Search

Liver X receptor alpha (LXRalpha) is a member of the nuclear receptor superfamily that is activated by oxysterols, and plays a pivotal role in regulating the metabolism, transport and uptake of cholesterol. Here, we demonstrate that LXRalpha also regulates the low-density lipoprotein receptor (LDLR) gene, which mediates the endocytic uptake of LDL cholesterol in the liver. An LXR agonist induced the expression of LDLR in cultured hepatoblastoma cells. Moreover, the LDLR promoter contained an LXR response element that was recognized by LXRalpha/RXRalpha (retinoid X receptor alpha) heterodimers in hepatoblastoma cells. These results suggest a novel pathway whereby LXRalpha might modulate cholesterol metabolism.

Scavenger receptors bind modified low-density lipoproteins (LDL) on a collagen-like domain which possesses a lysine cluster at the car☐y end. We previously constructed a receptor model peptide containing the lysine cluster. In the present study, we evaluated the ligand specificity of the receptor model peptide. It selectively bound modified-LDLs, and not LDL. The binding of acetylated-LDL (Ac-LDL) was inhibited by dextran sulfate, fucoidan, and sulfatides in a manner similar to that of the natural receptor. Both polyguanylic and polyinosinic acids inhibit the Ac-LDL binding whereas polycytidylic acid did not. These results indicate that the lysine cluster in the collagen-like domain has important roles in both ligand binding and ligand specificity.

Although PDZK1 is a well-known adaptor protein, the mechanisms for its role in transcriptional regulation are largely unknown. The peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-activated transcription factor that plays an important role in the regulation of lipid homeostasis. Previously, we established a tetracycline-regulated human cell line that can be induced to express PPARalpha and identified candidate target genes, one of which was PDZK1. In this study, we cloned and characterized the promoter region of the human pdzk1 gene and determined the PPAR response element. Finally, we demonstrate that endogenous PPARalpha regulates PDZK1 expression.

A novel low Mr GTP-binding protein cDNA was isolated from a rat megakaryocyte cDNA library with a synthetic oligonucleotide probe corresponding to an 8-amino acid sequence specific for c25KG, a GTP-binding protein previously isolated from human platelet cytosol fraction [(1989) J. Biol. Chem. 264, 17000–17005]. The cDNA has an open reading frame encoding a protein of 221 amino acids with a calculated Mr of 25068. The protein is designated as ram (ras-related gene from megakaryocyte) protein (ram p25). The amino acid sequence deduced from the ram cDNA contains the consensus sequences for GTP-binding and GTPase domains. ram p25 shares about 23%, 39% and 80% amino acid homology with the H-ras, smg25A and c25KG proteins, respectively. The 3.5-kb ram mRNA was detected abundantly in spleen cells.

The single-stranged region of the amino acid acceptor stem corresponding to the 3'-end of E. coli tRNA Met f was replaced by ligation of chemically synthesized ribooligonucleotides, in order to change the length of the single-stranded CCA terminus. The chemically synthesized ribooligomers, CCA, ACCA, AACCA and CAACCA, were ligated to nuclease-treated E. coli tRNA Met f , which lacked the ACCA sequence at the 3'-end. The methionine acceptor activities of these modified tRNAs were examined using E. coli methionyl-tRNA synthetase. Ligation of the chemically synthesized pentamer (AACCA) to the acceptor terminus restored the methionine acceptor activity, whereas ligation of the hexamer (CAACCA) or trimer (CCA) to the acceptor terminus did not Modification of the acceptor terminus had no effect on the formylation of accepted methionine.

A putative collagen structure from macrophage scavenger receptors binds to a wide range of ligands. In order to elucidate the ligand's binding mode, this collagen structure was constructed using short peptides. This was accomplished by the reaction of a tri-bromoacetylated branched peptide with a purified unprotected 25-residue peptide, which contained Cys, 4 repeats of the triplet, Gly-Pro-Hyp, and 12 residues from the bovine macrophage scavenger receptor (residues 332 to 343). The three identical 25-residue peptides are linked at the N-terminus. CD and NMR spectra of the N-terminus cross-linked tripeptide show that it forms a collagen structure below 10°C and an extended structure at high temperature with a midpoint of 20° C.