Activation of mTOR pathway in myeloid-derived suppressor cells stimulates cancer cell proliferation and metastasis in lal−/− mice

Abstract

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. Lysosomal acid lipase (LAL) deficiency causes systemic expansion and infiltration of MDSCs in multiple organs and subsequent inflammation. 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 from lal−/− mice directly stimulated B16 melanoma cell in vitro proliferation, and in vivo growth and metastasis. Cytokines i.e., IL-1β and TNFα from MDSCs are required for B16 melanoma cell proliferation 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. Knockdown of mTOR, Raptor or Rictor in lal−/− MDSCs 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 ability to directly stimulate cancer cell proliferation, and overcome immune rejection of cancer metastasis in allogeneic mice through modulation of the mTOR pathway, which provides a mechanistic basis for targeting MDSCs to reduce the risk of cancer metastasis. Therefore, MDSCs possess dual functions to facilitate cancer metastasis: suppress immune surveillance, and stimulate cancer cell proliferation and growth.

Hepatocyte-Specific Expression of Human Lysosome Acid Lipase Corrects Liver Inflammation and Tumor Metastasis in lal Mice

“…32 When tested in the lal À/À mouse model, we have recently discovered that LAL deficiencyeinduced inflammation plays crucial roles at all stages of tumor development. 24 In lal À/À mice, B16 melanoma metastasized in the liver and lung of allogeneic lal À/À mice, which was suppressed in allogeneic lal þ/þ mice due to immune rejection. Interestingly and importantly, in addition to the immune suppressive function, we found that MDSCs from lal À/À mice alone directly stimulated B16 melanoma cell in vitro proliferation and in vivo growth and metastasis.…”

“…Interestingly and importantly, in addition to the immune suppressive function, we found that MDSCs from lal À/À mice alone directly stimulated B16 melanoma cell in vitro proliferation and in vivo growth and metastasis. 24 Cytokines (ie, IL-1b, IL-6, and TNF-a) from lal À/À MDSCs are required for B16 melanoma proliferation. 24 In addition to MDSCs, it seems that hepatocytes were also responsible for production of tumor-promoting cytokines as found here.…”

“…24 Cytokines (ie, IL-1b, IL-6, and TNF-a) from lal À/À MDSCs are required for B16 melanoma proliferation. 24 In addition to MDSCs, it seems that hepatocytes were also responsible for production of tumor-promoting cytokines as found here. hLAL-specific expression in the hepatocytes reduced expression of tumor-promoting cytokines (Figures 8 and 9), as well as MDSCs ( Figure 6).…”

“…Primers of mouse IL-6, mouse granulocyte-macrophage colony-stimulating factor (GM-CSF), mouse macrophage colony-stimulating factor, mouse tumor necrosis factor (TNF)-a, mouse IL-2, mouse IL-4, mouse IL-17, mouse interferon (IFN)-g, mouse monocyte chemotactic protein-1 (MCP-1), mouse chemokine ligand (CCL)-3, mouse CCL4, mouse CCL5, mouse CXCL10, and glyceraldehyde-3-phosphate dehydrogenase for RT-qPCR were described previously. 23,24 Cytokine Measurement by ELISA…”

“…24 To evaluate the effects of hLAL in hepatocytes on tumor metastasis, B16 melanoma cells were injected into the tail veins of LAP-Tg/KO triple mice to assess the metastatic potential. Two weeks after injection, more B16 melanoma colonies were observed in the livers and lungs of doxycycline-treated LAP-Tg/KO triple mice compared with those in untreated mice with statistical significance (Figure 5 ½F5 ½F5 , A and B).…”