IKKβ serves as a central intermediate signaling molecule in the activation of the NF-κB pathway. However, the precise mechanism for the termination of IKKβ activity is still not fully understood. Using a functional genomic approach, we have identified two protein serine/threonine phosphatases, PPM1A and PPM1B, as IKKβ phosphatases. Overexpression of PPM1A or PPM1B results in dephosphorylation of IKKβ at Ser177 and Ser181 and termination of IKKβ-induced NF-κB activation. PPM1A and PPM1B associate with the phosphorylated form of IKKβ, and the interaction between PPM1A/PPM1B and IKKβ is induced by TNFα in a transient fashion in the cells. Furthermore, knockdown of PPM1A and PPM1B expression enhances TNFα-induced IKKβ phosphorylation, NF-κB nuclear translocation and NF-κB-dependent gene expression. These data suggest that PPM1A and PPM1B play an important role in the termination of TNFα-mediated NF-κB activation through dephosphorylating and inactivating IKKβ.

The very-low-density-lipoprotein receptor (VLDLR) is a recently described lipoprotein receptor that shows considerable similarity to the low-density-lipoprotein receptor (LDLR). This receptor has been suggested to be important for the metabolism of apoprotein-E-containing triacylglycerolrich lipoproteins, such as very-low-density-lipoprotein (VLDL), P-migrating VLDL and intermediate-density lipoprotein. cDNA clones that code for the VLDLR were isolated from a mouse heart cDNA library. The deduced amino acid sequence predicts a mature protein of 846 amino acids preceded by a 27-residue signal peptide. Three mRNA species for the VLDLR with sizes of 3.9, 4.5 and 7.9 kilobases were present in high concentration in heart and muscle, which utilize triacylglycerols as an energy source. VLDLR mRNA is also detected in decreasing amounts in kidney, brain, ovary, testis, lung and adipose tissue. It is essentially absent in liver and small intestine. The amino acid sequence of the VLDLR is highly conserved among rabbit, human and mouse. VLDLR contains five structural domains very similar to those in LDLR, except that the ligand-binding domain in VLDLR has an eightfold repeat instead of a sevenfold repeat in LDLR. Sequence conservation among animal species is much higher for the VLDLR than the LDLR. Sequences of the VLDLR from three vertebrate species and the LDLR from five vertebrate species were aligned and a phylogenetic tree was reconstructed. Although both receptors contain five domains and share amino acid sequence similarity, our computations showed that they diverged before the divergence between mammals and amphibians. In addition, sequence comparison of both receptor sequences suggests that the rabbit is evolutionarily closer to man than to the mouse. These results are consistent with the hypothesis that the VLDLR and the LDLR have evolved from a common ancestral gene to play distinct roles in lipoprotein metabolism and that the metabolic handling of triacylglycerol by the body via the VLDLR is a highly conserved mechanism.Lipoprotein receptors play pivotal roles in the metabolism of triacylglycerols and cholesterol . The best characterized of the lipoprotein receptors is the low-density-lipoprotein receptor (LDLR). The LDLR binds to the apolipoprotein (apo) B-100-containing low-density-lipoproteins (LDL), as well as apoprotein-Econtaining lipoproteins such as intermediate-density lipoproteins (IDL), P-migrating very-low-density lipoproteins @-VLDL) and a cholesterol-induced high-density lipoprotein, which contains apoE as its sole apolipoprotein (Esser et al.,

Although the LXXLL motif of nuclear receptor (NR) coactivators is essential for interaction with NRs, its role has not been assessed in unbiased animal models. The nuclear receptor coactivator 6 (NCoA6; also AIB3, PRIP, ASC-2, TRBP, RAP250, or NRC) is a coactivator containing an N-terminal LXXLL-1 (L1) and a C-terminal L2. L1 interacts with many NRs, while L2 interacts with the liver X receptor ␣ (LXR␣) and the estrogen receptor ␣ (ER␣). We generated mice in which L2 was mutated into AXXAL (L2m) to disrupt its interaction with LXR␣ and ER␣. NCoA6L2m/L2m mice exhibited normal reproduction, mammary gland morphogenesis, and ER␣ target gene expression. In contrast, when treated with an LXR␣ agonist, lipogenesis and the LXR␣ target gene expression were significantly reduced in NCoA6L2m/L2m mice. The induction of Cyp7A1 expression by a high-cholesterol diet was impaired in NCoA6 L2m/L2m mice, which reduced bile acid synthesis in the liver and excretion in the feces and resulted in cholesterol accumulation in the liver and blood. These results demonstrate that L2 plays a tissue-and NR-specific role: it is required for NCoA6 to mediate LXR␣-regulated lipogenesis and cholesterol/bile acid homeostasis in the liver but not required for ER␣ function in the mammary gland.The nuclear receptor (NR) superfamily consists of hormoneinducible transcription factors. Through regulation of gene expression, NRs control numerous biological events in development, growth, sexual maturation, reproduction, and metabolic homeostasis. Recent studies have identified a number of transcriptional coactivators and corepressors that modulate NR transcriptional activities and determine the expression levels of their target genes (28,36,41). Multiple coactivators usually form functional protein complexes which facilitate chromatin remodeling, general transcription factor assembly, RNA polymerase II recruitment, and transcriptional initiation. Mechanistically, most coactivators do not bind to DNA, and their recruitment to a gene promoter/enhancer is dependent on their interaction with specific agonist-bound NRs. Most coactivators contain one or more LXXLL (L, leucine; X, any amino acid) ␣-helix motifs required for interaction with the ligand-binding domain of NRs (10,26,27,37). Although the molecular basis and requirement of the LXXLL motif for NR coactivator function have been investigated using biochemical and cell culture experiments (10,26,27,37), the physiological function and the tissue and NR specificities of the LXXLL motif of coactivators have not been carefully assessed in an animal model using an unbiased molecular genetic approach.The nuclear receptor coactivator 6 (NCoA-6; also AIB3, PRIP, ASC-2, TRBP, RAP250, or NRC) is an NR coactivator amplified and overexpressed in some breast, colon, and lung cancers (4,9,16,20,24,25,50). Biochemical and cell culturebased experiments have demonstrated that NCoA6 interacts with and coactivates many NRs, including estrogen receptor ␣ (ER␣) and liver X receptor ␣ (LXR␣) (15,24,25). NCoA6 may enhance NR...

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