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The expression of the transcription factor GATA3 in FOXP3+ regulatory T (Treg) cells is crucial for their physiological function in limiting inflammatory responses. Although other studies have shown how T cell receptor (TcR) signals induce the up-regulation of GATA3 expression in Treg cells, the underlying mechanism that maintains GATA3 expression in Treg cells remains unclear. Here, we show how USP21 interacts with and stabilizes GATA3 by mediating its deubiquitination. In a T cell line model, we found that TcR stimulation promoted USP21 expression, which was further up-regulated in the presence of FOXP3. The USP21 mutant C221A reduced its capacity to stabilize GATA3 expression, and its knockdown led to the down-regulation of GATA3 protein expression in Treg cells. Furthermore, we found that FOXP3 could directly bind to the USP21 gene promoter and activated its transcription upon TcR stimulation. Finally, USP21, GATA3, and FOXP3 were found up-regulated in Treg cells that were isolated from asthmatic subjects. In summary, we have identified a USP21-mediated pathway that promotes GATA3 stabilization and expression at the post-translational level. We propose that this pathway forms an important signaling loop that stabilizes the expression of GATA3 in Treg cells. [ABSTRACT FROM AUTHOR]

Parathyroid hormone-related protein (PTHrP) regulates proliferation and differentiation of osteoblastic cells via binding to the parathyroid hormone receptor (PTH-1R). The cAMP-dependent protein kinase A pathway governs the majority of these effects, but recent evidence also implicates the MAPK pathway. MC3T3-E1 subclone 4 cells (MC4) were treated with the MAPK inhibitor U0126 and PTHrP. In differentiated MC4 cells, osteocalcin and bone sialoprotein gene expression were both down-regulated by PTHrP and also by inhibition of the MAPK pathway. PTHrP-mediated down-regulation of PTH-1R mRNA and up-regulation of c-fos mRNA were MAPK-independent, whereas PTHrP stimulation of fra-2 and interleukin-6 (IL-6) mRNA was MAPK-dependent. Luciferase promoter assays revealed that regulation of IL-6 involved the cAMP-dependent protein kinase A and MAPK pathways with a potential minor role of the protein kinase C pathway, and a promoter region containing an activator protein-1 site was necessary for PTHrP-induced IL-6 gene transcription. An alternative pathway, through cAMP/Epac/Rap1/MAPK, mediated ERK phosphorylation but was not sufficient for IL-6 promoter activation. Phosphorylation of the transcription factor CREB was also necessary but not sufficient for PTHrP-mediated IL-6 promoter activity. Most interesting, a bidirectional effect was found with PTHrP increasing phosphorylated ERK in undifferentiated MC4 cells but decreasing phosphorylated ERK in differentiated cells. These data indicate that inactivation of the MAPK pathway shows differential regulation of PTHrP-stimulated activator protein-1 members, blocks PTHrPstimulated IL-6, and synergistically down-regulates certain osteoblastic markers associated with differentiation. These novel findings indicate that the MAPK pathway plays a selective but important role in the actions of PTHrP. [ABSTRACT FROM AUTHOR]

CD4+ T-cells in systemic lupus erythematosus (SLE) patients show altered T-cell receptor signaling, which utilizes Fc-receptor γ-chain FcRγ-Syk. A role for FcγRIIIa activation from immune complex (IC) ligation and sublytic terminal complement complex (C5b-9) in CD4+T-cell responses is not investigated. In this study, we show that the ICs present in SLE patients by ligating to FcγRIIIa on CD4+ T-cells phosphorylate Syk and provide a co-stimulatory signal to CD4+ T-cells in the absence of CD28 signal. This led to the development of pathogenic IL-17A+ and IFN-γhigh CD4+ T-cells in vitro. Cytokines IL-1β, IL-6, TGF-β1, and IL-23 were the only requirement for the development of both populations. SLE patients CD4+ T-cells that expressed CD25, CD69, and CD98 bound to ICs showed pSyk and produced IFN-γ and IL-17A. This FcγRIIIa-mediated co-signal differentially up-regulated the expression of IFN pathway genes compared with CD28 co-signal. FcγRIIIa-pSyk upregulated several toll-like receptor genes as well as the HMGB1 and MyD88 gene transcripts. ICs co-localized with these tolllike receptor pathway proteins. These results suggest a role for the FcγRIIIa-pSyk signal in modulating adaptive immune responses. [ABSTRACT FROM AUTHOR]

Akt is perhaps the most frequently activated oncoprotein human cancers. Overriding cell cycle checkpoint in combination with the inhibition of apoptosis are two principal requirements for predisposition to cancer. Here we show that the activation of Akt is sufficient to promote these two principal processes, by inhibiting Chk1 activation with concomitant inhibition of apoptosis. These activities of Akt cannot be recapitulated by the knockdown of Chk1 alone or by overexpression Bcl2. Rather the combination of Chk1 knockdown and Bcl2 overexpression is required to recapitulate Akt activities. Akt was shown to directly phosphorylate Chk1. However, we found Chk1 mutants in the Akt phosphorylation sites behave like wild-type Chk1 in mediating G2 arrest, suggesting that the phosphorylation of Chk1 by Akt is either dispensable for Chk1 activity insufficient by itself to exert an effect on Chk1 activity. Here report a new mechanism by which Akt affects G2 cell cycle arrest. We show that Akt inhibits BRCA1 function that induces G2 cell cycle arrest. Akt prevents the translocation of BRCA1 DNA damage foci and, thereby, inhibiting the activation of Chk1 following DNA damage. [ABSTRACT FROM AUTHOR]