Investigating The Role And Mechanism Of LKb1 In Brown Adipose Tissue

Adipose tissue dysfunction linked obesity leads to development of cardiovascular disease,type 2 diabetes,atherosclerosis,hepatic steatosis and some cancers.Brown/beige adipocyte dissipate the energy into heat by Ucp1 uncoupling to ATPase,which increases the systematic energy expenditure,improves glucose tolerance and insulin sensitivity.Lkb1 is the member of the serine/threonine kinases family,whose classical substrate is AMPK,as an energetic sensor in vivo.Lkb1 plays important roles in various biological processes including cellular energy metabolism,cell polarity,cancer initiation and progression,whether Lkb1 is involved in development of brown adipose tissue(BAT)and browning of white adipose tissue(WAT)remains unexplored.Thus,we established adipose tissue specific knockout Lkb1 mice by Adipoq-Cre/Loxp.Firstly,Ad-Lkb1 KO mice were analyzed at morphological and molecular level to reveal Lkb1 regualation on adipocyte thermogenic function and systematic energy expenditure.Further,the mechanism of Lkb1 upregulation on systemic energy metabolism and Ucp1 expression were explored by ChIP,CoIP and dual luciferase assay.In addition,adipocytes are not only critical regulators of systemic energy homeostasis but also play key roles in inflammation.Ad-Lkb1 KO mice exhibit brown fat expansion,which is associated with low-chronic inflammation.Unexpectedly,Ad-Lkb1 mice develop hindlimb paralysis and sciatic nerve degeneration.As this mouse is deleted Lkb1 restricted in adipocyte,including epineurial adipocyte,it's a good model to explore the interaction between adipocyte and peripheral nerve.Thus,we elucidate the pathology of this model by monitoring the mice activity,observing ultrastructure and measuring compound action potential(CAP).Furthermore,the microarray assay of brown fat is performed to reveal the differential gene expression profiles,combination with Clodrosome treatment,which identify the key role of brown adipocyte inflammation for sciatic nerve function.Lastly,we carry out rapamycin treatment and establishment of Lkb1 and mTOR double knockout mice to investigate the underlying mechanism of Lkb1 deletion caused brown adipocyte inflammation and sciatic nerve degeneration.The major result as followed:1.Lkb1 regulates brown fat expansion and thermogenesis(1)Deletion of Lkb1 significantly increases the mass of brown adipose tissue(BAT)and expression of brown specific genes.(2)Lkb1 ablation induces the browning of white adipose tissue and promotes the browning response of inguinal white adipose tissue by cold stimulation.(3)Ad-Lkb1 KO mice improve the glucose tolerance and insulin sensitivity.Also,KO mice have higher systematic energy metabolism indicated by increasing O2 consumption,CO2 production,heat production,compared to those of wild type.(4)Ad-Lkb1 KO mice protect against obesity and fatty liver induced by high fat diet dependent on Ucp1 function.(5)Adipose tissue specific knockout Lkb1 leads to nuclear localization of CRTC3,which binds to Cebp? to augment the transcription of Ucp1.This finding reveals the mechanism of Lkb1 KO upregulating thermogenic genes and inducing the browning of WAT.2.Ad-Lkb1 KO induces the brown fat cell inflammation and sciatic nerve degeneration.(1)Ad-Lkb1 mice develop the hindlimb dysfunction and this pathology is sciatic nerve degeneration,indicated by reducing the total number of axons,exhibiting the abnormality of axonal myelin sheath and decreasing the compound action potential.(2)Ad-Lkb1 specifically induces proinflammatory genes expression in brown adipose tissue and epineurial adipocyte,which recruits macrophage to damage sciatic nerve in both systematic and local inflammation manners.(3)Deletion of macrophage by Clodrosome delays the occurrence and alleviates the severity of hindlimb paralysis in KO mice,which confirms that inflammation leads to macrophage infiltration into sciatic nerve and subsequently axon degeneration.(4)Rapamycin treated KO mice and double KO mice are behaviorally normal and completely free of paralysis.Moreover,the inflammation in BAT and sciatic nerve,ultrastructure of axons and CAP are all rescued,which indicate that Lkb1 KO causes brown adipocyte inflammation and sciatic nerve degeneration through activation of mTOR pathway.These data indicated although increased mass of BAT promotes energy expenditure,the chronic inflammation from brown adipocyte caused by BAT expassion should not be ignorced.