Conditional Disruption of the Peroxisome Proliferator-Activated Receptor γ Gene in Mice Results in Lowered Expression of ABCA1, ABCG1, and apoE in Macrophages and Reduced Cholesterol Efflux

Disruption of the peroxisome proliferator-activated receptor γ (PPARγ) gene causes embryonic lethality due to placental dysfunction. To circumvent this, a PPARγ conditional gene knockout mouse was produced by using the Cre-loxP system. The targeted allele, containing loxP sites flanking exon 2 of the PPARγ gene, was crossed into a transgenic mouse line expressing Cre recombinase under the control of the alpha/beta interferon-inducible (MX) promoter. Induction of the MX promoter by pIpC resulted in nearly complete deletion of the targeted exon, a corresponding loss of full-length PPARγ mRNA transcript and protein, and marked reductions in basal and troglitazone-stimulated expression of the genes encoding lipoprotein lipase, CD36, LXRα, and ABCG1 in thioglycolate-elicited peritoneal macrophages. Reductions in the basal levels of apolipoprotein E (apoE) mRNA in macrophages and apoE protein in total plasma and high-density lipoprotein (HDL) were also observed in pIpC-treated PPARγ-MXCre+ mice. Basal cholesterol efflux from cholesterol-loaded macrophages to HDL was significantly reduced after disruption of the PPARγ gene. Troglitazone selectively inhibited ABCA1 expression (while rosiglitazone, ciglitazone, and pioglitazone had little effect) and cholesterol efflux in both PPARγ-deficient and control macrophages, indicating that this drug can exert paradoxical effects on cholesterol homeostasis that are independent of PPARγ. Together, these data indicate that PPARγ plays a critical role in the regulation of cholesterol homeostasis by controlling the expression of a network of genes that mediate cholesterol efflux from cells and its transport in plasma.