Chen Liang, Wang Peixiang, Andrade Cristiano F, Zhao Ian Y, Dubé Philip E, Brubaker Patricia L, Liu Mingyao, Jin Tianru
Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network, Toronto, Ontario.
FEBS J. 2005 Jun;272(11):2746-59. doi: 10.1111/j.1742-4658.2005.04694.x.
Cdx-2 is a transactivator for the proglucagon gene in pancreatic and intestinal endocrine cells. Cdx-2 is also expressed in differentiated intestinal epithelia of nonendocrine origin. Cdx-2-/- mice are embryonic lethal, while Cdx-2+/- mutants show multiple malfunctions including the formation of intestinal polyps. Within the polyps, the remaining wild type Cdx-2 allele ceases its expression, while the expression of both Cdx-2 and proglucagon in the endocrine cells remains unaltered, indicating that Cdx-2 could be haplo-insufficient for nonendocrine cells, but not for proglucagon producing endocrine cells. We propose that mechanisms underlying Cdx-2 expression and auto-regulation [Xu F, Li H & Jin T (1999), J Biol Chem274, 34310-34316] differ in these two types of cells. We show here that forskolin and cAMP upregulate Cdx-2 expression in proglucagon producing cells, but not in colon cancer cells and primary intestinal cell cultures. It is unlikely that the activation is mainly mediated by PKA, because the activation was observed in a PKA deficient cell line. Co-transfecting a dominant negative Ras expression plasmid substantially repressed the Cdx-2 promoter, in contrast to a previous finding that Ras is a negative factor for Cdx-2 expression in colon cancer cells. Furthermore, forskolin activated ERK1/2 phosphorylation in the endocrine cells, and attenuation of ERK1/2 phosphorylation by its inhibitor is associated with attenuated Cdx-2 expression. Finally, an Epac pathway specific cAMP analogue stimulated both ERK1/2 phosphorylation and Cdx-2 expression. Taken together, our observations suggest that Cdx-2 expression is regulated by the second messenger cAMP, cell-type specifically, via the Epac pathway.
Cdx-2是胰腺和肠道内分泌细胞中胰高血糖素原基因的反式激活因子。Cdx-2也表达于非内分泌来源的分化肠道上皮细胞中。Cdx-2基因敲除小鼠胚胎致死，而Cdx-2杂合突变体表现出多种功能异常，包括肠息肉的形成。在息肉内，剩余的野生型Cdx-2等位基因停止表达，而内分泌细胞中Cdx-2和胰高血糖素原的表达保持不变，这表明Cdx-2对非内分泌细胞可能是单倍体不足，但对产生胰高血糖素的内分泌细胞并非如此。我们推测，这两种细胞中Cdx-2表达和自调节的机制[Xu F, Li H & Jin T (1999), J Biol Chem274, 34310-34316]有所不同。我们在此表明，福斯高林和cAMP上调产生胰高血糖素细胞中的Cdx-2表达，但在结肠癌细胞和原代肠细胞培养物中则不然。这种激活不太可能主要由PKA介导，因为在PKA缺陷细胞系中也观察到了这种激活。与之前发现Ras是结肠癌细胞中Cdx-2表达的负因子相反，共转染显性负性Ras表达质粒可显著抑制Cdx-2启动子。此外，福斯高林激活内分泌细胞中的ERK1/2磷酸化，其抑制剂减弱ERK1/2磷酸化与Cdx-2表达减弱相关。最后，一种Epac途径特异性cAMP类似物刺激ERK1/2磷酸化和Cdx-2表达。综上所述，我们的观察结果表明，Cdx-2表达受第二信使cAMP的调节，具有细胞类型特异性，通过Epac途径实现。
