tretinoin and benzyloxycarbonylleucyl-leucyl-leucine-aldehyde

Mutations in PARK2 (or parkin) are responsible for 50% of cases of autosomal-recessive juvenile-onset Parkinson's disease (PD). To date, 21 alternative splice variants of the human gene have been cloned. Yet most studies have focused on the full-length protein, whereas the spectrum of the parkin isoforms expressed in PD has never been investigated. In this study, the role of parkin proteins in PD neurodegeneration was explored for the first time by analyzing their expression profile in an in vitro model of PD. To do so, undifferentiated and all-trans-retinoic-acid (RA)-differentiated SH-SY5Y cells (which thereby acquire a PD-like phenotype) were exposed to PD-mimicking neurotoxins: 1-methyl-4-phenylpyridinium (MPP

Topics: 1-Methyl-4-phenylpyridinium; Alternative Splicing; Carbonyl Cyanide m-Chlorophenyl Hydrazone; Cell Differentiation; Cell Line; Cell Survival; Down-Regulation; Humans; In Vitro Techniques; Leupeptins; Models, Biological; Neurotoxins; Oxidopamine; Parkinson Disease; Protein Isoforms; Signal Transduction; Tretinoin; Ubiquitin-Protein Ligases

Acute promyelocytic leukemia (APL) is a distinctive subtype of acute myeloid leukemia (AML) in which the hybrid protein promyelocytic leukemia protein/retinoic acid receptor α (PML/RARα) acts as a transcriptional repressor impairing the expression of genes that are critical to myeloid cell mutation. We aimed at explaining the molecular mechanism of green tea polyphenol epigallocatechin-3-gallate (EGCG) enhancement of ATRA-induced APL cell line differentiation. Tumor suppressor phosphatase and tensin homolog (PTEN) was found downregulated in NB4 cells and rescued by proteases inhibitor MG132. A significant increase of PTEN levels was found in NB4, HL-60 and THP-1 cells upon ATRA combined with EGCG treatment, paralleled by increased myeloid differentiation marker CD11b. EGCG in synergy with ATRA promote degradation of PML/RARα and restores PML expression, and increase the level of nuclear PTEN. Pretreatment of PTEN inhibitor SF1670 enhances the PI3K signaling pathway and represses NB4 cell differentiation. Moreover, the induction of PTEN attenuated the Akt phosphorylation levels, pretreatment of PI3K inhibitor LY294002 in NB4 cells, significantly augmented the cell differentiation and increased the expression of PTEN. These results therefore indicate that EGCG targets PML/RARα oncoprotein for degradation and potentiates differentiation of promyelocytic leukemia cells in combination with ATRA via PTEN.

Topics: Catechin; Cell Differentiation; Chromones; Drug Resistance, Neoplasm; Gene Expression Regulation, Neoplastic; HL-60 Cells; Humans; Leukemia, Promyelocytic, Acute; Leupeptins; Morpholines; Phenanthrenes; Promyelocytic Leukemia Protein; Proteolysis; PTEN Phosphohydrolase; Retinoic Acid Receptor alpha; Tretinoin

Several evidences indicate stimulation of peroxisome proliferator activated receptor γ (PPARg), promotes neuronal differentiation. This study was conducted to testify the prominence of PPARγ during neural differentiation of human embryonic stem cells (hESCs).. PPARγ expression level was assessed during neural differentiation of hESCs. Meanwhile, the level of endogenous miRNAs, which could be engaged in regulation of PPARγ expression, was measured. Next, natural and synthetic components of PPARγ agonists and antagonist were implemented on neural progenitor formation during neural differentiation of hESCs.. Data showed an increasing wave of PPARγ expression level when human neural progenitors (NPs) were formed upon retinoic acid treatment. Interestingly, there was no significant difference in the amount of PPARγ proteins during the differentiation of hESCs that is inconsistent with what we observed for RNA level. Our results indicated that miRNAs are not involved in the regulation of PPARγ expression, while proteasome-mediated degradation may to some degree be involved in this process. Among numerous treatments, PPARγ inactivation during NPs formation significantly decreased expression of NP markers.. We conclude that a ground state of PPARγ activity is required for NP formation of hESCs during early neural differentiation. However, high expression and activity of PPARγ could not enhance the required neural differentiation, whereas the PPARγ inactivation could negatively influence NP formation from hESCs by antagonist.

Topics: Cells, Cultured; Gene Expression; Human Embryonic Stem Cells; Humans; Leupeptins; MicroRNAs; Neural Stem Cells; Neurogenesis; PPAR gamma; Proteasome Endopeptidase Complex; Tretinoin

Production of IL-6 constituted the major cause of death in the ATRA trial called retinoic acid syndrome (RAS). LAP and LIP are active and inactive isoforms of C/EBPβ, respectively. Inactive LIP dimerized with LAP to eliminate its activity. Following treatment with ATRA, CHOP expression was increased and dimerized with LIP more preferentially than LAP to rescue function of LAP. Oroxylin A has been reported to activate CHOP, a key mediator of unfolded protein response (UPR) pathway, and resulted in apoptosis. Interestingly, we found that low concentration of oroxylin A (≦ 40 μM) showed no apoptosis effect on NB4 and HL-60 cells and decreased the CHOP protein level via promoting its degradation. MG132 was utilized to conform the effect of oroxylin A on degrading CHOP. Our results showed that oroxylin A decreased the level of IL-6 secretion of NB4 cells with or without ATRA treatment while the effect was eliminated by C/EBPβ siRNA. We conclude that oroxylin A possessed abilities of inhibiting the ATRA-induced IL-6 production via modulation of LAP/LIP/CHOP in leukemia cell lines, which could providing a therapeutic strategy for RAS.

Topics: Antineoplastic Agents; Apoptosis; Blotting, Western; CCAAT-Enhancer-Binding Protein-beta; Cell Line, Tumor; Dose-Response Relationship, Drug; Down-Regulation; Flavonoids; Gene Expression; HL-60 Cells; Humans; Interleukin-6; K562 Cells; Leupeptins; RNA Interference; Syndrome; Transcription Factor CHOP; Tretinoin; U937 Cells

Neuroblastma cell lines contain a side-population of cells which express stemness markers. These stem-like cells may represent the potential underlying mechanism for resistance to conventional therapy and recurrence of neuroblastoma in patients.. To develop novel strategies for targeting the side-population of neurobastomas, we analyzed the effects of 13-cis-retinoic acid (RA) combined with the proteasome inhibitor MG132. The short-term action of the treatment was compared with effects after a 5-day recovery period during which both chemicals were withdrawn. RA induced growth arrest and differentiation of SH-SY5Y and SK-N-BE(2) neuroblastoma cell lines. Inhibition of the proteasome caused apoptosis in both cell lines, thus, revealing the critical role of this pathway in the regulated degradation of proteins involved in neuroblastoma proliferation and survival. The combination of RA with MG132 induced apoptosis in a dose-dependent manner, in addition to promoting G2/M arrest in treated cultures. Interestingly, expression of stem cell markers such as Nestin, Sox2, and Oct4 were reduced after the recovery period of combined treatment as compared with untreated cells or treated cells with either compound alone. Consistent with this, neurosphere formation was significantly impaired by the combined treatment of RA and MG132.. Given that stem-like cells are associated with resistant to conventional therapy and are thought to be responsible for relapse, our results suggest that dual therapy of RA and proteasome inhibitor might be beneficial for targeting the side-population of cells associated residual disease in high-risk neuroblastoma.

Topics: Apoptosis; Blotting, Western; Cell Differentiation; Cell Line, Tumor; Cell Proliferation; Dose-Response Relationship, Drug; Drug Synergism; Flow Cytometry; G2 Phase Cell Cycle Checkpoints; Humans; Leupeptins; Microscopy, Confocal; Neoplastic Stem Cells; Nestin; Neuroblastoma; Octamer Transcription Factor-3; Proteasome Inhibitors; Side-Population Cells; SOXB1 Transcription Factors; Time Factors; Tretinoin

Epidermolytic ichthyosis (EI) is an autosomal dominant epidermal skin fragility disorder caused by mutations in keratin 1 and 10 (K1 and K10) genes. Mutated keratins form characteristic aggregates in vivo and in vitro. Some patients benefit from retinoid therapy, although the mechanism is not fully understood. Our aim was to demonstrate whether retinoids affect the formation of keratin aggregates in immortalized EI cells in vitro. EI keratinocytes were seeded on cover slips, pre-treated or not with retinoids, heat-stressed, and keratin aggregate formation monitored. K10 aggregates were detected in 5% of cells in the resting state, whereas heat stress increased this proportion to 25%. When cells were pre-incubated with all-trans-retinoic acid (ATRA) or retinoic acid receptor (RAR)-α agonists the aggregates decreased in a dose-dependent manner. Furthermore, ATRA decreased the KRT10 transcripts 200-fold as well as diminished the ratio of mutant to wild-type transcripts from 0.41 to 0.35, thus providing a plausible rational for retinoid therapy of EI due to K10 mutations.

Topics: Cells, Cultured; Cysteine Proteinase Inhibitors; Hot Temperature; HSC70 Heat-Shock Proteins; Humans; Hyaluronan Receptors; Hyperkeratosis, Epidermolytic; Keratin-10; Keratinocytes; Keratolytic Agents; Leupeptins; Male; Mutation; Natural Cytotoxicity Triggering Receptor 2; p38 Mitogen-Activated Protein Kinases; Receptors, Retinoic Acid; Retinoids; RNA, Messenger; Tretinoin; Ubiquitin

c-myb null (knockout) embryonic stem cells (ESC) can differentiate into cardiomyocytes but not contractile smooth muscle cells (SMC) in embryoid bodies (EB).. To define the role of c-Myb in SMC differentiation from ESC.. In wild-type (WT) EB, high c-Myb levels on days 0-2 of differentiation undergo ubiquitin-mediated proteosomal degradation on days 2.5-3, resurging on days 4-6, without changing c-myb mRNA levels. Activin-A and bone morphogenetic protein 4-induced cardiovascular progenitors were isolated by FACS for expression of vascular endothelial growth factor receptor (VEGFR)2 and platelet-derived growth factor receptor (PDGFR)α. By day 3.75, hematopoesis-capable VEGFR2+ cells were fewer, whereas cardiomyocyte-directed VEGFR2+/PDGFRα+ cells did not differ in abundance in knockout versus WT EB. Importantly, highest and lowest levels of c-Myb were observed in VEGFR2+ and VEGFR2+/PDGFRα+ cells, respectively. Proteosome inhibitor MG132 and lentiviruses enabling inducible expression or knockdown of c-myb were used to regulate c-Myb in WT and knockout EB. These experiments showed that c-Myb promotes expression of VEGFR2 over PDGFRα, with chromatin immunopreciptation and promoter-reporter assays defining specific c-Myb-responsive binding sites in the VEGFR2 promoter. Next, FACS-sorted VEGFR2+ cells expressed highest and lowest levels of SMC- and fibroblast-specific markers, respectively, at days 7-14 after retinoic acid (RA) as compared with VEGFR2+/PDGFRα+ cells. By contrast, VEGFR2+/PDGFRα+ cells cultured without RA beat spontaneously, like cardiomyocytes between days 7 and 14, and expressed cardiac troponin. Notably, RA was required to more fully differentiate SMC from VEGFR2+ cells and completely blocked differentiation of cardiomyocytes from VEGFR2+/PDGFRα+ cells.. c-Myb is tightly regulated by proteosomal degradation during cardiovascular-directed differentiation of ESC, expanding early-stage VEGFR2+ progenitors capable of RA-responsive SMC formation.

Topics: Activins; Animals; Binding Sites; Biomarkers; Bone Morphogenetic Protein 4; Cell Differentiation; Cell Separation; Chromatin Immunoprecipitation; Cysteine Proteinase Inhibitors; Embryonic Stem Cells; Fibroblast Growth Factor 2; Flow Cytometry; Gene Expression Regulation, Developmental; Genes, Reporter; HEK293 Cells; Humans; K562 Cells; Leupeptins; Mice; Muscle, Smooth, Vascular; Mutation; Myocardial Contraction; Myocytes, Cardiac; Myocytes, Smooth Muscle; Promoter Regions, Genetic; Proteasome Endopeptidase Complex; Proteasome Inhibitors; Proto-Oncogene Proteins c-myb; Receptor, Platelet-Derived Growth Factor alpha; RNA Interference; RNA, Messenger; Time Factors; Transfection; Tretinoin; Troponin; Ubiquitination; Vascular Endothelial Growth Factor Receptor-2

All-trans retinoic acid (ATRA) is a promising therapeutic agent, but exhibits low efficacy against human cancers. We investigated the effect of sphingosine-1-phosphate (S1P) on ATRA activity in human colon cancer HT-29 cells. S1P antagonized ATRA activity on HT-29 cell proliferation and retinoic acid receptor beta (RARβ) expression. S1P treatment or transient co-transfection with SphK2 expression vector antagonized ATRA-induced RARβ promoter activity. Proteasome inhibition prevented S1P-induced modulation of ATRA activity. Overall, S1P antagonized ATRA's inhibitory effects by down-regulating RARβ expression, likely via the proteasome-dependent pathway. Decreasing S1P production or inhibiting SphK2 activity could enhance the efficacy of retinoids in cancer treatments.

Topics: Cell Proliferation; Colonic Neoplasms; Down-Regulation; HT29 Cells; Humans; Leupeptins; Lysophospholipids; Proteasome Inhibitors; Receptors, Retinoic Acid; Sphingosine; Tretinoin

We found that the levels of all general transcription factors (GTFs) for RNA polymerase II decreased in F9 cells when the cells were subjected to a differentiation procedure. Different from other GTFs, decrease of TFIIB during the differentiation was suppressed by addition of a proteasome inhibitor, MG132. The half-life of TFIIB in the differentiated cells was remarkably reduced compared with that in the undifferentiated cells. Moreover, it was demonstrated that TFIIB is a poly-ubiquitinated protein. Results of this study suggest that components of the transcription machinery decreased in accordance with cell differentiation and that TFIIB is specifically and rapidly degraded by the ubiquitin-proteasome pathway.

Topics: Animals; Blotting, Western; Cell Differentiation; Cell Line, Tumor; Chlorocebus aethiops; COS Cells; Cysteine Proteinase Inhibitors; Immunoprecipitation; Leupeptins; Mice; Polyubiquitin; Proteasome Endopeptidase Complex; Proteasome Inhibitors; Time Factors; Transcription Factor TFIIB; Transcription Factors, General; Transfection; Tretinoin; Ubiquitination

Histones are key components of chromatin. We investigated histone H2A-immunoreactive proteins in acute monocytic leukemia THP-1 cells using three polyclonal antibodies raised against peptides corresponding to distinct regions of H2A. Two unknown immunoreactive proteins (9- and 12-kDa proteins), H2A (14kDa) and ubiquitinated H2A (23kDa) were found in the cell lysates prepared by immediate direct addition of SDS-PAGE sample buffer to the cells as well as in the nuclear and chromatin fractions. However, they were not found in the cytoplasmic fraction. The unknown proteins were successfully purified by immunoaffinity chromatography from the cell nucleus extract and identified as 9-kDa H2A(1-87) and 12-kDa H2A(1-114), suggesting that both were produced by limited proteolysis of intact H2A(1-129). The truncated forms of H2A probably persisted as chromatin constituents, since the stability of H2A(1-87) in the chromatin fraction was sensitive to treatment with micrococcal nuclease, and H2A(1-114) was solubilized with lower ionic strength from the chromatin fraction obtained by micrococcal nuclease treatment. Truncated H2A proteins in THP-1 cells were transiently increased in amount by short-term treatment with phorbol 12-myristate 13-acetate or all-trans-retinoic acid, both of which induce macrophage-like differentiation. Furthermore, these increases were suppressed by preceding treatment with carbobenzoxy-l-leucyl-l-leucyl-l-leucinal (MG132) but not with carbobenzoxy-l-isoleucyl-gamma-t-butyl-l-glutamyl-l-alanyl-l-leucinal (PSI), both of which are generally known as proteasome inhibitors. Our results suggest that histone H2A is cleaved at least at two sites by protease(s) that remain obscure, and might affect chromatins in the early stage of THP-1 cell differentiation.

Topics: Antibodies; Antineoplastic Agents; Carcinogens; Cell Differentiation; Cell Line, Tumor; Chromatin; Histones; Humans; Leupeptins; Monocytes; Protein Processing, Post-Translational; Tetradecanoylphorbol Acetate; Tretinoin; Ubiquitins

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

Topics: Animals; Catalysis; Cell Survival; Cells, Cultured; Colchicine; Gene Expression Regulation; HeLa Cells; Hepatocytes; Humans; Leupeptins; Microtubules; Proteasome Inhibitors; Rats; Receptors, Retinoic Acid; Retinoic Acid Receptor alpha; Retinoic Acid Receptor gamma; RNA, Messenger; Thermodynamics; Transcription, Genetic; Transglutaminases; Tretinoin; Tubulin Modulators

To seek a novel therapeutic approach to neuroblastoma (NBL), we used three NBL cell lines (SK-N-DZ, NH12, and SK-N-SH) to examine the underlining molecular mechanisms of cellular reactions and sensitivity to all-trans-retinoic acid (ATRA). SK-N-DZ cells expressed relatively high levels of retinoic acid receptor alpha (RAR-alpha) and underwent ATRA-induced cell death that was blocked by an RAR-alpha antagonist. By contrast, RAR-alpha expression gradually decreased in NH12 and SK-N-SH cells, which did not experience increased cell death in response to ATRA. We report here the ubiquitin-dependent down-regulation of RAR-alpha expression during ATRA treatment. Our data suggest that SK-N-DZ cells have a defect in RAR-alpha down-regulation, resulting in sustained high expression of RAR-alpha that confers high sensitivity to ATRA. Accordingly, treatment with a proteasome inhibitor dramatically increased ATRA-induced cell death in NH12 and SK-N-SH cell lines. Our results reveal the crucial involvement of the RAR-alpha signaling pathway in NBL cell death and show that three NBL cell lines are differentially sensitive to ATRA. These data suggest a potential novel therapy for NBL involving retinoic acid treatment combined with the inhibition of RAR-alpha degradation.

Topics: Benzoates; Cell Division; Cell Line, Tumor; Chromans; Humans; Leupeptins; Neuroblastoma; Proteasome Inhibitors; Receptors, Retinoic Acid; Retinoic Acid Receptor alpha; RNA, Messenger; Tretinoin; Ubiquitin

PML-RARalpha protein, the leukemogenic product of t(15,17) in acute promyelocytic leukemia, is cleaved into a truncated form termed deltaPML-RARalpha during all-trans retinoic acid (ATRA)-induced differentiation of NB4 cells. DeltaPML-RARalpha is not formed in ATRA differentiation resistant NB4 subclones. As(2)O(3) inhibits deltaPML-RARalpha formation and differentiation-induction when given in combination with ATRA. Treatment with hexamethylene bisacetamide (HMBA) combined with ATRA enhances ATRA-induced differentiation in ATRA-insensitive NB4-CI and arsenic-resistant NB4/As cells, and is associated with stabilization of PML-RARalpha protein and increased deltaPML-RARalpha formation. Unlike forced expression of PML-RARalpha, forced deltaPML-RARalpha expression based on an estimated deletion of the N-terminal PML portion does not repress RARE-tk-luc reporter activity mediated by endogenous retinoic acid receptors. The cleavage of PML-RARalpha is blocked by RARalpha antagonist Ro-41-5253 and cycloheximide and therefore requires a RARalpha transactivation-dependent pathway. Proteasome inhibitor MG-132 and caspase inhibitor Z-VAD-FMK do not block ATRA-induced PML-RARalpha cleavage and differentiation. These data suggest that (a) ATRA treatment induces PML-RARalpha cleavage by induction of unknown enzymes independent of proteasome- and caspase-mediated pathways; (b) deltaPML-RARalpha might function differently from both PML-RARalpha and RARalpha; (c) failure to cleave PML-RARalpha and form deltaPML-RARalpha after ATRA treatment may contribute to ATRA resistance in APL cells.

Topics: Acetamides; Antineoplastic Agents; Arsenic; Blotting, Western; Cell Differentiation; Cell Line; Cycloheximide; Cysteine Endopeptidases; Cysteine Proteinase Inhibitors; Dose-Response Relationship, Drug; Enzyme Inhibitors; Humans; Leukemia, Promyelocytic, Acute; Leupeptins; Luciferases; Multienzyme Complexes; Neoplasm Proteins; Oncogene Proteins, Fusion; Plasmids; Proteasome Endopeptidase Complex; Protein Synthesis Inhibitors; Transcriptional Activation; Transfection; Tretinoin; Tumor Cells, Cultured

To bind DNA and to be retained in the nucleus, PBX proteins must form heterodimeric complexes with members of the MEINOX family. Therefore the balance between PBX and MEINOX must be an important regulatory feature. We show that overexpression of PREP-1 influences the level of PBX-2 protein maintaining the PREP-1-PBX balance. This effect has important functional consequences. F9 teratocarcinoma cells stably transfected with PREP-1 had an increased DNA binding activity to a PREP-PBX-responsive element. Because PREP-1 binds DNA efficiently only when dimerized to PBX, the increased DNA binding activity suggests that the level of PBX might also have increased. Indeed PREP-1-overexpressing cells had a higher level of PBX-2 and PBX-1b proteins. PBX-2 increase did not depend on increased mRNA level or a higher rate of translation but rather because of a protein stabilization process. Indeed, PBX-2 level drastically decreased after 3 h of cycloheximide treatment in control but not in PREP-1-overexpressing cells and the proteasome inhibitor MG132 prevented PBX-2 decay in control cells. Hence, dimerization with PREP-1 appears to decrease proteasomal degradation of PBX-2. Retinoic acid induces differentiation of F9 teratocarcinoma cells with a cascade synthesis of HOX proteins. In PREP-1-overexpressing cells, HOXb1 induction was more sustained (3 days versus 1 day) and the induced level of MEIS-1b, another TALE (three amino acid loop extension) protein involved in embryonal development, was higher. Thus an increase in PREP-1 leads to changes in the fate-determining HOXb1 and has therefore important functional consequences.

Topics: Amino Acid Sequence; Blotting, Northern; Cell Differentiation; Cell Nucleus; Cycloheximide; Cysteine Endopeptidases; Cytoplasm; Dimerization; DNA; Homeodomain Proteins; Immunoblotting; Kinetics; Leupeptins; Models, Biological; Molecular Sequence Data; Multienzyme Complexes; Myeloid Ecotropic Viral Integration Site 1 Protein; Neoplasm Proteins; Proteasome Endopeptidase Complex; Protein Binding; Protein Biosynthesis; Protein Synthesis Inhibitors; Proto-Oncogene Proteins; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Teratocarcinoma; Time Factors; Transfection; Tretinoin; Tumor Cells, Cultured

Most studies have reported an up-regulation of retinoic acid receptor (RAR) mRNA expression by all-trans retinoic acid (RA). We aimed to study the effect of RA on RAR protein levels in MCF-7 human breast cancer cells. Incubation of these cells with 10(-6) M RA induced a rapid breakdown of both RARalpha and RARgamma in spite of the accumulation of their mRNAs. Proteasome specific inhibitors blocked the RA-induced breakdown of RARs. Furthermore, RA enhanced the formation of the complex between RARalpha and ubiquitin in a concentration- and time-dependent manner, suggesting the involvement of ubiquitin and proteasome in this reaction. Retinoid X receptor alpha (RXRalpha) was also decreased, albeit to a lesser extent, in RA-treated cells. Use of synthetic receptor agonists and antagonists clearly showed that the effect of the retinoid on the breakdown of the retinoid receptors is receptor-ligand agonist-dependent and blunted by the antagonist. An electrophoretic mobility shift assay, using nuclear extracts from RA-treated cells, showed that a reduction in complex formation with hormone response elements correlated with the reduction of RAR and RXR protein. These data suggest that RA induces the breakdown of RARs through a process involving ubiquitination and that this phenomenon causes a reduction in the formation of DNA-receptor complexes.

Topics: Antineoplastic Agents; Breast Neoplasms; Cysteine Endopeptidases; Cysteine Proteinase Inhibitors; DNA; Humans; Leupeptins; Multienzyme Complexes; Nuclear Proteins; Proteasome Endopeptidase Complex; Receptors, Retinoic Acid; Response Elements; Retinoic Acid Receptor alpha; Retinoic Acid Receptor gamma; Tretinoin; Tumor Cells, Cultured; Ubiquitins