Myostatin (MSTN) is a transforming growth factor-β (TGF-β) family member that normally acts to limit muscle mass (for review, see [4
]). Mutations in the Mstn
gene have been shown to result in dramatic increases in muscle mass in multiple species [5
], and inhibitors of MSTN signalling have been shown to cause increases in muscle growth when administered systemically to adult mice [12
]. As a result, there have been extensive efforts directed at developing strategies and agents capable of modulating this signalling pathway for applications in a wide range of clinical settings. The finding that overexpression of MSTN in mice could lead to the development of a cachexia-like syndrome characterized by an extensive loss of fat and muscle [16
] raised two important questions about potential therapeutic applications of targeting this pathway in patients with cachexia. First, does the inappropriate activation of this pathway play a causative role in the development of cachexia in humans? Second, whether or not this signalling pathway is involved in the aetiology of cachexia, can blocking this pathway to preserve muscle mass thereby reduce morbidity and mortality in patients with cachexia?
Benny-Klimek et al
] and Zhou et al
] examined the effect of blocking the MSTN pathway in mice bearing cachexia-inducing tumours. Previous studies had shown that MSTN signals by binding initially to activin type II receptors [17
] and that a soluble form of the activin type IIB receptor (ActRIIB or ACVR2B), consisting of its ligand binding domain fused to an immunoglobulin Fc domain, can inhibit signalling of MSTN and other TGF-β family secreted proteins that signal via the ActRII receptors, such as the activins [15
]. Moreover, the soluble receptor had been shown to cause significant muscle growth (40%-60% in just 2 weeks) when administered systemically to adult mice [15
], presumably by acting as a 'trap' of the circulating ligands, binding them in serum and, thereby, preventing binding and activation of the cellular receptor complexes (Figure ).
Figure 1 Inhibition of myostatin (MSTN) and activin signalling by the soluble activin type IIB receptor (ActRIB). MSTN and activin signal to target cells by binding initially to the two activin type II receptors, ActRIIA and/or ActRIIB (also called Acvr2 and Acvr2b, (more ...)
Either by inoculating mice with Chinese hamster ovary cells expressing this soluble receptor [2
] or by injecting mice directly with the purified fusion protein [3
], the two groups showed that blocking this signalling pathway was effective in preserving muscle mass in a wide range of muscle groups, as well as in maintaining forelimb grip strength in mice bearing various tumour cells known to induce wasting. Interestingly, this protective effect was not observed when a different pharmacological agent was used to block this pathway, namely, the deacetylase inhibitor, trichostatin A (TSA). Although previous studies had shown that TSA could increase the expression of the MSTN antagonist, follistatin [18
], Benny-Klimek et al
. found that treatment of tumour-bearing mice with TSA could not prevent muscle loss even at doses capable of inducing muscle growth in normal mice. In addition to these effects on skeletal muscle, Zhou et al
. reported that treatment with the soluble receptor could also prevent cardiac muscle atrophy in tumour-bearing mice. This finding is particularly significant, as there have been concerns that blocking MSTN signalling for clinical applications might have adverse effects on cardiac function. Perhaps the most spectacular result was the finding by Zhou et al
. that the soluble receptor was capable of increasing survival of mice inoculated with colon-26 (C26) carcinoma cells even though the intervention had no effect on actual tumour growth. Hence, these studies have provided exciting and compelling data that blocking muscle wasting per se
can have significant beneficial effects on both morbidity and mortality in the setting of cancer and that agents capable of blocking this signalling pathway may be effective means of achieving this end.
What is less clear is whether these studies get us any closer to understanding the role that this signalling pathway may play in the aetiology of cachexia. The fact that muscle mass is preserved by blocking this pathway does not necessarily mean that overactivity of the pathway is responsible for inducing wasting. Blocking MSTN signalling with agents like the soluble receptor is known to induce significant muscle growth, and it could be that these anabolic effects may simply be compensating for the muscle wasting that is being induced by activation of other pathways. In this respect, Zhou et al
. found Mstn
messenger RNA (mRNA) levels in muscle to be elevated by about two-fold in C26 tumour-bearing mice. However, Benny-Klimek et al
. showed using two different tumour lines (Lewis lung carcinoma and B16F10 melanoma) that Mstn
knockout mice are not only susceptible to tumour-induced wasting but, for reasons that are unclear, actually appear to exhibit an exaggerated response. Zhou et al
. further examined the role of this signalling pathway by focusing on the possibility that the culprit in cancer cachexia may not be MSTN itself but activins, which are TGF-β family members capable of signalling through the same receptors as MSTN (for review, see [20
]). Previous studies had shown that several TGF-β family members, including activins, are as active as MSTN in inhibiting myoblast differentiation, acting through the ActRIIB pathway [21
], and that electroporation of an activin A expression cassette directly into muscle can induce myofiber atrophy [22
], suggesting that activin A and MSTN may be capable of activating the same signalling cascade leading to wasting. Zhou et al
. present two sets of studies examining the possible role that activins may play in inducing cancer cachexia.
In one set of studies, they utilized a genetic model of tumourigenesis in which the normal balance of inhibin/activin signalling had been disrupted by a targeted mutation in the Inha
gene, which encodes the inhibin-α subunit. Inhibins and activins, which generally have counteracting biological activities, are dimers that differ with respect to their subunit composition, with inhibins consisting of α and β subunits and activins consisting of just β subunits. Hence, mice lacking inhibin-α have excess levels of activin signalling, and previous studies have demonstrated that these mice develop both gonadal and adrenal tumours and exhibit a cachexia-like syndrome characterized by severe weight loss, hepatocellular necrosis and gastric mucosal atrophy [23
]. Zhou et al
. show that these mice also develop skeletal muscle atrophy and that this muscle wasting can be blocked by administering the soluble activin type IIB receptor. Although these studies demonstrate that excess activin activity can ultimately lead to muscle loss, additional studies are needed in order to help us to understand the relevance of these findings to what may be happening in cancer cachexia. In particular, it will be important to determine the extent to which the effects on muscle seen in this model reflect excess signalling of activin directly to muscle versus activation of atrophy-inducing pathways as a result of tumour development. The interpretation of these studies is somewhat complicated by the fact that the development of tumours in these mice is itself dependent on activin signalling; that is, blocking activin activity using a soluble form of a different activin type II receptor had previously been shown to block not only the cachexia-like syndrome in these mice but also tumour progression [25
In a second set of studies, Zhou et al. surveyed a number of human tumours and identified several that express high levels of activin A. They went on to show that two of these tumour lines could induce muscle loss when inoculated into nude mice and that this wasting process could, again, be blocked by administering the soluble receptor. Although these findings were consistent with the model that increased activin signalling played a causative role in inducing wasting, it will be important to carry out additional studies to further characterize these human tumours with respect to their ability to induce cachexia. For example, is there is correlation between expression levels of activin in the tumour lines and their ability to induce wasting? Similarly, does blocking activin expression in a given tumour line abrogate its ability to induce wasting? Furthermore, as in the case of the tumours that developed in the inhibin-α knockout mice, Zhou et al. reported that the soluble receptor also suppressed the growth of the human tumours in mice, making it difficult to attribute the wasting process to the direct effects of activin on muscle.