Angiotensin-(1-7) Attenuates Skeletal Muscle Fibrosis and Stiffening in a Mouse Model of Extremity Sarcoma Radiation Therapy

Background: Radiation-induced fibrosis (RIF) of musculoskeletal tissue is a common complication of radiation therapy for extremity soft-tissue sarcoma, with no standardized strategy for prevention and treatment. Angiotensin-(1-7) (Ang-[1-7]), a well-tolerated endogenous heptapeptide hormone with antitumor and antifibrotic properties, was tested as a radioprotectant for RIF and stiffening of irradiated muscles. Methods: Male CD-1 mice were randomized to one of three treatment groups: control, simulated sarcoma radiation therapy to the gastrocnemius and soleus muscles, or radiation therapy along with continuous Ang-(1-7) delivery initiated three days before radiation therapy. The biologically equivalent dose of radiation (∼100.3 Gy) absorbed by normal musculature during the course of radiation therapy for extremity sarcoma was delivered by means of four dose fractions of 7.3 Gy over two weeks. Fibrosis (n = 5 per group) and mechanical properties (n = 4 to 6 per group) of the muscles were measured at six weeks and four months after radiation therapy, and the intramuscular concentration of the profibrotic cytokines transforming growth factor-beta (TGF-β) and connective tissue growth factor (CTGF) (n = 8 to 10 per group) were measured at six weeks. Results: Interstitial (p < 0.01) and perivascular (p < 0.05) fibrosis increased significantly in the muscles treated with radiation therapy alone versus the nonirradiated controls at both six weeks (interstitial, +89%; perivascular, +112%) and four months (interstitial, +154%; perivascular, +88%). The muscles treated with radiation alone also exhibited increased tension (p < 0.01) versus nonirradiated controls at both six weeks (+779%) and four months (+1761%) when placed under 5% strain, and at four months (+1390%; p < 0.001) under 10% strain. At four months, muscle stiffness had increased in the mice treated with radiation therapy alone (+90%; p = 0.002) compared with nonirradiated controls. TGF-β production was also greater in this group at six weeks (+37%; p = 0.06) versus control. Ang-(1-7) administration prevented RIF and stiffening, with no differences observed for any other outcome between those receiving radiation therapy with Ang-(1-7) and the nonirradiated controls. Likewise, Ang-(1-7) mitigated the increase in TGF-β and CTGF concentration from radiation therapy. Conclusions: Ang-(1-7) attenuated RIF, stiffening, and production of profibrotic cytokines that were elevated in mouse skeletal muscles after simulated radiation therapy for extremity sarcoma. Clinical Relevance: Ang-(1-7) may serve as a potential therapy for the prevention of RIF in patients who require radiation therapy as adjuvant treatment for soft-tissue sarcoma.