Functions And Mechanisms Of TNF? And IL-1?-induced NF-?B Activation Regulated By USP15

The inflammatory response is a violent immune response.Organisms tightly regulate the induction and resolution of the inflammatory response to resist pathogens,perform self-repair,and restore the homeostasis.Tumor necrosis factor ?(TNF?)and interleukin 1?(IL-1?)-induced nuclear factor ?B(NF-?B)activation play key roles in inflammation,immunity,and cancer development.Many proteins participate in the regulation of NF-?B activation by targeting the adaptors in the NF-?B activation.Here,we identified that a deubiquitinating enzymes(DUB)?? ubiquitin-specific protease15(USP15)stabilized TAK1 binding protein 2(TAB2)and TAK1 binding protein 3(TAB3)in different ways to promote the activation of the transforming growth factor-? activated kinase-1(TAK1)-TABs complex,thereby promoting TNF? and IL-1?-induced NF-?B activation.Previous results in our research group showed that overexpression of USP15 enhanced the activation of inhibitor of NF-?B kinases(IKKs)and degradation of inhibitor of NF-?B ?(I?B?)induced by TNF? and IL-1?,as well as downstream gene transcription.Moreover,we found that USP15 also activated three main members of mitogen activated protein kinase(MAPKs)?? c-Jun N-terminal kinase(JNK),extracellular regulated MAP kinase(ERK)and p38.And knockdown of USP15 inhibited the activation of JNK,ERK and p38.Through reporter assay and Western blot experiments,we found that USP15 specifically stabilized exogenous or endogenous TAB2/3,and re-expressing USP15 in USP15-RNAi stable cells rescued the accelerated degradation of TAB2/3 induced by TNF?.However USP15 did not have these effects on the other members of the TAK1-TABs complex ?? TAK1,TAB1(TAK1 binding protein 1),or tumor necrosis factor receptor(TNFR)associated factor 6(TRAF6).Through co-immuno-precipitation(co-IP)experiments,we further confirmed that USP15 interacted with TAB2/3 and TNF? could enhance the interaction between them.More researches confirmed that USP15 up-regulated TAB2/3 expression by inhibiting their protein degradation,and the ubiquitin carboxyl-terminal hydrolase(UCH)domain of USP15 played an important role in this function.Interestingly USP15 inhibited the degradation of TAB2 and TAB3 via different processes.USP15 stabilized TAB2 by inhibiting its ubiquitin-proteasomal and lysosomal degradation,while stabilized TAB3 by inhibiting its autophagy-lysosomal and lysosomal degradation.Through ubiquitination-assay experiments,we also found that USP15 could specifically cleavage of lysine 48-linked TAB2 ubiquitination but not TAB3 in a catalytic-dependent way.At the same time,through Western blot experiments and confocal microscopy assay,it was found that USP15 inhibited NBR1-mediated selective autophagic degradation of TAB3 in a catalytic-independent way.Moreover,we further found that USP15 reduced colocalization of TAB2/3 and LAMP1(lysosomal associated membrane protein 1),which consequently inhibited the lysosomal degradation pathway of TAB2 and TAB3 in a catalytic-independent way.In summary,we found that USP15 targets the TAK1-TABs complex to enhance TNF? and IL-1?-induced NF-?B activation by stabilizing TAB2/3 differently.Apart from deubiquitination-dependently inducing cleavage of lysine 48-linked TAB2 ubiquitination,USP15 also DUB-independently inhibited lysosome-associated TAB2 degradation.For TAB3,USP15 inhibited NBR1-mediated selective autophagic TAB3 degradation independent of its deubiquitinating activity.Firstly,our results indicated TAB2/3 as the new targets of USP15,and the functional diversity of USP15 which carried out function by enzyme dependent and independent ways.Finally as the NF-?B activation is involved in a lot of physiological and pathological processes such as cell survival,growth and development,immune response and cancer development,USP15 or TAB2/3 can be a potential target for drug design.These drugs could regulate the expression of USP15 or activation of TAK1-TABs complex mediated by TAB2/3,in order to regulate the NF-?B activation to achieve therapeutic effects.