C/EBPbeta regulates transcription factors critical for proliferation and survival of multiple myeloma cells

Oncogenic KRAS impairs EGFR antibodies' efficiency by C/EBPβ-dependent suppression of EGFR expression

Oncogenic KRAS mutations in colorectal cancer (CRC) are associated with lack of benefit from epidermal growth factor receptor (EGFR)-directed antibody (Ab) therapy. However, the mechanisms by which constitutively activated KRAS (KRAS(G12V)) impairs effector mechanisms of EGFR-Abs are incompletely understood. Here, we established isogenic cell line models to systematically investigate the impact of KRAS(G12V) on tumor growth in mouse A431 xenograft models as well as on various modes of action triggered by EGFR-Abs in vitro. KRAS(G12V) impaired EGFR-Ab-mediated growth inhibition by stimulating receptor-independent downstream signaling. KRAS(G12V) also rendered tumor cells less responsive to Fc-mediated effector mechanisms of EGFR-Abs-such as complement-dependent cytotoxicity (CDC) and Ab-dependent cell-mediated cytotoxicity (ADCC). Impaired CDC and ADCC activities could be linked to reduced EGFR expression in KRAS-mutated versus wild-type (wt) cells, which was restored by small interfering RNA (siRNA)-mediated knockdown of KRAS4b. Immunohistochemistry experiments also revealed lower EGFR expression in KRAS-mutated versus KRAS-wt harboring CRC samples. Analyses of potential mechanisms by which KRAS(G12V) downregulated EGFR expression demonstrated significantly decreased activity of six distinct transcription factors. Additional experiments suggested the CCAAT/enhancer-binding protein (C/EBP) family to be implicated in the regulation of EGFR promoter activity in KRAS-mutated tumor cells by suppressing EGFR transcription through up-regulation of the inhibitory family member C/EBPβ-LIP. Thus, siRNA-mediated knockdown of C/EBPβ led to enhanced EGFR expression and Ab-mediated cytotoxicity against KRAS-mutated cells. Together, these results demonstrate that KRAS(G12V) signaling induced C/EBPβ-dependent suppression of EGFR expression, thereby impairing Fc-mediated effector mechanisms of EGFR-Abs and rendering KRAS-mutated tumor cells less sensitive to these therapeutic agents.

Targeting of the innate immunity/inflammation as complementary anti-tumor therapies

Different types of cancer take advantage of inflammatory components to improve their life-span in the organs. A sustenance of growth factors and cytokines (e.g. interleukin (IL)-1, tumor necrosis factor, IL-6, vascular endothelial growth factor) supports malignant cell progression and contributes to suppress the body immune defense. Strategies to modulate the host micro-environment offer new approaches for anti-cancer therapies. For these reasons new molecules with anti-tumor and anti-inflammatory features (e.g. trabectedin) are looked at with new eyes in the light of the crucial link between inflammation and cancer.

Targeting the microtubular network as a new antimyeloma strategy

We identified nocodazole as a potent antimyeloma drug from a drug screening library provided by the Multiple Myeloma Research Foundation. Nocodazole is a benzimidazole that was originally categorized as a broad-spectrum anthelmintic drug with antineoplastic properties. We found that nocodazole inhibited growth and induced apoptosis of primary and multiresistant multiple myeloma cells cultured alone and in the presence of bone marrow stromal cells. Nocodazole caused cell-cycle prophase and prometaphase arrest accompanied by microtubular network disarray. Signaling studies indicated that increased expression of Bim protein and reduced X-linked inhibitor of apoptosis protein and Mcl-1(L) levels were involved in nocodazole-induced apoptosis. Further investigation showed Bcl-2 phosphorylation as a critical mediator of cell death, triggered by the activation of c-jun-NH(2) kinase (JNK) instead of p38 kinase or extracellular signal-regulated kinases. Treatment with JNK inhibitor decreased Bcl-2 phosphorylation and subsequently reduced nocodazole-induced cell death. Nocodazole combined with dexamethasone significantly inhibited myeloma tumor growth and prolonged survival in a human xenograft mouse model. Our studies show that nocodazole has potent antimyeloma activity and that targeting the microtubular network might be a promising new treatment approach for multiple myeloma.

C/EBPα, C/EBPα oncoproteins, or C/EBPβ preferentially bind NF-κB p50 compared with p65, focusing therapeutic targeting on the C/EBP:p50 interaction

Canonical nuclear factor kappaB (NF-κB) activation signals stimulate nuclear translocation of p50:p65, replacing inhibitory p50:p50 with activating complexes on chromatin. C/EBP interaction with p50 homodimers provides an alternative pathway for NF-κB target gene activation, and interaction with p50:p65 may enhance gene activation. We previously found that C/EBPα cooperates with p50, but not p65, to induce Bcl-2 transcription and that C/EBPα induces Nfkb1/p50, but not RelA/p65, transcription. Using p50 and p65 variants containing the FLAG epitope at their N- or C-termini, we now show that C/EBPα, C/EBPα myeloid oncoproteins, or the LAP1, LAP2, or LIP isoforms of C/EBPβ have markedly higher affinity for p50 than for p65. Deletion of the p65 transactivation domain did not increase p65 affinity for C/EBPs, suggesting that unique residues in p50 account for specificity, and clustered mutation of HSDL in the "p50 insert" lacking in p65 weakens interaction. Also, in contrast to Nfkb1 gene deletion, absence of the RelA gene does not reduce Bcl-2 or Cebpa RNA in unstimulated cells or prevent interaction of C/EBPα with the Bcl-2 promoter. Saturating mutagenesis of the C/EBPα basic region identifies R300 and nearby residues, identical in C/EBPβ, as critical for interaction with p50. These findings support the conclusion that C/EBPs activate NF-κB target genes via contact with p50 even in the absence of canonical NF-κB activation and indicate that targeting C/EBP:p50 rather than C/EBP:p65 interaction in the nucleus will prove effective for inflammatory or malignant conditions, alone or synergistically with agents acting in the cytoplasm to reduce canonical NF-κB activation.

Identification of novel myeloma-specific XBP1 peptides able to generate cytotoxic T lymphocytes: a potential therapeutic application in multiple myeloma

The purpose of these studies was to identify HLA-A2⁺ immunogenic peptides derived from XBP1 antigens to induce a multiple myeloma (MM)-specific immune response. Six native peptides from non-spliced XBP1 antigen and three native peptides from spliced XBP1 antigen were selected and evaluated for their HLA-A2 specificity. Among them, XBP1_184–192, XBP1 SP_196–204 and XBP1 SP_367–375 peptides showed the highest level of binding affinity, but not stability to HLA-A2 molecules. Novel heteroclitic XBP1 peptides, YISPWILAV or YLFPQLISV, demonstrated a significant improvement in HLA-A2 stability from their native XBP1_184–192 or XBP1 SP_367–375 peptide, respectively. Cytotoxic T lymphocytes generated by repeated stimulation of CD3⁺ T cells with each HLA-A2-specific heteroclitic peptide showed an increased percentage of CD8⁺ (cytotoxic) and CD69⁺/CD45RO⁺ (activated memory) T cells and a lower percentage of CD4⁺ (helper) and CD45RA⁺/CCR7⁺ (naïve) T cells, which were distinct from the control T cells. Functionally, the CTLs demonstrated MM-specific and HLA-A2-restricted proliferation, IFN-γ secretion and cytotoxic acivity in response to MM cell lines and importantly, cytotoxicty against primary MM cells. These data demonstrate the distinct immunogenic characteristics of unique heteroclitic XBP1 peptides which induce MM-specific CTLs and highlights their potential application for immunotherapy to treat the patients with MM or its pre-malignant condition.

CCAAT/enhancer-binding protein-β participates in oxidized LDL-enhanced proliferation in 3T3-L1 cells

Increased circulating oxidized LDL (oxLDL) have been found in obese subjects. Obesity is characterized by an excess of fat mass resulting from an increase in adipocyte number and size. The generation of new adipocytes is a tightly controlled process where multiple factors acting in a signaling cascade follow a precise temporal expression pattern; oxLDL appear to have a role in the impairment of this process. The purpose of this study was to examine the effects of oxLDL on the mechanisms involved in the proliferative stage of the differentiation process in 3T3-L1 cells. After hormonal induction, 3T3-L1 cells undergo approximately two rounds of mitotic clonal expansion (MCE), a process required for adipogenesis. CCAAT/enhancer-binding protein β (C/EBPβ) is immediately expressed after induction, and plays a crucial role in MCE, but its expression must decrease to allow preadipocytes to mature into adipocytes. We found that, in the presence of stimuli to differentiate, oxLDL induced a higher proliferation rate in this cell line, associated with a sustained up-regulation of C/EBPβ, which remained activated inside the nucleus for several days. RNAi-mediated knockdown of C/EBPβ 24 h after oxLDL treatment counteracted the increase in proliferation rate. Both C/EBPβ expression and proliferation processes appear to be influenced by cAMP/protein kinase A (PKA) and extracellular signal-regulated kinases1/2 (ERK1/2) pathways. OxLDL treatment led to increased levels of cAMP, and to a strong, prolonged phosphorylation of ERK1/2 and C/EBPβ. The addition of cAMP and PKA inhibitors, SQ22536 and H-89, respectively, reduced proliferation only in oxLDL-treated cells, whereas the addition of ERK1/2 inhibitor U0126 blocked proliferation in both control and oxLDL-treated cells. C/EBPβ nuclear expression and DNA-binding activity were reduced by U0126, under all tested conditions. These findings show that the altered expression pattern of C/EBPβ is involved in the increase in the number of proliferating cells induced by oxLDL, in hormone-stimulated 3T3-L1 cells.

Bortezomib induction of C/EBPβ mediates Epstein-Barr virus lytic activation in Burkitt lymphoma

Epstein-Barr virus (EBV) is associated with a variety of lymphoid malignancies. Bortezomib activates EBV lytic gene expression. Bortezomib, a proteasome inhibitor, leads to increased levels of CCAAT/enhancer-binding proteinβ (C/EBPβ) in a variety of tumor cell lines. C/EBPβ activates the promoter of the EBV lytic switch gene ZTA. Bortezomib treatment leads to increased binding of C/EBP to previously recognized binding sites in the ZTA promoter. Knockdown of C/EBPβ inhibits bortezomib activation of EBV lytic gene expression. Bortezomib also induces the unfolded protein response (UPR), as evidenced by increases in ATF4, CHOP10, and XBP1s and cleavage of ATF6. Thapsigargin, an inducer of the UPR that does not interfere with proteasome function, also induces EBV lytic gene expression. The effects of thapsigargin on EBV lytic gene expression are also inhibited by C/EBPβ knock-down. Therefore, C/EBPβ mediates the activation of EBV lytic gene expression associated with bortezomib and another UPR inducer.

[Progress of transcription factor CCAAT enhancer binding protein β]

CCAAT enhancer binding protein β (C/EBP β) belongs to CCAAT enhancer binding protein (C/EBP) family, which is a subfamily of basic leucine zipper (bZIP) protein family. C/EBP family plays important roles in many processes such as cell differentiation, metabolism, and development. In this paper, the structure, expression regulation, and function of C/EBP β were reviewed.

IMiD immunomodulatory compounds block C/EBP{beta} translation through eIF4E down-regulation resulting in inhibition of MM

Immunomodulatory derivatives of thalidomide (IMiD compounds), such as pomalidomide and lenalidomide, are highly active in multiple myeloma (MM) treatment. However, the precise mechanisms of action and resistance in MM are unresolved. Here we show that IMiD compounds down-regulate CCAAT/enhancer-binding protein-β (C/EBPβ) resulting in abrogation of cell proliferation. Overexpression of C/EBPβ rescued MM cells from IMiD-induced inhibition of proliferation, indicating that C/EBPβ is critical in mediating antiproliferative effects. IMiD-induced decrease of C/EBPβ protein led to impaired transcription of interferon regulatory factor 4 (IRF4). Down-regulation of IRF4 by lenalidomide was confirmed by longitudinal studies of bone marrow samples from 23 patients obtained before and during lenalidomide treatment using CD138⁺/IRF4⁺ double labeling. In contrast to down-regulation of C/EBPβ protein, IMiD compounds did not alter C/EBPβ mRNA levels or protein stability, suggesting translational regulation of C/EBPβ. We could demonstrate that C/EBPβ protein expression is under eIF4E-translational control in MM. Furthermore, inhibition of the eIF4E-C/EBPβ axis by IMiD compounds was not observed in IMiD-resistant MM cells. However, targeting translation at a different level by inhibiting eukaryotic translation initiation factor 4E-binding protein 1 phosphorylation overcame resistance, suggesting that this pathway is critical and might be a target to overcome drug resistance.

Cigarette smoke induces C/EBP-β-mediated activation of miR-31 in normal human respiratory epithelia and lung cancer cells

BACKGROUND

Limited information is available regarding mechanisms by which miRNAs contribute to pulmonary carcinogenesis. The present study was undertaken to examine expression and function of miRNAs induced by cigarette smoke condensate (CSC) in normal human respiratory epithelia and lung cancer cells.

METHODOLOGY

Micro-array and quantitative RT-PCR (qRT-PCR) techniques were used to assess miRNA and host gene expression in cultured cells, and surgical specimens. Software-guided analysis, RNA cross-link immunoprecipitation (CLIP), 3' UTR luciferase reporter assays, qRT-PCR, focused super-arrays and western blot techniques were used to identify and confirm targets of miR-31. Chromatin immunoprecipitation (ChIP) techniques were used to evaluate histone marks and transcription factors within the LOC554202 promoter. Cell count and xenograft experiments were used to assess effects of miR-31 on proliferation and tumorigenicity of lung cancer cells.

RESULTS

CSC significantly increased miR-31 expression and activated LOC554202 in normal respiratory epithelia and lung cancer cells; miR-31 and LOC554202 expression persisted following discontinuation of CSC exposure. miR-31 and LOC554202 expression levels were significantly elevated in lung cancer specimens relative to adjacent normal lung tissues. CLIP and reporter assays demonstrated direct interaction of miR-31 with Dickkopf-1 (Dkk-1) and DACT-3. Over-expression of miR-31 markedly diminished Dkk-1 and DACT3 expression levels in normal respiratory epithelia and lung cancer cells. Knock-down of miR-31 increased Dkk-1 and DACT3 levels, and abrogated CSC-mediated decreases in Dkk-1 and DACT-3 expression. Furthermore, over-expression of miR-31 diminished SFRP1, SFRP4, and WIF-1, and increased Wnt-5a expression. CSC increased H3K4Me3, H3K9/14Ac and C/EBP-β levels within the LOC554202 promoter. Knock-down of C/EBP-β abrogated CSC-mediated activation of LOC554202. Over-expression of miR-31 significantly enhanced proliferation and tumorigenicity of lung cancer cells; knock-down of miR-31 inhibited growth of these cells.

CONCLUSIONS

Cigarette smoke induces expression of miR-31 targeting several antagonists of cancer stem cell signaling in normal respiratory epithelia and lung cancer cells. miR-31 functions as an oncomir during human pulmonary carcinogenesis.

CCAAT/enhancer-binding protein β and NF-κB mediate high level expression of chemokine genes CCL3 and CCL4 by human chondrocytes in response to IL-1β

A large set of chemokines is highly up-regulated in human chondrocytes in response to IL-1β (Sandell, L. J., Xing, X., Franz, C., Davies, S., Chang, L. W., and Patra, D. (2008) Osteoarthr. Cartil. 16, 1560-1571). To investigate the mechanism of transcriptional regulation, deletion constructs of selected chemokine gene promoters, the human CCL3 (MIP-1α) and CCL4 (MIP-1β), were transfected into human chondrocytes with or without IL-1β. The results show that an IL-1β-responsive element is located between bp -300 and -140 of the CCL3 promoter and between bp -222 and -100 of the CCL4 promoter. Because both of these elements contain CCAAT/enhancer-binding protein β (C/EBPβ) motifs, the function of C/EBPβ was examined. IL-1β stimulated the expression of C/EBPβ, and the direct binding of C/EBPβ to the C/EBPβ motif was confirmed by EMSA and ChIP analyses. The -300 bp CCL3 promoter and -222 bp CCL4 promoter were strongly up-regulated by co-transfection with the C/EBPβ expression vector. Mutation of the C/EBPβ motif and reduction of C/EBPβ expression by siRNA decreased the up-regulation. Additionally, another cytokine-related transcription factor, NF-κB, was also shown to be involved in the up-regulation of chemokines in response to IL-1β, and the binding site was identified. The regulation of C/EBPβ and NF-κB was confirmed by the inhibition by C/EBPβ and NF-κB and by transfection with C/EBPβ and NF-κB expression vectors in the presence or absence of IL-1β. Taken together, our results suggest that C/EBPβ and NF-κB are both involved in the IL-1β-responsive up-regulation of chemokine genes in human chondrocytes. Time course experiments indicated that C/EBPβ gradually and steadily induces chemokine up-regulation, whereas NF-κB activity was highest at the early stage of chemokine up-regulation.

Type 10 17β-hydroxysteroid dehydrogenase expression is regulated by C/EBPβ in HepG2 cells

17β-hydroxysteroid dehydrogenases (HSD17Bs) are enzymes that stereospecifically reduce or oxidize a keto- or hydroxy group at C17 of the steroid scaffold, respectively. Fourteen mammalian HSD17Bs have been identified so far. We previously described that the HSD17B8 gene is regulated by C/EBPβ in the hepatocarcinoma cell line HepG2. Here, we analyze the 5'-flanking region of 14 promoters (HSD17B1-14) looking for CCAAT boxes and binding sites for CCAAT enhancer binding factors (C/EBPs). All promoters were found to have binding sites for these transcription factors except HSD17B1. Ectopic expression of C/EBPα or C/EBPβ in HepG2 cells showed that HSD17B11 expression was induced by both transcription factors while HSD17B10 expression was only induced by C/EBPβ. The first 500bp of the 5'-flanking region of both genes contain two putative binding sites for C/EBPs. Gene reporter assays showed that C/EBPβ transactivated HSD17B10 but not HSD17B11. Additional experiments showed that several isoforms of C/EBPβ are involved in HSD17B10 regulation. Mutation of the CCAAT box located at -30/-19 induced HSD17B10 promoter activity when only LIP was expressed, while impaired LAP-induced HSD17B10 transactivation in HepG2 cells when LAP isoforms are expressed. Taken together, our findings reveal that HSD17B10 is regulated by several isoforms of C/EBPβ in HepG2 cells.

Epigenetic down-regulation of the tumor suppressor gene PRDM1/Blimp-1 in diffuse large B cell lymphomas: a potential role of the microRNA let-7

PRDM1/Blimp-1, a master regulator for B cell terminal differentiation, is a putative tumor suppressor in diffuse large B cell lymphomas (DLBCL). Inactivating mutations of PRDM1 have been previously identified in a subset of nongerminal center B cell-like (GCB) DLBCL. We investigated the presence of alternative mechanisms of down-regulating PRDM1 in a cohort of 25 primary DLBCL and six DLBCL cell lines. While some DLBCL, predominantly the GCB-type, showed low levels of both PRDM1alpha mRNA and protein, presumably as a result of direct transcription repression, discordant expressions between the two were identified in a subset of DLBCL without PRDM1 mutations, the primarily non-GCB type, consistent with translational down-regulation. This subset of DLBCL exhibits relatively high PRDM1alpha mRNA levels but low levels of PRDM1. Data obtained from expression analysis, luciferase reporter assays, and transfection experiments support a role of targeting of PRDM1 by microRNA let-7 family in mediating this down-regulation. Let-7, in particular let-7b, is overexpressed in DLBCL relative to normal GCB cells, suggesting that it is deregulated. Thus, abnormal epigenetic down-regulation of PRDM1 by let-7 and other microRNAs may represent an alternative mechanism of reducing normal PRDM1 function in a subset of DLBCL with relatively high PRDM1alpha mRNA expression and unmutated PRDM1. These findings provide further evidence for an important role of impairment of terminal B cell differentiation in DLBCL pathogenesis.

XBP1s levels are implicated in the biology and outcome of myeloma mediating different clinical outcomes to thalidomide-based treatments

Immunoglobulin production by myeloma plasma cells depends on the unfolded protein response for protein production and folding. Recent studies have highlighted the importance of IRE1alpha and X box binding protein 1 (XBP1), key members of this pathway, in normal B-plasma cell development. We have determined the gene expression levels of IRE1alpha, XBP1, XBP1UNSPLICED (XBP1u), and XBP1SPLICED (XBP1s) in a series of patients with myeloma and correlated findings with clinical outcome. We show that IRE1alpha and XBP1 are highly expressed and that patients with low XBP1s/u ratios have a significantly better overall survival. XBP1s is an independent prognostic marker and can be used with beta2 microglobulin and t(4;14) to identify a group of patients with a poor outcome. Furthermore, we show the beneficial therapeutic effects of thalidomide in patients with low XBP1s/u ratios. This study highlights the importance of XBP1 in myeloma and its significance as an independent prognostic marker and as a predictor of thalidomide response.