Retinoic Acid Inhibits Monocyte to Macrophage Survival and Differentiation

The role of β-carotene and vitamin A in atherogenesis: Evidences from preclinical and clinical studies

Atherosclerotic cardiovascular disease (ASCVD) is the principal contributor to myocardial infarction, the leading cause of death worldwide. Epidemiological and mechanistic studies indicate that β-carotene and its vitamin A derivatives stimulate lipid catabolism in several tissues to reduce the incidence of obesity, but their roles within ASCVD are elusive. Herein, we review the mechanisms by which β-carotene and vitamin A modulate ASCVD. First, we summarize the current knowledge linking these nutrients with epidemiological studies and lipoprotein metabolism as one of the initiating factors of ASCVD. Next, we focus on different aspects of vitamin A metabolism in immune cells such as the mechanisms of carotenoid uptake and conversion to the vitamin A metabolite, retinoic acid. Lastly, we review the effects of retinoic acid on immuno-metabolism, differentiation, and function of macrophages and T cells, the two pillars of the innate and adaptive immune response in ASCVD, respectively. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

All trans-retinoic acid abrogates the pro-tumorigenic phenotype of prostate cancer tumor-associated macrophages

Tumor-associated macrophages (TAMs) are a prominent cell type of the tumor stroma and stimulate malignant cell growth, survival and metastasis. The present manuscript demonstrates that prostate cancer cell-derived factors induce a pro-tumoral TAM-like phenotype characterized by increased proliferation and increased expression of pro-angiogenic, immunosuppressive and pro-metastatic factors. These effects were abrogated by all trans-retinoic acid (ATRA), a clinically available molecule with known immune-modulating properties. Furthermore, ATRA inhibited the cancer cell-stimulated proliferation of the pro-tumoral macrophages and restored their cytotoxic capacity towards prostate cancer cells. These findings suggest the use of ATRA as an immunomodulating agent to block the activity of prostate cancer TAMs.

Biological pathways and potential targets for prevention and therapy of chronic allograft nephropathy

Renal transplantation (RT) is the best option for patients with end-stage renal disease, but the half-life is limited to a decade due to progressive deterioration of renal function and transplant failure from chronic allograft nephropathy (CAN), which is the leading cause of transplant loss. Extensive research has been done to understand the pathogenesis, the biological pathways of fibrogenesis, and potential therapeutic targets for the prevention and treatment of CAN. Despite the advancements in the immunosuppressive agents and patient care, CAN continues to remain an unresolved problem in renal transplantation. The aim of this paper is to undertake a comprehensive review of the literature on the pathogenesis, biological pathways of RT fibrogenesis, and potential therapeutic targets for the prevention and therapy of CAN.

SB1578, a novel inhibitor of JAK2, FLT3, and c-Fms for the treatment of rheumatoid arthritis

SB1578 is a novel, orally bioavailable JAK2 inhibitor with specificity for JAK2 within the JAK family and also potent activity against FLT3 and c-Fms. These three tyrosine kinases play a pivotal role in activation of pathways that underlie the pathogenesis of rheumatoid arthritis. SB1578 blocks the activation of these kinases and their downstream signaling in pertinent cells, leading to inhibition of pathological cellular responses. The biochemical and cellular activities of SB1578 translate into its high efficacy in two rodent models of arthritis. SB1578 not only prevents the onset of arthritis but is also potent in treating established disease in collagen-induced arthritis mice with beneficial effects on histopathological parameters of bone resorption and cartilage damage. SB1578 abrogates the inflammatory response and prevents the infiltration of macrophages and neutrophils into affected joints. It also leads to inhibition of Ag-presenting dendritic cells and inhibits the autoimmune component of the disease. In summary, SB1578 has a unique kinase spectrum, and its pharmacological profile provides a strong rationale for the ongoing clinical development in autoimmune diseases.

All-trans retinoic acid up-regulates Prostaglandin-E Synthase expression in human macrophages

All-trans retinoic acid (ATRA) is a potent retinoid, which has been used successfully in different clinical settings as a potential drug to treat COPD and emphysema. In the present study, we analyzed genes modulated by ATRA by performing mRNA expression array analysis on alveolar macrophages after treatment with ATRA. Here we observed a 375-fold up-regulation of Prostaglandin-E Synthase (microsomal PGES-1, NM_004878 PTGES) which mediates the conversion of prostaglandin H(2) (PGH(2)) to Prostaglandin E(2) (PGE(2)). We furthermore studied the expression of PTGES after treatment with ATRA in human monocyte-derived macrophages (MDMs) and bronchoalveolar lavage (BAL) cells. ATRA up-regulated PTGES mRNA expression in MDMs generated with M-CSF by 2500-fold whereas in M-CSF+IL-13 macrophages the up-regulation was only 20-fold. Similarly, ATRA up-regulated PTGES mRNA expression by factor 1524 in BAL cells. The up-regulation of PTGES mRNA expression by ATRA is both time and dose dependent. IL-13 suppressed the ATRA induced PTGES expression at both mRNA and protein level in MDM and BAL cells. We also observed that LPS acts synergistically with ATRA in MDMs and strongly induces PTGES expression. ATRA had little impact on cyclooxygenase-1 and -2 (COX-1 and -2) expression as compared to PTGES expression under the same experimental conditions. Furthermore, we observed an induction of PGE(2) levels by ATRA in BAL cells. These data indicate that ATRA is a potent inducer of PTGES expression in human macrophages but not in alternatively activated macrophages and suggest that the eicosanoid pathway is important for ATRA action in macrophages.

9-cis-Retinoic acid (9cRA), a retinoid X receptor (RXR) ligand, exerts immunosuppressive effects on dendritic cells by RXR-dependent activation: inhibition of peroxisome proliferator-activated receptor gamma blocks some of the 9cRA activities, and precludes them to mature phenotype development

At nanomolar range, 9-cis-retinoic acid (9cRA) was able to interfere in the normal differentiation process from human monocyte to immature dendritic cell (DC) and produced a switch in mature DCs to a less stimulatory mode than untreated cells. 9cRA-treated mature DCs secreted high levels of IL-10 with an IL-12 reduced production. The phenotypic alterations unleashed by 9cRA were similar but not identical to other specific retinoid X receptor (RXR) agonists and to those already reported for rosiglitazone, a PPARgamma activator, on DCs. The simultaneous addition of 9cRA and rosiglitazone on DCs displayed additive effects. Moreover, addition to cultures of GW9662, a specific inhibitor of PPARgamma, or the RXR pan-antagonist HX603, blocked these changes. All these results suggest an activation of PPARgamma-RXR and other RXR containing dimers by 9cRA in DCs. Finally, both GW9662 and HX603 by themselves altered the maturation process unleashed by TNFalpha, poly(I:C) or LPS on human DCs further suggesting that the heterodimer PPARgamma-RXR must fulfill a significant role in the physiological maturation process of these cells in addition to the repressing effects reported till now for this nuclear receptor.

Effects of inflammatory cytokines IL-1beta, IL-6, and TNFalpha on the intracellular localization of retinoid receptors in Schwann cells

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

Therapeutic targets in the treatment of allograft fibrosis

The dramatic improvements in short-term graft survival and acute rejection rates could only have been dreamed of 20 years ago. Late graft loss following kidney transplantation is now the critical issue of this decade. Frequently, graft loss is associated with the development of tubular atrophy and interstitial fibrosis within the kidney (i.e. chronic allograft nephropathy; CAN). Major treatment strategies in this disorder are non-specific and the focus of intervention has been on limiting injurious events. Following graft injury is a fibrogenesis phase featuring both proliferative and infiltrative responses mediated by chemokines, cytokines and growth factors. In particular, TGFbeta has been strongly implicated in the pathogenesis of chronic injury and epithelial-mesenchymal transformation (EMT) may be part of this process. The cascade of events results in matrix accumulation, due to either increased production and/or reduced degradation of matrix. Recent investigations into the pathogenesis of tissue fibrosis have suggested a number of new strategies to ameliorate matrix synthesis. While the majority of therapies have focused on TGFbeta, this may not be an ideal maneuver in transplant settings and alternative targets identified in other fibrotic diseases will be discussed. Attacking graft fibrosis should be a new focus in organ transplantation.

Differential regulation of CC chemokine receptors by 9-cis retinoic acid in the human mast cell line, HMC-1

Mast cells are well known as effector cells in a variety of inflammatory diseases, including asthma as well as other allergic disorders. The precise role of 9-cis retinoic acid (9CRA) in mast cells is not understood despite the accepted fact that 9CRA regulates inflammatory responses and neutrophil differentiation. In this study, we investigated the effects of 9CRA on the expression of CC chemokine receptors in the human mast cell line, HMC-1. 9CRA selectively inhibits the CCR2 mRNA level and increases the CCR3 mRNA level in both a time and dose dependent manner. Other CC chemokine receptors, including CCR1, CCR4 and CCR5 are not altered by treatment with 9CRA. Both TNF-alpha and LPS, known pro-inflammatory molecules, have no effect on mRNA levels of CC chemokine receptors. For surface expression, 9CRA decreased the CCR2 level but had no effect on the CCR3 level. 9CRA inhibited the chemotactic activity in response to the CCR2-dependent chemokine, MCP-1/CCL2 but not in response to CCR3-specific chemokine, eotaxin/CCL11. 9CRA decreased spontaneous homotype clustering. Therefore, our results demonstrate that 9CRA differentially decreases both CCR2 expression and chemotactic ability of HMC-1 cells, and may regulate the inflammatory effects of mast cells.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

13-cis retinoic acid inhibits development and progression of chronic allograft nephropathy

Chronic allograft nephropathy is characterized by chronic inflammation and fibrosis. Because retinoids exhibit anti-proliferative, anti-inflammatory, and anti-fibrotic functions, the effects of low and high doses of 13-cis-retinoic acid (13cRA) were studied in a chronic Fisher344-->Lewis transplantation model. In 13cRA animals, independent of dose (2 or 20 mg/kg body weight/day) and start (0 or 14 days after transplantation) of 13cRA administration, serum creatinine was significantly lower and chronic rejection damage was dramatically reduced, including subendothelial fibrosis of preglomerular vessels and chronic tubulointerstitial damage. The number of infiltrating mononuclear cells and their proliferative activity were significantly diminished. The mRNA expression of chemokines (MCP-1/CCL2, MIP-1alpha/CCL3, IP-10/CXCL10, RANTES/CCL5) and proteins associated with fibrosis (plasminogen activator inhibitor-1, transforming growth factor-beta1, and collagens I and III) were strikingly lower in treated allografts. In vitro, activated peritoneal macrophages of 13cRA-treated rats showed a pronounced decrease in protein secretion of inflammatory cytokines (eg, tumor necrosis factor-alpha, interleukin-6). The suppression of the proinflammatory chemokine RANTES/CCL5 x 13cRA in fibroblasts could be mapped to a promoter module comprising IRF-1 and nuclear factor-kappaB binding elements, but direct binding of retinoid receptors to promoter elements could be excluded. In summary, 13cRA acted as a potent immunosuppressive and anti-fibrotic agent able to prevent and inhibit progression of chronic allograft nephropathy.

Differential effects of 9-cis retinoic acid on expression of CC chemokine receptors in human monocytes

9-cis Retinoic acid (9-CRA) is a lipophilic molecule that binds to the retinoid X receptor (RXR). Although retinoic acid (RA) has been known to regulate neutrophil differentiation, a specific role for 9-CRA in chemokine-mediated cellular processes remains obscure. We investigated the effects of 9-CRA on expression of CC chemokine receptors (CCRs) in human monocytic THP-1 cells and peripheral blood monocytes. RNase protection assay was performed to examine the mRNA levels of CCRs in 9-CRA-treated THP-1 cells. mRNA expression of CCR1 and CCR2 was induced in both a dose and time dependent manner. CCR1 and CCR2 mRNA expression began to increase from 6h after a 100nM 9-CRA treatment and reached a maximal level at 12h. Surface expression of CCRs was monitored by flow cytometry. CCR1 and CCR2 surface expression increased in 9-CRA-treated THP-1 cells, but not in untreated cells. Calcium mobilization and chemotactic activity were determined to examine the effect of 9-CRA on cell movement. The intracellular Ca(2+) concentration and the chemotactic activity increased in 9-CRA-treated cells in response to the CCR1-dependent chemokines Lkn-1, MIP-1alpha, and RANTES, and the CCR2-specific chemokine MCP-1. Increased surface expression of CCR1 and the Ca(2+) influx due to 9-CRA were confirmed in peripheral blood monocytes. Taken together, 9-CRA increases the expression levels of mRNA and protein of both CCR1 and CCR2, and the cell migration ability in THP-1 cells and peripheral blood monocytes, indicating that 9-CRA may regulate inflammatory processes through an increased response to CCR1- and CCR2-dependent chemokines.

DNA array analysis of altered gene expression in human leukocytes stimulated with soluble and particulate forms of Candida cell wall β-glucan

We previously reported that 1,3-beta-d-glucan derived from Candida albicans, a pathogenic fungus was obtained by oxidation of the cell wall with sodium hypochlorite (NaClO). It could be solubilized by treatment with dimethylsulfoxide (DMSO). We found that OX-CA and CSBG showed significantly different levels of activity toward leukocytes. Here, we have used cDNA microarrays to analyze the mRNA expression of 1176 genes in PBMCs stimulated with Candida cell wall glucan and considered the difference in the activation mechanism of OX-CA and CSBG. Total mRNA showed a significant change for 147 out of 1176 arrayed genes on stimulation with OX-CA and CSBG for 4 h. Among those genes, 62 were common, 26 were OX-CA-specific and 59 were CSBG-specific. Many of these up-regulated genes encode effectors with well-characterized proinflammatory properties. The expression of genes related with signal transduction differed in the particulate and soluble glucans derived from C. albicans having exactly the same primary structure. This fact suggested that each glucan induced specific biological activity through a different activation mechanism. This study using cDNA microarrays to analyze a broad spectrum of mRNA expression provides information on the biological activity of Candida cell wall glucan as a potential pathogenic factor.

Isotretinoin ameliorates renal damage in experimental acute renal allograft rejection

BACKGROUND

Retinoic acids, derivatives of vitamin A, act through retinoid receptors that are expressed in renal and immunocompetent cells (B and T cells; monocytes and macrophages). In experimental models of glomerulonephritis and renal interstitial disease, retinoids were shown to reduce both glomerular and tubular damage and inflammation. We therefore examined whether retinoids reduce cellular rejection and renal damage in a model of acute renal allograft rejection.

METHODS

Kidneys of Fisher rats (F344, RT11v1) were orthotopically grafted to Lewis rats (RT11). Animals were killed 7 or 14 days after transplantation. Rats undergoing transplantation were treated with isotretinoin (13 cis-retinoic acid) at a low dose of 2 mg/kg body weight per day (LD isotretinoin) or at a high dose of 20 mg/kg body weight per day (HD isotretinoin) or with vehicle.

RESULTS

At day 14, albuminuria was reduced by approximately 70% (vehicle: 1.1+/-0.2 mg/24 hr vs. LD isotretinoin: 0.32+/-0.1 mg/24 hr; P<0.001). At days 7 and 14 serum creatinine levels were significantly higher in the vehicle-treated group than in the LD and HD isotretinoin-treated rats (P<0.05). Both LD and HD isotretinoin significantly reduced acute vascular injury compared with vehicle-treated rats (score at day 14: LD isotretinoin 20.1+/-5.1 vs. vehicle 57.8+/-9.9, P<0.01), acute glomerular injury (score: LD isotretinoin 6.8+/-1.0 vs. vehicle 10.6+/-0.9 P<0.05), and the number of glomerular monocytes and macrophages and cytotoxic T cells. Isotretinoin also significantly lessened tubulointerstitial damage, tubulointerstitial cell proliferation, and the number of cells infiltrating the tubulointerstitium.

CONCLUSIONS

Isotretinoin significantly ameliorated functional, vascular, glomerular, and tubulointerstitial lesions in acute graft rejection. Although the current study did not definitely eliminate the possibility that isotretinoin only delayed the rejection process, retinoic acid derivatives may provide a new approach in the treatment of acute rejection injury.

Vitamin D(3) and analogues modulate the expression of CSF-1 and its receptor in human dendritic cells

The active vitamin D(3)-metabolite 1,25(OH)(2)D(3) inhibits the interleukin 4/granulocyte-macrophage colony-stimulating factor (IL-4/GM-CSF)-induced differentiation of human monocytes into dendritic cells without altering survival. Colony-stimulating factor 1 (CSF-1) is an important survival factor for cells of the monocytic lineage. We therefore investigated whether the inhibitory activity of 1,25(OH)(2)D(3) is paralleled by a regulation of CSF-1 and its receptor. Purified human monocytes were cultured together with IL-4/GM-CSF in the presence of 1,25(OH)(2)D(3), its analogue tacalcitol, the low-affinity vitamin D receptor ligand 24,25(OH)(2)D(3), or the solvent ethanol for up to 5 days. Expression of CSF-1, CSF-1R, and GM-CSF mRNA was measured by RT-PCR. Protein secretion for CSF-1 was measured by ELISA, expression of CSF-1R by flow cytometry. The results showed that 1,25(OH)(2)D(3) and tacalcitol significantly up-regulated CSF-1 mRNA-expression and protein secretion in a dose-dependent manner. The effect of 1,25(OH)(2)D(3) occurred already after 1h of pre-treatment. In contrast, CSF-1R mRNA- and cell surface-expression was down-regulated simultaneously. The solvent ethanol and 24,25(OH)(2)D(3) were without effect. GM-CSF mRNA expression was not modulated in 1,25(OH)(2)D(3)-treated cells. These data point towards a distinct and specific regulation of CSF-1 and its receptor by 1,25(OH)(2)D(3) and its analogue tacalcitol in human monocytes which parallels the inhibition of differentiation into dendritic cells without altering survival.

Isotretinoin alleviates renal damage in rat chronic glomerulonephritis

BACKGROUND

Retinoids, derivatives of vitamin A, have strong anti-inflammatory and antiproliferative properties. We previously demonstrated that the pan-agonists all-transretinoic acid (RA) and isotretinoin (13-cis RA) alleviate renal damage in rat acute glomerulonephritis (GN) induced by anti-Thy-1.1 mAb OX-7.

METHODS

The present study examined the effects of low dose and high dose treatment with isotretinoin in the chronic glomerulonephritis model, Thy-GN. Thy-GN was induced by a single intravenous injection of monoclonal antibody (mAb) 1-22-3 in uninephrectomized Wistar rats (N = 7 to 10 per group). Control and nephritic groups were treated with vehicle (veh), low dose isotretinoin (2 mg/kg body wt), or high dose isotretinoin (10 mg/kg body wt). The experiment was terminated 60 days after induction of Thy-GN.

RESULTS

In animals with Thy-GN, isotretinoin abrogated the increase in blood pressure and significantly reduced albuminuria. Glomerulosclerosis index, glomerular and interstitial cell counts, as well as the area of the interstitial space were significantly lower in nephritic rats treated with low and high dose isotretinoin compared to vehicle-treated nephritic controls. Treatment with isotretinoin also significantly reduced the number of glomerular and interstitial macrophages. The increase of transforming growth factor (TGF)-beta1, TGF receptor II and prepro-endothelin-1 gene expression in vehicle-treated nephritic rats was significantly attenuated by isotretinoin.

CONCLUSIONS

Treatment with isotretinoin significantly reduces glomerular and interstitial damage in rats with chronic glomerulonephritis as indicated by different functional and histological markers. Retinoids may provide a novel therapeutic option for the treatment of glomerulonephritis.

CAM-cytarabine, aclarubicin plus macrophage colony-stimulating factor in the treatment of acute myelogenous leukemia with trilineage dysplasia: usefulness of in vitro apoptosis in leukemic cells

A 67-year-old woman was treated for acute myelogenous leukemia with trilineage dysplasia (AML-TLD) by combination chemotherapy with cytarabine, aclarubicin plus macrophage colony-stimulating factor (M-CSF) (referred to as CAM therapy). Complete remission was achieved after two courses of CAM therapy. After coculture of her bone marrow mononuclear cells with M-CSF in vitro, differentiation of leukemic cells into macrophages with apoptotis was observed. This case confirms an earlier report that an effect of M-CSF inducible by differentiation with apoptotic phenomena, against human leukemic cells was shown both in vitro and in vivo when achieving complete remission.

Sources and sink of retinoic acid in the embryonic chick retina: distribution of aldehyde dehydrogenase activities, CRABP-I, and sites of retinoic acid inactivation

Previous experiments in mice and zebrafish led to the hypothesis that an asymmetric distribution of the transcriptional activator retinoic acid (RA) causes ventral-dorsal polarity in the vertebrate eye anlage. A high concentration of RA in the ventral retinal neuroepithelium has been suggested to induce developmental events that finally establish topographic order in the retinotectal projection along the vertical eye axis. In the present study we have investigated potential sources and sinks of RA during embryonic development of the chick retina. At embryonic day (E)1 to E2, when the spatial determination of the eye primordia takes place, no RA synthesis by aldehyde dehydrogenases was detectable, and neither immunoreactivity for retinaldehyde dehydrogenase RALDH-2 nor for cellular retinoic acid binding protein CRABP-I was observed. These components of RA signal transduction appeared in the eye between E3 and E5. At later stages, RA-measurements with a reporter cell line showed highest synthesis in the retinal pigment epithelium (RPE) and at the ventral and dorsal poles of the retina. RA degradation occurred mostly in a horizontal region in the middle of the retina with only small differences along the nasal-temporal axis. CRABP-I immunoreactivity appeared first in differentiating retinal ganglion cells with no indication of a spatial gradient across the ventral-dorsal eye axis. RA-production depended on three NAD+-dependent enzyme activities, which could be competitively inhibited by citral. One enzyme, located in the dorsal retina (corresponding to mouse RALDH-1), and one enzyme in the RPE (RALDH-2) were aldehyde dehydrogenases of the same molecular weight (monomers about 55 kDa) but with different isoelectric points (6.5-6.9; 4.9-5.4). The third RA-synthesizing activity (pI 6.0-6.3) was limited to the ventral retina, and likely corresponded to mouse RALDH-3. The restricted localization of retinoid-metabolizing activities along the dorsal-ventral axis of the embryonic chick retina does support the idea that RA is involved in dorsal-ventral eye patterning. However, the late time of appearance of aldehyde dehydrogenase activities and CRABP-I points to functions in cellular differentiation that are distinct from the initiation of the dorsal-ventral polarity.

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.

CD137 Induces Proliferation and Endomitosis in Monocytes

Peripheral monocytes are short-lived and are replenished from hematopoietic stem cells whose proliferation is believed to be confined to the bone marrow. Human peripheral monocytes are assumed not to be able to proliferate. In this study we show that CD137 (ILA/4-1BB), a member of the tumor necrosis factor receptor family, induces a widespread and profound proliferation of human peripheral monocytes. Macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor are essential, but not sufficient for proliferation. Additional soluble autocrine factors induced by CD137 are required. Induction of proliferation is mediated via reverse signaling through a CD137 ligand, expressed constitutively by peripheral monocytes. The ability of CD137 to induce proliferation in human peripheral monocytes is not shared by any other known molecule.

CD137 Induces Proliferation and Endomitosis in Monocytes

Peripheral monocytes are short-lived and are replenished from hematopoietic stem cells whose proliferation is believed to be confined to the bone marrow. Human peripheral monocytes are assumed not to be able to proliferate. In this study we show that CD137 (ILA/4-1BB), a member of the tumor necrosis factor receptor family, induces a widespread and profound proliferation of human peripheral monocytes. Macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor are essential, but not sufficient for proliferation. Additional soluble autocrine factors induced by CD137 are required. Induction of proliferation is mediated via reverse signaling through a CD137 ligand, expressed constitutively by peripheral monocytes. The ability of CD137 to induce proliferation in human peripheral monocytes is not shared by any other known molecule.