Novel Methods of Mycobacterial Control via Manipulation of Host Lipid Bioavailability
Lipids represent an important source of nutrition for infecting mycobacteria, accumulating within the necrotic core of granulomas and present in foamy macrophages associated with mycobacterial infection. In order to better understand the timing, process and importance of lipid accumulation, we developed methods for direct in vivo visualization and quantification of lipid accumulation using the zebrafish-M. marinum model of infection. We find that neutral lipids accumulate cell autonomously in mycobacterium-infected macrophages in vivo during early infection, with detectable levels of accumulation by two days post-infection. Reducing available free cholesterol and neutral lipids during early infection via treatment with ezetimibe, an FDA-approved drug, resulted in a reduction of bacterial growth in vivo. The effect of ezetimibe in reducing bacterial growth was dependent on the mce4 operon, a key bacterial determinant of lipid utilization. Thus, in vivo, lipid accumulation can occur cell autonomously at early timepoints of mycobacterial infection, and this early lipid accumulation confers a growth advantage to infecting mycobacteria.
This accumulation represents a perturbation of the normal homeostatic mechanisms by which intracellular lipids are tightly controlled. Under homeostatic conditions macrophages are central to the function of returning excess lipids to the liver where they are processed for excretion via the digestive tract. This function of macrophages, termed reverse cholesterol transport, results from the uptake of excess extracellular lipids, followed by the coordinated efflux of these lipids through the transport proteins ABCA1 and ABCG1 and packaging into HDL particles containing ApoAI and ApoAII. This process is controlled via the action of nuclear receptors including PPARγ, PPAR-α, and LXRα.
We performed RNA-seq analysis of gene expression in macrophages both uninfected and infected with mycobacteria and observed a profound down-regulation of all the major lipoproteins. The HDL associated lipoproteins ApoAI and ApoAII were the most profoundly down regulated. Based on this observation we sought to investigate the role of nuclear receptors involved in intracellular lipid sensing and control of apolipoprotein expression. We determined that agonism and antagonism of LXRα signaling decrease and increase infection burden during in vivo studies respectively. We further found that the ApoAI agonizing fibrate drugs fenofibrate and gemfibrozil reduce mycobacterial infection in vivo. This work demonstrates that lipid lowering agents already approved for use in humans might function as relevant adjuvant therapies toward treatment of mycobacterial infections.
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