Ca2+-independent phospholipase A2ß-derived PGE2 contributes to osteogenesis.

Bone modeling can be modulated by lipid signals such as arachidonic acid (AA) and its cyclooxygenase 2 (COX2) metabolite, prostaglandin E2 (PGE2), which are recognized mediators of optimal bone formation. Hydrolysis of AA from membrane glycerophospholipids is catalyzed by phospholipases A2 (PLA2s). We reported that mice deficient in the Ca2+- independent PLA2beta (iPLA2ß), encoded by Pla2g6, exhibit a low bone phenotype, but the cause for this remains to be identified. Here, we examined the mechanistic and molecular roles of iPLA2ß in bone formation using bone marrow stromal cells and calvarial osteoblasts from WT and iPLA2ß-deficient mice, and the MC3T3-E1 osteoblast precursor cell line. Our data reveal that transcription of osteogenic factors (Bmp2, Alpl, and Runx2) and osteogenesis are decreased with iPLA2ß-deficiency. These outcomes are corroborated and recapitulated in WT cells treated with a selective inhibitor of iPLA2 ß (10 µM S-BEL), and rescued in iPLA2ß-deficient cells by additions of 10 µM PGE2. Further, under osteogenic conditions we find that PGE2 production is through iPLA2ß activity and that this leads to induction of Runx2 and iPLA2ß transcription. These findings reveal a strong link between osteogenesis and iPLA2ß-derived lipids and raise the intriguing possibility that iPLA2ß-derived PGE2 participates in osteogenesis and in the regulation of Runx2 and also iPLA2ß.

Polarization of Macrophages toward M2 Phenotype Is Favored by Reduction in iPLA2ß (Group VIA Phospholipase A2).

Macrophages are important in innate and adaptive immunity. Macrophage participation in inflammation or tissue repair is directed by various extracellular signals and mediated by multiple intracellular pathways. Activation of group VIA phospholipase A2 (iPLA2ß) causes accumulation of arachidonic acid, lysophospholipids, and eicosanoids that can promote inflammation and pathologic states. We examined the role of iPLA2ß in peritoneal macrophage immune function by comparing wild type (WT) and iPLA2ß-/- mouse macrophages. Compared with WT, iPLA2ß-/- macrophages exhibited reduced proinflammatory M1 markers when classically activated. In contrast, anti-inflammatory M2 markers were elevated under naïve conditions and induced to higher levels by alternative activation in iPLA2ß-/- macrophages compared with WT. Induction of eicosanoid (12-lipoxygenase (12-LO) and cyclooxygenase 2 (COX2))- and reactive oxygen species (NADPH oxidase 4 (NOX4))-generating enzymes by classical activation pathways was also blunted in iPLA2ß-/- macrophages compared with WT. The effects of inhibitors of iPLA2ß, COX2, or 12-LO to reduce M1 polarization were greater than those to enhance M2 polarization. Certain lipids (lysophosphatidylcholine, lysophosphatidic acid, and prostaglandin E2) recapitulated M1 phenotype in iPLA2ß-/- macrophages, but none tested promoted M2 phenotype. These findings suggest that (a) lipids generated by iPLA2ß and subsequently oxidized by cyclooxygenase and 12-LO favor macrophage inflammatory M1 polarization, and (b) the absence of iPLA2ß promotes macrophage M2 polarization. Reducing macrophage iPLA2ß activity and thereby attenuating macrophage M1 polarization might cause a shift from an inflammatory to a recovery/repair milieu.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases.

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2ß) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Getting to the heart of the sphingolipid riddle.

Obesity, Type 2 Diabetes, and Metabolic Syndrome induce dyslipidemia resulting in inundation of peripheral organs with fatty acids. These not only serve as substrates for energy production, but also contribute to aberrant production of bioactive lipids. Moreover, lipid metabolism is affected in many cardiac disorders including heart failure, ischemia reperfusion injury, and others. While lipids serve crucial homeostatic roles, perturbing biosynthesis of lipid mediators leads to aberrant cell signaling, which contributes to maladaptive cardiovascular programs. Bioactive sphingolipids, in particular, have been implicated in pathophysiology in the heart and vasculature by a variety of studies in cells, animal models, and humans. Because of the burgeoning interest in sphingolipid-driven biology in the cardiovascular system, it is necessary to discuss the experimental considerations for studying sphingolipid metabolism and signaling, emphasizing the caveats to some widely available experimental tools and approaches. Additionally, there is a growing appreciation for the diversity of ceramide structures generated via specific enzymes and bearing disparate cellular functions. While targeting these individual species and enzymes constitutes a major advance, studies show that sphingolipid synthesis readily adapts to compensate for experimental targeting of any individual pathway, thereby convoluting data interpretation. Furthermore, though some molecular mechanisms of sphingolipid action are known, signaling pathways impacted by sphingolipids remain incompletely understood. In this review, we discuss these issues and highlight recent studies as well as future directions that may extend our understanding of the metabolism and signaling actions of these enigmatic lipids in the cardiovascular context.