Study For A UCP1-independent Thermogenersis Pathway Mediated By SERCA2 And Comparison Among Species

Adipocytes are classified into white adipocytes,brown adipocytes and beige adipocytes.White adipose tissue stores energy as triglyceride,while beige and brown adipose tissue contains abundant mitochondria in cells,which burns energy and generates heat through non-shivering thermogenesis.Some studies show that norepinephrine released by sympathetic nerves acts on ?3-adrenergic receptors in brown and beige adipocytes,which activate heat producing in these adipocytes.UCP1 is a protein located on the mitochondrial inner membrane.It has been well studied that brown adipocytes process thermogenesis through UCP1.But in beige adipocytes,the mechanisms of UCP1-regulated heat generation remain elusive.This study focus on the metabolism of beige adipocytes under norepinephrine stimulation,helping us to elucidate the role of beige fat in the regulation of whole-body energy expenditure and homeostasis.Our study found that the norepinephrine stimulated oxygen consumption rate in UCP1-KO mice cannot be distinguished from WT group,which means that there is a UCP1 independent thermogenesis pathway existed in mouse beige adipocytes.This compensatory effect rescued the energy deficiency-caused by lacking of UCP1 expression.We further on proved that,compared to WT cells,the gene expression of Serca2 and Ckmt1 are compensatory upregulated in UCP1-KO cells.SERCA2 is Ca2+-ATPase which transfers Ca2+from the cytosol to the lumen of the(SR)at the expense of ATP hydrolysis.CKMT1 is a key enzyme which catalyzes Creatine cycle inside of mitochondria.The activation of these two genes can promote ATP consumption inside the cells.Based on pharmacological inhibition experiment,we showed that the activation of SERCA2 protein was a key regulator for the norepinephrine stimulated cell respiration in UCP1-KO cells.And genetic inhibition of SERCA2(knockdown and knockout)significantly impair the response to norepinephrine stimulation in UCP1-KO cells.Therefore,we can conclude that SERCA2-mediated energy expenditure pathway is a new UCP1-independent thermogenesis mechanism in mouse beige adipocytes,and the function of SERCA2 protein is important on beige adipocytes energy expenditure and homeostasis.In addition,Creatine cycle also plays a role in mouse beige adipocytes energy metabolism.To study whether SERCA2 mediated-Ca2+cycle and Creatine cycle have regulatory role on thermogenesis and energy metabolism except for rodent species,our study turned to reveal the mechanism of UCP1-independent thermogenesis pathway in human and porcine beige adipocytes.In UCP1-KO human beige adipocytes,either pharmacological inhibition of SERCA2 activity or Creatine cycle can totally block cell response to norepinephrine stimulation on respiration,which indicated that SERCA2 and Creatine cycle play equal important role on energy metabolism and thermogenesis in UCP1-independent way.The results in Porcine beige adipocytes supported what we found in mouse and human,that pharmacological and genetic inhibition of SERCA2 blocked norepinephrine stimulated cell respiration.However,Ckmt2 had very low expression and Ckmt1 was undetectable in porcine beige adipocytes,which indicated that Creatine cycle might not be taken as thermogenesis pathway.This study report that SERCA2-Ca2+cycle is an evolutionary conservative UCP1-independent thermogenesis pathway existed in human,mouse and porcine beige adipocytes.This pathway mediates the energy metabolism in beige adipocytes and the response to norepinephrine stimulation in UCP1-independent mechanism.The investigation of this thermogenesis mechanism provided clue and knowledge for new treatment of human obesity and cold-resistant pig species breeding research.