Regulation of parathyroid hormone-related peptide production in normal human mammary epithelial cells in vitro

We have examined the expression and production of parathyroid hormone-related peptide (PTHRP) in primary cultures of normal human mammary epithelial cells (HMEC) derived from nonlactating breast tissue. In response to growth factors such as insulin, insulin-like growth factor I (IGF-I), and epidermal growth factor (EGF), immunoreactive PTHRP was released into conditioned medium, and PTHRP mRNA rapidly increased. In contrast, hydrocortisone and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] inhibited these effects in a dose-dependent manner. Addition of prolactin (PRL) in the presence or absence of insulin, IGF-I, or EGF did not influence PTHRP production during the time course studied. To investigate whether these factors were acting at the transcriptional level, we performed nuclear run-on assays and demonstrated that IGF-I increased PTHRP gene transcription whereas hydrocortisone and 1,25(OH)2D3 inhibited this effect. These studies therefore demonstrate that IGF-I, EGF, 1,25(OH)2D3, and hydrocortisone modulate PTHRP expression in HMEC and that these effects occur in part at the level of gene transcription. Additionally, PRL, a known stimulator of PTHRP expression in vivo, has no effect in this in vitro model.