Retinoid signaling is attenuated by proteasome-mediated degradation of retinoid receptors in human keratinocyte HaCaT cells

Distinct functional modes of SUMOylation for retinoid X receptor alpha

The present study investigated human retinoid X receptor alpha (hRXRα) as a substrate for modification with small ubiquitin like modifier (SUMO) and how members of the protein inhibitor of activated STAT (PIAS) family may impact upon this process. In agreement with a previous study, we validate Ubc9 to facilitate SUMOylation of hRXRα at lysine 108 but note this modification to occur for all isoforms rather than specifically with SUMO1 and to preferentially occur with the unliganded form of hRXRα. SUMOylation of hRXRα is significantly enhanced through PIAS4-mediated activity with lysine 245 identified as a specific SUMO2 acceptor site modified in a PIAS4-dependent fashion. While individual mutations at lysine 108 or 245 modestly increase receptor activity, the combined loss of SUMOylation at both sites significantly potentiates the transcriptional responsiveness of hRXRα suggesting both sites may cooperate in a DNA element-dependent context. Our findings highlight combinatorial effects of SUMOylation may regulate RXRα-directed signalling in a gene-specific fashion.

An ATRActive future for differentiation therapy in AML

The success of all-trans retinoic acid (ATRA) therapy in acute promeylocytic leukemia (APL) has spawned numerous attempts to translate the paradigm of differentiation therapy to non-APL acute myelocytic leukemia (AML). However, the results of clinical trials have been overall disappointing. In this review we discuss the mechanism of retinoic acid signaling and the results of major clinical trials that have attempted to incorporate ATRA into AML regimens. We discuss recent evidence that indicate that the retinoic acid signaling pathway may be dysfunctional in AML. Preliminary studies suggest that targeting the pathways that modify retinoic acid receptor activity may reactivate the dormant retinoic acid-signaling pathway. Such strategies may revive the ability of ATRA to induce myeloid differentiation and apoptosis in non-APL AML.

Expression of Ski can act as a negative feedback mechanism on retinoic acid signaling

BACKGROUND

Retinoic acid signaling is essential for many aspects of early development in vertebrates. To control the levels of signaling, several retinoic acid target genes have been identified that act to suppress retinoic acid signaling in a negative feedback loop. The nuclear protein Ski has been extensively studied for its ability to suppress transforming growth factor-beta (TGF-β) signaling but has also been implicated in the repression of retinoic acid signaling.

RESULTS

We demonstrate that ski expression is up-regulated in response to retinoic acid in both early Xenopus embryos and in human cell lines. Blocking retinoic acid signaling using a retinoic acid antagonist results in a corresponding decrease in the levels of ski mRNA. Finally, overexpression of SKI in human cells results in reduced levels of CYP26A1 mRNA, a known target of retinoic acid signaling.

CONCLUSIONS

Our results, coupled with the known ability of Ski to repress retinoic acid signaling, demonstrate that Ski expression is a novel negative feedback mechanism acting on retinoic acid signaling.

Organotypic modeling of human keratinocyte response to peroxisome proliferators

Peroxisome proliferators (PPs) are a diverse chemical group including hypolipidemic drugs and some fatty acids. Their stimulation of PP-activated receptors (PPARs) and subsequent control of gene expression regulates metabolism and differentiation in many cells. PPs have multiple opportunities to target human epidermal keratinocytes because of delivery through dietary, clinical, and/or topical exposure routes. PPAR knockout mice and PP treatment of mouse skin or human keratinocytes in monolayer culture have established some effects for PPs in cutaneous differentiation. However, incomplete epidermal maturation characteristic of monolayer keratinocytes and rodent-specific effects may limit our full understanding of human keratinocyte responses to PPs. To address these issues, we investigated PP influence on primary human keratinocytes in organotypic cultures that recapitulate biochemical markers of epidermis. We found that the PPARα agonists clofibrate, docasohexaenoic acid, and WY-14,643 produced mild to moderate keratinocyte hyperplasia, increased stratification (particularly of granular and cornified layers), and enhanced levels of the differentiation markers filaggrin, ABCA12, and phosphorylated HSP27. Several PP effects generated in the organotypic system, however, were distinct from those previously reported for rodent skin and human keratinocyte monolayer cultures, suggesting that the species and growth context of target cells can impact exposure outcomes. Given the utility of organotypic cultures for modeling the epidermis, studies in this system may bridge the gap between the rodent assays and clinical studies of human epidermal responses to PPs.

Retinoid X receptor α and retinoids are key regulators in apoptosis of trophoblasts of patients with recurrent miscarriages

The retinoid X receptor α (RXRα) is a nuclear hormone receptor that is able to bind other nuclear receptors in a heterodimeric complex, thereby activating gene transcription. Recently, we identified enhanced expression of RXRα in extravillous trophoblasts (EVT) and villous trophoblasts (VT) of miscarried placentas. In addition, an increased number of apoptotic EVT was present in miscarried placentas. In this study, on the basis of immunocytochemical analysis, western blots, and quantitative real-time reverse transcription PCR, we could demonstrate a reduced expression of RXRα in choriocarcinoma cell lines and in human VTs after stimulation with the retinoids 9-cis-retinoic acid and all-trans-retinoic acid and the prostaglandin 15-deoxy-Δ(12,14)-prostaglandin J(2). Furthermore, a simultaneous expression of RXRα and the apoptotic marker M30 CytoDEATH in EVT of miscarried placentas from the first trimester was shown. In EVT of control placentas from legal termination of pregnancies, no co-expression of RXRα and M30 could be detected. A likely conclusion is that RXRα plays an important role in the induction of apoptosis. Downregulation of RXRα, as observed in the tested choriocarcinoma cells and trophoblasts, might serve as a protection against apoptosis and miscarriage. In conclusion, RXRα represents a potential target in the treatment of recurrent miscarriages.

Retinoic acid receptor-mediated signaling protects cardiomyocytes from hyperglycemia induced apoptosis: role of the renin-angiotensin system

Diabetes mellitus (DM) is a primary risk factor for cardiovascular diseases and heart failure. Activation of the retinoic acid receptor (RAR) and retinoid X receptor (RXR) has an anti-diabetic effect; but, a role in diabetic cardiomyopathy remains unclear. Using neonatal and adult cardiomyocytes, we determined the role of RAR and RXR in hyperglycemia-induced apoptosis and expression of renin-angiotensin system (RAS) components. Decreased nuclear expression of RARα and RXRα, activation of apoptotic signaling and cell apoptosis was observed in high glucose (HG) treated neonatal and adult cardiomyocytes and diabetic hearts in Zucker diabetic fatty (ZDF) rats. HG-induced apoptosis and reactive oxygen species (ROS) generation was prevented by both RAR and RXR agonists. Silencing expression of RARα and RXRα, by small interference RNA, promoted apoptosis under normal conditions and significantly enhanced HG-induced apoptosis, indicating that RARα and RXRα are required in regulating cell apoptotic signaling. Blocking angiotensin type 1 receptor (AT(1) R); but, not AT(2) R, attenuated HG-induced apoptosis and ROS generation. Moreover, HG induced gene expression of angiotensinogen, renin, AT(1) R, and angiotensin II (Ang II) synthesis were inhibited by RARα agonists and promoted by silencing RARα. Activation of RXRα, downregulated the expression of AT(1) R; and RXRα silencing accelerated HG induced expression of angiotensinogen and Ang II synthesis, whereas there was no significant effect on renin gene expression. These results indicate that reduction in the expression of RARα and RXRα has an important role in hyperglycemia mediated apoptosis and expression of RAS components. Activation of RAR/RXR signaling protects cardiomyocytes from hyperglycemia, by reducing oxidative stress and inhibition of the RAS.

Effects of narrow-band ultraviolet B and tazarotene therapy on keratinocyte proliferation and TIG3 expression

Tazarotene plus narrow-band ultraviolet B (NB-UVB) therapy has been shown to enhance the efficacy in treating patients with psoriasis, while the mechanism is not clear. The present study aims to investigate the alteration of cell proliferation and TIG3 in cultured normal human keratinocytes after NB-UVB and/or tazarotene treatment. Keratinocytes were exposed to NB-UVB, then incubated with or without tazarotene, and then cell proliferation was detected by methyl thiazoleterazolium colorimetric assay and TIG3 mRNA expression and protein production was examined by real-time reverse transcription polymerase chain reaction and immunocytochemistry, respectively. The results show that keratinocyte proliferation was inhibited and TIG3 mRNA expression and protein production were elevated by tazarotene at a dose higher than 0.1 micromol/L. In NB-UVB single irradiating groups, only 200 mJ/cm2 NB-UVB inhibited keratinocyte proliferation, and none of the irradiated groups had an effect on TIG3 expression. Moreover, tazarotene plus NB-UVB have stronger effects than those separately. These results indicate NB-UVB plus tazarotene may have synergistic effects on inhibiting keratinocyte proliferation and elevating TIG3 expression, which may have some implications for the understanding of how to treat psoriasis patients with tazarotene plus NB-UVB.

Synergistic effects of acitretin and narrow-band UVB on inducing the expression of heparin-binding epidermal-growth-factor-like growth factor in normal human keratinocytes

It has been reported that heparin-binding epidermal-growth-factor-like growth factor (HB-EGF) plays an important role in regulating keratinocyte growth after retinoic acid treatment. In clinic, there are synergistic effects of retinoic acid plus ultraviolet (UVB) on improving efficacy and reducing the side effects in treating psoriasis, while the mechanism is not fully clear. Therefore, this study was taken to examine the alteration of cell proliferation and HB-EGF expression in cultured normal human keratinocytes after acitretin and/or narrow-band UVB (NB-UVB) exposure. After 12 h incubation with acitretin (0.1-1 micromol/L) and/or following NB-UVB (50-100 mJ/cm2) irradiation in normal human keratinocytes, the proliferation potency of the cells was evaluated by MTT colorimetric assay, and HB-EGF protein and mRNA was detected by cytoimmunochemistry and real-time quantitative reverse transcription polymerase chain reaction (RT-PCR), respectively. Our results revealed 1 micromol/L acitretin combined with 100 mJ/cm2 NB-UVB showed strongest inhibition of keratinocyte proliferation and highest induction of HB-EGF mRNA expression (19.7% and 8.6-fold, respectively), the others were 1 micromol/L acitretin (14.4%, 7.1-fold), 0.1 micromol/L acitretin (10.2%, 3.2-fold), 100 mJ/cm2 NB-UVB (8.7%, 2.5-fold) and 50 mJ/cm2 NB-UVB (5.4%, 1.8-fold). Pattern of HB-EGF protein expression was similar to mRNA expression in keratinocytes upon above cultural conditions. Our results suggest acitretin or NB-UVB single treatment can dose-dependently inhibit keratinocyte proliferation and induce HB-EGF expression and there are synergistic effects when they are combined.

Expression, protein stability and transcriptional activity of retinoic acid receptors are affected by microtubules interfering agents and all-trans-retinoic acid in primary rat hepatocytes

Cellular signaling by glucocorticoid receptor and aryl hydrocarbon receptor is restricted by microtubules interfering agents (MIAs). This leads to down-regulation of drug metabolizing enzymes and drug interactions. Here we investigated the effects of all-trans-retinoic acid (ATRA) and MIAs, i.e. colchicine, nocodazole and taxol on the regulation of retinoic acid receptor (RAR) genes in primary cultures of rat hepatocytes. ATRA (1microM) down-regulated RARalpha and RARgamma mRNAs (decrease 23% and 41%, respectively) whereas it up-regulated RARbeta mRNA (4.3-fold induction). All MIAs diminished the expression of RARs in dose-dependent manner; the potency of MIAs increased in order NOC Collapse

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MESH Headings

• Animals
• Catalysis/drug effects
• Cell Survival/drug effects
• Cells, Cultured
• Colchicine/pharmacology
• Gene Expression Regulation/drug effects
• HeLa Cells
• Hepatocytes/cytology
• Hepatocytes/drug effects
• Hepatocytes/metabolism
• Humans
• Leupeptins/pharmacology
• Microtubules/drug effects
• Proteasome Inhibitors
• RNA, Messenger/genetics
• RNA, Messenger/metabolism
• Rats
• Receptors, Retinoic Acid/genetics
• Receptors, Retinoic Acid/metabolism
• Retinoic Acid Receptor alpha
• Thermodynamics
• Transcription, Genetic/drug effects
• Transglutaminases/metabolism
• Tretinoin/pharmacology
• Tubulin Modulators/pharmacology
• Retinoic Acid Receptor gamma

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Grants Collapse

Affiliation(s)

Zdenek Dvorák Institute of Medical Chemistry and Biochemistry, Faculty of Medicine, Palacký University Olomouc, Hnevotínská 3, 77515 Olomouc, Czech Republic.
Radim Vrzal
Jitka Ulrichová
Dana Macejová
Slavomíra Ondková
Július Brtko

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Retinoic acid attenuates promyelocytic leukemia protein-induced cell death in breast cancer cells by activation of the ubiquitin–proteasome pathway

All-trans-retinoic acid and the tumor suppressor promyelocytic leukemia protein (PML) are potent regulators of the growth of cancer cells. This study investigates the individual and combined effects of PML, when overexpressed by the recombinant PML adenovirus, and all-trans-retinoic acid on the proliferation of human estrogen-receptor negative SKBR-3 and estrogen-receptor positive MCF-7 breast cancer cell lines. All-trans-retinoic acid caused a significant degree of cell death in SKBR-3 cells and MCF-7 cells, and PML elicited a similar incidence of or slightly more cell death in MCF-7 cells. Dual-treated cells displayed significantly less cell death than did single-treated cells in the same cell line. We concluded that PML and all-trans-retinoic acid cause cell death by different pathways: PML activates ERK1/2, p38 MAPK, and p21; arrests the cell cycle; and later causes cell death; and all-trans-retinoic acid activates proteasome function, caspase cleavage, and apoptosis. The combined use of all-trans-retinoic acid and PML gene therapy may not be the best treatment for patients with cancer, because the ubiquitinylation of PML and its subsequent proteasome-dependent degradation by retinoic acids occur before overexpressed PML exhibits tumor-suppressive activity.

Accumulation of retinoid X receptor-alpha in uterine leiomyomas is associated with a delayed ligand-dependent proteasome-mediated degradation and an alteration of its transcriptional activity

An alteration of the retinoid pathway can influence the development of uterine leiomyomas in animal models, and retinoids have shown efficacy in inhibiting the growth of this benign tumor both in vitro and in vivo. However, the underlying mechanisms and biological implications are unclear. The present study was based on the demonstration of an accumulation of full-length retinoid X receptor alpha (RXRalpha) in leiomyomas that was not associated with a modification of its gene expression. This accumulation was shown to increase the transcription of the RXR-responsive gene cellular retinoic acid binding protein II (CRABP-II) and to be linked to the cellular redistribution of the receptor and to its retarded degradation via the ubiquitin/proteasome pathway. Accordingly, treatment with a specific proteasome inhibitor but not with protease inhibitors strongly inhibited the degradation of full-length RXRalpha in cells deriving from both myometrium and leiomyoma, but the formation of RXRalpha/ubiquitin conjugates was differentially regulated between the two cell types. Moreover, full-length RXRalpha accumulated in leiomyomas was abnormally phosphorylated at serine/threonine residues relative to myometrial tissue. The ligand to RXRalpha, 9-cis-retinoic acid, induced the receptor breakdown in smooth muscle cells deriving from both normal and tumor tissue, whereas a MAPK-specific inhibitor was able to reduce RXRalpha levels only in leiomyoma cells. These results suggest that switching of the ubiquitin/proteasome-dependent degradation of RXRalpha by phosphorylation in leiomyomas may be responsible for the accumulation of the receptor and the consequent dysregulation of retinoic acid target genes. The ability of retinoids to modify this molecular alteration may be the rationale for their use in the treatment of leiomyomas.

The ubiquitin-proteasome system at the crossroads of stress-response and ageing pathways: a handle for skin care?

The regulation of gene expression at the transcriptional level has been considered for long as the main mechanism of cellular adaptive responses. Since the turn of the century, however, it is becoming clear that higher organisms developed a complex, sensitive and maybe equally important network of regulatory pathways, relying largely on protein interactions, post-translational modifications and proteolysis. Here we review the involvement of the ubiquitin-proteasome pathway of protein degradation at different levels of cellular life in relation with ageing, and with a special focus on skin. It comes out that the ubiquitin system plays a major role in signal transduction associated with stress and ageing, in skin in particular through the control of retinoid and NF-kappaB pathways. The understanding of specific proteolytic targeting by E3 ubiquitin-ligases paves the way for a new generation of active molecules that may control particular steps of normal and pathological ageing.

Phosphorylation of the retinoid x receptor at the omega loop, modulates the expression of retinoic-acid-target genes with a promoter context specificity

The retinoid response is mediated by two classes of nuclear receptors, the retinoic acid receptors (RARalpha, beta, and gamma) and the retinoid X receptors (RXRalpha, beta, and gamma) which act as ligand-dependent heterodimeric RAR/RXR transcription activators. Like most transcription factors, RARs and RXRs are regulated by phosphorylation processes. Here, we report that stress agents induce RXRalpha phosphorylation, subsequently to the activation of the stress-activated protein kinases cascade (JNKs). This phosphorylation process concerns three residues located in the N-terminal AF-1 domain of RXRalpha and one located in the omega loop of the Ligand Binding Domain. To decipher how stress-induced RXRalpha phosphorylation influences the transcription of RA-target genes, we used a ribotoxic stress agent, anisomycin, which activates signaling kinases without promoting DNA or protein damages, at subinhibitory concentrations. Taking advantage of vectors expressing recombinant RXRalpha mutated at its phosphorylation sites and of F9 cell lines re-expressing the same RXRalpha mutants in an RXRalpha null background, we provide evidence that stress signaling modulates RAR/RXRalpha-mediated transcription, through the phosphorylation of RXRalpha at the residue located in the Omega loop, in a promoter context-dependent manner.

Controlling nuclear receptors: the circular logic of cofactor cycles

Nuclear receptors regulate many biologically important processes in development and homeostasis by their bimodal function as repressors and activators of gene transcription. A finely tuned modulation of the transcriptional activities of nuclear receptors is crucial for determining highly specific and diversified programmes of gene expression. Recent studies have provided insights into the molecular mechanisms that are required to switch between repression and activation functions, the combinatorial roles of the multiple cofactor complexes that are required for mediating transcriptional regulation, and the central question of how several different signalling pathways can be integrated at the nuclear level to achieve specific profiles of gene expression.

Retinoic acid and arsenic trioxide cooperate for apoptosis through phosphorylated RXR alpha

Arsenite trioxide (As2O3) induces apoptosis in several cell lines by disturbing key signal transduction pathways through its oxidative properties. Here, we report that As2O3 also induces the phosphorylation of the retinoid receptor RXRalpha, subsequent to oxidative damages and the activation of the stress-activated protein kinases cascade (JNKs). We also report that RA amplifies both As2O3-induced phosphorylation of RXRalpha and apoptosis. Taking advantage of 'rescue' F9 cell lines expressing RXRalpha mutated at its phosphorylation sites, in an RXRalpha null background, we provide evidence that RXRalpha is a key element involved in that potentiating effect. Finally, we demonstrate that As2O3 also abrogates the transactivation of RA-target genes.

Regulation of Ultraviolet B Radiation-Mediated Activation of AP1 Signaling by Retinoids in Primary Keratinocytes

The main cause of skin cancer and photo-aging is chronic exposure to ultraviolet B (UVB) radiation. Such damage can be ameliorated by retinoid treatment. UVB-radiation-induced skin carcinogenesis is associated with the induction of activator protein 1 (AP1) signaling and factors, namely FOS and JUN family members. We investigated the effects of several retinoids, all-trans-retinoic acid (tRA), 9-cis-retinoic acid (cRA), and N-(4-hydroxyphenyl)-retinamide (HPR), on UVB-induced damage in primary mouse keratinocytes. In addition, the interplay between UVB radiation, retinoid receptors, and AP1 signaling was assessed using Western blot analysis and ribonuclease protection and gene reporter assays. Exposure of keratinocytes to UVB radiation caused a down-regulation of the retinoid receptor protein levels in a proteasome-mediated manner. In contrast, FOS and JUN proteins were transiently induced shortly after exposure to UVB radiation. Retinoid treatment caused a dose-dependent reduction in the levels of retinoid receptor proteins. When irradiated cells were treated with retinoids, no significant effects on AP1 protein expression were noted. Interestingly, pretreatments with tRA and cRA, but not HPR, suppressed UVB-radiation-induced AP1 activity by more than 50%, whereas post-treatment failed to produce similar effects. Our findings indicate that the inhibition of AP1 activity by retinoids explains, at least in part, the chemopreventive potential of retinoids in UV-radiation-associated epidermal damage.

Global analysis of genes differentially expressed in branching and non-branching regions of the mouse embryonic lung

During development, the proximal and distal regions of respiratory tract undergo distinct processes that ultimately give rise to conducting airways and alveoli. To gain insights into the genetic pathways differentially activated in these regions when branching morphogenesis is initiating, we characterized their transcriptional profiles in murine rudiments isolated at embryonic (E) day 11.5. By using oligonucleotide microarrays, we identified 83 and 128 genes preferentially expressed in branching and non-branching regions, respectively. The majority of these genes (85%) had not been previously described in the lung, or in other organs. We report restricted expression patterns of 22 of these genes were by in situ hybridization. Among them in the lung potential components of the Wnt, TGF beta, FGF and retinoid pathways identified in other systems, and uncharacterized genes, such as translocases, small GTPases and splicing factors. In addition, we provide a more detailed analysis of the expression pattern and regulation of a representative gene from the distal (transforming growth factor, beta induced) and proximal (WW domain-containing protein 2) regions. Our data suggest that these genes may regulate focal developmental events specific of each of these regions during respiratory tract formation.

The expression of immediate early gene X-1 (IEX-1) is differentially induced by retinoic acids in NB4 and KG1 cells: possible implication in the distinct phenotype of retinoic acid-responsive and -resistant leukemic cells

In a cell-type- and stimulus-dependent fashion, the early response gene immediate early gene X-1 (IEX-1) is involved in growth control and modulation of apoptosis. The present study demonstrates that, in the two acute promyelocytic leukemia (APL) cell lines NB4 and KG1, exhibiting distinct responsiveness to retinoic acids (RAs), IEX-1 expression is rapidly (30-60 min) induced by all-trans- or cis-RA and independently of other signal transduction mediators, such as TNFalpha, NF-kappaB or MAP kinases. In NB4 cells (expressing PML-RARalpha), this increase is transient and completely reversible, along with a cell cycle arrest, ongoing differentiation and lower sensitivity to anti-cancer-drug-induced apoptosis. In contrast, the RA-induced IEX-1 expression in KG1 cells (expressing PLZF-RARalpha) persists over days, along with continued cell cycle progression and increased apoptotic sensitivity. Furthermore, two functional RA-response elements in the IEX-1 promoter were identified by gel shift and luciferase reporter gene assays. IEX-1 might be a rather unique transcriptional target of the two X-RARalpha fusion receptors exhibiting distinct responsiveness to RAs. Following a different time course of direct transcriptional induction by PML-RARalpha and PLZF-RARalpha in NB4 and KG1 cells, respectively, IEX-1 expression may be involved in the modified actions of these receptors and the distinct phenotypes of APL cells.