Effect of N-terminal modified analogs of growth hormone on collagen synthesis in avian skin fibroblasts

Human growth hormone (hGH) inhibits α1(I) collagen gene expression in cultured avian skin fibroblasts resulting in a decrease in the amount of collagenase-digestible proteins (CDP) in the medium. In addition, a synergism exists between GH and insulin-like growth factor-I (IGF-I) in their effect on CDP. Four N-terminal modified hGH analogs were tested for their ability to affect collagen metabolism in these cells. The truncated analog Des-7 hGH(R8M, D11A) was found to be a strong antagonist of the hGH-induced inhibition of the collagen synthesis but by itself did not inhibit collagen α1(I) gene expression or modify the CDP appearance in the medium. Some synergism between Des-7 hGH and IGF-I was observed. The analog Met-hGH(R19H, L20P), in which Arg¹⁹ was replaced by histidine, and Leu²⁰ by proline was only partially potent compared with the native hormone in causing inhibition of collagen gene expression, in attenuating CDP appearance in the medium, and in antagonizing hGH. However, this analog was as potent as hGH in its ability to synergize with IGF-I. The importance of His¹⁸ was assessed by testing the response to Met-hGH(H18D), in which His¹⁸ was replaced by Asp, and to Met-hGH(H18Q), in which His¹⁸ was replaced by glutamine (as in chicken GH sequence). Substitution of His¹⁸ by a negatively charged amino acid abolished all the hormone activities tested whereas substitution with glutamine restored only part of the activity. These results demonstrate the importance of the N-terminal sequence of the GH molecule and suggest that various effects of GH are transduced through diverse pathways, each produced by structurally different sites in the hormone molecule.