Characterization and hormonal modulation of immunoreactive thiamin carrierprotein secreted by adult vat Leydig cells in vitro

Leydig cells isolated from adult rats and maintained under defined conditio ns in culture secrete a protein of molecular weight (M-r) 70 000 which is i mmunologically similar to chicken thiamin carrier protein (TCP). Synthesis of immunoreactive TCP by these cells is demonstrated by immunoprecipitation of [S-35]methionine incorporated, newly synthesized proteins with monoclon al and polyclonal antibodies to chicken TCP. The amount of immunoreactive T CP secreted into the culture supernatant is quantitated by using a specific radioimmunoassay. Under the influence of LH, secretion of immunoreactive T CP is enhanced 3-fold and can be inhibited by up to 70% with aromatase inhi bitor (1,4,6-androstatrien-3,17-dione). Cyclic AMP acts as a second messeng er in the sequence of events involved in LH-induced elevation of immunoreac tive TCP in Leydig cells. The effects of exogenous estradiol-17 beta and di ethylstilbestrol are comparable in terms of stimulation of secretion immuno reactive TCP by these cells. Tamoxifen brought about a 70% decrease in the elevated levels of immunoreactive TCP. These results suggest that estrogen mediates immunoreactive TCP induction in hormonally stimulated adult rat Le ydig cells.