The imbalance that renders greater
The imbalance that renders greater lipid uptake toxic is unclear, but the comparison of our in vitro studies with those in mice illustrates the importance of TAG storage versus accumulation of less non-polar molecules. Protective effects of ACSL1 and PPARγ have been reported in other cell types. Particularly, PPARγ reduces inflammation in macrophages . PPARγ overexpression was more effective in reducing palmitate-induced ER stress than ACSL1 overexpression. While ACSL1 merely catalyzes esterification of fatty acids with CoA, the initial step in intracellular fatty ABT mg metabolism, PPARγ regulates key lipogenic pathways and regulates lipid droplet coat protein expression needed to facilitate TAG storage . Our data do not exclude that the stronger protective effects of PPARγ versus ACSL1 overexpression are due to factors other than increased facilitation of TAG storage, such as increased utilization of NEFA for β-oxidation.
PPARγ actions in the heart have beneficial but sometimes also toxic actions. We recently showed that pharmacologic and genetic activation of cardiac PPARγ in mice treated with lipopolysaccharide (LPS) corrected cardiac dysfunction, further supporting the beneficial effects of increased PPARγ activity . However, ectopic expression of ACSL1 or PPARγ in the heart eventually results in cardiac hypertrophy, myofibrillar disorganization, interstitial fibrosis, and left-ventricular dysfunction , , , indicating that prolonged overexpression of these proteins has negative consequences. Similarly, although treatment with some PPAR agonists alleviates lipid-induced toxicity by increasing uptake of circulating lipids by adipose tissue , more robust activation of PPARγ can cause cardiac hypertrophy . Although increased cardiac ACSL1 and PPARγ levels may not be beneficial in the long-term, this study does suggest that short-term stimulation of ACSL1 and/or PPARγ may be beneficial in conditions of increased lipid supply.
To our surprise, we also found that the release of FAs from stored TAGs led to ER stress. Besides free FAs other toxic lipids could be released from lipid droplets due to ATGL action, e.g. DAG. The aggravating effect of ATGL in ER stress may explain the increased CHOP mRNA levels observed in hearts of αMHC-PPARγ mice ; αMHC-PPARγ mice have increased cardiac ATGL expression . Most studies of lipid-induced cellular and organ toxicity have focused on lipid entry leading to excess accumulation. In fact, others showed that release of lipids from lipid droplets by ATGL leads to the production of non-toxic molecules, specifically diacylglycerols that do not activate PKC . Although we cannot be sure which ATGL-generated lipids lead to ER stress, our observations that ATGL activity causes non-toxic lipid treatments such as oleic acid to induce ER stress is novel and might explain some of the inflammatory and cachectic effects of cancers that are prevented in ATGL knockout mice . The reduction in PPAR-mediated gene expression responsible for cardiac FA oxidation in ATGL knockout mice and the correction of cardiac dysfunction by PPAR agonists that induce FA oxidation  is strong evidence that accumulation of non-metabolized lipid species is responsible, at least in part, for heart dysfunction. Our data with ATGL overexpression in lipid-loaded cells show that toxic lipids can be released from stored TAG. Therefore, our data suggest that TAG storage without excess lipolysis is needed to maintain normal cardiomyocyte function.
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Acknowledgements This study was supported by the National Heart, Lung, and Blood Institute (NHLBI) Grants HL45095, HL73029 (I.J.G.) and HL112853 (K.D.). M.B. was supported by a Dr. E. Dekker Student Fellowship of the Netherlands Heart Foundation.
Introduction Porcine reproductive and respiratory syndrome (PRRS) is inarguably the most economically devastating disease affecting the global swine industry. The disease is typically characterized by reproductive failures in pregnant sows and acute respiratory disease in young piglets (Lunney et al., 2010). In the United States alone, PRRS is estimated to cause more than $600 million in production losses each year (Neumann et al., 2005; Holtkamp et al., 2013). The causative agent of the disease, PRRS virus (PRRSV), was identified and characterized first in the Netherlands (Meulenberg et al., 1993) and subsequently in the United States (Collins et al., 1992; Snijder et al., 2013). Following the acute phase of infection, PRRSV can establish a persistent infection lasting for several months. Persistently-infected animals continuously shed virus and are therefore important sources of transmission to naïve pigs within the herd (Snijder et al., 2013). PRRSV infection suppresses all facets of the host’s immune response, characterized by weak cellular and humoral immune responses that are suboptimal compared to other swine pathogens. PRRSV is also a poor inducer of the antiviral type I interferons (IFNs), important cytokines that bridge the innate and adaptive immune responses (Sun et al., 2012a). Due to the extensive genetic diversity of PRRSV, current available vaccines only offer limited protection to heterologous strains (Ke and Yoo, 2017; Renukaradhya et al., 2015). Only with a better understanding of the molecular mechanisms of PRRSV immune evasion will we be able to design more effective vaccines.