The functional network of the Arabidopsis plastoglobule proteome based on quantitative proteomics and genome-wide coexpression analysis

Plastoglobules (PGs) in chloroplasts are thylakoid-associated monolayer lipoprotein particles containing prenyl and neutral lipids and several dozen proteins mostly with unknown functions. An integrated view of the role of the PG is lacking. Here, we better define the PG proteome and provide a conceptual framework for further studies. The PG proteome from Arabidopsis (Arabidopsis thaliana) leaf chloroplasts was determined by mass spectrometry of isolated PGs and quantitative comparison with the proteomes of unfractionated leaves, thylakoids, and stroma. Scanning electron microscopy showed the purity and size distribution of the isolated PGs. Compared with previous PG proteome analyses, we excluded several proteins and identified six new PG proteins, including an M48 metallopeptidase and two Absence of bc1 complex (ABC1) atypical kinases, confirmed by immunoblotting. This refined PG proteome consisted of 30 proteins, including six ABC1 kinases and seven fibrillins together comprising more than 70% of the PG protein mass. Other fibrillins were located predominantly in the stroma or thylakoid and not in PGs; we discovered that this partitioning can be predicted by their isoelectric point and hydrophobicity. A genome-wide coexpression network for the PG genes was then constructed from mRNA expression data. This revealed a modular network with four distinct modules that each contained at least one ABC1K and/or fibrillin gene. Each module showed clear enrichment in specific functions, including chlorophyll degradation/senescence, isoprenoid biosynthesis, plastid proteolysis, and redox regulators and phosphoregulators of electron flow. We propose a new testable model for the PGs, in which sets of genes are associated with specific PG functions.     

Prostaglandin A2 influences gene expression in an established insect cell line (BCIRL-HzAM1) cells

Prostaglandins (PGs) and other eicosanoids are oxygenated metabolites of arachidonic acid and two other C(20) polyunsaturated fatty acids. While most well studied in mammals, PGs exert important actions in insects and virtually all other invertebrates. We have been researching the mechanisms of PG actions in established insect cell lines and reported earlier that two PGs, PGA(1) and PGE(1), influence gene and protein expression in HzAM1 cells. Here we report on further experiments with three 2-series PGs, PGA(2), PGE(2) and PGF(2α). In separate experiments we treated cells with each of the three PGs for 12 and 24h and then analyzed cell lysates by 2-D electrophoresis. Analysis of the gels by Delta2D software showed that PGA(2) influenced expression of 60 proteins while PGE(2) and PGF(2α) treatments led to expression changes for only a few proteins. All spots representing changes in protein expression were processed for analysis by MALDI TOF/TOF mass spectrometry. Bioinformatic analysis of the resulting sequences yielded in silico identifications of all proteins. The apparent changes in some proteins were confirmed by quantitative PCR, which demonstrated that changes in protein expression were parallel to changes in mRNA expression. We assorted the proteins into functional categories, including 1/cell structure and function; 2/cell protection and immunity; 3/energetics and metabolism; 4/nucleotide processing; 5/protein action and processing and 6/signal transduction. These findings substantially extend our idea that one mechanism of PG actions in insect cells is the modulation of gene and protein expression.     

Identification of endo- and exo-polygalacturonase activity in Lygus hesperus (Knight) salivary glands

Polygalacturonase (PG) activity found in the salivary gland apparatus of the western tarnished plant bug (WTPB, Lygus hesperus Knight) has been thought to be the main chemical cause of the damage inflicted by this mirid when feeding on its plant hosts. Early viscosity and thermal stability studies of the PG activity in L. hesperus protein extracts were difficult to interpret. Thus, it has been suggested that one or more PG protein(s) with different hydrolytic modes of action are produced by this mirid. In order to understand the quantitative complexity of the WTPB salivary PG activity, PG purification from a protein extract from salivary glands excised from L. hesperus insects was performed using affinity and ion exchange chromatography. To elucidate the qualitative complexity of the purified PGs, the digestion products generated by the PGs were separated using high performance anion exchange chromatography with pulsed amperometric detection. At least five PG proteins were detected; these differing in terms of their glycosylation, mass-to-charge ratios, and/or molecular mass. The characterization of the products generated by these PGs showed that endo- and exo-acting PGs are produced by WTPB. Although none of the PGs was purified to homogeneity, the present work provides biochemical evidence of a multiplicity of PGs that degrade the pectin component of the plant tissue in different fashions. The implications of these findings affect the understanding of WTPB feeding damage and, potentially, help identify ways to control this important crop pest. Arch. Insect Biochem. Physiol. 2008. © 2008 Wiley-Liss, Inc.     

Intron loss mediated structural dynamics and functional differentiation of the polygalacturonase gene family in land plants

The polygalacturonase (PG) gene family is one of the largest gene families in plants. PGs are involved in various plant development steps. The evolutionary processes accounting for the functional divergence and the specialized functions of PGs in land plants are unclear. Whole sets of PG genes were retrieved from the genome web sites of model organisms in algae and land plants. The number of PG genes was expanded by lineage-specific manner with the biological complexity of the organism. Differentiation of PGs was related with phylogenetic hierarchy such as presence of rhamno-PGs from algae to plants, endo- and exo-PGs in land plants, exo-PGs in flowering plants. Gene structure analysis revealed that land plant PG genes resulted from differential intron gain and loss, with the latter event predominating. Differential intron losses partitioned the PGs into separate clades to be expressed differentially during plant development. Intron position and phase were not conserved between PGs of algae and land plants but conserved among PG genes of land plants from moss to vascular plants, indicating that the current introns in the PGs in land plants appeared after the split between unicellular algae and multicelluar land plants. The results demonstrate that the functional divergence and differentiation of PGs in land plants is attributable to intron losses.     

An accumulation of proteoglycans in scarred fascia

A little is known about proteoglycan (PG) changes, occuring in the course of scarring of tissues another than skin. The aim of present study was biochemical characterization of glycosaminoglycans (GAGs) and proteoglycans (PGs) of normal and scarred fascia. Samples of normal fascia lata were taken at autopsy from 23 individuals and samples of scarred fascia lata were removed from 23 patients at reoperations for femoral fracture. The obtained tissues were divided into two samples: first of them was submitted to GAG isolation and the second one to PG isolation.GAGs were extracted by extensive papain digestion followed by the fractionation using cetylpyridinium chloride. In order to qualitative and quantitative characterization GAGs were submitted to electrophoresis on cellulose acetate before and after treatment with enzymes, specifically depolymerizing some kinds of GAGs. PGs were extracted using 4 M guanidine HCl followed by purification by forming complexes with Alcian blue. PGs were submitted to gel permeation chromatography on Sepharose 4B. In order to obtain core proteins PGs were depolymerized with chondroitinase ABC. The purified PGs and their core proteins were separated with sodium dodecyl sulphate/polyacrylamide gel electrophoresis (SDS/PAGE). It was found that total GAGs content was significantly elevated in scarred fascia. Both types of fascia contained chondroitin-, dermatan- and heparan sulphates and hyaluronic acid. Dermatan sulphates (DS) were the predominant GAGs of normal and scarred fascia. The contents of all GAG types were increased in scarred fascia. Both types of fascia contained two kinds of dermatan sulphate proteoglycans (DSPGs); first being similar to biglycan and the second one similar to decorin, as it was judged by molecular weight of their native molecules and core proteins as well as type of GAG components. Densitometric analysis showed that decorin is a predominant DSPG in both fascia types, but in scarred tissue the ratio of biglycan to decorin is considerably higher. Moreover, in scarred fascia a large chondroitin sulphate proteoglycan (CSPG) was also observed. The obtained results have shown that the scar formation is accompanied by quantitative and qualitative alterations in GAGs/PGs resembling those observed in hypertrophic skin scars. The biochemical modification of the scarred fascia lata may partly explain the clinically manifested damage to biomechanical properties of this tissue.     

Non-specific interaction of proteoglycans with surfaces and matrices

Evidence is presented that reversible non-specific adsorption of proteoglycans (PGs) to surfaces and matrices is an inherent property of the PGs. This adsorption is dependent on the intact PG structure as the glycosaminoglycans (GAGs), which are isolated after papain digestion of the PG show no such non-specific adsorption. The interaction of the PG with surfaces and matrices is also highly dependent on the internal milieu and can be both inhibited and enhanced by such factors as the ionic composition and concentration, pH, detergents and chaotropic reagents such as guanidine hydrochloride (Gdn-HC1). It is suggested that this inherent stickiness of the PGs allows them to function like a reversible fluid adhesant in the connective tissues. This weak binding force thus not only aids in maintaining the integrity of the connective tissues, but its reversible nature may provide for easy movement of other materials through the connective tissue matrix.     

Immunohistochemical characterization of extracellular matrix components of yolk sac tumors

Proteoglycans (PGs) were isolated from yolk sac tumor and chondroitin sulfate large PG (core molecule with a molecular weight congruent to 200,000) and small PG (core molecule with a molecular weight congruent to 50,000) were detected. Immunohistochemical localization of PGs in three yolk sac tumors was investigated using monoclonal antibodies raised against both small and large PGs, which were purified from human ovarian fibroma capsule and a yolk sac tumor, respectively. The localization of large PG was observed to be distinct from that of small PG. A markedly positive reaction for antibody against large PG was observed in myxomatous areas, perivascular and perivesicular portions; hyaline globules were the most intensely reactive. In the areas showing a polyvesicular vitelline tumor pattern, the compact connective tissue stroma consisted of small PGs. It is conceivable that large PGs are synthesized by immature mesenchymal cells and also by epithelial-like cells as a basement membrane component, whereas small PGs are synthesized by mature fibroblastic cells synthesizing collagen. Immunohistochemical localization of other extracellular matrix components (laminin, fibronectin, type I-IV collagen) was also studied in relation to PG localization.     

The role of prostaglandins in the aetiology and treatment of erectile dysfunction.

Both animal and human penile tissue synthesize prostaglandins (PGs). Furthermore, intracavernous injection of certain PGs elicits erection in men with erectile dysfunction (ED). It is also well established that PGs are involved in the pathophysiology of atherosclerosis and diabetes mellitus (DM). Since atherosclerosis and DM are major risk factors for ED, it has been suggested that the disruption of PG synthesis in penile tissues and related vasculature may play a role in the pathogenesis of ED. In this review, we discuss the role of PGs in normal penile erection as well as on the pathophysiology and treatment.     

The proteoglycan decorin is synthesized and secreted by differentiated myotubes

Proteoglycans (PGs) are important components of the skeletal muscle extracellular matrix (ECM). Skeletal muscles are composed of muscle fibers and mononucleated cells. The latter are known to synthesize and secrete several PGs. Rat skeletal muscle ECM contains a chondrotin/dermatan sulfate PG which was immunoprecipitated by antibodies against rat decorin. The synthesis and secretion of PGs by a mouse cell line was analyzed during in vitro differentiation. PGs were characterized by biochemical and immunological techniques including immunocytolocalization experiments. At least three different PGs are synthesized and secreted by differentiated myotubes: a 220 to 460 kDa heparan sulfate, a 250 to 310 kDa chondroitin/dermatan sulfate, and a 75 to 130 kDa chondroitin/dermatan sulfate. This latter PG was specifically immunoprecipitated with antibodies against rat fibroblast decorin. Indirect immunocytolocalization analysis revealed that decorin was localized inside the cells, with a strong reaction around the nuclei. During differentiation the relative proportions of some PGs changed. Thus, a decrease in the relative proportion of the heparan sulfate PG was observed, whereas a significant increase in the relative proportion of decorin was detected. No change in the large chondroitin/dermatan PG was seen during the differentiation process. The possible cell sources of decorin found in rat skeletal muscle ECM are discussed.     

Hormonal mechanisms underlying termination of larval diapause by juvenile hormone in the bamboo borer, Omphisa fuscidentalis

Topical application of methoprene, a juvenile hormone analogue (JHA), induces pupation by activating the prothoracic glands (PGs) in diapausing larvae of the bamboo borer, Omphisa fuscidentalis. To determine the minimum stimulation period for PG activation, we transplanted PGs of JHA-treated larvae (donors) into non-treated larvae (recipients) on successive days after JHA treatment and observed the recipients for pupation. JHA stimulation for 1 day was sufficient to induce pupation. In recipient larvae, the hemolymph ecdysteroid titer increased transiently on day 18 after transplantation and significantly on days 24-28, prior to pupation. Secretory activity of recipient PGs increased transiently on day 16 and days 22-28. Because the recipient PG activity was too low to account for an increased ecdysteroid titer, the JHA-stimulated donor PGs must produce the major part of hemolymph ecdysteroids. In addition, the ecdysteroid produced by the donor PGs might have stimulated the recipient PGs. We examined the possible involvement of two ecdysone receptor (EcR) isoforms, OfEcR-A and OfEcR-B1, in PG activation by JHA, and found that although both isoforms were up-regulated, accompanied by an increased ecdysteroid titer in the hemolymph, the isoform mRNA levels were not altered at all before the increase in PG secretory activity. Thus, EcR expression might not be involved in feedback activation of PGs.     

正常及损伤后修复的内皮细胞合成的蛋白聚糖

用自制尼龙刷将培养至汇合的人脐静脉内皮细胞刮伤后,造成规则的内皮细胞缺失区。继续培养可见,原有的内皮钿胞很快迁移到缺失区,并分裂增殖。约48小时新生的内皮细胞即将缺失区全部修复而形成新的汇合单层。以DEAE-Sephacel离子交换及Sepharose 6B凝胶过滤柱层析分析损伤后修复的内皮细胞合成的蛋白聚糖时发现所合成的蛋白聚糖总量减少;硫酸乙酰肝素蛋白聚糖合成相对减少、而硫酸软骨素及/或硫酸皮肤素蛋白聚糖合成相对增多。说明:伴随着内皮细胞的损伤后修复其蛋白聚糖的合成也有质和量的改变。     

Toll-like receptor 2-dependent bacterial sensing does not occur via peptidoglycan recognition

Toll-like receptor 2 (TLR2) has been shown to recognize several classes of pathogen-associated molecular patterns including peptidoglycan (PG). However, studies linking PG with TLR2 recognition have relied mainly on the use of commercial Staphylococcus aureus PG and have not addressed TLR2 recognition of other PG types. Using highly purified PGs from eight bacteria (Escherichia coli, Pseudomonas aeruginosa, Yersinia pseudotuberculosis, Helicobacter pylori, Bacillus subtilis, Listeria monocytogenes, Streptococcus pneumoniae and S. aureus), we show that these PGs are not sensed through TLR2, TLR2/1 or TLR2/6. PG sensing is lost after removal of lipoproteins or lipoteichoic acids (LTAs) from Gram-negative and Gram-positive cell walls, respectively. Accordingly, purified LTAs are sensed synergistically through TLR2/1. Finally, we show that elicited peritoneal murine macrophages do not produce tumour necrosis factor-alpha or interleukin-6 in response to purified PGs, suggesting that PG detection is more likely to occur intracellularly (through Nod1/Nod2) rather than from the extracellular compartment.     

High binding affinity of electronegative LDL to human aortic proteoglycans depends on its aggregation level

Electronegative LDL [LDL(-)] is an atherogenic subfraction of plasma LDL that has increased apolipoprotein E (apoE) and apoC-III content, high density, and increased susceptibility to aggregation. These characteristics suggest that LDL(-) could bind to proteoglycans (PGs); therefore, our aim was to evaluate its affinity to PGs. Binding of LDL(-) and native LDL [LDL(+)] to human aortic PGs was determined by precipitation of LDL-glycosaminoglycan complexes, LDL incubation in PG-coated microtiter wells, and affinity chromatography on PG column. All methods showed that LDL(-) had higher binding affinity to PGs than did LDL(+). PG capacity to bind LDL(-) was increased approximately 4-fold compared with LDL(+) in precipitation and microtiter assays. Chromatography on PG column showed LDL(-) to consist of two subpopulations, one with higher and one with lower PG binding affinity than LDL(+). Unexpectedly, the lower PG affinity subpopulation had increased apoE and apoC-III content. In contrast, the high PG affinity subpopulation presented phospholipase C (PLC)-like activity and increased aggregation. These results suggest that PLC-like activity could alter LDL lipid composition, thereby promoting particle aggregation and binding to PGs. This propensity of a subpopulation of LDL(-) to bind to PGs could facilitate its retention in the extracellular matrix of arterial intima and contribute to atherosclerosis progression.     

Uterine and placental prostaglandins and their modulation of oxytocin sensitivity and contractility in the parturient uterus

The parturient uterus develops a markedly enhanced sensitivity to the uterotonic action of oxytocin (OT). The mechanism leading to this enhanced OT sensitivity is not known. Our previous work suggested that prostaglandins (PGs) may be involved. To define the relationship between OT sensitivity and uterine PG production, we measured uterine sensitivity to OT by a quantitative dose-response procedure in rats on Days 19, 20, 21 and 22 of pregnancy and monitored uterine and placental tissue concentrations of PGF2 alpha and PGE2. In addition, we determined the effects of inhibition of endogenous PG synthesis on OT sensitivity and uterine contractility. We found that both OT sensitivity and spontaneous contractility are positively related to uterine PGF2 alpha production. An abrupt increase in OT sensitivity was observed on Days 21 and 22 of pregnancy. The increase in OT sensitivity was coincidental with the marked increase in PGF2 alpha production in the uterus on Days 21 and 22 of pregnancy. Suppression of in vivo PG synthesis caused a reduction in both spontaneous uterine contractility and OT-induced contractions. Uterine PGE2 concentrations and release were 3-5 times lower than PGF2 alpha. There were no significant fluctuations of uterine PGE2 concentration measured on these last 4 days of gestation. Placental PG levels were also found not to be related to uterine contractility. Placental PGE2 levels were higher than PGF2 alpha and may play a regulatory role in placental perfusion. However, placental PGs did not vary with gestational age.     

Examination of corneal proteoglycans and glycosaminoglycans by rotary shadowing and electron microscopy

Proteoglycans (PGs) from cornea and their relevant glycosaminoglycan (GAG) chains, dermatan sulphate (DS) and keratin sulphate (KS), were examined by electron microscopy following rotary shadowing, and compared with hyaluronan (HA), chondroitin sulphate (CS), alginate, heparin, heparan sulphate (HS) and methyl cellulose. Corneal DS PG had the tadpole shape previously seen in scleral DS FG, and the images from corneal KS PG could be interpreted similarly, although the GAG (KS) chains were very much fainter than those of DS PG GAG. Isolated GAG (KS, DS, CS, HA, etc.) examined in the same way showed images that decreased very significantly in clarity and contrast, in the sequence HA greater than DS greater than CS greater than KS. The presence of secondary and tertiary structures in the GAGs may be at least partly responsible for these variations. HA appeared to be double stranded, and DS frequently self-aggregated, KS and HS showed tendencies to coil into globular shapes. It is concluded that it is unsafe to assume the absence of GAGs, based on these techniques, and quantitative measurements of length may be subject to error. The results on corneal DS PG confirm and extend the hypothesis that PGs specifically associated with collagen fibrils are tadpole shaped.     

Age and renal prostaglandin inhibition during exercise and heat stress.

Aging is associated with a number of physiological changes that may cause the kidney to rely to a greater extent on vasodilatory PGs for normal functioning. Acute exercise has been shown to cause renal vasoconstriction that may be partially buffered by vasodilatory PGs. To determine the relative importance of renal PGs during exercise in older adults, we compared the renal effects of the PG inhibitor ibuprofen (1.2 g/day for 3 days) vs. a placebo control in a cohort of eight younger (24 +/- 2 yr) and eight older (64 +/- 2 yr) women during treadmill exercise ( approximately 57% maximal oxygen consumption) in the heat (36 degrees C). This over-the-counter dose of ibuprofen reduced renal PG (i.e., PGE2) excretion by 47% (P < 0. 05). Acute exercise in the heat caused dramatic decreases in glomerular filtration rate, renal blood flow, and sodium excretion in both age groups. PG inhibition was associated with greater decreases in urine production and free water clearance (P < 0.05). There were no drug-related declines in glomerular filtration rate or renal blood flow. We conclude that PG inhibition has only modest effects on renal function during exercise. Also, the lack of hemodynamic changes with PG inhibition indicates that healthy well-hydrated older women are not in a renal PG-dependent state.     

Prostaglandin synthases: Molecular characterization and involvement in prostaglandin biosynthesis

Prostaglandins (PGs) belong to a subclass of eicosanoids and are classified based on the structures of the cyclopentane ring and their number of double bonds in their hydrocarbon structures. PGs are important lipid mediators that are involved in inflammatory response. The biosynthesis of diverse PGs from unsaturated C20 fatty acids containing at least three double bonds such as dihomo-γ-linoleic acid (20:3^Δ8Z,11Z,14Z), arachidonic acid (20:4^{Δ5Z,8Z,11Z,14Z}), and eicosapentaenoic acid (20:5^{Δ5Z,8Z,11Z,14Z,17Z}) is enables by various PG synthases, including prostaglandin H synthase (PGHS), 15-hydroxyprostaglandin dehydrogenase (15-HPGD), PGES, PGDS, PGFS, PGIS, and thromboxane A synthase (TXAS). This review summarizes the biochemical properties, reaction mechanism, and active site details of PG synthases. Because PGs are involved in the immune system, an understanding of PG synthases is important in the design of new anti-inflammatory drugs. The biosynthesis of PGs in various organisms, such as mammals, corals, florideae (a class of red algae), yeast, and fungi, is also introduced. The expression of PG synthases in the microbial systems for the synthesis of PGs is discussed. Now, the biosynthesis of PGs from glucose or glycerol is possible using metabolically engineered cells expressing both unsaturated fatty acid-producing enzymes and PG synthases.     

Interactions between parathyroid hormone and prostaglandins on renal cortical cyclic AMP

Effects of parathyroid hormone (PTH) and several prostaglandins (PGs) on cyclic AMP (cAMP) metabolism were studied and compared in isolated renal cortical tubules from male hamsters. Both production and intracellular degradation of cAMP were increased by PTH and each of the PGs tested (PGE2, PGE1, PGI2). Production of cAMP was increased to similar levels by maximal concentrations of PTH and each PG, however, degradation of cAMP was significantly higher in response to PTH than with any of the PGs. This difference in intracellular degradation of cAMP was responsible for the much higher concentrations of cAMP in renal cortical tubules exposed to PGs (PGE1, PGE2, PGI2) than to PTH. Submaximal amounts of each PG produced additive increases in cAMP concentrations in the presence of maximal amounts of PTH. Additivity of the combined responses was lost, however, as the PGs concentrations reached their maxima. The results suggest that renal PGs (PGE2 and PGI2) may modulate the effects of PTH on cAMP concentrations in renal cortical tubules.     

Proteoglycan expression in bleomycin lung fibroblasts: role of transforming growth factor-beta(1) and interferon-gamma

Bleomycin (BM)-induced pulmonary fibrosis involves excess production of proteoglycans (PGs). Because transforming growth factor-beta(1) (TGF-beta(1)) promotes fibrosis, and interferon-gamma (IFN-gamma) inhibits it, we hypothesized that TGF-beta(1) treatment would upregulate PG production in fibrotic lung fibroblasts, and IFN-gamma would abrogate this effect. Primary lung fibroblast cultures were established from rats 14 days after intratracheal instillation of saline (control) or BM (1.5 units). PGs were extracted and subjected to Western blot analysis. Bleomycin-exposed lung fibroblasts (BLF) exhibited increased production of versican (VS), heparan sulfate proteoglycan (HSPG), and biglycan (BG) compared with normal lung fibroblasts (NLF). Compared with NLF, BLF released significantly increased amounts of TGF-beta(1). TGF-beta(1) (5 ng/ml for 48 h) upregulated PG expression in both BLF and NLF. Incubation of BLF with anti-TGF-beta antibody (1, 5, and 10 microg/ml) inhibited PG expression in a dose-dependent manner. Treatment of BLF with IFN-gamma (500 U. ml(-1) x 48 h) reduced VS, HSPG, and BG expression. Furthermore, IFN-gamma inhibited TGF-beta(1)-induced increases in PG expression by these fibroblasts. Activation of fibroblasts by TGF-beta(1) promotes abnormal deposition of PGs in fibrotic lungs; downregulation of TGF-beta(1) by IFN-gamma may have potential therapeutic benefits in this disease.     

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.