Function And Mechanism Of PPM1B In TGF-β/BMP Signaling Pathway

Transforming growth factor-β(TGF-β)mediated signaling pathway plays an important role in multiple physiological processes,such as cell proliferation,cell differentiation,apoptosis,cell migration,immune response and embryo development.SMAD family proteins are key factors in TGF-β signal transduction.SMAD proteins are classified into three types,Receptor Regulated SMAD(R-SMAD),Common Mediator SMAD(Co-SMAD)and Inhibitory SMAD(I-SMAD).After phosphorylation by TGF-β type I receptor,R-SMAD associates with Co-SMAD as a heterotrimer,which regulates the transcription of downstream genes through interaction with other cofactors in the nucleus.The phosphorylation of R-SMAD C terminal SXS motif by the type I receptor is a central event in the entire signaling pathway.Thus,its phosphorylation level is very crucial in the transduction of TGF-β signaling,which is controlled by Ser/Thr kinases and phosphatases.Several Ser/Thr phosphatases can dephosphorylate the phosphorylation of R-SMAD at the SXS motif,which terminates TGF-β signaling.Here we found that PPM1B(Protein phosphatase 1B),a phosphatase belonging to the PPM family,can downregulate the phosphorylation level of SMAD2/3 and SMAD1.PPM1 B plays a critical role in innate antiviral immunity,cell necrosis,and NF-κB signaling pathway by dephosphorylating TBK1,Rip3,and IKK beta.However,the function of PPM1 B in TGF-β signaling pathway is still unknown.Here we found that PPM1 B gene produces several mRNA isoforms,in which the mRNA level of PPM1 B isoform2 was upregulated by TGF-β in HaCaT cells.Both PPM1 B isoform1 and isoform2 can downregulate the phosphorylation level of SMAD2 and SMAD3.Meanwhile,PPM1 B phosphatase-dead mutant R179 G lost the function,which suggests PPM1 B dephosphorylates R-SMAD dependent on its phosphatase activity.Subsequently,we demonstrated that PPM1 B is an inhibitor of TGF-β signaling pathway by luciferase reporter assay and qPCR assay.We further found that PPM1 B exerts an inhibitory effect on the TGF-β signaling pathway through controlling the phosphorylation level of SMAD2 and SMAD3.On the other hand,PPM1 B also restricts BMP signaling through controlling the phosphorylation level of SMAD1,which suggests PPM1 B acts as a pan-phosphatase for all R-SMADs.Similarly,BMP2 also increases the expression of PPM1 B isoform2.In summary,we found that TGF-β/BMP regulates PPM1 B expression,which in turn,PPM1 B might dephosphorylate R-SMAD,thereby inhibiting TGF-β/BMP signaling pathway.The entire process forms a negative feedback regulation system.This study reveals a new mechanism of TGF-β/BMP signaling regulation and enriches a new function of PPM1 B,which might provide potential drug targets for human diseases related to TGF-β/BMP signaling.