ATBF1异构体的选择性表达及其抑癌作用的分子机制

ATBF1(AT motif binding factor 1)基因是一个新发现的抑癌基因,其表达产物是目前发现的分子量最大的转录调节因子,它能和甲胎蛋白（alpha fetoprotein, AFP）基因增强子AT富聚区结合,调节AFP的转录.ATBF1基因表达过程中,由于转录本mRNA的选择性剪接,可产生ATBF1-A和ATBF1-B两种异构体,这两种异构体对AFP表达的调节具有相互对抗作用.ATBF1-A是ATBF1基因的主要表达形式,其能抑制癌细胞生长,而ATBF1-B则能促进癌细胞增殖.本文分析ATBF1异构体如何调控AFP表达及其作用的多样性,阐述ATBF1表达下调对肿瘤细胞生长和侵袭产生的影响;探讨ATBF1异构体抑癌作用的可能机制和选择性应用ATBF1异构体治疗肿瘤的科学意义.     

ATBF1研究综述

AT模体结合因子1(ATBF1)是一个新发现的抑癌基因,从人肝癌细胞HuH-7中分离得到。ATBF1可与甲胎蛋白基因增强子中AT富含原件结合,其表达产物是目前发现的分子量最大的转录调节因子。ATBF1基因表达过程中,通过选择性剪接产生ATBF1-A和ATBF1-B两种mRNA,这两种mRNA对AFP表达的调节具有相互对抗作用。ATBF1-A是ATBF1基因的主要表达形式,能抑制癌细胞生长;而ATBF1-B则能促进癌细胞增殖。ATBF1作为抑癌基因,为肿瘤的治疗带来新希望,但目前学术界对ATBF1的研究仍然有限。本文重点对ATBF1在神经系统、乳腺癌、胃癌、肝癌、结直肠癌以及其他肿瘤中的研究作综述,以期进一步明确ATBF1的抑癌机制。     

Intestinal peptidases form functional complexes with the neutral amino acid transporter B(0)AT1

The brush-border membrane of the small intestine and kidney proximal tubule are the major sites for the absorption and re-absorption of nutrients in the body respectively. Transport of amino acids is mediated through the action of numerous secondary active transporters. In the mouse, neutral amino acids are transported by B(0)AT1 [broad neutral ((0)) amino acid transporter 1; SLC6A19 (solute carrier family 6 member 19)] in the intestine and by B(0)AT1 and B(0)AT3 (SLC6A18) in the kidney. Immunoprecipitation and Blue native electrophoresis of intestinal brush-border membrane proteins revealed that B(0)AT1 forms complexes with two peptidases, APN (aminopeptidase N/CD13) and ACE2 (angiotensin-converting enzyme 2). Physiological characterization of B(0)AT1 expressed together with these peptidases in Xenopus laevis oocytes revealed that APN increased the substrate affinity of the transporter up to 2.5-fold and also increased its surface expression (V(max)). Peptide competition experiments, in silico modelling and site-directed mutagenesis of APN suggest that the catalytic site of the peptidase is involved in the observed changes of B(0)AT1 apparent substrate affinity, possibly by increasing the local substrate concentration. These results provide evidence for the existence of B(0)AT1-containing digestive complexes in the brush-border membrane, interacting differentially with various peptidases, and responding to the dynamic needs of nutrient absorption in the intestine and kidney.     

Characterization and histochemical localization of the rat intestinal Na(+)-D-glucose cotransporter by monoclonal antibodies

The localization of the Na(+)-D-glucose cotransporter in rat small intestine was investigated with four monoclonal antibodies which were raised against porcine renal brush-border membrane proteins. The antibodies alter high affinity phlorizin binding or Na+ gradient-dependent D-glucose uptake in kidney and intestine. In both organs, the antibodies react with polypeptides with apparent molecular weights of 75,000 and 47,000. In pig kidney, these polypeptides were identified as components of the Na(+)-D-glucose cotransporter (Koepsell, H., K. Korn, A. Raszeja-Specht, S. Bernotat-Danielowski, D. Ollig, J. Biol. Chem. 263, 18419-18429 (1988)). The electron microscopic localization of antibody binding was investigated by immunogold labeling of ultrathin plastic sections. In villi and crypts of duodenum, jejunum and ileum the antibodies bound specifically to brush-border membranes of enterocytes and did not react with the basolateral membranes. The density of antigenic sites in brush-border membranes was highest in jejunum, intermediate in ileum and lowest in duodenum. On the tip, the middle and the basis of the villi the density of antigenic sites was similar. The data demonstrate homologous Na(+)-D-glucose cotransporters in kidney and intestine. They suggest that during maturation of the enterocytes when the total area of brush-border membrane increases, the concentration of the Na(+)-D-glucose cotransporter in the brush-border membrane remains constant. However, we found that different segments of small intestine not only contain different surface areas of the transporter-containing brush-border membrane per intestinal length but also different densities of the transporter within the brush-border membrane.     

美国有机农业发展及对我国的启示

美国有机农业起步较早、发展迅速,已经成为全球有机农产品第一大消费市场,分析美国有机农业起源、发展、标识管理和财政支持对发展我国有机农业、提高我国农产品质量安全具有重要借鉴意义。     

Homeotic factor ATBF1 induces the cell cycle arrest associated with neuronal differentiation

The present study aimed to elucidate the function of AT motif-binding factor 1 (ATBF1) during neurogenesis in the developing brain and in primary cultures of neuroepithelial cells and cell lines (Neuro 2A and P19 cells). Here, we show that ATBF1 is expressed in the differentiating field in association with the neuronal differentiation markers beta-tubulin and MAP2 in the day E14.5 embryo rat brain, suggesting that it promotes neuronal differentiation. In support of this, we show that ATBF1 suppresses nestin expression, a neural stem cell marker, and activates the promoter of Neurod1 gene, a marker for neuronal differentiation. Furthermore, we show that in Neuro 2A cells, overexpressed ATBF1 localizes predominantly in the nucleus and causes cell cycle arrest. In P19 cells, which formed embryonic bodies in the floating condition, ATBF1 is mainly cytoplasmic and has no effect on the cell cycle. However, the cell cycle was arrested when ATBF1 became nuclear after transfer of P19 cells onto adhesive surfaces or in isolated single cells. The nuclear localization of ATBF1 was suppressed by treatment with caffeine, an inhibitor of PI(3)K-related kinase activity of ataxa-telangiectasia mutated (ATM) gene product. The cytoplasmic localization of ATBF1 in floating/nonadherent cells is due to CRM1-dependent nuclear export of ATBF1. Moreover, in the embryonic brain ATBF1 was expressed in the cytoplasm of proliferating stem cells on the ventricular zone, where cells are present at high density and interact through cell-to-cell contact. Conversely, in the differentiating field, where cell density is low and extracellular matrix is dense, the cell-to-matrix interaction triggered nuclear localization of ATBF1, resulting in the cell cycle arrest. We propose that ATBF1 plays an important role in the nucleus by organizing the neuronal differentiation associated with the cell cycle arrest.