| Objective:Acetaminophen(APAP)overdose causes acute liver injury(ALI),acute liver failure(ALF),and even death.At present,N-acetylcysteine(NAC)is commonly used to treat APAP-induced hepatotoxicity in clinic,but adverse reactions limit its application.Therefore,it is necessary to develop new intervention targets and drugs.In previous studies,we confirmed that Nicotinamide(NAM)could alleviate APAP-induced acute liver injury,but the underlying mechanism is not yet clear.Nicotinamide adenine dinucleotide(NAD),which can be synthesized from NAM as a precursor,has two states:the oxidized(NAD~+)and the reduced(NADH),and participates in many critical cell processes.In addition,NAD~+can also be used as a substrate for the enzymes to regulate their activities.In recent years,alterations of NAD~+levels in diseases have attracted more and more attention.Thus NAD~+may be a potential target for conteracting APAP-induced acute liver injury,and its precursor NAM may play a protective role by affecting liver NAD~+levels.Method:Male Balb/c mice were intraperitoneally injected with different concentrations(100 mg/kg,300 mg/kg,and 500 mg/kg)of APAP to mimic an acute liver injury.The level of NAD~+in liver was detected by spectrophotometry,and two important proteins expressions in the synthesis and consumption of NAD~+,Nicotinamide phosphoribosyltransferase(NAMPT)and poly(ADP-ribose)polymerase 1(Poly(ADP-ribose)polymerase-1,PARP-1),were detected by Western blot to explore the role of NAD~+in acute liver injury.Furthermore,NAM pretreated APAP-bearing mice to observe the effects on liver NAD~+levels and protein expressions of NAMPT and PARP-1.At the same time,to clarify whether NAM exerts a protective effect,the serum ALT and AST levels were detected by Lai's method,the liver index was measured,and gross appearance and morphological changes under the microscope were observed.Ultimately,the expressions of crucial molecules in antioxidant signaling pathway,Nuclear factor erythroid 2-related factor 2(Nrf2)and Kelch-like ECH-associated protein-1(Keap1),were measured by Western blot,and the important indicators of oxidative stress:malondialdehyde(MDA),myeloperoxid(MPO),superoxide dismutase(SOD),glutathione(GSH)were also evaluated.Results:(1)After APAP exposure,the hepatic NAD~+levels were decreased in a dose-dependent manner,which were companied with the downregulation of NAMPT and upregulation of PARP-1.(2)Pretreatment of NAM improved hepatic NAD~+reduction due to APAP administration via upregulating NAMPT and downregulating PARP-1 protein expression,respectively.(3)NAM attenuated serum ALT and AST levels,decreased liver index,and partially improved liver abnormal morphology.(4)APAP reduced Nrf2 protein expression,while upregulated Keap1 protein level,and NAM treatment improved Nrf2 downregulation and repressed Keap1upregulation.Furthermore,NAM triggered Nrf2 translocation from cytoplasm to nucleus which evidenced Nrf2 activation.Ultimately,pretreatment with NAM reduced the increase of MDA and MPO caused by APAP exposure.In contrast,reduced SOD and GSH levels were improved.Conclusion:The results show that APAP can reduce liver NAD~+levels due to downregulation of NAMPT and upregulation of PARP-1,which means reduced synthesis and elevated consumption of NAD~+.NAM has the potential to promote NAMPT and inhibit PARP-1 expression,and improves liver NAD~+levels.NAM can activate the Nrf2 signaling pathway,and enhance antioxidant capacity to alleviate APAP-induced acute liver injury... |
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