Estrogen has a Protective Role in Influenza A Virus Infection

Summary of:

Robinson, D.P., Hall, O.J., Nilles, T.L., Bream, J.H., and Klein, S.L. (2014). 17β-estradiol Protects Females against Influenza by Recruiting Neutrophils and Increasing Virus-Specific CD8 T Cell Responses in the Lungs. Journal of Virology. Vol. 88:9 p. 4711-4720.

Influenza viruses cause contagious respiratory illness with a sudden onset of high fever, cough, sore throat, runny or stuffy nose, body aches, headache, and fatigue. There are 3 types of influenza viruses (A, B, and C) with influenza A and B viruses causing seasonal epidemics of disease. The Centers for Disease Control (CDC) estimates that flu-associated deaths range from 3000-49,000 each year depending on length and severity of the season (see the CDC website for more general information about the flu virus, Moreover, pulmonary inflammatory diseases such as influenza are known to differentially affect males and females with women suffering a worse outcome. Prior evidence indicates that estrogen impacts the function of various immune cells and therefore may contribute to the differential outcomes between men and women following respiratory infections. The authors of this paper aimed to examine the role of estradiol (E2, one of the 2 major biologically active estrogens in non-pregnant humans) in both the host response to the influenza A virus (IAV) and the ability of IAV to replicate in the host. It was hypothesized that estradiol alters the recruitment and activity of immune cells therefore affecting the outcome of IAV infection. In order to test this hypothesis, female C57BL/6 mice were ovariectomized and implanted with either E2 or placebo capsules followed by infection with IAV. E2 treatment resulted in less morbidity as compared to placebo-treated females. E2 did not affect virus replication, but increased the levels of chemokines in lung homogenates, increased recruitment of neutrophils to the lungs, and increased the levels of interferon-γ and tumor necrosis factor-α released from virus-specific CD8 T cells. These effects were dependent on neutrophils because depletion of neutrophils in females treated with E2 increased morbidity, reduced chemokine production, and reduced CD8 T cell IFN-g production. In summary, sustained E2 levels affect the host response to IAV infection through a neutrophil-dependent mechanism leading to improved outcomes. This work sets the stage for further mechanistic studies related to how E2 regulates these effects and supports the consideration of E2 levels in the treatment of IAV infected patients.

Understanding the Complex Role of Isoprenoid Depletion in MKD

Summary of:

Burgh, Pervolaraki, Turkenburg, Waterham, Frenkel, & Boes. Unprenylated RhoA Contributes to IL-1b Hypersecretion in Mevalonate Kinase Deficiency Model through Stimulation of Rac1 Activity. The Journal of Biological Chemistry Vol. 289, No. 40, pp. 27757-27765, October 2014.

Hereditary autoinflammatory disorders are caused by genetic mutations in molecules involved in regulating the innate immune response. They are characterized by recurrent and usually short attacks of joint pain, rashes, abdominal pain, and fever. One category of these diseases includes hyperimmunoglobulinemia D and periodic fever syndrome (HIDS) and mevalonic aciduria (MA) and is caused by mutations in mevalonate kinase, an enzyme that acts upstream in the mevalonate pathway (Figure 1). Mevalonate kinase is responsible for the production of both non-sterol (farnesylpyrophosphate (FPP) and geranylgeranylpyrophosphate (GGPP) and sterol isoprenoids (cholesterol). Taken together, these disorders are referred to as mevalonate kinase deficiency (MKD). Patients with MKD suffer from periodic fever episodes, which have been attributed, at least in part, to depletion GGPP and subsequent uncontrolled release of the cytokine interleukin (IL)-1b from monocytes and macrophages.

Figure 1 (Clin Cancer Res, July 1, 2012 vol. 18 no. 13 3524-3531)


How does GGPP control IL-1b release? GGPP is a lipid moiety that is responsible for the prenylation of proteins including the Rho and Rab families of small GTPases, which cycle between a GTP bound active state and a GDP bound inactive state. Modification of these proteins by GGPP is involved in subcellular localization (membrane attachment) and the control of GTP hydrolysis via interaction with guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs), and GDP dissociation inhibitors (GDIs). GEFs facilitate the exchange of GDP for GTP to generate the activated form. GAPs increase the GTPase activity thereby causing inactivation. Finally, GDIs interact with the prenylated, GDP bound form to inhibit GEF/GAP binding and membrane localization (Figure 2). Lack of prenylation by GGPP has the potential to either activate (for example by blocking GDI interaction) or inhibit (for example by disrupting proper localization) GTPases depending on the circumstances.


Figure 2 (Genes & Dev. 2002, 16: 1587-1609)

In MKD cell culture models, Rac1, a member of the Rho family of GTPases, is known to have reduced prenylation by GGPP leading to increased activity, which has previously been reported to mediate IL-1b hypersecretion. However, activation of Rac1 does not account for all MKD related phenotypes and the exact mechanism of RacI activation has not been determined. This article explores the role of protein prenylation in the function of another Rho family member, RhoA, and hypothesizes that lack of RhoA prenylation contributes to some MKD phenotypes. In order to test this hypothesis, a cell culture model of MKD was used where THP-1 human moncytic cells were treated with simvastatin, a drug that inhibits the enzyme directly upstream of mevalonate kinase, HMG-CoA reductase. As previously observed, simvastatin treatment resulted in increased GTP bound Rac1, indicative of activation. However, simvastatin treatment resulted in decreased GTP-RhoA, indicative of decreased activation. In addition, inhibition of RhoA with C3 transferase (an inhibitor more specific for RhoA than simvastatin) resulted in increased GTP-bound (and therefore activated) Rac1 and IL-1b release. Taken together, these results indicate that decreased prenylation of RhoA leads to inactivation, which subsequently leads to increased Rac1 activation and IL-1b release. Ultimately, this study increased our understanding of molecular mechanisms related to MKD pathogenesis by adding another piece to the puzzle related to IL-1b hypersecretion therefore uncovering a novel target for treatment.