Subclinical hypothyroid men characterized by a rise in only thyroid stimulating hormone (TSH) levels and normal thyroid hormone levels showed a fall in their serum progesterone and testosterone levels. This suggested a role of TSH in regulating Leydig cell steroidogenesis. Therefore, we investigated the direct role of TSH on steroid production and secretion using a mouse Leydig tumour cell line, MLTC-1. MLTC-1 cells were treated with different doses of TSH isolated from porcine pituitary as well as recombinant TSH. Steroid secretion was measured by radioimmunoassay. The mRNA levels of steroidogenic enzymes were quantitated by real time PCR whereas the corresponding protein levels were determined by Western blot. In MLTC-1 cells, pituitary TSH as well as recombinant TSH inhibited progesterone and testosterone secretion in a dose dependent manner. The inhibitory action of TSH on steroid secretion was unique and not mimicked by other anterior pituitary hormones including FSH and ACTH. Recombinant TSH showed no effect on StAR and CYP11A1, the enzymes catalysing the non-steroidogenic and steroidogenic rate-limiting steps of steroid synthesis respectively. Recombinant TSH was shown to inhibit steroidogenesis in MLTC-1 cells by inhibiting the 3β hydroxy steroid dehydrogenase mRNA and protein levels, the enzyme that catalyses the conversion of pregnenolone to progesterone. This inhibitory effect of TSH is probably direct as both mRNA and protein of the TSH receptor were shown to be present in the MLTC-1 cells.

Keywords: Leydig cells; TSH/TSH receptor; steroidogenesis.