Denise M. Ray

Prostaglandins as modulators of immunity

Prostaglandins are potent lipid molecules that affect key aspects of immunity. The original view of prostaglandins was that they were simply immunoinhibitory. This review focuses on recent findings concerning prostaglandin E2 (PGE2) and the PGD2 metabolite 15-deoxy-Delta(12,14)-PGJ2, and their divergent roles in immune regulation. We will highlight how these two seminal prostaglandins regulate immunity and inflammation, and play an emerging role in cancer progression. Understanding the diverse activities of these prostaglandins is crucial for the development of new therapies aimed at immune modulation.

The Peroxisome Proliferator-Activated Receptor γ (PPARγ) Ligands 15-Deoxy-Δ12,14-Prostaglandin J2 and Ciglitazone Induce Human B Lymphocyte and B Cell Lymphoma Apoptosis by PPARγ-Independent Mechanisms

Peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor important for adipogenesis and more recently has been shown to be an anticancer target. PPARγ ligands, including the endogenous ligand 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2) and synthetic ligands like ciglitazone and troglitazone, all induce apoptosis in normal and malignant human B lymphocytes, but the dependency of PPARγ for apoptosis induction is unknown. In this study, we used a PPARγ dominant-negative approach and a small molecule irreversible PPARγ antagonist and found that these inhibitors prevented PPARγ activation but did not prevent B cell apoptosis induced by 15d-PGJ2 or ciglitazone. In addition, a PPARγ agonist that is a structural analog of 15d-PGJ2, and lacks the electrophilic carbon of the 15d-PGJ2 cyclopentenone ring, activated PPARγ but did not kill B lymphocytes, further supporting a non-PPARγ-mediated mechanism. To further investigate the apoptotic mechanism, the effects of 15d-PGJ2 and ciglitazone on reactive oxygen species were investigated. 15d-PGJ2, but not ciglitazone, potently induced reactive oxygen species in B lymphocytes, implicating the reactive nature of the 15d-PGJ2 structure in the apoptosis mechanism. In addition, 15d-PGJ2 caused an almost complete depletion of intracellular glutathione. Moreover, incubation with glutathione reduced ethyl ester, an antioxidant, prevented apoptosis induced by 15d-PGJ2, but not by ciglitazone. These findings indicate that the expression of PPARγ may not be predictive of whether a normal or malignant B lineage cell is sensitive to PPARγ agonists. Furthermore, these new findings support continued investigation into the use of PPARγ agonists as agents to attenuate normal B cell responses and as anti-B cell lymphoma agents.

More Than Structural Cells, Fibroblasts Create and Orchestrate the Tumor Microenvironment

The tumor microenvironment comprises many cell types including infiltrating immune cells such as lymphocytes, endothelial cells and a complex stroma consisting mainly of fibroblasts. Fibroblasts are heterogeneous and consist of Thy-1+ and Thy-1− subsets that define different biosynthetic and differentiation potential. They produce mediators linked to carcinogenesis and metastasis, including Cox-2 and PGE2, both of which are also increased in most cancers. This review will highlight the emerging role of the complex fibroblastic stroma in establishing a microenvironment supporting malignant transformation, tumor growth and attenuation of host anti-tumor immune responses.

Peroxisome proliferator-activated receptor γ and retinoid X receptor transcription factors are released from activated human platelets and shed in microparticles

Peroxisome proliferator-activated receptor gamma (PPARgamma) and its ligands are important regulators of lipid metabolism, inflammation, and diabetes. We previously demonstrated that anucleate human platelets express the transcription factor PPARgamma and that PPARgamma ligands blunt platelet activation. To further understand the nature of PPARgamma in platelets, we determined the platelet PPARgamma isoform(s) and investigated the fate of PPARgamma following platelet activation. Our studies demonstrated that human platelets contain only the PPARgamma1 isoform and after activation with thrombin, TRAP, ADP or collagen PPARgamma is released from internal stores. PPARgamma release was blocked by a cytoskeleton inhibitor, Latrunculin A. Platelet-released PPARgamma was complexed with the retinoid X receptor (RXR) and retained its ability to bind DNA. Interestingly, the released PPARgamma and RXR were microparticle associated and the released PPARgamma/RXR complex retained DNA-binding ability. Additionally, a monocytic cell line, THP-1, is capable of internalizing PMPs. Further investigation following treatment of these cells with the PPARgamma agonist, rosiglitazone and PMPs revealed a possible transcellular mechanism to attenuate THP-1 activation. These new findings are the first to demonstrate transcription factor release from platelets, revealing the complex spectrum of proteins expressed and expelled from platelets, and suggests that platelet PPARgamma has an undiscovered role in human biology.

Platelets as a Novel Target for PPARγ Ligands

Peroxisome proliferator-activated receptor γ (PPARγ) is an important transcription factor for lipid and glucose metabolism. Currently, the PPARγ ligands rosiglitazone and pioglitazone are used for the treatment of type 2 diabetes mellitus because they are potent insulin sensitizers. Recently, PPARγ has emerged as an important anti-inflammatory factor. Platelets, anucleate cells involved in hemostasis, have also been implicated as key contributors to inflammation, because they produce many pro-inflammatory and pro-atherogenic mediators when activated. Surprisingly, it was discovered recently that platelets contain PPARγ and that PPARγ ligands, both natural and synthetic, inhibit platelet activation and release of bioactive mediators. In particular, release of soluble CD40 ligand (sCD40L) and thromboxane (TXA2) was inhibited by PPARγ ligands in thrombin-activated platelets. CD40L signaling induces pro-inflammatory processes in many cell types, and increased blood levels of sCD40L are closely associated with inflammation, diabetes, and cardiovascular disease. Targeting platelet PPARγ will, therefore, be an important treatment strategy for the attenuation of chronic inflammatory processes and prevention of thrombus formation.

Ligand- and Species-Dependent Activation of PPARα

Peroxisome proliferator-activated receptor alpha (PPARα) is mainly expressed in liver and involved in lipid metabolism. Oxidation of certain fatty acids in peroxisomes is under PPARα control. A wide variety of lipid molecules activate PPARα as well as the fibric acid derivative clofibrate. In the present study, we evaluated the differential activation of PPARα with several agonist ligands through its expression and DNA binding in both rat (McA-RH7777) and human (HepG2) hepatoma cell lines. In McA-RH7777 cells, clofibrate alone mediated a higher induction of PPARα expression than linoleic acid. In contrast, linoleic acid was the most effective ligand in HepG2 cells and treatment with clofibrate plus linoleic acid did not further increase PPARα expression. PPRE-binding activity of PPARα in ligand-treated cells was also increased in a parallel manner. We suggest that ligand-induced PPARα activation might give rise to differential species-dependent responses.