Date of Award
Doctor of Philosophy (PhD)
Metastasis, the spread of cancer cells from a primary tumor to a distal organ, represents the predominant cause of mortality in patients with solid tumors. However, the molecular mechanisms underpinning this multistep progression are poorly defined. Successful metastasis requires a cancer cell to acquire multiple capabilities, including the ability to migrate, invade, and co-opt the microenvironment in the distal organ for colonization. Many of these phenotypes require the actions of a family of lipids called phosphoinositides (PIs). Although structurally small molecules and minor components of the cellular lipidome, PIs are critical mediators of many cellular processes through their localization, abundance, and recruitment of effector proteins. The actions of PIs are orchestrated by PI regulator proteins that bind to or act upon each PI. Identifying and characterizing PI regulator proteins that promote metastasis could elucidate novel cellular pathways and enable the development of therapeutic approaches targeting mechanisms unique to metastatic disease. In this thesis, I describe work delineating the molecular mechanisms of three PI regulator proteins in promoting breast cancer metastasis. These proteins were initially identified as putative targets of the metastasis suppressor miRNAs, miR-126 and miR- 335. We identify PITPNC1 as amplified in nearly half human breast cancers, and overexpressed in metastatic melanoma, breast, and colon cancer. Biochemical and cellbiological experiments reveal that PITPNC1 binds to phosphatidyl inositol 4-phosphate (PI4P) in the Golgi. Through this binding, PITPNC1 recruits RAB1B to the Golgi, which in turn recruits GOLPH3. GOLPH3 facilitates elongation of the Golgi structure, enhancing vesicular release. Through this pathway, PITPNC1 increases the secretion of a set of pro-angiogenic and pro-invasive proteins including ADAM10, FAM3C, HTRA1, MMP1, and PDGFA. In the second half of this thesis, I characterize the molecular mechanism of PTPRN2 and PLCβ1 in driving breast cancer metastasis by enhancing cellular migration. I find that increased expression of PTPRN2 and PLCβ1 associates with human metastatic relapse. PTPRN2 and PLCβ1 enzymatically reduce plasma membrane phosphatidyl inositol 4,5-bisphosphate (PI(4,5)P2) through two independent mechanisms. Reduction of plasma membrane PI(4,5)P2 abundance releases the PI(4,5)P2-binding protein cofilin from its membrane-bound inactive state into the cytoplasm. Cytoplasmic cofilin binds and severs actin, generating free barbed ends in actin filaments and inducing actin polymerization. PTPRN2 and PLCβ1-mediated actin remodeling dynamics increasecellular migration, a key metastatic phenotype.
Sengelaub, Caitlin Anne, "Protein Regulators of Phosphoinositides as Promoters of Cancer Metastasis" (2016). Student Theses and Dissertations. 304.