Regulation of [beta]-catenin by Gli1 in epithelial transformation

by Li, Xingnan.

Gli family members-mediated continuous Hedgehog (Hh) pathway activity plays a role in the growth of a number of human cancers, including the common malignancy of skin, basal cell carcinoma (BCC), and other highly aggressive tumor types. However, the mechanism by which Gli proteins act to promote the outgrowth of cancer in response to stimulation of Hh signaling is poorly understood. Identified as zinc finger transcriptional factors, the Gli family of proteins has been shown to be involved in multiple cellular processes through activating its target genes. Therefore, to study the function of Gli target genes is important for our understanding in the pathologies of Hh-associated cancers and would provide valuable knowledge for design of therapeutic strategies. Gli1 can rapidly induce Snail transcripts in vitro, and co-expression of Snail with Gli1 was observed in anagen hair follicles and human skin tumors. Loss-of-function alleles of Snail inhibited transformation by Gli1 in RK3E epithelial cells, while enforced expression of wild type (WT) Snail promoted the outgrowth of transformed foci. In vivo, induction of Snail by a conditional Gli1 transgene was associated with cell proliferation in the interfollicular epidermis and with loss of E-cadherin in epithelial displasia. These observations identify Snail as an early responsive gene of Gli1 in the skin and as a limiting effector of Gli1-mediated epithelial transformation. ii E-cadherin is normally complexed with ?-catenin in adherens junctions. Loss of E-cadherin during developmental epithelial-mesenchymal transitions (EMT) may contribute to switching ?-catenin from its role of cell-cell adhesion to its role of nuclear signaling transmission. In the current study, increased ?-catenin was identified in Gli1-transformed cells in vitro and in the E-cadherin-deficient, BCC-like lesions induced by Gli1 in transgenic mice. In addition to the observation that E-cadherin played a selective role of suppression during Gli1-mediated transformation in vitro, we found that Snail loss-of-function was rescued by down-regulation of E-cadherin. Alleles of E-cadherin modulated transformation by Gli1 concordantly with their ability to promote or inhibit nuclear localization of ?-catenin, and inhibition of Wnt pathway activity by dominant negative Tcf4 selectively blocked transformation by Gli1. Taken together, these data identify the Shh-Gli1 pathway as a key regulator to switch ?-catenin to nuclear signaling in epithelial cells and cancers. iii