Targeted mutation reveals a central role for SR-BI in hepatic selective uptake of high density lipoprotein cholesterol

Scavenger receptor BI (SR-BI) is a cell surface receptor that binds high density lipoproteins (HDL) and mediates selective uptake of HDL cholesteryl esters (CE) in transfected cells. To address the physiological role of SR-BI in HDL cholesterol homeostasis, mice were generated bearing an SR-BI promoter mutation that resulted in decreased expression of the receptor in homozygous mutant (designated SR-BI att) mice. Hepatic expression of the receptor was reduced by 53% with a corresponding increase in total plasma cholesterol levels of 50–70% in SR-BI att mice, attributable almost exclusively to elevated plasma HDL. In addition to increased HDL-CE, HDL phospholipids and apo A-1 levels were elevated, and there was an increase in HDL particle size in mutant mice. Metabolic studies using HDL bearing nondegradable radiolabels in both the protein and lipid components demonstrated that reducing hepatic SR-BI expression by half was associated with a decrease of 47% in selective uptake of CE by the liver, and a corresponding reduction of 53% in selective removal of HDL-CE from plasma. Taken together, these findings strongly support a pivotal role for hepatic SR-BI expression in regulating plasma HDL levels and indicate that SR-BI is the major molecule mediating selective CE uptake by the liver. The inverse correlation between plasma HDL levels and atherosclerosis further suggests that SR-BI may influence the development of coronary artery disease.

The ribosome-in-pieces: Binding of elongation factor EF-G to oligoribonucleotides that mimic the sarcin/ricin and thiostrepton domains of 23S ribosomal RNA

An oligoribonucleotide (a 27-mer) that mimics the sarcin/ricin (S/R) domain of Escherichia coli 23S rRNA binds elongation factor EF-G; the Kd is 6.9 μM, whereas for binding to ribosomes it is 0.7 μM. Binding saturates when EF-G and the S/R RNA are equimolar; at saturation 70% of the input RNA is in complexes with EF-G. Binding of EF-G to S/R RNA does not require GTP but is inhibited by GDP; the inhibition by GDP is overcome by GTP. The effects of mutations of the S/R domain nucleotides G2655, A2660, and G2661 suggest that EF-G recognizes the conformation of the RNA rather than the identity of the nucleotides. EF-G also binds to an oligoribonucleotide (an 84-mer) that has the thiostrepton region of 23S rRNA; however, EF-G binds independently to S/R and thiostrepton oligoribonucleotides.

A neurotensin antagonist, SR 48692, inhibits colonic responses to immobilization stress in rats.

We previously reported that short-term immobilization stress of rats causes increased colonic mucin release, goblet cell depletion, prostaglandin E2 secretion, and colonic mast cell activation, as well as increased colonic motility. The purpose of this study was to investigate whether neurotensin (NT), a peptide expressed in both brain and digestive tract, participates in these responses. Rats were pretreated with SR 48692 (1 mg/kg, i.p.), an NT antagonist, 15 min before immobilization (30 min). The administration of the antagonist significantly inhibited stress-mediated secretion of colonic mucin, prostaglandin E2, and a product of rat mast cells, rat mast cell protease II (P < 0.05), but did not alter the increase in fecal pellet output caused by immobilization stress. Immobilization stress also resulted in a quantifiable decrease in the abundance of NT receptor mRNA in rat colon compared with that in colonic tissues from nonimmobilized rats as measured by densitometric analysis of in situ hybridization studies (P < 0.03). We conclude that the peptide NT is involved in colonic goblet cell release and mucosal mast cell activation after immobilization stress.