Toxoplasma gondii Proliferation Require Down-Regulation of Host Nox4 Expression via Activation of PI3 Kinase/Akt Signaling Pathway

Abstract

Toxoplasma gondii results in ocular toxoplasmosis characterized by chorioretinitis with inflammation and necrosis of the neuroretina, pigment epithelium, and choroid. After invasion, T. gondii replicates in host cells before cell lysis, which releases the parasites to invade neighboring cells to repeat the life cycle and establish a chronic retinal infection. The mechanism by which T. gondii avoids innate immune defense, however, is unknown. Therefore, we determined whether PI3K/Akt signaling pathway activation by T. gondii is essential for subversion of host immunity and parasite proliferation. T. gondii infection or excretory/secretory protein (ESP) treatment of the human retinal pigment epithelium cell line ARPE-19 induced Akt phosphorylation, and PI3K inhibitors effectively reduced T. gondii proliferation in host cells. Furthermore, T. gondii reduced intracellular reactive oxygen species (ROS) while activating the PI3K/Akt signaling pathway. While searching for the main source of these ROS, we found that NADPH oxidase 4 (Nox4) was prominently expressed in ARPE-19 cells, and this expression was significantly reduced by T. gondii infection or ESP treatment along with decreased ROS levels. In addition, artificial reduction of host Nox4 levels with specific siRNA increased replication of intracellular T. gondii compared to controls. Interestingly, these T. gondii-induced effects were reversed by PI3K inhibitors, suggesting that activation of the PI3K/Akt signaling pathway is important for suppression of both Nox4 expression and ROS levels by T. gondii infection. These findings demonstrate that manipulation of the host PI3K/Akt signaling pathway and Nox4 gene expression is a novel mechanism involved in T. gondii survival and proliferation.

Toxoplasma gondii GRA7-Induced TRAF6 Activation Contributes to Host Protective Immunity

“…Innate immune responses triggered by TLR signaling are mediated through the generation of intracellular ROS, which are now being recognized as important secondary messengers (21,22,24 activation of the PI3K/AKT signaling pathway in human retinal epithelium cells (19). Additionally, a recent study showed that T. gondii induced the NOX1-dependent production of excessive ROS, which contributed to mitochondrial structural damage and mitochondrial dysfunction in placentas, as well as the cleavage of caspase-9 and caspase-3, which resulted in the apoptosis of trophoblasts (35).…”

“…Although intracellular ROS as a secondary messenger can regulate infection signaling against T. gondii (19,26), ROS formation in response to toxoplasma antigens is poorly characterized. We first determined whether stimulation with rGRA7 leads to ROS generation in murine BMDMs.…”

“…There is a growing body of evidence suggesting that reactive oxygen species (ROS) contribute to diverse signaling processes, including TLR-induced innate immune responses, antimicrobial activity, and inflammation (8,19,20). It was demonstrated that NADPH oxidase (NOX)-derived ROS plays an essential role in inflammatory responses and has antiparasite activity against T. gondii (19,20).…”

“…It was demonstrated that NADPH oxidase (NOX)-derived ROS plays an essential role in inflammatory responses and has antiparasite activity against T. gondii (19,20). Additionally, ROS generation with T. gondii is essential for mammalian innate immunity and acts through the phosphatidylinositol 3-kinase (PI3K)/AKT (19), endoplasmic reticulum (ER) stress, apoptosis, or JNK pathway (21). However, the roles of TRAF6 and ROS in the regulation of GRA7-induced intracellular signaling and innate immune responses are largely uncharacterized.…”

Toxoplasma gondiiHsp90: potential roles in essential cellular processes of the parasite

“…Moreover, it is still unknown whether there is a connection between host cell and tachyzoites in the modulation of the parasite replication process. The treatment of host cells with phosphatidylinositide 3-kinases (PI3K kinases) inhibitors such as LY294002 and wortmannin has been shown to reduce tachyzoite replication (Zhou et al 2013). TgHsp90 PPI analysis has shown several putative kinases and phosphatases that could be involved in cell signalling (Table S1).…”