Investigate the Role of the B55δ Regulatory Subunit of Protein Phosphatase 2A in the Tumorigenesis and Metastasis of Colorectal Cancer

Abstract

Colorectal cancer (CRC) is the third most common type of cancer and the third most common cause of cancer deaths in the world CRC patients with distant metastases at the time of diagnosis are associated with worse prognosis Therefore it is urgent to find more biomarkers and therapeutic targets for improving detection diagnosis and prognosis and survival for patients with CRC Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase in eukaryotic cells and mostly acts as a tumor suppressor in human cancer The PP2A holoenzyme is composed of a structural A subunit a catalytic C subunit and a variable regulatory B subunit The regulatory B subunits determine the substrate specificity and subcellular localization of PP2A The regulatory B subunits have been categorized into four families B (B55 or PR55) B' (B56 or PR61) B” (PR72) and B'” (PR93 or PR110) The B55δ subunit a member of B55 family and encoded by the PPP2R2D gene plays a crucial role in regulating mitosis in the cell cycle Data mining of expression profiles of B55δ showed differential expression levels of B55δ in different types of cancer and reduced expression levels of B55δ were found in tumor parts as compared to that of the normal counterparts in datasets of CRC specimens However higher expression levels of B55δ in tumor tissues were associated with reduced survival rate in a subset of CRC patients Additionally differential expression levels of B55δ were found in different CRC cell lines We hypothesized that B55δ may play a dual role in colorectal tumor progression In agreement with the known role of B55δ in cell cycle growth curve analysis showed that stable B55δ overexpression reduced proliferation of both HCT116 and SW620 cells compared to cells with vector only B55δ overexpression or knockdown did not affect steady-state viability of HCT116 and SW620 cells Similarly using colony formation assay both HCT116 and SW620 cells with B55δ overexpression showed reduced colony number compared to that of cells with vector only Unexpectedly the colony number of SW620 cells with stable knockdown of B55δ expression was also reduced compared to that of cells with vector only Furthermore B55δ overexpression increased the number of both SW620 and HCT116 cells with epithelial-like morphology and B55δ knockdown increased the number of SW620 and HCT116 cells with mesenchymal morphology as compared to that of cells stably expressing vector However transwell migration assay showed that B55δ overexpression increased the migration ability whereas B55δ knockdown reduced the migration ability of both HCT116 and SW620 cells In addition transwell invasion assay showed that B55δ overexpression increased the invasion ability whereas B55δ knockdown reduced the invasion ability of SW620 cells We further analyzed several EMT markers to elucidate whether B55δ regulates molecular changes of EMT of SW620 cells The data showed that E-cadherin is negatively regulated by B55δ in both HCT116 and SW620 cells but Snail is both negatively and positively regulated by B55δ in SW620 cells In summary our results showed that B55δ inhibits proliferation of CRC HCT116 and SW620 cells but down-regulates the level of E-cadherin and promotes migration and invasion of CRC cells Our finding suggests that it may suppress tumorigenesis in early stages of CRC progression but promote metastasis in later stages of CRC progression

Date of Award	2019
Original language	English
Supervisor	Chi-Wu Chiang (Supervisor)