The importin/exportin transport system provides the machinery involved in nucleocytoplasmic transport. Alterations of the levels of importins and exportins may play crucial roles in development, differentiation and transformation. Employing human leukaemia HL-60 cells, we and others have revealed the differentiation-associated changes in the protein and gene expression of these factors. The recent finding that a switch to the importin-α subtype triggers neural differentiation of embryonic stem cells underscores the importance of nucleocytoplasmic transport factors in cellular events. This review focuses on current research into the roles of importins and exportins in cell differentiation.

Rat hepatoma H4IIE cells were stimulated with dexamethasone and dibutyryl cAMP to increase gene expressions of gluconeogenic enzymes, glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Inclusion of catechin-rich green tea beverage (GTB) in the culture medium reduced the up-regulation of these genes as well as that of hepatocyte nuclear factor 4 alpha (HNF4α) gene. GTB was fractionated into chloroform-soluble (Fraction I), ethyl acetatesoluble (Fraction II), methanol-soluble (Fraction III) and residual (Fraction IV) fractions. Fractions II and III containing catechins caused an attenuation of the up-regulated expression of these genes as well as the down-regulation of HNF4α gene expression. Fraction IV had a synergistic effect on the up-regulation by dexamethasone/dibutyryl cAMP of the PEPCK gene expression and upregulated HNF4α gene expression. These results suggest that GTB down-regulated the expression of the HNF4α gene to cause the down-regulated gene expression of gluconeogenic enzymes. One reason why GTB did not down-regulate hepatic PEPCK gene expression in previous animal experiments may be that the component(s) acting to up-regulate PEPCK gene expression was more effective in vivo than in cultured cells.Tea is one of the world's most popular beverages and has been regarded to possess anti-cancer, antiobesity, anti-atherosclerotic, anti-diabetic, anti-bacterial, and anti-viral effects (4,7,8,17). Recently, it has been demonstrated that (−)-epigallocatechin gallate (EGCG), a major component of green tea catechins, represses glucose production in rat hepatoma H4IIE cells through down-regulation of the gene expression of gluconeogenic enzymes, glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK), suggesting a beneficial effect of green tea on type 2 diabetes, since the disease is associated with enhanced glucose production in the post-absorptive state (16). Previously, we reported that the administration of catechin-rich green tea beverage (GTB) for 4 weeks caused a reduction in the gene expression of G6Pase, but not PEPCK, in the rat liver (1). The findings suggest that some component(s) of the green tea beverage other than catechins might affect the gene expression of PEPCK. We have also posed the possibility that forkhead box transcription factor 1a (Foxo1a) and hepatocyte nuclear factor 4α (HNF4α) may contribute to the down-regulated expression of G6Pase in the liver of GTB-treated rats (1).To answer these questions, we examined effects of GTB on the gene expression of gluconeogenic enzymes in rat hepatoma H4IIE cells using the quantitative real-time reverse transcription-polymerase chain reaction (Q-PCR). Our results suggest that the down-regulation of HNF4α by GTB is relat-

Employing the DNA microarray technique, we previously reported the alteration in gene expression of nucleocytoplasmic transport factors, importins and exportins, induced by 1,25-dihydroxyvitamin D3 (DVD) in human leukemia HL-60 cells. Here, we used the quantitative reverse transcription-polymerase chain reaction method to confirm such previous findings, and compared them with those from the cells treated with all-trans-retinoic acid (ATRA). The results indicated that the gene expression of the transport factors examined was mostly down-regulated following differentiation induced by DVD and ATRA, but importin α5 gene expression was up-regulated in either case. The differences were found in the gene expression of importin α3 and exportin 6 between the cells after treatments with DVD and ATRA. These variations may be related to the difference between HL-60 cell lineages differentiating into monocytes/macrophages and granulocytes. The present findings provide further evidence to support the important roles of importins and exportins in cell differentiation.In eukaryotic cells, the nucleus is separated from the cytoplasm by the nuclear envelope. Macromolecules such as RNA transcripts generated in the nucleus are exported from the nucleus to the ribosomes in the cytoplasm, and proteins synthesized in the cytoplasm, such as histones, polymerases, and transcription factors, are imported into the nucleus. The importin/exportin transport system is involved in the nucleocytoplasmic transport of such cargo molecules (1-4, 6-8). In this system, proteins to be imported or exported have generally a nuclear localization signal (NLS) sequence or a nuclear export signal (NES) sequence. Importin α binds to NLS within protein cargoes and links them to importin β to enter into the nucleus. In some cases, the cargo molecule with NLS directly binds to β and is transported into the nucleus.In an analogous fashion, exportin recognizes NES in the cargo protein and the complex is exported from the nucleus by binding to the GTP-bound form of guanine nucleotide binding protein Ran. There are at least 18 importin and 6 exportin genes in humans and 15 importin and 6 exportin genes in mice. Alteration of the expression level of importins and exportins would affect the transport efficiency, and play a crucial role in development, differentiation, and transformation.It is well-known that human promyelocytic leukemia HL-60 cells can be induced to differentiate into monocytes/macrophages and granulocytes by 1,25-dihydroxyvitamin D3 (DVD) and all-trans-retinoic acid (ATRA), respectively (5). Employing the DNA microarray technique, we previously reported the alteration in gene expression of importins and exportins induced by DVD in HL-60 cells (11). In the present study, we used the quantitative real-time polymerase chain reaction (Q-PCR) method to con-

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Many biological activities of green tea have been attributed to a major constituent, (minus;)-epigallocatechin gallate (EGCG). We previously reported that EGCG and an EGCG-free fraction derived from green tea modulated the gene expression of gluconeogenic enzymes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, in the mouse liver. EGCG is also known to affect the gene expression of enzymes related to lipid metabolism. However, it remains to be examined whether or not a constituent other than EGCG contributes to the change in gene expression of these enzymes. In this study, we prepared an EGCG-free water-soluble fraction (GT-W), and examined its effects on the hepatic gene expression of lipogenic enzymes in mice. The results of quantitative real-time PCR assays indicated that the dietary administration of GT-W for 4 weeks reduced the hepatic gene expression of lipogenic enzymes: fatty acid synthase, hydroxymethylglutaryl coenzyme A reductase, and acetyl-coenzyme A carboxylase alpha. Also, the gene expression of sterol regulatory element-binding transcription factor (Srebf)1 and/or Srebf2 was reduced, suggesting that the reduction of Srebfs contributed to the down-regulation of the lipogenic enzymes, since these transcription factors bind the promoter region to enhance their expression. The plasma levels of triglycerides and cholesterol were reduced with statistical significance in the group given a diet containing GT-W. These results suggest that in addition to EGCG, green tea contains some component(s) which may help to prevent arteriosclerosis and obesity.