Hedgehog Signaling and Adult Liver Regeneration

描述(申请人提供)：调节慢性肝损伤修复的机制知之甚少。成熟肝细胞在慢性损伤过程中增殖能力下降。为了弥补这一点，肝上皮祖细胞群体扩大和分化以取代死亡的细胞。类似的反应也发生在间充质中，使肝星状细胞(HSC)群体中有增殖的肌纤维母细胞。当间充质-上皮细胞(M-E)的相互作用协调上皮细胞和间充质细胞的平衡扩张和分化时，成功的肝脏结构重建就发生了。然而，缺陷的重塑会导致肝脏结构的紊乱，导致肝硬变和肿瘤。涉及Hedgehog(HH)信号通路的M-E相互作用调节某些成体组织的修复。我们最近发现，健康的成人肝脏含有能够产生HH配体并对其做出反应的细胞。有趣的是，这种HH反应性群体包括未成熟的肝上皮细胞和肝星状细胞。这两种细胞类型在肝脏修复中都发挥着重要作用，提出了以下假设：Hedgehog信号介导的间质-上皮相互作用调节成年肝脏的再生。我们发现，损伤相关因子，如PDGF-BB，促进肌成纤维细胞生长，产生Sonic Hedgehog(Shh)，以自动调节其活性。肝损伤还会扩大产生印度刺猬(IHH)的未成熟胆管细胞的数量，同时显著减少HH抑制剂HIP的肝脏表达。伴随而来的是基质和上皮细胞群的扩张，这些细胞群表达HH靶基因，如PTC和/或Gli。双重免疫荧光染色显示，IHH的表达相对局限于胆管细胞，而上皮细胞和基质细胞都表达HH靶基因。祖细胞群体似乎相对丰富，有HH反应的细胞。随着肝损伤的缓解，纤维化、肌成纤维细胞和上皮祖细胞退化，HH途径的活性逐渐减弱。这些数据支持我们的假设，并证明了进一步努力描述控制成人肝脏HH活性的机制，并阐明HH途径在调节成人肝脏对损伤的反应中所起的作用。因此，我们的目标是确定：1)HSC的激活如何改变HH配体的产生和反应；2)不同类型的肝上皮细胞是否影响它们对HSC衍生的HH配体的反应，或者它们诱导HSC产生HH配体的能力；以及3)调节HH信号活性是否改变肝损伤后的再生。公共卫生相关性：这项工作的结果将扩大目前对维持和恢复成人肝脏结构的复杂动态平衡机制的理解。这些知识可能会对肝病的诊断、预防和治疗产生影响，因此具有重要的临床意义。

DESCRIPTION (provided by applicant): The mechanisms that regulate repair of chronic liver injury are poorly understood. Mature hepatocytes proliferative capacity declines during chronic injury. To compensate for this, hepatic epithelial progenitor populations expand and differentiate to replace dying cells. Similar responses occur in the mesenchyme, enriching hepatic stellate cell (HSC) populations with proliferating myofibroblastic cells. Successful reconstruction of liver architecture occurs when mesenchymal-epithelial (M-E) interactions orchestrate balanced expansion and differentiation of both epithelial and mesenchymal cells. However, defective remodeling leads to disorganization of hepatic architecture, resulting in cirrhosis and neoplasia. M-E interactions that involve the Hedgehog (Hh) signaling pathway modulate repair of some adult tissues. We recently discovered that healthy adult livers contain cells that are capable of both producing and responding to Hh ligands. Interestingly, this Hh-reactive population includes immature liver epithelial cells and hepatic stellate cells. Both cell types play important roles in liver repair, suggesting the following HYPOTHESIS: Hedgehog signaling-mediated mesenchymal-epithelial interactions regulate regeneration of adult livers. We found that injury-related factors, such as PDGF-BB, promote the outgrowth of myofibroblastic cells that produce Sonic hedgehog (Shh) to auto-regulate their viability. Liver injury also expands populations of immature bile ductular cells that produce Indian hedgehog (Ihh), while significantly reducing hepatic expression of the Hh inhibitor, Hip. This is accompanied by expansion of stromal and epithelial cell populations that express Hh-target genes, such as Ptc and/or Gli. Double immunofluorescence staining reveals that Ihh expression is relatively restricted to bile ductular cells, while both epithelial and stromal cells express Hh-target genes. Progenitor populations seem to be relatively enriched with Hh-responsive cells. As liver damage resolves, fibrosis, myofibroblastic cells, and epithelial progenitors regress and Hh-pathway activity gradually subsides. These data support our hypothesis and justify further efforts to delineate mechanisms that control Hh activity in adult livers and clarify the role of the Hh pathway in regulating how adult livers respond to injury. Thus, our Aims are to determine: 1) how the activation of HSC alters the production of- and response to- Hh ligands; 2) if the phenotype of different types of liver epithelial cells influences their response to HSC-derived Hh ligands, or their ability to elicit HSC production of Hh ligands; and 3) if modulating Hh signaling activity alters regeneration following liver injury. PUBLIC HEALTH RELEVANCE: Results from this work will extend current understanding about the complex homeostatic mechanisms that maintain and restore liver architecture in adults. Such knowledge is likely to impact upon liver disease diagnosis, prevention and treatment and thus, has important clinical implications.