as other androgenic effects. However, the clinical applications of synthetic androgens are limited by their virilizing effects on women and children. As testosterone has negligible oral bioactivity due to rapid degradation in the gut and liver, many synthetic androgens have been developed to be orally active—a pharmacological feature which is almost exclusively due to the introduction of a 17 α-alkyl substituent into the molecule. However, oral 17 α-alkylated steroid androgens (e.g. methyltestosterone, fluoxymesterone, oxymetholone, and oxandrolone) are hepatotoxic due to the class-specific effects of the 17 α-alkyl group and are therefore not suitable for long-term androgen treatment [5]. Oxandrolone is an orally active synthetic 17-α alkylated androgen with greater potency than testosterone [12, 13]. In addition to being a potent androgen, oxandrolone has additional effects antagonizing glucocorticoid receptor effects [13]. In the absence of any safe oral androgens in the USA, oxandrolone has been used in major burn injury and various other pediatric growth and clinical situations as a synthetic androgen, despite the hepatotoxicity risk, whereas in other Western countries oral testosterone products are available and free from hepatotoxicity. In any situation where oxandrolone has proved clinically effective, its use could be replaced by safer non-hepatotoxic androgens, such as testosterone or DHT.
Experimental murine models to study androgenic effects on cutaneous wound healing
*Orchidectomy in mice
*Global knockout mouse models
*Cell-specific knockout mouse models
Effects of androgens on the cutaneous wound-healing process
*Androgens and inflammation
*Androgens in cell-recruitment phases of wound repair
*Androgens modify matrix remodeling
Androgens in major burn injury wound healing
In contrast to cutaneous injury, major burn injury initiates both cutaneous local wound-healing processes as well as a systemic hypermetabolic response. Patients with a 20% total body surface area burn become hypermetabolic, experiencing a significant increase in resting energy expenditure, which is largely driven by elevated levels of circulating catecholamines, corticosteroids, and pro-inflammatory cytokines following the burn injury. Furthermore, hypermetabolic burns patients can suffer from endocrine dysfunction, immune compromise, insulin resistance, and whole-body catabolism [62, 63]. These clinical features are associated with delayed recovery, prolonged hospital admission, and increased morbidity and mortality [62, 63]. Currently available treatments to ameliorate hypermetabolism in major burn patients include early excision and closure of the wound, nutritional support, or pharmacologic modalities, such as androgens and other anabolic hormones [64]. Clinical studies have reported that testosterone and synthetic androgen oxandrolone can enhance recovery from burn injury [65]. Androgen-treated burns patients have been reported to maintain more lean body mass and have improved body composition and hepatic protein synthesis during the acute postburn phase [66, 67]. Although there are reports of improved patient outcomes, mainly in children with major burns, the role of androgens in major burn injury wound healing is not clear. Androgen treatments have not been generally adopted due to the lack of clarity of the mechanism of ameliorating the hypermetabolic response, as well as the risks of hepatotoxicity from a 17 α-alkylated androgen and unwanted virilization of women and children.
Testosterone treatment
Serum testosterone levels are decreased significantly in burn patients in an intensive care unit during the first week post-injury [68]. Whether decreased serum testosterone contributes to the burn-induced catabolic stress state is not known. Testosterone administration has been reported to have positive effects on patients with burn injuries and mainly manifests through better preservation of muscle [69]. Testosterone administration has been shown to reduce the rate of protein breakdown and increase the net protein balance in muscle during the acute burn injury phase, mitigating protein catabolism of the systemic hypermetabolic response to major burn injury. Protein synthesis did not change with testosterone treatment and was attributed to the limited availability of precursor amino acids [69]. Testosterone treatment has therefore been proposed to provide an antagonistic balance to the hypermetabolic serving to reduce protein breakdown without necessarily increasing protein synthesis. Current studies investigating the effects of testosterone treatment following burn injury have been limited by small sample sizes, lack of placebo control, and a limited range of important clinical outcomes being assessed (e.g. wound healing, length of hospital stay). Furthermore, many of these studies have only been conducted in male burn patients.
Oxandrolone in treating major burn injuries
Major burn injury causes significant depletion in lean body mass, resulting in complications such as infection, failure to thrive, and endocrine, immune, and nutritional deficiencies due to increased metabolic demands and inefficient energy utilization. Previous studies have reported that the administration of oxandrolone had no significant effect on resting expenditure rate or basal metabolic rate when measured by indirect calorimetry, suggesting that oxandrolone does not reduce the burn-induced hypermetabolic state [1, 70–72]. However, some clinical studies found that combining oxandrolone with exercise consistently improved the gross parameters of lean body mass, total body mass, bone mineral composition, strength and reduced length of hospital stay in patients with major burn injuries (Table 4) [1, 70–82]. A majority of these studies noted some benefit to net muscle deposition or fractional synthesis rate in oxandrolone-treated patients compared to controls via either an increase in protein synthesis or a decrease in protein breakdown (Table 4) [1, 65, 69–71, 76, 77, 80, 82–87]. Increased protein synthesis in patients treated with oxandrolone may be associated with the upregulation of genes governing transcription factors, growth factors and muscle-associated proteins, including myosin, a light chain, and calmodulin, while also downregulating phosphatase I inhibitor [82, 88].
Oxandrolone also stimulates upregulation of supplementary systemic anabolic hormones, including insulin-like growth factor-1 and thyroid hormones, all of which promote protein synthesis, increasing lean body mass [72, 78, 79]. Moreover, oxandrolone has been demonstrated to increase inflammatory markers and hepatic acute-phase proteins, such as ferritin, haptoglobin, C-reactive protein, and α2- macroglobin, returning the levels to normal at a faster rate compared to controls [71]. It has also been claimed that oxandrolone may inhibit glucocorticoid action via ARs, thus limiting systemic catabolism and proteolysis [71]. Whether the use of oxandrolone in clinical studies will compromise adrenal response to severe injury remains to be investigated. However, the effects of oxandrolone, and that of safer, alternative androgens, on local wound-healing processes and systemic induced hypermetabolism has not been demonstrated and therefore require further investigation, particularly to better understand the role of androgen action in major burn injury recovery. Proponents of oxandrolone claim it has decreased side effects compared to testosterone but few studies with testosterone are reported and none have compared it to oxandrolone [70, 74, 77, 81, 85, 89]. The major concerns for oxandrolone is unwanted virilization of women and children and hepatoxicity, both of which remain to be clarified [65, 77, 79].
DHT in treating major burn injuries
DHT is the most potent natural androgen, with higher affinity and greater molar potency in the transactivation of the AR. DHT lacks the hepatotoxicity of 17 α-alkylated synthetic androgens, highlighting its overlooked potential as an important therapeutic treatment option. Aside from a few studies showing the effect of DHT treatment on increasing mouse skeletal muscle size and strength, research on the application of DHT in burn injury wound healing is limited [90, 91]. In our recent study, the effect of DHT treatment in major burn injury healing was examined in a burn injury mouse model [92, 93]. Our data showed that DHT has a positive impact on both local wound healing and metabolic catabolic responses, which differs from results reported after cutaneous injury. Mice that received DHT implantations had a faster healing rate, particularly in the early stages of the healing process. While those circulating immune cells then infiltrate into the wound site and help with the resolution of inflammation. The increase in monocyte chemoattractant protein-1 (MCP-1) level at the wound area and systemically recruits more monocytes to the blood circulation and the wound site. These monocytes were found to differentiate to macrophages, which are involved in removing bacteria, preventing infection, and contributing to collagen disposition once the inflammation is resolved [93] (Figure 5). Therefore, as a safer, non-hepatoxic androgen, DHT was approved for the first time to induce acceleration of the inflammatory turnover both locally and systemically as the key in promoting major burn injury wound healing without any adverse effects.
Conclusions
This review summarizes the current understanding of the role of androgens in both cutaneous and major burn injury wound healing. The inhibitory role of androgens in cutaneous wound healing has been well-studied with androgens modifying the inflammation and delaying the proliferation phase. In major burn injury, a systemic hypermetabolic state develops and the administration of testosterone or oxandrolone has been clinically reported to improve maintenance of body weight, increase muscle protein metabolism and shorten hospital stay. However, at present, the role of safe androgens in major burn injury wound healing is poorly understood and further investigation into the therapeutic potential of androgens for major burn injury patients are warranted.
Wound healing is a complex process involving four overlapping phases: hemostasis, inflammation, cell recruitment, and matrix remodeling. In mouse models, surgical, pharmacological, and genetic approaches targeting androgen actions in the skin have shown that androgens increase interleukin-6 and tumor necrosis factor-α production and reduce wound re-epithelization and matrix deposition, retarding cutaneous wound healing. Similarly, clinical studies have shown that cutaneous wound healing is slower in men compared to women. However, in major burn injury, which triggers not only local wound-healing processes but also systemic hypermetabolism, the role of androgens is poorly understood. Recent studies have claimed that a synthetic androgen, oxandrolone, increases protein synthesis, improves lean body mass, and shortens the length of hospital stay. However, the possible mechanisms by which oxandrolone regulates major burn injury have not been reported. In this review, we summarize the current findings on the roles of androgens in cutaneous and major burn wound healing, as well as androgens as a potential therapeutic treatment option for patients with major burn injuries.
Background
Cutaneous injuries and small and major burn injuries trigger a wound-healing process that consists of several highly integrated and overlapping phases, including inflammation, cell recruitment, matrix deposition, epithelization, and tissue remodeling. In addition to local wound repair, large or major burns also stimulate a systemic hypermetabolic catabolic condition and pathophysiological stress response. If not corrected in time, the hypermetabolic response results in a catabolic state, characterized by weight reduction, a negative nitrogen balance, loss of lean body mass, impaired wound healing and sepsis [1]. These syndromes are associated with delayed recovery, prolonged hospital admission, and increased morbidity and mortality.
In men, testosterone is the predominant circulating androgen, which, in target cells, can be rapidly, irreversibly, and almost completely converted to the more potent androgen dihydrotestosterone (DHT) by 5α-reductases, with type 1 and type 2 is highly expressed in the skin [2, 3]. Androgens are known to exacerbate inflammation and the tissue remodeling phases in cutaneous (non-burn) wound healing. In contrast to the retarding effect of androgens in cutaneous wound healing, a few clinical studies reported using the androgen analog oxandrolone in the treatment of major burn injuries in children, increased protein synthesis improved lean body mass, and shortened length of hospital stay. In the present article, we discuss the experimental models used to study androgenic effects in wound healing, if and how the actions of androgens modify the wound-healing process under a pathophysiological stress response and the role of androgens in treating major burn injuries.
Review
Androgens and androgen action
Androgens are a group of 19-carbon steroid hormones produced de novo mainly in testes and, to a much lesser extent, other steroidogenic tissues (ovaries, adrenals, and placenta) by the conversion of pro-androgen precursors in peripheral tissues including the liver, skin, adipose tissue, breast and prostate [4, 5]. Androgens have both virilizing and anabolic effects on their target tissues, mediated by binding to and activating the androgen receptor (AR) [5–7].
Testosterone and dihydrotestosterone
The major circulating androgen in male mammals is testosterone, which is secreted by testicular Leydig cells. After testicular secretion, a small proportion of circulating testosterone (∼5–10%) is irreversibly converted to the more potent androgen DHT by 5α-reductase enzymes [5, 6]. Testosterone also forms the obligate precursor for the synthesis of estradiol by the enzyme aromatase, thereby diversifying the effects of testosterone due to its activation of estrogen receptors (Figure 1). Two isoforms of 5α-reductases, namely type 1 and type 2 5αreductase, convert testosterone to DHT, with type 1 predominantly being expressed in the skin [2, 3] (Figure 1).
Synthetic androgens
Since the discovery of testosterone as the principal mammalian testicular-derived androgen in the mid-1930s, thousands of synthetic steroidal androgens have been produced, and in the late 1990s, the first non-steroidal androgen was synthesized. Testosterone is used exclusively for replacement therapy in men having pathological hypogonadism or disorders of the hypothalamus, pituitary, or testes, which reduce endogenous testosterone production [5, 8– 11]. By contrast, synthetic androgens are used for numerous non-reproductive disorders to exploit their effects on muscle size and strength, and hemoglobin function, as well as other androgenic effects. However, the clinical applications of synthetic androgens are limited by their virilizing effects on women and children. As testosterone has negligible oral bioactivity due to rapid degradation in the gut and liver, many synthetic androgens have been developed to be orally active—a pharmacological feature which is almost exclusively due to the introduction of a 17 α-alkyl substituent into the molecule. However, oral 17 α-alkylated steroid androgens (e.g. methyltestosterone, fluoxymesterone, oxymetholone, and oxandrolone) are hepatotoxic due to the class-specific effects of the 17 α-alkyl group and are therefore not suitable for long-term androgen treatment [5]. Oxandrolone is an orally active synthetic 17-α alkylated androgen with greater potency than testosterone [12, 13]. In addition to being a potent androgen, oxandrolone has additional effects antagonizing glucocorticoid receptor effects [13]. In the absence of any safe oral androgens in the USA, oxandrolone has been used in major burn injury and various other pediatric growth and clinical situations as a synthetic androgen, despite the hepatotoxicity risk, whereas in other Western countries oral testosterone products are available and free from hepatotoxicity. In any situation where oxandrolone has proved clinically effective, its use could be replaced by safer non-hepatotoxic androgens, such as testosterone or DHT.
Experimental murine models to study androgenic effects on cutaneous wound healing
*Orchidectomy in mice
*Global knockout mouse models
*Cell-specific knockout mouse models
Effects of androgens on the cutaneous wound-healing process
*Androgens and inflammation
*Androgens in cell-recruitment phases of wound repair
*Androgens modify matrix remodeling
Androgens in major burn injury wound healing
In contrast to cutaneous injury, major burn injury initiates both cutaneous local wound-healing processes as well as a systemic hypermetabolic response. Patients with a 20% total body surface area burn become hypermetabolic, experiencing a significant increase in resting energy expenditure, which is largely driven by elevated levels of circulating catecholamines, corticosteroids, and pro-inflammatory cytokines following the burn injury. Furthermore, hypermetabolic burns patients can suffer from endocrine dysfunction, immune compromise, insulin resistance, and whole-body catabolism [62, 63]. These clinical features are associated with delayed recovery, prolonged hospital admission, and increased morbidity and mortality [62, 63]. Currently available treatments to ameliorate hypermetabolism in major burn patients include early excision and closure of the wound, nutritional support, or pharmacologic modalities, such as androgens and other anabolic hormones [64]. Clinical studies have reported that testosterone and synthetic androgen oxandrolone can enhance recovery from burn injury [65]. Androgen-treated burns patients have been reported to maintain more lean body mass and have improved body composition and hepatic protein synthesis during the acute postburn phase [66, 67]. Although there are reports of improved patient outcomes, mainly in children with major burns, the role of androgens in major burn injury wound healing is not clear. Androgen treatments have not been generally adopted due to the lack of clarity of the mechanism of ameliorating the hypermetabolic response, as well as the risks of hepatotoxicity from a 17 α-alkylated androgen and unwanted virilization of women and children.
Testosterone treatment
Serum testosterone levels are decreased significantly in burn patients in an intensive care unit during the first week post-injury [68]. Whether decreased serum testosterone contributes to the burn-induced catabolic stress state is not known. Testosterone administration has been reported to have positive effects on patients with burn injuries and mainly manifests through better preservation of muscle [69]. Testosterone administration has been shown to reduce the rate of protein breakdown and increase the net protein balance in muscle during the acute burn injury phase, mitigating protein catabolism of the systemic hypermetabolic response to major burn injury. Protein synthesis did not change with testosterone treatment and was attributed to the limited availability of precursor amino acids [69]. Testosterone treatment has therefore been proposed to provide an antagonistic balance to the hypermetabolic serving to reduce protein breakdown without necessarily increasing protein synthesis. Current studies investigating the effects of testosterone treatment following burn injury have been limited by small sample sizes, lack of placebo control, and a limited range of important clinical outcomes being assessed (e.g. wound healing, length of hospital stay). Furthermore, many of these studies have only been conducted in male burn patients.
Oxandrolone in treating major burn injuries
Major burn injury causes significant depletion in lean body mass, resulting in complications such as infection, failure to thrive, and endocrine, immune, and nutritional deficiencies due to increased metabolic demands and inefficient energy utilization. Previous studies have reported that the administration of oxandrolone had no significant effect on resting expenditure rate or basal metabolic rate when measured by indirect calorimetry, suggesting that oxandrolone does not reduce the burn-induced hypermetabolic state [1, 70–72]. However, some clinical studies found that combining oxandrolone with exercise consistently improved the gross parameters of lean body mass, total body mass, bone mineral composition, strength and reduced length of hospital stay in patients with major burn injuries (Table 4) [1, 70–82]. A majority of these studies noted some benefit to net muscle deposition or fractional synthesis rate in oxandrolone-treated patients compared to controls via either an increase in protein synthesis or a decrease in protein breakdown (Table 4) [1, 65, 69–71, 76, 77, 80, 82–87]. Increased protein synthesis in patients treated with oxandrolone may be associated with the upregulation of genes governing transcription factors, growth factors and muscle-associated proteins, including myosin, a light chain, and calmodulin, while also downregulating phosphatase I inhibitor [82, 88].
Oxandrolone also stimulates upregulation of supplementary systemic anabolic hormones, including insulin-like growth factor-1 and thyroid hormones, all of which promote protein synthesis, increasing lean body mass [72, 78, 79]. Moreover, oxandrolone has been demonstrated to increase inflammatory markers and hepatic acute-phase proteins, such as ferritin, haptoglobin, C-reactive protein, and α2- macroglobin, returning the levels to normal at a faster rate compared to controls [71]. It has also been claimed that oxandrolone may inhibit glucocorticoid action via ARs, thus limiting systemic catabolism and proteolysis [71]. Whether the use of oxandrolone in clinical studies will compromise adrenal response to severe injury remains to be investigated. However, the effects of oxandrolone, and that of safer, alternative androgens, on local wound-healing processes and systemic induced hypermetabolism has not been demonstrated and therefore require further investigation, particularly to better understand the role of androgen action in major burn injury recovery. Proponents of oxandrolone claim it has decreased side effects compared to testosterone but few studies with testosterone are reported and none have compared it to oxandrolone [70, 74, 77, 81, 85, 89]. The major concerns for oxandrolone is unwanted virilization of women and children and hepatoxicity, both of which remain to be clarified [65, 77, 79].
DHT in treating major burn injuries
DHT is the most potent natural androgen, with higher affinity and greater molar potency in the transactivation of the AR. DHT lacks the hepatotoxicity of 17 α-alkylated synthetic androgens, highlighting its overlooked potential as an important therapeutic treatment option. Aside from a few studies showing the effect of DHT treatment on increasing mouse skeletal muscle size and strength, research on the application of DHT in burn injury wound healing is limited [90, 91]. In our recent study, the effect of DHT treatment in major burn injury healing was examined in a burn injury mouse model [92, 93]. Our data showed that DHT has a positive impact on both local wound healing and metabolic catabolic responses, which differs from results reported after cutaneous injury. Mice that received DHT implantations had a faster healing rate, particularly in the early stages of the healing process. While those circulating immune cells then infiltrate into the wound site and help with the resolution of inflammation. The increase in monocyte chemoattractant protein-1 (MCP-1) level at the wound area and systemically recruits more monocytes to the blood circulation and the wound site. These monocytes were found to differentiate to macrophages, which are involved in removing bacteria, preventing infection, and contributing to collagen disposition once the inflammation is resolved [93] (Figure 5). Therefore, as a safer, non-hepatoxic androgen, DHT was approved for the first time to induce acceleration of the inflammatory turnover both locally and systemically as the key in promoting major burn injury wound healing without any adverse effects.
Conclusions
This review summarizes the current understanding of the role of androgens in both cutaneous and major burn injury wound healing. The inhibitory role of androgens in cutaneous wound healing has been well-studied with androgens modifying the inflammation and delaying the proliferation phase. In major burn injury, a systemic hypermetabolic state develops and the administration of testosterone or oxandrolone has been clinically reported to improve maintenance of body weight, increase muscle protein metabolism and shorten hospital stay. However, at present, the role of safe androgens in major burn injury wound healing is poorly understood and further investigation into the therapeutic potential of androgens for major burn injury patients are warranted.
