Properties of Lipid Rafts in Two Epigenetically Distinct Subtypes of the Oncogenic Cell‐line SW13

Abstract

IntroductionMutant GOF p53 is an epigenetic regulator which promotes oncogenesis. Many cancers with a GOF mutation accumulate cholesterol and inhibition of cholesterol synthesis with statin treatment reverses some of the pro‐oncogenic properties of GOF p53. Our central hypothesis is that lipid raft signaling is altered by cholesterol accumulation in cells expressing a GOF p53. To address the role of cholesterol in promoting oncogenesis, we use the SW13 cell line bearing a GOF p53 (H193Y). This cell line has two epigenetically distinct cell subtypes that differentially express genes coding for GPI anchors, cholesterol biosynthesis, and sphingolipids, all of which are components of lipid rafts. In addition, the SW13 subtypes have different oncogenic profiles. The SW13(Vim−) subtype has an epithelial morphology and is highly proliferative. The SW13(Vim+) subtype has a mesenchymal like phenotype and a higher metastatic potential. When treated with histone deacetylase inhibitors (HDACi), SW13(Vim−) appear to adopt a SW13(Vim+) phenotype. Therefore, we treat with HDACi to control the subtype conversion and shRNA to knockdown GOF p53 expression. This experimental approach allows the composition of raft fraction to be correlated to GOF p53 expression and oncogenic properties of each SW13 subtype, with and without p53 expression.MethodsProliferation rate, MMP expression, and migration of SW13(Vim−), SW13(Vim+), shRNA SW13(Vim−), and shRNA SW13(Vim+) were quantitated by EdU assays and in situ zymography respectively. We have isolated lipid raft fractions by the detergent resistant enrichment technique from SW13 cells of each subtype, with and without GOF p53 expression knockdown by shRNA. Proteins were identified by mass spectrometry analysis. Fluorescent cholesterol assays and p53 western blots were used to quantify cholesterol levels and p53 protein expression, respectively.ResultsData suggest that there is a shift from planar rafts in the SW13(Vim−) to caveolar rafts in the SW13(Vim+) line. Although the contribution of GOF p53 to this shift is under study, evidence suggests that raft composition is altered in the shRNA lines, in which GOF p53 has been knocked down by ~90%. In addition, p53 knockdown increases the proliferation rate in SW13(Vim−) (p = 0.01) but does not affect SW13(Vim+). In contrast, p53 knockdown decreases MMP activity in SW13(Vim+) (p = 0.05) but not SW13(Vim−).ConclusionsOur data support the hypothesis that lipid raft signaling is significantly altered between the epigenetically distinct SW13 subtypes and that these differences may correlate to oncogenic potential. In addition, statin treatment, which reverses some of the pro oncogenic properties of GOF p53, may do so, in part, by altering raft composition and signaling.