THE INDIRECT PARTICIPATION OF GROWTH-HORMONE IN THE THYMOCYTE PROLIFERATION SYSTEM

The specific binding sites for growth hormone was recognized on both t hymic lymphocytes (thymocytes) and thymus epithelial cells. The somato tropic action of GH is considered to be mediated mainly by insulin-lik e growth factor-1 (IGF-1) and partially by GH itself. In this study, e ffect of GH and IGF-1 on DNA synthetic activity of thymocytes was exam ined. By 48-hrs. culture with IGF-1 and 24-hrs. H-3-TdR pulse labeling , significant enhancement of DNA synthetic activity of thymocytes was detected, while the enhancement in the culture with GH was very weak. It is considered that IGF-1 acts on the cells in G0/G1 phase in cell c ycle. The effect of IGF-1 on thymocyte proliferation was examined by u sing the thymocytes incubated 12 hrs. before IGF-1 stimulation and 3-h rs. H-3-TdR pulse labeling. The optimal condition to induce thymocyte proliferation was 15-hrs. culture with 380 ng/ml of IGF-1. Furthermore , 10 ng/ml or higher concentration of GH significantly increases IGF-1 release from TECs in confluent state. These results suggest that GH i ndirectly participates in thymus growth by increasing IGF-1 release fr om TECs, which enhances thymocyte proliferation.