Robert Rottapel

Regulation of NF-κB Signaling by Pin1-Dependent Prolyl Isomerization and Ubiquitin-Mediated Proteolysis of p65/RelA

The transcription factor NF-kappaB is activated by the degradation of its inhibitor IkappaBalpha, resulting in its nuclear translocation. However, the mechanism by which nuclear NF-kappaB is subsequently regulated is not clear. Here we demonstrate that NF-kappaB function is regulated by Pin1-mediated prolyl isomerization and ubiquitin-mediated proteolysis of its p65/RelA subunit. Upon cytokine treatment, Pin1 binds to the pThr254-Pro motif in p65 and inhibits p65 binding to IkappaBalpha, resulting in increased nuclear accumulation and protein stability of p65 and enhanced NF-kappaB activity. Significantly, Pin1-deficient mice and cells are refractory to NF-kappaB activation by cytokine signals. Moreover, the stability of p65 is controlled by ubiquitin-mediated proteolysis, facilitated by a cytokine signal inhibitor, SOCS-1, acting as a ubiquitin ligase. These findings uncover two important mechanisms of regulating NF-kappaB signaling and offer new insight into the pathogenesis and treatment of some human diseases such as cancers.

The MAGUK family protein CARD11 is essential for lymphocyte activation.

Members of the MAGUK family proteins cluster receptors and intracellular signaling molecules at the neuronal synapse. We report that genetic inactivation of the MAGUK family protein CARD11/Carma1/Bimp3 results in a complete block in T and B cell immunity. CARD11 is essential for antigen receptor- and PKC-mediated proliferation and cytokine production in T and B cells due to a selective defect in JNK and NFkappaB activation. Moreover, B cell proliferation and JNK activation were impaired upon stimulation of TLR4 with lipopolysaccharide, indicating that CARD11 is involved in both the innate and adaptive immune systems. Our results show that the same family of molecules are critical regulators of neuronal synapses and immune receptor signaling.

A point mutation in CD28 distinguishes proliferative signals from survival signals

Upon interaction with its ligand, B7, CD28 becomes phosphorylated on tyrosines. One tyrosine in particular (Y170 in mouse CD28, Y173 in human CD28) has received much attention. This is because it permits CD28 to recruit SH2-containing signaling molecules, including phosphoinositide 3 kinase, Grb2 and Gads. Using mice we employed a transgenic approach to express a tyrosine→phenylalanine mutant form of CD28 that uncouples these SH2-mediated interactions from CD28. The CD28 mutant is unable to up-regulate expression of the prosurvival protein Bcl-xL, rendering the T cells more susceptible to radiation-induced death. Nonetheless, this mutated form of CD28 still prevents the induction of anergy and promotes T cell proliferation, interleukin 2 secretion and B cell help. Thus, we describe a single point mutation within the CD28 cytoplasmic domain that uncouples signals required for proliferation and survival.

SHP-1 Binds and Negatively Modulates the c-Kit Receptor by Interaction with Tyrosine 569 in the c-Kit Juxtamembrane Domain

ABSTRACT The SH2 domain-containing SHP-1 tyrosine phosphatase has been shown to negatively regulate a broad spectrum of growth factor- and cytokine-driven mitogenic signaling pathways. Included among these is the cascade of intracellular events evoked by stem cell factor binding to c-Kit, a tyrosine kinase receptor which associates with and is dephosphorylated by SHP-1. Using a series of glutathioneS-transferase (GST) fusion proteins containing either tyrosine-phosphorylated segments of the c-Kit cytosolic region or the SH2 domains of SHP-1, we have shown that SHP-1 interacts with c-Kit by binding selectively to the phosphorylated c-Kit juxtamembrane region and that the association of c-Kit with the larger of the two SHP-1 isoforms may be mediated through either the N-terminal or C-terminal SHP-1 SH2 domain. The results of binding assays with mutagenized GST-Kit juxtamembrane fusion proteins and competitive inhibition assays with phosphopeptides encompassing each c-Kit juxtamembrane region identified the tyrosine residue at position 569 as the major site for binding of SHP-1 to c-Kit and suggested that tyrosine 567 contributes to, but is not required for, this interaction. By analysis of Ba/F3 cells retrovirally transduced to express c-Kit receptors, phenylalanine substitution of c-Kit tyrosine residue 569 was shown to be associated with disruption of c-Kit–SHP-1 binding and induction of hyperproliferative responses to stem cell factor. Although phenylalanine substitution of c-Kit tyrosine residue 567 in the Ba/F3–c-Kit cells did not alter SHP-1 binding to c-Kit, the capacity of a second c-Kit-binding tyrosine phosphatase, SHP-2, to associate with c-Kit was markedly reduced, and the cells again showed hyperproliferative responses to stem cell factor. These data therefore identify SHP-1 binding to tyrosine 569 on c-Kit as an interaction pivotal to SHP-1 inhibitory effects on c-Kit signaling, but they indicate as well that cytosolic protein tyrosine phosphatases other than SHP-1 may also negatively regulate the coupling of c-Kit engagement to proliferation.

Socs1 binds to multiple signalling proteins and suppresses Steel factor-dependent proliferation

We have identified Socs1 as a downstream component of the Kit receptor tyrosine kinase signalling pathway. We show that the expression of Socs1 mRNA is rapidly increased in primary bone marrow‐derived mast cells following exposure to Steel factor, and Socs1 inducibly binds to the Kit receptor tyrosine kinase via its Src homology 2 (SH2) domain. Previous studies have shown that Socs1 suppresses cytokine‐mediated differentiation in M1 cells inhibiting Janus family kinases. In contrast, constitutive expression of Socs1 suppresses the mitogenic potential of Kit while maintaining Steel factor‐dependent cell survival signals. Unlike Janus kinases, Socs1 does not inhibit the catalytic activity of the Kit tyrosine kinase. In order to define the mechanism by which Socs1‐mediated suppression of Kit‐dependent mitogenesis occurs, we demonstrate that Socs1 binds to the signalling proteins Grb‐2 and the Rho‐family guanine nucleotide exchange factors Vav. We show that Grb2 binds Socs1 via its SH3 domains to putative diproline determinants located in the N‐terminus of Socs1, and Socs1 binds to the N‐terminal regulatory region of Vav. These data suggest that Socs1 is an inducible switch which modulates proliferative signals in favour of cell survival signals and functions as an adaptor protein in receptor tyrosine kinase signalling pathways.

Putting out the fire: coordinated suppression of the innate and adaptive immune systems by SOCS1 and SOCS3 proteins.

Summary: The mounting of an effective immune response requires the coordinated function of both the innate and the adaptive arm of the immune system. Cells from both types of immunity respond to antigenic stimuli through a variety of surface and intracellular receptors and produce cytokines that tightly orchestrate the inflammatory response. The operation of feedback control mechanisms that regulate the duration and the amplitude of antigenic and cytokine receptor signaling is therefore required to prevent hyper‐activation of the immune system that could lead to tissue destruction or autoimmunity. Suppressor of cytokine signaling (SOCS) proteins have been identified as a negative feedback loop to cytokine signaling. Recently, the generation of genetically engineered mouse models permitted the evaluation of their function in different processes of the immune responses. In this article, we review new insights into the modular structure of SOCS proteins and the function of SOCS1 and SOCS3 to negatively regulate activation and/or differentiation pathways in macrophages, dendritic cells, and T lymphocytes. Thus, SOCS family proteins are components of an emerging immunoregulatory mechanism that maintains the coordinated balance of both innate and adaptive immune responses.

Autoinhibition of the Kit Receptor Tyrosine Kinase by the Cytosolic Juxtamembrane Region

ABSTRACT Genetic studies have implicated the cytosolic juxtamembrane region of the Kit receptor tyrosine kinase as an autoinhibitory regulatory domain. Mutations in the juxtamembrane domain are associated with cancers, such as gastrointestinal stromal tumors and mastocytosis, and result in constitutive activation of Kit. Here we elucidate the biochemical mechanism of this regulation. A synthetic peptide encompassing the juxtamembrane region demonstrates cooperative thermal denaturation, suggesting that it folds as an autonomous domain. The juxtamembrane peptide directly interacted with the N-terminal ATP-binding lobe of the kinase domain. A mutation in the juxtamembrane region corresponding to an oncogenic form of Kit or a tyrosine-phosphorylated form of the juxtamembrane peptide disrupted the stability of this domain and its interaction with the N-terminal kinase lobe. Kinetic analysis of the Kit kinase harboring oncogenic mutations in the juxtamembrane region displayed faster activation times than the wild-type kinase. Addition of exogenous wild-type juxtamembrane peptide to active forms of Kit inhibited its kinase activity in trans, whereas the mutant peptide and a phosphorylated form of the wild-type peptide were less effective inhibitors. Lastly, expression of the Kit juxtamembrane peptide in cells which harbor an oncogenic form of Kit inhibited cell growth in a Kit-specific manner. Together, these results show the Kit kinase is autoinhibited through an intramolecular interaction with the juxtamembrane domain, and tyrosine phosphorylation and oncogenic mutations relieved the regulatory function of the juxtamembrane domain.

CD28-dependent Activation of Protein Kinase B/Akt Blocks Fas-mediated Apoptosis by Preventing Death-inducing Signaling Complex Assembly

The T cell costimulatory molecule CD28 is important for T cell survival, yet both the signaling pathways downstream of CD28 and the apoptotic pathways they antagonize remain poorly understood. Here we demonstrate that CD4+ T cells from CD28-deficient mice show increased susceptibility to Fas-mediated apoptosis via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. Protein kinase B (PKBα/Akt1) is an important serine/threonine kinase that promotes survival downstream of PI3K signals. To understand how PI3K-mediated signals downstream of CD28 contribute to T cell survival, we examined Fas-mediated apoptosis in T cells expressing an active form of PKBα. Our data demonstrate that T cells expressing active PKB are resistant to Fas-mediated apoptosis in vivo and in vitro. PKB transgenic T cells show reduced activation of caspase-8, BID, and caspase-3 due to impaired recruitment of procaspase-8 to the death-inducing signaling complex (DISC). Similar alterations are seen in T cells from mice which are haploinsufficient for PTEN, a lipid phosphatase that regulates phosphatidylinositol-3,4,5-trisphosphate (PIP3) and influences PKBα activity. These findings provide a novel link between CD28 and an important apoptosis pathway in vivo, and demonstrate that PI3K/PKB signaling prevents apoptosis by inhibiting DISC assembly.

Structural Basis and Sequence Rules for Substrate Recognition by Tankyrase Explain the Basis for Cherubism Disease

The poly(ADP-ribose)polymerases Tankyrase 1/2 (TNKS/TNKS2) catalyze the covalent linkage of ADP-ribose polymer chains onto target proteins, regulating their ubiquitylation, stability, and function. Dysregulation of substrate recognition by Tankyrases underlies the human disease cherubism. Tankyrases recruit specific motifs (often called RxxPDG “hexapeptides”) in their substrates via an N-terminal region of ankyrin repeats. These ankyrin repeats form five domains termed ankyrin repeat clusters (ARCs), each predicted to bind substrate. Here we report crystal structures of a representative ARC of TNKS2 bound to targeting peptides from six substrates. Using a solution-based peptide library screen, we derive a rule-based consensus for Tankyrase substrates common to four functionally conserved ARCs. This 8-residue consensus allows us to rationalize all known Tankyrase substrates and explains the basis for cherubism-causing mutations in the Tankyrase substrate 3BP2. Structural and sequence information allows us to also predict and validate other Tankyrase targets, including Disc1, Striatin, Fat4, RAD54, BCR, and MERIT40.

The tumor suppressor activity of SOCS-1.

SOCS-1 is an inducible SH2-containing inhibitor of Jak kinases and as such can potently suppress cytokine signaling. SOCS-1 deficient mice die within the first three weeks of life from a myeloproliferative disorder driven by excessive interferon signaling. We report here that SOCS-1 inhibits proliferation signals induced by a variety of oncogenes active within the hematopoietic system. Ectopic expression of SOCS-1 abolished proliferation mediated by a constitutively active form of the KIT receptor, TEL-JAK2, and v-ABL, and reduced metastasis from BCR-ABL transformed cells. SOCS-1, however, did not interfere with v-SRC or RASV12 mediated cellular transformation. A mutant form of SOCS-1 unable to bind through its SH2 domain to tyrosine phosphorylated proteins could still inhibit KIT, but not TEL-JAK2, indicating multiple mechanisms for SOCS-1-mediated tumor suppression. We show that the steady state levels of TEL-JAK2 and to a greater extent v-ABL are diminished in the presence of SOCS-1. Lastly, we show that SOCS-1 −/− fibroblasts are more sensitive than wild type fibroblasts to either spontaneous or oncogene-induced transformation. These data suggest that loss-of-function of SOCS-1 may collaborate with a variety of hematopoietic oncogenes to facilitate tumor progression.