Required for lipopolysaccharide (LPS)-induced, TLR4-mediated activation of the MAPK/ERK pathway in macrophages, thus being critical for production of the pro-inflammatory cytokine TNF-alpha (TNF) during immune responses. Involved in the regulation of T-helper cell differentiation and IFNG expression in T-cells. Involved in mediating host resistance to bacterial infection through negative regulation of type I interferon (IFN) production. Transduces CD40 and TNFRSF1A signals that activate ERK in B-cells and macrophages, and thus may play a role in the regulation of immunoglobulin production. May also play a role in the transduction of TNF signals that activate JNK and NF-kappa-B in some cell types. In adipocytes, activates MAPK/ERK pathway in an IKBKB-dependent manner in response to IL1B and TNF, but not insulin, leading to induction of lipolysis. Plays a role in the cell cycle. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Forms a ternary complex with NFKB1/p105 and TNIP2. Interacts with NFKB1; the interaction increases the stability of MAP3K8 but inhibits its MEK phosphorylation activity, whereas loss of interaction following LPS stimulation leads to its degradation. Interacts with CD40 and TRAF6; the interaction is required for ERK activation. Interacts with KSR2; the interaction inhibits ERK and NF-kappa-B activation. Expressed in bone marrow-derived macrophages, peritoneal macrophages, splenocytes and 3T3-L1 fibroblasts and differentiated adipocytes (at protein level). Highly expressed in adult submandibular gland, thymus, spleen and newborn digestive tract. Up-regulated by Il12 in T-lymphocytes. Up-regulated during in vitro adipocyte differentiation. Up-regulated in epididymal adipose tissue of obese mice. Autophosphorylated. Mutant mice develop normally and show histologically normal bone marrow, thymus, spleen and lymph nodes. Mice are resistant to the induction of endotoxin shock due to defect in the induction of Tnf in response to LPS. Mice display impaired host defense against T.gondii with reduced parasite clearance and decreased Ifng production. Mice also show increased susceptibility to M.tuberculosis and L.monocytogenes infection. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.