Unmodified prolactin (PRL) and S179D PRL-initiated bioluminescence resonance energy transfer between homo- and hetero-pairs of long and short human PRL receptors in living human cells

We have used bioluminescence resonance energy transfer (BRET) to examine the interaction between human prolactins (PRLs) and the long (LF) and two short isoforms (SF1a and SF1b) of the human PRL receptor in living cells. cDNA sequences encoding the LF, SF1a, and SF1b were subcloned into codon-humanized vectors containing cDNAs for either Renilla reniformis luciferase (Rluc) or a green fluorescent protein (GFP(2)) with a 12- or 13-amino acid linker connecting the parts of the fusion proteins. Transfection into human embryonic kidney 293 cells demonstrated maintained function of Rluc and GFP(2) when linked to the receptors, and confocal microscopy demonstrated the localization of tagged receptors in the plasma membrane by 48 h after transfection. All three tagged receptors transduced a signal, with the LF and SF1a stimulating, and SF1b inhibiting, promoter activity of an approximately 2.4-kb beta-casein-luc construct. Both unmodified PRL (U-PRL) and the molecular mimic of phosphorylated PRL, S179D PRL, induced BRET with all combinations of long and short receptor isoforms except SF1a plus SF1b. No BRET was observed with the site two-inactive mutant, G129R PRL. This is the first demonstration, 1) that species homologous PRL promotes both homo- and hetero-interaction of most long and short PRLR pairs in living cells, 2) that both U-PRL and S179D PRL are active in this regard, and 3) that there is some aspect of SF1a-SF1b structure that prevents this particular hetero-receptor pairing. In addition, we conclude that preferential pairing of different receptor isoforms is not the explanation for the different signaling initiated by U-PRL and S179D PRL.     

Trichostatin A inhibits beta-casein expression in mammary epithelial cells.

Many aspects of cellular behavior are defined by the content of information provided by association of the extracellular matrix (ECM) and with cell membrane receptors. When cultured in the presence of laminin-containing ECM and prolactin (Prl), normal mammary epithelial cells express the milk protein beta-casein. We have previously found that the minimal ECM- and Prl-responsive enhancer element BCE-1 was only active when stably integrated into chromatin, and that trichostatin A (TSA), a reagent that leads to alterations in chromatin structure, was able to activate the integrated enhancer element. We now show that endogenous beta-casein gene, which is controlled by a genetic assembly that is highly similar to that of BCE-1 and which is also activated by incubation in ECM and Prl, is instead inhibited by TSA. We provide evidence that the differing response of beta-casein and BCE-1 to TSA is neither due to an unusual effect of TSA on mammary epithelial cells, nor to secondary consequences from the expression of a separate gene, nor to a particular property of the BCE-1 construct. As a component of this investigation, we also showed that ECM mediated rapid histone deacetylation in mammary epithelial cells. These results are discussed in combination with previous work showing that TSA mediates the differentiation of many types of cancer cells but inhibits differentiation of some nonmalignant cell types.     

Immunocytochemical technique for detection of prolactin (PRL) and growth hormone (GH) in hyperplastic and neoplastic lesions of dog prostate and mammary gland.

An immunocytochemical technique was described to test for immunoreactive prolactin (PRL) and growth hormone (GH) in spontaneous and experimentally induced hyperplastic and neoplastic lesions of the prostate and mammary gland. The dog was used as an animal model. The specificity and validity of the immunocytochemical staining procedure and of the antisera to canine PRL and canine GH can be regarded as established for the demonstration of PRL- and GH-dependent staining respectively. In mammary and prostatic tissues, both endogenous PRL and GH as well as intracellular free binding sites (for exogenous PRL and GH) were detected immunocytochemically. The technique presented seems to be an important tool to localize putative target sites for pituitary hormones in hormone-dependent hyperplasia and neoplasia.     

A novel polymorphism of the porcine prolactin gene (PRL)

Prolactin is a protein hormone playing a role in the maintenance of pregnancy in the pig by action on corpora lutea cells and possibly initiating production of progesterone. The prolactin gene is 10 kb in size and is composed of 5 exons and 4 introns. The present work is a report of the swine PRL gene--comparative DNA sequence analysis and the SNP revealed in the promoter region. Based on the bovine prolactin gene, three primer pairs were designed using the Primer3 on-line software. The overlapping fragments covered about 400 nucleotides of the promoter and 78 nucleotides of exon 1. The fragments were amplified; two of them were sequenced and deposited in the GenBank database (AY341908 and AY905690). All fragments were analyzed using multitemperature SSCP (MSSCP) technique. Only one fragment appeared to show a different MSSCP pattern. The samples of differing MSSCP conformers were sequenced and the C499T transition was identified in the 5'UTR region of the gene. The HphI restriction enzyme appeared to recognize the novel SNP. The alignment for homology analysis was performed with porcine, bovine (X01452) and human (NM_000948) DNA sequences available in GenBank database, using BLAST software. The comparative homology analysis results varied in dependence on the species and functional region of the gene.     

Control of mammary gland fibroblast growth by insulin, growth hormone and prolactin

Fibroblasts isolated from guinea pig mammary glands were cultured in 96 well culture plates in the presence of various concentrations of insulin, growth hormone and prolactin. Insulin (30 micrograms/ml increased uptake of tritiated thymidine by 30%. Higher concentrations of insulin did not result in any further increase in thymidine uptake. Growth hormone alone did not alter thymidine uptake in concentrations of 0 to 250 ng/ml. 300 ng/ml gave thymidine uptake of 136% of controls. In the presence of 20 g/ml insulin, growth hormone (250 ng/ml) increased thymidine uptake to approximately double that of controls. Prolactin alone (300 ng/ml decreased thymidine uptake by 19%. Insulin increased thymidine uptake, but the negative effect of prolactin was still evident above 150 ng/ml.     

Characterization and regulation of the gene expression of amino acid transport system A (SNAT2) in rat mammary gland

Amino acid transport via system A plays an important role during lactation, promoting the uptake of small neutral amino acids, mainly alanine and glutamine. However, the regulation of gene expression of system A [sodium-coupled neutral amino acid transporter (SNAT)2] in mammary gland has not been studied. The aim of the present work was to understand the possible mechanisms of regulation of SNAT2 in the rat mammary gland. Incubation of gland explants in amino acid-free medium induced the expression of SNAT2, and this response was repressed by the presence of small neutral amino acids or by actinomycin D but not by large neutral or cationic amino acids. The half-life of SNAT2 mRNA was 67 min, indicating a rapid turnover. In addition, SNAT2 expression in the mammary gland was induced by forskolin and PMA, inducers of PKA and PKC signaling pathways, respectively. Inhibitors of PKA and PKC pathways partially prevented the upregulation of SNAT2 mRNA during adaptive regulation. Interestingly, SNAT2 mRNA was induced during pregnancy and to a lesser extent at peak lactation. beta-Estradiol stimulated the expression of SNAT2 in mammary gland explants; this stimulation was prevented by the estrogen receptor inhibitor ICI-182780. Our findings clearly demonstrated that the SNAT2 gene is regulated by multiple pathways, indicating that the expression of this amino acid transport system is tightly controlled due to its importance for the mammary gland during pregnancy and lactation to prepare the gland for the transport of amino acids during lactation.     

Proteolytic modification of rat prolactin by subcellular fractions of the lactating rat mammary gland

The current study explored prolactin proteolysis by rat lactating mammary gland. 125I-labelled rat prolactin was incubated with tissue fractions of lactating mammary gland and the extent of prolactin degradation and fragment formation was visualized and densitometrically quantitated from autoradiographs derived from SDS-polyacrylamide gel electrophoresis under reducing conditions. At pH 4.5, the 25 000 X g pellet of mammary gland converted intact prolactin (23 kDa band) to proteolytic fragments (8-16 kDa bands) in a time- and tissue concentration-dependent fashion similar to that reported previously for rat ventral prostate. The prolactin-degrading and -fragmenting activity in lactating mammary gland was 5-10-times that observed for ventral prostate, the most active male tissue. This activity at acid pH was also demonstrable in other fractions of mammary gland but appeared to predominate in the cytosol. The above activities in mammary gland virtually disappeared at pH 7.4, appeared sensitive to aspartate and sulfhydryl proteinase inhibitors, and insensitive to serine and metalloenzyme proteinase inhibitors. The distribution of this activity could not be correlated with a particular enzyme marker. These characteristics of mammary gland activity differed significantly from those reported previously for prostate. When electrophoresis was conducted under non-reducing conditions, prolactin proteolysis in prostate and mammary gland was primarily associated with the formation of a more slowly migrating product (24 kDa band) with little spontaneous 8-16 kDa fragment formation. Re-electrophoresis of the 24 kDa band under reducing conditions resulted in the appearance of the 8 and 16 kDa fragments. In conclusion, prolactin is proteolytically modified by prostate and lactating mammary gland to a variant of intact hormone (24 kDa band) with a cleavage site in its large loop, by two or more widely distributed, acid-dependent proteinases. Lactating mammary gland, the principal target for prolactin, has the capacity to cleave the hormone in its loop at rates higher than any other tissue examined to date.     

Persistent changes in gene expression induced by estrogen and progesterone in the rat mammary gland.

Epidemiological studies have consistently shown that an early full-term pregnancy is protective against breast cancer. We hypothesize that the hormonal milieu that is present during pregnancy results in persistent changes in the pattern of gene expression in the mammary gland, leading to permanent changes in cell fate that determine the subsequent proliferative response of the gland. To investigate this hypothesis, we have used suppression subtractive hybridization to identify genes that are persistently up-regulated in the glands of E- and progesterone (P)-treated Wistar-Furth rats 28 d after steroid hormone treatment compared with age-matched virgins. Using this approach, a number of genes displaying persistent altered expression in response to previous treatment with E and P were identified. Two markers have been characterized in greater detail: RbAp46 and a novel gene that specifies a noncoding RNA (designated G.B7). Both were persistently up-regulated in the lobules of the regressed gland and required previous treatment with both E and P for maximal persistent expression. RbAp46 has been implicated in a number of complexes involving chromatin remodeling, suggesting a mechanism whereby epigenetic factors responsible for persistent changes in gene expression may be related to the determination of cell fate. These results provide the first support at the molecular level for the hypothesis that hormone-induced persistent changes in gene expression are present in the involuted mammary gland.     

Insulin-like growth factor-II regulates PTEN expression in the mammary gland

The tumor suppressor PTEN is altered in many cancers, including breast cancer, but only a handful of factors are known to control its expression. PTEN plays a vital role in cell survival and proliferation by regulating Akt phosphorylation, a key component of the phosphatidylinositol 3 kinase (PI3K) pathway. Here we show that insulin-like growth factor-II (IGF-II), which signals through PI3K, regulates PTEN expression in the mammary gland. IGF-II injection into mouse mammary gland significantly increased PTEN expression. Transgenic IGF-II expression also increased mammary PTEN protein, leading to reductions in Akt phosphorylation, epithelial proliferation, and mammary morphogenesis. IGF-II induced PTEN promoter activity and protein levels and this involved the immediate early gene egr-1. Thus, we have identified a novel negative feedback loop within the PI3K pathway where IGF-II induces PTEN expression to modulate its physiologic effects.