Mismatch repair protein MLH1 suppresses replicative stress in BRCA2-deficient breast tumors

Loss of BRCA2 (breast cancer 2) is lethal for normal cells. Yet it remains poorly understood how, in BRCA2 mutation carriers, cells undergoing loss of heterozygosity overcome the lethality and undergo tissue-specific neoplastic transformation. Here, we identified mismatch repair gene mutL homolog 1 (MLH1) as a genetic interactor of BRCA2 whose overexpression supports the viability of Brca2-null cells. Mechanistically, we showed that MLH1 interacts with Flap endonuclease 1 (FEN1) and competes to process the RNA flaps of Okazaki fragments. Together, they restrained the DNA2 nuclease activity on the reversed forks of lagging strands, leading to replication fork (RF) stability in BRCA2-deficient cells. In these cells, MLH1 also attenuated R-loops, allowing the progression of stable RFs, which suppressed genomic instability and supported cell viability. We demonstrated the significance of their genetic interaction by the lethality of Brca2-mutant mice and inhibition of Brca2-deficient tumor growth in mice by Mlh1 loss. Furthermore, we described estrogen as inducing MLH1 expression through estrogen receptor α (ERα), which might explain why the majority of BRCA2 mutation carriers develop ER-positive breast cancer. Taken together, our findings reveal a role of MLH1 in relieving replicative stress and show how it may contribute to the establishment of BRCA2-deficient breast tumors.

Stabilization of E-cadherin adhesions by COX-2/GSK3β signaling is a targetable pathway in metastatic breast cancer

Metastatic progression of epithelial cancers can be associated with epithelial-mesenchymal transition (EMT) including transcriptional inhibition of E-cadherin (CDH1) expression. Recently, EM plasticity (EMP) and E-cadherin-mediated, cluster-based metastasis and treatment resistance have become more appreciated. However, the mechanisms that maintain E-cadherin expression in this context are less understood. Through studies of inflammatory breast cancer (IBC) and a 3D tumor cell "emboli" culture paradigm, we discovered that cyclooxygenase 2 (COX-2; PTGS2), a target gene of C/EBPδ (CEBPD), or its metabolite prostaglandin E2 (PGE2) promotes protein stability of E-cadherin, β-catenin, and p120 catenin through inhibition of GSK3β. The COX-2 inhibitor celecoxib downregulated E-cadherin complex proteins and caused cell death. Coexpression of E-cadherin and COX-2 was seen in breast cancer tissues from patients with poor outcome and, along with inhibitory GSK3β phosphorylation, in patient-derived xenografts (PDX) including triple negative breast cancer (TNBC).Celecoxib alone decreased E-cadherin protein expression within xenograft tumors, though CDH1 mRNA levels increased, and reduced circulating tumor cell (CTC) clusters. In combination with paclitaxel, celecoxib attenuated or regressed lung metastases. This study has uncovered a mechanism by which metastatic breast cancer cells can maintain E-cadherin-mediated cell-to-cell adhesions and cell survival, suggesting that some patients with COX-2+/E-cadherin+ breast cancer may benefit from targeting of the PGE2 signaling pathway.

Abstract P1-06-03: Promotion of E-cadherin-mediated tumor cell adhesion by COX-2/GSK3β signaling is a targetable mechanism of metastatic breast cancer

Purpose: Metastatic progression and treatment resistance of breast cancer has been associated with epithelial-mesenchymal transition (EMT), including downregulation of E-cadherin (CDH1) gene expression, which can be initiated by inflammatory signaling such as by COX-2 (PTGS2). However, E-cadherin expression is maintained in many advanced breast cancers, including inflammatory BC (IBC), where it plays an essential role in forming tumor cell emboli within the cancer parenchyma and dermal lymph vasculature and which predict poor outcomes. Thus, the study of IBC offers an opportunity to understand the mechanisms of IBC and other aggressive BCs that lead to E-cadherin-associated cluster-based metastasis, which has recently received heightened recognition. Here, we have investigated the mechanisms that sustain E-cadherin expression in metastatic breast cancer to identify new targeted treatment options. Study Design and Methods: In vitro emboli formation assays and gene/protein expression studies with IBC (SUM149, IBC-3) and non-IBC (SUM159, MCF-7) cell lines along with gene silencing, overexpression, and pharmacological interventions. Analysis of experimental (SUM149) and spontaneous (orthotopic patient-derived xenograft, PDX) lung metastases and of circulating tumor cells (CTCs) in xenograft models treated with celecoxib (Cxb) and/or paclitaxel by imaging cytometry, immunohistochemistry, and/or Western analysis. Results: By analyzing the transcription factor C/EBPδ (CEBPD) and cells derived from inflammatory breast cancer (IBC), an aggressive breast cancer subtype that often presents with E-cadherin-dependent tumor cell emboli, we discovered that COX-2, unexpectedly, sustained E-cadherin protein expression without changing its mRNA levels. Using an in vitro tumor cell emboli culture paradigm (3D), we found that COX-2 or its metabolite PGE2 increased AKT activity and the inhibitory phosphorylation on GSK3β that prevents degradation of p120 catenin (CTNND1), a stabilizer of E-cadherin adhesion complexes. Conversely, the COX-2 inhibitor celecoxib downregulated E-cadherin specifically at the protein level and caused cell death in 3D. Co-expression of E-cadherin and COX-2 was seen in breast cancer patients with poor outcomes and, along with inhibitory GSK3β phosphorylation, in patient-derived xenografts (PDX) of metastatic triple-negative breast cancers (TNBC). Celecoxib alone decreased E-cadherin protein expression within xenograft primary tumors, reduced circulating tumor cells (CTCs) and clusters, and sensitized lung metastases to paclitaxel treatment. Conclusions: Our study uncovered a novel function of COX-2/PGE2 in promoting E-cadherin protein expression and cell-cell adhesions that are relevant for tumor cell cluster-based metastasis. Indeed, COX-2 inhibition reduced CTC clusters in a xenograft model, and sentized established metastases to chemotherapy. These results suggest that patients with COX-2+/E-cadherin+ metastastic BC, including IBC, may specifically benefit from targeting the PGE2 pathway in cobniation therapy approaches.

Citation Format: Kuppusamy Balamurugan, Saadiya Sehareen, Savitri Krishnamurthy, Shikha Sharan, Wei Tang, Naoto Ueno, Stefan Ambs, Dipak Poria, Esta Sterneck. Promotion of E-cadherin-mediated tumor cell adhesion by COX-2/GSK3β signaling is a targetable mechanism of metastatic breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-06-03.

Abstract 2031: The PERK arm of the unfolded protein response induces CXCL8 and CCL20 through C/EBPδ

Cancer cells often experience endoplasmic reticulum (ER) stress due to either activated oncogenes, microenvironmental conditions or therapeutic interventions. ER stress initiates an adaptive unfolded protein response (UPR) program, mediated by PERK, IRE1 and ATF6 transmembrane sensors, which alleviates the stress and promotes cells survival. However, if unmitigated, the UPR proceeds to initiation of cell death. Cancer cells often usurp the UPR pathways that contribute to cell survival and metastasis through intra- and intercellular mechanisms. Intracellular mechanisms of cellular adaptation are well-characterized; however, the intercellular consequences of ER stress are less understood. Here, we show that in cell lines representing melanoma, breast and prostate cancer, the transcription factor C/EBPδ (CEBPD) is an early ER stress response factor that is induced by the PERK arm of the UPR. CEBPD induction occurs in part through activation of PERK-mediated non-canonical JAK/STAT3 signaling. Transcriptional profiling in MDA-MB-435 melanoma cells revealed that C/EBPδ supports the expression of genes involved in cell intrinsic adaptations such as chaperones, components of ER associated degradation, and apoptosis inhibitors. In addition, C/EBPδ promotes expression of the inflammatory mediators CXCL8 and CCL20, which are known to modulate the tumor microenvironment through the receptors present on various host cells. Depletion of CEBPD by siRNA or inhibition of PERK kinase reduced CXCL8 and CCL20 transcripts and secreted proteins upon ER stress induction by thapsigargin. Bioinformatic analysis and the ENCODE ChIP-seq data suggest direct interaction of C/EBPδ with CXCL8 and CCL20 promoter/enhancer, indicative of direct transcriptional activation of the genes. Indeed, ectopic expression of C/EBPδ, but not a mutant lacking the transactivation domain, was sufficient to induced CXCL8 and CCL20 transcription in HEK293 cells in the absence of ER stress. Consistent with these results, conditioned media from ER-stressed MDA-MB-435 cells induced the gene expression of proinflammatory cytokines IL-6, IL-12, TNFα and IL-1RA in HL-60 promyelocytic cells while suppressing IL-10 expression. Taken together, these data show that the PERK arm of the UPR activates JAK/STAT3 pathway to induce expression of C/EBPδ and thereby enhance secretion of immunomodulators and transmit UPR-induced inflammatory signals to immune cells in the tumor microenvironment. This pathway may contribute to the overall outcome of the UPR on cancer cell survival, metastasis and response to anticancer therapies.

Citation Format: Dipak Kumar Poria, Namratha Sheshadri, Shikha Sharan, Esta Sterneck. The PERK arm of the unfolded protein response induces CXCL8 and CCL20 through C/EBPδ [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2031.

The STAT3 inhibitor Stattic acts independently of STAT3 to decrease histone acetylation and modulate gene expression

Signal transducer and activator of transcription 3 (STAT3) is an important transcription factor involved in many physiological functions including embryonic development and immune responses and is often activated under pathological conditions such as cancer. Strategies to inactivate STAT3 are being pursued as potential anticancer therapies and have led to the identification of Stattic (6-nitrobenzo[b]thiophene-1,1-dioxide) as a "specific" STAT3 inhibitor that is often used to interrogate STAT3-mediated gene expression in vitro and in vivo. Here, we show that Stattic exerts many STAT3-independent effects on cancer cells, calling for reassessment of results previously ascribed to STAT3 functions. Studies of the STAT3-deficient prostate cancer cell line PC-3 (PC3) along with STAT3-proficient breast cancer cell lines (MDA-MB-231, SUM149) revealed that Stattic attenuated histone acetylation and neutralized effects of the histone deacetylase (HDAC) inhibitor romidepsin. In PC3 cells, Stattic alone inhibited gene expression of CCL20 and CCL2, but activated expression of TNFA, CEBPD, SOX2, and MYC. In addition, we found that Stattic promoted autophagy and caused cell death. These data point to profound epigenetic effects of Stattic that are independent of its function as a STAT3 inhibitor. Our results demonstrate that Stattic directly or indirectly reduces histone acetylation and suggest reevaluation of Stattic and related compounds as polypharmacological agents through multipronged cytotoxic effects on cancer cells.

Abstract 2611: Comparison of 3D culture conditions to develop an in vitro model for breast cancer cell emboli formation

Inflammatory breast cancer (IBC) is a rare type of breast cancer accounting for 2-4% of all breast cancer patients, but accounts for 7-10% of breast cancer related mortality. IBC is characterized by extensive skin invasion and the formation of tumor cell emboli within the breast and dermal lymphatic system. Characteristics of emboli include cell to cell adhesions mediated by E-cadherin, encoded by the gene CDH1. Studies from our laboratory show that CEBPD is expressed in IBC patient emboli and promotes emboli formation in vitro. Mechanistically we found that CEBPD promotes cell-cell adhesion within emboli at least in part by supporting the expression of E-cadherin adhesion complex proteins. In this study, we compared an in vitro model for intra-lymphatic emboli formation with conditions of tumor sphere formation to begin to address the role of fluid shear in emboli formation and emboli-specific molecular events, if any. We conducted experiments under slow rocking and non-rocking culture conditions with 4 different culture media. We found that the mechanical shear forces (achieved by low speed rocking) generated large, well-compacted emboli-like structures within 24 hours of suspension culture. In the absence of fluid shear, the cells assembled into smaller and more irregular aggregates. In agreement with this, our preliminary data suggest that E-cadherin expression is modestly higher under fluid shear conditions in addition to overall increase under 3D conditions. We are now validating the mechanisms for higher expression of emboli in 3D culture conditions and are designing experiments to address the functional relevance of the pathways for IBC emboli biology. Understanding the molecular mechanisms of emboli formation may provide valuable insights into the uniquely aggressive nature of IBC.

Citation Format: Ariana Vitale, Esta Sterneck, Balamurugan Kuppusamy. Comparison of 3D culture conditions to develop an in vitro model for breast cancer cell emboli formation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2611.

Slug and E-Cadherin: Stealth Accomplices?

During physiological epithelial-mesenchymal transition (EMT), which is important for embryogenesis and wound healing, epithelial cells activate a program to remodel their structure and achieve a mesenchymal fate. In cancer cells, EMT confers increased invasiveness and tumor-initiating capacity, which contribute to metastasis and resistance to therapeutics. However, cellular plasticity that navigates between epithelial and mesenchymal states and maintenance of a hybrid or partial E/M phenotype appears to be even more important for cancer progression. Besides other core EMT transcription factors, the well-characterized Snail-family proteins Snail (SNAI1) and Slug (SNAI2) play important roles in both physiological and pathological EMT. Often mentioned in unison, they do, however, differ in their functions in many scenarios. Indeed, Slug expression does not always correlate with complete EMT or loss of E-cadherin (CDH1). For example, Slug plays important roles in mammary epithelial cell progenitor cell lineage commitment and differentiation, DNA damage responses, hematopoietic stem cell self-renewal, and in pathologies such as pulmonary fibrosis and atherosclerosis. In this Perspective, we highlight Slug functions in mammary epithelial cells and breast cancer as a “non-EMT factor” in basal epithelial cells and stem cells with focus reports that demonstrate co-expression of Slug and E-cadherin. We speculate that Slug and E-cadherin may cooperate in normal mammary gland and breast cancer/stem cells and advocate for functional assessment of such Slug⁺/E-cadherin^low/+ (SNAI2⁺/CDH1^low/+) “basal-like epithelial” cells. Thus, Slug may be regarded as less of an EMT factor than driver of the basal epithelial cell phenotype.

C/EBPδ links IL-6 and HIF-1 signaling to promote breast cancer stem cell-associated phenotypes

To improve cancer patient outcome significantly, we must understand the mechanisms regulating stem-like cancer cells, which have been implicated as a cause of metastasis and treatment resistance. The transcription factor C/EBPδ can exhibit pro- and anti-tumorigenic activities, but the mechanisms underlying the complexity of its functions are poorly understood. Here, we identify a role for breast cancer cell intrinsic C/EBPδ in promoting phenotypes that have been associated with cancer stem cells (CSC). While C/EBPδ expression is not abundant in most metastatic breast cancers, our data support a pro-tumorigenic role of C/EBPδ when expressed in subsets of tumor cells and/or through transient activation by the tumor microenvironment or loss of substrate adhesion. Using genetic mouse models and human breast cancer cell lines, we show that deletion or depletion of C/EBPδ reduced expression of stem cell factors and stemnness markers, sphere formation and self-renewal, along with growth of tumors and established experimental metastases in vivo. C/EBPδ is also known as a mediator of the innate immune response, which is enhanced by hypoxia and interleukin-6 (IL-6) signaling, two conditions that also play important roles in cancer progression. Our mechanistic data reveal C/EBPδ as a link that engages two positive feed-back loops, in part by directly targeting the IL-6 receptor (IL6RA) gene, and, thus, amplifying IL-6 and HIF-1 signaling. This study provides a molecular mechanism for the synergism of tumor micro-environmental conditions in cancer progression with potential implications for the targeting of cancer stem cells.

Scientific Summary from the Morgan Welch MD Anderson Cancer Center Inflammatory Breast Cancer (IBC) Program 10th Anniversary Conference

In 2006, a remarkable collaboration between University of Texas MD Anderson Cancer Center clinicians and Texas and New Mexico State legislators led to the formation of a dedicated IBC Research Program and Clinic at MD Anderson. This initiative provided funding and infrastructure to foster coordination of an IBC World Consortium of national and international experts, and launch the first ever IBC international conference in 2008, which brought together experts from around the world to facilitate collaborations and accelerate progress. Indeed great progress has been made since then. National and international experts in IBC convened at the 10^th Anniversary Conference of the MD Anderson IBC Clinic and Research Program and presented the most extensive sequencing analysis to date comparing IBC to non-IBC, gene- and protein-based immunoprofiling of IBC versus non-IBC patients, and converging lines of evidence on the specific role of the microenvironment in IBC. Novel models, unique metabolic mechanisms, and prominent survival pathways have been identified and were presented. Multiple clinical trials based on the work of the last decade are in progress or in development. The important challenges ahead were discussed. This progress and a coordinated summary of these works are presented herein.

The C/EBPδ protein is stabilized by estrogen receptor α activity, inhibits SNAI2 expression and associates with good prognosis in breast cancer

Hypoxia and inflammatory cytokines like interleukin-6 (IL-6, IL6) are strongly linked to cancer progression, and signal in part through the transcription factor Ccaat/enhancer-binding protein δ (C/EBPδ, CEBPD), which has been shown to promote mesenchymal features and malignant progression of glioblastoma. Here we report a different role for C/EBPδ in breast cancer. We found that the C/EBPδ protein is expressed in normal breast epithelial cells and in low-grade cancers. C/EBPδ protein (but not mRNA) expression correlates with estrogen receptor (ER+) and progesterone receptor (PGR) expression and longer progression-free survival of breast cancer patients. Specifically in ER+ breast cancers, CEBPD-but not the related CEBPB-mRNA in combination with IL6 correlated with lower risk of progression. Functional studies in cell lines showed that ERα promotes C/EBPδ expression at the level of protein stability by inhibition of the FBXW7 pathway. Furthermore, we found that C/EBPδ attenuates cell growth, motility and invasiveness by inhibiting expression of the SNAI2 (Slug) transcriptional repressor, which leads to expression of the cyclin-dependent kinase inhibitor CDKN1A (p21CIP1/WAF1). These findings identify a molecular mechanism by which ERα signaling reduces the aggressiveness of cancer cells, and demonstrate that C/EBPδ can have different functions in different types of cancer. Furthermore, our results support a potentially beneficial role for the IL-6 pathway specifically in ER+ breast cancer and call for further evaluation of the role of intra-tumoral IL-6 expression and of which cancers might benefit from current attempts to target the IL-6 pathway as a therapeutic strategy.

Abstract 2017: The CEBPD transcription factor, a marker of good prognosis in breast cancer, becomes a signaling hub for promotion of cancer cell stemness by hypoxia and interleukin-6

The transcription factor C/EBPdelta (CEBPD) is expressed in normal mammary epithelial cells and high expression in breast cancer correlates with hormone receptors and good prognosis (Mendoza-Villanueva et al., submitted). In the MMTV-Neu mouse model, deletion of CEBPD increases tumor multiplicity, but “paradoxically” reduces metastatic progression, suggesting that CEBPD has dual tumor suppressing and metastasis promoting functions (Balamurugan et al., 2010). Here, we identify a molecular mechanism for the tumor promoting function of CEBPD that may reach beyond breast cancer. We show that CEBPD serves as a signaling hub that integrates and amplifies pathways that promote stemness in breast cancer, glioblastoma and embryonic cancer cells. This work identifies a mechanism by which hypoxia and inflammation promote cancer stem-like cells (CSCs), which are implicated in tumor progression and resistance. Our earlier studies showed that CEBPD promotes inflammatory signaling and hypoxia adaptation by inhibiting the expression of FBXW7 (Balamurugan et al., 2010 and 2013), which can target several oncoproteins for degradation. Using a variety of established breast and glioblastoma tumor cell lines/xenografts, primary mouse mammary tumor cells and tissues, recent patient-derived glioblastoma cell lines, as well as embryonic stem/cancer cell lines, we find that deletion or depletion of CEBPD reduced the number of stem cells as determined by surface markers, sphere formation, limiting dilution xenografts, and by expression of stemness genes. Mechanistic studies in culture models show that CEBPD is expressed in CSCs and connects and amplifies both hypoxia (HIF-1) and inflammation (IL-6/STAT3) pathways to increase Notch1/NICD expression and cell stemness. CEBPD induces NICD expression transcriptionally through HIF-1 and at the level of the protein by inhibition of FBXW7-mediated degradation. In addition to its known role as a target of STAT3 and activator of IL-6 gene expression, we identified an additional role for CEBPD in promoting expression of the IL-6 receptor. Furthermore, we found that CEBPD also directly activates the expression of stemness-promoting genes such as OCT4, Sox2, KLF4, Myc, Nanog, CD44 and CD133. Taken together, this study provides novel insights into the molecular mechanisms that promote cancer cell stemness. Because CEBPD integrates and amplifies multiple stemness promoting pathways, CEBPD may represent a unique point of vulnerability in cancer stem cells, strongly suggesting that pharmacological inhibition of CEBPD signaling may effectively target CSCs. Results from current efforts to pharmacologically downregulate CEBPD in cancer stem cells will also be presented.

Citation Format: Kuppusamy Balamurugan, Daniel Mendoza-Villanueva, Barbara Rath, Philip Tofilon, Shikha Sharan, Esta Sterneck. The CEBPD transcription factor, a marker of good prognosis in breast cancer, becomes a signaling hub for promotion of cancer cell stemness by hypoxia and interleukin-6. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2017.

Abstract A17: The CEBPD transcription factor: A signaling hub for promotion of cancer cell stemness

Our previous studies showed that the transcription factor CCAAT/enhancer binding protein delta (CEBPD) promotes tumor metastasis in a mouse model of mammary tumorigenesis (MMTV-Neu) (Balamurugan et al., EMBO J, 2010). Here we show that CEBPD serves as a signaling hub, which integrates and amplifies signaling pathways that promote stemness in breast cancer and glioblastoma as well as in embryonic carcinoma stem cells. This work identifies a mechanism by which hypoxia and inflammation promote cancer stem-like cells (CSCs), which are implicated in tumor progression, metastasis, and therapy resistance.

We have shown in our earlier studies that CEBPD promotes inflammatory signaling and hypoxia adaptation by inhibiting the expression of the F-box protein FBXW7, a bona fide tumor suppressor (Balamurugan et al., EMBO J 2010; Nature Communications 2013). Because FBXW7 can target the stem cell factor Notch for degradation, we hypothesized that CEBPD can promote cell stemness. We tested this hypothesis using a variety of established breast and glioblastoma tumor cell lines/xenografts, primary mouse mammary tumor cells and tissues, recent patient-derived glioblastoma cell lines, as well as embryonic stem/cancer cell lines. Deletion or depletion of CEBPD reduced the number of stem cells as determined by surface markers, sphere formation, limiting dilution xenografts, and by expression of stemness genes. Mechanistic studies in culture models show that CEBPD is expressed in CSCs and connects and amplifies both hypoxia (HIF-1) and inflammation (IL-6/STAT3) pathways to increase Notch1 activity and cell stemness. Furthermore, we found that CEBPD also directly activates the expression of stemness associated genes such as OCT4, Sox2, KLF4, Myc, Nanog, CD44 and CD133. Taken together, this study provides novel insights into the molecular mechanisms that promote cancer cell stemness, and identifies a central role for CEBPD in integrating HIF-1α,

IL-6/STAT3, and Notch1 signaling, while also activating expression of stemness factors. These results strongly suggest that pharmacological inhibition of CEBPD signaling may effectively target CSCs.

Citation Format: Balamurugan Kuppusamy, Daniel Mendoza-Villaneuva, Glenn Summers, Shikha Sharan, Esta Sterneck. The CEBPD transcription factor: A signaling hub for promotion of cancer cell stemness. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr A17.

Abstract 1913: C/EBPdelta links hypoxia and inflammation to the promotion of tumor cell stemness through inhibition of FBXW7: A molecular target for HDAC inhibitor action

Hypoxia and inflammation are linked to progression of epithelial cancers by promoting epithelial-mesenchymal transition (EMT) and cancer stem cells (CSCs). A better understanding of the mechanisms by which these conditions augment tumor progression is necessary to combat metastases and treatment resistance. The transcription factor C/EBPδ promotes hypoxia adaptation and inflammatory signaling by inhibiting expression of FBXW7, which targets many oncoproteins (e.g. Notch, c-myc) and also C/EBPδ for degradation (Balamurugan et al., EMBO J, 2010; Nature Commun 2013). FBXW7 is also known to regulate self-renewal, differentiation and quiescence of neural and hematopoietic stem cells. Therefore, we hypothesized that C/EBPδ supports the development of CSCs through inhibition of FBXW7 and that this pathway has potential as therapeutic target.

We investigated the roles of C/EBPδ and FBXW7 through overexpression and silencing approaches in human breast tumor cell lines and primary mouse mammary tumors; followed by gene expression analyses and assays for stem cell-like functions under hypoxia and in response to interleukin 6 (IL-6). We found that C/EBPδ augments _ while FBXW7 inhibits _ hypoxia- and IL-6-induced formation of mammospheres, a surrogate assay for stem cell function, and expression of markers for EMT and CSCs. Furthermore, C/EBPδ was necessary for hypoxia-induced expression of IL-6, providing a link between these pathways. Indeed, silencing of C/EBPδ delayed the outgrowth of MCF-7 xenograft tumors, indicating that C/EBPδ supports CSC development.

Because C/EBPδ can interact with histone deacetylases (HDACs), we tested the effect of HDAC inhibitors (HDACi) that are in clinical trials for breast cancer (e.g. SAHA). HDACi inhibited C/EBPδ expression and induced FBXW7 expression. Furthermore, HDACi reduced mammosphere formation specifically under hypoxia, and this effect could be partially reversed by FBXW7 knockdown. These results suggest a therapeutic benefit of HDACi for breast cancer through activation of FBXW7 and inhibition of C/EBPδ followed by reduction in CSCs. Future investigations will explore drug combinations with HDACi to maximize the effectiveness of the FBXW7 tumor suppressor pathway under hypoxia and inflammatory conditions.

Citation Format: Kuppusamy Balamurugan, Glenn H. Summers, Shikha Sharan, Esta Sterneck. C/EBPdelta links hypoxia and inflammation to the promotion of tumor cell stemness through inhibition of FBXW7: A molecular target for HDAC inhibitor action. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1913. doi:10.1158/1538-7445.AM2014-1913

C/EBPδ deficiency sensitizes mice to ionizing radiation-induced hematopoietic and intestinal injury

Knowledge of the mechanisms involved in the radiation response is critical for developing interventions to mitigate radiation-induced injury to normal tissues. Exposure to radiation leads to increased oxidative stress, DNA-damage, genomic instability and inflammation. The transcription factor CCAAT/enhancer binding protein delta (Cebpd; C/EBPδ is implicated in regulation of these same processes, but its role in radiation response is not known. We investigated the role of C/EBPδ in radiation-induced hematopoietic and intestinal injury using a Cebpd knockout mouse model. Cebpd−/− mice showed increased lethality at 7.4 and 8.5 Gy total-body irradiation (TBI), compared to Cebpd+/+ mice. Two weeks after a 6 Gy dose of TBI, Cebpd−/− mice showed decreased recovery of white blood cells, neutrophils, platelets, myeloid cells and bone marrow mononuclear cells, decreased colony-forming ability of bone marrow progenitor cells, and increased apoptosis of hematopoietic progenitor and stem cells compared to Cebpd+/+ controls. Cebpd−/− mice exhibited a significant dose-dependent decrease in intestinal crypt survival and in plasma citrulline levels compared to Cebpd+/+ mice after exposure to radiation. This was accompanied by significantly decreased expression of γ-H2AX in Cebpd−/− intestinal crypts and villi at 1 h post-TBI, increased mitotic index at 24 h post-TBI, and increase in apoptosis in intestinal crypts and stromal cells of Cebpd−/− compared to Cebpd+/+ mice at 4 h post-irradiation. This study uncovers a novel biological function for C/EBPδ in promoting the response to radiation-induced DNA-damage and in protecting hematopoietic and intestinal tissues from radiation-induced injury.

The many faces of C/EBPδ and their relevance for inflammation and cancer

The CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is a transcription factor that modulates many biological processes including cell differentiation, motility, growth arrest, proliferation, and cell death. The diversity of C/EBPδ's functions depends in part on the cell type and cellular context and can have opposing outcomes. For example, C/EBPδ promotes inflammatory signaling, but it can also inhibit pro-inflammatory pathways, and in a mouse model of mammary tumorigenesis, C/EBPδ reduces tumor incidence but promotes tumor metastasis. This review highlights the multifaceted nature of C/EBPδ's functions, with an emphasis on pathways that are relevant for cancer and inflammation, and illustrates how C/EBPδ emerged from the shadow of its family members as a fascinating “jack of all trades.” Our current knowledge on C/EBPδ indicates that, rather than being essential for a specific cellular process, C/EBPδ helps to interpret a variety of cues in a cell-type and context-dependent manner, to adjust cellular functions to specific situations. Therefore, insights into the roles and mechanisms of C/EBPδ signaling can lead to a better understanding of how the integration of different signaling pathways dictates normal and pathological cell functions and physiology.

Positional variations in mammary gland development and cancer

Most mammals develop their mammary glands in pairs of which the two counterparts are symmetrically displaced away from the ventral midline. Based on this symmetry and the same functional outcome as a milk-producing organ, the mammary glands are easily presumed to be mere copies of one another. Based on our analysis of published data with inclusion of new results related to mammary development and pathology in mice, we argue that this presumption is incorrect: Between and within pairs, mammary glands differ from one another, and tumor incidence and biology depend on the position along the anterior-posterior and the left-right axis as well. This insight has implications for experimental designs with mouse models and for data extrapolation between mammary glands within and between species. We suggest that improved documentation of location-specific mammary gland features will lead to more insights into the molecular mechanisms of mammary gland development and cancer biology in both mice and humans.

Abstract 3464: CEBPD (C/EBPδ) acts as a tumor suppressor in hormone receptor positive breast cancer cells and may serve as biomarker to predict the need for adjuvant therapy

The transcription factor CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is expressed in normal breast epithelial cells and down-regulated in breast cancer tissue and cell lines. In the MMTV-c-Neu mouse mammary tumor model, loss of CEBPD leads to increased tumor incidence but reduced tumor metastasis, suggesting dual functions as a tumor suppressor and tumor promoter.

To investigate the role of CEBPD in human breast cancer, we analyzed tissue microarrays (TMA) by immunostaining. Analysis of 79 breast tumor samples confirmed overall downregulation of CEBPD in cancer but also revealed that CEBPD was expressed in 50% of estrogen receptor (ER) positive (+) breast tumors and associated with longer disease-specific patient survival (P=0.034, log-rank test). Furthermore, CEBPD was correlated with expression of progesterone receptor (PR; P=0.0007). Results from an independent TMA of 292 tumors from patients that were randomized to no adjuvant treatment or two years of tamoxifen (TAM) treatment confirmed that CEBPD expression was an independent good prognostic marker for ER(+) patients (progression-free survival, Multivariate Cox analysis P=0.041). Within the subgroup with high PR labeling, low or no expression of CEBPD was associated with worst prognosis if untreated and associated with significant response to TAM (P=0.034). On the other hand, patients with high CEBPD had better prognosis but did not further benefit from TAM. These data indicate that CEBPD expression is significantly associated with ER/PR(+) breast cancer and has potential as prognostic and/or predictive biomarker.

To investigate the functions of CEBPD we silenced its expression in the ER/PR(+) MCF-7 breast tumor cell line. We found that CEBPD attenuates proliferation and promotes estrogen-dependence. In this cell culture model, CEBPD augmented MCF-7 cell sensitivity to TAM. Furthermore, we found that CEBPD promotes cell-cell adhesion through expression of alpha-catenin. These observations are consistent with a tumor suppressor-like function for CEBPD in ER/PR(+) cancer cells. Currently, we are using ChIP-Seq and mRNA-Seq approaches in MCF-7 cells to identify a CEBPD-dependent gene signature and test its prognostic value in breast cancer gene expression data. Our long-term goal is to contribute to the development of more precise multiparameter predictors of outcome and/or response through the combination of biological assays and analysis of gene expression signatures.

Citation Format: Daniel Y. Mendoza-Villanueva, Suryun Kim, H.Raza Ali, Shikha Sharan, Tapasree R. Sarkar, Carlos Caldas, Göran Landberg, Esta Sterneck. CEBPD (C/EBPδ) acts as a tumor suppressor in hormone receptor positive breast cancer cells and may serve as biomarker to predict the need for adjuvant therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3464. doi:10.1158/1538-7445.AM2013-3464

Abstract 3955: The tumor suppressor C/EBPdelta promotes hypoxia-induced EMT and expression of stem cell markers through inhibition of FBXW7: Implications for mammary tumor metastasis

The transcription factor CCAAT/enhancer binding protein delta (C/EBPδ) exhibits tumor-suppressor like-qualities such as downregulation of cyclin D1, and promotion of growth arrest and apoptosis. However, using a mouse model of mammary tumorigenesis (MMTV-Neu), we had found that C/EBPδ has dual functions. C/EBPδ suppresses primary tumor incidence while also augmenting metastatic progression. The pro-metastatic function of C/EBPδ can be correlated with a role in hypoxia adaptation and expression of pro-inflammatory genes and macrophage activation. C/EBPδ exerts both of these functions through inhibition of FBXW7, a component of SCF E3 ubiquitin ligase complex, which targets substrates such as c-myc and Notch for degradation (Balamurugan et al, 2010; Balamurugan et al., under revision). Hypoxia is believed to promote tumor progression in part by inducing epithelial-mesenchymal transition (EMT), which contributes to the generation of stem-like cancer cells (CSCs). In the present study, we found that hypoxia-induced EMT and expression of CSCs markers were compromised in Cebpd null primary tumor cells and in human breast tumor cells when C/EBPδ expression was silenced. On the other hand, silencing of the C/EBPδ-repressed gene FBXW7 was sufficient to induce a mesenchymal-like phenotype. Loss of FBXW7 has been shown to promote acquisition of stemness. In line with this notion, we found that overexpression of FBXW7 in the breast tumor cell line MDA-MB-468 reduced expression of aldehyde dehydrogenase 1 (Aldh1), C/EBPδ and the active form of NOTCH (NICD). Together, these data suggest that C/EBPδ augments - whereas its target FBXW7 prevents - EMT and breast cancer stem cell development, which may be an additional mechanism by which C/EBPδ promotes metastatic progression. Our current investigations are aimed at understanding how the different functions of C/EBPδ are integrated depending on cellular environment and context that results in its tumor promoting function despite its tumor suppressor role.

Citation Format: Balamurugan Kuppusamy, Esta Sterneck. The tumor suppressor C/EBPdelta promotes hypoxia-induced EMT and expression of stem cell markers through inhibition of FBXW7: Implications for mammary tumor metastasis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3955. doi:10.1158/1538-7445.AM2013-3955

The tumour suppressor C/EBPδ inhibits FBXW7 expression and promotes mammary tumour metastasis

Inflammation and hypoxia are known to promote the metastatic progression of tumours. The CCAAT/enhancer-binding protein-δ (C/EBPδ, CEBPD) is an inflammatory response gene and candidate tumour suppressor, but its physiological role in tumourigenesis in vivo is unknown. Here, we demonstrate a tumour suppressor function of C/EBPδ using transgenic mice overexpressing the Neu/Her2/ERBB2 proto-oncogene in the mammary gland. Unexpectedly, this study also revealed that C/EBPδ is necessary for efficient tumour metastasis. We show that C/EBPδ is induced by hypoxia in tumours in vivo and in breast tumour cells in vitro, and that C/EBPδ-deficient cells exhibit reduced glycolytic metabolism and cell viability under hypoxia. C/EBPδ supports CXCR4 expression. On the other hand, C/EBPδ directly inhibits expression of the tumour suppressor F-box and WD repeat-domain containing 7 gene (FBXW7, FBW7, AGO, Cdc4), encoding an F-box protein that promotes degradation of the mammalian target of rapamycin (mTOR). Consequently, C/EBPδ enhances mTOR/AKT/S6K1 signalling and augments translation and activity of hypoxia-inducible factor-1α (HIF-1α), which is necessary for hypoxia adaptation. This work provides new insight into the mechanisms by which metastasis-promoting signals are induced specifically under hypoxia.

CEBPD reverses RB/E2F1-mediated gene repression and participates in HMDB-induced apoptosis of cancer cells

PURPOSE

Recent evidence indicates that a tumor suppressor gene CEBPD (CCAAT/enhancer-binding protein delta) is downregulated in many cancers including cervical cancer, which provides a therapeutic potential associated with its reactivation. However, little is known for CEBPD activators and the effect of reactivation of CEBPD transcription upon anticancer drug treatment. In this study, we identified a novel CEBPD activator, 1-(2-hydroxy-5-methylphenyl)-3-phenyl-1,3-propanedione (HMDB). The purpose of this study is to characterize the mechanism of HMDB-induced CEBPD activation and its potential effect in cancer therapy.

EXPERIMENTAL DESIGN

Methylation-specific PCR assay, reporter assay, and chromatin immunoprecipitation (ChIP) assay were performed to dissect the signaling pathway of HMDB-induced CEBPD transcription. Furthermore, a consequence of HMDB-induced CEBPD expression was linked with E2F1 and retinoblastoma (RB), which discloses the scenario of CEBPD, E2F1, and RB bindings and transcriptional regulation on the promoters of proapoptotic genes, PPARG2 and GADD153. Finally, the anticancer effect of HMDB was examined in xenograft mice.

RESULTS

We demonstrate that CEBPD plays an essential role in HMDB-mediated apoptosis of cancer cells. HMDB up-regulates CEBPD transcription through the p38/CREB pathway, thus leading to transcriptional activation of PPARG2 and GADD153. Furthermore, increased level of CEBPD attenuates E2F1-induced cancer cell proliferation and partially rescues RB/E2F1-mediated repression of PPARG2 and GADD153 transcription. Moreover, HMDB treatment attenuates the growth of A431 xenografts in severe combined immunodeficient mice mice.

CONCLUSIONS

These results clearly demonstrate that HMDB kills cancer cells through activation of CEBPD pathways and suggest that HMDB can serve as a superior chemotherapeutic agent with limited potential for adverse side effects.

C/EBP{delta} targets cyclin D1 for proteasome-mediated degradation via induction of CDC27/APC3 expression

The transcription factor CCAAT/enhancer binding protein delta (C/EBPdelta, CEBPD, NFIL-6beta) has tumor suppressor function; however, the molecular mechanism(s) by which C/EBPdelta exerts its effect are largely unknown. Here, we report that C/EBPdelta induces expression of the Cdc27 (APC3) subunit of the anaphase promoting complex/cyclosome (APC/C), which results in the polyubiquitination and degradation of the prooncogenic cell cycle regulator cyclin D1, and also down-regulates cyclin B1, Skp2, and Plk-1. In C/EBPdelta knockout mouse embryo fibroblasts (MEF) Cdc27 levels were reduced, whereas cyclin D1 levels were increased even in the presence of activated GSK-3beta. Silencing of C/EBPdelta, Cdc27, or the APC/C coactivator Cdh1 (FZR1) in MCF-10A breast epithelial cells increased cyclin D1 protein expression. Like C/EBPdelta, and in contrast to cyclin D1, Cdc27 was down-regulated in several breast cancer cell lines, suggesting that Cdc27 itself may be a tumor suppressor. Cyclin D1 is a known substrate of polyubiquitination complex SKP1/CUL1/F-box (SCF), and our studies show that Cdc27 directs cyclin D1 to alternative degradation by APC/C. These findings shed light on the role and regulation of APC/C, which is critical for most cellular processes.

Abstract 2558: CEBPD induces SOD1 and cisplatin resistance in urothelial carcinoma

Bladder cancer is the fourth most common type of cancer in men (ninth in women) in the United States. Cisplatin is an effective agent against the most common subtype, urothelial carcinoma. However, the development of chemotherapy resistance is a severe clinical problem for the successful treatment of cancers. A better understanding of the cellular and molecular events in response to cisplatin treatment and the development of resistance are critical to improve the therapeutic options for patients. Here, we report that expression of the CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ, NFIL-6β) is induced by cisplatin in the human bladder urothelial carcinoma NTUB1 cell line and is specifically elevated in a cisplatin resistant subline. Expression of CEBPD reduces cisplatin-induced reactive oxygen species (ROS) and apoptosis in NTUB1 cells by inducing the expression of Cu/Zn-superoxide dismutase (SOD1) via direct promoter trans-activation. Several reports have implicated CEBPD as a tumor suppressor gene. This study reveals a novel role for CEBPD in conferring drug resistance, suggesting that it can also be pro-oncogenic. Furthermore, our data suggest that SOD inhibitors, which are already used as antiangiogenic agents, may be suitable for combinatorial chemotherapy to prevent or treat cisplatin resistance in bladder and possibly other cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2558.

Transcriptional up-regulation of SOD1 by CEBPD: a potential target for cisplatin resistant human urothelial carcinoma cells

Bladder cancer is the fourth most common type of cancer in men (ninth in women) in the United States. Cisplatin is an effective agent against the most common subtype, urothelial carcinoma. However, the development of chemotherapy resistance is a severe clinical problem for the successful treatment of this and other cancers. A better understanding of the cellular and molecular events in response to cisplatin treatment and the development of resistance are critical to improve the therapeutic options for patients. Here, we report that expression of the CCAAT/enhancer binding protein delta (CEBPD, C/EBPdelta, NF-IL6beta) is induced by cisplatin in the human bladder urothelial carcinoma NTUB1 cell line and is specifically elevated in a cisplatin resistant subline. Expression of CEBPD reduced cisplatin-induced reactive oxygen species (ROS) and apoptosis in NTUB1 cells by inducing the expression of Cu/Zn-superoxide dismutase (SOD1) via direct promoter transactivation. Several reports have implicated CEBPD as a tumor suppressor gene. This study reveals a novel role for CEBPD in conferring drug resistance, suggesting that it can also be pro-oncogenic. Furthermore, our data suggest that SOD inhibitors, which are already used as anti-angiogenic agents, may be suitable for combinatorial chemotherapy to prevent or treat cisplatin resistance in bladder and possibly other cancers.

MAPK3/1 (ERK1/2) in ovarian granulosa cells are essential for female fertility

A surge of luteinizing hormone (LH) from the pituitary gland triggers ovulation, oocyte maturation, and luteinization for successful reproduction in mammals. Because the signaling molecules RAS and ERK1/2 (extracellular signal-regulated kinases 1 and 2) are activated by an LH surge in granulosa cells of preovulatory follicles, we disrupted Erk1/2 in mouse granulosa cells and provide in vivo evidence that these kinases are necessary for LH-induced oocyte resumption of meiosis, ovulation, and luteinization. In addition, biochemical analyses and selected disruption of the Cebpb gene in granulosa cells demonstrate that C/EBPbeta (CCAAT/Enhancer-binding protein-beta) is a critical downstream mediator of ERK1/2 activation. Thus, ERK1/2 and C/EBPbeta constitute an in vivo LH-regulated signaling pathway that controls ovulation- and luteinization-related events.

CAATT/enhancer-binding proteins alpha and delta interact with NKX2-1 to synergistically activate mouse secretoglobin 3A2 gene expression

Secretoglobin (SCGB) 3A2 is a small molecular weight secreted protein predominantly expressed in lung airways. We previously demonstrated that the expression of SCGB3A2 is regulated by homeodomain transcription factor NKX2-1. Here we show that CCAAT/enhancer-binding proteins, C/EBPalpha and C/EBPdelta, regulate mouse Scgb3a2 gene transcription in vivo and in vitro by binding to specific sites located in the Scgb3a2 promoter and the activity is synergistically enhanced through cooperative interaction with NKX2-1. Six C/EBP binding sites lie within 500 bp of the Scgb3a2 gene promoter, of which two sites, located at -44 to -54 bp and -192 to -201 bp, appear to be critical for the synergistic activation of Scgb3a2 gene transcription with NKX2-1. All three transcription factors, C/EBPalpha, C/EBPdelta, and NKX2-1, are expressed in the epithelial cells of airways, particularly the bronchus, where high expression of SCGB3A2 is found. The expression of these transcription factors markedly increases toward the end of gestation, which coincides with the marked increase of SCGB3A2, suggesting the importance of C/EBPalpha and C/EBPdelta, and their synergistic interaction with NKX2-1 in mouse Scgb3a2 gene transcription and lung development.

SLC5A8 triggers tumor cell apoptosis through pyruvate-dependent inhibition of histone deacetylases

Tumor cells up-regulate glycolysis but convert pyruvate into lactate instead of oxidizing it. Here, we show that pyruvate, but not lactate, is an inhibitor of histone deacetylases (HDAC) and an inducer of apoptosis in tumor cells and that SLC5A8, a Na(+)/monocarboxylate cotransporter, is obligatory for this process. We found that SLC5A8 is expressed in nontransformed breast epithelial cell lines but silenced by DNA methylation in tumor cell lines. The down-regulation of the gene is also evident in primary breast tumors. When MCF7 breast tumor cells are transfected with SLC5A8 cDNA, the cells undergo pyruvate-dependent apoptosis. Butyrate and propionate also induce apoptosis in SLC5A8-expressing cells, whereas lactate does not. The differential ability of these monocarboxylates to cause apoptosis in SLC5A8-expressing MCF7 cells correlates with their ability to inhibit HDACs. Apoptosis induced by SLC5A8/pyruvate in MCF7 cells is associated with up-regulation of p53, Bax, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), TRAIL receptor (TRAILR) 1, and TRAILR2 and down-regulation of Bcl2 and survivin. Lactate dehydrogenase isozymes are differentially expressed in nontransformed cells and tumor cells such that the latter convert pyruvate into lactate. Silencing of SLC5A8 coupled with conversion of pyruvate into lactate in tumor cells correlates with increased HDAC activity in these cells compared with nontransformed cells. Our studies thus identify pyruvate as a HDAC inhibitor and indicate that the Na(+)-coupled pyruvate transport underlies the tumor-suppressive role of SLC5A8. We propose that tumor cells silence SLC5A8 and convert pyruvate into lactate as complementary mechanisms to avoid pyruvate-induced cell death.

c/ebpδ Null Mouse as a Model for the Double Knock-out of slc5a8 and slc5a12 in Kidney

slc5a8 and slc5a12 represent the high affinity and low affinity Na+/lactate co-transporters, respectively, in the kidney. Here we show that these transporters are expressed in the apical membrane of the proximal tubular cells in mouse kidney, indicating that these transporters are likely to mediate the first step in the renal reabsorption of lactate. Interestingly, the renal expression of both transporters is almost completely ablated in mice homozygous for the deletion of the transcription factor c/ebpdelta. This effect is tissue-specific since the expression of the transporters is not affected in non-renal tissues. The functional role of C/EBPdelta in the expression of SLC5A8 and SLC5A12 is demonstrable in HEK293 cells in reporter assays using gene-specific promoters. The ablation of the transporters in the kidney is accompanied by a marked increase in urinary excretion of lactate as well as a decrease in blood levels of lactate in c/ebpdelta-/- mice. These data provide evidence for an obligatory role for slc5a8 and slc5a12 in the renal absorption of lactate. In addition, we show that urinary excretion of urate is significantly elevated in c/ebpdelta-/- mice even though the expression of URAT1, the transporter responsible for the apical membrane uptake of urate in renal proximal tubule, is not altered. These data provide in vivo evidence for the functional coupling between lactate reabsorption and urate reabsorption in the kidney. Thus, the fortuitous double knock-out of slc5a8 and slc5a12 in kidney in c/ebpdelta-/- mice reveals the physiologic role of these transporters in the renal handling of lactate and urate.

Conditional ablation of C/EBP beta demonstrates its keratinocyte-specific requirement for cell survival and mouse skin tumorigenesis

The CCAAT/enhancer binding protein beta (C/EBP beta) is implicated in the regulation of many different molecular and physiological processes. Mice with a germline deletion of C/EBP beta (C/EBP beta(-/-)) display phenotypes in a multitude of cell types and organ systems, including skin where C/EBP beta(-/-) mice exhibit increased apoptosis in epidermal keratinocytes in response to carcinogen treatment and are completely resistant to carcinogen-induced skin tumorigenesis. To determine the contribution of systemic versus cell autonomous functions of C/EBP beta to specific phenotypes, mice with a conditional 'floxed' C/EBP beta null allele were generated. Epidermal-specific deletion of C/EBP beta was achieved by Cre recombinase expression from a keratin 5 (K5) promoter. Similar to C/EBP beta(-/-) mice, K5-Cre;C/EBP beta(fl/fl) mice were completely refractory to 7,12 dimethylbenz[a]anthracene (DMBA)-induced skin tumorigenesis and these mice displayed increased DMBA-induced apoptosis in epidermal keratinocytes compared to wild-type mice. In contrast, mice lacking the related gene, C/EBP delta, were not resistant to DMBA-induced skin tumorigenesis, indicating a unique role of C/EBP beta in skin tumor development. Our findings demonstrate that C/EBP beta exerts an essential, keratinocyte-intrinsic role in cell survival in response to carcinogen treatment and the elimination of C/EBP beta in keratinocytes is sufficient to confer complete resistance of the skin to chemical carcinogenesis.

C/EBPdelta is a crucial regulator of pro-apoptotic gene expression during mammary gland involution

The STAT3 transcription factor is an important initiator of mammary gland involution in the mouse. This work shows that the STAT3 target gene CCAAT/enhancer binding protein delta (C/EBPdelta) is a crucial mediator of pro-apoptotic gene expression events in mammary epithelial cells. In the absence of C/EBPdelta, involution is delayed, the pro-apoptotic genes encoding p53, BAK, IGFBP5 and SGP2/clusterin are not activated, while the anti-apoptotic genes coding for BFL1 and Cyclin D1 are not repressed. Consequently, p53 targets such as survivin, BRCA1, BRCA2 and BAX are not regulated appropriately and protease activation is delayed. Furthermore, expression of MMP3 and C/EBPdelta during the second phase of involution is perturbed in the absence of C/EBPdelta. In HC11 cells, C/EBPdelta alone is sufficient to induce IGFBP5 and SGP2. It also suppresses Cyclin D1 expression and cooperates with p53 to elicit apoptosis. This study places C/EBPdelta between STAT3 and several pro- and anti-apoptotic genes promoting the physiological cell death response in epithelial cells at the onset of mammary gland involution.

Repression of the inhibin alpha-subunit gene by the transcription factor CCAAT/enhancer-binding protein-beta

Inhibin is a dimeric peptide hormone produced in ovarian granulosa cells that suppresses FSH synthesis and secretion in the pituitary. Expression of inhibin alpha- and beta-subunit genes in the rodent ovary is positively regulated by FSH and negatively regulated after the preovulatory LH surge. We have investigated the role of the transcription factor CCAAT/enhancer-binding protein-beta (C/EBPbeta) in repressing the inhibin alpha-subunit gene. C/EBPbeta knockout mice fail to appropriately down-regulate inhibin alpha-subunit mRNA levels after treatment with human chorionic gonadotropin, indicating that C/EBPbeta may function to repress inhibin gene expression. The expression and regulation of C/EBPbeta were examined in rodent ovary, and these studies show that C/EBPbeta is expressed in ovary and granulosa cells and is induced in response to human chorionic gonadotropin. Transient cotransfections with an inhibin promoter-luciferase reporter in a mouse granulosa cell line, GRMO2 cells, show that C/EBPbeta is capable of repressing both basal and forskolin-stimulated inhibin gene promoter activities. An upstream binding site for C/EBPbeta in the inhibin alpha-subunit promoter was identified by electrophoretic mobility shift assays, which, when mutated, results in elevated inhibin promoter activity. However, C/EBPbeta also represses shorter promoter constructs lacking this site, and this component of repression is dependent on the more proximal promoter cAMP response element (CRE). Electrophoretic mobility shift assays show that C/EBPbeta effectively competes with CRE-binding protein for binding to this atypical CRE. Thus, there are two distinct mechanisms by which C/EBPbeta represses inhibin alpha-subunit gene expression in ovarian granulosa cells.

Comparison of mammary gland involution between 129S1 and C57BL/6 inbred mouse strains: differential regulation of Bcl2a1, Trp53, Cebpb, and Cebpd expression

Genetic engineering has made the mouse an invaluable tool to address the function of individual genes in a targeted manner. Over the last decade it has become apparent that the genetic mouse strain background can significantly influence the phenotype of an engineered mouse. Therefore, it is essential to characterize the biology of the different wild-type background strains. In this study, we have compared mouse mammary gland involution in the 129S1 and C57BL/6 inbred strains and report significant differences at the molecular level with differential expression of Bcl2a1 (Bfl1), Trp53 (p53), Cebpb (C/EBP beta), and Cebpd (C/EBP delta). The C57BL/6 strain exhibits dynamic responses with induction of Trp53 and Cebpd and concomitant downregulation of Bcl2a1 during the first phase of involution. In contrast, expression of these genes does not change significantly in 129S1 mice. During the second phase, C57BL/6 glands contain more Cebpb than 129S1 glands. Nevertheless, involution proceeds morphologically with similar kinetics in both strains. The data demonstrate that the genetic response of mammary tissue varies significantly between 129S1 and C57BL/6. These results may provide a basis for the interpretation of strain-specific phenotypes in engineered mice and underline the importance of pure strains for large-scale expression studies with mutant mice.

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.

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.

An essential role for a MEK-C/EBP pathway during growth factor-regulated cortical neurogenesis

Mammalian neurogenesis is determined by an interplay between intrinsic genetic mechanisms and extrinsic cues such as growth factors. Here we have defined a signaling cascade, a MEK-C/EBP pathway, that is essential for cortical progenitor cells to become postmitotic neurons. Inhibition of MEK or of the C/EBP family of transcription factors inhibits neurogenesis while expression of a C/EBPbeta mutant that is a phosphorylation-mimic at a MEK-Rsk site enhances neurogenesis. C/EBP mediates this positive effect by direct transcriptional activation of neuron-specific genes such as Talpha1 alpha-tubulin. Conversely, inhibition of C/EBP-dependent transcription enhances CNTF-mediated generation of astrocytes from the same progenitor cells. Thus, activation of a MEK-C/EBP pathway enhances neurogenesis and inhibits gliogenesis, thereby providing a mechanism whereby growth factors can selectively bias progenitors to become neurons during development.

CCAAT/enhancer binding protein-beta is a mediator of keratinocyte survival and skin tumorigenesis involving oncogenic Ras signaling

The basic leucine zipper transcription factor CCAAT/enhancer binding protein-beta (C/EBPbeta) is expressed in many cell types, including keratinocytes. C/EBPbeta activity can be increased by phosphorylation through pathways stimulated by oncogenic Ras, although the biological implications of Ras-C/EBPbeta signaling are not currently understood. We report here that C/EBPbeta-nullizygous mice are completely refractory to skin tumor development induced by a variety of carcinogens and carcinogenesis protocols, including 7,12-dimethylbenz[a]anthracene-initiation/12-O-tetradecanoylphorbol 13-acetate promotion, that produce tumors containing oncogenic Ras mutations. No significant differences in TPA-induced epidermal keratinocyte proliferation were observed in C/EBPbeta-null versus wild-type mice. However, apoptosis was significantly elevated (17-fold) in the epidermal keratinocytes of 7,12-dimethylbenz[a]anthracene-treated C/EBPbeta-null mice compared with wild-type mice. In v-Ha-ras transgenic mice, C/EBPbeta deficiency also led to greatly reduced skin tumor multiplicity and size, providing additional evidence for a tumorigenesis pathway linking Ras and C/EBPbeta. Oncogenic Ras potently stimulated C/EBPbeta to activate a C/EBP-responsive promoter-reporter in keratinocytes and mutating an ERK1/2 phosphorylation site (T188) in C/EBPbeta abolished this Ras effect. Finally, we observed that C/EBPbeta participates in oncogenic Ras-induced transformation of NIH 3T3 cells. These findings indicate that C/EBPbeta has a critical role in Ras-mediated tumorigenesis and cell survival and implicate C/EBPbeta as a target for tumor inhibition.

Feedback inhibition of the retinaldehyde dehydrogenase gene ALDH1 by retinoic acid through retinoic acid receptor alpha and CCAAT/enhancer-binding protein beta

Aldehyde dehydrogenase 1 (ALDH1) plays a major role in the biosynthesis of retinoic acid (RA), a hormone required for several essential life processes. Recent evidence, using the aryl hydrocarbon receptor-null mouse, suggests that elevated hepatic RA down-regulates ALDH1 in a unique feedback pathway to control RA biosynthesis. To determine the mechanism of suppression of the ALDH1 gene by RA, transactivation studies were carried out in Hepa-1 mouse hepatoma cells. RA decreased expression of an ALDH1-CAT construct containing -2536 base pairs of DNA upstream of the transcription start site. Retinoic acid receptor alpha (RARalpha) transactivates the ALDH1 gene promoter through a complex with an RA response-like element (RARE) located at -91/-75 bp, which bound to the RARalpha/retinoid X receptor beta heterodimer. CCAAT/enhancer-binding protein (C/EBPbeta) also transactivates the ALDH1 gene promoter through a CCAAT box located 3' and directly adjacent to the RARE, and the ALDH1 gene is down-regulated in C/EBPbeta-null mouse liver. Exposure of Hepa-1 cells to RA results in a decrease in C/EBPbeta mRNA levels; however, there was no difference in mRNA and protein levels between wild-type and AHR-null mouse liver. These data support a model in which the RARalpha and C/EBPbeta activate the ALDH1 gene promoter through the RARE and C/EBP response elements, and in Hepa-1 cells, high levels of RA inhibit this activation by decreasing cellular levels of C/EBPbeta.

C/EBPbeta modulates the early events of keratinocyte differentiation involving growth arrest and keratin 1 and keratin 10 expression

The epidermis is a stratified squamous epithelium composed primarily of keratinocytes that become postmitotic and undergo sequential changes in gene expression during terminal differentiation. The expression of the transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) within mouse epidermis and primary keratinocytes has recently been described; however, the function of C/EBPbeta within the epidermal keratinocyte is unknown. We report here that transient transfection of mouse primary keratinocytes with a C/EBP-responsive promoter-reporter construct resulted in a sevenfold increase in luciferase activity when keratinocytes were switched to culture conditions that induce growth arrest and differentiation. Forced expression of C/EBPbeta in BALB/MK2 keratinocytes inhibited growth, induced morphological changes consistent with a more differentiated phenotype, and upregulated two early markers of differentiation, keratin 1 (K1) and keratin 10 (K10) but had a minimal effect on the expression of late-stage markers, loricrin and involucrin. Analysis of the epidermis of C/EBPbeta-deficient mice revealed a mild epidermal hyperplasia and decreased expression of K1 and K10 but not of involucrin and loricrin. C/EBPbeta-deficient primary keratinocytes were partially resistant to calcium-induced growth arrest. Analysis of terminally differentiated spontaneously detached keratinocytes or those induced to differentiate by suspension culture revealed that C/EBPbeta-deficient keratinocytes displayed striking decreases in K1 and K10, while expression of later-stage markers was only minimally altered. Our results demonstrate that C/EBPbeta plays an important role in the early events of stratified squamous differentiation in keratinocytes involving growth arrest and K1 and K10 expression.

STAT6, NF-kappaB and C/EBP in CD23 expression and IgE production

STAT6, NF-kappaB (p50) and C/EBPbeta transcription factors (TF) were examined with respect to CD23 regulation. Electrophoretic mobility shift assay (EMSA), competition and supershift analysis demonstrated that STAT6 binds the CD23a promoter but with a lower affinity than the consensus site. STAT6-/- mice were analyzed for CD23 levels and showed reduced expression after CD40 ligand trimer (CD40LT) stimulation. However, normal CD23 expression and even some IgE production was induced in STAT6-/- mice with CD40LT/IL-4. EMSA analysis indicated that the CD23a STAT site was bound by a protein in nuclear extracts from CD40+/-IL-4-stimulated STAT6-/-B cells. Western blot analysis of these nuclear extracts demonstrated the presence of STAT3 and STAT5, suggesting that these STATs can induce CD23 in this situation. Further supporting evidence was obtained by showing that IL-2 and IL-4 both synergize with CD40 in an identical manner for CD23 induction on STAT6-/- B cells. EMSA analysis of the two putative NF-kappaB sites confirmed binding to both, although one site bound with a higher affinity than the second. Analysis of p50-/-mice indicated that this subunit was not necessary for CD23 induction or CD40/IL-4-induced IgE production. Finally, no role for C/EBP was observed in CD23 induction by EMSA or by CD23 induction analysis in C/EBPbeta-/- mice, whereas the absence of C/EBP, did have an effect on IgE production and lipopolysaccharide-induced B cell proliferation. Based on these data, a model is presented which suggests that CD23 superinduction results from STAT and NF-kappaB interaction.

Selectively enhanced contextual fear conditioning in mice lacking the transcriptional regulator CCAAT/enhancer binding protein delta

CCAAT/enhancer binding protein delta (C/EBPdelta) is a transcriptional regulator implicated in the hepatic acute phase response and in adipogenic and myeloid cell differentiation. We found that C/EBPdelta is widely expressed in the peripheral and central nervous systems, including neurons of the hippocampal formation, indicating a role in neural functions. To examine the role of C/EBPdelta in vivo, we generated mice with a targeted deletion of the C/EBPdelta gene. This mutation does not interfere with normal embryonic and postnatal development. Performance in a battery of behavioral tests indicates that basic neurological functions are normal. Furthermore, performance in a Morris water maze task suggests that C/EBPdelta mutant mice have normal spatial learning. However, in the contextual and auditory-cue-conditioned fear task, C/EBPdelta null mice displayed significantly more conditioned freezing to the test context than did wild-type controls, but equivalent conditioning to the auditory cue. These data demonstrate a selectively enhanced contextual fear response in mice carrying a targeted genomic mutation and implicate C/EBPdelta in the regulation of a specific type of learning and memory.

The C/EBPbeta transcription factor regulates epithelial cell proliferation and differentiation in the mammary gland

Studies of C/EBPbeta-deficient mice have demonstrated a pivotal role for this transcription factor in hematopoiesis, adipogenesis, and ovarian function. Here we show that C/EBPbeta is also essential for normal development and function of the mammary gland. Ductal morphogenesis in virgin C/EBPbeta-deficient mice was disrupted, with ducts displaying reduced growth and branching. To distinguish whether the effect of C/EBPbeta deficiency on mammary epithelium is indirect or cell autonomous, we performed ovarian and mammary gland transplants. Transplants of wild-type ovaries into mutant females partially restored ductal morphogenesis during puberty but failed to support mammopoiesis during pregnancy. At term, mutant mice harboring wild-type ovaries exhibited reduced alveolar proliferation and impaired epithelial cell differentiation, including a complete absence of milk protein expression. Mammary gland transplant experiments demonstrated that development of C/EBPbeta-deficient epithelium was defective within a wild-type stroma and host background. Cell proliferation during pregnancy was reduced and differentiation, as measured by the activity of milk protein genes, was inhibited. However, wild-type epithelium developed in a C/EBPbeta-deficient stroma. Thus, C/EBPbeta plays an essential, cell autonomous role in the proliferation and differentiation of mammary secretory epithelial cells and is required for the activation of milk protein genes.

C/EBPalpha is a regulator of the UDP glucuronosyltransferase UGT2B1 gene

The rat UDP glucuronosyltransferase, UGT2B1, is expressed in the liver where it glucuronidates steroids, environmental toxins, and carcinogens. A region between -88 and -111 base pairs upstream from the UGT2B1 gene transcription start site contains a CCAAT enhancer binding protein (C/EBP)-like element and was previously shown by Dnase I footprint analysis to bind to proteins in both rat liver and human hepatoma (HepG2) cell nuclear extracts. In this study, the importance of this region in the regulation of the UGT2B1 gene was assessed by functional and DNA binding assays. Varying lengths of the UGT2B1 gene promoter, with and without the C/EBP-like element, were fused to the chloramphenicol acetyltransferase reporter gene and transfected into HepG2 cells. Transcriptional activity of the UGT2B1 promoter construct containing the C/EBP-like element was strongly elevated in the presence of a cotransfected C/EBPalpha expression vector. In contrast, no change was observed when an expression vector encoding C/EBPbeta was cotransfected with the UGT2B1 promoter constructs. Introduction of point mutations into the C/EBP-like element prevented any C/EBPalpha-mediated increase in chloramphenicol acetyltransferase activity. Gel shift analyses demonstrated that the C/EBP-like element binds a complex of nuclear proteins present in both HepG2 cells and rat liver. The presence of C/EBPalpha in this complex was confirmed by supershift analysis with antiserum to this factor. These data strongly suggest that the liver-enriched factor C/EBPalpha binds to, and activates, the UGT2B1 gene promoter. The importance of C/EBPalpha in the regulation of the homologous mouse UGT2B1 gene was also assessed in vivo. Transcripts homologous to UGT2B1 were detected in the livers of mice containing intact c/ebpalpha and c/ebpbeta genes and in mice containing a homozygous null mutation in the c/ebpbeta gene. In contrast, these transcripts were not detected in mice with a disrupted hepatic c/ebpalpha gene. These data extend the findings with the rat UGT2B1 gene promoter and establish that C/EBPalpha, but not C/EBPbeta, is an essential transcriptional regulator of the homologous UGT2B1 gene in the mouse.

CCAAT/enhancer binding protein beta is a neuronal transcriptional regulator activated by nerve growth factor receptor signaling

CCAAT/enhancer binding protein beta (C/EBPbeta) is a transcriptional regulator of the basic leucine zipper family. By in situ hybridization analysis, we found that C/EBPbeta is widely expressed in the CNS of adult mice, including cells of the hippocampus and dentate gyrus and cerebellar Purkinje and granule cells. Expression of C/EBPbeta had also been reported in the PC12 cell line, which undergoes differentiation to neuron-like cells in response to nerve growth factor (NGF). We show that C/EBPbeta mRNA expression increases while protein levels decrease during differentiation of PC12 cells. In transactivation assays, C/EBPbeta activity was stimulated by NGF receptor signaling. Mutations of a phosphorylation site for mitogen-activated protein kinase in C/EBPbeta affected its capacity to transactivate in a promoter-specific manner. Our data identify the C/EBPbeta protein and gene as direct downstream targets of the NGF receptor and suggest a role for C/EBPbeta in neurotrophin signaling in the brain.

Interleukin-6-specific activation of the C/EBPdelta gene in hepatocytes is mediated by Stat3 and Sp1

C/EBPdelta (CCAAT/enhancer binding protein delta) has been implicated as a regulator of acute-phase response (APR) genes in hepatocytes. Its expression increases dramatically in liver during the APR and can be induced in hepatic cell lines by interleukin-6 (IL-6), an acute-phase mediator that activates transcription of many APR genes. Here we have investigated the mechanism by which C/EBPdelta expression is regulated by IL-6 in hepatoma cells. C/EBPdelta promoter sequences to -125 bp are sufficient for IL-6 inducibility of a reporter gene and include an APR element (APRE) that is essential for IL-6 responsiveness. DNA binding experiments and transactivation assays demonstrate that Stat3, but not Stat1, interacts with this APRE. Two Sp1 sites, one of which is adjacent to the APRE, are required for IL-6 induction and transactivation by Stat3. Thus, Stat3 and Sp1 function cooperatively to activate the C/EBPdelta promoter. Replacement of the APRE with Stat binding elements (SBEs) from the ICAM-1 or C/EBPbeta promoter, both of which recognize both Stat1 and Stat3, confers responsiveness to gamma interferon, a cytokine that selectively activates Stat1. Sequence comparisons suggest that the distinct Stat binding specificities of the C/EBPdelta and C/EBPbeta SBEs are determined primarily by a single base pair difference. Our findings indicate that the cytokine specificity of C/EBPdelta gene expression is governed by the APRE sequence.

An essential role for C/EBPbeta in female reproduction

A large number of intercellular signaling molecules have been identified that orchestrate female reproductive physiology. However, with the exception of steroid hormone receptors, little information exists about the transcriptional regulators that mediate cellular responses to these signals. The transcription factor C/EBP beta (CCAAT/enhancer-binding protein beta) is expressed in ovaries and testes, as well as many other tissues of adult mice. Here we show that mice carrying a targeted deletion of the C/EBP beta gene exhibit reproductive defects. Although these animals develop normally and males are fertile, adult females are sterile. Transplantation of normal ovaries into mutant females restored fertility, thus localizing the primary reproductive defect to the ovary proper. In normal ovaries, C/EBP beta mRNA is specifically induced by luteinizing hormone (LH/hCG) in the granulosa layer of preovulatory antral follicles. C/EBP beta-deficient ovaries lack corpora lutea and fail to down-regulate expression of the prostaglandin endoperoxidase synthase 2 and P450 aromatase genes in response to gonadotropins. These findings demonstrate that C/EBP beta is essential for periovulatory granulosa cell differentiation in response to LH. C/EBP beta is thus established as a critical downstream target of G-protein-coupled LH receptor signaling and one of the first transcription factors, other than steroid hormone receptors, known to be required for ovarian follicle development in vivo.

The ability of C/EBP beta but not C/EBP alpha to synergize with an Sp1 protein is specified by the leucine zipper and activation domain

The rat CYP2D5 P-450 gene is activated in the liver during postnatal development. We previously showed that liver-specific transcription of the CYP2D5 gene is dictated by a proximal promoter element, termed 2D5, that is composed of a binding site for Sp1 or a related factor, and an adjacent cryptic C/EBP (CCAAT/enhancer-binding protein) site. Despite the fact that both C/EBP alpha and C/EBP beta are expressed abundantly in liver, only C/EBP beta is capable of stimulating the 2D5 promoter in HepG2 hepatocarcinoma cells. In addition, activation of the 2D5 promoter by C/EBP beta is completely dependent on the presence of the Sp1 site. Domain switch experiments reveal that C/EBP beta proteins containing either the leucine zipper or the activation domain of C/EBP alpha are unable to stimulate the 2D5 promoter yet are fully capable of transactivating an artificial promoter bearing a high-affinity C/EBP site. Thus, the leucine zipper and the activation domain of C/EBP beta are absolutely required to support transactivation of the 2D5 promoter. Using Drosophila cells that lack endogenous Sp1 activity, we show that the serine/threonine- and glutamine-rich activation domains A and B of Sp1 are required for efficient cooperatively with C/EBP beta. Furthermore, analysis of c/ebp beta-deficient mice shows that mutant animals are defective in expression of a murine CYP2D5 homolog in hepatic cells, confirming the selective ability of C/EBP beta to activate this liver-specific P-450 gene in vivo. Our findings illustrate that two members of a transcription factor family can achieve distinct target gene specificities through differential interactions with a cooperating Sp1 protein.

Interleukin-6 induces expression of peripherin and cooperates with Trk receptor signaling to promote neuronal differentiation in PC12 cells

In contrast to the intensively studied nerve growth factor (NGF)-related family of cytokines, relatively little is known about the mechanisms of neurotrophic activity elicited by the cytokine interleukin-6 (IL-6). We have examined the mechanisms of IL-6-induced neuronal differentiation of the pheochromocytoma cell line PC12. IL-6 independently induced the expression of peripherin, identifying this gene as the first neuronal-specific target of IL-6. However, IL-6 alone failed to elicit neurite outgrowth in PC12 cells and instead required low levels of Trk/NGF receptor tyrosine kinase activity to induce neuronal differentiation. The cooperating Trk signal could be provided by either overexpression of Trk or exposure to low concentrations of NGF. IL-6 also functioned cooperatively with basic fibroblast growth factor to promote PC12 differentiation. IL-6 and Trk/NGF synergized in enhancing tyrosine phosphorylation of the Erk-1 mitogen-activated protein kinase and in activating expression of certain NGF target genes. NGF also induced expression of the gp80 subunit of the IL-6 receptor, providing another potential mechanism of cooperativity between NGF and IL-6 signaling. We propose that IL-6 functions as an enhancer of NGF signaling rather than as an autonomous neuronal differentiation signal. Moreover, our results demonstrate that a Trk receptor-specific cellular response can be achieved in the absence of NGF through amplification of its basal signaling activity by the IL-6 receptor system.