Dissecting A Function Of The BHD Syndrome Gene In Cellular Iron Metabolism

Mutations of the FLCN(folliculin)gene can cause the Birt-Hogg-Dubé(BHD)syndrome.The major symptoms of BHD include fibrous hair follicle tumors,lung cysts,and kidney cancer.FLCN can regulate a variety of cellular processes,such as cell migration,cell adhesion,protein transportation,energy and nutrient metabolism,mitochondria and lysosome biosynthesis,but the precise reasons of BHD caused by FLCN loss remain to be elucidated.The FLCN gene is widely conserved from the unicellular yeast to mammals.This indicates FLCN probably regulates some fundamental cellular physiological processes.In a previous study,we identified FLCN as a binding protein of Rab11 A,which is a key regulator of the recycling endosomal transport.Specifically,we found FLCN promotes the binding of Rab11 A to the transmembrane amino acid transporter PAT1.This allows PAT1 to actively move onto the cell surface and thus inhibits PAT1 transport to the lysosome,thereby preventing the release of the lysosomal amino acids into the cytoplasm.As a result,the high luminal amino acids can induce the m TORC1 signaling pathway.A surprising funding of this study is that FLCN can also promote the binding of Rab11 A to the transferrin receptor TfR1(also called CD71).This gave us a hint that FLCN may regulate iron homeostasis by controlling the TfR1 intracellular transport process.Iron is an essential micronutrient for nearly all the living organisms.It is a cofactor of many enzymes involved in some important physiological processes,such as oxygen storage and transportation,energy generation,and DNA synthesis.Not surprisingly,abnormal iron metabolism can lead to many disorders.Then the question arises,is FLCN involved in iron metabolism,or in another word,is BHD a disease of abnormal iron metabolism? In order to address this point,we performed this research,focusing on the cellular iron metabolism process.We used both mammalian cell cultures and a Drosophila BHD animal model,and applied various cell biology,biochemistry,and molecular biology research methods.The main research results are as follows:1.FLCN can bind with TfR1,and this interaction is enhanced by iron deficiency;2.FLCN promotes the TfR1-Rab11 A interaction,which was weakened by FLCN loss;3.FLCN regulates the TfR1 transport;the recycling of TfR1 to the cell surface was delayed by FLCN loss;4.FLCN regulates iron homeostasis;loss of FLCN decreased the iron level in a variety of cell types;5.FLCN regulates iron level through Rab11A;inducing the Rab11 A activity can increase iron level in FLCN-/-cells;6.FLCN regulates iron metabolism in Drosophila.Iron rich food can rescue the phenotypes of the Drosophila BHD larvae;7.FLCN regulates the iron-dependent ferroptosis;FLCN-/-cells are less prone to ferroptosis;overexpression of FLCN made cells sensitive to ferroptosis induction;8.FLCN regulates the stability of TfR1 protein;loss of FLCN induces TfR1 degradation through the ERAD pathway.These results suggest a model.That is,FLCN regulates the intracellular transport of TfR1 by promoting the TfR1-Rab11 A interaction,facilitating iron absorption from the extracellular environment.In the absence of FLCN,the TfR1-Rab11 A interaction is weakened.As a result,the transport of TfR1 to the cell surface is delayed.This leads to decreased iron absorption ability and the cellular iron pool is decreased.FLCN can regulate iron metabolism in different types of mammalian cells and in Drosophila,suggesting this might be a general function of FLCN.Because the FLCN deficient cells have decreased iron levels,these cells are resistant to ferroptosis induction.We speculate that this might be a new tumor suppressor mechanism of FLCN.Our research laid the foundation for investigating the relationship between BHD and iron homeostasis in future studies.