Itraconazole not only impairs lipid homeostasis, but also affects different signaling pathways, including mammalian target of rapamycin (mTOR), hedgehog, and Wnt signaling pathways that are hijacked by a broad range of viruses to drive the production of infectious particles.
The mammalian target of rapamycin (mTOR) signaling cascade is a pivotal signaling pathway that regulates apoptosis and counteracts stress-induced autophagy (such as, e.g., that elicited by viruses). Although different cellular locations for mTOR complex 1 (mTORC1) and mTORC2 have been reported, mTORC1 lysosomal localization appears critical for its ability to sense and respond to cell starvation
[52]. Cholesterol was recently identified to promote the recruitment of mTORC1 to the lysosomal membrane
[53] and the mTOR signaling cascade is regulated in a cholesterol-dependent manner
[54].
Several viruses have evolved strategies to subvert the mTORC1 signaling network to drive their replication and propagation
[55][56][57][58][59][60][61][62][63]. The Semliki Forest virus (SFV), Sindbis virus (SINV), and Chikungunya virus (CHIKV), members of the alphavirus family, cause different diseases but have in common that they encode nonstructural proteins (nsP)
[64]. The activation of the PI3K/Akt/mTOR pathway is mediated through the phosphorylated and membrane-attached protein nsP3, which forms the viral replication complex upon virus internalization
[65][66][67]. The activation of mTOR signaling is also fundamental for infection with the
Flaviviridae West Nile virus (WNV), Japanese encephalitis virus (JEV) and dengue virus (DENV)
[68][69][70].
Flaviviridae infection increases mTOR activity through a PI3K-dependent mechanism to maintain translation of its positive-sense RNA genome and also delays WNV-induced apoptosis
[58][71][72]. The hepatitis C virus, another
Flaviviridae member, increases phosphorylation of mTOR through the nonstructural protein 5A (NS5A)
[73]. NS5A seems to activate PI3K/Akt signaling by directly binding PI3K
[74][75]. The activation of the mTORC1 pathway by HCV has been linked to antiapoptotic signals that ensure cell survival and maintain persistence by promoting steady-state levels of virus replication
[76][77]. Among the β-herpesvirus, the human cytomegalovirus (HCMV) maintains mTORC1 activation
[78][79] through the expression of the two HCMV immediate early proteins, IEP72 and IEP86
[80]. Another persistent virus that tightly regulates mTOR signaling pathways is the human immunodeficiency virus type 1 (HIV-1). In dendritic cells, the HIV-1 envelope glycoprotein activates mTOR to prevent autophagy and to increase virus infection. Pharmacological treatment with rapamycin decreased viral spreading
[81]. Another study implied that the HIV-1 protein Nef initiates mTOR activation which can be blocked by inhibitors of mTOR or PI3K
[82][83], suggesting that drugs that modify the mTORC1 signaling pathway could act as anti-HIV-1 agents
[84][85].
Well-balanced mTOR signaling is vitally important for IAV infection
[86][87]. Thus, the pharmacological inhibition of the mTOR signaling axis might serve as potential antiviral target. In contrast to other azoles, itraconazole additionally inhibits mTOR signaling through affecting the upstream 5′-AMP-dependent protein kinase (AMPK)
[88], which is activated upon an increased AMP/ATP ratio and serves as a regulator of cellular energy levels
[89]. Once activated, AMPK inhibits mTOR signaling
[90]. The activation of AMPK through itraconazole is a result of direct binding and inhibition of the mitochondrial Voltage-Dependent Anion Channel 1 (VDAC1), a critical regulator of mitochondrial metabolism, resulting in a drop in cellular energy levels
[91]. Itraconazole treatment also impairs vascular endothelial growth factor receptor 2 (VEGFR2) functionality in endothelial cells, which is mostly due to altered VEGFR2 glycosylation, trafficking, and signaling
[92]. As some viruses like human papillomaviruses or hepatitis viruses promote angiogenesis to facilitate optimal supply by nutrients
[93][94][95], this might be an additional beneficial effect of itraconazole in antiviral strategy.