Histone 3 Trimethylation of IGFBP-7 Gene Promoter by Expression of D5 Stat5a in Breast Epithelial Cells.

As a very important transcription factor, signal transducer and activator of transcription 5a (Stat5a) has been reported to be involved in human reproductive cancers such as breast, prostate and ovarian cancer. However, up to date, the exact role of Stat5a in breast cancer is still not clear. The data reported are conflicting. D5 Stat5a is a variant of Stat5a we cloned recently. This newly cloned variant behave like its full length counterpart in terms of dimerization, being activated by prolactin and nuclear translocation, however it also behave differently in terms of effect on cell proliferation and interaction with other transcription factors. In the present study, we examined its effect on cell proliferation of cultured breast cancer cells (MCF-10A and MCF-7) by using adenovirus-mediated gene transfer and MTS technology. Also, quantitative real time polymerase chain reaction (qRT-PCR), chromatin immunoprecipitation assay (ChIP) and Western blot were used to probe the possible changes of insulin-like growth factor binding protein-7 (IGFBP-7) expression including mRNA and protein, and the epigenetic changes with overexpression of this newly cloned variant. The results clarified that D5 Stat5a (1) behaves as a promoting factor to the cell proliferation of MCF-10A and MCF-7, (2) induces enhancer of zeste homology 2 (EZH2) expression in breast epithelial cells, as well as histone 3 trimethylation of IGFBP-7 promoter region, and (3) lower IGFBP-7 expression was detected in breast cancer tissue. Taking together, we concluded that the mitogenic effect of D5 Stat5a on breast cells is, at least partly, through up-regulation of histone methyltransferase, EZH2, and therefore inhibiting IGFBP-7 expression by increasing H3K27Me3 of IGFBP-7 promoter region.

Knockdown of receptor tyrosine kinase-like orphan receptor 2 inhibits cell proliferation and colony formation in osteosarcoma cells by inducing arrest in cell cycle progression.

Osteosarcoma (OS) is the most common malignant tumor of the bone, with a high mortality rate and poor prognosis. Receptor tyrosine kinase-like orphan receptor 2 (ROR2) has been reported to be dysregulated in human malignancies. More recently, ROR2 has been demonstrated to promote OS cell migration and invasion. However, the role of ROR2 in the regulation of OS cell proliferation, as well as the underlying molecular mechanism, remains unclear. The present study aimed to investigate the underlying mechanism of ROR2 in osteosarcoma growth. Reverse transcription-quantitative polymerase chain reaction analysis and western blot analysis were used to examine the mRNA and protein expression. MTT assay, colony formation assay and cell cycle analysis were conducted to explore the function of ROR2 in osteosarcoma cells. In the present study, the expression of ROR2 was found to be frequently upregulated in OS tissues compared with matched adjacent normal tissues. It was also upregulated in the OS cell lines Saos-2, MG-63 and U-2 OS, relative to normal osteoblast hFOB 1.19 cells. Knockdown of ROR2 expression by transfection with ROR2-specific siRNA markedly inhibited the proliferation and colony formation of OS cells. Data from the cell cycle distribution assay revealed an accumulation of ROR2-knockdown cells in the G0/G1 phase, indicating that knockdown of ROR2 leads to an arrest in cell cycle progression. Mechanistic investigation revealed that the protein levels of c-myc, a target gene of the Wnt signaling, as well as cyclin D1, cyclin E and cyclin-dependent kinase 4 were markedly reduced in the ROR2-knockdown OS cells, suggesting that the inhibitory effect of ROR2 knockdown on OS cell proliferation is associated with the Wnt signaling pathway. In summary, the current study indicates an important role for ROR2 in the proliferation of OS cells. Therefore, ROR2 may be a promising therapeutic target in OS.

Blockade of estrogen-stimulated proliferation by a constitutively-active prolactin receptor having lower expression in invasive ductal carcinoma.

A comprehensive understanding of prolactin's (PRL's) role in breast cancer is complicated by disparate roles for alternatively-spliced PRL receptors (PRLR) and crosstalk between PRL and estrogen signaling. Among PRLRs, the short form 1b (SF1b) inhibits PRL-stimulated cell proliferation. In addition to ligand-dependent PRLRs, constitutively-active varieties, missing the S2 region of the extracellular domain (ΔS2), naturally occur. Expression analysis of the ΔS2 version of SF1b (ΔS2SF1b) showed higher expression in histologically-normal contiguous tissue versus invasive ductal carcinoma. To determine the function of ΔS2SF1b, a T47D breast cancer line with inducible expression was produced. Induction of ΔS2SF1b blocked estrogen-stimulated cell proliferation. Unlike intact SF1b, induction of ΔS2SF1b had no effect on PRL-mediated activation of Stat5a. However induction inhibited estrogen's stimulatory effects on serine-118 phosphorylation of estrogen receptor α, serine-473 phosphorylation of Akt, serine-9 phosphorylation of GSK3ß, and c-myc expression. In addition, induction of ΔS2SF1b increased expression of the cell cycle-inhibiting protein, p21. Thus, increased expression of ΔS2SF1b, such as we demonstrate occurs with the selective PRLR modulator, S179D PRL, would create a physiological state in which estrogen-stimulated proliferation was inhibited, but differentiative responses to PRL were maintained.

Expression of a constitutively active prolactin receptor causes histone trimethylation of the p53 gene in breast cancer.

BACKGROUND: Prolactin (PRL) is a pituitary polypeptide hormone characterized by multiple biological actions including stimulation of growth in the prostate and formation of secretory alveoli and stimulation of milk protein gene expression in the mammary gland. PRL exerts its effect by dimerizing its receptor (PRLR) on the plasma membrane and regulating gene expression through the JAK-Stat signal pathway. We have previously described a natural variant of the PRLR in which the S2 subdomain of the extracellular domain is missing (Delta S2). Delta S2 PRLRs are dimerized in the absence of PRL and have constitutive activity in the promotion of breast cancer cell growth. Enhancer of zeste homolog 2 (EZH2), as one of the histone-modifying enzymes, is a key factor regulating gene expression by epigenetic modification. We hypothesized that these constitutive activated Delta S2 PRLRs played a pathogenic role in breast cancer in part through alterations in the expression of EZH2 and the trimethylation of histone 3 on lysine 27 (H3K27Me3). METHODS: In order to verify the clinical significance and to establish the link between Delta S2 PRLR expression and epigenetic change, EZH2, H3K27Me3, and Delta S2 PRLR were detected in both normal and cancerous human breast tissues. Also, overexpression of Delta S2 PRLR in breast epithelial cells was achieved by infection with adenovirus carrying the cDNA. Western blotting and chromatin immunoprecipitation (ChIP assay) and acid histone extraction were applied to detect the expression of EZH2 and the trimethylation of histone 3, respectively. RESULTS: In breast tissue, higher EZH2 expression and higher H3K27Me3 were found associated with higher Delta S2 expression in breast cancer samples. In breast epithelial cells, overexpression of Delta S2 PRLR increased EZH2 methyltransferase mRNA and protein, induced EZH2 methyltransferase recruitment to chromatin, increased the trimethylation of H3K27Me3, and decreased the expression of p53 gene. CONCLUSIONS: Delta S2 PRLR plays an important pathogenic role in breast cancer through epigenetic modification. Elevated expression of Delta S2 PRLR, achieved by alternate splicing of the pre-mRNA of the full-length form, is a new mechanism contributing to human breast cancer.

An N-terminal splice variant of human Stat5a that interacts with different transcription factors is the dominant form expressed in invasive ductal carcinoma.

We have identified a new variant of human Stat5a, found at higher ratios to full-length Stat5a in invasive ductal carcinoma versus contiguous normal tissue. The variant, missing exon 5, inhibits p21 and Bax production and increases cell number. After prolactin stimulation, only full-length Stat5a interacts with the vitamin D and retinoid X receptors, whereas only Δ5 Stat5a interacts with activating protein 1-2 and specificity protein 1. Prolactin also oppositely regulates interaction of the two Stat5a forms with ß-catenin. We propose that a change in splicing leading to upregulation of this new isoform is a pathogenic aspect of invasive ductal carcinoma.

Histone trimethylation of the p53 gene by expression of a constitutively active prolactin receptor in prostate cancer cells.

Prolactin (PRL) is a pituitary polypeptide hormone characterized by multiple biological actions including stimulation of growth in the prostate, breast and ovarian epithelial cells. A sizable body of reports has presented evidences to indicate the involvement of PRL in the pathogenic process of cancers of the reproductive system, such as prostate and breast cancers. PRL exerts its effects by dimerizing its receptor (PRLR) on the plasma membrane, and initiating cellular Jak-Stat signal pathway. We have previously cloned from prostate cancer cells a natural variant of PRLR in which the S2 subdomain of the extracellular domain is missing (ΔS2). Our preliminary data showed that ΔS2 PRLR was able to dimerize and to constitutively activate the ß-casein promoter (in the absence of its ligand, PRL) in breast and prostate epithelial cells. Enhancer of zeste homologue 2 (EZH2), an important histone-modifying enzyme, is able to trimethylate histone 3 on lysine 27 (H3K27Me3), consequently leading to gene silencing, especially silencing of tumor suppressor genes such as p53. We hypothesized that ΔS2 PRLR played an important pathogenic role in prostate cancer through, at least partly, alterations in the expression of EZH2 and the trimethylation of histone 3 on lysine 27. In the present study, overexpression of ΔS2 PRLR in prostate epithelial cells was achieved by infection with an adenoviral vector carrying the cDNA. The viable cell number overexpressing ΔS2 PRLR was assessed using MTS reagent. Western blot, chromatin immunoprecipitation (ChIP) assay and acid histone extraction were applied to detect expression of EZH2 as well as trimethylation of histone 3, respectively. In prostate epithelial cells, overexpression of ΔS2 PRLR increased the levels of EZH2 methyltransferase mRNA and protein, induced EZH2 methyltransferase recruitment to chromatin, increased the trimethylation of histone 3 on lysine 27, and decreased expression of the p53 gene. We concluded that ΔS2 PRLR plays an important pathogenic role in prostate cancer through epigenetic covalent modification leading to chromatin remodeling. Hypertrimethylation on H3K27 of the p53 gene promoter region due to elevated expression of ΔS2 PRLR by alternative splicing of the pre-mRNA in its full-length form might serve as a new mechanism underlying human prostate cancer.

[Progress in prolactin receptor research].

Prolactin (PRL) is secreted by lactotrophs in the anterior pituitary and some extra-pituitary tissues such as breast, lacrimal gland, uterus, thymus and spleen, etc. Since PRL is closely related to growth hormone (GH) and placental lactogens (PL), it has been broadly accepted that PRL, GH and PL are resulted from the duplication of an ancestral gene. PRL regulates hundreds of biological functions by endocrine, paracrine and autocrine manners. Prolactin initiates its effects by binding to its receptor (PRLR). PRLR belongs to the class I cytokine receptor superfamily. Up to now, three membrane--PRLRs have been clarified. They are long form (LF), intermediate form (IF) and short form (SF) including SFla and SFlb. All PRLRs are derived from a primary transcript of common gene through alternative splicing mechanism. Although the extracellular domain (ECD) and the transmembrane domain (TD) of LF, IF and SF are equal, different isoforms of PRLR exert different function through different intracellular domain. It has been well documented that abnormity of PRLR is closely related to the pathogenesis, progression and prognosis of cancers including breast cancer. Several PRLR antagonists have been well designed and evidenced to have the potential to be important therapeutics.

Prolactin increases survival and migration of ovarian cancer cells: importance of prolactin receptor type and therapeutic potential of S179D and G129R receptor antagonists.

Variably-spliced prolactin receptors (PRLRs) and PRL are expressed by the ovarian cancer cell lines, TOV-112D, OV-90 and TOV-21G. Incubation in the PRLR antagonists, G129R- or S179D-PRL, or anti-PRL reduced cell number, indicating a functional autocrine PRL growth loop. Added PRL promoted, and the antagonists decreased, cell migration. When cells were stressed, added PRL decreased apoptosis and increased survival, and the antagonists had the opposite effect. Cells expressing higher long:short PRLR ratios had increased growth, survival and migration in response to PRL. Results suggest that PRLR antagonists may be therapeutically beneficial in ovarian cancer.

Short form 1b human prolactin receptor down-regulates expression of the long form.

Alternative splicing produces different human prolactin (PRL) receptors. These include a long form (LF) and two short forms (SF1a and SF1b). The SFs of the receptor can act as dominant negatives for PRL effector function through the LF. This is proposed to be due to LF-SF heterodimerization and resultant interference with LF-LF dimer signaling. We, along with others, have provided evidence for LF-SF heterodimerization of the human receptors in support of this mechanism, along with others. However, to further investigate the ways SF may influence LF function, we co-transfected human embryonic kidney 293 cells with vectors coding for tagged (green fluorescent protein (GFP) or luciferase) LF alone or plus untagged SF1b and measured LF-GFP intensity, LF-luciferase activity, and LF mRNA 48 h later. Equal amounts of SF1b cDNA decreased LF-GFP fluorescence intensity, LF-luciferase activity, and LF mRNA by 80-100%. Similar co-transfections with untagged LF had no significant effect on tagged LF expression. Use of hygromycin showed degradation of already formed protein was the same for LF-luciferase alone and LF-luciferase with SF1b. Inhibition of mRNA synthesis, on the other hand, showed that SF1b expression accelerated LF mRNA degradation two- to three-fold. SF1b also down-regulated expression of endogenous LF mRNA in T47D breast cancer cells and opposed an increase in cell number resulting from transfection with extra LF alone. These results demonstrate a previously unrecognized mechanism whereby SF1b affects the end result of signaling through the LF receptor. The effects on cell number also support the concept that the LF:SF1b ratio may be relevant to tumor growth.

S2 deletion variants of human PRL receptors demonstrate that extracellular domain conformation can alter conformation of the intracellular signaling domain.

Using spacers between the C-termini of the long (LF) or short (SF) human prolactin receptors and luciferase/GFP such that bioluminescence resonance energy transfer (BRET) occurred minimally in intact versions of these receptors in the absence of ligand, we have monitored the BRET signal after deletion of regions of the extracellular domain (ECD). Deletion of S2 produced ligand-independent BRET for only those pairings normally occurring in the presence of ligand with the intact receptor. Deletion of the similarly sized S1, or S1 plus S2, produced no ligand-independent or -dependent BRET. When deleted receptors were transfected into human breast (T47D) or prostate (DU145) cancer cells incubated in the absence of added prolactin (PRL) and presence of anti-PRL, expression of the DeltaS2LF resulted in increased cell number, whereas expression of the intact receptor did not. When endogenous beta-casein expression was examined in T47D cells, the DeltaS2LF and DeltaS2F1a both showed ligand-independent activation of transcription, again not duplicated by the intact receptor. Paired with evidence in the literature for predimerization of PRLRs, these results demonstrate that altered ECD conformation, and not just a change in bulk, produces altered conformation of the intracellular signaling region of the receptors, supporting the concept that ligand binding to the ECD of intact predimerized receptors could initiate signaling. In addition, the current work supports a dual proliferative and differentiative role for the LF receptor, but only a differentiative role for the SF1a receptor. Naturally occurring DeltaS2 PRL receptors (PRLR) were also found in normal and cancerous human cells. This additionally suggests a heretofore unappreciated ligand-independent role for PRLRs.

Different forms of prolactin have opposing effects on the expression of cell cycle regulatory proteins in differentiated mammary epithelial cells.

Prolactin (PRL) is a hormone that contributes to both the growth and differentiation of mammary epithelial cells, activities likely to impact breast cancer in opposite ways. Whether PRL causes growth or differentiation has been solely attributed to the coexisting steroidal environment, with PRL stimulating mammary gland growth during pregnancy, and then milk production after the postpartum drop in estrogen and progesterone. However, previous work from our laboratory has shown that the form of PRL may also be an important factor. During pregnancy, unmodified PRL (U-PRL) promotes mammary growth, while an increase in phosphorylated PRL, or administration of a molecular mimic of phosphorylated PRL (S179D PRL), inhibits growth. Unknown, however, is whether these forms of PRL have opposite effects on growth in the absence of steroids and whether effects are directly on mammary epithelial cells. To mimic the glandular epithelium in vitro, we used contact-inhibited, differentiated cells and showed that even with these minimally growing cells that treatment with U-PRL caused increased expression of cyclin D1 and cyclin-dependent kinase 4, increased activity of both cdk4 and cdk2, while having no effect on the inhibitory protein, p21. S179D PRL, by contrast, had no effect on cyclin D1 and cdk4 expression, but increased p21 expression and expression of the vitamin D receptor (VDR). We conclude that increased U-PRL or decreased phosphorylated PRL can directly affect cell cycle control proteins in relatively differentiated mammary epithelial cells, thereby implicating the balance between these two forms of PRL in the early promotion of breast cancer.

Two wrongs can make a right: dimers of prolactin and growth hormone receptor antagonists behave as agonists.

Prolactin (PRL) and GH have two distinct binding sites (site 1 with high affinity; site 2 with low affinity) that each interact with a PRL receptor (PRLR) to form a functional receptor dimer that activates signal transduction. The G129R mutation in PRL and the G120R mutation in GH disrupt the structural integrity of site 2 such that the ligands retain the ability to bind to the first receptor with high affinity, but act as receptor antagonists. In this study, we examined the ability of monomeric and dimeric forms of these ligands, human (h) PRL and hGH, and their antagonists (hPRL-G129R and hGH-G120R) to 1) bind to PRLRs; 2) induce conformational changes in PRLRs; 3) activate signaling pathways associated with the PRLR; and 4) mediate cell proliferation in vitro. In contrast to monomeric hPRL-G129R, homodimeric hPRL-G129R induced PRLR dimerization; activated Janus family of tyrosine kinases 2/signal transducer and activator of transcription 5, Ras/Raf/MAPK kinase/Erk, and phosphatidylinositol 3-kinase/Akt signaling; and stimulated Nb2 cell proliferation. Similarly, homodimeric hGH-G120R was able to mediate signaling via the PRLR and to stimulate Nb2 cell proliferation. These experiments demonstrate that a ligand must have two functional binding sites, but that these may be site 1 plus site 2 or two site 1's, to elicit receptor-mediated signal transduction. The size of the ligand plays less of a role in receptor activation, suggesting that the extracellular portion of the PRLR (and possibly the GH receptor) is rather flexible and can accommodate larger ligands. These findings may have implications for designing multifunctional therapeutics that target this class of cytokine receptors.

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.