5. Pleotropic Effects of Statins—Inhibition of Infection
Statins interrupt specific stages of host cell invasion through non-cholesterol-dependent pleiotropic effects. The centrality of isoprenoid depletion in these effects is evidenced by the restoration of invasion during simvastatin treatment if Fpp or GGpp are replenished. Invasion was not restored by replenishing cholesterol
[14] with this concentration of simvastatin, a lower concentration than that of early work showing apoptotic responses to simvastatin
[34]. Multiple effects are rendered through small GTPases, including CDC42, a CaaX domain-containing host cell protein that relies on post-translational prenylation for membrane localization
[35][36]. As prenylation decreases following treatment with simvastatin, CDC42 becomes sequestered in the cytoplasm, no longer anchored at the host cell membrane
[14]. This loss of membrane localization appears central to several downstream effects.
Although CDC42 remains sequestered within the cytosol, simvastatin stimulates GTP-loading within the CDC42 activation site
[37]. In the active, GTP-bound state, mislocated CDC42 is available for coupling with cytosolic PI3Kp85α. Coupled to GTP-bound CDC42 in its cytosolic location, PI3Kp85α is sequestered away from the host cell membrane
[14]. The loss of membrane localization of PI3Kp85α potentially results in loss of membrane anchoring for the PI3K110 catalytic subunits that rely on PI3Kp85α for membrane localization. The loss of PI3K110 catalytic subunit access to membrane-bound phosphoinositide would have a resultant loss of formation of the cell-signaling molecule PIP
3. Evidence of this disruption is that simvastatin treatment limits actin stress fiber disassembly, the endocytic process dependent on PIP
3 binding α-actinin.
Simvastatin reduces host cell binding to fibronectin
[38]. Moreover, simvastatin decreases uptake of the β1 integrin complex from the cell surface and decreases nascent formation of these complexes by limiting recycling of the β1 component to the host cell membrane
[37]. Thus, pleiotropic effects of simvastatin, as a host-directed therapeutic, limits
S. aureus invasion into host cells through decreased synthesis of isoprenoid intermediates, sequestration of RAC, RHO, and CDC42 in the cytosol, decreased membrane localization of these small-GTPases coupled to PI3Kp85α, reduced actin stress fiber depolymerization, decreased host cell binding to fibronectin, and decreased internalization and recycling of β1-integrin receptor complexes to the host cell surface.
In vivo, simvastatin treatment aids in clearing pulmonary infection by invasive
S. aureus [39]. Similar to findings by Merx et al. in the host response to the endogenous murine microbiome
[40][41], simvastatin decreases lung bacterial burden by exogenously administered
S. aureus and decreases lethality. Simvastatin blunts pulmonary pathogenesis and the inflammatory response to infection, in addition to lowering markers of the inflammatory response both within lung tissue and systemically. Thus, in vivo evidence supports the potential efficacy of statin use for limiting pulmonary infection.
Although pleiotropic effects of statins include inhibition of infection, the use of statins in the critically ill has been challenged due to altered pharmacokinetics within this patient population that renders statins more toxic
[42][43][44][45][46]. The usefulness of statins in the treatment of infection has also been questioned following clinical trials demonstrating their limited efficacy
[24][25] and concerns that observational studies may have overestimated their therapeutic benefit
[27][28]. However, benefit is in evidence in patient populations undergoing statin therapy prior to the onset of infection
[21][29][47]. This finding speaks to a potential underlying mechanism of statin efficacy reliant on pleiotropic effects following a reduction in the levels of isoprenoid intermediates that would require a prior statin regimen for efficacy to be achieved.
6. Emerging Approaches in Treatment Strategies—ML141
Alternative small molecule inhibitors have been examined that might limit host cell invasion yet circumvent adverse effects and limitations associated with statins. In characterizing the underlying mechanism of simvastatin, RAC, RHO, and CDC42 had emerged as potential molecular targets central to host cell invasion by
S. aureus [14]. Earlier work had shown CDC42 is activated during
S. aureus invasion
[15] and CDC42 acting upstream of both RAC and RHO
[35]. We therefore examined the role of CDC42 by using site-directed mutagenesis to encode valine in place of cysteine within the canonical CAAX prenylation site of CDC42. This inhibition of prenylation within this single host protein diminished invasion by more than 90%, suggesting a central role for CDC42 in invasion
[14]. To examine this possibility, we used ML141, a small molecule inhibitor with specificity for hCDC42
[48].
7. ML141′s Mechanism of Action
ML141 differs from simvastatin in its target and mechanism of action. While simvastatin demonstrates specificity for HMG-CoA reductase at the early, rate limiting step of cholesterol/isoprenoid biosynthesis and thereby indirectly decreases the prenylation of CDC42, RAC, and RHO
[14][49][50], ML141 demonstrates specificity for CDC42
[35][48]. Acting as an allosteric inhibitor, ML141 dissociates GTP and GDP from the CDC42 active site through rapid, reversible inhibition. Longer-term treatment elicits cellular responses similar to those observed previously in CDC42
-/- mouse embryonic fibroblasts (MEF)
[51][52]. These downstream effects may be mediated in part through the impaired coupling of GTP-bound CDC42 with downstream effector proteins such as members of the WASp and PI3K families
[53][54][14][55][37].