Abstract
Several in vivo studies have reported the presence of immunoreactive transforming growth factor-beta's (TGF-beta's) in testicular cells at defined stages of their differentiation. The most pronounced changes in TGF-beta1 and TGF-beta2 immunoreactivity occurred during spermatogenesis. In the present study we have investigated whether germ cells and Sertoli cells are able to secrete bioactive TGF-beta's in vitro, using the CCl64 mink lung epithelial cell line as bioassay for the measurement of TGF-beta. In cellular lysates, TGF-beta bioactivity was only observed following heat-treatment, indicating that within these cells TGF-beta is present in a latent form. To our surprise, active TGF-beta could be detected in the culture supernatant of germ cells and Sertoli cells without prior heat-treatment. This suggests that these cells not only produce and release TGF-beta in a latent form, but that they also release a factor which can convert latent TGF-beta into its active form. Following heat-activation of these culture supernatant's, total TGF-beta bioactivity increased 6- to 9-fold. Spermatocytes are the cell type that releases most bioactive TGF-beta during a 24 h culture period, although round and elongated spermatids and Sertoli cells also secrete significant amounts of TGF-beta. The biological activity of TGF-beta could be inhibited by neutralizing antibodies against TGF-beta1 (spermatocytes and round spermatids) and TGF-beta2 (round and elongating spermatids). TGF-beta activity in the Sertoli cell culture supernatant was inhibited slightly by either the TGF-beta1 and TGF-beta2 neutralizing antibody. These in vitro data suggest that germ cells and Sertoli cells release latent TGF-beta's. Following secretion, the TGF-beta's are converted to a biological active form that can interact with specific TGF-beta receptors. These results strengthen the hypothesis that TGF-beta's may play a physiological role in germ cell proliferation/differentiation and Sertoli cell function.