Search

In cancer, elevated activin levels promote cachectic wasting of muscle, irrespective of tumor progression. In excess, activins A and B use the myostatin signaling pathway in muscle, triggering a decrease in protein synthesis and an increase in protein degradation, which ultimately leads to atrophy. Recently, we demonstrated that local delivery of engineered activin and myostatin propeptides (natural inhibitors of these growth factors) could induce profound muscle hypertrophy in healthy mice. Additionally, the expression of these propeptides effectively attenuated localized muscle wasting in models of dystrophy and cancer cachexia. In this study, we examined whether a systemically administered recombinant propeptide could reverse activin A-induced cachectic wasting in mice. Chinese hamster ovary cells stably expressing activin A were transplanted into the quadriceps of nude mice and caused an 85-fold increase in circulating activin A levels within 12 days. Elevated activin A induced a rapid reduction in body mass (-16%) and lean mass (-10%). In agreement with previous findings, we demonstrated that adeno-associated virus-mediated delivery of activin propeptide to the tibialis anterior muscle blocked activin-induced wasting. In addition, despite massively elevated levels of activin A in this model, systemic delivery of the propeptide significantly reduced activin-induced changes in lean and body mass. Specifically, recombinant propeptide reversed activin-induced wasting of skeletal muscle, heart, liver, and kidneys. This is the first study to demonstrate that systemic administration of recombinant propeptide therapy effectively attenuates tumor-derived activin A insult in multiple tissues., (Copyright © 2019 Endocrine Society.)

Cachexia is a life-threatening wasting syndrome lacking effective treatment, which arises in many cancer patients. Although ostensibly induced by multiple tumor-produced cytokines (tumorkines), their functional contribution to initiation and progression of this syndrome has proven difficult to determine. In this study, we used adeno-associated viral vectors to elevate circulating levels of the tumorkines IL6 and/or activin A in animals in the absence of tumors as a tactic to evaluate hypothesized roles in cachexia development. Mice with elevated levels of IL6 exhibited 8.1% weight loss after 9 weeks, whereas mice with elevated levels of activin A lost 11% of their body weight. Co-elevation of both tumorkines to levels approximating those observed in cancer cachexia models induced a more rapid and profound body weight loss of 15.4%. Analysis of body composition revealed that activin A primarily triggered loss of lean mass, whereas IL6 was a major mediator of fat loss. Histologic and transcriptional analysis of affected organs/tissues (skeletal muscle, fat, and liver) identified interactions between the activin A and IL6 signaling pathways. For example, IL6 exacerbated the detrimental effects of activin A in skeletal muscle, whereas activin A curbed the IL6-induced acute-phase response in liver. This study presents a useful model to deconstruct cachexia, opening a pathway to determining which tumorkines are best targeted to slow/reverse this devastating condition in cancer patients. Cancer Res; 76(18); 5372-82. ©2016 AACR., (©2016 American Association for Cancer Research.)

Soluble activin type II receptors (ActRIIA/ActRIIB), via binding to diverse TGF-β proteins, can increase muscle and bone mass, correct anemia or protect against diet-induced obesity. While exciting, these multiple actions of soluble ActRIIA/IIB limit their therapeutic potential and highlight the need for new reagents that target specific ActRIIA/IIB ligands. Here, we modified the activin A and activin B prodomains, regions required for mature growth factor synthesis, to generate specific activin antagonists. Initially, the prodomains were fused to the Fc region of mouse IgG2A antibody and, subsequently, "fastener" residues (Lys(45), Tyr(96), His(97), and Ala(98); 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 (IC(50) 5 nmol/l) and specifically inhibited activin A signaling in vitro, and activin A-induced muscle wasting in vivo. Interestingly, the modified activin B prodomain inhibited both activin A and B signaling in vitro (IC(50) ~2 nmol/l) and in vivo, suggesting it could serve as a general activin antagonist. Importantly, unlike soluble ActRIIA/IIB, the modified prodomains did not inhibit myostatin or GDF-11 activity. To underscore the therapeutic utility of specifically antagonising activin signaling, we demonstrate that the modified activin prodomains promote significant increases in muscle mass.

In models of cancer cachexia, inhibiting type IIB activin receptors (ActRIIBs) reverse muscle wasting and prolongs survival, even with continued tumor growth. ActRIIB mediates signaling of numerous TGF-β proteins; of these, we demonstrate that activins are the most potent negative regulators of muscle mass. To determine whether activin signaling in the absence of tumor-derived factors induces cachexia, we used recombinant serotype 6 adeno-associated virus (rAAV6) vectors 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 10 wk, 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-injected mice correlated inversely with elevated serum activin A levels (7- to 24-fold). Mechanistically, we show that activin A reduces muscle mass and function by stimulating the ActRIIB pathway, leading to deleterious consequences, including increased transcription of atrophy-related ubiquitin ligases, decreased Akt/mTOR-mediated protein synthesis, and a profibrotic response. Critically, we demonstrate that the muscle wasting and fibrosis that ensues in response to excessive activin levels is fully reversible. These findings highlight the therapeutic potential of targeting activins in cachexia.

Throughout development, activin A signaling stimulates proliferation and inhibits differentiation of testicular Sertoli cells. A decline in activin levels at puberty corresponds with the differentiation of Sertoli cells that is required to sustain spermatogenesis. In this study, we consider whether terminally differentiated Sertoli cells can revert to a functionally immature phenotype in response to activin A. To increase systemic activin levels, the right tibialis anterior muscle of 7-wk-old C57BL/6J mice was transduced with an adeno-associated virus (rAAV6) expressing activin A. We show that chronic activin signaling reduces testis mass by 23.5% compared with control animals and induces a hypospermatogenic phenotype, consistent with a failure of Sertoli cells to support spermatogenesis. We use permeability tracers and transepithelial electrical resistance measurements to demonstrate that activin potently disrupts blood-testis-barrier function in adult mice and ablates tight junction formation in differentiated primary Sertoli cells, respectively. Furthermore, increased activin signaling reinitiates a program of cellular proliferation in primary Sertoli cells as determined by 5-ethynyl-2'-deoxyuridine incorporation. Proliferative cells reexpress juvenile markers, including cytokeratin-18, and suppress mature markers, including claudin-11. Thus, activin A is the first identified factor capable of reprogramming Sertoli cells to an immature, dedifferentiated phenotype. This study indicates that activin signaling must be strictly controlled in the adult in order to maintain Sertoli cell function in spermatogenesis.