Carbaprostacyclin, a PPARδ agonist, ameliorates excess lipid accumulation in diabetic rat placentas
Introduction
The altered metabolic environment in maternal diabetes during pregnancy impairs fetal and placental development (Jawerbaum and Gonzalez, 2006, Melamed and Hod, 2009). Placental alterations in morphology, function and metabolism are found in both human and experimental diabetic pregnancies (Capobianco et al., 2008, Daskalakis et al., 2008, Desoye and Shafrir, 1996, Yu et al., 2008). Impaired nutrient transfer through the placenta leads to lipid overaccumulation in the placenta and the fetus, a feature that characterizes the macrosomic fetus in both human and various experimental models (Diamant et al., 1982, Herrera and Amusquivar, 2000, Khan, 2007). In addition, a pro-inflammatory environment is a feature in diabetic pregnancies, and several markers of pro-inflammation, including increased lipid peroxidation, have been observed both in fetuses and in the placenta (Jawerbaum and Gonzalez, 2006, Pustovrh et al., 2005, Radaelli et al., 2003).
The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that regulate the expression of several genes involved in metabolic homeostasis, developmental processes and the regulation of pro-inflammatory processes (Bensinger and Tontonoz, 2008, Desvergne et al., 2004, Wieser et al., 2008). PPAR ligands are well known to induce the expression of several genes involved in both peroxisomal and mitochondrial β-oxidation, and to amplify their signalling pathways through the increase of PPAR expression (Limor et al., 2008, Nagasawa et al., 2006, Reddy and Rao, 2006).
There are three PPAR isotypes identified (PPARα, PPARγ and PPARδ) which form a functional transcriptional unit upon heterodimerization with retinoid X receptors and activation by their ligands (Hihi et al. 2002). PPAR ligands are various fatty acids, with greater preference for monounsaturated and polyunsaturated fatty acids. Indeed, the arachidonic acid metabolites leukotriene B4 (LTB4), 15deoxyΔ12,14prostaglandinJ2 (15dPGJ2) and prostacyclin (PGI2) have been shown to respectively activate PPARα, PPARγ and PPARδ (Forman et al., 1997, Hihi et al., 2002, Wise, 2003). Synthetic analogues of PGI2 such as carbaprostacyclin (cPGI2) are also able to bind and act as agonists of PPARδ (Forman et al., 1997, Wise, 2003). Although it is well known that PGI2 can classically act through membrane prostacyclin receptors (IP), PGI2 activation of PPARδ has been shown to be involved in several biological processes such as angiogenesis, lipid homeostasis in cardiomyocytes and skeletal muscle, stem cell proliferation, decidualization and embryo implantation (Barish et al., 2006, de Lange et al., 2008, He et al., 2008, Kim and Han, 2008, Lim and Dey, 2000, Wang et al., 2007).
We have previously found reduced PPARδ and PGI2 concentrations in the embryos from diabetic rats during early organogenesis, which are alterations related to impairments in the synthesis of lipids and in the production of PGE2 needed for proper embryo morphogenesis (Higa et al. 2007). Reduced concentrations of PGI2 have also been found in term placentas from diabetic patients and diabetic experimental models, an alteration related to the impaired vascular function of the placenta (Jawerbaum et al., 1997, Kuhn et al., 1990, Saldeen et al., 1998).
PPARδ is highly expressed in the syncytial trophoblast layer in the human placenta and its expression increases during labour (Berry et al., 2003, Wang et al., 2002). In human trophoblast cells cultured in vitro, cPGI2 regulates the expression of 11-β-hydroxysteroid dehydrogenase type 2, an enzyme that metabolizes maternal derived glucocorticoids and is related to fetal growth (Julan et al. 2005).
PPARδ is expressed in the three layers of the mouse placenta and regulates the differentiation of trophoblast giant cells, which play a critical role in the establishment of the placental structure, fulfil an important endocrine function, and are an important site of lipid accumulation during development (Nadra et al. 2006). PPARδ null mice are not viable due to severe impairments in placental development, as a result of an altered differentiation of the trophoblast giant cells, spongiotrophoblasts and glycogen trophoblast cells (Barak et al., 2002, Nadra et al., 2006, Wang et al., 2007). These developmental alterations are different from those found in the PPARγ null mice, in which defective placental vascularization leads to an impaired fetal cardiac development and function (Barak et al. 1999). PPARα null mice are viable but present several metabolic impairments and an increased abortion rate (Yessoufou et al. 2006). These data suggest that the three PPAR isotypes have specific roles in placental development (Barak et al. 2008).
We have recently identified PPARγ and PPARα as regulators of lipid metabolic processes in the rat placenta (Capobianco et al., 2008, Martinez et al., 2008). Moreover, in a mild diabetic rat model obtained through neonatal administration of streptozotocin (Portha et al. 1979), thoroughly characterized throughout gestation (Jawerbaum and Gonzalez 2005), the concentrations of PPARγ and PPARα isoforms and the concentrations of 15dPGJ2 and LTB4 (PPARγ and PPARα respective endogenous ligands) are altered in the placenta, and the activation of these nuclear receptors results in different metabolic functions in the diabetic placenta (Capobianco et al., 2008, Martinez et al., 2008). The aim of the present study was to assess the expression of PPARδ, the concentrations of PGI2 and the effect of carbaprostacyclin (a PGI2 analogue that activates PPARδ) in lipid concentrations, synthesis, catabolism and peroxidation in placentas of pregnant control and diabetic rats at day 13.5 of gestation. The results revealed that the PPARδ agonist cPGI2 also has a specific regulatory function in the regulation of lipid homeostasis in both control and diabetic placentas.
Section snippets
Animals
Albino Wistar rats bred in the laboratory were fed ad libitum with commercial rat chow (Asociación Cooperativa Argentina, Buenos Aires, Argentina). At 2 days of age neonates were injected with either streptozotocin (90 mg/kg s.c., Sigma-Aldrich) in citrate buffer (0.05 M, pH 4.5) (diabetic experimental model) or buffer alone (controls) (Portha et al. 1979). The reproductive characteristics of this diabetic experimental model have been reported previously (Jawerbaum and Gonzalez 2005). Control and
Prostacyclin concentrations and PPARδ expression
In the diabetic experimental model evaluated, pregnant rats on day 13.5 of gestation showed marked hyperglycemia (control: 100 ± 11 mg/dl, diabetic: 221 ± 25 mg/dl; p < 0.001) and triglyceridemia (control: 1.06 ± 0.15 g/l, diabetic: 2.06 ± 0.41 g/l; p < 0.05). In the placentas from diabetic animals there were no changes in PPARδ mRNA when compared to controls (Fig. 1A). PPARδ protein content was also similar in placentas from control and diabetic rats (Fig. 1B). The concentrations of PGI2, an endogenous PPARδ
Discussion
In the present study, we identified a novel role of cPGI2, a PPARδ agonist, as a regulator of lipid metabolism and peroxidation in the placentas from diabetic rats. These results, together with the observed reduction in PGI2 concentration in the placentas from these diabetic animals, contribute to the understanding of the altered regulatory pathways of placental lipid metabolism that could lead to fetal impairments in maternal diabetes.
Indeed, placental lipid transfer is critical for normal
Conclusions
The present work provides evidence of abnormal PGI2 concentrations in the diabetic rat placenta in a post-implantation stage, a reduction that may lead to alterations in its signalling pathway through PPARδ, as no changes in the expression of this nuclear receptor were observed. Besides, the PPARδ agonist cPGI2 has been shown to be a potent regulator of lipid concentrations, synthesis, catabolism and peroxidation and to upregulate PPARδ mRNA, providing clues that PPARδ activation and/or
Conflict of interest statement
There is no conflict of interest.
Acknowledgements
This work was supported by grants from Agencia de Promoción Científica y Tecnológica de Argentina (PICT 05-32268 and PICT 06-00084).
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