We previously observed that Gadd45β/MyD118, a member of the Gadd45 family of inducible factors, showed the strongest immediate-early induction common to two distinctive proliferation responses of the liver: (1) regeneration induced by surgical partial hepatectomy and (2) hyperplasia induced by the primary mitogen TCPOBOP, a ligand of the constitutive androstane receptor (CAR). Gadd45β is known to be stimulated by nuclear factor (NF) κB, which is activated by tumor necrosis factor alpha (TNFα) in the early response to partial hepatectomy. We therefore investigated whether TNFα and NFκB also stimulated Gadd45β as part of the response to CAR ligands, or whether activation occurred by an alternative pathway. TCPOBOP effects were characterized in three mouse genotypes: wild-type, TNFR1−/−, and TNFR1−/−TNFR2−/−. The results showed that TCPOBOP did not activate NFκB in any of the mice, but a strong induction of Gadd45β messenger RNA was observed in all three genotypes, where TCPOBOP also induced CyP2b10, a classical target gene of activated CAR, and cyclin D1, a proliferation linked gene. Thus, the absence of TNFR signaling and induction of NFκB did not impair CAR-mediated gene induction. Moreover, hepatocyte proliferation was strongly induced, and at significantly higher levels than wild type, in both TNFR1−/− and TNFR1−/−TNFR2−/− mice. Further studies evaluated TCPOBOP-induced gene expression in CAR−/− mice, by microarray expression profiling and Northern blot. The induced changes in gene expression, including the stimulation of Gadd45β, were almost completely abolished—hence all were mediated via CAR activation. In conclusion, in the liver, Gadd45β can be induced by a distinctive pathway that requires CAR and is independent of TNFα-NFκB. The greater induction of proliferation in TNFR-null mice suggests negative cross-talk between the CAR and TNFα-NFκB controls that regulate proliferation. (HEPATOLOGY 2005.)