Abstract A11: Myeloid-derived suppressor cells suppress immune surveillance and stimulate cancer cell proliferation/metastasis through activation of mTOR pathway in lal-/- mice

Inflammation critically contributes to cancer metastasis, in which myeloid-derived suppressor cells (MDSCs) are an important participant. Although MDSCs are known to suppress immune surveillance, their roles in directly stimulating cancer cell proliferation and metastasis currently remain unclear. MDSCs development and homeostasis is controlled by lysosomal acid lipase (LAL), a critical enzyme in the metabolic signaling pathway that hydrolyzes cholesteryl esters (CE) and triglycerides (TG) in lysosomes. Lysosomal acid lipase (LAL) deficiency causes systemic expansion and infiltration of MDSCs in multiple organs. LAL-deficient (lal-/-) MDSCs arise from dysregulated production of progenitor cells in the bone marrow. In humans, increased CD14+CD16+ and CD14+CD33+ MDSC subsets have been reported with heterozygote carriers of LAL mutations. Patients with mutations in the LAL gene has been reported to be associated with carcinogenesis. In the LAL-deficient (lal-/-) mouse model, melanoma metastasized massively in allogeneic lal-/- mice, which was suppressed in allogeneic lal+/+ mice due to immune rejection. Here we report for the first time that MDSCs isolated from lal-/- mice directly stimulated B16 melanoma cell proliferation in vitro, and growth and metastasis in vivo. Cytokines i.e., IL-1β and TNFα from MDSCs are required for B16 melanoma cell proliferative function in vitro. Myeloid-specific expression of human LAL (hLAL) in lal-/- mice rescues these malignant phenotypes in vitro and in vivo. The tumor-promoting function of lal-/- MDSCs is mediated, at least in part, through over-activation of the mammalian target of rapamycin (mTOR) pathway. mTOR is a serine/threonine protein kinase that regulates cell growth, proliferation, migration, survival, protein synthesis, and transcription in response to growth factors and mitogens. mTOR is the catalytic subunit of two distinctive complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). Unique accessory proteins, regulatory-associated protein of mTOR (RAPTOR), and rapamycin-insensitive companion of mTOR (RICTOR) define the TORC1 and TORC2 complexes, respectively. Affymetrix GeneChip microarray analysis and Ingenuity Pathway Analysis of gene transcripts revealed up-regulation of multiple genes in the mTOR signaling pathway in lal-/- MDSCs. We have shown that inhibition of mTOR in lal-/- MDSCs increased apoptosis, decreased ATP synthesis, decreased ROS production, and recovered impairment of mitochondrial membrane potential. As a result, lal-/- MDSCs reversed the increased cell proliferation, corrected enhanced lal-/- MDSCs development from lineage negative progenitor cells, and reduced systemic MDSC expansion. More importantly, knockdown of mTOR, Raptor or Rictor in lal-/- MDSCs reversed the immune suppression on T cell proliferation and function, as well as suppressed their stimulation on proliferation of cancer cells (including B16 melanoma, LLC and Tramp-C2 cancer cells). Our results indicate that LAL plays a critical role in regulating MDSCs through modulation of the mTOR pathway. MDSCs possess dual functions to overcome immune rejection and facilitate cancer metastasis: 1) suppress immune surveillance, and 2) stimulate cancer cell proliferation, growth, and metastasis. Grant support: This work was supported by National Institutes of Health Grants CA138759, CA152099 (to C. Y.) and HL087001 (to H. D.). Citation Format: Hong Du, Ting Zhao, Xinchun Ding, Katlin Walls, Cong Yan. Myeloid-derived suppressor cells suppress immune surveillance and stimulate cancer cell proliferation/metastasis through activation of mTOR pathway in lal-/- mice. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr A11.