Functions of SMAD4 in development and cancers

我们对突变小鼠的初步分析揭示了SMAD基因在多个生物过程中的不同功能。我们发现SMAD2和SMAD4是原肠胚形成所必需的，SMAD5是血管生成所必需的，SMAD3是建立粘膜免疫反应和骨骼正常发育所必需的。我们最近的研究重点是SMAD4，它是tgf - β信号的一种常见介质。SMAD4也被称为DPC4（在胰腺癌基因座4中缺失）。SMAD4突变已在胰腺癌、结肠癌、胆管细胞癌、胃息肉病和腺癌中被检测到。我们的研究表明，SMAD4对小鼠的胚胎发育至关重要，因为SMAD4的缺失会导致E67时胚胎外膜形成受损和外胚层增殖减少而致死。smad4杂合小鼠由于单倍功能不全而发生胃息肉病和胃癌。使用SMAD4- co小鼠，我们已经证明SMAD4缺乏可导致乳腺组织、皮肤、肝脏、前胃和结肠的肿瘤形成。肝细胞中SMAD4的缺失也会导致铁积累，这是一种类似人类血色素沉着病的疾病。

Our preliminary analysis on the mutant mice has revealed distinct functions of SMAD genes in multiple biological processes. We showed that SMAD2 and SMAD4 are needed for gastrulation, SMAD5 for angiogenesis, and SMAD3 for establishment of the mucosal immune response and proper development of the skeleton. Our recent effort is focused on SMAD4, which is a common mediator of TGF-beta signals. SMAD4 is also called DPC4 (deleted in pancreastic cancer locus 4). Mutatons of SMAD4 has been detected in pancreatic cancer, colon cancer, cholangiocellular carcinoma, gastric polyposis, and adenocarcinomas. Our study indicates that SMAD4 is essential for embryonic development in mice, as loss of SMAD4 results in lethality at E67 due to impaired extraembryonic membrane formation and decreased epiblast proliferation. SMAD4-heterozygous mice developed gastric polyposis and cancer due to haploinsufficiency. Using SMAD4-Co mice, we have demonstrated that SMAD4 deficiency could cause tumor formation in mammary tissue, skin, liver, forestomach, and colon. Deletion of SMAD4 in hepatic cells also results in iron accumulation, a disease mimicking human hemochromatosis. In the past year, we focused on studying SMAD4 in cholangiocellular carcinoma (CC). CC is the second most common primary liver cancer, and is associated with a poor prognosis. It has been shown that CCs harbor alterations of a number of tumor-suppressor genes and oncogenes, yet key regulators for tumorigenesis remain unknown. Here we have generated a mouse model that develops CC with high penetrance using liver-specific targeted disruption of tumor suppressors SMAD4 and PTEN. In the absence of SMAD4 and PTEN, hyperplastic foci emerge exclusively from bile ducts of mutant mice at 2 months of age and continue to grow, leading to tumor formation in all animals at 4-7 months of age. We show that CC formation follows a multistep progression of histopathological changes that are associated with significant alterations, including increased levels of phosphorylated AKT, FOXO1, GSK-3beta, mTOR, and ERK and increased nuclear levels of cyclin D1. We further demonstrate that SMAD4 and PTEN regulate each other through a novel feedback mechanism to maintain an expression balance and synergistically repress CC formation. Finally, our analysis of human CC detected PTEN inactivation in a majority of p-AKT-positive CCs, while about half also lost SMAD4 expression. These findings elucidate the relationship between SMAD4 and PTEN and extend our understanding of CC formation.