LPS-stimulated MAPK Signaling

Lipopolysaccharide (LPS/endotoxin) is a component of the outer membrane of Gram-negative bacteria that potently promotes the activation of macrophages and microglia cells, which are important sensors of infection by bacteria, fungi, and viruses, in both the periphery and the CNS. There are several known LPS-Binding Proteins (LBP) present on macrophage membranes, including CD14, CD11b/18, and TLR. CD14 lacks a transmembrane domain and is unable to initiate signals on its own. Thus, the TLR4 has emerged as a potential signaling partner for LPS/CD14 interactions.The key downstream pathway for LPS-induced signaling events is the MAPK cascade that leads to several functional responses. The MAPK family is composed of the ERK1/2, p38 and SAPK/JNK pathways. The primary substrates for JNK are c-Jun, c-Fos and ATF2, which induce gene expression by binding to the Activator Protein-1 binding site in the promoters. MEKK1/4 or Raf activates (through phosphorylation) MEK1/2 or MKK3/4/7, which in turn phosphorylates a specific tyrosine and threonine residue on a MAPK. At least three members of the MKK superfamily are capable of activating p38 MAPK. MKK3, MKK4, and MKK6 activate p38 MAPK and MKK7 activates JNK. This family of kinases is important in a wide spectrum of cell functions including proliferation, apoptosis, cytokine biosynthesis, and cytoskeletal reorganization. The activated MAPK are responsible for phosphorylating and activating numerous transcription factors which function to stimulate the synthesis of various inflammatory proteins including the cytokines TNF-a, IL-1b and IL-6. The MAPK are also involved in the transcriptional regulation of NOS2 and COX2. LPS-mediated signaling also activate various small molecular weight GTPases including Rac, CDC42, PAK1 and Ras. Also occurring early after LPS stimulation is the activation of the PI3K and subsequent activation of PKC. PC-PLC may also stimulate PKC activation. Subsequently activation of the MAPK kinase kinases including MEKK1/4, Raf1, ASK1 and TAK1 and the activation of MAPK kinases such as the MKKs and MEK1/2 takes place. In the nucleus these lead to the recruitment of the transcriptional coactivators CREB and Elk1. Elk1 is a part of a TCF that binds together with SRF to the Serum Response Element (SRE). CREB can form homodimers or heterodimers with other members of the ATF family, including ATF1 by binding to cAMP Response Element (CRE). The MAP kinase can directly phosphorylate various transcription factors or can work through activation of kinases like MSK1. TAK1 activates two kinase cascades, one leads to the activation of JNK, and the other cascade leads to the activation of the IKK through NIK, which phosphorylate I-κB. This phosphorylation triggers ubiquitination and subsequent degradation of I-κB, resulting in the release of NF-κB subunits p50 and p65 that translocate to the nucleus and induce the transcriptional activation of a wide variety of inflammatory and immune response genes.