Mitogen-activated protein kinases (MAPKs) are key regulators of signal transduction and cell responses. Abnormalities in MAPKs are associated with multiple diseases. Dual-specificity phosphatases (DUSPs) dephosphorylate many key signaling molecules, including MAPKs, leading to the regulation of duration, magnitude, or spatiotemporal profiles of MAPK activities. Hence, DUSPs need to be properly controlled. Protein post-translational modifications, such as ubiquitination, phosphorylation, methylation, and acetylation, play important roles in the regulation of protein stability and activity. Ubiquitination is critical for controlling protein degradation, activation, and interaction. For DUSPs, ubiquitination induces degradation of eight DUSPs, namely, DUSP1, DUSP4, DUSP5, DUSP6, DUSP7, DUSP8, DUSP9, and DUSP16. In addition, protein stability of DUSP2 and DUSP10 is enhanced by phosphorylation. Methylation-induced ubiquitination of DUSP14 stimulates its phosphatase activity. In this review, we summarize the knowledge of the regulation of DUSP stability and ubiquitination through post-translational modifications.

Systemic lupus erythematosus (SLE) is a complex autoimmune disease that is characterized by systemic inflammation and multiple organ failures. Dysregulation of T cells plays a critical role in SLE pathogenesis. Our previous study indicates that JKAP (also named DUSP22) inhibits T-cell activation and that JKAP knockout mice develop spontaneous autoimmunity; therefore, we investigated whether JKAP downregulation is involved in SLE patients. JKAP protein levels in purified T cells were examined by immunoblotting using blood samples from 43 SLE patients and 32 healthy controls. SLE patients showed significantly decreased JKAP protein levels in peripheral blood T cells compared to healthy controls. JKAP protein levels in peripheral blood T cells were inversely correlated with SLE disease activity index (SLEDAI) and anti-dsDNA antibody levels. JKAP downregulation in T cells was highly correlated with daily urinary protein amounts and with poor renal outcome in lupus nephritis patients. Notably, the diagnostic power of JKAP downregulation in T cells for active lupus nephritis was higher than those of serum anti-dsDNA antibody, C3, and C4 levels. Moreover, T-cell-specific transgenic mice expressing a dominant-negative JKAP mutant developed spontaneous autoimmune nephritis. Furthermore, JKAP-deficient T cells overproduced complement components, soluble ICAM-1, and soluble VCAM-1 in the kidney; these cytokines have been reported to be involved in lupus nephritis. Taken together, JKAP downregulation in T cells is a novel diagnostic and prognostic biomarker for SLE nephritis.

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T cell activation is dependent upon phosphorylation of MAPKs, which play a critical role in the regulation of immune responses. Dual-specificity phosphatase 14 (DUSP14; also known as MKP6) is classified as a MAPK phosphatase. The in vivo functions of DUSP14 remain unclear. Thus, we generated DUSP14-deficient mice and characterized the roles of DUSP14 in T cell activation and immune responses. DUSP14 deficiency in T cells resulted in enhanced T cell proliferation and increased cytokine production upon T cell activation. DUSP14 directly interacted with TGF-β–activated kinase 1 (TAK1)-binding protein 1 (TAB1) and dephosphorylated TAB1 at Ser438, leading to TAB1–TAK1 complex inactivation in T cells. The phosphorylation levels of the TAB1–TAK1 complex and its downstream molecules, including JNK and IκB kinase, were enhanced in DUSP14-deficient T cells upon stimulation. The enhanced JNK and IκB kinase activation in DUSP14-deficient T cells was attenuated by TAB1 short hairpin RNA knockdown. Consistent with that, DUSP14-deficient mice exhibited enhanced immune responses and were more susceptible to experimental autoimmune encephalomyelitis induction. Thus, DUSP14 negatively regulates TCR signaling and immune responses by inhibiting TAB1 activation.