Sertoli cell-secreted protein(s) stimulates DNA synthesis in purified rat Leydig cells in vitro

We have examined the effects of Sertoli cell-secreted proteins (SCSP) on [³H]thymidine incorporation by purified preparations (> 96%) of rat Leydig cells to determine whether Sertoli cells influence DNA synthesis in these cells in vitro. Incubation of Leydig cells isolated from testes of rats of ages 16 to 90 days with SCSP (M(r) > 10,000) induced significant dose-, time- and age-related increases in [³H]thymidine incorporation by the cells. A dose-response curve to SCSP showed that as little as 0.2 μg SCSP/ml consistently induced a small but significant increase (31% and 10% above control; P < 0.001) in [³H]thymidine incorporation by Leydig cells isolated from immature (26 days) and mature (70 days) rats, respectively. The maximum response (230% and 48% above control) was obtained with a concentration of 18 μg SCSP/ml in cells isolated from immature and mature rats, respectively. Hydroxyurea, a specific inhibitor of replicative DNA synthesis, significantly (P < 0.001) inhibited both basal and SCSP-induced [³H]thymidine incorporation in Leydig cells from immature and adult rats without affecting the viability of the cells. Incubation of immature rat Leydig cells in SCSP for 48 h also stimulated a 3-fold increase in cell number. The component of the crude SCSP which stimulated Leydig cell [³H]thymidine incorporation is trypsin-sensitive, heat-stable, and adsorbs to a heparin-agarose affinity column but not to concanavalin A-Sepharose. The secretion of this factor(s) by Sertoli cells is stimulated independently by FSH and testosterone. These results demonstrate for the first time that cultured Sertoli cells secrete a protein(s) which, in vitro, stimulates rat Leydig cell replicative DNA synthesis.