The regulation of self-renewal in normal human urothelial cells

The urinary tract is lined by a mitotically-quiescent, but highly regenerative epithelium, the urothelium. The mechanisms regulating urothelial regeneration are incompletely understood although autocrine stimulation of the Epidermal Growth Factor Receptor (EGFR) signalling pathway has been implicated. The hypothesis developed in this thesis is that urothelial homeostasis is regulated through resolution of interactive signal transduction networks downstream of local environmental cues, such as cell:cell contact. Here, canonical Wnt signalling was examined as a candidate key pathway due to the pivotal role of β-catenin in both nuclear transcription and intercellular adherens junctions.
Normal human urothelial (NHU) cells isolated from surgical biopsies were grown as finite cell lines in monolayer culture. mRNA analysis from proliferating cultures inferred all components for a functional autocrine-activated canonical Wnt cascade were present. In proliferating cells, β-catenin was nuclear and Axin2 expression provided an objective hallmark of β-catenin/TCF transcription factor activity. This endogenous activity was not mediated by Wnt receptor activation, as Wnt ligand was produced in inactive (non-palmitylated) form in serum-free culture, but instead, β-catenin activation was driven via EGFR-mediated phosphorylation of GSK3β and inhibition of the β-catenin destruction complex. In quiescent, contact–inhibited cultures, β-catenin was seen to re-localise to the adherens junctions and GSK3β activity was re-established. Knock-down of β-catenin using RNA interference led to significant changes in p-ERK and p-AKT activity as well as an increase in E-cadherin protein expression. The results presented in this thesis identifies β-catenin as a central component of a bi-directional feedback loop between growth factor-mediated cell signalling and cell:cell contact and provides preliminary evidence that de-regulation of the mechanisms that control β-catenin regulation and EGFR signalling are important in neoplastic growth.