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

Cleaved PINK1 induces neuronal plasticity through PKA-mediated BDNF functional regulation

Mutations in PTEN-induced kinase 1 (PINK1) lead to early onset autosomal recessive Parkinson's disease in humans. In healthy neurons, full-length PINK1 (fPINK1) is post-translationally cleaved into different lower molecular weight forms, and cleaved PINK1 (cPINK1) gets shuttled to the cytosolic compartments to support extra-mitochondrial functions. While numerous studies have exemplified the role of mitochondrially localized PINK1 in modulating mitophagy in oxidatively stressed neurons, little is known regarding the physiological role of cPINK1 in healthy neurons. We have previously shown that cPINK1, but not fPINK1, modulates the neurite outgrowth and the maintenance of dendritic arbors by activating downstream protein kinase A (PKA) signaling in healthy neurons. However, the molecular mechanisms by which cPINK1 promotes neurite outgrowth remain to be elucidated. In this report, we show that cPINK1 supports neuronal development by modulating the expression and extracellular release of brain-derived neurotrophic factor (BDNF). Consistent with this role, we observed a progressive increase in the level of endogenous cPINK1 but not fPINK1 during prenatal and postnatal development of mouse brains and during development in primary cortical neurons. In cultured primary neurons, the pharmacological activation of endogenous PINK1 leads to enhanced downstream PKA activity, subsequent activation of the PKA-modulated transcription factor cAMP response element-binding protein (CREB), increased intracellular production and extracellular release of BDNF, and enhanced activation of the BDNF receptor-TRKβ. Mechanistically, cPINK1-mediated increased dendrite complexity requires the binding of extracellular BDNF to TRKβ. In summary, our data support a physiological role of cPINK1 in stimulating neuronal development by activating the PKA-CREB-BDNF signaling axis in a feedforward loop.

RFX1: a promising therapeutic arsenal against cancer

Regulatory factor X1 (RFX1) is an evolutionary conserved transcriptional factor that influences a wide range of cellular processes such as cell cycle, cell proliferation, differentiation, and apoptosis, by regulating a number of target genes that are involved in such processes. On a closer look, these target genes also play a key role in tumorigenesis and associated events. Such observations paved the way for further studies evaluating the role of RFX1 in cancer. These studies were indispensable due to the failure of conventional chemotherapeutic drugs to target key cellular hallmarks such as cancer stemness, cellular plasticity, enhanced drug efflux, de-regulated DNA repair machinery, and altered pathways evading apoptosis. In this review, we compile significant evidence for the tumor-suppressive activities of RFX1 while also analyzing its oncogenic potential in some cancers. RFX1 induction decreased cellular proliferation, modulated the immune system, induced apoptosis, reduced chemoresistance, and sensitized cancer stem cells for chemotherapy. Thus, our review discusses the pleiotropic function of RFX1 in multitudinous gene regulations, decisive protein–protein interactions, and also its role in regulating key cell signaling events in cancer. Elucidation of these regulatory mechanisms can be further utilized for RFX1 targeted therapy.

Manipulation of retinoic acid signaling in the nucleus accumbens shell alters rat emotional behavior

Novel targets for depression and anxiety disorders are necessary for the development of more effective pharmacotherapeutics. Our previous study found that the retinoic acid (RA) signaling pathway is the signaling pathway most enhanced in the nucleus accumbens (NAc) shell, a region important for depression, anxiety, and addiction. Genetic manipulations of RA signaling in the NAc affecting addiction-related behavior prompted our study of the role of retinoic acid signaling in depression-related and anxiety-related behavior using in vivo RNA interference. Knockdown of the retinoic acid degradation enzyme cytochrome p450 family 26 subfamily b member 1 (Cyp26b1) in the nucleus accumbens shell increased depression-related behavior while decreasing anxiety-like behavior. Knockdown of the retinoic acid binding protein, cellular RA binding protein 2 (Crabp2), also increased depression-related behavior. Knockdown of another RA binding partner fatty acid binding protein 5 (Fabp5), did not alter these behaviors. These results further support the contention that RA signaling in the NAc shell can affect emotional behavior and that targeting some components of this pathway could be a promising avenue for developing novel treatments for depression and anxiety.

Neuronal ELAV proteins enhance mRNA stability by a PKCalpha-dependent pathway

More than 1 in 20 human genes bear in the mRNA 3' UTR a specific motif called the adenine- and uridine-rich element (ARE), which posttranscriptionally determines its expression in response to cell environmental signals. ELAV (embryonic lethal abnormal vision) proteins are the only known ARE-binding factors that are able to stabilize the bound mRNAs, thereby positively controlling gene expression. Here, we show that in human neuroblastoma SH-SY5Y cells, neuron-specific ELAV (nELAV) proteins (HuB, HuC, and HuD) are up-regulated and redistributed by 15 min of treatment with the activators of PKC phorbol esters and bryostatin-1. PKC stimulation also induces nELAV proteins to colocalize with the translocated PKCalpha isozyme preferentially on the cytoskeleton, with a concomitant increase of nELAV threonine phosphorylation. The same treatment promotes stabilization of growth-associated protein 43 (GAP-43) mRNA, a well known nELAV target, and induces an early increase in GAP-43 protein concentration, again only in the cytoskeletal cell fraction. Genetic or pharmacological inactivation of PKCalpha abolishes nELAV protein cytoskeletal up-regulation, GAP-43 mRNA stabilization, and GAP-43 protein increase, demonstrating the primary role of this specific PKC isozyme in the cascade of nELAV recruitment. Finally, in vivo PKC activation is associated with an up-regulation of nELAV proteins in the hippocampal rat brain. These findings suggest a model for gene expression regulation by nELAV proteins through a PKCalpha-dependent pathway that is relevant for the cellular programs in which ARE-mediated control plays a pivotal role.

Retinoic acid-induced Golgi apparatus disruption in F2000 fibroblasts: a model for enhanced intracellular retrograde transport

Retinoic acid (RA) can transform the Golgi apparatus (GA) into a diffuse vacuolar aggregate and increase the toxicity of some immunotoxins that enter into cells by receptor-mediated endocytosis. An ultramorphological study of the RA-induced GA disruption was performed on F2000 fibroblasts. Cultures were treated with 0.11 to 30 microM RA for 7-180 min. The endocytosis of Limax flavus agglutinin-peroxidase conjugate (LFA), and the interactions between a phorbol ester (PMA) and RA concerning GA disruption, were examined. Exposure to 0.33 microM RA for 20 min transformed the GA into vacuolar aggregate. These vacuoles were not involved in endocytosis since they remained unstained after endocytosis of LFA. However, the lysosomes were involved in endocytosis, as they were strongly stained. Therefore, a RA-induced shift towards lysosomal routing of the entered LFA was presumed. Exposure to PMA made cells resistant to the Golgi-disturbing effects of RA, indicating that protein kinase C plays an important role in this process.

Interleukin-1beta enhances retinoic acid-mediated expression of bone-type alkaline phosphatase in rat IEC-6 cells

We previously showed that vitamin A upregulated the expression of bone-type alkaline phosphatase (ALP) in fetal rat small intestine and rat intestinal IEC-6 cells. In this study, we examined interactions between retinoic acid (RA) and several growth factors/cytokines on the isozyme expression in IEC-6 cells. Epidermal growth factor and interleukins (ILs)-2, -4, -5, and -6 completely blocked the RA-mediated increase in ALP activity. In contrast, IL-1beta markedly increased the activity, protein, and mRNA of the bone-type ALP only when RA was present. IL-1beta and/or RA did not change the type 1 IL-1 receptor transcript level, whereas IL-1beta enhanced the RA-induced expressions of retinoic acid receptor-beta (RAR-beta) and retinoid X receptor-beta (RXR-beta) mRNAs and RA-mediated RXR response element binding. The synergism of IL-1beta and RA on ALP activity was completely blocked by protein kinase C (PKC) inhibitors. Our results suggest that IL-1beta may modify the ALP isozyme expression in small intestinal epithelial cells by stimulating PKC-dependent, RAR-beta- and/or RXR-beta-mediated signaling pathways.

Serine 157, a retinoic acid receptor alpha residue phosphorylated by protein kinase C in vitro, is involved in RXR.RARalpha heterodimerization and transcriptional activity

Retinoic acid (RA) regulation of cellular proliferation and differentiation is mediated, at least in part, through two related nuclear receptors, RAR and RXR. RA-induced modulation of gene expression leads generally to cellular differentiation, whereas stimulation of the protein kinase C (PKC) signaling pathway is associated with cellular proliferation. Pursuant to our discovery that prolonged activation of PKCs induced a strong decrease in RA responsiveness of a retinoid-inducible reporter gene, we have further investigated the connections between these two signaling pathways. We demonstrate that PKC isoforms alpha and gamma are able to phosphorylate human RARalpha (hRARalpha) in vitro on a single serine residue located in the extended DNA binding domain (T box). The introduction of a negative charge at this position (serine 157) strongly decreased hRARalpha transcriptional activity, whereas a similar mutation at other PKC consensus phosphorylation sites had no effect. The effect on transcriptional activation was correlated with a decrease in the capacity of hRARalpha to heterodimerize with hRXRalpha. Thus hRARalpha is a direct target for PKCalpha and gamma, which may control retinoid receptor transcriptional activities during cellular proliferation and differentiation.

Aging decreases retinoic acid and triiodothyronine nuclear expression in rat liver: exogenous retinol and retinoic acid differentially modulate this decreased expression

The expression of nuclear receptors of retinoic acid (RAR) and triiodothyronine (TR) was analyzed in the liver of rats aged 2.5 (young), 6 (adult) and 24 (aged) months. In aged rats, decreased binding properties, binding capacity (Cmax) and affinity (Ka), of nuclear receptors were observed. This resulted, at least in part, from decreased transcription of receptor genes in that the amount of their mRNA also decreased. Moreover, the activity of malic enzyme (ME) and tissue transglutaminase (tTG), whose genes are TR and RAR responsive, respectively, was reduced in aged rats. These results are in agreement with the decreased binding capacity of these receptors. An inducer-related increase of RAR and TR expression was observed 24 h after a single dose of retinoic acid administration (5 mg/kg), while retinol administration (retinyl palmitate, 13 mg/kg) was without incidence on nuclear receptor expression in aged rats.

Retinoic acid stimulates growth hormone synthesis in human somatotropic adenoma cells: characterization of its nuclear receptors

In order to gain a better understanding on the possible role of retinoic acid (RA) on human GH secretion, we have characterized the expression of its nuclear receptors in somatotropic adenoma cell extracts. By immunoblotting with rabbit polyclonal antibodies directed against RAR alpha, beta, and gamma and RXR alpha and beta, we could only detect the presence of RAR alpha and RXR alpha proteins. The predominant expression of RXR alpha was confirmed at the mRNA level by Northern and slot-blot analysis. We then investigated the effect of RA on GH synthesis in cell culture of adenomatous somatotrophs. In cultured cells, RA (1 microM) stimulated GH secretion, increased intracellular GH content and GH mRNA levels within 72 h, suggesting a modulation of GH synthesis by RA.

Inositol prevents folate-resistant neural tube defects in the mouse

Clinical trials demonstrate that up to 70% of neural tube defects (NTDs) can be prevented by folic acid supplementation in early pregnancy, whereas the remaining NTDs are resistant to folate. Here, we show that a second vitamin, myo-inositol, is capable of significantly reducing the incidence of spinal NTDs in curly tail mice, a genetic model of folate-resistant NTDs. Inositol increases flux through the inositol/lipid cycle, stimulating protein kinase C activity and upregulating expression of retinoic acid receptor beta, specifically in the caudal portion of the embryonic hindgut. This reduces the delay in closure of the posterior neuropore, the embryonic defect that is known to lead directly to spina bifida in curly tail embryos. Our findings reveal a molecular pathway of NTD prevention and suggest the possible efficacy of combined treatment with folate and inositol in overcoming the majority of human NTDs.

Multifactorial mechanism for the potentiation of cisplatin (CDDP) cytotoxicity by all-trans retinoic acid (ATRA) in human ovarian carcinoma cell lines

All-trans retinoic acid (ATRA) has been previously shown to inhibit the proliferation of some human ovarian carcinoma cell lines, and this inhibition was accompanied by cellular changes that were indicative of differentiation (Caliaro et al, 1994). In this work, a pretreatment of these adenocarcinoma cells with ATRA, for their respective doubling time, enhanced cisplatin (CDDP) cytotoxicity in the cell ines that were sensitive to its antiproliferative effect, but not in the ATRA-resistant ones. Results were assessed using median effect analysis in two ATRA-sensitive cell lines (OVCCR1 and NIHOVCAR3 cells) and in one ATRA-insensitive cell line (IGROV1 cells). Synergy between these two agents was observed only in cells sensitive to ATRA, regardless of their relative sensitivity to CDDP. Potential mechanisms for this synergy were investigated. ATRA did not increase the cellular platinum content, did not decrease the cellular glutathione and had no influence on the metallothionein IIA mRNA levels in NIHOVCAR3 cells. Moreover, the protein kinase C (PKC) activity was modulated by this differentiating agent in all cell lines tested, indicating that this activity was not directly involved in this potentiation. However, an ATRA inhibition of glutathione-S-transferase activity associated with an increase in the total DNA adducts formation could explain the potentiation of the CDDP cytotoxicity observed in NIHOVCAR3 cells. Finally, the ATRA modulation of the epidermal growth factor (EGF) receptor mRNA level could also be implicated in this synergy.

Phosphorylation of eIF-4E and initiation of protein synthesis in P19 embryonal carcinoma cells

Mitogenic stimulation of protein synthesis is accompanied by an increase in eIF-4E phosphorylation. The effect on protein synthesis by induction of differentiation is less well known. We treated P19 embryonal carcinoma cells with the differentiating agent retinoic acid and found that protein synthesis increased during the first hour of addition. However, the phosphorylation state, as well as the turnover of phosphate on eIF-4E, remained unchanged. Apparently, the change in protein synthesis after RA addition is regulated by another mechanism than eIF-4E phosphorylation. By using P19 cells overexpressing the EGF receptor, we show that the signal transduction pathway that leads to phosphorylation of eIF-4E is present in P19 cells; the EGF-induced change in phosphorylation of eIF-4E in these cells is likely to be regulated by a change in eIF-4E phosphatase activity. These results suggest that the onset of retinoic acid-induced differentiation is triggered by a signal transduction pathway which involves changes in protein synthesis, but not eIF-4E phosphorylation.

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.