MON-475 Growth Hormone Induces Colon Tissue DNA Damage Independently of IGF-1

Environmental insults and aging lead to DNA damage, which, if unrepaired, results in chromosomal instability and tumorigenesis. We have shown that GH suppresses phosphorylation of ATM and its target proteins p53, Chk2, and H2AX in DNA damaged non-tumorous human colon cells. We show that GH decreased DNA repair by non-homologous end-joining and DNA damage accumulation, enhancing cell transformation as evidenced by increased growth in soft agar. Prolonged high circulating GH levels in mice bearing human GH-secreting colon cell xenografts resulted in 60% increase in unrepaired DNA damage and increased number of metastatic lesions. We sought to elucidate whether GH effects on DNA damage are mediated through induced IGF-1. In non-tumorous human colon cells treated with etoposide, GH but not IGF-1 showed enhanced DNA damage. Stable disruption of IGF-1R by lentivirus expressing shRNA in vitro, or treatment with IGF-1R inhibitor PPP in vitro and in vivo, markedly induced GH receptor expression, rendering cells more sensitive to effects of GH. Suppressing IGF-1R in normal human colon cells triggered DNA damage, while when cells with disrupted IGF-1R were treated with GH, levels of DNA damage were further increased, indicating that GH effects on DNA damage were not mediated by IGF-1R, but likely by induced GH receptor. Similar results were obtained when 3-dimensional human intestinal organoid cultures were treated with PPP. To confirm these observations in vivo, nude mice were injected with GH-secreting xenografts and 7 weeks later treated with PPP (20 mg/kg, i.p., twice daily, total 4 injections). Similar to results observed in vitro, in mice with high circulating GH colon DNA damage were significantly increased when IGF-1R was disrupted by PPP, and GH receptor levels were again induced. To further confirm that GH effects on DNA damage are directly mediated by GH signaling, GHR(-/-) mice were injected with PPP to block IGF-1R, resulting in increased colon DNA damage in WT mice, but animals lacking GHR did not respond to induced DNA damage in the face of IGF-1 suppression. The results imply that GH directly induces DNA damage independently of IGF-1.