Activation of PPARdelta alters lipid metabolism in db/db mice.

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARalpha and PPARgamma, relatively little is known about the most widely expressed PPAR subtype, PPARdelta. Here we show that treatment of insulin resistant db/db mice with the PPARdelta agonist L-165041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L-165041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARdelta ligand, but was increased by a PPARgamma agonist. These data suggest both that PPARdelta is involved in the regulation of cholesterol metabolism in db/db mice and that PPARdelta ligands could potentially have therapeutic value.

A fluorescent cholesterol analog traces cholesterol absorption in hamsters and is esterified in vivo and in vitro.

The fluorescent cholesterol analog 22-(N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (fluoresterol) was characterized as a tool for exploring the biochemistry and cell biology of intestinal cholesterol absorption. Hamsters absorbed fluoresterol in a concentration- and time-dependent manner, with an efficiency of about 15-30% that of cholesterol. Fluoresterol absorption was blocked by compounds known to inhibit cholesterol absorption, implying that fluoresterol interacts with those elements of the normal pathway for cholesterol absorption on which the inhibitors act. Confocal microscopy of small intestinal tissue demonstrated that fluoresterol was taken up by absorptive epithelial cells and packaged into lipoprotein particles, suggesting a normal route of intracellular trafficking. Uptake of fluoresterol was confirmed by biochemical analysis of intestinal tissue, and a comparison of [(3)H] cholesterol and fluoresterol content in the mucosa suggested that fluoresterol moved through the enterocytes more rapidly than did cholesterol. This interpretation was supported by measurements of fluoresterol esterification in the mucosa. Four hours after hamsters were given fluoresterol and [(3)H]cholesterol orally, 44% of the fluoresterol in the intestinal mucosa was esterified, compared to 8% of the [(3)H]cholesterol. Caco-2 cells took up 2- to 5-fold more [(3)H]cholesterol than fluoresterol from bile acid micelles, and esterified 21-24% of the fluoresterol but only 1-4% of the [(3)H]cholesterol. Thus fluoresterol apparently interacts with the proteins required for cholesterol uptake, trafficking, and processing in the small intestine.

LDL oxidation by activated monocytes: characterization of the oxidized LDL and requirement for transition metal ions.

Monocytes can be activated by incubation with opsonized zymosan (Zop), and under these conditions can oxidize low density lipoprotein (LDL). We have characterized the biochemical changes in the lipoprotein after this oxidation. We found that monocyte-oxidized LDL has increased mobility on agarose gels, increased absorbance at 234 nm, increased content of lysophosphatidylcholine, and fluorescence at 430 nm when excited at 350 nm. All these features were somewhat less pronounced in monocyte-oxidized LDL than in LDL oxidized by 5 micrometer CuSO4. Under appropriate conditions, Zop-stimulated monocytes oxidized LDL to a form recognized by macrophage scavenger receptors. Monocytes stimulated by Zop produced superoxide and also oxidized LDL, whereas monocytes stimulated by phorbol ester produced slightly more superoxide but did not oxidize LDL. We found that the chelators EDTA and diethylenetriaminepentaacetic acid inhibited LDL oxidation by Zop-stimulated monocytes, implying a requirement for transition metal ions. We found that Zop contained approximately 5 nmol iron per mg, probably as Fe3+. Zop stripped of its iron supported superoxide production by monocytes, but did not support LDL oxidation. Furthermore, Fe2+ appeared in the medium when monocytes were incubated with Zop, but not with iron-stripped Zop. Taken together, these results imply that monocytes stimulated by Zop are able to oxidize LDL only because of contaminating iron in the commercial zymosan preparations. and requirement for transition metal ions.