No abstract

The PTEN tumor suppressor gene modulates several cellular functions, including cell migration, survival, and proliferation [1] by antagonizing phosphatidylinositol 3-kinase (PI 3-kinase)-mediated signaling cascades. Mechanisms by which the expression of PTEN is regulated are, however, unclear. The ligand-activated nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) [2] has been shown to regulate differentiation and/or cell growth in a number of cell types [3, 4, 5], which has led to the suggestion that PPARgamma, like PTEN [1, 6], could act as a tumor suppressor. PPARgamma has also been implicated in anti-inflammatory responses [7, 8], although downstream mediators of these effects are not well defined. Here, we show that the activation of PPARgamma by its selective ligand, rosiglitazone, upregulates PTEN expression in human macrophages, Caco2 colorectal cancer cells, and MCF7 breast cancer cells. This upregulation correlated with decreased PI 3-kinase activity as measured by reduced phosphorylation of protein kinase B. One consequence of this was that rosiglitazone treatment reduced the proliferation rate of Caco2 and MCF7 cells. Antisense-mediated disruption of PPARgamma expression prevented the upregulation of PTEN that normally accompanies monocyte differentiation and reduced the proportion of macrophages undergoing apoptosis, while electrophoretic mobility shift assays showed that PPARgamma is able to bind two response elements in the genomic sequence upstream of PTEN. Our results demonstrate a role for PPARgamma in regulating PI 3-kinase signaling by modulating PTEN expression in inflammatory and tumor-derived cells.

No abstract

The peroxisome proliferator-activated receptors (PPARs) are a family of fatty acid-activated transcription factors which control lipid homeostasis and cellular differentiation. PPAR␣ (NR1C1) controls lipid oxidation and clearance in hepatocytes and PPAR␥ (NR1C3) promotes preadipocyte differentiation and lipogenesis. Drugs that activate PPAR␣ are effective in lowering plasma levels of lipids and have been used in the management of hyperlipidemia. PPAR␥ agonists increase insulin sensitivity and are used in the management of type 2 diabetes. In contrast, there are no marketed drugs that selectively target PPAR␦ (NR1C2) and the physiological roles of PPAR␦ are unclear. In this report we demonstrate that the expression of PPAR␦ is increased during the differentiation of human macrophages in vitro. In addition, a highly selective agonist of PPAR␦ (compound F) promotes lipid accumulation in primary human macrophages and in macrophages derived from the human monocytic cell line, THP-1. Compound F increases the expression of genes involved in lipid uptake and storage such as the class A and B scavenger receptors (SRA, CD36) and adipophilin. PPAR␦ activation also represses key genes involved in lipid metabolism and efflux, i.e. cholesterol 27-hydroxylase and apolipoprotein E. We have generated THP-1 sublines that overexpress PPAR␦ and have confirmed that PPAR␦ is a powerful promoter of macrophage lipid accumulation. These data suggest that PPAR␦ may play a role in the pathology of diseases associated with lipidfilled macrophages, such as atherosclerosis, arthritis, and neurodegeneration.

Abstract-We studied the expression of lipoprotein-associated phospholipase A 2 (Lp-PLA 2 ), an enzyme capable of hydrolyzing platelet-activating factor (PAF), PAF-like phospholipids, and polar-modified phosphatidylcholines, in human and rabbit atherosclerotic lesions. Oxidative modification of low-density lipoprotein, which plays an important role in atherogenesis, generates biologically active PAF-like modified phospholipid derivatives with polar fatty acid chains. PAF is known to have a potent proinflammatory activity and is inactivated by its hydrolysis. On the other hand, lysophosphatidylcholine and oxidized fatty acids released from oxidized low-density lipoprotein as a result of Lp-PLA 2 activity are thought to be involved in the progression of atherosclerosis. Using combined in situ hybridization and immunocytochemistry, we detected Lp-PLA 2 mRNA and protein in macrophages in both human and rabbit atherosclerotic lesions. Reverse transcriptase-polymerase chain reaction analysis indicated an increased expression of Lp-PLA 2 mRNA in human atherosclerotic lesions. In addition, Ϸ6-fold higher Lp-PLA 2 activity was detected in atherosclerotic aortas of Watanabe heritable hyperlipidemic rabbits compared with normal aortas from control rabbits. It is concluded that (1) macrophages in both human and rabbit atherosclerotic lesions express Lp-PLA 2 , which could cleave any oxidatively modified phosphatidylcholine present in the lesion area, and (2) modulation of Lp-PLA 2 activity could lead to antiatherogenic effects in the vessel wall.