Differentiation of T-helper lymphocytes: selective regulation by members of the STAT family of transcription factors

Multifarious determinants of cytokine receptor signaling specificity

Cytokines play crucial roles in regulating immune homeostasis. Two important characteristics of most cytokines are pleiotropy, defined as the ability of one cytokine to exhibit diverse functionalities, and redundancy, defined as the ability of multiple cytokines to exert overlapping activities. Identifying the determinants for unique cellular responses to cytokines in the face of shared receptor usage, pleiotropy, and redundancy will be essential in order to harness the potential of cytokines as therapeutics. Here, we discuss the biophysical (ligand-receptor geometry and affinity) and cellular (receptor trafficking and intracellular abundance of signaling molecules) parameters that contribute to the specificity of cytokine bioactivities. Whereas the role of extracellular ternary complex geometry in cytokine-induced signaling is still not completely elucidated, cytokine-receptor affinity is known to impact signaling through modulation of the stability and kinetics of ternary complex formation. Receptor trafficking also plays an important and likely underappreciated role in the diversification of cytokine bioactivities but it has been challenging to experimentally probe trafficking effects. We also review recent efforts to quantify levels of intracellular signaling components, as second messenger abundance can affect cytokine-induced bioactivities both quantitatively and qualitatively. We conclude by discussing the application of protein engineering to develop therapeutically relevant cytokines with reduced pleiotropy and redirected biological functionalities.

Silibinin inhibits constitutive activation of Stat3, and causes caspase activation and apoptotic death of human prostate carcinoma DU145 cells

Transcription factor signal transducer and activator of transcription (Stat)-3 is activated constitutively in prostate cancer (PCA) suggesting that its disruption could be an effective approach to control this malignancy. Here we assessed whether silibinin, a flavanone from Silybum marianum with proven anticancer efficacy in various cancer models, inhibits Stat3 activation in DU145 cells, and if it does, what is the biological fate of the cells? At 50 muM or higher concentrations for 24 or 48 h, silibinin concentration dependently reduced constitutive Stat3 phosphorylation at Tyr705 and Ser727 residues under both serum and serum-starved conditions. Constitutively active Stat3-DNA binding was also inhibited concentration dependently by silibinin; however, apoptotic death together with caspase and poly(ADP-ribose) polymerase (PARP) cleavage was observed by silibinin only under serum-starved conditions suggesting that additional survival pathways are active under serum conditions. In other studies, cells were treated with various specific pharmacological inhibitors where phosphorylation of Stat3 was not reduced by epidermal growth factor receptor and Mitogen activated protein/extracellular signal regulate kinase kinase (MEK1/2) inhibitors, suggesting lack of significant roles of these in Stat3 activation in DU145 cells. Janus kinase (JAK)-1 and JAK2 inhibitors strongly reduced Stat3 phosphorylation but did not result in apoptotic cell death. Interestingly, JAK1 inhibitor only in combination with silibinin resulted in a complete reduction in Stat3 phosphorylation at Tyr705, activated caspase-9 and caspase-3, and caused strong PARP cleavage and apoptotic death of DU145 cells. Given a critical role of Stat3 activation in PCA, our results showed that silibinin inhibits constitutively active Stat3 and induces apoptosis in DU145 cells, and thus might have potential significance in therapeutic intervention of this deadly malignancy.

Signaling by IL-12 and IL-23 and the immunoregulatory roles of STAT4

Produced in response to a variety of pathogenic organisms, interleukin (IL)-12 and IL-23 are key immunoregulatory cytokines that coordinate innate and adaptive immune responses. These dimeric cytokines share a subunit, designated p40, and bind to a common receptor chain, IL-12R beta 1. The receptor for IL-12 is composed of IL-12R beta 1 and IL-12R beta 2, whereas IL-23 binds to a receptor composed of IL-12R beta 1 and IL-23R. Both cytokines activate the Janus kinases Tyk2 and Jak2, the transcription factor signal transducer and activator of transcription 4 (STAT4), as well as other STATs. A major action of IL-12 is to promote the differentiation of naive CD4+ T cells into T-helper (Th) 1 cells, which produce interferon (IFN)-gamma, and deficiency of IL-12, IL-12R subunits or STAT4 is similar in many respects. In contrast, IL-23 promotes end-stage inflammation. Targeting IL-12, IL-23, and their downstream signaling elements would therefore be logical strategies for the treatment of immune-mediated diseases.

Cutting edge: Chandra, a novel four-transmembrane domain protein differentially expressed in helper type 1 lymphocytes

Development of naive Th cells into Th1 and Th2 effector populations requires coordinated expression of a complex set of genes. In this study, we have identified a novel four-transmembrane domain protein, Chandra, that is differentially expressed in Th1 cells. Chandra expression is observed in STAT4- and IFN-gamma-deficient mice, indicating that Chandra is not an IL-12- or IFN-gamma-responsive gene. Interestingly, Chandra mRNA is detected in anti-CD3-activated T cells from STAT6-deficient mice in the absence of any differentiation conditions. Furthermore, neutralization of IL-4 signaling is sufficient to induce transcription of Chandra in anti-CD3-activated T cells from wild-type mice, demonstrating that STAT6 signaling is required to repress Chandra expression in activated T cells and Th2 subsets. This is the first demonstration of a differentially expressed four-transmembrane domain protein in Th1 cells.

Repression of IL-4-Induced Gene Expression by IFN-γ Requires Stat1 Activation

IFN-γ antagonizes many physiological responses mediated by IL-4, including the inhibition of IL-4-induced IgE production. This event is largely mediated at the level of transcription. We observed that the IL-4 response element of the germline epsilon promoter is sufficient to confer IFN-γ-mediated repression onto a reporter construct. The inhibitory effects were observed in both lymphoid and nonlymphoid cell lines. Stat1, which is activated by IFN-γ, cannot recognize the Stat6-specific IL-4 response element in the ε promoter. Hence, competitive DNA binding does not seem to be the underlying mechanism for the inhibitory effect. This is supported by the observation that inhibition is not seen at early time points, but requires prolonged IFN-γ treatment. IFN-γ stimulation results in a loss of IL-4-induced Stat6 tyrosine phosphorylation, nuclear translocation, and DNA binding. Using the fibrosarcoma cell line U3A, which lacks Stat1, we demonstrated that the transcription activation function of Stat1 is required for the IFN-γ-mediated repression. Repression was restored by overexpression of Stat1α, but not Stat1β, in U3A cells. Treatment with IFN-γ, but not IL-4, specifically up-regulates the expression of SOCS-1 (silencer of cytokine signaling), a recently characterized inhibitor of cytokine signaling pathways, such as IL-6 and IFN-γ. Overexpression of SOCS-1 effectively blocks IL-4-induced Stat6 phosphorylation and transcription. This suggests that IFN-γ-mediated repression of IL-4-induced transcription is at least in part mediated by SOCS-1.

Identification of a STAT4 binding site in the interleukin-12 receptor required for signaling

The specificity of the various STAT SH2 domains for different tyrosine-containing peptides enables cytokines to activate different signaling pathways and to induce distinct patterns of gene expression. We show that STAT4 has a unique peptide specificity and binds to the peptide sequence pYLPSNID (where pY represents phosphotyrosine). This motif is found at tyrosine residue 800 in the beta2 subunit of the interleukin-12 receptor and is required for DNA binding and transcriptional activity of STAT4. Our data demonstrate that transfection of interleukin-12 receptor beta1 and beta2 subunits is sufficient for STAT4 activation but not for STAT1 or STAT3 activation.

Synergistic Activation of the Germline ε Promoter Mediated by Stat6 and C/EBPβ

Transcription of the Ig H chain germline transcripts is a prerequisite for class switching. Expression of the ε germline transcript is induced by IL-4 and requires the integrity of a composite IL-4 response element. The element is bound by the IL-4-inducible transcription factor Stat6 and one or more members of the CAAT/enhancer-binding protein (C/EBP) family, a constitutively expressed class of transcription factors. Here, we show that Stat6 and C/EBPβ cooperate to synergistically activate transcription from the ε element. The effect was most pronounced in lymphoid cells, and the activation domains of both proteins were required to achieve this synergy. Although other members of the C/EBP family are able to bind the element, very little cooperativity was seen with C/EBPα and none with C/EBPγ. In fact, C/EBPγ was able to inhibit IL-4-induced reporter activity. Stat6 and C/EBPβ bind the IL-4 response element simultaneously. The fast dissociation rate apparent when Stat6 binds this DNA element alone is slowed when C/EBPβ binds at the neighboring site. These data suggest a mechanism whereby C/EBPβ stabilizes Stat6 binding at this element, thereby increasing the likelihood that both of their activation domains will interact, possibly with other factors, to activate transcription in an IL-4-dependent manner.