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.
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.
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.