Activation of the nuclear transcription factor NF-kappaB by inflammatory cytokines requires the successive action of NF-kappaB-inducing kinase (NIK) and an IKB-kinase (IKK) complex composed of IKKalpha and IKKbeta. Here we show that the Akt serine-threonine kinase is involved in the activation of NF-kappaB by tumour necrosis factor (TNF). TNF activates phosphatidylinositol-3-OH kinase (PI(3)K) and its downstream target Akt (protein kinase B). Wortmannin (a PI(3)K inhibitor), dominant-negative PI(3)K or kinase-dead Akt inhibits TNF-mediated NF-kappaB activation. Constitutively active Akt induces NF-kappaB activity and this effect is blocked by dominant-negative NIK. Conversely, NIK activates NF-kappaB and this is blocked by kinase-dead Akt. Thus, both Akt and NIK are necessary for TNF activation of NF-kappaB. Akt mediates IKKalpha phosphorylation at threonine 23. Mutation of this amino acid blocks phosphorylation by Akt or TNF and activation of NF-kappaB. These findings indicate that Akt is part of a signalling pathway that is necessary for inducing key immune and inflammatory responses.

MicroRNAs (miRNAs) are small endogenous noncoding RNAs that suppress gene expression at the post-transcriptional level. In the past decade, miRNAs have been extensively studied in a number of different human cancers. MiRNAs have been identified to act both as oncogenes and as tumor suppressors. In addition, miRNAs are associated with the intrinsic resistance of cancer to various forms of therapy, and they are implicated in both tumor progression and metastasis. The characterization of the specific alterations in the patterns of miRNA expression in cancer has great potential for identifying biomarkers for early cancer detection, as well as for potential therapeutic intervention in cancer treatment. In this chapter, we describe the ever-expanding role of miR-21 and its target genes in different cancers, and provide insight into how this oncogenic miRNA regulates cancer cell proliferation, migration, and apoptosis by suppressing the expression of tumor suppressors.

STAT (signal transducers and activators of transcription) proteins undergo cytokine-dependent phosphorylation on serine and tyrosine. STAT3, a transcription factor for acute phase response genes, was found to act as an adapter molecule in signal transduction from the type I interferon receptor. STAT3 bound to a conserved sequence in the cytoplasmic tail of the IFNAR1 chain of the receptor and underwent interferon-dependent tyrosine phosphorylation. The p85 regulatory subunit of phosphatidylinositol 3-kinase, which activates a series of serine kinases, bound to phosphorylated STAT3 and subsequently underwent tyrosine phosphorylation. Thus, STAT3 acts as an adapter to couple another signaling pathway to the interferon receptor.

Chemokines and inflammatory cytokines are key regulators of immunity and inflammation during viral infections. Hepatitis C virus (HCV) is a hepatotropic RNA virus frequently associated with chronic liver inflammation and hepatocellular carcinoma. Intrahepatic levels of chemokines and cytokines are elevated in chronic HCV infections, but the underlying mechanisms remain unclear. We find that Toll like receptor-3 (TLR3) senses HCV infection in cultured hepatoma cells, leading to NF-κB activation and the production of numerous chemokines and inflammatory cytokines, such as RANTES, MIP-1α, MIP-1β, IP-10 and IL-6. The chemokine/cytokine induction occurred late in HCV infection and was abrogated when HCV was UV-inactivated prior to infection, indicating a dependence on cellular recognition of HCV replication products. Gel shift and chromatin immunoprecipitation assays revealed that NF-κB plays a pivotal role in HCV-induced chemokine/cytokine transcription. Mutations specifically disrupting the dsRNA binding activity of TLR3 ablated the chemokine/cytokine response to HCV infection, indicating that HCV dsRNA was the pathogen associated molecular pattern triggering TLR3 signaling. In vitro synthesized HCV dsRNAs with a minimal length of ~80-100 bp activated TLR3-dependent chemokine expression, regardless of the genome position from which they derive. In contrast, HCV ssRNAs, including those derived from the structured 3′NTR highly potent for RIG-I activation, failed to do so. Moreover, robust production of chemokines and inflammatory cytokines was also observed in primary human hepatocytes following stimulation with extracellular poly-I:C, a TLR3 ligand. Conclusion Our data suggest that TLR3-mediated chemokine and inflammatory cytokine responses may play an important role in host immune responses to HCV and the pathogenesis of HCV-associated liver diseases.