Down-regulation of the tumor suppressor gene retinoic acid receptor beta2 through the phosphoinositide 3-kinase/Akt signaling pathway

The retinoic acid receptor beta2 (RARbeta2) is a potent, retinoid-inducible tumor suppressor gene, which is a critical molecular relay for retinoid actions in cells. Its down-regulation, or loss of expression, leads to resistance of cancer cells to retinoid treatment. Up to now, no primary mechanism underlying the repression of the RARbeta2 gene expression, hence affecting cellular retinoid sensitivity, has been identified. Here, we demonstrate that the phosphoinositide 3-kinase/Akt signaling pathway affects cellular retinoid sensitivity, by regulating corepressor recruitment to the RARbeta2 promoter. Through direct phosphorylation of the corepressor silencing mediator for retinoic and thyroid hormone receptors (SMRT), Akt stabilized RAR/SMRT interaction, leading to an increased tethering of SMRT to the RARbeta2 promoter, decreased histone acetylation, down-regulation of the RARbeta2 expression, and impaired cellular differentiation in response to retinoid. The phosphoinositide 3-kinase/Akt signaling pathway, an important modulator of cellular survival, has thus a direct impact on cellular retinoid sensitivity, and its deregulation may be the triggering event in retinoid resistance of cancer cells.     

维甲酸信号通路的生物学进展

本文阐述了受维甲酸信号调控的基因表达多样性的分子机制,描述了维甲酸受体的特征、维甲酸反应元件的多态性、转录中介因子包括辅活化因子和辅抑制因子在维甲酸受体介导的转录调控中的作用,主要的维甲酸应答基因及维甲酸在肿瘤治疗中的应用。     

Regulation of mucin gene expression by CREB via a nonclassical retinoic acid signaling pathway

Vitamin A and its metabolite retinoic acid (RA) are essential elements for normal lung development and the differentiation of lung epithelial cells. We previously showed that RA rapidly activated cyclic AMP response element-binding protein (CREB) in a nonclassical manner in normal human tracheobronchial epithelial (NHTBE) cells. In the present study, we further demonstrated that this nonclassical signaling of RA on the activation of CREB plays a critical role in regulating the expression of airway epithelial cell differentiation markers, the MUC2, MUC5AC, and MUC5B genes. We found that RA rapidly activates the protein kinase Calpha isozyme and transmits the activation signal to CREB via the Raf/MEK/extracellular signal-regulated kinase/p90 ribosomal S6 kinase (RSK) pathway. Activated RSK translocated from the cytoplasm to the nucleus, where it phosphorylates CREB. Activated CREB then binds to a cis-acting replication element motif on the promoter (at nucleotides [nt] -878 to -871) of the MUC5AC gene. The depletion of CREB using small interfering RNA abolished not only the RA-induced MUC5AC but also RA-induced MUC2 and MUC5B. Taken together, our findings demonstrate that CREB activation via this nonclassical RA signaling pathway may play an important role in regulating the expression of mucin genes and mediating the early biological effects of RA during normal mucous differentiation in NHTBE cells.     

Regulation of retinoic acid signaling during lung morphogenesis

Little is known about how retinoic acid (RA) synthesis, utilization and metabolism are regulated in the embryonic lung and how these activities relate to lung pattern formation. Here we report that early lung bud formation and subsequent branching morphogenesis are characterized by distinct stages of RA signaling. At the onset of lung development RA signaling is ubiquitously activated in primary buds, as shown by expression of the major RA-synthesizing enzyme, RALDH-2 and activation of a RARE-lacZ transgene. Nevertheless, further airway branching appears to require downregulation of RA pathways by decreased synthesis, increased RA degradation in the epithelium via P450RAI-mediated metabolism, and inhibition of RA signaling in the mesenchyme by COUPTF-II expression. These mechanisms controlling local RA signaling may be critical for normal branching, since we show that manipulating RA levels in vitro to maintain RA signaling activated as in the initial stage, leads to an immature lung phenotype characterized by failure to form typical distal buds. We show that this phenotype likely results from RA interfering with the establishment of a distal signaling center, altering levels and distribution of Fgf10 and Bmp4, genes that are essential for distal lung formation. Furthermore, RA upregulates P450RAI expression, suggesting the presence of feedback mechanisms controlling RA availability. Our study illustrates the importance of regional mechanisms that control RA availability and utilization for correct expression of pattern regulators and normal morphogenesis during lung development.     

Tributyltin and triphenyltin inhibit osteoclast differentiation through a retinoic acid receptor-dependent signaling pathway

Organotin compounds, such as tributyltin (TBT) and triphenyltin (TPT), have been widely used in agriculture and industry. Although these compounds are known to have many toxic effects, including endocrine-disrupting effects, their effects on bone resorption are unknown. In this study, we investigated the effects of organotin compounds, such as monobutyltin (MBT), dibutyltin (DBT), TBT, and TPT, on osteoclast differentiation using mouse monocytic RAW264.7 cells. MBT and DBT had no effects, whereas TBT and TPT dose-dependently inhibited osteoclast differentiation at concentrations of 3-30 nM. Treatment with a retinoic acid receptor (RAR)-specific antagonist, Ro41-5253, restored the inhibition of osteoclastogenesis by TBT and TPT. TBT and TPT reduced receptor activator of nuclear factor-kappaB ligand (RANKL) induced nuclear factor of activated T cells (NFAT) c1 expression, and the reduction in NFATc1 expression was recovered by Ro41-5253. Our results suggest that TBT and TPT suppress osteoclastogenesis by inhibiting RANKL-induced NFATc1 expression via an RAR-dependent signaling pathway.     

The prevention of adipose differentiation of 3T3-L1 cells caused by retinoic acid is elicited through retinoic acid receptor alpha

Retinoids, especially all-trans retinoic acid (RA), have been shown to inhibit the differentiation of preadipose cells. It is important to human health, especially to obesity, that the regulatory system for the differentiation of adipocytes is well defined. Previously, we have shown that retinoic acid receptor (RAR) γ2 gene expression is up-regulated by RA in 3T3-L1 preadipose cells. In this study, the RAR system was dissected and the RA-regulated function in 3T3-L1 cells was assigned to one given receptor. We used three synthetic retinoids; (1) Ro 41–5253, a selective RAR α antagonist, (2) Ch 55, an RAR α, β and γ agonist, and (3) Am 80, an RAR α and β agonist, which has less affinity to RAR γ. Ro 41–5253 reverted RA-induced inhibition of the differentiation of 3T3-L1 cells. However, there was no significant reversion in RA-induced RAR γ mRNA level by treatment with Ro 41–5253. In the case of RAR agonists, both Am 80 and Ch 55 strongly inhibited the differentiation of 3T3-L1 cells. However, Am 80 weakly increased RAR γ mRNA content less than did Ch 55. These findings suggest, that RAR α is involved in the prevention of adipose differentiation by RA in 3T3-L1 cells. Moreover, there seems no causal relationship between the prevention of adipose differentiation by RA and the up-regulation of RAR γ2 gene expression by RA in 3T3-L1 cells. We have shown the functional heterogeneity of RA action through different RARs in 3T3-L1 cells.     

Improved hematopoietic differentiation of human pluripotent stem cells via estrogen receptor signaling pathway

Background

Aside from its importance in reproduction, estrogen (E2) is known to regulate the proliferation and differentiation of hematopoietic stem cells in rodents. However, the regulatory role of E2 in human hematopoietic system has not been investigated. The purpose of this study is to investigate the effect of E2 on hematopoietic differentiation using human pluripotent stem cells (hPSCs).

Results

E2 improved hematopoietic differentiation of hPSCs via estrogen receptor alpha (ER-α)-dependent pathway. During hematopoietic differentiation of hPSCs, ER-α is persistently maintained and hematopoietic phenotypes (CD34 and CD45) were exclusively detected in ER-α positive cells. Interestingly, continuous E2 signaling is required to promote hematopoietic output from hPSCs. Supplementation of E2 or an ER-α selective agonist significantly increased the number of hemangioblasts and hematopoietic progenitors, and subsequent erythropoiesis, whereas ER-β selective agonist did not. Furthermore, ICI 182,780 (ER antagonist) completely abrogated the E2-induced hematopoietic augmentation. Not only from hPSCs but also from human umbilical cord bloods, does E2 signaling potentiate hematopoietic development, suggesting universal function of E2 on hematopoiesis.

Conclusions

Our study identifies E2 as positive regulator of human hematopoiesis and suggests that endocrine factors such as E2 influence the behavior of hematopoietic stem cells in various physiological conditions.

     

Regulation of IGF-I receptor signaling in tumor cells.

Signals from the IGF-IR and other members of the IR family contribute to the growth, survival, adhesion, and motility of tumor cells. These signals are initiated through recruitment of adapter proteins including the IRS family and Shc proteins, and are mediated through the PI3-kinase, mitogen activated protein (MAP) kinase and stress-activated protein kinase (SAPK) pathways. Regulation of signaling responses from the IGF-IR involves the actions of regulatory adapter proteins including RACK1 and Grb10 that recruit or sequester cytoplasmic proteins, and the actions of phosphatases including tyrosine PTP-1B, PTEN, and PP2A. This review focuses on the signaling pathways that are activated by the IGF-IR in tumor cells, the mechanisms of regulation of these pathways by adapter proteins and phosphatases, and how modulation of IGF-IR signaling could contribute to cancer progression.     

All hematopoietic cells develop from hematopoietic stem cells through Flk2/Flt3-positive progenitor cells

While it is clear that a single hematopoietic stem cell?(HSC) is capable of giving rise to all other hematopoietic cell types, the differentiation paths beyond HSC remain controversial. Contradictory reports on?the lineage potential of progenitor populations have questioned their physiological contribution of progenitor populations to multilineage differentiation. Here, we established a lineage tracing mouse model that enabled direct assessment of differentiation pathways in?vivo. We provide definitive evidence that differentiation into all hematopoietic lineages, including megakaryocyte/erythroid cell types, involves Flk2-expressing non-self-renewing progenitors. A Flk2+ stage was used during steady-state hematopoiesis, after irradiation-induced stress and upon HSC transplantation. In contrast, HSC origin and maintenance do not include a Flk2+ stage. These data demonstrate that HSC specification and maintenance are Flk2 independent, and that hematopoietic lineage separation occurs downstream of Flk2 upregulation.     

The potential signaling pathway between peroxisome proliferator-activated receptor gamma and retinoic acid receptor alpha in renal interstitial fibrosis disease

Abstract

Peroxisome proliferator-activated receptorγ (PPARγ) can regulate the process of cell apoptosis and is related to the progression of renal disorders. Retinoic acid receptor alpha (RARα) is one of the nuclear receptors involved in a variety of kidney diseases. Renal interstitial fibrosis (RIF) is a common denominator of chronic kidney disease (CKD). This study investigated whether a potential signaling pathway existed between PPARγ and RARα in RIF rats with unilateral ureteral obstruction (UUO). The rats were randomly divided into four groups: a model group subjected to UUO (GU), and three other groups treated with rosiglitazone sodium (GRS), GW9662 and dimethyl sulfoxide (DMSO), n?=?40, respectively. Renal tissues were collected two and four weeks after post-surgery. The relevant indicators were detected. In comparison with the GU group, the expressions of PPARγ and RARα (protein and mRNA) were increased in the GRS group, and decreased in the GW9662 group (all p?<?0.01). The RIF index, mRNA and protein expression of transforming growth factor-β1 (TGF-β1), and the protein expressions of collagen-IV (Col-IV) and fibronectin (FN) in the GRS group were more markedly reduced than those in the GU group; their levels in the GW9662 group were elevated (all p?<?0.01). PPARγ or RARα was negatively correlated to the RIF index, TGF-β1, Col-IV and FN. PPARγ was positively correlated with RARα (all p?<?0.01). In conclusion, PPARγ agonist can elevate the expression of PPARγ or RARα in RIF rats. There might be a potential signaling pathway between PPARγ and RARα in RIF disease.     

Zinc enhances intestinal epithelial barrier function through the PI3K/AKT/mTOR signaling pathway in Caco-2 cells

Zinc plays an important role in maintaining intestinal barrier function as well as modulating cellular signaling recognition and protein kinase activities. The phosphatidylinositol 3-kinase (PI3K) cascade has been demonstrated to affect intercellular integrity and tight junction (TJ) proteins. The current study investigated the hypothesis that zinc regulates intestinal intercellular junction integrity through the PI3K/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway. A transwell model of Caco-2 cell was incubated with 0, 50 and 100 μM of zinc at various time points. Transepithelial electrical resistance (TEER), paracellular permeability, TJ proteins, cell proliferation, differentiation and cell damage were measured. Compared with controls, 50 and 100 μM of zinc increased cell growth at 6, 12 and 24 h and the expression of proliferating cell nuclear antigen at 24 h. Zinc (100 μM) significantly elevated TEER at 6–24 h and reduced TJ permeability at 24 h, accompanied by the up-regulation of alkaline phosphatase (AP) activity and zonula occludens (ZO)-1 expression. In addition, zinc (100 μM) affected the PI3K/AKT/mTOR pathway by stimulating phosphorylation of AKT and the downstream target mTOR. Inhibition of PI3K signaling by LY294002 counteracted zinc promotion, as shown by a decrease in AP activity, TEER, the abundance of ZO-1 and phosphorylation of AKT and mTOR. Additionally, TJ permeability and the expression of caspase-3 and LC3II (markers of cell damage) were increased by addition of PI3K inhibitor. In conclusion, the activation of PI3K/AKT/mTOR signaling by zinc is involved in improving intestinal barrier function by enhancing cell differentiation and expression of TJ protein ZO-1.