| Epithelia provide a protective barrier for the organs they encase, yet the cells compromising the epithelia constantly turn over via cell death and cell division. Our lab has identified a process by which cells are removed from an epithelial layer without compromising its barrier function called 'epithelial cell extrusion'. To extrude, a cell emits the bioactive lipid Sphingosine 1 Phosphate (S1P), which binds the S1P2 receptor in the neighboring cells and triggers them to form an actomyosin ring that contracts to squeeze the cell out of the epithelial layer. Although extrusion removes apoptotic cells in response to damage, typically during homeostasis, epithelia extrude live cells that later die by anoikis. Wild type cells extrude predominantly apically and are eliminated through the lumen; however, we found that some cells extrude basally, back into the tissue the epithelium encases. Because most cells extrude while alive and many aggressive tumors override anoikis signaling to enhance survival outside the epithelium, the direction a cell extrudes has important consequences to its later fate. Transformed cells that extrude apically would be eliminated through the lumen; however, those extruding basally could potentially invade into the stroma. My goal was to elucidate the mechanisms that control the direction a cell extrudes from epithelia. My first study showed that apical extrusion requires microtubules and that they target p115 RhoGEF in the cells surrounding an extruding cell to the basolateral surface of the live/extruded cell interface to activate Rho-mediated actin/myosin contraction basolaterally. Blocking this mechanism by microtubule disrupters causes cells to instead contract apically and extrude basally. In my second study, I found that cells expressing oncogenic K-Ras shift extrusion basally and enable extruded cells to survive and proliferate. In this situation, basal extrusion results from degradation of S1P due to autophagy that is highly upregulated in oncogenic K-Ras cells. Although S1P2 also becomes slightly downregulated in K-Ras expressing cells too, disruption of the autophagy flux is sufficient to rescue both S1P levels and apical extrusion. Similarly, several aggressive tumors that are driven by K-Ras mutations lack S1P2. S1P2 deficient epithelia are extrusion deficient, which results in neoplastic masses that are chemoresistant, basal extrusion, and poor barrier function both in vitro and in vivo, all properties that could contribute to tumor formation and progression. Currently, I am developing an in vivo model system in zebrafish epidermis to investigate if basal cell extrusion can drive invasion of oncogenic K-Ras cells. The transparency of zebrafish allows us for the first time to directly follow extrusion, invasion, and migration of individual cells without invasive techniques intrinsic to most current cancer models. |
