Retinoid target genes in acute promyelocytic leukemia

SIRT1 ISGylation accelerates tumor progression by unleashing SIRT1 from the inactive state to promote its deacetylase activity

ISG15 is an interferon-stimulated ubiquitin-like protein (UBL) with multifaceted roles as a posttranslational modifier in ISG15 conjugation (ISGylation). However, the mechanistic consequences of ISGylation in cancer have not been fully elucidated, largely due to a lack of knowledge on the ISG15 target repertoire. Here, we identified SIRT1, a nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase, as a new target for ISGylation. SIRT1 ISGylation impairs the association of SIRT1 with its negative regulator, deleted in breast cancer 1 (DBC1), which unleashes SIRT1 from its inactive state and leads to an increase in its deacetylase activity. Importantly, SIRT1 ISGylation promoted lung cancer progression and limited lung cancer cell sensitivity to DNA damage-based therapeutics in vivo and in vitro models. The levels of ISG15 mRNA and protein were significantly higher in lung cancer tissues than in adjacent normal tissues. Accordingly, elevated expression of SIRT1 and ISG15 was associated with poor prognosis in lung cancer patients, a finding that could be translated for lung cancer patient stratification and disease outcome evaluation. Taken together, our findings provide a mechanistic understanding of the regulatory effect of SIRT1 ISGylation on tumor progression and therapeutic efficacy in lung cancer.

Advances in cutaneous squamous cell carcinoma

Human malignancies arise predominantly in tissues of epithelial origin, where the stepwise transformation from healthy epithelium to premalignant dysplasia to invasive neoplasia involves sequential dysregulation of biological networks that govern essential functions of epithelial homeostasis. Cutaneous squamous cell carcinoma (cSCC) is a prototype epithelial malignancy, often with a high tumour mutational burden. A plethora of risk genes, dominated by UV-induced sun damage, drive disease progression in conjunction with stromal interactions and local immunomodulation, enabling continuous tumour growth. Recent studies have identified subpopulations of SCC cells that specifically interact with the tumour microenvironment. These advances, along with increased knowledge of the impact of germline genetics and somatic mutations on cSCC development, have led to a greater appreciation of the complexity of skin cancer pathogenesis and have enabled progress in neoadjuvant immunotherapy, which has improved pathological complete response rates. Although measures for the prevention and therapeutic management of cSCC are associated with clinical benefit, the prognosis remains poor for advanced disease. Elucidating how the genetic mechanisms that drive cSCC interact with the tumour microenvironment is a current focus in efforts to understand, prevent and treat cSCC.

A comparative study of anti-leukemic effects of kaempferol and epigallocatechin-3-gallate (EGCG) on human leukemia HL-60 cells

OBJECTIVE

Acute promyelocytic leukemia (APL) is among the most threatening hematological malignant cancers. Defects in cell growth and apoptotic pathways lead to the pathogenesis of the disease as well as its resistance to therapy; therefore, it is a good model for examining pro-apoptotic agents. The present study compared the molecular mechanism induced by kaempferol and epigallocatechin gallate (EGCG) as well as all-trans retinoic acid (ATRA), in HL-60 leukemia cells during five days.

MATERIALS AND METHODS

Cell viability was determined by resazurin assay following treatment with ATRA (10 µM), EGCG, and kaempferol (12.5-100 µM), and apoptosis was detected by the ANX V/PI kit. Moreover, the levels of genes involved in apoptosis (PI3K, AKT, BCL2, BAX, P21, PTEN, CASP3, CASP8, and CASP9) and multi-drug resistance (MDR, ABCB1 and ABCC1) were assessed by using real-time PCR test.

RESULTS

Based on the findings, kaempferol decreased cell viability and increased apoptosis in HL60 cells more than EGCG. Apoptosis was induced via extrinsic and intrinsic pathways in HL60 cells by kaempferol and EGCG. In addition, kaempferol and EGCG increased apoptosis and inhibited MDR in a concentration- and time-dependent manner.

CONCLUSION

Kaempferol at high concentrations can be taken into consideration for treating patients with APL as compared with EGCG.

The in vivo ISGylome links ISG15 to metabolic pathways and autophagy upon Listeria monocytogenes infection

ISG15 is an interferon-stimulated, ubiquitin-like protein, with anti-viral and anti-bacterial activity. Here, we map the endogenous in vivo ISGylome in the liver following Listeria monocytogenes infection by combining murine models of reduced or enhanced ISGylation with quantitative proteomics. Our method identifies 930 ISG15 sites in 434 proteins and also detects changes in the host ubiquitylome. The ISGylated targets are enriched in proteins which alter cellular metabolic processes, including upstream modulators of the catabolic and antibacterial pathway of autophagy. Computational analysis of substrate structures reveals that a number of ISG15 modifications occur at catalytic sites or dimerization interfaces of enzymes. Finally, we demonstrate that animals and cells with enhanced ISGylation have increased basal and infection-induced autophagy through the modification of mTOR, WIPI2, AMBRA1, and RAB7. Taken together, these findings ascribe a role of ISGylation to temporally reprogram organismal metabolism following infection through direct modification of a subset of enzymes in the liver.

Strategies to generate functionally normal neutrophils to reduce infection and infection-related mortality in cancer chemotherapy

Neutrophils form an essential part of innate immunity against infection. Cancer chemotherapy-induced neutropenia (CCIN) is a condition in which the number of neutrophils in a patient's bloodstream is decreased, leading to increased susceptibility to infection. Granulocyte colony-stimulating factor (GCSF) has been the only approved treatment for CCIN over two decades. To date, CCIN-related infection and mortality remain a significant concern, as neutrophils generated in response to administered GCSF are functionally immature and cannot effectively fight infection. This review summarizes the molecular regulatory mechanisms of neutrophil granulocytic differentiation and innate immunity development, dissects the biology of GCSF in myeloid expansion, highlights the shortcomings of GCSF in CCIN treatment, updates the recent advance of a selective retinoid agonist that promotes neutrophil granulocytic differentiation, and evaluates the benefits of developing GCSF biosimilars to increase access to GCSF biologics versus seeking a new mode to fundamentally advance GCSF therapy for treatment of CCIN.

Study of the mechanisms of crocetin-induced differentiation and apoptosis in human acute promyelocytic leukemia cells

Crocetin, the major carotenoid in saffron, exhibits potent anticancer effects. However, the antileukemic effects of crocetin are still unclear, especially in primary acute promyelocytic leukemia (APL) cells. In the current study, the potential antipromyelocytic leukemia activity of crocetin and the underlying molecular mechanisms were investigated. Crocetin (100 µM), like standard anti-APL drugs, all-trans retinoic acid (ATRA, 10 µM) and As₂ O ₃ (arsenic trioxide, 50 µM), significantly inhibited proliferation and induced apoptosis in primary APL cells, as well as NB4 and HL60 cells. The effect was associated with the decreased expressions of prosurvival genes Akt and BCL2, the multidrug resistance (MDR) proteins, ABCB1 and ABCC1 and the inhibition of tyrosyl-DNA phosphodiesterase 1 (TDP1), while the expressions of proapoptotic genes CASP3, CASP9, and BAX/BCL2 ratio were significantly increased. In contrast, crocetin at relatively low concentration (10 µM), like ATRA (1 µM) and As ₂ O ₃ (0.5 µM), induced differentiation of leukemic cells toward granulocytic pattern, and increased the number of differentiated cells expressing CD11b and CD14, while the number of the immature cells expressing CD34 or CD33 was decreased. Furthermore, crocetin suppressed the expression of clinical marker promyelocytic leukemia/retinoic acid receptor-α ( PML/RARα) in NB4 and primary APL cells, and reduced the expression of histone deacetylase 1 ( HDAC1) in all leukemic cells. The results suggested that crocetin can be considered as a candidate for future preclinical and clinical trials of complementary APL treatment.

Evidence for the ISG15-Specific Deubiquitinase USP18 as an Antineoplastic Target

Ubiquitination and ubiquitin-like posttranslational modifications (PTM) regulate activity and stability of oncoproteins and tumor suppressors. This implicates PTMs as antineoplastic targets. One way to alter PTMs is to inhibit activity of deubiquitinases (DUB) that remove ubiquitin or ubiquitin-like proteins from substrate proteins. Roles of DUBs in carcinogenesis have been intensively studied, yet few inhibitors exist. Prior work provides a basis for the ubiquitin-specific protease 18 (USP18) as an antineoplastic target. USP18 is the major DUB that removes IFN-stimulated gene 15 (ISG15) from conjugated proteins. Prior work discovered that engineered loss of USP18 increases ISGylation and in contrast to its gain decreases cancer growth by destabilizing growth-regulatory proteins. Loss of USP18 reduced cancer cell growth by triggering apoptosis. Genetic loss of USP18 repressed cancer formation in engineered murine lung cancer models. The translational relevance of USP18 was confirmed by finding its expression was deregulated in malignant versus normal tissues. Notably, the recent elucidation of the USP18 crystal structure offers a framework for developing an inhibitor to this DUB. This review summarizes strong evidence for USP18 as a previously unrecognized pharmacologic target in oncology. Cancer Res; 78(3); 587-92. ©2018 AACR.

Kaempferol increases apoptosis in human acute promyelocytic leukemia cells and inhibits multidrug resistance genes

Acute promyelocytic leukemia (APL) is one of the most life-threatening hematological malignancies. Defects in the cell growth and apoptotic pathways are responsible for both disease pathogenesis and treatment resistance. Therefore, pro-apoptotic agents are potential candidates for APL treatment. Kaempferol is a flavonoid with antioxidant and anti-tumor properties. This study was designed to investigate the cytotoxic, pro-apoptotic, and differentiation-inducing effects of kaempferol on HL-60 and NB4 leukemia cells. Resazurin assay was used to determine cell viability following treatment with kaempferol (12.5-100 μM) and all-trans retinoic acid (ATRA; 10 μM; used as a positive control). Apoptosis and differentiation were also detected using propidium iodide and NBT staining techniques, respectively. Furthermore, the expression levels of genes involved in apoptosis (PI3 K, AKT, BCL2, BAX, p53, p21, PTEN, CASP3, CASP8, and CASP9), differentiation (PML-RAR and HDAC1), and multi-drug resistance (ABCB1 and ABCC1) were determined using quantitative real-time PCR. The protein expressions of Bax/Bcl2 and casp3 were confirmed using Western blot. The results showed that kaempferol decreased cell viability and increased subG1 population in the tested leukemic cells. This effect was associated with decreased expression of Akt, BCL2, ABCB1, and ABCC1 genes, while the expression of CASP3 and BAX/BCL-2 ratio were significantly increased at both gene and protein levels. Kaempferol promoted apoptosis and inhibited multidrug resistance in a concentration-dependent manner, without any differential effect on leukemic cells. In conclusion, this study suggested that kaempferol may be utilized as an appropriate alternative for ATRA in APL patients.

Prolonged intracellular accumulation of light-inducible nanoparticles in leukemia cells allows their remote activation

Leukaemia cells that are resistant to conventional therapies are thought to reside in protective niches. Here, we describe light-inducible polymeric retinoic acid (RA)-containing nanoparticles (NPs) with the capacity to accumulate in the cytoplasm of leukaemia cells for several days and release their RA payloads within a few minutes upon exposure to blue/UV light. Compared to NPs that are not activated by light exposure, these NPs more efficiently reduce the clonogenicity of bone marrow cancer cells from patients with acute myeloid leukaemia (AML) and induce the differentiation of RA-low sensitive leukaemia cells. Importantly, we show that leukaemia cells transfected with light-inducible NPs containing RA can engraft into bone marrow in vivo in the proximity of other leukaemic cells, differentiate upon exposure to blue light and release paracrine factors that modulate nearby cells. The NPs described here offer a promising strategy for controlling distant cell populations and remotely modulating leukaemic niches.

Effects of the overexpression of IFITM5 and IFITM5 c.-14C>T mutation on human osteosarcoma cells

The present study aimed to investigate the effects of overexpression of interferon-induced transmembrane protein 5 (IFITM5) and IFITM5 c.-14C>T mutation on osteogenic differentiation, and the proliferation, migration and invasion of SaOS2 cells. SaOS2 cells were transfected with plasmids containing wild type IFITM5 (W) or IFITM5 containing the c.-14C>T mutation (MU). The mRNA and protein expression levels of IFITM5 in SaOS2 cells were respectively detected by reverse transcription quantitative polymerase chain reaction and western blotting. The proliferative, migratory and invasive ability of SaOS2 cells was also examined. In addition, the expression levels of osteogenic differentiation markers alkaline phosphatase (ALP), osteocalcin (OCN) and runt-related transcription factor 2 (Runx2) were detected. Mineralized nodules were detected by Alizarin Red S staining and were quantified by measuring absorbance. The mRNA and protein expression levels of IFITM5 were high in cells transfected with IFITM5 and IFITM5 c.-14C>T mutation, and were higher in cells transfected with IFITM5 c.-14C>T mutation. There was no difference in proliferation between the control group (C) and the W and MU groups. However, overexpression of IFITM5 and IFITM5 c.-14C>T mutation increased apoptotic rate, decreased invasive capacity, increased the expression of ALP, OCN and Runx2, and increased the number of mineralized nodules following osteogenic induction. In addition, compared with C and W groups, cells transfected with IFITM5 c.-14C>T mutation exhibited decreased migratory ability. In conclusion, overexpression of IFITM5 and IFITM5 c.-14C>T mutation promotes tumor cell apoptosis, inhibits tumor invasion and promotes osteogenic differentiation. These findings may provide a theoretical basis for the development of a novel treatment method that targets IFITM5, and provides a platform for the potential treatment of human osteosarcoma.

In vitro models of cancer stem cells and clinical applications

Cancer cells, stem cells and cancer stem cells have for a long time played a significant role in the biomedical sciences. Though cancer therapy is more effective than it was a few years ago, the truth is that still none of the current non-surgical treatments can cure cancer effectively. The reason could be due to the subpopulation called “cancer stem cells” (CSCs), being defined as those cells within a tumour that have properties of stem cells: self-renewal and the ability for differentiation into multiple cell types that occur in tumours.

The phenomenon of CSCs is based on their resistance to many of the current cancer therapies, which results in tumour relapse. Although further investigation regarding CSCs is still needed, there is already evidence that these cells may play an important role in the prognosis of cancer, progression and therapeutic strategy. Therefore, long-term patient survival may depend on the elimination of CSCs. Consequently, isolation of pure CSC populations or reprogramming of cancer cells into CSCs, from cancer cell lines or primary tumours, would be a useful tool to gain an in-depth knowledge about heterogeneity and plasticity of CSC phenotypes and therefore carcinogenesis. Herein, we will discuss current CSC models, methods used to characterize CSCs, candidate markers, characteristic signalling pathways and clinical applications of CSCs. Some examples of CSC-specific treatments that are currently in early clinical phases will also be presented in this review.

A novel single cell method to identify the genetic composition at a single nuclear body

Gene loci make specific associations with compartments of the nucleus (e.g. the nuclear envelope, nucleolus, and transcription factories) and this association may determine or reflect a mechanism of genetic control. With current methods, it is not possible to identify sets of genes that converge to form a “gene hub” as there is a reliance on loci-specific probes, or immunoprecipitation of a particular protein from bulk cells. We introduce a method that will allow for the identification of loci contained within the vicinity of a single nuclear body in a single cell. For the first time, we demonstrate that the DNA sequences originating from a single sub-nuclear structure in a single cell targeted by two-photon irradiation can be determined, and mapped to a particular locus. Its application to single PML nuclear bodies reveals ontologically related loci that frequently associate with each other and with PML bodies in a population of cells, and a possible nuclear body targeting role for specific transcription factor binding sites.

RIG-G inhibits the proliferation of NB4 cells and propels ATRA-induced differentiation of APL cells

RIG-G (retinoic acid-induced gene G) was originally identified in ATRA (all-trans retinoic acid)-treated NB4 acute promyelocytic leukemia (APL) cells. It was induced to expression by ATRA along with the differentiation of the cells. However, little is known about its role(s). Here, we established a RIG-G stably expression transformant of NB4 cells. By using the transformant, we showed that expression of RIG-G in NB4 cells not only arrested the cells at G1/G0 transition phase and inhibited their proliferation, but also markedly drive the maturation of NB4 cells in the presence of very low concentration of ATRA (10(-9)mol/L). What's more, by detecting the expression of RIG-G in fresh primary bone marrow mononuclear cells of APL patients in different morbid states, we found high RIG-G expression level in complete remission patients, while low level in untreated or relapsed patients. These results indicated that RIG-G level was high in maturated cells and low in blast cells, and suggested that RIG-G might play a role in the differentiation of bone marrow hemocytes in vivo.

Expression of the miR-302/367 cluster in glioblastoma cells suppresses tumorigenic gene expression patterns and abolishes transformation related phenotypes

Cellular transformation is initiated by the activation of oncogenes and a closely associated developmental reprogramming of the epigenetic landscape. Transcription factors, regulators of chromatin states and microRNAs influence cell fates in development and stabilize the phenotypes of normal, differentiated cells and of cancer cells. The miR‐302/367 cluster, predominantly expressed in human embryonic stem cells (hESs), can promote the cellular reprogramming of human and mouse cells and contribute to the generation of iPSC. We have used the epigenetic reprogramming potential of the miR‐302/367 cluster to “de‐program” tumor cells, that is, hift their gene expression pattern towards an alternative program associated with more benign cellular phenotypes. Induction of the miR‐302/367 cluster in extensively mutated U87MG glioblastoma cells drastically suppressed the expression of transformation related proteins, for example, the reprogramming factors OCT3/4, SOX2, KLF4 and c‐MYC, and the transcription factors POU3F2, SALL2 and OLIG2, required for the maintenance of glioblastoma stem‐like tumor propagating cells. It also diminished PI3K/AKT and STAT3 signaling, impeded colony formation in soft agar and cell migration and suppressed pro‐inflammatory cytokine secretion. At the same time, the miR‐302/367 cluster restored the expression of neuronal markers of differentiation. Most notably, miR‐302/367 cluster expressing cells lose their ability to form tumors and to establish liver metastasis in nude mice. The induction of the miR‐302/367 cluster in U87MG glioblastoma cells suppresses the expression of multiple transformation related genes, abolishes the tumor and metastasis formation potential of these cells and can potentially become a new approach for cancer therapy.

What's new?

The transformation of normal cells into malignant cells shares many similarities with the reprogramming of somatic cells into pluripotent cells, raising the possibility that reprogramming factors may be used to counteract cellular transformation. This study demonstrates that reversion of transformation and normalization of cellular properties can be achieved in highly‐aberrant glioblastoma cells through the expression of the miR‐302/367 cluster. miR‐302/367 drastically changes the gene expression pattern and abolishes transformation‐related phenotypes in a coordinated fashion. miR‐302/367 prevents tumor and metastasis formation and restores features of neuronal differentiation. Such “deprogramming” of tumor cells could potentially become a new concept for cancer therapy.

Critical role of retinoid/rexinoid signaling in mediating transformation and therapeutic response of NUP98-RARG leukemia

While the nucleoporin 98-retinoic acid receptor gamma (NUP98-RARG) is the first RARG fusion protein found in acute leukemia, its roles and the molecular basis in oncogenic transformation are currently unknown. Here, we showed that homodimeric NUP98-RARG not only acquired unique nuclear localization pattern and ability of recruiting both RXRA and wild-type NUP98, but also exhibited similar transcriptional properties as RARA fusions found in acute promyelocytic leukemia (APL). Using murine bone marrow retroviral transduction/transformation assay, we further demonstrated that NUP98-RARG fusion protein had gained transformation ability of primary hematopoietic stem/progenitor cells, which was critically dependent on the C-terminal GLFG domain of NUP98 and the DNA binding domain (DBD) of RARG. In contrast to other NUP98 fusions, cells transformed by the NUP98-RARG fusion were extremely sensitive to all-trans retinoic acid (ATRA) treatment. Interestingly, while pan-RXR agonists, SR11237 and LGD1069 could specifically inhibit NUP98-RARG transformed cells, mutation of the RXR interaction domain in NUP98-RARG had little effect on its transformation, revealing that therapeutic functions of rexinoid can be independent of the direct biochemical interaction between RXR and the fusion. Together, these results indicate that deregulation of the retinoid/rexinoid signaling pathway has a major role and may represent a potential therapeutic target for NUP98-RARG-mediated transformation.

Hippo-YAP signaling pathway: A new paradigm for cancer therapy

In the past decades, the Hippo signaling pathway has been delineated and shown to play multiple roles in the control of organ size in both Drosophila and mammals. In mammals, the Hippo pathway is a kinase cascade leading from Mst1/2 to YAP and its paralog TAZ. Several studies have demonstrated that YAP/TAZ is a candidate oncogene and that other members of the Hippo pathway are tumor suppressive genes. The dysregulation of the Hippo pathway has been observed in a variety of cancers. This review chronicles the recent progress in elucidating the function of Hippo signaling in tumorigenesis and provide a rich source of potential targets for cancer therapy.

Hyperoside, a flavonoid compound, inhibits proliferation and stimulates osteogenic differentiation of human osteosarcoma cells

Osteosarcoma, one of the most common malignant bone tumours, is generally considered a differentiation disease caused by genetic and epigenetic disruptions in the terminal differentiation of osteoblasts. Novel therapies based on the non-cytotoxic induction of cell differentiation-responsive pathways could represent a significant advance in treating osteosarcoma; however, effective pharmaceuticals to induce differentiation are lacking. In the present study, we investigated the effect of hyperoside, a flavonoid compound, on the osteoblastic differentiation of U2OS and MG63 osteosarcoma cells in vitro. Our results demonstrated that hyperoside inhibits the proliferation of osteosarcoma cells by inducing G₀/G₁ arrest in the cell cycle, without causing obvious cell death. Cell migration assay further suggested that hyperoside could inhibit the invasion potential of osteosarcoma cells. Additionally, osteopontin and runt-related transcription factor 2 protein levels and osteocalcin activation were upregulated dramatically in hyperoside-treated osteosarcoma cells, suggesting that hyperoside may stimulates osteoblastic differentiation in osteosarcoma cells. This differentiation was accompanied by the activation of transforming growth factor (TGF)-β and bone morphogenetic protein-2, suggesting that the hyperoside-induced differentiation involves the TGF-β signalling pathway. To our knowledge, this study is the first to evaluate the differentiation effect of hyperoside in osteosarcoma cells and assess the possible potential for hyperoside treatment as a future therapeutic approach for osteosarcoma differentiation therapy.

Tumour-stroma crosstalk in the development of squamous cell carcinoma

Squamous cell carcinoma (SCC) represents one of the most frequently diagnosed tumours and contributes significant mortality worldwide. Recent deep sequencing of cancer genomes has identified common mutations in SCC arising across different tissues highlighting perturbation of squamous differentiation as a key event. At the same time significant data have been accumulating to show that common tumour-stroma interactions capable of driving disease progression are also evident when comparing SCC arising in different tissues. We and others have shown altered matrix composition surrounding SCC can promote tumour development. This review focuses on some of the emerging data with particular emphasis on SCC of head and neck and skin with discussion on the potential tumour suppressive properties of a normal microenvironment. Such data indicate that regardless of the extent and type of somatic mutation it is in fact the tumour context that defines metastatic progression.

Role of cell cycle regulatory molecules in retinoic acid- and vitamin D3-induced differentiation of acute myeloid leukaemia cells

The important role of cell cycle regulatory molecules in all trans-retinoic acid (ATRA)- and vitamin D3-induced growth inhibition and differentiation induction has been intensively studied in both acute myeloid leukaemia primary cells and a variety of leukaemia cell lines. Cyclin-dependent kinases (CDK)-activating kinase has been demonstrated to interact with retinoic acid receptor (RAR)α in acute promyelocytic leukaemia cells, and inhibition of CDK-activating kinase by ATRA causes hypophosphorylation of PML-RARα, leading to myeloid differentiation. In many cases, downregulation of CDK activity by ATRA and vitamin D3 is a result of elevated p21- and p27-bound CDKs. Activation of p21 is regulated at the transcriptional level, whereas elevated p27 results from both (indirectly) transcriptional activation and post-translational modifications. CDK inhibitors (CKIs) of the INK family, such as p15, p16 and p18, are mainly involved in inhibition of cell proliferation, whereas CIP/KIP members, such as p21, regulate both growth arrest and induction of differentiation. ATRA and vitamin D3 can also downregulate expression of G1 CDKs, especially CDK2 and CDK6. Inhibition of cyclin E expression has only been observed in ATRA- but not in vitamin D3-treated leukaemic cells. In vitro, not only dephosphorylation of pRb but also elevation of total pRb is required for ATRA and vitamin D3 to suppress growth and trigger their differentiation. Finally, sharp reduction in c-Myc has been observed in several leukaemia cell lines treated with ATRA, which may regulate expression of CDKs and CKIs.

Prohibitin 2 represents a novel nuclear AKT substrate during all-trans retinoic acid-induced differentiation of acute promyelocytic leukemia cells

The AKT/PKB kinase is essential for cell survival, proliferation, and differentiation; however, aberrant AKT activation leads to the aggressiveness and drug resistance of many human neoplasias. In the human acute promyelocytic leukemia cell line NB4, nuclear AKT activity increases during all-trans retinoic acid (ATRA)-mediated differentiation. As nuclear AKT activity is associated with differentiation, we sought to identify the nuclear substrates of AKT that were phosphorylated after ATRA treatment. A proteomics-based search for nuclear substrates of AKT in ATRA-treated NB4 cells was undertaken by using 2D-electrophoresis/mass spectrometry (MS) in combination with an anti-AKT phospho-substrate antibody. Western blot analysis, an in vitro kinase assay, and/or site-directed mutagenesis were performed to further characterize the MS findings. MS analysis revealed prohibitin (PHB)-2, a multifunctional protein involved in cell cycle progression and the suppression of oxidative stress, to be a putative nuclear substrate of AKT. Follow-up studies confirmed that AKT phosphorylates PHB2 on Ser-91 and that forced expression of the PHB2(S91A) mutant results in a rapid loss of viability and apoptotic cell death. Activation of nuclear AKT during ATRA-mediated differentiation results in the phosphorylation of several proteins, including PHB2, which may serve to coordinate nuclear-mitochondrial events during differentiation.

Repression of exogenous gene expression by the retinoic acid target gene G0S2

The G0/G1 switch gene 2 (G0S2) is rapidly induced by all-trans-retinoic acid (RA)-treatment of acute promyelocytic leukemia (APL) and other cells. G0S2 regulates lipolysis via inhibition of adipose triglyceride lipase (ATGL). This study found that retinoic acid receptor (RAR), but not retinoid X receptor (RXR) agonists induced G0S2 expression in APL cells. Novel G0S2 functions were uncovered that included repression of exogenous gene expression and transcriptional activity. Transient G0S2 transfection repressed the activities of multiple reporter constructs (including the retinoid-regulated species RARβ, UBE1L and G0S2); this occurred in diverse cell contexts. This inhibition was antagonized by siRNA-mediated G0S2 knockdown. To determine the inhibitory effects were not due to transient G0S2 expression, G0S2 was stably overex-pressed in cells without appreciable basal G0S2 expression. As expected, this repressed transcriptional activities. Intriguingly, transfection of G0S2 did not affect endogenous RARβ, UBE1L or G0S2 expression. Hence, only exogenously expressed genes were affected by G0S2. The domain responsible for this repression was localized to the G0S2 hydrophobic domain (HD). This was the same region responsible for the ability of G0S2 to inhibit ATGL activity. Whether an interaction with ATGL accounted for this new G0S2 activity was studied. Mimicking the inhibition of ATGL by oleic acid treatment that increased lipid droplet size or ATGL siRNA knockdown did not recapitulate G0S2 repressive effects. Engineered gain of ATGL expression did not rescue G0S2 transcriptional repression either. Thus, transcriptional repression by G0S2 did not depend on the ability of G0S2 to inhibit ATGL. Subcellular localization studies revealed that endogenous and exogenously-expressed G0S2 proteins were localized to the cytoplasm, particularly in the perinuclear region. Expression of a mutant G0S2 species that lacked the HD domain altered cytosolic G0S2 localization. This linked G0S2 subcellular localization to G0S2 transcriptional repression. The potential mechanisms responsible for this G0S2 repression are examined.

Solid tumor differentiation therapy - is it possible?

Genetic and epigenetic events within a cell which promote a block in normal development or differentiation coupled with unregulated proliferation are hallmarks of neoplastic transformation. Differentiation therapy involves the use of agents with the ability to induce differentiation in cells that have lost this ability, i.e. cancer cells. The promise of differentiation-based therapy as a viable treatment modality is perhaps best characterized by the addition of retinoids in the treatment of acute promyelocytic leukemia (APML) revolutionizing the management of APML and dramatically improving survival. However, interest and application of differentiationbased therapy for the treatment of solid malignancies have lagged due to deficiencies in our understanding of differentiation pathways in solid malignancies. Over the past decade, a differentiation-based developmental model for solid tumors has emerged providing insights into the biology of various solid tumors as well as identification of targetable pathways capable of re-activating blocked terminal differentiation programs. Furthermore, a variety of agents including retinoids, histone deacetylase inhibitors (HDACI), PPARγ agonists, and others, currently in use for a variety of malignancies, have been shown to induce differentiation in solid tumors. Herein we discuss the relevancy of differentiation-based therapies in solid tumors, using soft tissue sarcomas (STS) as a biologic and clinical model, and review the preclinical data to support its role as a promising modality of therapy for the treatment of solid tumors.

The plasticity and potential of leukemia cell lines to differentiate into dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells that orchestrate the innate and adaptive immune systems to induce immunity. DCs are significant in maintaining immune tolerance towards self-antigens, organ transplantation and allergic responses. DCs are powerful adjuvants for eliciting T-cell immunity and are therefore considered primary targets for inducing immune responses in the prevention and treatment of infectious diseases and cancer. DCs have been increasingly applied in the immunotherapy of cancer worldwide during the last decade; however, a number of the highly specialized biological characteristics of DCs remain to be elucidated. Previous studies of human DCs have been constrained by certain difficulties, therefore the majority of studies have been carried out using in vitro model systems. Suitable cell lines with dendritic-like properties may provide valuable tools for the study of DC physiology and pathology. In the current review, various human DC line differentiation models are discussed. Certain cell lines provide valuable tools for studying the specific aspects of DC biology, despite variations in cell biological and immunological features when compared with primary DCs.

Terminal differentiation and loss of tumorigenicity of human cancers via pluripotency-based reprogramming

Pluripotent cells can be derived from various types of somatic cells by nuclear reprogramming using defined transcription factors. It is, however, unclear whether human cancer cells can be similarly reprogrammed and subsequently terminally differentiated with abrogation of tumorigenicity. Here, using sarcomas we show that human-derived complex karyotype solid tumors: (1) can be reprogrammed into a pluripotent-like state as defined by all in vitro criteria used to define pluripotent stem cells generated from somatic cells; (2) can be terminally differentiated into mature connective tissue and red blood cells; and (3) terminal differentiation is accompanied with loss of both proliferation and tumorigenicity. We go on to perform the first global DNA promoter methylation and gene expression analyses comparing human cancers to their reprogrammed counterparts and report that reprogramming/differentiation results in significant epigenetic remodeling of oncogenes and tumor suppressors, while not significantly altering the differentiation status of the reprogrammed cancer cells, in essence dedifferentiating them to a state slightly before the mesenchymal stem cell differentiation stage. Our data demonstrate that direct nuclear reprogramming can restore terminal differentiation potential to human-derived cancer cells, with simultaneous loss of tumorigenicity, without the need to revert to an embryonic state. We anticipate that our models would serve as a starting point to more fully assess how nuclear reprogramming overcomes the multitude of genetic and epigenetic aberrancies inherent in human cancers to restore normal terminal differentiation pathways. Finally, these findings suggest that nuclear reprogramming may be a broadly applicable therapeutic strategy for the treatment of cancer.

Comparative proteomics in acute myeloid leukemia

The term proteomics was used for the first time in 1995 to describe large-scale protein analyses. At the same time proteomics was distinguished as a new domain of the life sciences. The major object of proteomic studies is the proteome, i.e. the set of all proteins accumulating in a given cell, tissue or organ. During the last years several new methods and techniques have been developed to increase the fidelity and efficacy of proteomic analyses. The most widely used are two-dimensional electrophoresis (2DE) and mass spectrometry (MS). In the past decade proteomic analyses have also been successfully applied in biomedical research. They allow one to determine how various diseases affect the pattern of protein accumulation. In this paper, we attempt to summarize the results of the proteomic analyses of acute myeloid leukemia (AML) cells. They have increased our knowledge on the mechanisms underlying AML development and contributed to progress in AML diagnostics and treatment.

Blockade of the ubiquitin protease UBP43 destabilizes transcription factor PML/RARα and inhibits the growth of acute promyelocytic leukemia

More effective treatments for acute promyelocytic leukemia (APL) are needed. APL cell treatment with all-trans-retinoic acid (RA) degrades the chimeric, dominant-negative-acting transcription factor promyelocytic leukemia gene (PML)/RARα, which is generated in APL by chromosomal translocation. The E1-like ubiquitin-activating enzyme (UBE1L) associates with interferon-stimulated gene ISG15 that binds and represses PML/RARα protein. Ubiquitin protease UBP43/USP18 removes ISG15 from conjugated proteins. In this study, we explored how RA regulates UBP43 expression and the effects of UBP43 on PML/RARα stability and APL growth, apoptosis, or differentiation. RA treatment induced UBE1L, ISG15, and UBP43 expression in RA-sensitive but not RA-resistant APL cells. Similar in vivo findings were obtained in a transgenic mouse model of transplantable APL, and in the RA response of leukemic cells harvested directly from APL patients. UBP43 knockdown repressed PML/RARα protein levels and inhibited RA-sensitive or RA-resistant cell growth by destabilizing the PML domain of PML/RARα. This inhibitory effect promoted apoptosis but did not affect the RA differentiation response in these APL cells. In contrast, elevation of UBP43 expression stabilized PML/RARα protein and inhibited apoptosis. Taken together, our findings define the ubiquitin protease UBP43 as a novel candidate drug target for APL treatment.

Potential of retinoic acid derivatives for the treatment of corticotroph pituitary adenomas

Cushing's disease is a severe clinical condition caused by hypersecretion of corticosteroids due to excessive ACTH secretion from a pituitary adenoma. This complex endocrine disorder still represents a major challenge for the physician in terms of efficient treatment. In the last years there was only little progress in elucidating the molecular mechanisms responsible for the constitutive and autonomous ACTH secretion of pituitary corticotrophinomas. As a consequence, no effective drug therapy is currently available, particularly if surgical excision is not successful. In the present article we examine recent studies that have investigated the therapeutic potential of retinoic acid receptors as nuclear receptor targets for the treatment of Cushing's disease. Retinoic acid is an efficient drug used for the treatment of different types of cancers and it proved to act in animal models of Cushing's disease. The efficiency of this treatment in patients with this disorder still needs to be tested in clinical trials.

UBE1L represses PML/RAR{alpha} by targeting the PML domain for ISG15ylation

Acute promyelocytic leukemia (APL) is characterized by expression of promyelocytic leukemia (PML)/retinoic acid (RA) receptor alpha (RARalpha) protein and all-trans-RA-mediated clinical remissions. RA treatment can confer PML/RARalpha degradation, overcoming dominant-negative effects of this oncogenic protein. The present study uncovered independent retinoid degradation mechanisms, targeting different domains of PML/RARalpha. RA treatment is known to repress PML/RARalpha and augment ubiquitin-activating enzyme-E1-like (UBE1L) protein expression in NB4-S1 APL cells. We previously reported RA-induced UBE1L and the IFN-stimulated gene, 15-kDa protein ISG15ylation in APL cells. Whether the ubiquitin-like protein ISG15 directly conjugates with PML/RARalpha was not explored previously and is examined in this study. Transient transfection experiments with different PML/RARalpha domains revealed that RA treatment preferentially down-regulated the RARalpha domain, whereas UBE1L targeted the PML domain for repression. As expected, ubiquitin-specific protease 18 (UBP43/USP18), the ISG15 deconjugase, opposed UBE1L but not RA-dependent PML/RARalpha degradation. In contrast, the proteasomal inhibitor, N-acetyl-leucinyl-leucinyl-norleucinal, inhibited both UBE1L- and RA-mediated PML/RARalpha degradation. Notably, UBE1L induced ISG15ylation of the PML domain of PML/RARalpha, causing its repression. These findings confirmed that RA triggers PML/RARalpha degradation through different domains and distinct mechanisms. Taken together, these findings advance prior work by establishing two pathways converge on the same oncogenic protein to cause its degradation and thereby promote antineoplastic effects. The molecular pharmacologic implications of these findings are discussed.

Alpha-fetoprotein, stem cells and cancer: how study of the production of alpha-fetoprotein during chemical hepatocarcinogenesis led to reaffirmation of the stem cell theory of cancer

Identification of the cells in the liver that produce alpha-fetoprotein during development, in response to liver injury and during the early stages of chemical hepatocarcinogenesis led to the conclusion that maturation arrest of liver-determined tissue stem cells was the cellular process that gives rise to hepatocellular carcinomas. When the cellular changes in these processes were compared to that of the formation of teratocarcinomas, the hypothesis arose that all cancers arise from maturation arrest of tissue-determined stem cells. This was essentially a reinterpretation of the embryonal rest theory of cancer whereby tissue stem cells take the role of embryonal rests. A corollary of the stem cell theory of the origin of cancer is that cancers contain the same functional cell populations as normal tissues: stem cells, transit-amplifying cells and mature cells. Cancer stem cells retain the essential feature of normal stem cells: the ability to self-renew. Growth of cancers is due to continued proliferation of cancer transit-amplifying cells that do not differentiate to mature cells (maturation arrest). On the other hand, cancer stem cells generally divide very rarely and contribute little to tumor growth. However, the presence of cancer stem cells in tumors is believed to be responsible for the properties of immortalization, transplantability and resistance to therapy characteristic of cancers. Current therapies for cancer (chemotherapy, radiotherapy, antiangiogenesis and differentiation therapy) are directed against the cancer transit-amplifying cells. When these therapies are discontinued, the cancer reforms from the cancer stem cells. Therapy directed toward interruption of the cell signaling pathways that maintain cancer stem cells could lead to new modalities to the prevention of regrowth of the cancer.

Cyclin degradation for cancer therapy and chemoprevention

Cancer is characterized by uncontrolled cell division resulting from multiple mutagenic events. Cancer chemoprevention strategies aim to inhibit or reverse these events using natural or synthetic pharmacologic agents. Ideally, this restores normal growth control mechanisms. Diverse classes of compounds have been identified with chemopreventive activity. What unites many of them is an ability to inhibit the cell cycle by specifically modulating key components. This delays division long enough for cells to respond to mutagenic damage. In some cases, damage is repaired and in others cellular damage is sufficient to trigger apoptosis. It is now known that pathways responsible for targeting G1 cyclins for proteasomal degradation can be engaged pharmacologically. Emergence of induced cyclin degradation as a target for cancer therapy and chemoprevention in pre-clinical models is discussed in this article. Evidence for cyclin D1 as a molecular pharmacologic target and biological marker for clinical response is based on experience of proof of principle trials.

Expression of retinoic acid receptors in human endometrial carcinoma

The retinoids (vitamin A and its biologically active derivatives) are essential for the health and survival of the individual. Several studies have reported a strong rationale for the use of retinoids in cancer treatment and chemoprevention. It has been discovered that expression of retinoic acid receptor (RAR) beta is frequently silenced in epithelial carcinogenesis, which has led to the hypothesis that RAR beta could act as a tumor suppressor. However, the status of RAR beta in human endometrial carcinoma has not been examined. In the present study, we initially studied the effects of retinoic acid on cell proliferation and the expression of RAR alpha, RAR beta, and RAR gamma using AM580 (a RAR-specific agonist) in the Ishikawa endometrial cancer cell line. We also examined the expression of RAR in human eutopic endometrium (30 cases), endometrial hyperplasia (28 cases), and endometrial carcinoma (103 cases) using immunohistochemistry. Finally, we correlated these findings with the clinicopathological parameters. In vitro, cell growth was inhibited and RAR beta and RAR gamma mRNA was significantly induced by AM580, compared with vehicle controls, whereas RAR alpha mRNA was significantly attenuated by AM580, compared with vehicle. RAR beta was detected predominantly in endometrial hyperplasia, compared with endometrial carcinoma. No statistically significant correlation was obtained between the expression of any other RAR subtypes and clinicopathological parameters in human endometrial carcinoma. The results of our study demonstrate that AM580 inhibits cell growth and induces RAR beta mRNA expression in the Ishikawa cell line, and the expression level of RAR beta in endometrial carcinoma is significantly lower than that in endometrial hyperplasia. AM580 might therefore be considered as a potential treatment for endometrial carcinoma.

[Retinoid metabolism and cancer]

Retinoids play important roles in cell differentiation and apoptosis, notably in epithelial tissues. Their utility in cancer therapy has been demonstrated in specific cancer types. Use of retinoic acid (RA) in the treatment of acute promyelocytic leukemia was the first successful example of retinoid-based differentiation therapy. RA has since been evaluated for treatment of other cancers, revealing variable effectiveness. The observation that expression of enzymes involved in RA biosynthesis is suppressed during tumorigenesis suggests that intra-tumor depletion in RA levels may contribute to tumor development and argues for the use of retinoids in cancer treatment. However, the induction of RA-inactivating enzymes is one of the mechanisms that may limit the efficacy of retinoid therapy and contribute to acquired resistance to RA treatment, suggesting that retinoic acid metabolism blocking agents may be effective agents in differentiation therapy.

RNAi screen identifies UBE2D3 as a mediator of all-trans retinoic acid-induced cell growth arrest in human acute promyelocytic NB4 cells

All-trans retinoic acid (ATRA) has been widely used in differentiation therapy for acute promyelocytic leukemia (APL). ATRA binds to retinoic acid receptor (RAR) and triggers the formation of the transcription coactivator complex, which leads to changes in gene expression, APL cell-cycle arrest and differentiation, and clinical remission. The mechanisms responsible for ATRA's beneficial effects are still ill-defined. Here, we conducted a large-scale, unbiased short hairpin RNA (shRNA) screen aiming to identify mediators of ATRA-induced differentiation and growth arrest of APL cells. Twenty-six proteins were identified. They cover a wide range of cellular functions, including gene expression, intracellular signaling, cell death control, stress responses, and metabolic regulation, indicating the complexity of ATRA-induced cell growth control and differentiation in APL. One of these proteins, the ubiquitin-conjugating enzyme UBE2D3, is up-regulated in ATRA-treated acute promyelocytic NB4 cells. UBE2D3 is physically associated with cyclin D1 and mediates ATRA-induced cyclin D1 degradation. Knocking down UBE2D3 by RNA interference (RNAi) leads to blockage of ATRA-induced cyclin D1 degradation and cell-cycle arrest. Thus, our results highlight the involvement of the ubiquitin-mediated proteolysis pathway in ATRA-induced cell-cycle arrest and provide a novel strategy for modulating ATRA-elicited cellular effects.

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.

Leukemia: stem cells, maturation arrest, and differentiation therapy

Human myeloid leukemias provide models of maturation arrest and differentiation therapy of cancer. The genetic lesions of leukemia result in a block of differentiation (maturation arrest) that allows myeloid leukemic cells to continue to proliferate and/or prevents the terminal differentiation and apoptosis seen in normal white blood cells. In chronic myeloid leukemia, the bcr-abl (t9/22) translocation produces a fusion product that is an activated tyrosine kinase resulting in constitutive activation cells at the myelocyte level. This activation may be inhibited by imatinib mesylate (Gleevec, STI-571), which blocks the binding of ATP to the activated tyrosine kinase, prevents phosphorylation, and allows the leukemic cells to differentiate and undergo apoptosis. In acute promyelocytic leukemia, fusion of the retinoic acid receptor-alpha with the gene coding for promyelocytic protein, the PML-RAR alpha (t15:17) translocation, produces a fusion product that blocks the activity of the promyelocytic protein, which is required for formation of the granules of promyelocytes and prevents further differentiation. Retinoic acids bind to the retinoic acid receptor (RAR alpha) component of the fusion product, resulting in degradation of the fusion protein by ubiquitinization. This allows normal PML to participate in granule formation and differentiation of the promyelocytes. In one common type of acute myeloid leukemia, which results in maturation arrest at the myeloid precursor level, there is a mutation of FLT3, a transmembrane tyrosine kinase, which results in constitutive activation of the IL-3 receptor. This may be blocked by agents that inhibit farnesyl transferase. In each of these examples, specific inhibition of the genetically altered activation molecules of the leukemic cells allows the leukemic cells to differentiate and die. Because acute myeloid leukemias usually have mutation of more than one gene, combinations of specific inhibitors that act on the effects of different specific genetic lesions promises to result in more effective and permanent treatment.

Ubiquitin and ubiquitin-like proteins in cancer pathogenesis

Ubiquitin and ubiquitin-like proteins (Ubls) are signalling messengers that control many cellular functions, such as cell proliferation, apoptosis, the cell cycle and DNA repair. It is becoming apparent that the deregulation of ubiquitin pathways results in the development of human diseases, including many types of tumours. Here we summarize the common principles and specific features of ubiquitin and Ubls in the regulation of cancer-relevant pathways, and discuss new strategies to target ubiquitin signalling in drug discovery.

Validating cancer drug targets

A cancer drug target is only truly validated by demonstrating that a given therapeutic agent is clinically effective and acts through the target against which it was designed. Nevertheless, it is desirable to declare an early-stage drug target as 'validated' before investing in a full-scale drug discovery programme dedicated to it. Although the outcome of validation studies can guide cancer research programmes, strictly defined universal validation criteria have not been established.

Target for cancer therapy: proliferating cells or stem cells

Tumor stem cells are quiescent and, therefore, resistant to therapy, yet harbor the capacity to replenish a tumor after therapy. Therefore, it is tempting to explain all therapeutic failures by the persistence of tumor stem cells. Yet, this explanation is relevant only to initial stages of stem-cell-dependent tumors (such as chronic myeloid leukemia) that, actually, are well controlled by therapy. In advanced cancers that poorly respond to therapy, quiescent tumor stem cells play a negligible role. Instead, proliferating cells determine disease progression, prognosis, therapeutic failures, and resistance to therapy. And therapy fails not because it eliminates only proliferating tumor cells, but because it does not eliminate them. With noticeable exceptions, it is the proliferating cell that should be targeted, whereas resting cancer cells including stem and dormant cells need to be targeted only when they 'wake up'. Finally, I discuss a strategy of selectively killing dominant proliferating clones, including proliferating stem-like and drug-resistant cancer cells, while sparing normal cells.

Cancer Stem Cells and Differentiation Therapy

Cancers arise from stem cells in adult tissues and the cells that make up a cancer reflect the same stem cell --> progeny --> differentiation progression observed in normal tissues. All adult tissues are made up of lineages of cells consisting of tissue stem cells and their progeny (transit-amplifying cells and terminally differentiated cells); the number of new cells produced in normal tissue lineages roughly equals the number of old cells that die. Cancers result from maturation arrest of this process, resulting in continued proliferation of cells and a failure to differentiate and die. The biological behavior, morphological appearance, and clinical course of a cancer depend on the stage of maturation at which the genetic lesion is activated. This review makes a comparison of cancer cells to embryonic stem cells and to adult tis sue stem cells while addressing two basic questions: (1) Where do cancers come from?, and (2) How do cancers grow? The answers to these questions are critical to the development of approaches to the detection, prevention, and treatment of cancer.

Differential expression of dendritic cell markers by all-trans retinoic acid on human acute promyelocytic leukemic cell line

Dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) for naive T cells and play an important role in cancer immunology. All-trans retinoic acid (ATRA) is known to be a differentiating agent in the treatment of acute promyelocytic leukemia (APL). In this study, we investigated whether ATRA can differentiate the retinoic acid (RA)-sensitive promyelocytic leukemic cell line, NB4, to DC-like cells and whether these differentiated cells can activate T cells. NB4 cells were differentiated to myeloid cells by 4, 6, and 8 days of ATRA treatment. NB4 cells up-regulated markers found in DCs, including HLA-DR, costimulatory molecules (CD80 and CD86), adhesion molecules (CD40), and chemokine receptors (CCR6) when cultured for 8 days in the presence of 1 microM ATRA. Upregulation of CD83 was also detected on the surface of ATRA-treated NB4 cells versus untreated cells. The addition of cytokines alone, such as GM-CSF or CD40 ligand, did not affect the expression of CD83 in untreated NB4 cells but they up-regulated CD83 in ATRA-treated cells. CD11b was coexpressed with CD80, CD83, and CD86 in ATRA-treated NB4 cells. In a functional assay, ATRA-treated NB4 cells stimulated T cell proliferation when challenged with Staphylococcus enterotoxin B. These results suggest that the differentiation of NB4 cells by ATRA causes the cells to express DC markers, and that ATRA-differentiated NB4 cells are able to present antigens to T cells.

Systems analysis of transcriptome and proteome in retinoic acid/arsenic trioxide-induced cell differentiation/apoptosis of promyelocytic leukemia

Understanding the complexity and dynamics of cancer cells in response to effective therapy requires hypothesis-driven, quantitative, and high-throughput measurement of genes and proteins at both spatial and temporal levels. This study was designed to gain insights into molecular networks underlying the clinical synergy between retinoic acid (RA) and arsenic trioxide (ATO) in acute promyelocytic leukemia (APL), which results in a high-quality disease-free survival in most patients after consolidation with conventional chemotherapy. We have applied an approach integrating cDNA microarray, 2D gel electrophoresis with MS, and methods of computational biology to study the effects on APL cell line NB4 treated with RA, ATO, and the combination of the two agents and collected in a time series. Numerous features were revealed that indicated the coordinated regulation of molecular networks from various aspects of granulocytic differentiation and apoptosis at the transcriptome and proteome levels. These features include an array of transcription factors and cofactors, activation of calcium signaling, stimulation of the IFN pathway, activation of the proteasome system, degradation of the PML-RARalpha oncoprotein, restoration of the nuclear body, cell-cycle arrest, and gain of apoptotic potential. Hence, this investigation has provided not only a detailed understanding of the combined therapeutic effects of RA/ATO in APL but also a road map to approach hematopoietic malignancies at the systems level.

Molecular Mechanisms of Hormonal Activity. II. Kinase Systems. Systems with Intracellular Receptors. Transactivation of STS

Hormone receptors and other components, functional mechanisms, and biological role of analyzed signal transduction systems (STS) are described. The recently revealed module principle of the structure and STS transactivation providing diversity and plasticity of regulation are highlighted. STS activities are significantly changed in many diseases. Novel promising pharmaceuticals targeted to certain components of STS increase in number from year to year. The data published by the beginning of January 2004 are summarized in this review.

Promyelocytic leukemia protein-induced growth suppression and cell death in liver cancer cells

The promyelocytic leukemia protein (PML), involved in the pathogenesis of acute promyelocytic leukemia, is a coactivator of p53 tumor suppressive functions. The ability of PML to inhibit growth and induce cell death in solid tumor cells, however, has not been determined. We therefore assayed the tumor suppressor activities of PML and compared them with those of p53 in four liver cancer cell lines. Following infection of cells with replication-deficient recombinant PML adenovirus, the exogenous PML localized in the nucleus and formed abnormally enlarged PML-nuclear bodies after 24 hours. In vitro growth curve analysis showed that the overexpressed PML initially induced a substantial G1 cell cycle arrest and triggered massive cell death in all tested cell lines, irrespective of their p53 status. PML-induced cell death decreased by about 30% in the presence of a broad caspase inhibitor, zVAD. The cell death effect of PML was higher than that induced by p53 over a longer period of time. As with p53, overexpression of PML was closely related to upregulation of p21 and decrease of cyclin D1 expression. Unexpectedly, retinoic acid (RA) antagonized rather than enhanced PML-triggered cell death. RA enhanced the expression of adenovirus-cytomegalovirus-promoted PML at both transcription and protein levels within 12 hours after treatment; however, the PML protein was significantly degraded in the presence of RA at days 3-5 postinfection. PML degradation was also observed in SK-BR3 breast cancer cells treated with RA. Taken together, our findings strongly support the hypothesis that PML acts as a strong independent cell death inducer and that RA conversely abolishes the therapeutic effects of the PML proteins through proteasomal degradation of the protein.

Stem cell origin of cancer and differentiation therapy

Our forefathers in pathology, on observing cancer tissue under the microscope in the mid-19th century, noticed the similarity between embryonic tissue and cancer, and suggested that tumors arise from embryo-like cells [Recherches dur le Traitement du Cancer, etc. Paris. (1829); Editoral Archiv fuer pathologische Anatomie und Physiologie und fuer klinische Medizin 8 (1855) 23]. The concept that adult tissues contain embryonic remnants that generally lie dormant, but that could be activated to become cancer was later formalized by Cohnheim [Path. Anat. Physiol. Klin. Med. 40 (1867) 1-79; Virchows Arch. 65 (1875) 64] and Durante [Arch. Memori ed Osservazioni di Chirugia Practica 11 (1874) 217-226], as the "embryonal rest" theory of cancer. An updated version of the embryonal rest theory of cancer is that cancers arise from tissue stem cells in adults. Analysis of the cellular origin of carcinomas of different organs indicates that there is, in each instance, a determined stem cell required for normal tissue renewal that is the most likely cell of origin of carcinomas [Lab. Investig. 70 (1994) 6-22]. In the present review, the nature of normal stem cells (embryonal, germinal and somatic) is presented and their relationships to cancer are further expanded. Cell signaling pathways shared by embryonic cells and cancer cells suggest a possible link between embryonic cells and cancer cells. Wilm's tumors (nephroblastomas) and neuroblastomas are presented as possible tumors of embryonic rests in children. Teratocarcinoma is used as the classic example of the totipotent cancer stem cell which can be influenced by its environment to differentiate into a mature adult cell. The observation that "promotion" of an epidermal cancer may be accomplished months or even years after the initial exposure to carcinogen ("initiation"), implies that the original carcinogenic event occurs in a long-lived epithelial stem cell population. The cellular events during hepatocarcinogenesis illustrate that cancers may arise from cells at various stages of differentiation in the hepatocyte lineage. Examples of genetic mutations in epithelial and hematopoietic cancers show how specific alterations in gene expression may be manifested as maturation arrest of a cell lineage at a specific stage of differentiation. Understanding the signals that control normal development may eventually lead us to insights in treating cancer by inducing its differentiation (differentiation therapy). Retinoid acid (RA) induced differentiation therapy has acquired a therapeutic niche in treatment of acute promyelocytic leukemia and the ability of RA to prevent cancer is currently under examination.

Activation of the p70 S6 kinase by all-trans-retinoic acid in acute promyelocytic leukemia cells

Although the mechanisms by which all-trans-retinoic acid (RA) regulates gene transcription are well understood, very little is known on the signaling events regulating RA-dependent initiation of mRNA translation. We examined whether the mammalian target of rapamycin (mTOR)/p70 S6 kinase pathway is activated by RA. RA treatment of sensitive cell lines resulted in phosphorylation/activation of mTOR and downstream induction of p70 S6 kinase activity. Such phosphorylation/activation of p70 S6 kinase was inducible in primary acute promyelocytic leukemia (APL) blasts and RA-sensitive NB-4 cells, but was defective in an NB-4 variant cell line (NB-4.007/6) that is resistant to the biologic effects of RA. The RA-dependent activation of p70 S6 kinase was also phosphatidylinositol 3' kinase (PI3'K)-dependent, and resulted in downstream phosphorylation of the S6 ribosomal protein on Ser235/236 and Ser240/244, events important for initiation of translation for mRNAs with oligopyrimidine tracts in their 5' untranslated region. RA treatment of leukemia cells also resulted in an mTOR-mediated phosphorylation of the 4E-BP1 repressor of mRNA translation, to induce its deactivation and dissociation from the eukaryotic initiation factor-4E (eIF-4E) complex. Altogether, these findings provide evidence for the existence of a novel RA-activated cellular pathway that regulates cap-dependent translation, and strongly suggest that this cascade plays a role in the induction of retinoid responses in APL cells.

Acute promyelocytic leukemia: where does it stem from?

A fundamental issue in cancer biology is the identification of the target cell in which the causative molecular lesion arises. Acute myeloid leukemia (AML) is thought to reflect the transformation of a primitive stem cell compartment. The resultant 'cancer stem cells' comprise only a minor portion of the leukemic clone but give rise through differentiation to more committed progenitors as well as differentiated blasts that constitute the bulk of the tumor. The maintenance of the leukemic clone is dependent on the self-renewal capacity of the cancer stem cell compartment, which is revealed by its ability to re-initiate leukemia in a transplant setting. The cellular basis of acute promyelocytic leukemia (APL) is however less clear. APL has traditionally been considered to be the most differentiated form of AML and to arise from a committed myeloid progenitor. Here we review apparently conflicting evidence pertaining to the cellular origins of APL and propose that this leukemia may originate in more than one cellular compartment. This view could account for many apparent inconsistencies in the literature to date. An understanding of the nature of the target cell involved in transformation of APL has important implications for biological mechanism and for clinical treatment.

Involvement of UBE1L in ISG15 conjugation during retinoid-induced differentiation of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) cases expressing the t(15,17) product, promyelocytic leukemia (PML)/retinoic acid receptor alpha (RARalpha), have clinical remissions through leukemic cell differentiation after all-trans-retinoic acid (RA) treatment. This differentiation therapy propelled interest in uncovering molecular mechanisms for RA-dependent APL differentiation. We previously identified the ubiquitin-activating enzyme-E1-like protein (UBE1L) as an RA-regulated target gene in APL that triggers PML/RARalpha degradation and apoptosis. This study reports that conjugation of the ubiquitin-like species, interferon-stimulated gene, 15-kDa protein (ISG15), also occurs during RA-induced APL differentiation. Knock-down of UBE1L expression inhibited this conjugation. RA treatment of APL and other RA-responsive leukemic cells induced expression of UBE1L and ISG15 as well as intracellular ISG15 conjugates. Notably, ISG15 conjugation did not occur in RA-resistant NB4-R1 APL cells. Induction of UBE1L and ISG15 along with ISG15 conjugation in RA-sensitive NB4-S1 APL cells were detected following treatment with specific retinoids and type I interferon (IFN). UBE1L and ISG15 mRNAs were co-expressed in normal human tissues that were examined. In contrast, UBE1L mRNA expression was markedly repressed in several cancer cell lines. A physical association was found between UBE1L and ISG15 in vivo. This required the conserved diglycine motif in the carboxyl terminus of ISG15. Targeting UBE1L expression with small inhibitory RNA or small hairpin RNA inhibited IFN and RA-induced ISG15 conjugation. Formation of ISG15 conjugates through induction of an activating enzyme represents a novel pharmacologic mechanism for regulation of this ubiquitin-related species. Taken together, the observed rela tionship between expression of UBE1L and ISG15, their physical association and coordinate regulation, and induced ISG15 conjugation during leukemic cell differentiation implicate an important role for these proteins in retinoid response.

Identification of the molecular requirements for an RAR alpha-mediated cell cycle arrest during granulocytic differentiation

Retinoids are potent inducers of cell cycle arrest and differentiation of numerous cell types, notably granulocytes. However the mechanisms by which retinoids mediate cell cycle arrest during differentiation remain unclear. We have used myeloid differentiation to characterize the molecular pathways that couple cell cycle withdrawal to terminal differentiation. Using primary cells from mice deficient for either the cyclin-dependent kinase inhibitor (CDKi) p27(Kip1), the Myc antagonist Mad1, or both Mad1 and p27(Kip1), we observed that signals mediated through retinoic acid receptor alpha (RAR alpha), but not RAR beta or gamma, required both Mad1 and p27(Kip1) to induce cell cycle arrest and to accelerate terminal differentiation of granulocytes. Although RAR alpha did not directly regulate Mad1 or p27(Kip1), the RAR alpha target gene C/EBP epsilon directly regulated transcription of Mad1. Induction of C/EBP epsilon activity in granulocytic cells led to rapid induction of Mad1 protein and transcript, with direct binding of C/EBP epsilon to the Mad1 promoter demonstrated through chromatin immunoprecipitation assay. These data demonstrate that cell cycle arrest in response to RAR alpha specifically requires Mad1 and p27(Kip1) and that Mad1 is transcriptionally activated by CCAAT/enhancer-binding protein epsilon (C/EBP epsilon). Moreover, these data demonstrate selectivity among the RARs for cell cycle arrest pathways and provide a direct mechanism to link differentiation induction and regulation of the Myc antagonist Mad1.