APC control of intestinal differentiation

我们已经证明，人类结肠腺瘤和结肠癌显示出严重缺乏维甲酸生物合成酶，APC通过控制视黄醇脱氢酶的表达来控制肠道细胞的分化。这些发现表明了一种新的模式，即APC通过控制维甲酸的生物合成来促进肠道细胞的分化，并表明APC的功能并不局限于其在调节规范的WNT信号中的既定作用。从机制上讲，我们已经确定转录共抑制因子C末端结合蛋白(CtBP)是一种新的APC调节蛋白，可以抑制视黄醇脱氢酶和肠道细胞分化。与APC对照的CtBP一致，取自FAP患者的人腺瘤与邻近未受累组织相比，显示出强烈的核CtBP染色。然而，令人惊讶的是，这些相同的切片几乎没有显示出核B-连环蛋白的证据，这表明CtBP的积累可能先于B-连环蛋白的核积累，并且B-连环蛋白的核积累可能除了失去APC之外还需要发生事件。与这种可能性一致的是，许多报告表明，在小腺瘤和大腺瘤中，(B-catenin)信号的激活似乎与k-ras癌基因的激活突变平行。此外，对携带人类FAP典型突变的小鼠的研究表明，k-ras的激活增加了息肉的大小和数量。这些数据表明，在大腺瘤形成过程中，k-ras的激活允许B-连环蛋白刺激的肠道细胞增殖。事实上，我们的初步数据显示，仅APC的丢失不足以促进斑马鱼的肠道细胞增殖。此外，致癌的k-ras促进B-连环蛋白在人类细胞和斑马鱼中的积累和随后的增殖。我们对FAP和未定义的高危家族的了解，以及我们使用斑马鱼作为遗传模型系统研究APC的专业知识，为我们提供了一个独特的机会来评估CtBP和k-ras在结肠腺瘤发生和发展中的作用。

We have shown that human colon adenomas and carcinomas show a profound deficiency of retinoic acid biosynthetic enzymes and that APC controls intestinal cell differentiation by controlling the expression of retinol dehydrogenases. These findings suggest a novel model wherein APC promotes enterocyte differentiation by controlling retinoic acid biosynthesis and indicates that the functions of APC are not limited to its well-established role in regulating canonical WNT signaling. Mechanistically, we have identified the transcriptional co-repressor, C-terminal binding protein (CtBP), as a novel, APC-regulated protein that suppresses retinol dehydrogenases and intestinal cell differentiation. Consistent with APC control of CtBP, human adenomas taken from FAP patients showed robust staining for nuclear CtBP in comparison to adjacent, uninvolved tissues. Surprisingly, however, these same sections showed little evidence of nuclear B-catenin suggesting that accumulation of CtBP may precede nuclear accumulation of B-catenin and that nuclear accumulation of B-catenin may require events in addition to loss of APC. Consistent with this possibility are a number of reports showing that activation of (B-catenin signaling in small versus large adenomas appears to parallel activating mutations in the k-ras oncogene. Further, work in mice carrying a mutation typical of those found in human FAP has shown that activation of k-ras increased polyp size and number. These data suggest that k-ras activation permits B-catenin-stimulated intestinal cell proliferation during the formation of a large adenoma. Indeed our preliminary data show that APC loss alone is insufficient to promote intestinal cell proliferation in zebrafish. Furthermore, oncogenic k-ras promotes the accumulation of nuclear B-catenin and subsequent proliferation in both human cells and zebrafish. Our access to both FAP and undefined high-risk families, as well as our expertise in using zebrafish as a genetic model system to study APC, provides us a unique opportunity to evaluate the contribution of CtBP and k-ras in the initiation and progression of colon adenomas.