Antagonism of Growth Hormone Receptor Suppresses Cancer Growth and Drug Resistance in Mice

Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) play important roles in different stages of progression and drug resistance in many types of cancers, including breast, colon, endometrial, liver cancer and melanoma. GH receptor (GHR) is highly expressed in melanoma and promotes cancer proliferation and multidrug efflux pumps mediated drug resistance. Knockdown of GHR in melanoma cells significantly increased their drug sensitivity in vitro. Thus, a GHR antagonist could become a therapeutic molecule in suppressing melanoma cancer growth and sensitizing the tumor to chemotherapy in vivo. Here, we used GHR antagonist (GHA) transgenic mice which constitutively express a GHA to specifically suppress GH/IGF-1 axis. We found have circulating IGF-1 level was significantly lowered in these mice as a result of GHR antagonism. Furthermore, the sera from the mice could inhibit the growth of melanoma cells in culture. Recombinant GHA produced in our laboratory was able to suppress the phosphorylation of STAT5, a well-established marker of GH action, and the phosphorylation of MAPK, a critical signaling component of cell growth. The GHA mice were intradermally inoculated with mouse melanoma cells (B16-F10) or subcutaneously inoculated with mouse liver cancer cells (Hepa1-6) to generate syngeneic mouse tumor models. We observed that tumor size and tumor weight were markedly reduced and that phosphorylation of STAT5 and MAPK was suppressed in the livers from these mice. In parallel, the activation of GH signaling and the expression level of various types of multidrug efflux pumps were reduced in these tumors. To test the effect of GHA on drug synergy, the GHA mice or WT controls with liver cancer cells were treated with sorafenib or vehicle. Sorafenib is an FDA-approved tyrosine kinase inhibitor, widely used to treat advanced hepatocarcinoma, but has a reduced efficacy in application due to the multidrug efflux pump ABCG2. In vitro, recombinant bovine GH increased the IC50 of sorafenib and the expression of ABCG2. In vivo, GHA mice treated with sorafenib had the smallest tumors compared with WT mice, or in mice treated with sorafenib or GHA alone. Furthermore, ABCG2 mRNA levels were also suppressed in the liver tumors from GHA mice. All these findings from functional and mechanistic investigations confirm that a GHA or Pegvisomant is effective in cancer treatment in vivo and may be a novel therapeutic strategy or molecule to suppress the tumor growth and to sensitize different types of cancers to anti-cancer therapies. Acknowledgments: This work was supported by the State of Ohio’s Eminent Scholar Program that includes a gift from Milton and Lawrence Goll to J.K.; NIH-R01AG059779, the AMVETS, Edison Biotechnology Institute and Diabetes Institute at Ohio University; OURC funding and Baker Fund to Y.Q.; the PURF Fund and the John J. Kopchick Molecular Cell Biology Undergraduate Student Fund to N.A and J.T.