Stable transfection of U937 cells with sense or antisense RXR-alpha cDNA suggests a role for RXR-alpha in the control of monoblastic differentiation induced by retinoic acid and vitamin D

9-cis-retinoic acid promotes cell adhesion through integrin dependent and independent mechanisms across immune lineages

Retinoids are essential in the proper establishment and maintenance of immunity. Although retinoids are implicated in immune related processes, their role in immune cell adhesion has not been well established. In this study, the effect of 9-cis-retinoic acid (9-cis-RA) on human hematopoietic cell adhesion was investigated. 9-cis-RA treatment specifically induced cell adhesion of the human immune cell lines HuT-78, NB4, RPMI 8866 and U937. Due to the prominent role of integrin receptors in mediating immune cell adhesion, we sought to evaluate if cell adhesion was integrin-dependent. By employing a variety of integrin antagonist including function-blocking antibodies and EDTA, we establish that 9-cis-RA prompts immune cell adhesion through established integrin receptors in addition to a novel integrin-independent process. The novel integrin-independent adhesion required the presence of retinoid and was attenuated by treatment with synthetic corticosteroids. Finally, we demonstrate that 9-cis-RA treatment of primary murine B-cells induces ex vivo adhesion that persists in the absence of integrin function. Our study is the first to demonstrate that 9-cis-RA influences immune cell adhesion through at least two functionally distinct mechanisms.

Normal hematopoiesis after conditional targeting of RXRalpha in murine hematopoietic stem/progenitor cells

Because of the retinoic acid receptor-alpha (RARalpha) gene's involvement in acute promyelocytic leukemia, the important role of RARs in hematopoiesis is now well established. However, relatively few studies of hematopoiesis have focused on the role of the retinoid X receptors (RXRs), the obligate heterodimeric partners of the RARs. We sought to establish whether conditional targeting of RXRalpha in early hematopoietic progenitors, ideally to the level of the hematopoietic stem cell (HSC), would compromise hematopoiesis. For hematopoietic targeting of RXRalpha, we characterized IFN-inducible MxCre mice for use in studying the role of RXRalpha in hematopoiesis. We established that MxCre executes recombination of loxP-flanked RXRalpha in hematopoietic progenitors immunophenotypically enriched for HSC, leading to widespread and sustained targeting of RXRalpha in hematopoietic cells. However, we found no evidence of hematologic compromise in mice lacking RXRalpha, suggesting that RXRalpha is dispensable for normal murine hematopoiesis. Nonetheless, RXRalpha null bone marrow cells cultured in methylcellulose form colonies more efficiently than bone marrow cells obtained from control mice. This result suggests that although RXRalpha is not required for murine hematopoiesis, there may be hematopoietic signaling pathways that respond selectively to RXRalpha or settings in which combined expression of RXR (alpha, beta, and gamma) is limiting.

Vitamin D receptor and retinoid X receptor interactions in motion

Vitamin D receptor (VDR) and retinoid X receptor (RXR) are members of the nuclear receptor superfamily and they bind target DNA sequences as heterodimers to regulate transcription. This article surveys the latest findings regarding the roles of dimerizing RXR in VDR function and emphasizes potential areas for future developments. We first highlight the importance of dimerization with RXR for both the ligand-independent (hair growth) and ligand-dependent functions of VDR (calcium homeostasis, bone development and mineralization, control of cell growth and differentiation). Emerging information regarding the regulatory control of dimerization based on biochemical, structural, and genetic studies is then presented. Finally, the main focus of this article is a new dynamic perspective of dimerization functions, based on recent research with fluorescent protein chimeras in living cells by microscopy. These studies revealed that both VDR and RXR constantly shuttle between the cytoplasm and the nucleus and between subnuclear compartments, and showed the transient nature of receptor--DNA and receptor--coregulator interactions. Because RXR dimerizes with most of the nuclear receptors, regulation of receptor dynamics by RXR has a broad significance.

Degradation of RXRs influences sensitivity of rat osteosarcoma cells to the antiproliferative effects of calcitriol

Several cell lines, including ROS17/2.8 rat osteosarcoma (ROS) cells, contain functional VDRs and RXRs but are resistant to the antiproliferative effects of calcitriol and retinoids. We explored the role of receptor degradation in this hormone resistance. Results of transactivation assays indicated that ROS cells contain insufficient amounts of RXR to activate a DR-1 reporter, and Western blot analyses of cell extracts showed that the degradation of RXR is accelerated and produces an aberrant 45-kDa RXR. We stably expressed functional fluorescent chimeras of VDR and RXR [green fluorescent protein (GFP)-VDR; yellow fluorescent protein (YFP)-RXR] to evaluate degradation mechanisms and the impact of excess receptor expression on antiproliferative effects. Microscopy showed a diminished expression of YFP-RXR in ROS cells compared with the expression in CV-1 cells. Treatment with inhibitors of proteasomal degradation (lactacystin and MG132) selectively enhanced GFP-VDR and YFP-RXR expression and also increased the endogenous levels of VDR and RXR. Expression of GFP-VDR had no effect on the sensitivity of ROS cells to calcitriol. Increases of RXR levels by YFP-RXR expression, drug treatments, or the combination of the two, however, restored the growth-inhibitory effects of calcitriol and 9-cis-RA and restored p21 induction by calcitriol. These studies revealed that an accelerated and aberrant RXR degradation could cause resistance to the antiproliferative effects of calcitriol and retinoids in ROS cells.

Lipopolysaccharide-induced switch between retinoid receptor (RXR) alpha and glucocorticoid attenuated response gene (GARG)-16 messenger RNAs in cultured rat microglia

Glucocorticoid-attenuated response genes (GARG) belong to a recently described family of genes responsive to the action of dexamethasone. Full-length cDNA of one member of this family, GARG16, has been cloned from rat microglia and regulation of its mRNA expression has been studied. Moreover, regulation of retinoid/retinoic acid activated transcription factor (RXR/RAR) mRNAs in mixed astrocyte and in purified microglia cultures has been investigated. RARbeta mRNA was undetectable in microglia by RT-PCR, whereas clearly present in the mixed cultures. RXRalpha, RARgamma, and GARG16 mRNAs were found in both culture systems. RXRalpha mRNA was strongly expressed in control microglia but rapidly declined upon treatment with LPS. Conversely, GARG16 mRNA was almost untraceable in control microglia but rapidly increased by LPS. Time-course studies revealed an oscillating behavior of expression of both mRNAs during the first 6 hr, which receded to control levels (RXRalpha high, GARG16 low) at 72 hr of LPS-treatment. Additionally, p38 MAPK and SEK phosphorylations peaked at 1 hr followed by steady declines, whereas MEK and c-Jun showed double peaks at 1+4 hr and 1+6 hr, respectively, before subsiding to control levels. This behavior was not observed in comparative studies with TNF-alpha, interleukin-10 (IL-10), or interferon-gamma inducible protein 10 (IP-10). Finally, inhibitors of p38 MAPK, p42/p44 ERK, and PKCalpha as well as the use of dexamethasone revealed major influences of the p38 MAPK-c-Jun-AP-1 signaling pathway on RXRalpha and GARG16 mRNA expressions. The counter regulatory control of GARG16 and RXRalpha mRNA expression is believed to be an example of a fine-tuned cellular mechanism to react to inflammatory stimuli.

Expression of retinoid receptors during human monocyte differentiation in vitro

1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)VD(3)) and retinoic acid (RA) modulate the activation of monocytes (MO) and their differentiation into macrophages (MAC). As these effects are mostly mediated by heterodimers or homodimers of the specific nuclear receptors for 1,25(OH)(2)VD(3) and RA, we investigated the expression of the retinoic acid receptors (RAR) alpha, beta, and gamma and the retinoid X-receptor (RXR) alpha in MO during differentiation into MAC or dendritic cells (DC). The mRNA of all investigated receptors except RARbeta was detected in short-term cultured MO. During differentiation of MO to MAC the mRNA expression of the RA receptors decreased. In contrast, along the differentiation pathway of MO to DC, only the mRNA expression of RARgamma declined, whereas RARalpha and RXRalpha were constantly expressed at a high level. Despite the strong expression of RARalpha and RXRalpha at mRNA level in MO-derived DC, the protein expression of the receptors was low in these cells. However, MO and MO-derived MAC showed a strong expression of these receptors at protein level. This suggests that a posttranscriptional or posttranslational mechanism of receptor regulation is occurring in these cells, and in particular in the DC. The inverse regulation of RA receptor expression and protein levels between MAC and DC may control the responsiveness of these cells to 1,25(OH)(2)VD(3) and RA.

Leukemia cell differentiation: cellular and molecular interactions of retinoids and vitamin D

1. The conventional approach to treatment of acute myeloid leukemia has been the use of chemotherapy, which although being cytotoxic to malignant clones, is also cytodestructive to normal cells. In addition, some leukemia cells develop resistance to chemotherapy and are therefore difficult to eradicate. 2. Differentiation therapy, whereby immature cells are induced to attain a mature phenotype by differentiation agents, has provided an alternative strategy in the treatment of hyperproliferative disorders. This has been highlighted by the use of all-trans retinoic acid (ATRA) in the treatment of acute promyelocytic leukemia (APL). 3. Another differentiation agent, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), directs monocytic maturation of normal and leukemic cells. Cellular studies have revealed that combinations of vitamin D derivatives and retinoids such as ATRA and 9-cis retinoic acid (9-cis RA) exhibit cooperative effects on differentiation in established leukemia cell lines such as HL-60, U937, and NB4. Furthermore, vitamin D compounds, although not able to induce apoptosis when used alone, potentiate apoptosis induced by 9-cis RA in HL-60 cells and differentially regulate the expression of the apoptosis-related gene products bcl-2 and bax. The molecular mechanisms involved in regulating differentiation and apoptosis by these agents are mediated through the interactions of the nuclear receptors for vitamin D (VDR), ATRA (RAR), and 9-cis RA (RXR), which are able to form homo- or heterodimeric complexes and transcriptionally activate or repress target gene expression. 4. There is evidence to suggest that nitric oxide may also play a role in leukemic cell differentiation and that 1,25(OH)2D3 may influence endogenous nitric oxide production either by directly increasing tumor necrosis factor-alpha (TNF-alpha) or through a secondary mediator such as the C-type lectin CD23.

Adrenomedullin production is correlated with differentiation in human leukemia cell lines and peripheral blood monocytes

We demonstrated that adrenomedullin (AM) is produced and secreted from human leukemia cell lines (THP-1 and HL-60) as well as peripheral blood granulocytes, lymphocytes, monocytes and monocyte-derived macrophages. Immunoreactive AM accumulated in the culture media of THP-1 and HL-60 cells increased according to their differentiation into macrophage-like cells. Retinoic acid exerted synergistic effects on AM secretion from THP-1 and HL-60 cells when administered with tumor necrosis factor-alpha, lipopolysaccharide or 12-O-tetradecanoyl phorbol-13-acetate. AM was shown to increase the scavenger receptor activity on THP-1 cells. Thus, monocytes/macrophages should be recognized as sources of AM, and the secreted AM may modulate the function of macrophages.