Institute of Biochemistry, Faculty of Medicine, University of Leipzig, Leipzig, Germany.
Cell Commun Signal. 2014 Feb 18;12:11. doi: 10.1186/1478-811X-12-11.
Hedgehog signaling plays an important role in embryonic development, organogenesis and cancer. In the adult liver, Hedgehog signaling in non-parenchymal cells has been found to play a role in certain disease states such as fibrosis and cirrhosis. However, whether the Hedgehog pathway is active in mature healthy hepatocytes and is of significance to liver function are controversial.
Two types of mice with distinct conditional hepatic deletion of the Smoothened gene, an essential co-receptor protein of the Hedgehog pathway, were generated for investigating the role of Hedgehog signaling in mature hepatocytes. The knockout animals (KO) were inconspicuous and healthy with no changes in serum transaminases, but showed a slower weight gain. The liver was smaller, but presented a normal architecture and cellular composition. By quantitative RT-PCR the downregulation of the expression of Indian hedgehog (Ihh) and the Gli3 transcription factor could be demonstrated in healthy mature hepatocytes from these mice, whereas Patched1 was upregulated. Strong alterations in gene expression were also observed for the IGF axis. While expression of Igf1 was downregulated, that of Igfbp1 was upregulated in the livers of both genders. Corresponding changes in the serum levels of both proteins could be detected by ELISA. By activating and inhibiting the transcriptional output of Hedgehog signaling in cultured hepatocytes through siRNAs against Ptch1 and Gli3, respectively, in combination with a ChIP assay evidence was collected indicating that Igf1 expression is directly dependent on the activator function of Gli3. In contrast, the mRNA level of Igfbp1 appears to be controlled through the repressor function of Gli3, while that of Igfbp2 and Igfbp3 did not change. Interestingly, body weight of the transgenic mice correlated well with IGF-I levels in both genders and also with IGFBP-1 levels in females, whereas it did not correlate with serum growth hormone levels.
Our results demonstrate for the first time that Hedgehog signaling is active in healthy mature mouse hepatocytes and that it has considerable importance for IGF-I homeostasis in the circulation. These findings may have various implications for mouse physiology including the regulation of body weight and size, glucose homeostasis and reproductive capacity.
Hedgehog 信号通路在胚胎发育、器官发生和癌症中起着重要作用。在成年肝脏中，已发现非实质细胞中的 Hedgehog 信号通路在某些疾病状态如纤维化和肝硬化中发挥作用。然而，Hedgehog 途径是否在成熟健康的肝细胞中活跃，以及对肝功能是否有意义，仍存在争议。
为了研究 Hedgehog 信号在成熟肝细胞中的作用，我们生成了两种具有明显条件性肝 Smoothened 基因缺失的小鼠，Smoothened 基因是 Hedgehog 途径的必需共受体蛋白。这些敲除动物（KO）没有明显的异常和健康表现，血清转氨酶没有变化，但体重增长较慢。肝脏较小，但呈现正常的结构和细胞组成。通过定量 RT-PCR 可以证明，这些小鼠的成熟健康肝细胞中 Indian hedgehog（Ihh）和 Gli3 转录因子的表达下调，而 Patched1 上调。IGF 轴的基因表达也发生了强烈的改变。Igf1 的表达下调，而 Igfbp1 的表达在雌雄两性肝脏中均上调。ELISA 可检测到相应的血清蛋白水平变化。通过 siRNA 分别针对 Ptch1 和 Gli3 激活和抑制培养的肝细胞中 Hedgehog 信号的转录输出，并结合 ChIP 实验，我们收集了证据表明 Igf1 表达直接依赖于 Gli3 的激活功能。相比之下，Igfbp1 的 mRNA 水平似乎受 Gli3 抑制功能的控制，而 Igfbp2 和 Igfbp3 的水平没有变化。有趣的是，转基因小鼠的体重与两性的 IGF-I 水平以及雌性的 IGFBP-1 水平密切相关，而与血清生长激素水平无关。
我们的研究结果首次证明 Hedgehog 信号在健康成熟的小鼠肝细胞中活跃，并对循环中的 IGF-I 稳态具有重要意义。这些发现可能对包括体重和体型调节、葡萄糖稳态和生殖能力在内的小鼠生理学具有多种意义。
