Retinoic acid modulation of transmembrane signaling. Analysis in F9 teratocarcinoma cells

Effect of retinoic acid on cell proliferation and differentiation as well as on lipid synthesis, lipoprotein secretion, and apolipoprotein biogenesis

Dietary vitamin A and its active metabolites are essential nutrients for many functions as well as potent regulators of gene transcription and growth. Although the epithelium of the small intestine is characterized by rapid and constant renewal and enterocytes play a central role in the absorption and metabolism of alimentary retinol, very little is known about the function of retinoids in the human gastrointestinal epithelium and mechanisms by which programs engage the cell cycle are poorly understood. We have assessed the effects of 10 microM 9- and 13-cis-retinoic acid (RA) on proliferation and differentiation processes, lipid esterification, apolipoprotein (apo) biogenesis and lipoprotein secretion along with nuclear factor gene transcription. Treatment of Caco-2 cells with RA at different concentrations and incubation periods revealed the reduction of thymidine incorporation in 60% preconfluent or 100% confluent cells. Concomitantly, RA 1) modulated D-type cyclins by reducing the mitogen-sensitive cyclin D1 and upregulating cyclin D3 expressions and 2) caused a trend of increase in p38 MAPK, which triggers CDX2, a central protein in cell differentiation. RA remained without effect on lipoprotein output and apo synthesis, even for apo A-I that possesses RARE in its promoter. RA, in combination with 22-hydroxycholesterol, could induce apo A-I gene expression without any impact on apo A-I mass. Only the gene expression of peroxisome proliferator-activated receptor (PPAR)beta, retinoic receptor (RAR)beta, and RARgamma was augmented and no alteration was noted in PPARalpha, PPARgamma, liver X receptor (LXR)alpha, LXRbeta, and retinoid X receptors. Taken together, these data highlight RA-induced cell differentiation via specific signaling without a significant impact on apo A-I synthesis.

Role of G proteins and ERK activation in hemin-induced erythroid differentiation of K562 cells

Heterotrimeric G proteins which couple extracellular signals to intracellular effectors play a central role in cell growth and differentiation. The pluripotent erythroleukemic cell line K562 that acquires the capability to synthesize hemoglobin in response to a variety of agents can be used as a model system for erythroid differentiation. Using Western blot analysis and RT-PCR, we studied alterations in G protein expression accompanying hemin-induced differentiation of K562 cells. We demonstrated the presence of G(alpha s), G(alpha i2) and G(alpha q) and the absence of G(alpha i1), G(alpha o) and G(alpha 16) in K562 cells. We observed the short form of G(alpha s) to be expressed predominantly in these cells. Treatment of K562 cells with hemin resulted in an increase in the levels of G(alpha s) and G(alpha q). On the other hand, the level of G(alpha i2) was found to increase on the third day after induction with hemin, followed by a decrease to levels lower of those of uninduced cells. The mitogen-activated protein kinase ERK1/2 pathway is crucial in the control of cell proliferation and differentiation. Both Gi- and Gq-coupled receptors stimulate MAPK activation. We therefore examined the phosphorylation of ERK1/2 during hemin-induced differentiation of K562 cells. Using anti-ERK1/2 antibodies, we observed that ERK2 was primarily phosphorylated in K562 cells. ERK2 phosphorylation increased gradually until 48 h and returned to basal values by 96 h following hemin treatment. Our results suggest that changes in G protein expression and ERK2 activity are involved in hemin-induced differentiation of K562 cells.

Activation of the beta-catenin/Lef-Tcf pathway is obligate for formation of primitive endoderm by mouse F9 totipotent teratocarcinoma cells in response to retinoic acid

The morphogen retinoic acid promotes the formation of primitive endoderm in mouse F9 teratocarcinoma cells as does the stimulation of the Frizzled-1 pathway. We investigated whether the beta-catenin/Lef-Tcf-sensitive transcriptional pathway activated by Frizzled-1 plays a role in the retinoic acid-induced pathway to primitive endoderm formation. An analysis of Lef-Tcf-sensitive transcription reveals increased transcription at 1 and 4 h post-treatment with retinoic acid. The stimulation of Lef-Tcf-sensitive transcription as well as the formation of primitive endoderm was accompanied by the stabilization of beta-catenin as observed in activation of the Frizzled-1 pathway. Transient transfection of F9 cells with an expression vector harboring a dominant-negative mutant of Tcf4 resulted in the attenuation of both the increase in Lef-Tcf-sensitive transcription and formation of primitive endoderm in response to the morphogen. Clones stably transfected to express the dominant-negative Tcf4 displayed a block in retinoic acid-induced activation of Lef-Tcf-sensitive transcription and primitive endoderm formation. These data reveal the obligate role of the beta-catenin/Lef-Tcf transcriptional pathway in the action of the morphogen retinoic acid.

Activation of a frizzled-2/beta-adrenergic receptor chimera promotes Wnt signaling and differentiation of mouse F9 teratocarcinoma cells via Galphao and Galphat

The frizzled gene family of putative Wnt receptors encodes proteins that have a seven-transmembrane-spanning motif characteristic of G protein-linked receptors, though no loss-of-function studies have demonstrated a requirement for G proteins for Frizzled signaling. We engineered a Frizzled-2 chimera responsive to beta-adrenergic agonist by using the ligand-binding domains of the beta(2)-adrenergic receptor. The expectation was that the chimera would be sensitive both to drug-mediated activation and blockade, thereby circumventing the problem of purifying soluble and active Wnt ligand to activate Frizzled. Expression of the chimera in zebrafish embryos demonstrated isoproterenol (ISO)-stimulated, propranolol-sensitive calcium transients, thereby confirming the beta-adrenergic nature of Wnt signaling by the chimeric receptor. Because F9 embryonic teratocarcinoma cells form primitive endoderm after stable transfection of Frizzled-2 chimera and stimulation with ISO, they were subject to depletion of G protein subunits. ISO stimulation of endoderm formation of F9 stem cells expressing the chimeric receptor was blocked by pertussis toxin and by oligodeoxynucleotide antisense to Galphao, Galphat2, and Gbeta2. Our results demonstrate the requirement of two pertussis toxin-sensitive G proteins, Galphao and Galphat, for signaling by the Frizzled-2 receptor.

Transcriptional regulation of the transforming growth factor-beta2 promoter by cAMP-responsive element-binding protein (CREB) and activating transcription factor-1 (ATF-1) is modulated by protein kinases and the coactivators p300 and CREB-binding protein

Transcription of the transforming growth factor-beta2 (TGF-beta2) gene is dependent on a cAMP-response element/activating transcription factor (CRE/ATF) site that is bound by CREB and ATF-1 as well as an E-box motif that is bound by upstream stimulatory factors 1 and 2 (USF1 and USF2). To identify additional factors involved in the expression of the TGF-beta2 gene, we employed F9 embryonal carcinoma (EC) cells, which express TGF-beta2 only after the cells differentiate. We show that overexpression of the transcription factors, CREB, ATF-1, USF1, and USF2 dramatically increases TGF-beta2 promoter activity in F9-differentiated cells. We further show that the coactivators p300 and CBP up-regulate the TGF-beta2 promoter when CREB and ATF-1 are expressed in conjunction with protein kinases that phosphorylate CREB on serine 133 and ATF-1 on serine 63. Importantly, we identify the presence of serine 133-phosphorylated CREB in the nucleus of F9-differentiated cells but not in the nucleus of F9 EC cells. This phosphorylated form is present in whole cell extracts of both the parental and differentiated cells, suggesting that nuclear accumulation of serine 133-phosphorylated CREB is regulated during differentiation of F9 EC cells and is likely to play an important role in the activation of the TGF-beta2 gene.

Activation of rat frizzled-1 promotes Wnt signaling and differentiation of mouse F9 teratocarcinoma cells via pathways that require Galpha(q) and Galpha(o) function

The frizzled gene family of putative Wnt receptors encodes proteins that have a seven transmembrane-spanning motif characteristic of G-protein-linked receptors, although no loss-of-function studies have demonstrated a requirement for G-proteins for Wnt signaling by the gene product of frizzled-1. Medium conditioned by mouse F9 teratocarcinoma stem cells stably transfected to express either Xenopus Wnt-5a or Wnt-8 was used to test primitive endoderm formation of F9 stem cells. F9 stem cells expressing the rat Frizzled-1 receptors demonstrated endoderm formation in response to conditioned medium containing Wnt-8 but not to medium containing Wnt-5a. Primitive endoderm formation stimulated by Wnt-8 acting on the rat Frizzled-1 receptor was blocked by treatment with pertussis toxin by depletion of either Galpha(o) or Galpha(q) via antisense oligodeoxynucleotides, as well as by inhibitors of protein kinase C (bisindoylmaleimide) and of mitogen-activated protein kinase kinase (PD98059). Our results demonstrate the requirement for G-protein subunits Galpha(o) (a pertussis toxin substrate) and Galpha(q) for signaling by Frizzled-1, and an obligate role for the protein kinase C (likely mediated through stimulation of Galpha(q)) and mitogen-activated protein kinase network at the level of mitogen-activated protein kinase kinase.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Molecular cloning and characterization of a novel retinoic acid-inducible gene that encodes a putative G protein-coupled receptor

The effects of retinoids such as all-trans-retinoic acid (ATRA) on cell growth, differentiation, and apoptosis are thought to be mediated by nuclear retinoid receptors, which are involved in ligand-dependent transcriptional activation of target genes. Using differential display, we identified the cDNA of a novel gene, designated retinoic acid-inducible gene 1 (RAIG1), which was induced by ATRA in the squamous carcinoma cell line UMSCC-22B. Two RAIG1 transcripts of 2.4 and 6.8 kilobase pairs, respectively, have the same ORF that encodes a 357-amino acid polypeptide. RAIG1 mRNA is expressed at high level in fetal and adult lung tissues. Induction of RAIG1 expression by ATRA is rapid (within 2 h) and dose-dependent in the range between 1 nM to 1 microM. The constitutive RAIG1 mRNA levels, which were low in three of five head and neck and four of six lung cancer cell lines, increased after ATRA treatment in most cell lines. The deduced RAIG1 protein sequence contains seven transmembrane domains, characteristic of G protein-coupled receptors. A fusion protein of RAIG1 and the green fluorescent protein was localized in the cell surface membrane and perinuclear vesicles in transiently transfected cells. RAIG1 was mapped to chromosome 12p12. 3-p13. Our results provide novel evidence for a possible interaction between retinoid and G protein signaling pathways.

Retinoic acid treatment induces apoptosis or expression of a more differentiated phenotype on different fractions of cultured fetal rat hepatocytes

The present study reports the effects of retinoic acid (RA) on cultured fetal rat hepatocytes. We show that RA treatment induces both differentiation and apoptosis. Hepatocytes cultured for 48 hours in the presence of 5 microl/L RA form junctional complexes in the areas of contact between neighboring cells and develop bile canaliculi, typical features of mature and well-differentiated cells. At the same time, about 20% of cells are induced to die by apoptosis, and the percentage of apoptotic cells increases according to the concentration of RA used and the duration of treatment. The induction of apoptosis, studied at the morphological and biochemical levels, revealed that, in our system, the classical compaction of chromatin occurs only during the final stages of the process; instead of the common marker of apoptosis, i.e., the "DNA ladder" pattern of fragmentation, megabase-sized fragments were found. These observations provide further evidence of the existence of fundamental differences in the mechanisms of apoptosis among cell types. To investigate the molecular mechanism of the effects of RA, we evaluated the expression of two proteins, c-myc and p53, which are known to be involved in both cell differentiation and apoptosis. The data obtained show that the amount of p53 remained unchanged after RA treatment. On the contrary, a dose-dependent reduction in c-myc levels was found, suggesting that RA action may be mediated by modulation of this oncogene. Our findings regarding the apoptosis-inducing effect of RA, which was not found in adult hepatocytes, suggest a possible relationship between this phenomenon and the proliferative capacity and/or differentiation state of hepatocytes.

Dietary factors in women with dysplasia colli uteri associated with human papillomavirus infection

The studies were carried out in a group of 228 female patients with normal cytological smear and 324 patients with cervical intraepithelial neoplasia (CIN). The applied method of human papillomavirus (HPV) identification, i.e., the HPV digene hybrid capture system, made it possible to select a control group consisting of 168 HPV-negative patients with normal Papanicolaou smears, as well as a group of 228 HPV-positive female patients with CIN. By examining the diet of the patients, it was shown that women with cervical dysplasia associated with high oncogenic risk of HPV infections consumed a smaller quantity of foods containing vitamin C, beta-carotene, and folacin. Our studies indicate a relationship between nutritional habits and the development of CIN associated with HPV infection.

Identification of a retinoic acid response domain involved in the activation of the beta 1-adrenergic receptor gene by retinoic acid in F9 teratocarcinoma cells

The density of beta 1-adrenergic receptors (beta 1-AR) is up-regulated upon differentiation of embryonic F9 teratocarcinoma cells by retinoic acid (RA) to the primitive endodermal phenotype. To identify the domains involved in RA-mediated activation of beta 1-AR gene transcription, three kb of 5'-flanking sequence of the beta 1-AR gene were ligated to a luciferase reporter gene and transiently transfected into F9 cells that were pre-exposed to 100 nM RA for 2 days. By generating deletions in the beta 1-AR promoter, a region between -125 and -100 was found to mediate a 3-fold induction in cells exposed to RA for an additional 2 days. Through site-directed mutagenesis of this region, it was determined that the RA responsive element (RARE) was organized as a direct repeat separated by 5 nucleotides in which the 5'-most AGGTCG half-site was between nucleotides -106 and -101 and the 3'-most AGGTCA half-site was between nucleotides -117 and -112. The RA receptor alpha (RAR alpha) isoform bound to the oligomer representing the sequences between -125 and -100 as a heterodimer complex with the retinoid X receptor alpha (RXR alpha). In a separate study, it was determined that the nucleotides between -125 and -100 are involved in thyroid hormone-mediated activation of the beta 1-AR gene in ventricular myocytes. Therefore, transcriptional activation of the beta 1-AR gene by thyroid hormone or RA involves a single binding site in the promoter.

Heterotrimeric G-proteins and development

Heterotrimeric G-proteins are well-known transducers of signaling from a populous class of heptihelical, membrane receptors to a smaller group of effector molecules that includes adenylylcyclases, cyclic GMP phosphodiesterases, phospholipases (type C beta), and various ion channels. Dramatic changes in specific G-protein subunits that coincide with commitment to highly-specialized cell types suggest a key role for these extrinsic membrane proteins in cell differentiation and development. Through analysis of the effects of gain-of-function and loss-of-function mutants, it has been possible to explore this new dimension in G-protein biology, intimately linking specific G-proteins to development. G-protein subunits are shown to be important molecular switches in the complex biological processes controlling both cellular differentiation and early development.

Differentiation of F9 teratocarcinoma stem cells to primitive endoderm is regulated by the Gialpha2/Gsalpha axis via phospholipase C and not adenylylcyclase

Morphogen-induced decline in Gialpha triggers F9 teratocarcinoma stem cells to progress to primitive endoderm via activation of protein kinase C and mitogen-activated protein kinase (Gao, P., and Malbon, C. C. (1996) J. Biol. Chem. 271, 9002-9008). Constitutive expression of Gialpha2 blocks, whereas expression of Gsalpha provokes, progression to primitive endoderm, permitting identification of the effectors of the response-utilizing chimera created between Gialpha2 and Gsalpha. N-terminal substitution of Gsalpha with Gialpha2 sequence to create chimera Gialpha2 (1-54)/Gsalpha produced a chimera that activated adenylylcyclase but abolished progression to primitive endoderm and activation of phospholipase C. C-terminal substitution of Gsalpha with Gialpha2 sequence to Gsalpha/Gialpha2 (320-355) enhanced the ability of Gsalpha to promote progression. The Q205L-activated mutant of Gialpha2 suppresses, whereas the G225T-activated mutant of Gsalpha strongly activates phospholipase C and progression in these cells. The N-terminal region of Gsalpha (residues 62-86) appears to act as a dominant switch for the Gsalpha- (activation) versus Gialpha2- (suppression) mediated control of phospholipase C and progression to primitive endoderm.

Repression of adipogenesis by adenylyl cyclase stimulatory G-protein alpha subunit is expressed within region 146-220

The heterotrimeric G-protein alpha subunit stimulatory with respect to adenylyl cyclase (Gsalpha) represses adipogenesis of 3T3-L1 mouse embryonic fibroblasts. Derepression occurs in response to inducers, to oligodeoxynucleotides antisense to Gsalpha, and to overexpression of heterotrimeric G-protein alpha subunit 2, inhibitory with respect to adenylyl cyclase (Gialpha2). Constitutive expression of Gsalpha blocks adipogenesis and was exploited as an assay, in which chimeras of Gialpha2 and Gsalpha were expressed stably in 3T3-L1 cells to define the region controlling adipogenesis. N-terminal analysis revealed region 146-220 of Gsalpha as a repressor of adipogenesis; substitution of Gialpha2 abolished the ability of the chimera to repress adipogenesis in response to inducers. Expression of a chimera in which the 146-235 region of Gsalpha was embedded in Gialpha2 fully repressed adipogenesis in response to the inducers. C-terminal analysis revealed no loss of function for truncated Gsalpha, lacking the terminal 38 residues. The repressor domain for adipogenesis maps to a region that includes switch domains I and II and is spatially distinct from the regions mapped for control of adenylyl cyclase.

Retinoids modulate the effect of PTH and calcitriol on EGF receptor expression in UMR 106-01 cells

Although osteoblast proliferation is a prominent feature of osteitis fibrosa, studies in vitro using osteoblast-like cells have shown that parathyroid hormone (PTH) impairs cell growth. Recent studies in our laboratory have shown that PTH increases epidermal growth factor (EGF) receptor expression in UMR 106-01 osteoblast-like cells, and thus, osteoblast proliferation may occur as a result of an enhanced response of the osteoblast to EGF. In the present studies we investigated the effect of calcitriol and the influence of retinoids on the regulation of EGF receptors. Calcitriol increased 125I-EGF binding 2.5-3-fold after 72 hours of incubation and was maximal at a calcitriol dose of 100 nM. Scatchard analysis showed that this effect was due to increased receptor number. In contrast, all-trans retinoic acid or 9-cis retinoic acid alone, even at 10 microM, caused less than a 50% increase in 125I-EGF binding. However, the effect of calcitriol was totally abolished in the presence of all-trans retinoic acid. 9-cis retinoic acid was equivalent with all-trans retinoic acid in this regard. In the presence of either retinoid, the stimulatory effect of PTH was totally eliminated and EGF binding was actually decreased below control values. Additional studies revealed that retinoic acid decreased PTH-stimulated cAMP generation in a dose-dependent manner. These data are consistent with our previous studies which showed that the effect of PTH on the induction of EGF receptors was mediated by a cAMP-dependent mechanism. The inhibition of the calcitriol effect by retinoids is consistent with the requirement of the retinoid-X-receptor (RXR) for binding of the vitamin D receptor (VDR) to its target sequences in DNA. These data indicate that EGF receptors in UMR 106-01 cells are up-regulated by PTH and calcitriol and that this process can be modulated by retinoids. Retinoids, therefore, may play a major role in the regulation of osteoblast function by PTH and calcitriol.

Morphogen-induced decline in Gialpha2 triggers F9 teratocarcinoma stem cell progression via phospholipase C and mitogen-activated protein kinase

The linkage between Gialpha2 and morphogen-induced promotion of F9 embryonic teratocarcinoma stem (F9 stem) cells to primitive endoderm was explored using probes of the mitogen-activated protein (MAP) kinase network. The morphogen-induced decline in Gialpha2 is shown to trigger activation of phospholipase C, thereby activating protein kinase C, MAP kinase, and cell progression to primitive endoderm. In the absence of retinoic acid, reduction-of-function mutants (Gialpha2-deficient) display the effects of morphogen, i.e. activation of phospholipase C, protein kinase C, MAP kinase, and progression to primitive endoderm. Gain-of-function mutants (expressing the Q205L activating-mutation of Gialpha2) displayed no activation of phospholipase C, protein kinase C, MAP kinase and no progression to primitive endoderm, even in the presence of retinoic acid. Selective inhibitors of protein kinase C, like the gain-of-function mutations, effectively block morphogen-induced progression to primitive endoderm. Morphogen triggers F9 stem cell progression by triggering Gialpha2 loss and thereby activation of downstream elements, including protein kinase C and MAP kinase.

Constitutively active mutant GS alpha (G225T) and null-mutant G alpha i-2 (G203T) induce primitive endoderm from stem cells

In F9 teratocarcinoma stem cells, retinoic acid induces a primitive endoderm-like phenotype and a sharp decline in G alpha i-2, a response mimicked by expression of RNA antisense to G alpha i-2 in the absence of this morphogen (D. C. Watkins, G. L. Johnson, and C. C. Malbon. Science Wash. DC 258: 1373-1375, 1992). The role of the GS alpha/G alpha i-2 axis in cellular differentiation was explored. In the absence of retinoic acid, F9 stem cells stably expressing a constitutively active mutant of GS alpha (G225T) progressed to the primitive endoderm phenotype, as judged by morphological and differentiation markers, such as tissue plasminogen activator. Although elevated in cells expressing G225T GS alpha, adenosine 3',5'-cyclic monophosphate does not mimic retinoic acid action and alone fails to induce stem cells to primitive endoderm. In the absence of retinoic acid, expression of a null mutant of G alpha i-2 (G203T) also induced stem cells to primitive endoderm. These observations establish G proteins in the GS alpha/G alpha i-2 axis as a control point for regulating progression to primitive endoderm independent of adenylate cyclase, in the present study's model of early mouse development.

GTP-binding proteins in plants: new members of an old family

Regulatory guanine nucleotide-binding proteins (G proteins) have been studied extensively in animal and microbial organisms, and they are divided into the heterotrimeric and the small (monomeric) classes. Heterotrimeric G proteins are known to mediate signal responses in a variety of pathways in animals and simple eukaryotes, while small G proteins perform diverse functions including signal transduction, secretion, and regulation of cytoskeleton. In recent years, biochemical analyses have produced a large amount of information on the presence and possible functions of G proteins in plants. Further, molecular cloning has clearly demonstrated that plants have both heterotrimeric and small G proteins. Although the functions of the plant heterotrimeric G proteins are yet to be determined, expression analysis of an Arabidopsis G alpha protein suggests that it may be involved in the regulation of cell division and differentiation. In contrast to the very few genes cloned thus far that encode heterotrimeric G proteins in plants, a large number of small G proteins have been identified by molecular cloning from various plants. In addition, several plant small G proteins have been shown to be functional homologues of their counterparts in animals and yeasts. Future studies using a number of approaches are likely to yield insights into the role plant G proteins play.

Isolation and characterization of retinoic acid-inducible cDNA clones in F9 cells: one of the early inducible clones encodes a novel protein sharing several highly homologous regions with a Drosophila polyhomeotic protein

To elucidate regulatory mechanisms triggering early mammalian differentiation, 17 retinoic acid (RA)-inducible clones were isolated from 1.4 x 10(5) plaques of cDNA libraries prepared from mouse embryonal carcinoma F9 cells, using the differential plaque hybridization method. Partial nucleotide sequences of these clones demonstrated that ten clones correspond to known genes. Interestingly, only 2 of the 17 clones are among the previously documented up-regulated genes. Therefore, there are many more unidentified genes up-regulated in the course of RA-induced differentiation of F9 cells. As RNAs hybridizable with one of the seven unidentified clones were induced in F9 cells after 3 h of RA treatment, we chose this 'Rae-28' clone as being representative of developmentally up-regulated unidentified clones and its properties were characterized. We determined the Rae-28 cDNA sequence and deduced the RAE-28 protein structure. The deduced RAE-28 protein shared several motifs and highly homologous regions with a Drosophila polyhomeotic protein. As the Drosophila polyhomeotic gene is involved in regulating morphogenesis, the rae-28 gene may participate in regulating early mammalian development.

Retinoic acid stimulates protein kinase A-associated G proteins during human teratocarcinoma differentiation

Retinoic acid (RA) treatment of F9 murine teratocarcinoma (TC) cells reduces the expression of the protein kinase A (PKA)-associated G protein, G alpha i2. The present study reveals interactions between the RA and PKA pathways during differentiation of the multipotent human TC cell line NTERA-2 clone D1 (abbreviated NT2/D1) which differ from prior reports in F9 TC cells. Compared to untreated NT2/D1 cells, differentiated NT2/D1 cells expressed increased levels of G alpha s and G alpha i1,2 proteins as shown by both immunoblot analysis and cholera toxin- and pertussis toxin-induced ADP ribosylation. To further explore cooperation between these pathways during human TC differentiation, we examined the effects of cyclic adenosine monophosphate (cAMP) on RA-responsive genes and of RA treatment on the transcriptional activation of a cAMP response element (CRE). Compared to RA alone, combined treatment with RA and cAMP augmented the expression of the RA nuclear receptor-beta (RAR-beta). Also, transient transfection assays revealed that cAMP and RA cooperated to enhance CRE transcriptional activation. The cAMP-induced enhancement of RA actions in NT2/D1 cells extended to immunophenotypic changes typical of the neuronal differentiation program induced by RA. In contrast to these findings in NT2/D1 cells, prior work in F9 TC cells showed that cAMP inhibits the RA-mediated augmentation of RAR-beta expression and switches the differentiation program from visceral to parietal endoderm. Thus, unlike murine TC cells, in human NT2/D1 cells RA stimulates PKA-associated G proteins and PKA pathway activation enhances RA-mediated TC differentiation.

Retinoic acid stimulates the protein kinase C pathway before activation of its beta-nuclear receptor during human teratocarcinoma differentiation

We previously reported that protein kinase C (PKC) stimulation through phorbol ester (TPA) treatment enhances the effects of all-trans retinoic acid (RA) on immunophenotypic differentiation and RA nuclear receptor (RAR) activation in the multipotential human teratocarcinoma (TC) cell line NTera-2/clone D1 (abbreviated NT2/D1). This study extends prior work in NT2/D1 cells by demonstrating that PKC pathway activation is an early effect of RA treatment which regulates RAR transcriptional activity. RA activated the PKC pathway prior to induction of RAR-beta expression at 6 h, which is an established early marker of RAR activation in NT2/D1 cells. RA caused a transient 1.3-fold increase in intracellular diacylglycerol (DG) at 2 min and a translocation of the gamma isozyme of PKC (PKC-gamma) within 5 min. Transient co-transfection studies provided evidence that PKC pathway activation plays a role in the regulation of RAR-beta expression. In these studies a constitutively active PKC-gamma augmented the RA-mediated transactivation of a luciferase reporter containing the native RAR-beta promoter which has a retinoic-acid-response element (RARE). These findings reveal that PKC pathway activation is an early step in RA-mediated human TC differentiation and that PKC-gamma can potentiate the effects of RA on RAR transcriptional activation.

Regulation of the differentiation of teratocarcinoma cells into primitive endoderm by G alpha i2

The amount of the heterotrimeric G protein subunit G alpha i2 decreases after the induction of F9 teratocarcinoma cells to become primitive endoderm in the presence of retinoic acid (RA). The reduction of the G alpha i2 protein in F9 cells by antisense RNA expression was associated with (i) loss of receptor-mediated inhibition of adenylyl cyclase; (ii) decreased cell doubling time; (iii) induction of a primitive, endoderm-like phenotype in the absence of RA; and (iv) production of the differentiation marker tissue-type plasminogen activator. Expression of a constitutively active, mutant G alpha i2 blocked RA-induced differentiation. These data suggest the involvement of G alpha i2 in the control of stem cell differentiation and provide insight into the involvement of G proteins in growth regulation.

Gs alpha availability to cholera toxin-catalysed ADP-ribosylation is decreased in membranes of retinoic acid-treated leukemic cell lines HL-60 and THP-1. A posttranslational effect

Retinoic acid (RA) induces HL-60 and THP-1 leukemic cell lines to differentiate into granulocyte-like and monocyte-like cells. Limited data are available concerning the effects of RA on components of the cyclic AMP pathway in human myeloid leukemic cells. We showed previously a decrease in adenylate cyclase activity in the presence of histamine, prostaglandin E1 and forskolin in RA-treated HL-60 cells as compared to untreated cells. We examined the elements of the signal transduction pathway utilized by RA in the human myeloid cell line HL-60 and the human monocytic cell line THP-1. We therefore studied the effect of RA on the activity of the stimulatory G-protein (Gs). We demonstrate that addition of RA to two human myeloid leukemia cell lines, HL-60 and THP-1, does not induce a reduction of the 2 subunit of Gs (Gs alpha) RNA or Gs alpha protein in the plasma membrane but leads to a rapid decrease in the cholera toxin (CTX)-catalysed ADP-ribosylation of Gs alpha. In addition, this effect seems to be specific to RA, since there was no modification in Gs alpha ADP-ribosylation in the membranes of cells treated with dimethyl sulfoxide (DMSO), another inducer of differentiation in HL-60 cells.