Rab7 GTPase controls lipid metabolic signaling in myeloid-derived suppressor cells

// Xinchun Ding 1 , Wenjing Zhang 1 , Ting Zhao 1 , Cong Yan 1, 2 , Hong Du 1, 2 1 Department of Pathology and Laboratory Medicine, Indiana University School of Medicine, Indianapolis, IN, USA 2 IU Simon Cancer Center, Indiana University School of Medicine, Indianapolis, IN, USA Correspondence to: Hong Du, email: hongdu@iupui.edu Cong Yan, email: coyan@iupui.edu Keywords: Rab7 GTPase, myeloid-derived suppressor cells, lipid metabolism, tumor growth Received: November 18, 2016      Accepted: March 09, 2017      Published: March 16, 2017 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.