Activation of innate immune receptors by host-derived factors exacerbates CNS damage, but the identity of these factors remains elusive. We uncovered an unconventional role for the microRNA let-7, a highly abundant regulator of gene expression in the CNS, in which extracellular let-7 activates the RNA-sensing Toll-like receptor (TLR) 7 and induces neurodegeneration through neuronal TLR7. Cerebrospinal fluid (CSF) from individuals with Alzheimer’s disease contains increased amounts of let-7b, and extracellular introduction of let-7b into the CSF of wild-type mice by intrathecal injection resulted in neurodegeneration. Mice lacking TLR7 were resistant to this neurodegenerative effect, but this susceptibility to let-7 was restored in neurons transfected with TLR7 by intrauterine electroporation of Tlr7(−/−) fetuses. Our results suggest that microRNAs can function as signaling molecules and identify TLR7 as an essential element in a pathway that contributes to the spread of CNS damage.

Nuclear factor kappa B (NF-κB) has been implicated in the regulation of cell proliferation, transformation, and tumor development. We provide evidence for a direct link between NF-κB activity and cell cycle regulation. NF-κB was found to stimulate transcription of cyclin D1, a key regulator of G1checkpoint control. Two NF-κB binding sites in the human cyclin D1 promoter conferred activation by NF-κB as well as by growth factors. Both levels and kinetics of cyclin D1 expression during G1phase were controlled by NF-κB. Moreover, inhibition of NF-κB caused a pronounced reduction of serum-induced cyclin D1-associated kinase activity and resulted in delayed phosphorylation of the retinoblastoma protein. Furthermore, NF-κB promotes G1-to-S-phase transition in mouse embryonal fibroblasts and in T47D mammary carcinoma cells. Impaired cell cycle progression of T47D cells expressing an NF-κB superrepressor (IκBαΔN) could be rescued by ectopic expression of cyclin D1. Thus, NF-κB contributes to cell cycle progression, and one of its targets might be cyclin D1.

The IjB kinase (IKK) complex is the signal integration hub for NF-jB activation. Composed of two serine-threonine kinases (IKKa and IKKb) and the regulatory subunit NEMO (also known as IKKc), the IKK complex integrates signals from all NF-jB activating stimuli to catalyze the phosphorylation of various IjB and NF-jB proteins, as well as of other substrates. Since the discovery of the IKK complex components about 15 years ago, tremendous progress has been made in the understanding of the IKK architecture and its integration into signaling networks. In addition to the control of NF-jB, IKK subunits mediate the crosstalk with other pathways, thereby extending the complexity of their biological function. This review summarizes recent advances in IKK biology and focuses on emerging aspects of IKK structure, regulation and function.

AP-1 family transcription factors have been implicated in the control of proliferation, apoptosis and malignant transformation. However, their role in oncogenesis is unclear and no recurrent alterations of AP-1 activities have been described in human cancers. Here, we show that constitutively activated AP-1 with robust c-Jun and JunB overexpression is found in all tumor cells of patients with classical Hodgkin's disease. A similar AP-1 activation is present in anaplastic large cell lymphoma (ALCL), but is absent in other lymphoma types. Whereas c-Jun is up-regulated by an autoregulatory process, JunB is under control of NF-kappa B. Activated AP-1 supports proliferation of Hodgkin cells, while it suppresses apoptosis of ALCL cells. Furthermore, AP-1 cooperates with NF-kappa B and stimulates expression of the cell-cycle regulator cyclin D2, proto-oncogene c-met and the lymphocyte homing receptor CCR7, which are all strongly expressed in primary HRS cells. Together, these data suggest an important role of AP-1 in lymphoma pathogenesis.

As part of the genotoxic stress response, cells activate the transcription factor NF-κB. The DNA strand break sensor poly(ADP-ribose)-polymerase-1 (PARP-1) and the kinase ataxia telangiectasia mutated (ATM) act as proximal signal mediators. PARP-1 assembles a nucleoplasmic signalosome, which triggers PIASy-mediated IKKγ SUMOylation. ATM-dependent IKKγ phosphorylation and subsequent ubiquitination were implicated to activate the cytoplasmic IκB kinase (IKK) complex by unknown mechanisms. We show that activated ATM translocates in a calcium-dependent manner to cytosol and membrane fractions. Through a TRAF-binding motif, ATM activates TRAF6, resulting in Ubc13-mediated K63-linked polyubiquitin synthesis and cIAP1 recruitment. The ATM-TRAF6-cIAP1 module stimulates TAB2-dependent TAK1 phosphorylation. Both nuclear PARP-1- and cytoplasmic ATM-driven signaling branches converge at the IKK complex to catalyze monoubiquitination of IKKγ at K285. Our data indicate that exported SUMOylated IKKγ acts as a substrate. IKKγ monoubiquitination is a prerequisite for genotoxic IKK and NF-κB activation, but also promotes cytokine signaling.