Doctor of Philosophy (PhD)
Robert E. Lewis
Colorectal cancer (CRC) originates from epithelial cells lining the colon or rectum of the gastrointestinal tract. Most cancer deaths result from a tumor spreading to distant organs; however epithelial cells do not normally migrate from their tissue of origin. To do so, epithelial cells undergo biochemical changes allowing them to acquire behavior similar to motile mesenchymal cells termed the epithelial-to-mesenchymal transition (EMT), which contributes to tumor invasion and metastasis. Our study demonstrated that CRC cells require a molecular scaffold, Kinase Suppressor of Ras 1 (KSR1), and ERK to promote the EMT-like phenotype through the preferential translation of Epithelial Stromal Interaction 1 (EPSTI1). Disruption of KSR1 or EPSTI1 significantly impairs cell migration and invasion in vitro, and reverses the EMT-like phenotype, in part, by decreasing the expression of N-cadherin and of the transcriptional repressors of E-cadherin expression, Zeb1, and Slug, allowing restoration of E-cadherin expression. In CRC cells lacking KSR1, ectopic EPSTI1 expression restored the E- to N-cadherin switch, migration, invasion.
Our data further determined that KSR1-dependent mRNAs with predicted binding sites for Serine/arginine-rich Splicing Factor 9 (SRSF9) were disproportionately depleted by KSR1 knockdown. KSR1 disruption inhibits SRSF9 protein stability. Like KSR1, SRSF9 knockdown inhibits invasive behavior by CRC cells coincident with its ability to decrease N-cadherin expression in CRC. We further demonstrated SRSF9 interacts with EPSTI1 mRNA and a specific splice form of EPSTI1 is optimum for the EMT-like phenotype. Collectively, our data demonstrate that KSR1-dependent induction of the EMT-like phenotype via selective post-transcriptional regulation of mRNAs reveals its underappreciated role in remodeling the translational landscape of CRC cells to promote their migratory and invasive behavior.
Finally, we examined the effect of MAPK pathway inhibition with and without KSR1 to evaluate the role of KSR1 as co-dependent signaling promoting Ras/MAPK pathways. Our data show that under anchorage independent conditions CRISPR/Cas9-mediated KSR1 knockout markedly enhances the sensitivity of CRC cells to RAF, MEK, and ERK inhibitors. Furthermore, resistance to the MEK inhibitor, trametinib in CRC cells is prevented by KSR1 knockout. These data reveal that KSR1 as an essential component of MEK activity and highlight the value of disrupting KSR1 to suppress drug resistance.
Rao, Chaitra, "Post-Transcriptional Control of the Epithelial-to-Mesenchymal Transition (EMT) in Ras-Driven Colorectal Cancers" (2022). Theses & Dissertations. 661.