Local signals induce CCAAT/enhancer binding protein-delta (C/EBP-delta) and C/EBP-beta mRNA expression in the involuting mouse mammary gland

The Multifaceted Role of STAT3 in Mammary Gland Involution and Breast Cancer

Since seminal descriptions of signal transducer and activator of transcription 3 (STAT3) as a signal transducer and transcriptional regulator, which is most usually activated by phosphorylation of a specific tyrosine residue, a staggering wealth of research has delineated the key role of this transcription factor as a mediator of mammary gland postlactational regression (involution), and paradoxically, a pro-survival factor in breast cancer and some breast cancer cell lines. STAT3 is a critical regulator of lysosomal-mediated programmed cell death (LM-PCD) during mammary gland involution, where uptake of milk fat globules, and consequent high levels of free fatty acids, cause permeabilisation of lysosomal vesicle membranes, in turn leading to cathepsin protease leakage and cell death. A recent proteomic screen of STAT3-induced changes in lysosomal membrane protein components has highlighted wide-ranging effects of STAT3, which may coordinate LM-PCD via the stimulation of endocytosis, intracellular trafficking, and lysosome biogenesis. In parallel, STAT3 regulates the acute phase response during the first phase of involution, and it contributes to shaping the pro-tumourigenic 'wound healing' signature of the gland during the second phase of this process. STAT3 activation during involution is important across species, although some differences exist in the progression of involution in dairy cows. In breast cancer, a number of upstream regulators can lead to STAT3 activation and the effects of phosphorylation of STAT3 are equally wide-ranging. Recent studies have implicated microRNAs in some regulatory pathways. In this review, we will examine the multifaceted role of STAT3 in mammary gland involution and tumourigenesis, incorporating a review of these fundamental processes in tandem with a discussion of recent developments in this field.

An evolutionarily conserved non-coding element in casein locus acts as transcriptional repressor

In mammals, the casein locus consists of stretches of non-coding DNA, the functions of most of which are unknown. These regions are believed to harbour elements responsible for spatio-temporally regulated expression of genes in this locus and so far, only a few such elements have been identified. In this study, we report a novel regulatory element in the casein locus. Comparative analysis of genomic DNA sequences of casein loci from different mammals identified a 147bp long evolutionarily conserved region (ECR) upstream of Odam, a gene in this locus. The ECR was found in close proximity of Odam gene in all the mammals examined. In-silico analysis predicted the ECR as a potential regulatory element. Functional analysis in different cell lines identified it as a unidirectional repressor element. From our findings we speculate that the ECR may be involved in the repression of the Odam expression in the mammary gland during lactation.

Transcriptomic analysis reveals key regulators of mammogenesis and the pregnancy-lactation cycle

An organ unique to mammals, the mammary gland develops 90% of its mass after birth and experiences the pregnancylactation-involution cycle (PL cycle) during reproduction. To understand mammogenesis at the transcriptomic level and using a ribo-minus RNA-seq protocol, we acquired greater than 50 million reads each for the mouse mammary gland during pregnancy (day 12 of pregnancy), lactation (day 14 of lactation), and involution (day 7 of involution). The pregnancy-, lactation- and involution-related sequencing reads were assembled into 17344, 10160, and 13739 protein-coding transcripts and 1803, 828, and 1288 non-coding RNAs (ncRNAs), respectively. Differentially expressed genes (DEGs) were defined in the three samples, which comprised 4843 DEGs (749 up-regulated and 4094 down-regulated) from pregnancy to lactation and 4926 DEGs (4706 up-regulated and 220 down-regulated) from lactation to involution. Besides the obvious and substantive up- and down-regulation of the DEGs, we observe that lysosomal enzymes were highly expressed and that their expression coincided with milk secretion. Further analysis of transcription factors such as Trps1, Gtf2i, Tcf7l2, Nupr1, Vdr, Rb1, and Aebp1, and ncRNAs such as mir-125b, Let7, mir-146a, and mir-15 has enabled us to identify key regulators in mammary gland development and the PL cycle.

DNER modulates adipogenesis of human adipose tissue-derived mesenchymal stem cells via regulation of cell proliferation

OBJECTIVES

In recent years, obesity has become a global epidemic, highlighting the necessity for basic research into mechanisms underlying growth of adipose tissue and differentiation of stem cells into adipocytes, in humans. For better understanding of cell signalling in adipogenesis, the role of DNER (delta/Notch-like EGF-related receptor) in adipogenic differentiation of human adipose tissue-derived mesenchymal stem cells (hAMSC) was investigated.

MATERIALS AND METHODS

To assess the role of DNER in hAMSC adipogenesis, hAMSCs were transfected with DNER small interfering RNA (siDNER). Real-time quantitative reverse transcriptase polymerase chain reactions to assess expression levels of adipogenesis-related genes regulated by siDNER, cell cycle and immunoblot analyses were performed.

RESULTS

First, it was determined that DNER mRNA was profoundly expressed in hAMSCs and reduced during adipogenic differentiation. Knockdown of DNER altered cell morphology, inhibited proliferation and increased frequency and efficiency of adipogenesis in hAMSC. Expression of CCAAT/enhancer-binding protein delta increased and proportion of cells in S phase decreased by knockdown of DNER, using specific siRNA. Moreover, adipocyte-specific genes including peroxisome proliferator-activated receptor gamma, fatty acid binding protein 4 and perilipin were up-regulated in siDNER compared to the siControl group during adipogenesis in hAMSC.

CONCLUSIONS

These results indicate that DNER knockdown in hAMSC accelerated onset of adipogenic differentiation by bypassing mitotic clonal expansion during the early stages of adipogenesis.

Identification and characterization of CCAAT/Enhancer Binding proteindelta (C/EBPdelta) target genes in G0 growth arrested mammary epithelial cells

BACKGROUND

CCAAT/Enhancer Binding Proteindelta (C/EBPdelta) is a member of the highly conserved C/EBP family of leucine zipper (bZIP) proteins. C/EBPdelta is highly expressed in G0 growth arrested mammary epithelial cells (MECs) and "loss of function" alterations in C/EBPdelta have been associated with impaired contact inhibition, increased genomic instability and increased cell migration. Reduced C/EBPdelta expression has also been reported in breast cancer and acute myeloid leukemia (AML). C/EBPdelta functions as a transcriptional activator, however, only a limited number of C/EBPdelta target genes have been reported. As a result, the role of C/EBPdelta in growth control and the potential mechanisms by which "loss of function" alterations in C/EBPdelta contribute to tumorigenesis are poorly understood. The goals of the present study were to identify C/EBPdelta target genes using Chromatin Immunoprecipitation coupled with a CpG Island (HCG12K) Array gene chip ("ChIP-chip") assay and to assess the expression and potential functional roles of C/EBPdelta target genes in growth control.

RESULTS

ChIP-chip assays identified approximately 100 C/EBPdelta target gene loci which were classified by gene ontology (GO) into cell adhesion, cell cycle regulation, apoptosis, signal transduction, intermediary metabolism, gene transcription, DNA repair and solute transport categories. Conventional ChIP assays validated the ChIP-chip results and demonstrated that 14/14 C/EBPdelta target loci were bound by C/EBPdelta in G0 growth arrested MCF-12A MECs. Gene-specific RT-PCR analysis also demonstrated C/EBPdelta-inducible expression of 14/14 C/EBPdelta target genes in G0 growth arrested MCF-12A MECs. Finally, expression of endogenous C/EBPdelta and selected C/EBPdelta target genes was also demonstrated in contact-inhibited G0 growth arrested nontransformed human MCF-10A MECs and in mouse HC11 MECs. The results demonstrate consistent activation and downstream function of C/EBPdelta in growth arrested human and murine MECs.

CONCLUSION

C/EBPdelta target genes were identified by a global gene array approach and classified into functional categories that are consistent with biological contexts in which C/EBPdelta is induced, such as contact-mediated G0 growth arrest, apoptosis, metabolism and inflammation. The identification and validation of C/EBPdelta target genes provides new insights into the mechanistic role of C/EBPdelta in mammary epithelial cell biology and sheds new light on the potential impact of "loss of function" alterations in C/EBPdelta in tumorigenesis.

A brain-specific homeobox gene, Bsx, is essential for proper postnatal growth and nursing

To investigate in vivo roles of a murine hypothalamic homeobox gene, Bsx, we generated and analyzed two mutant alleles, Bsx(DeltaHD) and Bsx(lacZ). Bsx(DeltaHD) lacks the homeodomain, and Bsx(lacZ) is an insertion of a lacZ reporter gene. Bsx-lacZ expression was detected in the hypothalamus and pineal gland and reiterates Bsx expression. Bsx homozygous mutant mice were born at the expected Mendelian ratio, but their growth was impaired. Offspring from Bsx homozygous mutant females exhibited a low survival rate due to a nursing defect. Mammary glands of the mutant females developed normally during pregnancy; however, they involuted quickly after parturition. These results demonstrate that Bsx is required for postnatal growth and maintenance of lactating mammary glands. Thus, mouse Bsx is likely involved in systemic control of suppression of apoptosis of postpartum mammary epithelial cells.

Odd-Skipped Related 2 gene transcription is regulated by CCAAT enhancer-binding protein δ in mesenchymal C3H10T1/2 cells

Odd-skipped related 2 (Osr2) gene is mouse homolog of Drosophila Odd-skipped gene involved with the pair-rule segmentation phenotype in Drosophila mutant embryos. In this study, to examine Osr2 expression regulation, the mouse Osr2 promoter region was cloned and characterized, and found to have two enhancer elements in the -1463/-1031 (distal) and -581/+3 (proximal) regions, and a repressor region (-4845/-1463, far distal). CCAAT/enhancer binding protein (C/EBP) binding sites were found in both the distal and proximal enhancer elements. Osr2 promoter activity was enhanced by C/EBPdelta, a member of the C/EBP family, in a dose-dependent manner. Electrophoresis mobility shift assays showed that purified GST-C/EBPdelta bound to distal (-1295/-1261) and proximal (-89/-55) C/EBP binding motifs. Chromatin immunoprecipitation demonstrated that acetylated histones H3, H4, and C/EBPdelta in the proximal region (-280/-43), but not the distal region (-1438/-1196), indicating that the Osr2 promoter proximal region was transcriptionally activated in C3H10T1/2 cells. Our results suggest that Osr2 expression is regulated by C/EBP regulatory elements.

CCAAT/Enhancer binding protein delta (c/EBPdelta) regulation and expression in human mammary epithelial cells: I. "Loss of function" alterations in the c/EBPdelta growth inhibitory pathway in breast cancer cell lines

"Loss of function" alterations in growth inhibitory signal transduction pathways are common in cancer cells. In this study, we show that growth arrest (GA) treatments--serum and growth factor withdrawal and growth inhibitory IL-6 family cytokines (Interleukin-6 and Oncostatin M (OSM))--increase STAT3 phosphorylation (pSTAT3), increase CCAAT enhancer binding protein delta (C/EBPdelta) gene expression and induce GA of primary, finite-lifespan human mammary epithelial cells (HMECs), and immortalized breast cell lines (MCF-10A and MCF-12A). In contrast, serum and growth factor withdrawal from human breast cancer cell lines (MCF-7, SK-BR-3, T-47D, and MDA-MB-231) for up to 48 h induced a relatively modest increase in pSTAT3 levels and C/EBPdelta gene expression and resulted in varying levels of GA. In most breast cancer cell lines, IL-6 family cytokine treatment increased pSTAT3 levels and C/EBPdelta gene expression, however, growth inhibition was cell line dependent. In addition to "loss of function" alterations in growth inhibitory pathways, breast cancer cell lines also exhibit "gain of function" alterations in growth signaling pathways. The Akt growth/ survival pathway is constitutively activated in T-47D and MCF-7 breast cancer cells. The Akt inhibitor LY 294,002 significantly enhanced T-47D growth inhibition by serum and growth factor withdrawal or IL-6 family cytokine treatment. Finally, we show that activation of the pSTAT3/C/EBPdelta growth control pathway is independent of estrogen receptor status. These results demonstrate that "loss of function" alterations in the pSTAT3/C/EBPdelta growth inhibitory signal transduction pathway are relatively common in human breast cancer cell lines. Defective activation of the pSTAT3/ C/EBPdelta growth inhibitory signal transduction pathway, in conjunction with constitutive activation of the Akt growth stimulatory pathway, may play a synergistic role in the etiology or progression of breast cancer.

C/EBPdelta is a downstream mediator of IL-6 induced growth inhibition of prostate cancer cells

BACKGROUND

Although a number of reports have investigated the effects of IL-6 family cytokines on prostate cell growth, there is limited information available identifying IL-6 inducible downstream effector genes and their function in growth control. Previous studies have demonstrated that IL-6 treatment results in the activation of signal transducer and activator of transcription3 (STAT3) in prostate cancer cells. The goal of this study was to investigate the influence of IL-6 treatment and activation of the Jak/STAT signal transduction pathway on C/EBPdelta gene expression and growth inhibition of human prostate cancer cells.

METHODS

Expression of C/EBPdelta and STAT3 activation were assayed using Northern and Western blotting techniques. Proliferation was assessed by [(3)H] thymidine incorporation, flow cytometry, and colony formation analyses. The analysis of the transcriptional regulation of C/EBPdelta was performed using luciferase-reporter constructs.

RESULTS

In this report, we demonstrate that IL-6 treatment induces STAT3 activation (pSTAT3), pSTAT3 binds to the human C/EBPdelta gene promoter and induces its expression. We also demonstrate that C/EBPdelta over-expression is capable of suppressing prostate cancer cell growth.

CONCLUSIONS

These results demonstrate that C/EBPdelta gene expression is increased in IL-6 treated LNCaP cells. Increased C/EBPdelta gene expression plays an important role in IL-6/STAT3 mediated growth arrest of LNCaP prostate cancer cells. Ongoing studies are investigating the mechanism by which C/EBPdelta controls prostate cancer cell growth and the potential role of C/EBPdelta in the survival and chemo resistance of prostate cancer metastasis. (c) 2004 Wiley-Liss, Inc.

Loss of CCAAT/enhancer binding protein delta promotes chromosomal instability

The transcription factor CCAAT/enhancer binding protein delta (Cebpd, also known as C/EBPdelta, CRP3, CELF, NF-IL6beta) is implicated in diverse cellular functions such as the acute phase response, adipocyte differentiation, learning and memory, and mammary epithelial cell growth control. Here, we report that lack of Cebpd causes genomic instability and centrosome amplifications in primary embryonic fibroblasts derived from 129S1 mice. Upon spontaneous immortalization, Cebpd-deficient fibroblasts acquire transformed features such as impaired contact inhibition and reduced serum dependence. These data identify a novel role for Cebpd in the maintenance of chromosomal stability and suggest a potential tumor suppressor function in vivo.

An AU-rich element in the 3' untranslated region of the C/EBP delta mRNA is important for protein binding during G0 growth arrest

Posttranscriptional regulation at the level of mRNA stability is becoming increasingly recognized as an important mechanism to control the levels of mRNAs that encode key cell fate determining proteins. Previous work from our laboratory demonstrated that C/EBPdelta is a highly unstable mRNA in G(0) growth arrested mammary epithelial cells. In this report we investigated trans-acting factor binding to the C/EBPdelta 3'-UTR and identified a cis-acting element important for this interaction. RNA electromobility shift assays (REMSAs) demonstrate that the C/EBPdelta mRNA 3'-UTR binds trans-acting factor(s) present in G(0) growth arrested mammary epithelial cell lysates. This binding was not detected in the presence of lysates from growing cells. UV-binding analysis detected a RNA/protein complex of approximately 35kDa following incubation of the full-length C/EBPdelta 3'UTR with lysates from G(0) growth arrested mammary epithelial cells. Competition assays indicate that a specific AU-rich region (U1) is necessary for trans-acting factor binding to the C/EBPdelta 3'-UTR. These studies have identified an AU-rich element located within the C/EBPdelta 3'-UTR that interacts with a putative G(0) growth arrest-specific trans-acting factor(s), which may regulate C/EBPdelta mRNA decay.

Posttranscriptional and posttranslational regulation of C/EBP delta in G0 growth-arrested mammary epithelial cells

Previous work from our laboratory demonstrated that CCAAT/enhancer-binding protein delta (C/EBP delta) functions in the initiation and maintenance of G(0) growth arrest in mouse mammary epithelial cells (MECs). In this report, we investigated the posttranscriptional and posttranslational regulation of C/EBP delta in G(0) growth-arrested mouse MECs. The results of transcriptional inhibitor studies demonstrated that the C/EBP delta mRNA exhibits a relatively short half-life in G(0) growth-arrested mouse MECs (t(1/2) approximately 35 min). In contrast, C/EBP delta mRNA has a longer half-life in G(0) growth-arrested mouse fibroblast cells (t(1/2) >100 min). Oligo/RNase H cleavage analysis and rapid amplification of cDNA ends-poly(A) test both confirmed the short C/EBP delta mRNA half-life observed in MECs and demonstrated that the C/EBP delta mRNA poly(A) tail is relatively short (approximately 100 nucleotides). In addition, the poly(A) tail length was not shortened during C/EBP delta mRNA degradation, which suggested a deadenylation-independent pathway. The C/EBP delta protein also exhibited a relatively short half-life in G(0) growth-arrested mouse MECs (t(1/2) approximately 120 min). The C/EBP delta protein was degraded in a ubiquitin-dependent manner, primarily in the nucleus, during G(0) growth arrest. In conclusion, these studies indicated that the C/EBP delta mRNA and protein content are under tight regulation in G(0) growth-arrested mouse MECs, despite the general concept that G(0) growth arrest is associated with a decrease in cellular activity.

Nulliparous CCAAT/enhancer binding proteindelta (C/EBPdelta) knockout mice exhibit mammary gland ductal hyperlasia

CCAAT/Enhancer binding proteins (C/EBPs) are a family of nuclear proteins that function in the control of cell growth, death, and differentiation. We previously reported that C/EBPdelta plays a key role in mammary epithelial cell G(0) growth arrest. In this report, we investigated the role of C/EBPdelta in mammary gland development and function using female mice homozygous for a targeted deletion of C/EBPdelta (C/EBPdelta -/-). C/EBPdelta -/- females develop normally and exhibit normal reproductive and lactational performance. Adult nulliparous C/EBPdelta -/- females, however, exhibit mammary epithelial cell growth control defects. The mean number of mammary ductal branches is significantly higher in adult nulliparous C/EBPdelta -/- females compared with C/EBPdelta +/+ (wild-type control) females (66.8 +/- 5.2 vs 42.9 +/- 6.3 branch points/field, P < 0.01). In addition, the mean total mammary gland cellular volume occupied by epithelium is significantly higher in adult nulliparous C/EBPdelta -/- females compared with C/EBPdelta +/+ controls (29.0 +/- 1.4 vs 20.4 +/- 1.3, P < 0.001). Our results showed that the BrdU labeling index was significantly higher in mammary epithelial cells from nulliparous C/EBPdelta -/- females compared with C/EBPdelta +/+ controls during the proestrus/estrus (4.55 +/- 0.70 vs 2.14 +/- 0.43, P < 0.01) and metestrus/diestrus (6.92 +/- 0.75 vs 3.98 +/- 0.43 P < 0.01) phases of the estrus cycle. In contrast, the percentage of mammary epithelial cells undergoing apoptosis during both phases of the estrus cycle did not differ between C/EBPdelta -/- and C/EBPdelta +/+ females. The increased epithelial cell content and proliferative capacity was restricted to the nulliparous C/EBPdelta -/- females as no differences in mammary gland morphology, ductal branching or total epithelial content were observed between multiparous C/EBPdelta -/- and C/EBPdelta +/+ females. These results demonstrate that C/EBPdelta plays a novel role in mammary epithelial cell growth control that appears to be restricted to the nulliparous mammary gland.

C/EBP-beta, C/EBP-delta, PU.1, AML1 genes: mutational analysis in 381 samples of hematopoietic and solid malignancies

Mutations of transcription factors are associated with the pathogenesis of cancer. Genomic DNA from 381 cancers and cell lines representing leukemias, lymphomas and a variety of solid tumors were examined for mutations of genes coding for the C/EBP-beta, C/EBP-alpha, PU.1, and AML1 transcription factors using single strand conformation polymorphism (SSCP) and direct DNA sequencing. Mutation of C/EBP-beta (a chronic myelogenous leukemia cell line, Kcl22) and C/EBP-delta (a Burkitt's lymphoma cell line, Raji) were found. Interestingly, the sample with a C/EBP-beta alterations had two missense (P236L and G252A) and two silent mutations in a highly conserved region of the gene. The C/EBP-delta alteration in Raji was a missense mutation (A177G). These findings suggest that mutations of the C/EBP-beta, C/EBP-delta, PU.1, and AML1 rarely contribute to the development of hematopoietic or solid cancers.

Expression and function of CCAAT/enhancer binding proteinbeta (C/EBPbeta) LAP and LIP isoforms in mouse mammary gland, tumors and cultured mammary epithelial cells

CCAAT/Enhancer binding proteins (C/EBPs) play important roles in the regulation of cell growth and differentiation. This study investigated the expression and function of C/EBPbeta isoforms in the mouse mammary gland, mammary tumors, and a nontransformed mouse mammary epithelial cell line (HC11). C/EBPbeta mRNA levels are 2-5-fold higher in mouse mammary tumors derived from MMTV/c-neu transgenic mice compared with lactating and involuting mouse mammary gland. The "full-length" 38 kd C/EBPbeta LAP ("Liver-enriched Activator Protein") isoform is the predominant C/EBPbeta protein isoform in mammary tumor whole cell lysates, however, the truncated 20 kd C/EBPbeta LIP ("Liver-enriched Inhibitory Protein") isoform is also present at detectable levels (mean LAP:LIP ratio 5.3:1). The mammary tumor C/EBPbeta LAP:LIP ratio decreases 70% (from 5.3:1 to 1.6:1) when lysate preparation is switched from a rapid whole cell lysis protocol to a multistep nuclear/cytoplasmic fractionation protocol. In contrast to mammary tumors, only the C/EBPbeta LAP isoform is detectable in the mammary gland whole cell and nuclear lysates; the truncated "LIP" isoform is undetectable regardless of isolation protocol. Ectopic over expression of C/EBPbeta LIP or C/EBPbeta LAP did not alter HC11 growth rates. However, C/EBPbeta LIP over expressing HC11 cells (LAP:LIP ratio of approximately 1:1) exhibited a consistent 2-4 h delay in G(0)/S phase transition. C/EBPbeta LIP overexpressing HC11 cells did not express beta-casein mRNA (mammary epithelial cell differentiation marker) in response to lactogenic hormones. This defect in beta-casein expression was not corrected by carrying out the differentiation protocol in the presence of an artificial extracellular matrix. These results demonstrate that the "full-length" C/EBPbeta LAP isoform is the predominant C/EBPbeta protein isoform expressed in mouse mammary gland in vivo and mouse mammary epithelial cell cultures in vitro. C/EBPbeta LIP detected in mammary tumor lysates may result from in vivo production or ex vivo isolation-induced proteolysis of C/EBPbeta LAP. Ectopic overexpression of C/EBPbeta LIP (LAP:LIP ratio of approximately 1:1) inhibits mammary epithelial cell differentiation (beta-casein expression).

Cooperative effects of STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) on beta-casein gene transcription are mediated by the glucocorticoid receptor

Beta-casein gene transcription is controlled primarily by a composite response element (CoRE) that integrates signaling from the lactogenic hormones, PRL, insulin, and hydrocortisone, in mammary epithelial cells. This CoRE contains binding sites for STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) and several half-sites for glucocorticoid receptor (GR). To examine how interactions among these three transcription factors might regulate beta-casein gene transcription, a COS cell reconstitution system was employed. Cooperative transactivation was observed when all three factors were expressed, but unexpectedly was not seen between STAT5 and C/EBPbeta in the absence of full-length, transcriptionally active GR. Cooperativity required the amino-terminal transactivation domain of C/EBPbeta, and neither C/EBPalpha nor C/EBPdelta was able to substitute for C/EBPbeta when cotransfected with STAT5 and GR. Different GR determinants were needed for transcriptional cooperation between STAT5 and GR as compared with those required for all three transcription factors. These studies provide some new insights into the mechanisms responsible for high level, tissue-specific expression conferred by the beta-casein CoRE.