Attenuation of Skeletal Muscle Wasting with Recombinant Human Growth Hormone Secreted from a Tissue-Engineered Bioartificial Muscle

Abstract

Skeletal muscle wasting is a significant problem in elderly and debilitated patients. Growth hormone (GH) is an anabolic growth factor for skeletal muscle but is difficult to deliver in a therapeutic manner by injection owing to its in vivo instability. A novel method is presented for the sustained secretion of recombinant human GH (rhGH) from genetically modified skeletal muscle implants, which reduces host muscle wasting. Proliferating murine C2C12 skeletal myoblasts stably transduced with the rhGH gene were tissue engineered in vitro into bioartificial muscles (C2-BAMs) containing organized postmitotic myofibers secreting 3–5 μg of rhGH/day in vitro. When implanted subcutaneously into syngeneic mice, C2-BAMs delivered a sustained physiologic dose of 2.5 to 11.3 ng of rhGH per milliliter of serum. rhGH synthesized and secreted by the myofibers was in the 22-kDa monomeric form and was biologically active, based on downregulation of a GH-sensitive protein synthesized in the liver. Skeletal muscle disuse atrophy was induced in mice by hindlimb unloading, causing the fast plantaris and slow soleus muscles to atrophy by 21 to 35% (p < 0.02). This atrophy was significantly attenuated 41 to 55% (p < 0.02) in animals that received C2-BAM implants, but not in animals receiving daily injections of purified rhGH (1 mg/kg/day). These data support the concept that delivery of rhGH from BAMs may be efficacious in treating muscle-wasting disorders. Numerous human disorders that result from alterations in the secretory levels of protein hormones and/or growth factors might be effectively treated if a physiological method for their chronic delivery were available. Cell-based delivery of such proteins from genetically modified cells that have been tissue engineered into long-lived implantable “devices” holds significant promise for the future. Most of these proteins turn over rapidly (on the order of minutes) and implanting a living device that constantly synthesizes and secretes new protein would have significant advantages over currently available methods, both from a cost and patient compliance basis. Differentiated skeletal myofibers are ideal for such applications because they are postmitotic and non-migratory, long-lived (from birth to death), and have a high protein synthetic capacity. While much research remains before the processes of genetic and tissue engineering can be combined to treat human disorders, the results from the present study show the feasibility of this technology for attenuation of skeletal muscle wasting.