Production of immunoreactive forms of growth hormone by the Burkitt tumor serum-free cell line sfRamos

Abstract

Tumor B-cell mitogenesis may be regulated by autocrine secretions that cross-react immunologically with neuroendocrine peptides. This hypothesis is supported by our recent report that immunoreactive human PRL (ir-hPRL) is produced by and required for the continuous growth of sfRamos, a Burkitt tumor serum-free cell line. Further support for the hypothesis is provided in this study. The data illustrate that antiserum immunoglobulin fraction G (IgG) to synthetic human GH (NIDDK anti-synth hGH IC-4), but not nonimmune serum IgG, completely inhibited sfRamos proliferation. In addition, anti-synth hGH, but not nonimmune serum, identified several polypeptides in sfRamos spent medium that were not in serum-free control medium, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions and the Western immunoblot procedure. The ir-hGH polypeptides were in the 35- to more than 300-kilodalton (kDa) range, with a 48-kDa polypeptide as a major component. A polypeptide with electrophoretic properties identical to those of the hGH monomer (22 kDa) was not detected, even with prior incubation of spent medium with N-ethylmaleimide and dithiothreitol. Pituitary preparations of hPRL and hGH, each with the monomer form as the major component, were neither mitogenic for growth-arrested cells nor did they synergize with spent medium to enhance growth, either alone or in combination, whereas spent medium with immunoreactive polypeptides larger than the monomer form markedly stimulated proliferation of growth-arrested cells. These findings demonstrate that sfRamos mitogenesis is potentially regulated by autocrine secretions with cross-immunoreactivity to two anterior pituitary hormones.