A Regulatory Pathway, Ecdysone-Transcription Factor Relish-Cathepsin L, Is Involved in Insect Fat Body Dissociation

Insect fat body is the organ for intermediary metabolism, comparable to vertebrate liver and adipose tissue. Larval fat body is disintegrated to individual fat body cells and then adult fat body is remodeled at the pupal stage. However, little is known about the dissociation mechanism. We find that the moth Helicoverpa armigera cathepsin L (Har-CL) is expressed heavily in the fat body and is released from fat body cells into the extracellular matrix. The inhibitor and RNAi experiments demonstrate that Har-CL functions in the fat body dissociation in H. armigera. Further, a nuclear protein is identified to be transcription factor Har-Relish, which was found in insect immune response and specifically binds to the promoter of Har-CL gene to regulate its activity. Har-Relish also responds to the steroid hormone ecdysone. Thus, the dissociation of the larval fat body is involved in the hormone (ecdysone)-transcription factor (Relish)-target gene (cathepsin L) regulatory pathway.

昆虫脂肪体（Insect fat body）是机体中间代谢的关键器官，其功能等价于脊椎动物的肝脏与脂肪组织。幼虫脂肪体先解离为单个脂肪细胞，成虫脂肪体则在蛹期完成重塑。然而，当前学界对其解离机制的认知仍较为匮乏。本研究发现，棉铃虫（Helicoverpa armigera）组织蛋白酶L（cathepsin L，下文简称Har-CL）在脂肪体中呈高表达状态，并可从脂肪细胞释放至细胞外基质（extracellular matrix）中。通过抑制剂实验与RNA干扰（RNAi）实验验证，Har-CL在棉铃虫幼虫脂肪体解离过程中行使核心功能。后续研究进一步鉴定出一种核蛋白为转录因子Har-Relish，该因子既往被证实参与昆虫免疫应答，且能特异性结合Har-CL基因的启动子区域以调控其转录活性。此外，Har-Relish还可响应类固醇激素蜕皮激素（ecdysone）的信号。综上，幼虫脂肪体的解离过程隶属于“激素（蜕皮激素）-转录因子（Relish）-靶基因（组织蛋白酶L）”的调控通路。