Bing Su

Plasmacytoid dendritic cells sense self-DNA coupled with antimicrobial peptide

Plasmacytoid dendritic cells (pDCs) sense viral and microbial DNA through endosomal Toll-like receptors to produce type 1 interferons. pDCs do not normally respond to self-DNA, but this restriction seems to break down in human autoimmune disease by an as yet poorly understood mechanism. Here we identify the antimicrobial peptide LL37 (also known as CAMP) as the key factor that mediates pDC activation in psoriasis, a common autoimmune disease of the skin. LL37 converts inert self-DNA into a potent trigger of interferon production by binding the DNA to form aggregated and condensed structures that are delivered to and retained within early endocytic compartments in pDCs to trigger Toll-like receptor 9. Thus, our data uncover a fundamental role of an endogenous antimicrobial peptide in breaking innate tolerance to self-DNA and suggest that this pathway may drive autoimmunity in psoriasis.

SIN1/MIP1 Maintains rictor-mTOR Complex Integrity and Regulates Akt Phosphorylation and Substrate Specificity

Mammalian target of rapamycin (mTOR) controls cell growth and proliferation via the raptor-mTOR (TORC1) and rictor-mTOR (TORC2) protein complexes. Recent biochemical studies suggested that TORC2 is the elusive PDK2 for Akt/PKB Ser473 phosphorylation in the hydrophobic motif. Phosphorylation at Ser473, along with Thr308 of its activation loop, is deemed necessary for Akt function, although the regulatory mechanisms and physiological importance of each phosphorylation site remain to be fully understood. Here, we report that SIN1/MIP1 is an essential TORC2/PDK2 subunit. Genetic ablation of sin1 abolished Akt-Ser473 phosphorylation and disrupted rictor-mTOR interaction but maintained Thr308 phosphorylation. Surprisingly, defective Ser473 phosphorylation affected only a subset of Akt targets in vivo, including FoxO1/3a, while other Akt targets, TSC2 and GSK3, and the TORC1 effectors, S6K and 4E-BP1, were unaffected. Our findings reveal that the SIN1-rictor-mTOR function in Akt-Ser473 phosphorylation is required for TORC2 function in cell survival but is dispensable for TORC1 function.

The mammalian target of rapamycin complex 2 controls folding and stability of Akt and protein kinase C

The target of rapamycin (TOR), as part of the rapamycin‐sensitive TOR complex 1 (TORC1), regulates various aspects of protein synthesis. Whether TOR functions in this process as part of TORC2 remains to be elucidated. Here, we demonstrate that mTOR, SIN1 and rictor, components of mammalian (m)TORC2, are required for phosphorylation of Akt and conventional protein kinase C (PKC) at the turn motif (TM) site. This TORC2 function is growth factor independent and conserved from yeast to mammals. TM site phosphorylation facilitates carboxyl‐terminal folding and stabilizes newly synthesized Akt and PKC by interacting with conserved basic residues in the kinase domain. Without TM site phosphorylation, Akt becomes protected by the molecular chaperone Hsp90 from ubiquitination‐mediated proteasome degradation. Finally, we demonstrate that mTORC2 independently controls the Akt TM and HM sites in vivo and can directly phosphorylate both sites in vitro. Our studies uncover a novel function of the TOR pathway in regulating protein folding and stability, processes that are most likely linked to the functions of TOR in protein synthesis.

The essential role of MEKK3 in TNF-induced NF-κB activation

Activation of IκB kinase (IKK) is the key step in stimulation of the transcription factor NF-κB, which regulates many genes in the inflammatory response pathway. The molecular mechanism that underlies IKK activation in response to tumor necrosis factor (TNF) is still unknown. Using mitogen-activated protein kinase kinase kinase 3 (MEKK3)-deficient fibroblast cells, we found that MEKK3 plays a critical role in TNF-induced NF-κB activation. We have shown that MEKK3 is required for IKK activation and functions downstream of receptor-interacting protein (RIP) and TNF receptor– associated factor 2. We have also shown that MEKK3 interacts with RIP and directly phosphorylates IKK. The kinase activity of MEKK3 is pivotal to its function and, therefore, MEKK3 links RIP and IKK in TNF-induced NF-κB activation.

Mekk3 is essential for early embryonic cardiovascular development.

The early development of blood vessels consists of two phases, vasculogenesis and angiogenesis, which involve distinct and also overlapping molecular regulators, but the intracellular signal transduction pathways involved in these processes have not been well defined. We disrupted Map3k3 −/− (also known as Mekk3), which encodes Mekk3, a member of the Mekk/Ste11 family, in mice. Map3k3 −/− embryos died at approximately embryonic day (E) 11, displaying disruption of blood vessel development and the structural integrity of the yolk sac. Angiogenesis was blocked at approximately E9.5 in mutant embryos. Map3k3 disruption did not alter the expression of the genes encoding Vegf-1, angiopoietin or their receptors. The development of embryonic, but not maternal, blood vessels in the placentas of Map3k3 −/− embryos was impaired, revealing an intrinsic defect in Map3k3−/− endothelial cells. Moreover, Mekk3 activated myocyte-specific enhancer factor 2C (Mef2c), a transcription factor crucial for early embryonic cardiovascular development through the p38 mitogen-activated protein kinase (Mapk) cascade. We conclude that Mekk3 is necessary for blood vessel development and may be a possible target for drugs that control angiogenesis.

mTORC2 can associate with ribosomes to promote cotranslational phosphorylation and stability of nascent Akt polypeptide

The mechanisms that couple translation and protein processing are poorly understood in higher eukaryotes. Although mammalian target of rapamycin (mTOR) complex 1 (mTORC1) controls translation initiation, the function of mTORC2 in protein synthesis remains to be defined. In this study, we find that mTORC2 can colocalize with actively translating ribosomes and can stably interact with rpL23a, a large ribosomal subunit protein present at the tunnel exit. Exclusively during translation of Akt, mTORC2 mediates phosphorylation of the nascent polypeptide at the turn motif (TM) site, Thr450, to avoid cotranslational Akt ubiquitination. Constitutive TM phosphorylation occurs because the TM site is accessible, whereas the hydrophobic motif (Ser473) site is concealed in the ribosomal tunnel. Thus, mTORC2 can function cotranslationally by phosphorylating residues in nascent chains that are critical to attain proper conformation. Our findings reveal that mTOR links protein production with quality control.

Evidence for direct activation of mTORC2 kinase activity by phosphatidylinositol 3,4,5-trisphosphate

mTORC2 (mammalian target of rapamycin complex 2) plays important roles in signal transduction by regulating an array of downstream effectors, including protein kinase AKT. However, its regulation by upstream regulators remains poorly characterized. Although phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) is known to regulate the phosphorylation of AKT Ser473, the hydrophobic motif (HM) site, by mTORC2, it is not clear whether PtdIns(3,4,5)P3 can directly regulate mTORC2 kinase activity. Here, we used two membrane-docked AKT mutant proteins, one with and the other without the pleckstrin homology (PH) domain, as substrates for mTORC2 to dissect the roles of PtdIns(3,4,5)P3 in AKT HM phosphorylation in cultured cells and in vitro kinase assays. In HEK293T cells, insulin and constitutively active mutants of small GTPase H-Ras and PI3K could induce HM phosphorylation of both AKT mutants, which was blocked by the PI3K inhibitor LY294002. Importantly, PtdIns(3,4,5)P3 was able to stimulate the phosphorylation of both AKT mutants by immunoprecipitated mTOR2 complexes in an in vitro kinase assay. In both in vivo and in vitro assays, the AKT mutant containing the PH domain appeared to be a better substrate than the one without the PH domain. Therefore, these results suggest that PtdIns(3,4,5)P3 can regulate HM phosphorylation by mTORC2 via multiple mechanisms. One of the mechanisms is to directly stimulate the kinase activity of mTORC2.

TAK1 Is Recruited to the Tumor Necrosis Factor-α (TNF-α) Receptor 1 Complex in a Receptor-interacting Protein (RIP)-dependent Manner and Cooperates with MEKK3 Leading to NF-κB Activation

Receptor-interacting protein (RIP) plays a critical role in tumor necrosis factor-α (TNF-α)-induced IκB kinase (IKK) activation and subsequent activation of transcription factor NF-κB. However, the molecular mechanism by which RIP mediates TNF-α-induced NF-κB activation is not completely defined. In this study, we have found that TAK1 is recruited to the TNF-α receptor complex in a RIP-dependent manner following the stimulation of TNF-α receptor 1 (TNF-R1). Moreover, a forced recruitment of TAK1 to TNF-R1 in the absence of RIP is sufficient to mediate TNF-α-induced NF-κB activation, indicating that the major function of RIP is to recruit its downstream kinases to the TNF-R1 complex. Interestingly, we also find that TAK1 and MEKK3 form a functional complex, in which TAK1 regulates autophosphorylation of MEKK3. The TAK1-mediated regulation of MEKK3 phosphorylation is dependent on the kinase activity of TAK1. Although TAK1-MEKK3 interaction is not affected by overexpressed TAB1, TAB1 is required for TAK1 activation and subsequent MEKK3 phosphorylation. Together, we conclude that TAK1 is recruited to the TNF-R1 complex via RIP and likely cooperates with MEKK3 to activate NF-κB in TNF-α signaling.

Interleukin-1 (IL-1)-induced TAK1-dependent Versus MEKK3-dependent NFκB Activation Pathways Bifurcate at IL-1 Receptor-associated Kinase Modification

Interleukin-1 (IL-1) receptor-associated kinase (IRAK) is phosphorylated after it is recruited to the receptor, subsequently ubiquitinated, and eventually degraded upon IL-1 stimulation. Although a point mutation changing lysine 134 to arginine (K134R) in IRAK abolished IL-1-induced IRAK ubiquitination and degradation, mutations of serines and threonines adjacent to lysine 134 to alanines ((S/T)A (131–144)) reduced IL-1-induced IRAK phosphorylation and abolished IRAK ubiquitination. Through the study of these IRAK modification mutants, we uncovered two parallel IL-1-mediated signaling pathways for NFκB activation, TAK1-dependent and MEKK3-dependent, respectively. These two pathways bifurcate at the level of IRAK modification. The TAK1-dependent pathway leads to IKKα/β phosphorylation and IKKβ activation, resulting in classical NFκB activation through IκBα phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKγ phosphorylation and IKKα activation, resulting in NFκB activation through IκBα phosphorylation and subsequent dissociation from NFκB but without IκBα degradation. These results provide significant insight to our further understanding of NFκB activation pathways.

TAK1 Negatively Regulates NF-κB and p38 MAP Kinase Activation in Gr-1+CD11b+ Neutrophils

Stringent control of NF-κB and mitogen-activated protein kinase (MAPK) signaling is critical during innate immune responses. TGF-β activated kinase-1 (TAK1) is essential for NF-κB activation in T and B cells but has precisely the opposite activity in myeloid cells. Specific deletion of TAK1 (Map3k7(ΔM/ΔM)) led to development of splenomegaly and lymphomegaly associated with neutrophilia. Compared with wild-type cells, TAK1-deficient neutrophils enhanced the phosphorylation of the kinases IKK, p38, and JNK and the production of interleukin-1β (IL-1β), IL-6, tumor necrosis factor-α (TNF-α), and reactive oxygen species (ROS) after lipopolysaccharide (LPS) stimulation. Map3k7(ΔM/ΔM) mice were significantly more susceptible to LPS-induced septic shock and produced higher amounts of IL-1β, IL-6, and TNF-α in plasma than do wild-type mice. Specific ablation of p38 rescued the phenotype and functional properties of Map3k7(ΔM/ΔM) mice. Our findings identify a previously unrecognized role of TAK1 as a negative regulator of p38 and IKK activation in a cell type-specific manner.