Role of Ganglioside GM3 in Metastatic Cancer Cells with Macrophage Properties
Metastasis is the process by which cancer cells disseminate from the primary neoplasm and invade surrounding tissue and distant organs, and is the primary cause of morbidity and mortality for cancer patients. Most conventional cancer therapies are ineffective in managing tumor metastasis. This has been due in large part to the absence of in vivo metastatic models that represent the full spectrum of metastatic disease. Here I identify three new spontaneously arising tumors in the inbred VM mouse strain, which has a relatively high incidence of CNS tumors. Two of the tumors (VM-M2 and VM-M3) reliably expressed all of the major biological processes of metastasis to include local invasion, intravasation, immune system survival, extravasation, and secondary tumor formation involving liver, kidney, spleen, lung, and brain. Metastasis was assessed through visual organ inspection, histology, immunohistochemistry, and bioluminescence imaging. The metastatic VM tumor cells also expressed multiple properties of macrophages including morphological appearance, surface adhesion, phagocytosis, gene expression (CD11b, Iba1, F4/80, CD68, CD45, and CXCR4) and total lipid composition (glycosphingolipids and phospholipids). The third tumor (VM-NM1) grew rapidly and expressed properties of neural stem/progenitor cells, but was neither invasive nor metastatic. This thesis research also examined the influence of a genelinked up-regulation of the simple ganglioside GM3 in the metastatic VM-M3 tumor. Ganglioside GM3 has been shown to have anti-invasive effects through its ability to modulate integrins and matrix metalloproteases. Additionally, GM3 was previously shown to be elevated in resting macrophages when compared to activated macrophages. The bioluminescent VM-M3 cells (M3/Fluc) contain mostly GM2, GM1, and GD1a with undetectable levels of GM3. Additionally, the M3/Fluc cells express GalNAc-T, a key enzyme for the synthesis of complex gangliosides from GM3, the precursor used for complex ganglioside biosynthesis. Stable transduction of the M3/Fluc tumor with a lentiviral vector containing a cDNA sequence targeting the GalNAc-T gene (Fluc-TNG), resulted in a knock-down of GalNAc-T expression and an up-regulation of GM3 compared to the control (Fluc-csh) transduced M3/Fluc tumor cells. In vivo, the Fluc-TNG cells were significantly less invasive when implanted in the brain and less metastatic when implanted in the flank when compared to the control Fluc-csh tumors. My data indicate that spontaneous brain tumors can arise from different cell types in VM mice and that the ganglioside GM3 can inhibit invasion and metastasis in metastatic cancer cells with macrophage properties. The new VM tumor model will be useful for defining the biological processes of cancer metastasis and for evaluating potential therapies for tumor management.