Targeting activins to counteract muscle wasting and cachexia

Cancer cachexia is a condition of severe frailty and fatigue, associated with the loss of skeletal muscle and fat mass. Cachexia constitutes a major unmet medical challenge, as up to 80% of patients with advanced cancers exhibit cachectic symptoms, and as many as 25% of cancer-related mortalities may derive from the complications of cachexia rather than direct tumour burden. This condition is attributed to abnormal metabolism and catabolism, ostensibly induced by tumour- and host-derived cytokines and factors. In multiple models of cancer cachexia, inhibiting the type IIA/B activin receptors (ActRIIA/B) reverses muscle wasting and prolongs survival, even with continued tumour growth. To determine whether activin signalling in the absence of tumour-derived factors induces cachexia, we used recombinant adeno-associated viral vectors serotype-6 (rAAV6) to increase circulating activin A levels in C57BL/6 mice. While mice injected with control vector gained ~10% of their starting body mass (3.8 ± 0.4 g) over ten weeks, mice injected with increasing doses of rAAV6:activin A exhibited weight loss in a dose-dependent manner, to a maximum of 12.4% (-4.2 ± 1.1 g). These reductions in body mass in rAAV6:activin A-injected mice correlated inversely with elevated serum activin A levels (7 to 24-fold). In mice with high serum activin A, the significant loss of skeletal muscle and fat mass correlated with decreases in size of muscle fibres and brown and white adipocytes. The observed liver atrophy was associated with increased hepatocyte inflammation. To determine the relative role of cancer-associated factors, we employed a similar approach to manipulate systemic levels of the pro-inflammatory cytokine IL-6. Cachectic symptoms were only observed with supraphysiological levels of IL-6, further supporting activins as major drivers of this disease. Critically, we demonstrate that the muscle wasting and fibrosis that ensues in response to excessive activin levels is fully reversible. Although soluble ActRIIA/B can reverse activin-induced muscle wasting and cachexia, the fact that they antagonise multiple TGF-β proteins limits their therapeutic potential and highlights the need for new reagents that target specific ActRIIA/B ligands. Here, we modified the activin A and activin B prodomains to generate specific activin antagonists. Initially, the prodomains were fused to the Fc region of mouse IgG2A antibody and, subsequently, “fastener” residues (Lys45, Tyr96, His97 and Ala98; activin A numbering) that confer latency to other TGF-β proteins were incorporated. For the activin A prodomain, these modifications generated a reagent that potently (IC50 5 nM) and specifically inhibited activin A signalling in vitro, and activin A-induced muscle wasting in vivo. Intriguingly, the modified activin B prodomain inhibited both activin A and B signalling in vitro (IC50 ~2 nM) and in vivo, suggesting it can serve as a general activin antagonist. Importantly, unlike soluble ActRIIA/B, the modified prodomains did not inhibit myostatin or GDF-11 activity. To support the therapeutic utility of specifically antagonising activin signalling in muscle wasting and cachexia, we demonstrated that the modified activin prodomains promote significant increases in muscle mass. In conclusion, we showed that activins are potent negative regulators of muscle mass, and activin-specific inhibitors may be novel therapeutics for muscle wasting.