MITOGENIC AND RECEPTOR ACTIVITIES OF HUMAN GROWTH HORMONE-108-129

We have selectively synthesized a number of peptides encompassing the region of helix 3 of growth hormone (GH). These peptides and native hu man (h) GH have been evaluated for mitogenic and receptor activities i n 3T3-F442A preadipocytes. In this system, wild type hGH is anti-mitog enic. In contrast, hGH 108-129 stimulated DNA synthesis while other GH -derived peptides were ineffective. hGH (L) 108-129 had an EC50 of abo ut 0.2 nM and was maximally effective at about 0.5 nm in stimulating [ H-3]thymidine incorporation in 3T3-F442A cells. hGH (L) 108-129 was mi togenically as active as insulin-like growth factor-I and more active than insulin. It was less effective than transforming growth factor-be ta. By cell cycle analysis, hGH (L) 108-129 increased the proportion o f cells in S/G2/M phases to 28%. hGH, when coincubated with hGH (L) 10 8-129, blocked the mitogenic response of the peptide. A monoclonal ant ibody to the GH receptor significantly reduced binding of I-125-hGH to its receptor but had no effect on binding of I-125-hGH (L) 108-129. A ffinity cross-linking of I-125-hGH to its receptor was not duplicated with I-125-hGH (L) 108-129. No other GH peptides or insulin competed f or binding of I-125-hGH 108-129. Scatchard analysis indicated a K(d) o f 5.2 nM with 5.6 x 10(5) binding sites/cell for hGH (L) 108-129. Thes e studies indicate that hGH (L) 108-129, a sequence encompassing helix 3 of hGH, acts by binding to a site other than the GH receptor and ev okes high mitogenic responses.