Modulation of macrophage antiviral responses by lipin-2type i interferon, inflammasome and cholesterol homeostasis

Resumen

Lipin-2 is a phosphatidic acid phosphatase with a key role in the metabolism of lipids. Mutations in the gene coding for lipin-2 were found to be responsible for the development of Majeed Syndrome, an autoinflammatory disorder governed by excessive production of IL-1β. To date, the factors that initiate the episodes of inflammation in these patients are unknown. The present work has characterized the involvement of lipin-2 in IL-1β production specifically during antiviral responses. Lipin-2 has been found to be necessary to restrain both the priming step and the inflammasome activation induced by poly(I:C), a dsRNA molecule widely used to mimic viral nucleic acids. Regarding the priming step, lipin-2 deficiency leads to an enhanced production of ROS, activation of MAPK and translocation of NF-ĸB to the nucleus, which results in a markedly higher upregulation of Il1b. Lipin-2 deficiency also impacts on NLRP3 inflammasome activation, by increasing the processing of caspase-1, pro-IL-1β and pro-IL-18. Additionally, lipin-2 controls mitochondria content and functionality, as lipin-2 deficient cells exhibit greater amount of mitochondria, which are fragmented, hyperpolarized, prone to damage and less functional. In fact, an excessive production of mitochondrial ROS has been found to contribute to the exacerbated levels of IL-1β produced by lipin-2 deficient cells. This work provides evidences that indicate that lipin-2 regulates also interferon-β production. Interestingly, IL-1β production is actually regulated by a mechanism mediated by interferon-β, as the ISG caspase-11 is responsible for the enhanced production of IL-1β in lipin-2 deficient cells. The higher production of interferon-β that results from lipin-2 deficiency also affects cholesterol metabolism, since lipin-2 deficient cells show lower expression of the genes coding for SREBP-2, LDLR and HMGCR in an interferon-dependent manner. In consequence, the uptake and synthesis of cholesterol are impaired in absence of lipin-2, which contributes to the reduced levels of cholesterol previously described for lipin-2 deficient cells. Lipin-2 has also been shown to control distribution of cellular cholesterol, which along with the impact on total cholesterol levels, could explain the effect that lipin-2 deficiency shows on IL-1β production. Finally, results in this work reveal that in the absence of lipin-2, infection with MCMV results in an enhanced replication capacity of the virus both in vitro and in vivo that parallels an exacerbated production of IL-1β. This finding points to the idea that viral infections could be involved in the initiation of recurrent episodes of acute inflammation that Majeed Syndrome patients suffer. Overall, the present work provides insights into the role of lipin-2 in regulating the antiviral response in macrophages by modulating type I interferon production, the activation of the inflammasome and probably cholesterol metabolism, and opens the possibility to develop new therapies for patients with Majeed Syndrome.