Effects of sex steroids on the proliferation of thymic epithelial cells in a culture model: A role of protein kinase C

Using a rat thymic epithelial cell line (TEC; IT‐45R1), the present study attempted to elucidate the mechanism of action of sex steroid hormones (SH) on the proliferation of TEC The findings were as follows: (a) the proliferation of TEC in response to SH was mediated through protein kinase C activity introduced as a result of interaction between SH and plasma‐borne inhibitors; (b) the strong inhibitory effect of SH on TEC proliferation might be mediated through the SH receptor pathway because the proliferative response was triggered by progesterone (P) and androgen (A), whereas the inhibitory response was triggered by P, A and oestrogen. These results clearly suggest that the control of TEC proliferation is a ‘shut‐off’ mechanism triggered by high plasma levels of SH. This further refers to the speculation that the development of the normal thymus may be due to a lack of this ‘shut‐off’ mechanism so that development occurs at the adequate plasma SH levels that are often observed before puberty. However, this development is inhibited at the high plasma SH levels after puberty and/or during pregnancy.