Involvement of CCAAT/enhancer-binding protein alpha in haptoglobin gene expression by all-trans-retinoic acid

Differentiation therapy of acute myeloid leukemia

Acute Myeloid Leukemia (AML) is a predominant acute leukemia among adults, characterized by accumulation of malignantly transformed immature myeloid precursors. A very attractive way to treat myeloid leukemia, which is now called 'differentiation therapy', was proposed as in vitro studies have shown that a variety of agents stimulate differentiation of the cell lines isolated from leukemic patients. One of the differentiation-inducing agents, all-trans retinoic acid (ATRA), which can induce granulocytic differentiation in myeloid leukemic cell lines, has been introduced into clinics to treat patients with acute promyelocytic leukemia (APL) in which a PML-RARA fusion protein is generated by a t(15;17)(q22;q12) chromosomal translocation. Because differentiation therapy using ATRA has significantly improved prognosis for patients with APL, many efforts have been made to find alternative differentiating agents. Since 1,25-dihydroxyvitamin D3 (1,25D) is capable of inducing in vitro monocyte/macrophage differentiation of myeloid leukemic cells, clinical trials have been performed to estimate its potential to treat patients with AML or myelodysplastic syndrome (MDS). Unfortunately therapeutic concentrations of 1,25D can induce potentially fatal systemic hypercalcemia, thus limiting clinical utility of that compound. Attempts to overcome this problem have focused on the synthesis of 1,25D analogs (VDAs) which retain differentiation inducing potential, but lack its hypercalcemic effects. This review aims to discuss current problems and potential solutions in differentiation therapy of AML.

Mechanisms regulating repression of haptoglobin production by peroxisome proliferator-activated receptor-gamma ligands in adipocytes

Obesity leads to inflammation of white adipose tissue involving enhanced secretion of cytokines and acute-phase proteins in response in part to the accumulation of excess lipids in adipocytes. Haptoglobin is an acute-phase reactant secreted by white adipose tissue and induced by inflammatory cytokines such as TNFalpha. In this study, we investigated the mechanisms regulating haptoglobin expression in adipocytes. Peroxisome proliferator-activated receptor (PPAR)-gamma agonists such as thiazolidinediones (TZDs) as well as non-TZD ligands can repress in vitro and in vivo haptoglobin expression in adipocytes and also prevent its induction by TNFalpha. This action requires direct involvement of PPAR gamma in regulating haptoglobin gene transcription because mutation of critical amino acids within helix 7 of the ligand-binding domain of PPAR gamma prevents repression of the haptoglobin gene by the synthetic ligands. Chromatin immunoprecipitation analysis shows active binding of PPAR gamma to a distal region of the haptoglobin promoter, which contains putative PPAR gamma binding sites. Additionally, PPAR gamma induces transcription of a luciferase reporter gene when driven by the distal promoter region of the haptoglobin gene, and TZD treatment significantly reduces the extent of this induction. Furthermore, the mutated PPAR gamma is incapable of enhancing luciferase activity in these in vitro reporter gene assays. In contrast to other adipokines repressed by TZDs such as resistin and chemerin, repression of haptoglobin does not require either CCAAT/enhancer-binding protein C/EBP alpha or the corepressors C-terminal binding protein 1 or 2. These data are consistent with a model in which synthetic PPAR gamma ligands selectively activate PPAR gamma bound to the haptoglobin gene promoter to arrest haptoglobin gene transcription.

Regulation of rat haptoglobin gene expression is coordinated by the nuclear matrix

Using computer stress-induced duplex destabilization (SIDD) analysis and binding experiments, we identified a S/MAR element (-599/-200 bp) (Hp-S/MAR) adjacent to the cis-element (-165/-56 bp) in the rat haptoglobin gene. We examined its functional interactions with the lamins and lamin-associated proteins in the basal state and during acute-phase (AP) response-induced increased transcription. Colocalization, electrophoretic mobility shift assay (EMSA), and re-electrophoresis of nucleoprotein complexes, South-Western and Western blot analysis and coimmunoprecipitation experiments revealed that the lamins, PARP-1, C/EBP beta, and Hp-S/MAR assembled higher order complexes through direct lamin-Hp-S/MAR and probably PARP-1-Hp-S/MAR interactions although C/EBP beta did not bind to the Hp-S/MAR but established direct interaction with PARP-1. The transition from constitutive to increased haptoglobin gene transcription during the AP response was associated with quantitative and qualitative changes in Hp-S/MAR-protein interactions, respectively, observed as increased association of the lamin(s) with the Hp-S/MAR and as the appearance of a 90 kDa Hp-S/MAR-binding protein. Also, during the AP response the contact between C/EBP beta and PARP-1 established in the basal state was lost. DNA chromatography with the haptoglobin cis-element and Western blot analysis suggests that PARP-1 was a coactivator during constitutive and elevated transcription. The results show that the lamin components of the nuclear matrix form a network of functional, dynamic protein-protein and protein-Hp-S/MAR associations with multiple partners, and underline the involvement of PARP-1 in the regulation of haptoglobin gene transcription. We concluded that the interplay of these interactions fine tunes haptoglobin gene expression to meet the changing requirements of liver cells.

C/EBPalpha and C/EBPvarepsilon induce the monocytic differentiation of myelomonocytic cells with the MLL-chimeric fusion gene

CCAAT/enhancer binding proteins (C/EBPs) have an important function in granulocytic differentiation, and are also involved in the leukemogenesis of acute myeloid leukemia (AML). Their involvement in myelomonocytic leukemia, however, is still unclear. Therefore, the expression and function of C/EBPs in myelomonocytic cells with MLL-fusion genes were investigated. Retinoic acid (RA) induced monocytic differentiation in the myelomonocytic cell lines with MLL-fusion genes, THP-1, MOLM-14 and HF-6 cells, accompanied by monocytic differentiation with the upregulation of C/EBPalpha and C/EBPepsilon. Monocytic differentiation by RA treatment was confirmed in primary AML cells using a clonogenic assay. When the activity of C/EBPalpha or C/EBPepsilon was introduced into HF-6 cells, their cellular growth was arrested through differentiation into monocytes with the concomitant marked downregulation of Myc. Cebpe mRNA was upregulated by the induction of C/EBPalpha-ER, but not vice versa, thus suggesting that C/EBPepsilon may have an important function in the differentiation process. Introduction of Myc isoforms into HF-6 cells partially antagonized the C/EBPs effects. These findings suggest that the ectopic expression of C/EBPepsilon, as well as C/EBPalpha, can induce the monocytic differentiation of myelomonocytic leukemic cells with MLL-fusion gene through the downregulation of Myc, thus providing insight into the development of novel therapeutic approaches.

Activation of retinoic acid receptor-alpha favours regulatory T cell induction at the expense of IL-17-secreting T helper cell differentiation

Autoimmunity is thought to reflect an imbalance between regulatory T helper lymphocytes (Treg) and pathogenic, IL-17-secreting T helper (Th17) cells. Induction of both adaptive Treg and Th17 cells requires signalling from TGF-beta. We now show that, in the context of TGF-beta signalling, all-trans retinoic acid (ATRA) leads to increased induction of CD4(+) T cells expressing the Treg specification factor forkhead box protein P3 (FoxP3) and decreased frequency of cells expressing IL-17, even in the presence of IL-6. Using a specific agonist and antagonist, as well as retroviral over-expression, we also provide evidence that the effects of ATRA are likely to be at least partially mediated by the nuclear retinoic acid receptor-alpha (RARalpha). These findings indicate that signalling through a specific nuclear retinoic acid receptor can favour the decision to adopt the Treg fate at the expense of Th17 fate. Specific agonists of RARalpha could, therefore, be considered candidates for the treatment of autoimmunity.

Pathophysiological roles of ASK1-MAP kinase signaling pathways

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogenactivated protein kinase (MAPK) kinase kinase that activates JNK and p38 kinases. ASK1 is activated by various stresses, such as reactive oxygen species (ROS), endoplasmic reticulum (ER) stress, lipopolysaccharide (LPS) and calcium influx which are thought to be responsible for the pathogenesis or exacerbations of various human diseases. Recent studies revealed the involvement of ASK1 in ROS- or ER stressrelated diseases, suggesting that ASK1 may be a potential therapeutic target of various human diseases. In this review, we focus on the current findings for the relationship between pathogenesis and ASK1-MAPK pathways.

Protein Kinase C-delta Stimulates Haptoglobin Secretion

Haptoglobin (Hp) is a glycoprotein that is produced by hepatic cells and secreted into the circulation. While studying the physiologic functions of Hp, we found that Hp synthesized in THP-1 monocytic cells was largely retained within cells, although Hp is considered a secretory protein. To investigate the molecular mechanism on Hp secretion in THP-1 cells, in the present study, we examined the effect of protein kinase C (PKC) on Hp secretion. When several inhibitors of PKC isoforms were tested, only Rottlerin, a specific inhibitor of PKC-delta, completely blocked Hp secretion from cells to culture medium. To confirm the role of PKC-delta in Hp secretion, Hp-overexpressing COS7 cells were transiently transfected with a wild-type or a dominantnegative mutant of the PKC-delta gene. Mutant PKC-delta significantly inhibited Hp secretion, whereas the wild-type gene slightly increased Hp secretion. These results demonstrate that the PKC-delta signal is involved in Hp secretion.

JWA is required for the antiproliferative and pro-apoptotic effects of all-trans retinoic acid in Hela cells

1. All-trans retinoic acid (ATRA) is known to inhibit cellular proliferation and induce differentiation and apoptosis. It usually activates gene expression by binding to a nuclear receptor that interacts with retinoic acid-response elements (RARE) and then activates the mitogen-activated protein kinase signal pathway. JWA, a newly identified ATRA-responsive gene, has recently been proposed as an important molecule for cellular differentiation induced by some chemicals, including ATRA. 2. To investigate the possible involvement of JWA in the inhibition of cellular proliferation and induction of apoptosis by ATRA, HeLa cells were stably transfected with sense or antisense JWA to establish cell lines that overexpressed or were deficient in JWA; ATRA (0.05-10 micromol/L) was used to induce cellular differentiation and apoptosis. 3. Western blot analysis revealed that ATRA caused increased expression of JWA in HeLa cells in a dose- and time-dependent manner, accompanied by activation of extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. However, ERK1/2 phosphorylation induced by ATRA was inhibited in JWA-deficient HeLa cells. In JWA-overexpressing HeLa cells, ATRA showed more significant antiproliferative effects and induced more apoptosis. 4. The reporter gene assay showed that ATRA (5 mmol/L) enhanced the transcriptional activity of JWA by interacting with its promoter in the region from -194 to +107 bp (P < 0.01). Bioinformatic analysis indicated that the JWA promoter did not contain RARE, but did contain two CCAAT boxes in this fragment spanning -194 to +107 bp, which may be responsive to the ATRA-activated nuclear transcription factor CCAAT/enhancer binding proteins (C/EBP) or interacting proteins. Therefore, ATRA-inhibited cellular proliferation and -induced apoptosis in HeLa cells may be dependent on JWA transactivation via its C/EBP-binding motifs. 5. These data indicate that the inhibition of proliferation and the induction of apoptosis by ATRA are dependent on JWA expression in HeLa cells. The findings may represent a novel mechanism by which the effects of ATRA in regulating cellular proliferation and apoptosis are mediated, at least in part, by JWA expression.

Retinoids induce lumen morphogenesis in mammary epithelial cells

Lumen formation is a fundamental step in the development of the structural and functional units of glandular organs, such as alveoli and ducts. In an attempt to elucidate the molecular signals that govern this morphogenetic event, we set up an in vitro system in which cloned mammary epithelial cells grown in collagen gels under serum-free conditions form solid, lumen-less colonies. Addition of as little as 0.1% donor calf serum (DCS) was sufficient to induce the formation of a central cavity. Among a number of serum constituents analyzed, retinol was found to mimic the effect of DCS in inducing lumen morphogenesis. Since the biological activities of retinol are largely dependent on its conversion to all-trans-retinoic acid (RA), we examined in more detail the effect of RA on lumen formation. RA induced the formation of lumen-containing colonies (cysts) in a concentration- and time-dependent manner, a half-maximal effect after 9 days of culture being observed with 100 pM RA. The pleiotropic effects of retinoids are mediated by nuclear retinoic acid receptors (RARs; alpha, beta and gamma) and retinoid X receptors (RXRs; alpha, beta and gamma). To identify the signaling pathway involved in RA-induced lumen formation, we used receptor-specific synthetic retinoids. TTNPB, a selective RAR agonist, promoted lumen morphogenesis, whereas RXR-selective ligands lacked this activity. Lumen formation was also induced at picomolar concentrations by Am-580, a synthetic retinoid that selectively binds the RARalpha receptor subtype. Moreover, co-addition of Ro 41-5253, an antagonist of RARalpha, abrogated the lumen-inducing activity of both RA and DCS, indicating that this biological response is mediated through an RARalpha-dependent signaling pathway. To gain insight into the mechanisms underlying RA-induced lumen formation, we assessed the potential role of matrix metalloproteinases (MMP). Using gelatin zymography, we observed a dose-dependent increase in latent and active forms of gelatinase B (MMP-9) upon RA treatment. In addition, lumen formation was abrogated by addition of the synthetic MMP inhibitor BB94, indicating that this morphogenetic process is likely to require MMP activity. Collectively, our results provide evidence that RA promotes lumen formation by mammary epithelial cells in vitro and suggest that it plays a similar role during mammary gland development in vivo.