The phosphoinositide 3-kinase/Akt pathway is essential for the retinoic acid-induced differentiation of F9 cells

Identifying Codium fragile extract components and their effects on muscle weight and exercise endurance

Codium fragile (CF) is a type of green algae consumed as kimchi in Asia. UPLC-QTOF-MS/MS analysis showed that CF contain lysophosphatidyl choline, canthaxanthin, retinoic acid, α-tocopherol, and unsaturated fatty acids, which reportedly improve skeletal muscle health. However, the effect of CF on skeletal muscle mass and function remains to be elucidated. In mice fed with CF extracts, exercise endurance and muscle weight increased. CF extracts enhanced protein synthesis and myogenic differentiation through the mTORC1 pathway. CF extracts also promoted oxidative muscle fiber formation and mitochondrial biogenesis through the PGC-1α-related signaling pathway. Upregulation of PGC-1α by CF extracts was abolished by EX527 SIRT1 inhibitor treatment. Changed signaling molecules in the CF extracts were partially regulated by canthaxanthin, a new compound in CF extracts, suggesting that canthaxanthin contribute synergistically to the effect of CF extracts. Therefore, CF is a potential food source for sport nutrition or prevention of sarcopenia.

Retinoic acid induces differentiation in neuroblastoma via ROR1 by modulating retinoic acid response elements

Neuroblastoma is the most common inheritable, solid neoplasm in children found under the age of 7 and accounts for approximately 7% of childhood cancers. A common treatment that has been prescribed for over a decade is retinoid therapy [using all-trans retinoic acid (RA)]. Treatment with this differentiating agent has been revealed to progress the cells from their stem-cell state to a mature neuronal state gaining classical neuronal characteristics, including the suppression of proliferation. However, the molecular mechanism underlying the action of RA treatment remains to be elucidated. In the present study, a novel mechanism of RA-induced differentiation via regulation of receptor tyrosine kinase-like orphan receptor 1 (ROR1) is reported. ROR1 is overexpressed in neuroblastoma but significantly downregulated in mature differentiated neurons. Hence, it was hypothesized that RA may modulate ROR1 leading to differentiation and termination of cancerous properties. Immunoblotting revealed that following RA treatment, ROR1 levels initially increased then sharply decreased by 96 h. This was paired with synaptophysin, a mature neuron marker, sharply increasing concurrently, providing evidence of differentiation by 96 h. Investigation of the ROR1 pathway confirmed ROR1-dependent downstream activation of the PI3K/AKT signaling axis, a growth pathway previously demonstrated to promote differentiation. Chromatin immunoprecipitation revealed an increase in RAR binding to the promoters of ROR1 and its endogenous ligand, Wnt5a. This research provided compelling evidence that RA is able to modulate the expression of ROR1 and Wnt5a to promote differentiation through the expression of synaptophysin. This data combined with the overarching data from the scientific community regarding proliferation and other proliferative factors in early-stage neurons provides a more in-depth model of the process of differentiation in neurons.

TGFβ Family Signaling Pathways in Pluripotent and Teratocarcinoma Stem Cells' Fate Decisions: Balancing Between Self-Renewal, Differentiation, and Cancer

The transforming growth factor-β (TGFβ) family factors induce pleiotropic effects and are involved in the regulation of most normal and pathological cellular processes. The activity of different branches of the TGFβ family signaling pathways and their interplay with other signaling pathways govern the fine regulation of the self-renewal, differentiation onset and specialization of pluripotent stem cells in various cell derivatives. TGFβ family signaling pathways play a pivotal role in balancing basic cellular processes in pluripotent stem cells and their derivatives, although disturbances in their genome integrity induce the rearrangements of signaling pathways and lead to functional impairments and malignant transformation into cancer stem cells. Therefore, the identification of critical nodes and targets in the regulatory cascades of TGFβ family factors and other signaling pathways, and analysis of the rearrangements of the signal regulatory network during stem cell state transitions and interconversions, are key issues for understanding the fundamental mechanisms of both stem cell biology and cancer initiation and progression, as well as for clinical applications. This review summarizes recent advances in our understanding of TGFβ family functions in naїve and primed pluripotent stem cells and discusses how these pathways are involved in perturbations in the signaling network of malignant teratocarcinoma stem cells with impaired differentiation potential.

Combinatorial knockout of RARα, RARβ, and RARγ completely abrogates transcriptional responses to retinoic acid in murine embryonic stem cells

All-trans-retinoic acid (RA), a potent inducer of cellular differentiation, functions as a ligand for retinoic acid receptors (RARα, β, and γ). RARs are activated by ligand binding, which induces transcription of direct genomic targets. However, whether embryonic stem cells respond to RA through routes that do not involve RARs is unknown. Here, we used CRISPR technology to introduce biallelic frameshift mutations in RARα, RARβ, and RARγ, thereby abrogating all RAR functions in murine embryonic stem cells. We then evaluated RA-responsiveness of the RAR-null cells using RNA-Seq transcriptome analysis. We found that the RAR-null cells display no changes in transcripts in response to RA, demonstrating that the RARs are essential for the regulation of all transcripts in murine embryonic stem cells in response to RA. Our key finding, that in embryonic stem cells the transcriptional effects of RA all depend on RARs, addresses a long-standing topic of discussion in the field of retinoic acid signaling.

Regulatory T Cells As Potential Targets for HIV Cure Research

T regulatory cells (Tregs) are a key component of the immune system, which maintain a delicate balance between overactive responses and immunosuppression. As such, Treg deficiencies are linked to autoimmune disorders and alter the immune control of pathogens. In HIV infection, Tregs play major roles, both beneficial and detrimental. They regulate the immune system such that inflammation and spread of virus through activated T cells is suppressed. However, suppression of immune activation also limits viral clearance and promotes reservoir formation. Tregs can be directly targeted by HIV, thereby harboring a fraction of the viral reservoir. The vital role of Tregs in the pathogenesis and control of HIV makes them a subject of interest for manipulation in the search of an HIV cure. Here, we discuss the origin and generation, homeostasis, and functions of Tregs, particularly their roles and effects in HIV infection. We also present various Treg manipulation strategies, including Treg depletion techniques and interventions that alter Treg function, which may be used in different cure strategies, to simultaneously boost HIV-specific immune responses and induce reactivation of the latent virus.

Vitamin A regulates Akt signaling through the phospholipid fatty acid composition

Protein kinases, including the serine/threonine kinase Akt, mediate manifold bioactivities of vitamin A, although the mechanisms behind the sustained kinase activation are diffuse. To investigate the role of cellular lipids as targetable factors in Akt signaling, we combined mass spectrometry-based lipidomics with immunologic detection of Akt (Ser473) phosphorylation. A screening campaign revealed retinol (vitamin A alcohol) and all-trans retinoic acid (vitamin A acid) (RA) as hits that time-dependently (≥24 h) deplete phosphatidylcholine-bound polyunsaturated fatty acids (PUFA-PCs) from NIH-3T3 mouse fibroblasts while inducing Akt activation (EC₅₀ ≈ 0.1-1 µM). Other mitogenic and stress-regulated kinases were hardly affected. Organized in a coregulated phospholipid subcluster, PUFA-PCs compensated for the RA-induced loss of cellular PUFA-PCs and diminished Akt activation when supplemented. The counter-regulation of phospholipids and Akt by RA was mimicked by knockdown of lysophosphatidylcholine acyltransferase-3 or the selective retinoid X receptor (RXR) agonist bexarotene and prevented by the selective RXR antagonist Hx531. Treatment of mice with retinol decreased the tissue ratio of PUFA-PC and enhanced basal Akt activation preferentially in brain, which was attributed to astrocytes in dissociated cortical cultures. Together, our findings show that RA regulates the long-term activation of Akt by changes in the phospholipid composition.-Pein, H., Koeberle, S. C., Voelkel, M., Schneider, F., Rossi, A., Thürmer, M., Loeser, K., Sautebin, L., Morrison, H., Werz, O., Koeberle, A. Vitamin A regulates Akt signaling through the phospholipid fatty acid composition.

Mechanisms Regulating Stemness and Differentiation in Embryonal Carcinoma Cells

Just over ten years have passed since the seminal Takahashi-Yamanaka paper, and while most attention nowadays is on induced, embryonic, and cancer stem cells, much of the pioneering work arose from studies with embryonal carcinoma cells (ECCs) derived from teratocarcinomas. This original work was broad in scope, but eventually led the way for us to focus on the components involved in the gene regulation of stemness and differentiation. As the name implies, ECCs are malignant in nature, yet maintain the ability to differentiate into the 3 germ layers and extraembryonic tissues, as well as behave normally when reintroduced into a healthy blastocyst. Retinoic acid signaling has been thoroughly interrogated in ECCs, especially in the F9 and P19 murine cell models, and while we have touched on this aspect, this review purposely highlights how some key transcription factors regulate pluripotency and cell stemness prior to this signaling. Another major focus is on the epigenetic regulation of ECCs and stem cells, and, towards that end, this review closes on what we see as a new frontier in combating aging and human disease, namely, how cellular metabolism shapes the epigenetic landscape and hence the pluripotency of all stem cells.

Resistance to the atypical retinoid ST1926 in SK-N-AS cells selected the subline rAS-ST with enhanced sensitivity to ATRA mediated by not conventional mechanisms: DNA damage, G2 accumulation and late telomerase inhibition

BACKGROUND AND PURPOSE

13-cis-Retinoic acid represents a well-established clinical strategy for the management of minimal residual disease of high risk neuroblastoma (NB) patients. However, the clinical efficacy on the overall survival of these patients remains limited, addressing the issue of better understanding the molecular mechanisms and intracellular pathways mediating Retinoic Acid (RA) clinical effects.

EXPERIMENTAL APPROACH

This work investigates the mechanism underlying the sensitivity/resistance to RA in NB by taking advantage of the paired SK-N-AS/rAS-ST cells showing different responsivity to ATRA. The subline rAS-ST was selected by inducing resistance to the novel retinoid ST1926 in the NB SK-N-AS cell line.

KEY RESULTS

Resistance to ST1926 was neither dependent on cellular uptake nor on multi-drug resistance phenotype. Rather, both delayed/lower DNA damage and apoptosis appeared involved in reduced sensitivity of rAS-ST cells to ST1926. This subline showed enhanced responsivity to ATRA compared to the wt counterpart, that was associated with enhanced RARα/β expression, DNA damage, G2 accumulation, PI3K/AKT pathway inhibition, cellular differentiation and delayed telomerase inhibition, without involvement of either p27/p53 or caspase-mediated apoptosis.

CONCLUSIONS AND IMPLICATIONS

The present data add important information to the understanding of RA sensitivity in NB, providing further insights towards a more efficacious clinical use of this drug.

Role of retinoic acid and platelet-derived growth factor receptor cross talk in the regulation of neonatal gonocyte and embryonal carcinoma cell differentiation

Neonatal gonocytes are direct precursors of spermatogonial stem cells, the cell pool that supports spermatogenesis. Although unipotent in vivo, gonocytes express pluripotency genes common with embryonic stem cells. Previously, we found that all-trans retinoic acid (RA) induced the expression of differentiation markers and a truncated form of platelet-derived growth factor receptor (PDGFR)β in rat gonocytes, as well as in F9 mouse embryonal carcinoma cells, an embryonic stem cell-surrogate that expresses somatic lineage markers in response to RA. The present study is focused on identifying the signaling pathways involved in RA-induced gonocyte and F9 cell differentiation. Mitogen-activated protein kinase kinase (MEK) 1/2 activation was required during F9 cell differentiation towards somatic lineage, whereas its inhibition potentiated RA-induced Stra8 expression, suggesting that MEK1/2 acts as a lineage specification switch in F9 cells. In both cell types, RA increased the expression of the spermatogonial/premeiotic marker Stra8, which is in line with F9 cells being at a stage before somatic-germline lineage specification. Inhibiting PDGFR kinase activity reduced RA-induced Stra8 expression. Interestingly, RA increased the expression of PDGFRα variant forms in both cell types. Together, these results suggest a potential cross talk between RA and PDGFR signaling pathways in cell differentiation. RA receptor-α inhibition partially reduced RA effects on Stra8 in gonocytes, indicating that RA acts in part via RA receptor-α. RA-induced gonocyte differentiation was significantly reduced by inhibiting SRC (v-src avian sarcoma [Schmidt-Ruppin A-2] viral oncogene) and JAK2/STAT5 (Janus kinase 2/signal transducer and activator of transcription 5) activities, implying that these signaling molecules play a role in gonocyte differentiation. These results suggest that gonocyte and F9 cell differentiation is regulated via cross talk between RA and PDGFRs using different downstream pathways.

Phosphatidylinositol 3-kinase mediates the ability of retinol to decrease colorectal cancer cell invasion

Previously, we showed that retinol (vitamin A) decreased both colorectal cancer cell invasion and phosphatidylinositol 3-kinase (PI3K) activity through a retinoic acid receptor-independent mechanism. Here, we determined if these phenomena were related by using parental HCT-116 cells that harbor 1 allele of wild-type PI3K and 1 allele of constitutively active (ca) PI3K and 2 mutant HCT-116 cell lines homozygous for caPI3K. In vitro, treatment of parental HCT-116 cells with 10 μM retinol reduced cell invasion whereas treatment of mutant HCT-116 cell lines with retinol did not. Treatment with 10 μM retinol also decreased the activity of matrixmetalloproteinase-9 and increased tissue inhibitor of matrixmetalloproteinase-I levels in parental, but not mutant, HCT-116 cells. Finally, parental or mutant cells were intrasplenically injected into athymic mice consuming diets with or without supplemental vitamin A. As expected, vitamin A supplementation tended (P = 0.18) to reduce the incidence of metastases in mice injected with the parental cell line and consuming the supplemented diet. In contrast, metastatic incidence was not affected (P = 1.00) by vitamin A supplementation in mice injected with mutant cells. These data indicate that the capacity of retinol to inhibit PI3K activity confers its ability to decrease colorectal cancer metastasis.

IGF antagonizes the Wnt/β-Catenin pathway and promotes differentiation of extra-embryonic endoderm

Mouse F9 teratocarcinoma cells are an established model for the differentiation of extra-embryonic endoderm (ExEn). Primitive endoderm, parietal and visceral endoderm can be generated by stimulation of F9 cells with retinoic acid and dibutyryl cyclic adenosine monophosphate. Here we show that Wnt/β-Catenin signaling is down-regulated during ExEn differentiation in F9 cells and that the inhibition of the Wnt pathway promotes differentiation of the three extra-embryonic endoderm lineages. Wnt inhibition is achieved through the IGF pathway, which is up-regulated during differentiation. IGF signaling antagonizes the Wnt pathway by stimulating transcription of axin2 and by stabilizing Axin1 protein. Both Axin1 and Axin2 are components of the β-Catenin destruction complex and act as intra-cellular inhibitors of the Wnt/β-Catenin pathway. The data presented reveal a mechanism which restricts pluripotency of undifferentiated cells and directs them toward extra-embryonic lineages.

Hepatoma-derived growth factor: from the bovine uterus to the in vitro embryo culture

Early in cow embryo development, hepatoma-derived growth factor (HDGF) is detectable in uterine fluid. The origin of HDGF in maternal tissues is unknown, as is the effect of the induction on developing embryos. Herein, we analyze HDGF expression in day 8 endometrium exposed to embryos, as well as the effects of recombinant HDGF (rHDGF) on embryo growth. Exposure to embryos did not alter endometrial levels of HDGF mRNA or protein. HDGF protein localized to cell nuclei in the luminal epithelium and superficial glands and to the apical cytoplasm in deep glands. After uterine passage, levels of embryonic HDGF mRNA decreased and HDGF protein was detected only in the trophectoderm. In fetal fibroblast cultures, addition of rHDGF promoted cell proliferation. In experiments with group cultures of morulae in protein-free medium containing polyvinyl alcohol, adding rHDGF inhibited blastocyst development and did not affect cell counts when the morulae were early (day 5), whereas it enhanced blastocyst development and increased cell counts when the morulae were compact (day 6). In cultures of individual day 6 morulae, adding rHDGF promoted blastocyst development and increased cell counts. Our experiments with rHDGF indicate that the growth factor stimulates embryonic development and cell proliferation. HDGF is synthesized similarly by the endometrium and embryo, and it may exert embryotropic effects by autocrine and/or paracrine mechanisms.

PI3K/Akt pathway regulates retinoic acid-induced Hox gene expression in F9 cells

Retinoic acid (RA), the most potent natural form of vitamin A, is a key morphogen in vertebrate development and a potent regulator of both adult and embryonic cell differentiation. Specifically, RA regulates clustered Hox gene expression during embryogenesis and is required to establish the anteroposterior body plan. The PI3K/Akt pathway was also reported to play an essential role in the process of RA-induced cell differentiation. Therefore, we tested whether the PI3K/Akt pathway is involved in RA-induced Hox gene expression in a F9 murine embryonic teratocarcinoma cells. To examine the effect of PI3K/Akt signaling on RA-induced initiation of collinear expression of Hox genes, F9 cells were treated with RA in the presence or absence of PI3K inhibitor LY294002, and time-course gene expression profiles for all 39 Hox genes located in four different clusters-Hoxa, Hoxb, Hoxc, and Hoxd-were analyzed. Collinear expression of Hoxa and -b cluster genes was initiated earlier than that of the -c and -d clusters upon RA treatment. When LY294002 was applied along with RA, collinear expression induced by RA was delayed, suggesting that the PI3K/Akt signaling pathway somehow regulates RA-induced collinear expression of Hox genes in F9 cells. The initiation of Hox collinear expression by RA and the delayed expression following LY294002 in F9 cells would provide a good model system to decipher the yet to be answered de novo collinear expression of Hox genes during gastrulation, which make the gastrulating cells to remember their positional address along the AP body axis in the developing embryo.

PI3K-AKT signaling is a downstream effector of retinoid prevention of murine basal cell carcinogenesis

Basal cell carcinoma (BCC) is the most common human cancer. We have demonstrated previously that topical application of the retinoid prodrug tazarotene profoundly inhibits murine BCC carcinogenesis via retinoic acid receptor γ-mediated regulation of tumor cell transcription. Because topical retinoids can cause adverse cutaneous effects and because tumors can develop resistance to retinoids, we have investigated mechanisms downstream of tazarotene's antitumor effect in this model. Specifically we have used (i) global expression profiling to identify and (ii) functional cell-based assays to validate the phosphoinositide 3-kinase (PI3K)/AKT/mTOR pathway as a downstream target pathway of tazarotene's action. Crucially, we have demonstrated that pharmacologic inhibition of this downstream pathway profoundly reduces murine BCC cell proliferation and tumorigenesis both in vitro and in vivo. These data identify PI3K/AKT/mTOR signaling as a highly attractive target for BCC chemoprevention and indicate more generally that this pathway may be, in some contexts, an important mediator of retinoid anticancer effects.

Akt-signal integration is involved in the differentiation of embryonal carcinoma cells

The mechanism by which Akt modulates stem cell homeostasis is still incompletely defined. Here we demonstrate that Akt phosphorylates special AT-rich sequences binding protein 1 (SATB1) at serine 47 and protects SATB1 from apoptotic cleavage. Meanwhile, Akt phosphorylates Oct4 at threonine 228 and Klf4 at threonine 399, and accelerates their degradation. Moreover, PI3K/Akt signaling enhances the binding of SATB1 to Sox2, thereby probably impairing the formation of Oct4/Sox2 regulatory complexes. During retinoic acid (RA)-induced differentiation of mouse F9 embryonal carcinoma cells (ECCs), the Akt activation profile as well as its substrate spectrum is strikingly correlated with the down-regulation of Oct4, Klf4 and Nanog, which suggests Akt activation is coupled to the onset of differentiation. Accordingly, Akt-mediated phosphorylation is crucial for the capability of SATB1 to repress Nanog expression and to activate transcription of Bcl2 and Nestin genes. Taken together, we conclude that Akt is involved in the differentiation of ECCs through coordinated phosphorylations of pluripotency/differentiation factors.

Retinoids and nuclear retinoid receptors in white and brown adipose tissues: physiopathologic aspects

Vitamin A, ingested either as retinol or β-carotene from animal- or plant-derived foods respectively, is a nutrient essential for many biological functions such as embryonic development, vision, immune response, tissue remodeling, and metabolism. Its main active metabolite is all

The phosphoinositide-3-kinase/Akt pathway mediates the transient increase in Nanog expression during differentiation of F9 cells

Nanog is a key determinant that maintains self-renewal and pluripotency of embryonic stem cells and represses their differentiation to endoderm. In this study, we examined the regulation of Nanog expression by phosphoinositide-3-kinase (PI3K)/Akt pathway during retinoic acid (RA)-induced differentiation of F9 embryonic carcinoma cells. Nanog protein expression was transiently upregulated up to 6 h after RA treatment and then declined. In agreement, a murine Nanog promoter reporter assay revealed that promoter activity increased during early stage of differentiation, but decreased when F9 cells became fully differentiated. RA treatment of F9 cells also led to a transient and parallel increase in both Akt and glycogen synthase kinase 3beta phosphorylations. Nanog expression was diminished in the early stage by LY294002, a PI3K inhibitor, but was not affected in the late stage despite considerable inhibition of Akt phosphorylation and endoderm marker expression by the inhibitor. These data suggest that RA-induced PI3K/Akt activation in the early stage of differentiation is required for Nanog expression, which becomes independent of PI3K/Akt signaling once the differentiation is established. Thus, Nanog expression appears to be differently regulated by the PI3K/Akt pathway depending on differentiation stage.

Retinoic acid-induced nNOS expression depends on a novel PI3K/Akt/DAX1 pathway in human TGW-nu-I neuroblastoma cells

Neuronal nitric oxide synthase (nNOS)-derived nitric oxide (NO) acts as a neurotransmitter and intracellular signaling molecule in the central and peripheral nervous system. NO regulates multiple processes like neuronal development, plasticity, and differentiation and is a mediator of neurotoxicity. The nNOS gene is highly complex with 12 alternative first exons, exon 1a-1l, transcribed from distinct promoters, leading to nNOS variants with different 5'-untranslated regions. Transcriptional control of the nNOS gene is not understood in detail. To investigate regulation of nNOS gene expression by retinoic acid (RA), we used the human neuroblastoma cell line TGW-nu-I as a model system. We show that RA induces nNOS transcription in a protein synthesis-dependent fashion. We identify the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway and the atypical orphan nuclear receptor DAX1 (NR0B1) as critical mediators involved in RA-induced nNOS gene transcription. RA treatment increases DAX1 expression via PI3K/Akt signaling. Upregulation of DAX1 expression in turn induces nNOS transcription in response to RA. These results identify nNOS as a target gene of a novel RA/PI3K/Akt/DAX1-dependent pathway in human neuroblastoma cells and stress the functional importance of the transcriptional regulator DAX1 for nNOS gene expression in response to RA treatment.

Cdx genes, inflammation and the pathogenesis of Barrett's metaplasia

Metaplasia is the conversion of one cell or tissue type to another and can predispose patients to neoplasia. Perhaps one of the best-known examples of metaplasia is Barrett's metaplasia (BM), a pathological condition in which the distal oesophageal epithelium switches from stratified squamous to intestinal-type columnar epithelium. BM predisposes to oesophageal adenocarcinoma and is the consequence of long-term acid bile reflux. The incidence of BM and oesophageal adenocarcinoma has risen dramatically in recent years. A key event in the pathogenesis of BM is the induction of oesophageal CDX2 expression. Importantly, recent data reveal the molecular mechanisms that link inflammation in the development of Barrett's metaplasia, CDX2 and the progression to cancer. This review highlights the relationship between inflammation, metaplasia and carcinogenesis.

Dynamic and combinatorial control of gene expression by nuclear retinoic acid receptors (RARs)

Nuclear retinoic acid receptors (RARs) are transcriptional regulators controlling the expression of specific subsets of genes in a ligand-dependent manner. The basic mechanism for switching on transcription of cognate target genes involves RAR binding at specific response elements and a network of interactions with coregulatory protein complexes, the assembly of which is directed by the C-terminal ligand-binding domain of RARs. In addition to this scenario, new roles for the N-terminal domain and the ubiquitin-proteasome system recently emerged. Moreover, the functions of RARs are not limited to the regulation of cognate target genes, as they can transrepress other gene pathways. Finally, RARs are also involved in nongenomic biological activities such as the activation of translation and of kinase cascades. Here we will review these mechanisms, focusing on how kinase signaling and the proteasome pathway cooperate to influence the dynamics of RAR transcriptional activity.

Synaptojanin-1 plays a key role in astrogliogenesis: possible relevance for Down's syndrome

There is increasing interest in gliogenesis as the relevance of glia to both brain development and pathology becomes better understood. However, little is known about this process. The use of multidimensional protein identification technology (MudPIT) to identify changes in phosphoprotein levels in rat neural precursor cells treated with cytokines or retinoic acid showed that phosphorylation of the catalytic subunit of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K p110alpha) and dephosphorylation of the inositol phosphatase synaptojanin-1 were common to the gliogenic stimuli. Although PI3K was found to be involved in both neuro- and astrogliogenesis, synaptojanin-1 was specifically involved in astrogliogenesis of neural precursor cells. The role of synaptojanin-1 in astrogliogenesis was further confirmed by analysis of neuron- and glia-specific markers in synaptojanin-1 knockout mouse brain. Additional experiments showed that the Sac1-like phosphatase domain of synaptojanin-1 is responsible for the observed astrogliogenic effect. Our results strongly indicate that phosphatidylinositol metabolism plays a key role in astrogliogenesis. The relevance of our findings for Down's syndrome pathology is discussed.

Retinol decreases phosphatidylinositol 3-kinase activity in colon cancer cells

Previously, we showed that retinol inhibited all-trans-retinoic acid (ATRA)-resistant human colon cancer cell invasion via a retinoic acid receptor-independent mechanism. Because phosphatidylinositol 3-kinase (PI3K) regulates cell invasion, the objective of the current study was to determine if retinol affected PI3K activity. Following 24 h of serum starvation, the ATRA resistant human colon cancer cell lines HCT-116 and SW620 were treated with 0, 1, or 10 microM retinol. Thirty minutes of retinol treatment resulted in a significant decrease in PI3K activity in both cell lines. To determine the mechanism by which retinol reduces PI3K activity, the levels and heterodimerization of the regulatory subunit, p85, and the catalytic subunit, p110, of PI3K were examined. Retinol treatment did not alter p85 or p110 protein levels or the heterodimerization of these subunits at any time point examined. To determine if retinol affected the ability of PI3K to phosphorylate the substrate, phosphatidylinositol (PI), PI3K was immunoprecipitated from control cells and incubated with 10 microg PI and increasing concentrations of retinol or 10 microg retinol and increasing concentrations of PI. Retinol decreased PI3K activity in a dose-responsive manner and increased PI suppressed the inhibitory effect of retinol on PI3K activity. Finally, the PI3K inhibitor, LY294002, mimicked the ability of retinol to decrease cell invasion. Computational modeling revealed that retinol may inhibit PI3K activity in a manner similar to that of wortmannin. Thus, a decrease in PI3K activity due to retinol treatment may confer the ability of retinol to inhibit ATRA-resistant colon cancer cell invasion.

Gene expression profiling elucidates a specific role for RARgamma in the retinoic acid-induced differentiation of F9 teratocarcinoma stem cells

The biological effects of all-trans-retinoic acid (RA), a major active metabolite of retinol, are mainly mediated through its interactions with retinoic acid receptor (RARs alpha, beta, gamma) and retinoid X receptor (RXRs alpha, beta, gamma) heterodimers. RAR/RXR heterodimers activate transcription by binding to RA-response elements (RAREs or RXREs) in the promoters of primary target genes. Murine F9 teratocarcinoma stem cells have been widely used as a model for cellular differentiation and RA signaling during embryonic development. We identified and characterized genes that are differentially expressed in F9 wild type (Wt) and F9 RARgamma-/- cells, with and without RA treatment, through the use of oligonucleotide-based microarrays. Our data indicate that RARgamma, in the absence of exogenous RA, modulates gene expression. Genes such as Sfrp2, Tie1, Fbp2, Emp1, and Emp3 exhibited higher transcript levels in RA-treated Wt, RARalpha-/- and RARbeta2-/- lines than in RA-treated RARgamma-/- cells, and represent specific RARgamma targets. Other genes, such as Runx1, were expressed at lower levels in both F9 RARbeta2-/- and RARgamma-/- cell lines than in F9 Wt and RARalpha-/-. Genes specifically induced by RA at 6h with the protein synthesis inhibitor cycloheximide in F9 Wt, but not in RARgamma-/- cells, included Hoxa3, Hoxa5, Gas1, Cyp26a1, Sfrp2, Fbp2, and Emp1. These genes represent specific primary RARgamma targets in F9 cells. Several genes in the Wnt signaling pathway were regulated by RARgamma. Delineation of the receptor-specific actions of RA with respect to cell proliferation and differentiation should result in more effective therapies with this drug.

Role of the translational repressor 4E-BP1 in the regulation of p21(Waf1/Cip1) expression by retinoids

The mechanisms by which retinoids regulate initiation of mRNA translation for proteins that mediate their biological effects are not known. We have previously shown that all-trans-retinoic acid (ATRA) induces mTOR-mediated activation of the p70 S6 kinase, suggesting the existence of a mechanism by which retinoids may regulate mRNA translation. We now demonstrate that treatment of acute promyelocytic leukemia (APL)-derived NB4 cells with ATRA results in dissociation of the translational repressor 4E-BP1 from the eukaryotic initiation factor eIF4E, and subsequent formation of eIF4G-eIF4E complexes. We also show that siRNA-mediated inhibition of 4E-BP1 expression enhances ATRA-dependent upregulation of p21(Waf1/Cip1), a protein that plays a key role in the induction of retinoid-dependent responses. Our data also establish that ATRA- or cis-RA-dependent p21(Waf1/Cip1) protein expression is enhanced in mouse embryonic fibroblasts with targeted disruption of the 4e-bp1 gene, in the absence of any effects on the transcriptional regulation of the p21(Waf1/Cip1) gene. Moreover, generation of ATRA- or cis-retinoic acid (cis-RA)-antiproliferative responses is enhanced in 4E-BP1 knockout cells. Altogether, these findings strongly suggest a key regulatory role for the translational repressor 4E-BP1 in the generation of retinoid-dependent functional responses.

Activated Kras, but not Hras or Nras, may initiate tumors of endodermal origin via stem cell expansion

The three closely related human Ras genes, Hras, Nras, and Kras, are all widely expressed, engage a common set of downstream effectors, and can each exhibit oncogenic activity. However, the vast majority of activating Ras mutations in human tumors involve Kras. Moreover, Kras mutations are most frequently seen in tumors of endodermally derived tissues (lung, pancreas, and colon), suggesting that activated Kras may affect an endodermal progenitor to initiate oncogenesis. Using a culture model of retinoic acid (RA)-induced stem cell differentiation to endoderm, we determined that while activated HrasV12 promotes differentiation and growth arrest in these endodermal progenitors, KrasV12 promotes their proliferation. Furthermore, KrasV12-expressing endodermal progenitors fail to differentiate upon RA treatment and continue to proliferate and maintain stem cell characteristics. NrasV12 neither promotes nor prevents differentiation. A structure-function analysis demonstrated that these distinct effects of the Ras isoforms involve their variable C-terminal domains, implicating compartmentalized signaling, and revealed a requirement for several established Ras effectors. These findings indicate that activated Ras isoforms exert profoundly different effects on endodermal progenitors and that mutant Kras may initiate tumorigenesis by expanding a susceptible stem/progenitor cell population. These results potentially explain the high frequency of Kras mutations in tumors of endodermal origin.

Rapid, nongenomic actions of retinoic acid on phosphatidylinositol-3-kinase signaling pathway mediated by the retinoic acid receptor

Retinoic acid (RA) treatment of SH-SY5Y neuroblastoma cells results in activation of phosphatidylinositol-3-kinase (PI3K) signaling pathway, and this activation is required for RA-induced differentiation. Here we show that RA activates PI3K and ERK1/2 MAPK signaling pathways through a rapid, nongenomic mechanism that does not require new gene transcription or newly synthesized proteins. Activation of PI3K by RA appears to involve the classical nuclear receptor, retinoic acid receptor (RAR), on the basis of the pharmacological profile of the activation, loss, and gain of function experiments with mouse embryo fibroblast-RAR(alpha beta gamma)(L-/L-) null cells, and the physical association between liganded RAR and PI3K activity. The association of RAR with the two subunits of PI3K was differentially regulated by the ligand. Immunoprecipitation experiments performed in SH-SY5Y cells showed stable association between RARalpha and p85, the regulatory subunit of PI3K, independently of the presence of RA. In contrast, ligand administration increased the association of p110, the catalytic subunit of PI3K, to this complex. The intracellular localization of RAR proved to be relevant for PI3K activation. A chimerical RAR fusing c-Src myristylation domain to the N terminus of RARalpha (Myr-RARalpha) was targeted to plasma membrane. Transfection of Myr-RARalpha to mouse embryo fibroblast-RAR(alpha beta gamma)(L-/L-) null cells and COS-7 cells results in strong activation of the PI3K signaling pathway, although both in the absence as well in the presence of RA. Our results support a mechanism in which ligand binding to RAR would play a major role in the assembly and intracellular location of a signaling complex involving RAR and the subunits of PI3K.

Identification and distribution of a novel platelet-derived growth factor receptor beta variant: effect of retinoic acid and involvement in cell differentiation

We have shown previously that neonatal testicular gonocytes express platelet-derived growth factor receptors (PDGFR) alpha and beta. We report the expression of a novel PDGFRbeta (V1-PDGFRbeta) transcript in gonocytes of 3-d-old rat testes. V1-PDGFRbeta nucleotide sequence spans from intron 6 to exon 23 of the PDGFRbeta gene, and is predicted to encode a protein lacking part of the extracellular domain. V1-PDGFRbeta transcripts are expressed preferentially in developing gonads. The embryonic teratocarcinoma F9 cells, in which differentiation is driven by retinoic acid (RA), express V1-PDGFRbeta, but not wild-type PDGFRbeta. Green fluorescent protein-tagged V1-PDGFRbeta localized mainly in cytosol of F9, MA-10, and COS-1 cells. FLAG and green fluorescent protein-tagged V1-PDGFRbeta displayed tyrosine kinase activities and contain phosphotyrosine residues, suggesting that V1-PDGFRbeta is a cytosolic tyrosine kinase. Treatment of F9 cells with RA induced V1-PDGFRbeta gene expression, concomitant with changes in morphology and increased mRNA expression of collagen IV and laminin B1, suggesting that V1-PFGRbeta is involved in cell differentiation. Similarly, treatment of postnatal d 3 rat gonocytes with RA induced a dose-dependent increase in V1-PDGFRbeta expression together with an increase in c-kit and Stra8, markers of more differentiated germ cells and a concomitant decrease in GFRalpha1, a marker of spermatogonial stem cells. However, an excess of V1-PDGFRbeta inhibited RA-mediated collagen IV and laminin B1 expression and altered both RA-dependent and RA-independent morphological changes in F9 cells, while increasing cell survival. These results suggest that the expression of V1-PDGFRbeta is tightly regulated during differentiation and that it may play an active role in germ cell differentiation.

International Union of Pharmacology. LX. Retinoic acid receptors

Retinoid is a term for compounds that bind to and activate retinoic acid receptors (RARalpha, RARbeta, and RARgamma), members of the nuclear hormone receptor superfamily. The most important endogenous retinoid is all-trans-retinoic acid. Retinoids regulate a wide variety of essential biological processes, such as vertebrate embryonic morphogenesis and organogenesis, cell growth arrest, differentiation and apoptosis, and homeostasis, as well as their disorders. This review summarizes the considerable amount of knowledge generated on these receptors.

Cytodifferentiation by retinoids, a novel therapeutic option in oncology: rational combinations with other therapeutic agents

Retinoic acid (RA) and derivatives are promising antineoplastic agents endowed with both therapeutic and chemopreventive potential. Although the treatment of acute promyelocytic leukemia with all-trans retinoic acid is an outstanding example, the full potential of retinoids in oncology has not yet been explored and a more generalized use of these compounds is not yet a reality. One way to enhance the therapeutic and chemopreventive activity of RA and derivatives is to identify rational combinations between these compounds and other pharmacological agents. This is now possible given the information available on the biochemical and molecular mechanisms underlying the biological activity of retinoids. At the cellular level, the antileukemia and anticancer activity of retinoids is the result of three main actions, cytodifferentiation, growth inhibition, and apoptosis. Cytodifferentiation is a particularly attractive modality of treatment and differentiating agents promise to be less toxic and more specific than conventional chemotherapy. This is the result of the fact that cytotoxicity is not the primary aim of differentiation therapy. At the molecular level, retinoids act through the activation of nuclear retinoic acid receptor-dependent and -independent pathways. The cellular pathways and molecular networks relevant for retinoid activity are modulated by a panoply of other intracellular and extracellular pathways that may be targeted by known drugs and other experimental therapeutics. This chapter aims to summarize and critically discuss the available knowledge in the field.