REGULATION OF PARATHYROID HORMONE-RELATED PEPTIDE PRODUCTION INVITRO BY THE RAT HYPERCALCEMIC LEYDIG-CELL TUMOR H-500

The transplantable rat Leydig cell tumor H-500 is known to cause hypercalcemia in vivo by the release of abundant PTH-related peptide (PTHRP) and to closely reproduce the human syndrome of malignancy-associated hypercalcemia. In the rat only a single messenger RNA species of 1.4 kilobases is expressed which encodes a peptide of 141 amino acid as the sole molecular form. We have examined in cultured rat Leydig tumor cells H-500, the capacity of multiple factors to regulate PTHRP messenger RNA expression and secretion. Both fetal bovine serum and epidermal growth factor stimulated PTHRP gene expression and secretion into conditioned culture medium. Dexamethasone and 1,25-dihydroxyvitamin D3 produced inhibition of PTHRP gene expression and secretion. Furthermore, in these testicular cells, after 12 h or more of incubation, testosterone produced a dose-dependent (10(-9)-10(-7) M) inhibition of PTHRP production. No significant difference in this inhibitory response was seen between testosterone and its 5alpha-reduced metabolite dihydrotestosterone whereas 17beta-estradiol, progesterone, LH, FSH, and PRL were ineffective. An androgen receptor antagonist Win 49596 blocked the androgen-mediated inhibition of PTHRP gene expression and secretion, but not that due to dexamethasone. Epidermal growth factor caused an increase, whereas androgen caused a decrease in PTHRP gene transcription. These studies demonstrated that growth factors, dexamethasone, and 1,25-dihydroxyvitamin D3 are broadly active regulatory agents of PTHRP production which cross species and tissue barriers. Testosterone may be a more selective modulator which can regulate PTHRP in tissues such as Leydig cell neoplasms which express the androgen receptor.