Abstract
Lysosomal acid lipase (LAL) is a critical neutral lipid metabolic enzyme that regulates metabolic reprogramming in myeloid-derived suppressor cells (MDSCs) through over-activation of mammalian target of rapamycin (mTOR). Affymetrix GeneChip microarray analysis of MDSCs from LAL deficient mouse (lal−/−) revealed upregulation of Rab7 GTPase protein, which belongs to a superfamily of small-molecular-weight GTPase known to regulate intracellular membrane trafficking from early to late endosomes and lysosomes. Here, the physical protein-protein interaction between Rab7 GTPase and mTOR has been detected by co-immunoprecipitation in the cell extract of wild type HD1A and lal−/− MDSC-like HD1B myeloid cell lines. The GST pull down assay using the recombinant GST-Rab7 GTPase fusion protein showed that Rab7 GTPase interacts with the mTOR N-terminal heat repeat domain. Rab7 GTPase siRNA knocking down reversed the altered lysosome/mTOR distribution and expression levels in HD1B cells. Rab7 GTPase siRNA knocking down in isolated bone marrow lal−/− MDSCs or HD1B cells not only reduced over-activation of mTOR and its downstream effector S6, but also decreased glucose consumption, decreased ROS over-production, and increased healthy mitochondria by membrane potential measurement. Inhibition of Rab7 GTPase led to reduced lal−/− MDSCs differentiation from bone marrow Lin− progenitor cells, reduced lal−/− MDSCs trans-endothelial migration, and reversed lal−/− MDSCs suppression of T cell proliferation. Furthermore, inhibition of Rab7 GTPase reduced lal−/− MDSCs ability to stimulate tumor cell proliferation in vitro, tumor growth in vivo, and tumor invasion. Together, these results showed that Rab7 GTPase is critically involved in MDSCs homeostasis and pathogenic functions.
Highlights
Sporadic tumor cells arise from various genetic or epigenetic defects
In an array of studies recently conducted in our laboratories, we have clearly demonstrated that mammalian target of rapamycin (mTOR) critically regulates multi-aspects of lal–/– myeloid-derived suppressor cells (MDSCs), including development, systemic expansion, trans-endothelial migration, immune suppression, and direct stimulation of tumor cell proliferation [3, 5,6,7, 14, 15]
Since both over-activation of the mTOR signaling pathway and increased Rab7 GTPase expression coexist in lal–/– MDSCs [10], we hypothesized that the mTOR signaling pathway is regulated by Rab7 GTPase
Summary
Sporadic tumor cells arise from various genetic or epigenetic defects. The immune surveillance system in the body detects and destroys newly formed tumor cells, and prevents them from growing and spreading. When immune surveillance is compromised, unchecked tumor cells proliferate and grow freely to form tumor [1]. One of manifestations of inflammation is the expansion of myeloid-derived suppressor cells (MDSCs) which are commonly defined by the markers CD11b (integrin α-M) and Gr-1 (Ly6-C/G) in mouse [4]. The hallmark of MDSCs is their ability to suppress immune surveillance (T cells, NK cells etc). We have made a major breakthrough and identified that MDSCs are able to directly stimulate tumor cell proliferation in vitro, and tumor growth and invasion in vivo [5,6,7,8]
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