5A, lane 4). of Tpl2 and ERK by the TLR4 ligand LPS. On the other hand, prolonged incubation of macrophages with lithium results in dramatic loss of p105 and inhibition of LPS-stimulated NF-B activation. Consequently, lithium both attenuates LPS-mediated pro-inflammatory gene induction and induces apoptosis in macrophages. These results provide novel insight into the anti-inflammatory function of lithium. Keywords:Lithium, Tpl2, ERK, NF-B, apoptosis, macrophages == 1. Introduction == Inflammation serves as an important innate immune mechanism against infections [1]. A typical inflammatory response is initiated through the exposure of innate immune cells, such as macrophages and neutrophils, to microbial products [2]. The surface of these host cells have pattern recognition receptors, most importantly the toll-like receptors (TLRs), which recognize various microbial products and trigger the production of a plethora of cytokines and other pro-inflammatory mediators. These soluble immune factors in turn mediate induction of inflammation at the site of an infection, thereby preventing the spread of pathogens and facilitating the recruitment of additional immune cells/factors to the infected tissue [1]. However, deregulated inflammatory responses are associated with various human diseases, including chronic inflammatory disorders and systemic acute inflammatory diseases such as septic shock [35]. A well-studied TLR member, TLR4, responds to lipopolysaccharide (LPS), a bacterial cell-wall component that is responsible for the induction of septic shock by gram-negative bacteria [6]. In response to LPS stimulation, TLR4 recruits signaling adaptors and elicits activation of receptor-proximal signaling molecules, such as the interleukin-1 receptor-associated kinases (IRAKs) and the ubiquitin ligase TRAF6 [7]. The TLR signals can then be amplified to initiate several NXY-059 (Cerovive) downstream signaling pathways, including those that lead to the activation of IB kinase (IKK) and three families of MAP kinases (MAPKs), the extracellular signal-regulated kinases (ERK), the c-Jun NH2-terminal kinases (JNK), and p38. The primary function of IKK is to mediate activation of NF-B, a transcription factor regulating genes involved in immune response, inflammation, and cell survival [8,9]. Like IKK, the MAPKs are important for TLR-mediated induction of various target genes. Activation of MAPKs is mediated by specific MAPK kinases (MAP2K), which in turn are activated by MAPK kinase kinases (MAP3K) [10]. In particular, MEK1 and MEK2 are the MAP2Ks of two major ERK members, ERK1 and ERK2. Although different MAP3Ks are involved in the activation of MEK1/2, Tpl2 is the specific MAP3K that mediates MEK1/2 activation by a subset of inflammatory stimuli, including TLR ligands and TNF- [11,12]. Recent studies from us and others demonstrate that the stability and activity of Tpl2 are tightly regulated by the NF-B1 precursor protein, p105 [13,14]. Tpl2 is expressed as a longer and shorter isoforms, Tpl2L and Tpl2S, both being stably associated with p105 [15]. Activation of Tpl2 by LPS is mediated through degradation of p105, which NXY-059 (Cerovive) in turn involves p105 phosphorylation by IKK [16,17]. Upon liberation from p105, Tpl2 accesses and phosphorylates MEK1/2, leading to activation of the downstream ERK signaling pathway. The activated Tpl2, particularly Tpl2L, is rapidly degraded, which may serve as a feedback mechanism that prevents persistent ERK activation. Genetic evidence suggests that the Tpl2/ERK signaling pathway may have both pro- and anti-inflammatory functions. Tpl2 positively regulates the posttranscriptional expression and secretion of the pro-inflammatory cytokine TNF- [11,18]. On the other hand, Tpl2 negatively regulates the transcriptional induction of IL-12 induced by both LPS and the TLR9 ligand, CpG [1921]. Additionally, Tpl2 mediates the production of prostaglandin E2 (PGE2) [22], an important anti-inflammatory lipid mediator that is generated during the resolving phase of an inflammation [23,24]. Thus, understanding the mechanism of Tpl2 activation is important for developing more effective and specific approaches to modulate inflammatory responses. The anti-depressant drug lithium is known to have immunomodulatory functions. Accumulating evidence suggests that lithium and other anti-depressant drugs have both immuno-stimulatory and immuno-suppressive functions [2528]. Regarding the latter function, both in vitro and in vivo studies have Rabbit polyclonal to AASS revealed strong anti-inflammatory effect of lithium [2830]. The anti-inflammatory function of lithium may be important for its anti-depressant efficacy, since inflammation contributes to the pathogenesis of major depression [31,32]. It is known that NXY-059 (Cerovive) depressed patients have increased levels of pro-inflammatory cytokines and acute phase proteins in the plasma. Furthermore, lithium treatment significantly reduces the number of monocytes, which may contribute to the reduction of pro-inflammatory mediators [28]. Although how lithium mediates immunomodulation is poorly understood, it has been suggested that the anti-inflammatory action of lithium involves activation of the transcription factor CREB [29,33,34]. It is thought that CREB inhibits the transactivation function of.