2 -activated transcriptional expression of the hPRLR through PIII. Estradiol produced in breast stroma and adipose tissue, which are major sources of estrogen in post-menopausal women, could up-regulate hPRLR gene expression and stimulate breast tumor growth.
Prolactin (PRL),2 a hormone of the lactogen/cytokine family, is produced in the anterior pituitary gland and exerts diverse cellular actions through its transmembrane receptors (PRLR) in several target tissues. Prolactin is a major factor in the proliferation and differentiation of breast epithelium and is the primary hormone in the stimulation and maintenance of lactation. Prolactin is a tumor promoter in rodents (1, 2) and has been implicated in the development of breast cancer. The hPRLR has several forms, including long form (stimulatory) and short form(s) (inhibitory) (3, 4), which are expressed in normal and tumoral breast tissue and in most human breast cancer cells (5-7), and prolactin exhibits synergistic actions with estrogens on the proliferation of these cells (8). There is local production of PRL in mammary epithelial cells, and increased expression of the PRLR long form occurs in a significant number of human mammary tumors (9 -12). A lower ratio of short (inhibitory forms)/long (activating form) receptors reported in breast tumor tissues could cause unopposed prolactin-mediated stimulatory actions of the long form and may contribute to breast tumor development and progression (7). Moreover, the PRL antagonist G129R was reported to cause apoptosis in breast cancer cells (8, 13). These findings, and the correlation between serum PRL and the incidence and progression of breast tumors (11,12,14), indicate that PRL has a role in human breast cancer. Stromal and adipose tissue are the major sources of estrogen in post-menopausal women, and could exert paracrine control of prolactin and prolactin receptor expression in adjacent mammary epithelial cells.Our previous studies on the hPRLR gene revealed its complex 5Đ genomic structure, with multiple (six) alternative non-coding exons 1 and promoter utilization (15,16). These include the preferentially utilized, generic promoter 1/exon-1 (PIII/hE1 3 ), which is also present in rat and mouse, and five human specific exon-1/promoters (hE1 N1-5 ) (15-17). These forms were found to be expressed in breast cancer cells, and variably in other tissues (16). Quantitative competitive reverse transcriptase-PCR analysis showed that E 2 induced increases of PRLR non-coding exon-1 hE1 3 (generic) mRNA transcripts directed by promoter III (hPIII) in breast cancer cells. Also, in transfection studies E 2 activated the hPIII promoter (18). This promoter contains functional Sp1 and C/EBP sites that bind Sp1/Sp3 and C/EBPâ€, respectively (16,19). The lack of a formal ERE in the hPIII promoter suggested that the effect of estradiol is mediated through association of the activated ER with relevant DNA binding transfactors. Thus, although there is not a classical ERE within the hPRLR promoter/5Đ-flanking region of hPIII, our ...