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. In vitro, activates MAPK/ERK pathway in response to IL1 in an IRAK1-independent manner, leading to up-regulation of IL8 and CCL4. 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. Isoform 1 shows some transforming activity, although it is much weaker than that of the activated oncogenic variant. 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 several normal tissues and human tumor-derived cell lines. Isoform 1 is activated specifically during the S and G2/M phases of the cell cycle. Up-regulated by IL12 in T-lymphocytes. Up-regulated in subcutaneous adipose tissue of obese individuals. Autophosphorylated (PubMed:8226782, PubMed:1833717). Isoform 1 undergoes phosphorylation mainly on Ser residues, and isoform 2 on both Ser and Thr residues (PubMed:8226782). Phosphorylated on Thr-290; the phosphorylation is necessary but not sufficient for full kinase activity in vitro and for the dissociation of isoform 1 from NFKB1, leading to its degradation (PubMed:15466476, PubMed:15699325). Phosphorylated on Ser-400 by IKBKB; the phosphorylation is required for LPS-stimulated activation of the MAPK/ERK pathway in macrophages (PubMed:17472361, PubMed:22988300). Can be converted to an oncogenic protein by proviral activation, leading to a C-terminally truncated protein with transforming activity. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily. A paper describing a role for this protein in IRAK1-independent activation of the MAPK/ERK pathway in response to IL1 has been retracted, because some of the experimental data could not be reproduced.