Rapid induction of cAMP/PKA pathway during retinoic acid-induced acute promyelocytic leukemia cell differentiation

Nanoparticle-encapsulated retinoic acid for the modulation of bone marrow hematopoietic stem cell niche

More effective approaches are needed in the treatment of blood cancers, in particular acute myeloid leukemia (AML), that are able to eliminate resistant leukemia stem cells (LSCs) at the bone marrow (BM), after a chemotherapy session, and then enhance hematopoietic stem cell (HSC) engraftment for the re-establishment of the HSC compartment. Here, we investigate whether light-activatable nanoparticles (NPs) encapsulating all-trans-retinoic acid (RA+NPs) could solve both problems. Our in vitro results show that mouse AML cells transfected with RA+NPs differentiate towards antitumoral M1 macrophages through RIG.1 and OASL gene expression. Our in vivo results further show that mouse AML cells transfected with RA+NPs home at the BM after transplantation in an AML mouse model. The photo-disassembly of the NPs within the grafted cells by a blue laser enables their differentiation towards a macrophage lineage. This macrophage activation seems to have systemic anti-leukemic effect within the BM, with a significant reduction of leukemic cells in all BM compartments, of animals treated with RA+NPs, when compared with animals treated with empty NPs. In a separate group of experiments, we show for the first time that normal HSCs transfected with RA+NPs show superior engraftment at the BM niche than cells without treatment or treated with empty NPs. This is the first time that the activity of RA is tested in terms of long-term hematopoietic reconstitution after transplant using an in situ activation approach without any exogenous priming or genetic conditioning of the transplanted cells. Overall, the approach documented here has the potential to improve consolidation therapy in AML since it allows a dual intervention in the BM niche: to tackle resistant leukemia and improve HSC engraftment at the same time.

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

Extirpating the cancer stem cell hydra: Differentiation therapy and Hyperthermia therapy for targeting the cancer stem cell hierarchy

Ever since the discovery of cancer stem cells (CSCs), they have progressively attracted more attention as a therapeutic target. Like the mythical hydra, this subpopulation of cells seems to contribute to cancer immortality, spawning more cells each time that some components of the cancer cell hierarchy are destroyed. Traditional modalities focusing on cancer treatment have emphasized apoptosis as a route to eliminate the tumor burden. A major problem is that cancer cells are often in varying degrees of dedifferentiation contributing to what is known as the CSCs hierarchy and cells which are known to be resistant to conventional therapy. Differentiation therapy is an experimental therapeutic modality aimed at the conversion of malignant phenotype to a more benign one. Hyperthermia therapy (HT) is a modality exploiting the changes induced in cells by the application of heat produced to aid in cancer therapy. While differentiation therapy has been successfully employed in the treatment of acute myeloid leukemia, it has not been hugely successful for other cancer types. Mounting evidence suggests that hyperthermia therapy may greatly augment the effects of differentiation therapy while simultaneously overcoming many of the hard-to-treat facets of recurrent tumors. This review summarizes the progress made so far in integrating hyperthermia therapy with existing modules of differentiation therapy. The focus is on studies related to the successful application of both hyperthermia and differentiation therapy when used alone or in conjunction for hard-to-treat cancer cell niche with emphasis on combined approaches to target the CSCs hierarchy.

A novel network pharmacology approach for leukaemia differentiation therapy using Mogrify®

Acute myeloid leukaemia (AML) is a rapidly fatal blood cancer that is characterised by the accumulation of immature myeloid cells in the blood and bone marrow as a result of blocked differentiation. Methods which identify master transcriptional regulators of AML subtype-specific leukaemia cell states and their combinations could be critical for discovering novel differentiation-inducing therapies. In this proof-of-concept study, we demonstrate a novel utility of the Mogrify® algorithm in identifying combinations of transcription factors (TFs) and drugs, which recapitulate granulocytic differentiation of the NB4 acute promyelocytic leukaemia (APL) cell line, using two different approaches. In the first approach, Connectivity Map (CMAP) analysis of these TFs and their target networks outperformed standard approaches, retrieving ATRA as the top hit. We identify dimaprit and mebendazole as a drug combination which induces myeloid differentiation. In the second approach, we show that genetic manipulation of specific Mogrify®-identified TFs (MYC and IRF1) leads to co-operative induction of APL differentiation, as does pharmacological targeting of these TFs using currently available compounds. We also show that loss of IRF1 blunts ATRA-mediated differentiation, and that MYC represses IRF1 expression through recruitment of PML-RARα, the driver fusion oncoprotein in APL, to the IRF1 promoter. Finally, we demonstrate that these drug combinations can also induce differentiation of primary patient-derived APL cells, and highlight the potential of targeting MYC and IRF1 in high-risk APL. Thus, these results suggest that Mogrify® could be used for drug discovery or repositioning in leukaemia differentiation therapy for other subtypes of leukaemia or cancers.

Concomitant inhibition of hedgehog signalling and activation of retinoid receptors abolishes bleomycin-induced lung fibrosis

Pulmonary fibrosis is a devastating disease with unknown treatment. All-trans retinoic acid (ATRA) attenuates bleomycin-induced lung fibrosis by different mechanistic pathways. However, the role of retinoid receptors in lung fibrosis is still unclear. Forskolin (FSK), a potent inhibitor for the revolutionary hedgehog (Hh) signalling pathway, has a promising antifibrotic effect on other organs such as the liver. This study investigates the interplay between the retinoid receptors modulation and the Hh signalling pathway in bleomycin (BLM)-induced pulmonary fibrosis. Rats were randomised and administrated a single dose of 7.5 mg/kg of BLM alone and with ATRA, FSK and both of them. The effects of FSK and ATRA on lung functions, oxidative stress markers (malondialdehyde [MDA], glutathione [GSH], superoxide dismutase [SOD] and catalase [CAT]), retinoid markers (retinoic acid receptors [RAR] and rexinoid X receptors [RXR]) and Hh signalling markers (patched homolog 1 [Ptch-1], Smoothened [Smo] and glioblastoma-2 [Gli-2]) were assessed. In single therapies, ATRA and FSK ameliorated BLM-induced lung fibrosis. On the contrary, a combination of both drugs synergistically reversed the effect of BLM-induced lung fibrosis, as indicated by the enhancement of lung functions and the decrease of the α-smooth muscle actin (α-SMA) expression and collagen deposition. Additionally, FSK and ATRA ameliorated oxidative stress and inflammation, reduced transforming growth factor β1 (TGF-β1) levels and reversed the effect of BLM on the mRNA expression of Ptch-1, Smo and Gli-2. FSK inhibited the Hh pathway and also activated protein kinase A (PKA) that is, in part, involved in phosphorylation of RAR/RXR heterodimer (a key step in retinoid receptor activation). The present results suggest that a combination of FSK and ATRA has a promising therapeutic value for lung fibrosis management.

Differentiation of acute myeloid leukemia (AML) cells with ATRA reduces 18F-FDG uptake and increases sensitivity towards ABT-737-induced apoptosis

Acute myeloid leukemia (AML) is a malignant disease of the bone marrow, comprising various subtypes. We have investigated seven different AML cell lines that showed different sensitivities toward the inducer of apoptosis ABT-737, with IC₅₀ concentrations ranging from 9.9 nM to 1.8 µM. Besides, the AML cell lines revealed distinct differences in ¹⁸F-FDG uptake ranging from 4.1 to 11.0%. Moreover, the Pearson coefficient (0.363) suggests a moderate correlation between ¹⁸F-FDG uptake and the IC₅₀ values of ABT-737. Differentiation of the AML cell lines NB-4 and AML-193 with all-trans-retinoic-acid (ATRA) induced a significant increase in sensitivity towards ABT-737 along with a reduced uptake of ¹⁸F-FDG. Therefore, ¹⁸F-FDG uptake could be predictive on sensitivity to treatment with ABT-737. Furthermore, because differentiation treatment of AML cells using ATRA reduced ¹⁸F-FDG uptake and increased sensitivity towards ABT-737, a combined treatment regimen with ATRA and ABT-737 might be a promising therapeutic option in the future.

Nuclear receptors in podocyte biology and glomerular disease

Nuclear receptors have a broad spectrum of biological functions in normal physiology and in the pathology of various diseases, including glomerular disease. The primary therapies for many glomerular diseases are glucocorticoids, which exert their immunosuppressive and direct podocyte protective effects via the glucocorticoid receptor (GR). As glucocorticoids are associated with important adverse effects and a substantial proportion of patients show resistance to these therapies, the beneficial effects of selective GR modulators are now being explored. Peroxisome proliferator-activated receptor-γ (PPARγ) agonism using thiazolidinediones has potent podocyte cytoprotective and nephroprotective effects. Repurposing of thiazolidinediones or identification of novel PPARγ modulators are potential strategies to treat non-diabetic glomerular disease. Retinoic acid receptor-α is the key mediator of the renal protective effects of retinoic acid, and repair of the endogenous retinoic acid pathway offers another potential therapeutic strategy for glomerular disease. Vitamin D receptor, oestrogen receptor and mineralocorticoid receptor modulators regulate podocyte injury in experimental models. Further studies are needed to better understand the mechanisms of these nuclear receptors, evaluate their synergistic pathways and identify their novel modulators. Here, we focus on the role of nuclear receptors in podocyte biology and non-diabetic glomerular disease.

Long Non-coding RNA Maternally Expressed 3 Increases the Expression of Neuron-Specific Genes by Targeting miR-128-3p in All-Trans Retinoic Acid-Induced Neurogenic Differentiation From Amniotic Epithelial Cells

MicroRNA (miR)-128-3p is a brain-enriched miRNA that participates in the regulation of neural cell differentiation and the protection of neurons, but the mechanisms by which miR-128-3p regulates its target and downstream genes to influence cell fate from adult stem cells are poorly understood. In this study, we show down-regulation of miR-128-3p during all-trans retinoic acid (ATRA)-induced neurogenic differentiation from amniotic epithelial cells (AECs). We investigated miR-128-3p in both the Notch pathway and in the expression of neuron-specific genes predicted to be involved in miR-128-3p signaling to elucidate its role in the genetic regulation of downstream neurogenic differentiation. Our results demonstrate that miR-128-3p is a negative regulator for the transcription of the neuron-specific genes β III-tubulin, neuron-specific enolase (NSE), and polysialic acid-neural cell adhesion molecule (PSA-NCAM) via targeting Jagged 1 to inhibit activation of the Notch signaling pathway. We also used bioinformatics algorithms to screen for miR-128-3p interactions with long non-coding (lnc) RNA and circular RNA as competing endogenous RNAs to further elucidate underlying down-regulated molecular mechanisms. The lncRNA maternally expressed 3 is up-regulated by the ATRA/cAMP/CREB pathway, and it, in turn, is directly down-regulated by miR-128-3p to increase the amount of neuron differentiation. Endogenous miRNAs are, therefore, involved in neurogenic differentiation from AECs and should be considered during the development of effective cell transplant therapies for the treatment of neurodegenerative disease.

Adenylate cyclase 7 regulated by miR-192 promotes ATRA-induced differentiation of acute promyelocytic leukemia cells

Adenylate cyclase 7 (AC7) has been reported to participate in various biological processes during cancer progression. However, the roles of AC7 in all-trans retinoic acid (ATRA)-induced differentiation of acute promyelocytic leukemia (APL) cells are still unknown. In this study, firstly, our results showed that AC7 affected intracellular cAMP level and influenced ATRA-induced differentiation of APL cells. Secondly, we revealed that miR-192 could directly target AC7 expression and knockdown of miR-192 promoted ATRA-induced APL cell differentiation by regulating AC7 expression. Furthermore, we found that AC7 expression was lower in patients with relapsed APL than that in patients with newly diagnosed APL, while miR-192 expression was relatively higher in patients with relapsed APL. Taken together, our results show that miR-192-mediated AC7 could play important roles in differentiation of APL cells, AC7 and miR-192 might be new biomarkers and therapeutic targets for patients with relapsed APL.

Dynamic intercellular transport modulates the spatial patterning of differentiation during early neural commitment

The initiation of heterogeneity within a population of phenotypically identical progenitors is a critical event for the onset of morphogenesis and differentiation patterning. Gap junction communication within multicellular systems produces complex networks of intercellular connectivity that result in heterogeneous distributions of intracellular signaling molecules. In this study, we investigate emergent systems-level behavior of the intercellular network within embryonic stem cell (ESC) populations and corresponding spatial organization during early neural differentiation. An agent-based model incorporates experimentally-determined parameters to yield complex transport networks for delivery of pro-differentiation cues between neighboring cells, reproducing the morphogenic trajectories during retinoic acid-accelerated mouse ESC differentiation. Furthermore, the model correctly predicts the delayed differentiation and preserved spatial features of the morphogenic trajectory that occurs in response to intercellular perturbation. These findings suggest an integral role of gap junction communication in the temporal coordination of emergent patterning during early differentiation and neural commitment of pluripotent stem cells.

The success and the challenge of all-trans retinoic acid in the treatment of cancer

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, plays important roles in cell proliferation, cell differentiation, apoptosis, and embryonic development. The effects of ATRA are mediated by nuclear retinoid receptors as well as non-genomic signal pathway, such as MAPK and PKA. The great success of differentiation therapy with ATRA in acute promyelocytic leukemia (APL) not only improved the prognosis of APL but also spurred the studies of ATRA in the treatment of other tumors. Since the genetic and physiopathological simplicity of APL is not common in human malignancies, the combination of ATRA with other agents (chemotherapy, epigenetic modifiers, and arsenic trioxide, etc) had been extensively investigated in a variety of tumors. In this review, we will discuss in details about ATRA and its role in cancer treatment.

All-trans retinoic acid upregulates the expression of ciliary neurotrophic factor in retinal pigment epithelial cells

Retinopathy of prematurity, a leading cause of visual impairment in low birth-weight infants, remains a crucial therapeutic challenge. Ciliary neurotrophic factor (CNTF) is a promyelinating trophic factor that promotes rod and cone photoreceptor survival and cone outer segment regeneration in the degenerating retina. Ciliary neurotrophic factor expression is regulated by many factors such as all-trans retinoic acid (ATRA). In this study, we found that ATRA increased CNTF expression in mouse retinal pigment epithelial (RPE) cells in a dose- and time-dependent manner, and PKA signaling pathway is necessary for ATRA-induced CNTF upregulation. Furthermore, we showed that ATRA promoted CNTF expression through CREB binding to its promoter region. In addition, CNTF levels were decreased in serum of retinopathy of prematurity children and in retinal tissue of oxygen-induced retinopathy mice. In mouse RPE cells cultured with high oxygen, CNTF expression and secretion were decreased, but could be recovered after treatment with ATRA. In conclusion, our data suggest that ATRA administration upregulates CNTF expression in RPE cells.

Retinoic acid enhances progesterone production via the cAMP/PKA signaling pathway in immature rat granulosa cells

Retinoic acid (RA) is a metabolite of vitamin A and has important roles in development, differentiation, and reproduction. Activin has been shown to regulate the RA pathway and affect granulosa cell (GC) proliferation, suggesting that RA is important for early follicle development. However, little is known about the effects of RA on GC functions, particularly steroidogenesis, during the early follicle stage. The aim of this study was to investigate the effects of all-trans-RA (atRA) on progesterone production in immature rat GCs cultured without gonadotropin. Our results demonstrated that atRA enhanced progesterone production by upregulating the levels of steroidogenic acute regulatory protein (StAR) and cytochrome P450scc (Cyp11a1) mRNAs, but not 3β-hydroxysteroid dehydrogenase mRNA in immature rat GCs. Additionally, analysis of the mechanisms through which atRA upregulated StAR and Cyp11a1 mRNAs revealed that atRA enhanced intracellular cAMP accumulation and phosphorylation of cAMP response-element binding protein (CREB). In addition, H-89, an inhibitor of protein kinase A (PKA), abolished the stimulatory effects of atRA, indicating that atRA enhanced progesterone synthesis through cAMP/PKA signaling. In conclusion, our data demonstrated that atRA has a crucial role in progesterone synthesis in rat GCs during the early follicle stage.

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atRA upregulated StAR and Cyp11a1 and enhanced progesterone production.

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atRA enhanced intracellular cAMP accumulation and phosphorylation of CREB.

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Inhibition of PKA abolished the stimulatory effects of atRA.

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atRA mediated progesterone synthesis in rat GCs during the early follicle stage.

Synergistic targeted therapy for acute promyelocytic leukaemia: a model of translational research in human cancer

Acute promyelocytic leukaemia (APL), the M3 subtype of acute myeloid leukaemia, was once a lethal disease, yet nowadays the majority of patients with APL can be successfully cured by molecularly targeted therapy. This dramatic improvement in the survival rate is an example of the advantage of modern medicine. APL is characterized by a balanced reciprocal chromosomal translocation fusing the promyelocytic leukaemia (PML) gene on chromosome 15 with the retinoic acid receptor α (RARα) gene on chromosome 17. It has been found that all-trans-retinoic acid (ATRA) or arsenic trioxide (ATO) alone exerts therapeutic effect on APL patients with the PML-RARα fusion gene, and the combination of both drugs can act synergistically to further enhance the cure rate of the patients. Here, we provide an insight into the pathogenesis of APL and the mechanisms underlying the respective roles of ATRA and ATO. In addition, treatments that lead to more effective differentiation and apoptosis of APL cells, including leukaemia-initiating cells, and more thorough eradication of the disease will be discussed. Moreover, as a model of translational research, the development of a cure for APL has followed a bidirectional approach of 'bench to bedside' and 'bedside to bench', which can serve as a valuable example for the diagnosis and treatment of other malignancies.

Multidrug resistance-associated protein 4 is a determinant of arsenite resistance

Although arsenic trioxide (arsenite, As^III) has shown a remarkable efficacy in the treatment of acute promyelocytic leukemia patients, multidrug resistance is still a major concern for its clinical use. Multidrug resistance-associated protein 4 (MRP4), which belongs to the ATP-binding cassette (ABC) superfamily of transporters, is localized to the basolateral membrane of hepatocytes and the apical membrane of renal proximal tubule cells. Due to its characteristic localization, MRP4 is proposed as a candidate in the elimination of arsenic and may contribute to resistance to As^III. To test this hypothesis, stable HEK293 cells overexpressing MRP4 or MRP2 were used to establish the role of these two transporters in As^III resistance. The IC₅₀ values of As^III in MRP4 cells were approximately 6-fold higher than those in MRP2 cells, supporting an important role for MRP4 in resistance to As^III. The capacity of MRP4 to confer resistance to As^III was further confirmed by a dramatic decrease in the IC₅₀ values with the addition of MK571, an MRP4 inhibitor, and cyclosporine A, a well-known broad-spectrum inhibitor of ABC transporters. Surprisingly, the sensitivity of the MRP2 cells to As^III was similar to that of the parent cells, although insufficient formation of glutathione and/or Se conjugated arsenic compounds in the MRP2 cells might limit transport. Given that MRP4 is a major contributor to arsenic resistance in vitro, further investigation into the correlation between MRP4 expression and treatment outcome of leukemia patients treated with arsenic-based regimens is warranted.

Myoepithelial and luminal breast cancer cells exhibit different responses to all-trans retinoic acid

PURPOSE

Breast cancer is the leading cause of death among women worldwide. The exact role of luminal epithelial (LEP) and myoephitelial (MEP) cells in breast cancer development is as yet unclear, as also how retinoids may affect their behaviour. Here, we set out to evaluate whether retinoids may differentially regulate cell type-specific processes associated with breast cancer development using the bi-cellular LM38-LP murine mammary adenocarcinoma cell line as a model.

MATERIALS AND METHODS

The bi-cellular LM38-LP murine mammary cell line was used as a model throughout all experiments. LEP and MEP subpopulations were separated using inmunobeads, and the expression of genes known to be involved in epithelial to mysenchymal transition (EMT) was assessed by qPCR after all-trans retinoic acid (ATRA) treatment. In vitro invasive capacities of LM38-LP cells were evaluated using 3D Matrigel cultures in conjunction with confocal microscopy. Also, in vitro proliferation, senescence and apoptosis characteristics were evaluated in the LEP and MEP subpopulations after ATRA treatment, as well as the effects of ATRA treatment on the clonogenic, adhesive and invasive capacities of these cells. Mammosphere assays were performed to detect stem cell subpopulations. Finally, the orthotopic growth and metastatic abilities of LM38-LP monolayer and mammosphere-derived cells were evaluated in vivo.

RESULTS

We found that ATRA treatment modulates a set of genes related to EMT, resulting in distinct gene expression signatures for the LEP or MEP subpopulations. We found that the MEP subpopulation responds to ATRA by increasing its adhesion to extracellular matrix (ECM) components and by reducing its invasive capacity. We also found that ATRA induces apoptosis in LEP cells, whereas the MEP compartment responded with senescence. In addition, we found that ATRA treatment results in smaller and more organized LM38-LP colonies in Matrigel. Finally, we identified a third subpopulation within the LM38-LP cell line with stem/progenitor cell characteristics, exhibiting a partial resistance to ATRA.

CONCLUSIONS

Our results show that the luminal epithelial (LEP) and myoephitelial (MEP) mammary LM38-P subpopulations respond differently to ATRA, i.e., the LEP subpopulation responds with increased cell cycle arrest and apoptosis and the MEP subpopulation responds with increased senescence and adhesion, thereby decreasing its invasive capacity. Finally, we identified a third subpopulation with stem/progenitor cell characteristics within the LM38-LP mammary adenocarcinoma cell line, which appears to be non-responsive to ATRA.

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

p62/SQSTM1 upregulation constitutes a survival mechanism that occurs during granulocytic differentiation of acute myeloid leukemia cells

The p62/SQSTM1 adapter protein has an important role in the regulation of several key signaling pathways and helps transport ubiquitinated proteins to the autophagosomes and proteasome for degradation. Here, we investigate the regulation and roles of p62/SQSTM1 during acute myeloid leukemia (AML) cell maturation into granulocytes. Levels of p62/SQSTM1 mRNA and protein were both significantly increased during all-trans retinoic acid (ATRA)-induced differentiation of AML cells through a mechanism that depends on NF-κB activation. We show that this response constitutes a survival mechanism that prolongs the life span of mature AML cells and mitigates the effects of accumulation of aggregated proteins that occurs during granulocytic differentiation. Interestingly, ATRA-induced p62/SQSTM1 upregulation was impaired in maturation-resistant AML cells but was reactivated when differentiation was restored in these cells. Primary blast cells of AML patients and CD34(+) progenitors exhibited significantly lower p62/SQSTM1 mRNA levels than did mature granulocytes from healthy donors. Our results demonstrate that p62/SQSTM1 expression is upregulated in mature compared with immature myeloid cells and reveal a pro-survival function of the NF-κB/SQSTM1 signaling axis during granulocytic differentiation of AML cells. These findings may help our understanding of neutrophil/granulocyte development and will guide the development of novel therapeutic strategies for refractory and relapsed AML patients with previous exposure to ATRA.

Growth arrest and forced differentiation of human primary glioblastoma multiforme by a novel small molecule

Glioblastoma multiforme is the most common malignant brain tumor in adults, with an average survival of less than one year due to its resistance to therapy. Recent studies reported that GBM initiates from CD133-expressing cancer stem cells (CSC). However, the efficacy of CSC targeting is limited. A newly developed approach in cancer treatment is the forced differentiation of cancer cells. Here, we show that the treatment of the novel small molecule, CG500354, into CD133-expressing human primary GBM cells induces growth arrest by cell cycle regulators, p53, p21, p27 and phase-specific cyclins, and neural differentiation, as confirmed by neural progenitor/precursor markers, nestin, GFAP and Tuj1. When GBM-derived cells caused the tumors in NOD/SCID mice, CG500354 induced GBM-derived cells differentiation into Tuj1 and GFAP expressing cells. We next demonstrated that CG500354 plays a tumor-suppressive role via cAMP/CREB signaling pathway. CG500354 increases not only the extracellular cAMP level but also the protein level of PKA and CREB. Additionally, both mimetic substances, Forskolin and Rolipram, revealed comparable results with CG500354. Our findings indicate that induction of growth arrest and neural differentiation via cAMP/CREB signaling pathway by CG500354 treatment suggests the novel targeting of PDE4D in the development of new drugs for brain tumor therapy.

cAMP protects acute promyelocytic leukemia cells from arsenic trioxide-induced caspase-3 activation and apoptosis

More recently, arsenic trioxide (ATO), was integrated into acute promyelocytic leukemia (APL) treatment, showing high efficacy and tolerability in patients with both ATRA-sensitive and ATRA-resistant APL. ATO could induce apoptosis at relatively high concentrations (0.5 to 2.0 micromol/L) and partial differentiation at low concentrations (0.1 to 0.5 micromol/L) in leukemic promyelocytes. It is known that cAMP agonists enhance low-dose ATO-induced APL cells differentiation. Less well appreciated was the possible interaction between relatively high-doses of ATO and enhanced levels of cAMP in APL cells. Here, we show that elevation of cAMP levels by forskolin inhibited ATO-mediated apoptosis in APL-derived NB4 cells, and this inhibition could be averted by cell permeable cAMP-dependent protein kinase inhibitor (14-22) amide. Inactivating phosphorylation of the proapoptotic protein Bad at Ser118 and phosphorylation of the CREB proto-oncogene at Ser133 were observed upon elevation of cAMP levels in NB4 cells. Phosphorylation of these PKA target proteins is known to promote cell survival in AML cells. The ability of cAMP to endow the APL cells with survival advantage is of particular importance when cAMP agonists may be considered as adjuncts to APL therapy.

MicroRNA-181a-mediated downregulation of AC9 protein decreases intracellular cAMP level and inhibits ATRA-induced APL cell differentiation

AC9 is one of the adenylate cyclase (AC) isoforms, which catalyze the conversion of ATP to cAMP, an important second messenger. We previously found that the integration of cAMP/PKA pathway with nuclear receptor-mediated signaling was required during all-trans retinoic acid (ATRA)-induced maturation of acute promyelocytic leukemia (APL) cells. Here we showed that AC9 could affect intracellular cAMP level and enhance the trans-activity of retinoic acid receptor. Knockdown of AC9 in APL cell line NB4 could obviously inhibit ATRA-induced differentiation. We also demonstrated that miR-181a could decrease AC9 expression by targeting 3′UTR of AC9 mRNA, finally controlling the production of intracellular cAMP. The expression of miR-181a itself could be inhibited by CEBPα, probably accounting for the differential expression of miR-181a in NB4 and ATRA-resistant NB4-R1 cells. Moreover, we found that AC9 expression was relatively lower in newly diagnosed or relapsed APL patients than in both complete remission and non-leukemia cases, closely correlating with the leukemogenesis of APL. Taken together, our studies revealed for the first time the importance of miR-181a-mediated AC9 downregulation in APL. We also suggested the potential value of AC9 as a biomarker in the clinical diagnosis and treatment of leukemia.

Synergistic activation of steroidogenic acute regulatory protein expression and steroid biosynthesis by retinoids: involvement of cAMP/PKA signaling

Both retinoic acid receptors (RARs) and retinoid X receptors (RXRs) mediate the action of retinoids that play important roles in reproductive development and function, as well as steroidogenesis. Regulation of steroid biosynthesis is principally mediated by the steroidogenic acute regulatory protein (StAR); however, the modes of action of retinoids in the regulation of steroidogenesis remain obscure. In this study we demonstrate that all-trans retinoic acid (atRA) enhances StAR expression, but not its phosphorylation (P-StAR), and progesterone production in MA-10 mouse Leydig cells. Activation of the protein kinase A (PKA) cascade, by dibutyrl-cAMP or type I/II PKA analogs, markedly increased retinoid-responsive StAR, P-StAR, and steroid levels. Targeted silencing of endogenous RARα and RXRα, with small interfering RNAs, resulted in decreases in 9-cis RA-stimulated StAR and progesterone levels. Truncation of and mutational alterations in the 5'-flanking region of the StAR gene demonstrated the importance of the -254/-1-bp region in retinoid responsiveness. An oligonucleotide probe encompassing an RXR/liver X receptor recognition motif, located within the -254/-1-bp region, specifically bound MA-10 nuclear proteins and in vitro transcribed/translated RXRα and RARα in EMSAs. Transcription of the StAR gene in response to atRA and dibutyrl-cAMP was influenced by several factors, its up-regulation being dependent on phosphorylation of cAMP response-element binding protein (CREB). Chromatin immunoprecipitation studies revealed the association of phosphorylation of CREB, CREB binding protein, RXRα, and RARα to the StAR promoter. Further studies elucidated that hormone-sensitive lipase plays an important role in atRA-mediated regulation of the steroidogenic response that involves liver X receptor signaling. These findings delineate the molecular events by which retinoids influence cAMP/PKA signaling and provide additional and novel insight into the regulation of StAR expression and steroidogenesis in mouse Leydig cells.

Retinoids and breast cancer: from basic studies to the clinic and back again

All-trans retinoic acid (ATRA) is the most important active metabolite of vitamin A controlling segmentation in the developing organism and the homeostasis of various tissues in the adult. ATRA as well as natural and synthetic derivatives, collectively known as retinoids, are also promising agents in the treatment and chemoprevention of different types of neoplasia including breast cancer. The major aim of the present article is to review the basic knowledge acquired on the anti-tumor activity of classic retinoids, like ATRA, in mammary tumors, focusing on the underlying cellular and molecular mechanisms and the determinants of retinoid sensitivity/resistance. In the first part, an analysis of the large number of pre-clinical studies available is provided, stressing the point that this has resulted in a limited number of clinical trials. This is followed by an overview of the knowledge acquired on the role played by the retinoid nuclear receptors in the anti-tumor responses triggered by retinoids. The body of the article emphasizes the potential of ATRA and derivatives in modulating and in being influenced by some of the most relevant cellular pathways involved in the growth and progression of breast cancer. We review the studies centering on the cross-talk between retinoids and some of the growth-factor pathways which control the homeostasis of the mammary tumor cell. In addition, we consider the cross-talk with relevant intra-cellular second messenger pathways. The information provided lays the foundation for the development of rational and retinoid-based therapeutic strategies to be used for the management of breast cancer.

AP2α transcriptional activity is essential for retinoid-induced neuronal differentiation of mesenchymal stem cells

Pre-activation of the retinoid signaling pathway by all-trans retinoic acid facilitates neuronal differentiation of mesenchymal stem cells. Using protein/DNA based screening assays, we identified activator protein 2α as an important downstream target of all-trans retinoic acid. Although all-trans retinoic acid treatment significantly increased activator protein 2α transcriptional activity, it did not affect its expression. Inhibition of activator protein 2α with dominant-negative mutants reduced ATRA-induced differentiation of mesenchymal stem cells into neurons and reversed its associated functional recovery of memory impairment in the cell-based treatment of a hypoxic-ischemic brain damage rat model. Dominant-negative mutants of activator protein 2α inhibited the expression of neuronal markers which were induced by retinoic acid receptor β activation. All-trans retinoic acid treatment increased phosphorylation of activator protein 2α and resulted in its nuclear translocation. This was blocked by siRNA-mediated knockdown of retinoic acid receptor β. Furthermore, we found that retinoic acid receptor β directly interacted with activator protein 2α. In summary, the regulation of all-trans retinoic acid on activator protein 2α transcriptional activity was mediated by activation of retinoic acid receptor β and subsequent phosphorylation and nuclear translocation of activator protein 2α. Our results strongly suggest that activator protein 2α transcriptional activity is essential for all-trans retinoic acid-induced neuronal differentiation of mesenchymal stem cells.

The emerging role of miR-375 in cancer

MicroRNAs (miRNAs) are evolutionarily conserved, small noncoding RNAs that are believed to play fundamental roles in various biological processes through regulation of gene expression at the level of posttranscription. MiR-375 was first identified as a pancreatic islet-specific miRNA regulating insulin secretion. However, further study revealed that miR-375 is a multifunctional miRNA participating in pancreatic islet development, glucose homeostasis, mucosal immunity, lung surfactant secretion and more importantly, tumorigenesis. Recently, miR-375 has been found significantly downregulated in multiple types of cancer, and suppresses core hallmarks of cancer by targeting several important oncogenes like AEG-1, YAP1, IGF1R and PDK1. The alteration of miR-375 in cancer is caused by a variety of mechanisms, including the dysregulation of transcription factors, aberrant promoter methylation and so on. Reduced expression of miR-375 in tissue or circulation may indicate the presence of neoplasia as well as a poor prognosis of many malignant cancers. Moreover, miR-375 stands for a promising direction for developing targeted therapies due to its capacity to inhibit tumor cell growth in vitro and in vivo. Here, we summarize the present understanding of the tumor suppressive role of miR-375 in cancer progression; the mechanisms underlying the dysregulation of miR-375; the potential use of miR-375 in prognosis and diagnosis and the therapeutic prospects of miR-375 in cancer.

All-trans retinoic acid modulates ORMDL3 expression via transcriptional regulation

All-trans retinoic acid (ATRA) is an active metabolite of Vitamin A, it shows protective effects on asthma, including maintains airway epithelial integrity, inhibits asthma effector cells differentiation, modulates immune response, et al. However, the promoting effect of ATRA on Th2 response has restricted the clinical application of ATRA in asthma treatment. ORMDL3 is a candidate gene of childhood onset asthma, and high-transcript of ORMDL3 is associated with the development of asthma. Here we show that ATRA increases ORMDL3 production in vitro via inducing PKA-dependent CREB phosphorylation which in turn binds to the CRE element in promoter region of ORMDL3 and initiates ORMDL3 transcription. This finding is in consistent with the previous reports that ATRA could regulate target genes without the presence of retinoic acid response element (RARE) in promoter region but through other signals such as PKA/CREB. Nevertheless, in the present study, the traditional signal pathway of ATRA, retinoic acid receptor (RAR) signal transduction pathway, indirectly modulated ORMDL3 expression. RAR-α agonist (Am-80) increased ORMDL3 production even though there was no RARE in ORMDL3 promoter, introns or 3′-downstream region. Besides, the signal of RAR might differ from that of ATRA since Am-80 failed to induce CREB activation. In conclusion, our data indicate that ATRA facilitates ORMDL3 production probable through PKA/CREB, and this may be a starting point for more detailed mechanism researches on ATRA and asthma.

Retinoic acid impairs estrogen signaling in breast cancer cells by interfering with activation of LSD1 via PKA

More than 70% of breast cancers in women require estrogens for cell proliferation and survival. 17β-estradiol (E2) effect on mammary target cells is almost exclusively mediated by its binding to the estrogen receptor-α (ERα) that joins chromatin where it assembles active transcription complexes. The proliferative and pro-survival action of estrogens is antagonized in most cases by retinoic acid (RA), even though the cognate retinoic acid receptor-α (RARα) cooperates with ERα on promoters of estrogen-responsive genes. We have examined at the molecular level the crosstalk between these nuclear receptors from the point of view of their control of cell growth and show here that RA reverts estrogen-stimulated transcription of the pivotal anti-apoptotic bcl-2 gene by preventing demethylation of dimethyl lysine 9 in histone H3 (HeK9me2). As we previously reported, this is obtained by means of E2-triggered activation of the lysine-specific demethylase 1 (LSD1), an enzyme that manages chromatin plasticity in order to allow specific movements of chromosomal regions within the nucleus. We find that E2 fuels LSD1 by inducing migration of the catalytic subunit of protein kinase A (PKA) into the nucleus, where it targets estrogen-responsive loci. RA rescues LSD1-dependent disappearance of H3K9me2 at bcl-2 regulatory regions upon the prevention of PKA assembly to the same sites.

Activation of both protein kinase A (PKA) type I and PKA type II isozymes is required for retinoid-induced maturation of acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) is characterized by granulopoietic differentiation arrest at the promyelocytic stage. In most cases, this defect can be overcome by treatment with all-trans-retinoic acid (ATRA), leading to complete clinical remission. Cyclic AMP signaling has a key role in retinoid treatment efficacy: it enhances ATRA-induced maturation in ATRA-sensitive APL cells (including NB4 cells) and restores it in some ATRA-resistant cells (including NB4-LR1 cells). We show that the two cell types express identical levels of the Cα catalytic subunit and comparable global cAMP-dependent protein kinase A (PKA) enzyme activity. However, the maturation-resistant NB4-LR1 cells have a PKA isozyme switch: compared with the NB4 cells, they have decreased content of the juxtanuclearly located PKA regulatory subunit IIα and PKA regulatory subunit IIβ, and a compensatory increase of the generally cytoplasmically distributed PKA-RIα. Furthermore, the PKA regulatory subunit II exists mainly in the less cAMP-responsive nonautophosphorylated state in the NB4-LR1 cells. By the use of isozyme-specific cAMP analog pairs, we show that both PKA-I and PKA-II must be activated to achieve maturation in NB4-LR1 as well as NB4 cells. Therefore, special attention should be paid to activating not only PKA-I but also PKA-II in attempts to enhance ATRA-induced APL maturation in a clinical setting.

8-CPT-cAMP/all-trans retinoic acid targets t(11;17) acute promyelocytic leukemia through enhanced cell differentiation and PLZF/RARα degradation

The refractoriness of acute promyelocytic leukemia (APL) with t(11;17)(q23;q21) to all-trans retinoic acid (ATRA)-based therapy concerns clinicians and intrigues basic researchers. By using a murine leukemic model carrying both promyelocytic leukemia zinc finger/retinoic acid receptor-α (PLZF/RARα) and RARα/PLZF fusion genes, we discovered that 8-chlorophenylthio adenosine-3', 5'-cyclic monophosphate (8-CPT-cAMP) enhances cellular differentiation and improves gene trans-activation by ATRA in leukemic blasts. Mechanistically, in combination with ATRA, 8-CPT-cAMP activates PKA, causing phosphorylation of PLZF/RARα at Ser765 and resulting in increased dissociation of the silencing mediator for retinoic acid and thyroid hormone receptors/nuclear receptor corepressor from PLZF/RARα. This process results in changes of local chromatin and transcriptional reactivation of the retinoic acid pathway in leukemic cells. Meanwhile, 8-CPT-cAMP also potentiated ATRA-induced degradation of PLZF/RARα through its Ser765 phosphorylation. In vivo treatment of the t(11;17) APL mouse model demonstrated that 8-CPT-cAMP could significantly improve the therapeutic effect of ATRA by targeting a leukemia-initiating cell activity. This combined therapy, which induces enhanced differentiation and oncoprotein degradation, may benefit t(11;17) APL patients.

Retinaldehyde dehydrogenase 1 coordinates hepatic gluconeogenesis and lipid metabolism

Recent data link vitamin A and its retinoid metabolites to the regulation of adipogenesis, insulin sensitivity, and glucose homeostasis. Retinoid metabolism is tightly controlled by an enzymatic network in which retinaldehyde dehydrogenases (Aldh1-3) are the rate-limiting enzymes that convert retinaldehyde to retinoic acid. Aldh1a1-deficient mice are protected from diet-induced obesity and hence diabetes. Here we investigated whether Aldh1a1 and the retinoid axis regulate hepatic glucose and lipid metabolism independent of adiposity. The impact of Aldh1a1 and the retinoid pathway on glucose homeostasis and lipid metabolism was analyzed in hepatocytes in vitro and in chow-fed, weight-matched Aldh1a1-deficient vs. wild-type (WT) mice in vivo. Aldh1a1-deficient mice displayed significantly decreased fasting glucose concentrations compared with WT controls as a result of attenuated hepatic glucose production. Expression of key gluconeogenic enzymes as well as the activity of Forkhead box O1 was decreased in Aldh1a1-deficient vs. WT livers. In vitro, retinoid or cAMP agonist stimulation markedly induced gluconeogenesis in WT but not Aldh1a1-deficient primary hepatocytes. Aldh1a1 deficiency increased AMP-activated protein kinase α activity, decreased expression of lipogenic targets of AMP-activated protein kinase α and significantly attenuated hepatic triacylglycerol synthesis. In metabolic cage studies, lean Aldh1a1-deficient mice manifested enhanced oxygen consumption and reduced respiratory quotient vs. WT controls, consistent with increased expression of fatty acid oxidation markers in skeletal muscle. Taken together, this work establishes a role for retinoid metabolism in glucose homeostasis in vivo and for Aldh1a1 as a novel determinant of gluconeogenesis and lipid metabolism independent of adiposity.

Metformin induces differentiation in acute promyelocytic leukemia by activating the MEK/ERK signaling pathway

Recent studies have shown that metformin, a widely used antidiabetic agent, may reduce the risk of cancer development. In this study, we investigated the antitumoral effect of metformin on both acute myeloid leukemia (AML) and acute promyelocytic leukemia (APL) cells. Metformin induced apoptosis with partial differentiation in an APL cell line, NB4, but only displayed a proapoptotic effect on several non-M3 AML cell lines. Further analysis revealed that a strong synergistic effect existed between metformin and all-trans retinoic acid (ATRA) during APL cell maturation and that metformin induced the hyperphosphorylation of extracellular signal-regulated kinase (ERK) in APL cells. U0126, a specific MEK/ERK activation inhibitor, abrogated metformin-induced differentiation. Finally, we found that metformin induced the degradation of the oncoproteins PML-RARα and c-Myc and activated caspase-3. In conclusion, these results suggest that metformin treatment may contribute to the enhancement of ATRA-induced differentiation in APL, which may deepen the understanding of APL maturation and thus provide insight for new therapy strategies.

Roflumilast enhances the renal protective effects of retinoids in an HIV-1 transgenic mouse model of rapidly progressive renal failure

Retinoic acid decreases proteinuria and glomerulosclerosis in several animal models of kidney disease by protecting podocytes from injury. Our recent in vitro studies suggest that all-trans retinoic acid induces podocyte differentiation by activating the retinoic acid receptor-α (RARα)/cAMP/PKA/CREB pathway. When used in combination with all-trans retinoic acid, an inhibitor of phosphodiesterase 4 further enhanced podocyte differentiation by increasing intracellular cAMP. Additionally, we found that Am580, a specific RARα agonist, has similar renal protective effects as all-trans retinoic acid in a rederived colony of HIV-1 transgenic mice with rapidly progressive renal failure (HIV-Tg) that mimics human HIV-associated nephropathy. Treatment with either the inhibitor of phoshodiesterase 4, roflumilast, or Am580 significantly reduced proteinuria, attenuated kidney injury, and improved podocyte differentiation in these HIV-Tg mice. Additional renal protective effects were found when roflumilast was combined with Am580. Consistent with the in vitro data, glomeruli from HIV-Tg mice treated with both Am580 and roflumilast had more active phosphorylated CREB than with either agent alone. Thus, phosphodiesterase 4 inhibitors could be used in combination with RARα agonists to provide additional renal protection.

Acute effects of all-trans-retinoic acid in ischemic injury

All-trans-retinoic acid (ATRA) is a vitamin A derivative that is important in neuronal patterning, survival, and neurite outgrowth. Neuroprotective effects of ATRA in ischemia have been demonstrated but its effects on glial swelling are not known. We investigated the relatively acute effects of ATRA on cell swelling in ischemic injury and on key features hypothesized to contribute to cell swelling including increased reactive oxygen species/reactive nitrogen species (ROS/RNS), depolarization of the inner mitochondrial membrane potential (ΔΨm), and increased intracellular calcium ([Ca2+]i). C6 glial cultures were subjected to 5 hr oxygen-glucose deprivation (OGD). ATRA was added to separate groups after the end of OGD. OGD increased cell volume by 43%, determined at 90 min after the end of OGD, but this increase was significantly attenuated by ATRA. OGD induced an increase in ROS/RNS production in the whole cell and mitochondria, as assessed by the fluorescent dyes CM-H2DCFDA and MitoTracker CM-H2-XROS at the end of OGD. The increase in mitochondrial ROS, but not cellular ROS, was significantly attenuated by ATRA. OGD also induced a 67% decline in mitochondrial ΔΨm but this decline was significantly attenuated by ATRA. OGD-induced increase in [Ca2+]i was also significantly attenuated by ATRA. Taken together with our previous results where calcium channel blockers reduced cell swelling, the effects of ATRA in attenuating swelling are possibly mediated through its effects in regulating [Ca2+]i. Considering the paucity of agents in attenuating brain edema in ischemia, ATRA has the potential to reduce brain edema and associated neural damage in ischemic injury.

From an old remedy to a magic bullet: molecular mechanisms underlying the therapeutic effects of arsenic in fighting leukemia

Arsenic had been used in treating malignancies from the 18th to mid-20th century. In the past 3 decades, arsenic was revived and shown to be able to induce complete remission and to achieve, when combined with all-trans retinoic acid and chemotherapy, a 5-year overall survival of 90% in patients with acute promyelocytic leukemia driven by the t(15;17) translocation-generated promyelocytic leukemia-retinoic acid receptor α (PML-RARα) fusion. Molecularly, arsenic binds thiol residues and induces the formation of reactive oxygen species, thus affecting numerous signaling pathways. Interestingly, arsenic directly binds the C3HC4 zinc finger motif in the RBCC domain of PML and PML-RARα, induces their homodimerization and multimerization, and enhances their interaction with the SUMO E2 conjugase Ubc9, facilitating subsequent sumoylation/ubiquitination and proteasomal degradation. Arsenic-caused intermolecular disulfide formation in PML also contributes to PML-multimerization. All-trans retinoic acid, which targets PML-RARα for degradation through its RARα moiety, synergizes with arsenic in eliminating leukemia-initiating cells. Arsenic perturbs a number of proteins involved in other hematologic malignancies, including chronic myeloid leukemia and adult T-cell leukemia/lymphoma, whereby it may bring new therapeutic benefits. The successful revival of arsenic in acute promyelocytic leukemia, together with modern mechanistic studies, has thus allowed a new paradigm to emerge in translational medicine.

Endoplasmic reticulum calcium pumps and cancer

Endoplasmic reticulum calcium homeostasis is involved in a multitude of signaling, as well as "house-keeping" functions that control cell growth, differentiation or apoptosis in every human/eukaryotic cell. Calcium is actively accumulated in the endoplasmic reticulum by Sarco/Endoplasmic Reticulum Calcium transport ATPases (SERCA enzymes). SERCA-dependent calcium transport is the only calcium uptake mechanism in this organelle, and therefore the regulation of SERCA function by the cell constitutes a key mechanism to adjust calcium homeostasis in the endoplasmic reticulum depending on the cell type and its state of differentiation. The direct pharmacological modulation of SERCA activity affects cell differentiation and survival. SERCA expression levels can undergo significant changes during cell differentiation or tumorigenesis, leading to modified endoplasmic reticulum calcium storage. In several cell types such as cells of hematopoietic origin or various epithelial cells, two SERCA genes (SERCA2 and SERCA3) are simultaneously expressed. Expression levels of SERCA3, a lower calcium affinity calcium pump are highly variable. In several cell systems SERCA3 expression is selectively induced during differentiation, whereas during tumorigenesis and blastic transformation SERCA3 expression is decreased. These observations point at the existence of a cross-talk, via the regulation of SERCA3 levels, between endoplasmic reticulum calcium homeostasis and the control of cell differentiation, and show that endoplasmic reticulum calcium homeostasis itself can undergo remodeling during differentiation. The investigation of the anomalies of endoplasmic reticulum differentiation in tumor and leukemia cells may be useful for a better understanding of the contribution of calcium signaling to the establishment of malignant phenotypes.

Arsenic trioxide cooperates with all trans retinoic acid to enhance mitogen-activated protein kinase activation and differentiation in PML-RARalpha negative human myeloblastic leukemia cells

Arsenic trioxide (ATO) synergistically promotes all trans retinoic acid (ATRA)-induced differentiation of PML-RARalpha negative HL-60 myeloblastic leukemia cells. In PML-RARalpha positive myeloid leukemia cells, ATO is known to cause degradation of PML-RARalpha with subsequent induced myeloid differentiation. We found that ATO by itself does not cause differentiation of the PML-RARalpha negative HL-60 cells, but enhances ATRA's capability to cause differentiation. ATO augmented ATRA-induced RAF/MEK/ERK axis signaling, expression of CD11b and p47(PHOX), and inducible oxidative metabolism. ATO enhanced ATRA-induced population growth retardation without evidence of apoptosis or enhanced G1/G0 growth arrest. Compared to ATRA-treated cells, the ATRA plus ATO-treated cells progressed more slowly through the cell cycle as detected by a slower rate of accumulation in G2/M following nocodazole treatment. Hoechst/PI staining showed that low-dose ATO did not induce apoptosis. In summary, our results indicate that ATO in conjunction with ATRA is of potential chemotherapeutic use in PML-RARalpha negative leukemias.

Leukemic transformation by the APL fusion protein PRKAR1A-RARα critically depends on recruitment of RXRα

PRKAR1A (R1A)–retinoic acid receptor-α (R1A-RARα) is the sixth RARα–containing fusion protein in acute promyelocytic leukemia (APL). Using the murine bone-marrow retroviral transduction/transformation assay, we showed that R1A-RARα fusion protein could transform bone-marrow progenitor/stem cells. In gel-shift assays, R1A-RARα was able to bind to a panel of retinoic acid response elements both as a homodimer and as a heterodimer with RXRα, and demonstrated distinct DNA-binding characteristics compared with wild-type RARα/RXRα or other X-RARα chimeric proteins. The ratio of R1A-RARα to RXRα proteins affected the retinoic acid response element interaction pattern of R1A-RARα/RXRα complexes. Studies comparing R1A-RARα with R1A-RARα(ΔRIIa) demonstrated that the RIIa protein interaction domain located within R1A was responsible for R1A-RARα homodimeric DNA binding and interaction with wild-type R1A protein. However, the RIIa domain was not required for R1A-RARα–mediated transformation because its deletion in R1A-RARα(ΔRIIa) did not compromise its transformation capability. In contrast, introduction of point mutations within the RARα portion of either R1A-RARα or R1A-RARα(ΔRIIa), previously demonstrated to eliminate RXRα interaction or treatment of transduced cells with RXRα shRNA or a RXRα agonist, reduced transformation capability. Thus, leukemic transformation by APL fusion protein PRKAR1A-RARα is critically dependent on RXRα, which suggests RXRα is a promising target for APL.

Connexin-based signaling in acute myelogenous leukemia (AML)

Normal and malignant hematopoiesis are regulated by intercellular communication in the hematopoietic microenvironments, and both soluble mediators as well as direct cell-cell contact play important functional roles. Gap junctions are complex membrane structures that transfer molecules between neighboring cells and thereby alter intracellular signaling and metabolism. The gap junction building blocks, the connexins, are also involved in gap junction-independent intercellular communication by forming hemichannels that transfer substances between the intra- and extracellular spaces. Connexins are furthermore involved in cell regulation as single molecules by modulating intracellular pathways and possibly gene transcription. The role of connexins in leukemogenesis and leukemic cell functions are not well characterized. In this review, we describe the known effects of gap junctions and connexins in acute myelogenous leukemia and the diverse potential of connexins in acute myelogenous leukemia chemosensitivity, intracellular signaling and cell death regulation.

Elucidation of the molecular mechanisms of a salicylhydrazide class of compounds by proteomic analysis

Previously, we described a series of salicylhydrazide compounds with potent anti-cancer activities against a panel of human cancer cell lines derived from different origins. Preclinical evaluation showing efficacy both in vitro and in vivo in human cancer models indicated that these agents may represent a promising class of anticancer drugs. In the present study, we performed an in-depth investigation on the underlying molecular mechanisms of the most potent compounds, SC21 and SC23, using a proteomic method and bioinformatics tools. We demonstrated that SC23 induced apoptosis through multiple signaling pathways. In particular, SC23 regulated the expression of Bcl-2, p21, acetylated histone H3 and beta-tubulin and the combined modulation of these proteins may result in the induction of apoptosis. We also examined the effect of SC21 and SC23 on cell cycle progression and found that both compounds arrested cells in S-phase in most cell lines tested. To better understand the signaling networks involved, we analyzed the SC21- and SC23-treated cell lysates by the Kinexus 628 antibody microarray. The results were interpreted with the aid of Ingenuity Pathway Analysis (IPA) software. It was found that SC21 interfered with JAK/STAT signaling and elicited apoptosis through Fas and caspases pathways. Unlike SC21, SC23 induced RAR activation and caused cell cycle arrest. The signaling networks identified by this work may provide the basis for future mechanistic studies. The validation of the proposed pathways and the elucidation of the signaling cross-talk are currently under way.

Hypoxia-HIF-1alpha-C/EBPalpha/Runx1 signaling in leukemic cell differentiation

Acute myeloid leukemia (AML), a class of prevalent hematopoietic malignancies, is caused by the acquisition of gene mutations that confer deregulated proliferation, impaired differentiation and a survival advantage of hematopoietic progenitors. More recently, we reported that cobalt chloride (CoCl(2))/iron chelator desferrioxamine (DFO)-mimicked hypoxia or moderate hypoxia (2% and 3% O(2)) can directly trigger differentiation of many subtypes of AML cells. Also, intermittent hypoxia significantly prolongs the survival of the transplanted leukemic mice with differentiation induction of leukemic cells. Additionally, these hypoxia-simulating agents selectively stimulate differentiation in acute promyelocytic leukemic cells induced by arsenic trioxide, an effective second-line drug for this unique type of leukemia. Based on this interesting evidence in vitro and in vivo, the ongoing investigations showed the role of hypoxia-inducible factor-1alpha (HIF-1alpha) protein through its non-transcriptional activity in myeloid cell differentiation, as evidenced by chemical interference, the conditional HIF-1alpha induction, the specific short hairpin RNAs (shRNAs) against HIF-1alpha and HIF-1beta, an essential partner for transcription activity of HIF-1. Furthermore, HIF-1alpha and two hematopoietic transcription factors CCAAT/enhancer binding protein alpha (C/EBPalpha) and Runx1/AML1 interact directly with each other. Such interactions increase the transcriptional activities of C/EBPalpha and Runx1/AML1, while C/EBPalpha competes with HIF-1beta for direct binding to HIF-1alpha protein, and significantly inhibits the DNA-binding ability of HIF-1. As a protein is rapidly responsive to all-trans retinoic acid (ATRA), a classical clinical differentiation-inducing drug for AML, HIF-1alpha also plays a role in ATRA-induced differentiation of leukemic cells.

Transcriptional repression of microRNA genes by PML-RARA increases expression of key cancer proteins in acute promyelocytic leukemia

Micro(mi)RNAs are small noncoding RNAs that orchestrate many key aspects of cell physiology and their deregulation is often linked to distinct diseases including cancer. Here, we studied the contribution of miRNAs in a well-characterized human myeloid leukemia, acute promyelocytic leukemia (APL), targeted by retinoic acid and trioxide arsenic therapy. We identified several miRNAs transcriptionally repressed by the APL-associated PML-RAR oncogene which are released after treatment with all-trans retinoic acid. These coregulated miRNAs were found to control, in a coordinated manner, crucial pathways linked to leukemogenesis, such as HOX proteins and cell adhesion molecules whose expressions are thereby repressed by the chemotherapy. Thus, APL appears linked to transcriptional perturbation of miRNA genes, and clinical protocols able to successfully eradicate cancer cells may do so by restoring miRNA expression. The identification of abnormal miRNA biogenesis in cancer may therefore provide novel biomarkers and therapeutic targets in myeloid leukemias.

PML/RARalpha fusion protein mediates the unique sensitivity to arsenic cytotoxicity in acute promyelocytic leukemia cells: Mechanisms involve the impairment of cAMP signaling and the aberrant regulation of NADPH oxidase

Acute promyelocytic leukemia (APL) cells are characterized by PML/RARalpha fusion protein, high responsiveness to arsenic trioxide (ATO)-induced cytotoxicity and an abundant generation of reactive oxygen species (ROS). In this study we investigated the association among these three features in APL-derived NB4 cells. We found that NADPH oxidase-derived ROS generation was more abundant in NB4 cells compared with monocytic leukemia U937 cells. By using PR9, a sub-line of U937 stably transduced with the inducible PML/RARalpha expression vectors, we attributed disparities on ROS generation and ATO sensitivity to the occurrence of PML/RARalpha fusion protein, since PML/RARalpha-expressing cells appeared higher NADPH oxidase activity, higher ROS level and higher sensitivity to ATO. On the other hand, the basal intensity of cAMP signaling pathway was compared between NB4 and U937 as well as between PR9 cells with or without PML/RARalpha, demonstrating that PML/RARalpha-expressing cells had an impaired cAMP signaling pathway which relieved its inhibitory effect on NADPH oxidase derived ROS generation. In summary, the present study demonstrated the correlation of PML/RARalpha with cAMP signaling pathway, NADPH oxidase and ROS generation in APL cells. PML/RARalpha that bestows NB4 cells various pathological features, paradoxically also endows these cells with the basis for susceptibility to ATO-induced cytotoxcity.

Retinoic acid utilizes CREB and USF1 in a transcriptional feed-forward loop in order to stimulate MKP1 expression in human immunodeficiency virus-infected podocytes

Nef-induced podocyte proliferation and dedifferentiation via mitogen-activated protein kinase 1,2 (MAPK1,2) activation plays a role in human immunodeficiency virus (HIV) nephropathy pathogenesis. All-trans retinoic acid (atRA) reverses the HIV-induced podocyte phenotype by activating cyclic AMP (cAMP)/protein kinase A (PKA) and inhibiting MAPK1,2. Here we show that atRA, through cAMP and PKA, triggers a feed-forward loop involving CREB and USF1 to induce biphasic stimulation of MKP1. atRA stimulated CREB and USF1 binding to the MKP1 gene promoter, as shown by gel shifting and chromatin immunoprecipitation assays. CREB directly mediated the early phase of atRA-induced MKP1 stimulation; whereas the later phase was mediated by CREB indirectly through induction of USF1. These findings were confirmed by a reporter gene assay using the MKP1 promoter with mutation of CRE or Ebox binding sites. Consistent with these findings, the biological effects of atRA on podocytes were inhibited by silencing either MKP1, CREB, or USF1 with small interfering RNA. atRA also induced CREB phosphorylation and MKP1 expression and reduced MAPK1,2 phosphorylation in kidneys of HIV type 1-infected transgenic mice. We conclude that atRA induces sustained activation of MKP1 to suppress Nef-induced activation of the Src-MAPK1,2 pathway, thus returning the podocyte to a more differentiated state. The mechanism involves a feed-forward loop where activation of one transcription factor (TF) (CREB) leads to induction of a second TF (USF1).

Retinoic acid and the heart

Retinoic acid (RA), the active derivative of vitamin A, by acting through retinoid receptors, is involved in signal transduction pathways regulating embryonic development, tissue homeostasis, and cellular differentiation and proliferation. RA is important for the development of the heart. The requirement of RA during early cardiovascular morphogenesis has been studied in targeted gene deletion of retinoic acid receptors and in the vitamin A-deficient avian embryo. The teratogenic effects of high doses of RA on cardiovascular morphogenesis have also been demonstrated in different animal models. Specific cardiovascular targets of retinoid action include effects on the specification of cardiovascular tissues during early development, anteroposterior patterning of the early heart, left/right decisions and cardiac situs, endocardial cushion formation, and in particular, the neural crest. In the postdevelopment period, RA has antigrowth activity in fully differentiated neonatal cardiomyocytes and cardiac fibroblasts. Recent studies have shown that RA has an important role in the cardiac remodeling process in rats with hypertension and following myocardial infarction. This chapter will focus on the role of RA in regulating cardiomyocyte growth and differentiation during embryonic and the postdevelopment period.

Acute promyelocytic leukemia: from highly fatal to highly curable

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia. Morphologically, it is identified as the M3 subtype of acute myeloid leukemia by the French-American-British classification and cytogenetically is characterized by a balanced reciprocal translocation between chromosomes 15 and 17, which results in the fusion between promyelocytic leukemia (PML) gene and retinoic acid receptor alpha (RARalpha). It seems that the disease is the most malignant form of acute leukemia with a severe bleeding tendency and a fatal course of only weeks. Chemotherapy (CT; daunorubicin, idarubicin and cytosine arabinoside) was the front-line treatment of APL with a complete remission (CR) rate of 75% to 80% in newly diagnosed patients. Despite all these progresses, the median duration of remission ranged from 11 to 25 months and only 35% to 45% of the patients could be cured by CT. Since the introduction of all-trans retinoic acid (ATRA) in the treatment and optimization of the ATRA-based regimens, the CR rate was raised up to 90% to 95% and 5-year disease free survival (DFS) to 74%. The use of arsenic trioxide (ATO) since early 1990s further improved the clinical outcome of refractory or relapsed as well as newly diagnosed APL. In this article, we review the history of introduction of ATRA and ATO into clinical use and the mechanistic studies in understanding this model of cancer targeted therapy.