The Gli code: an information nexus regulating cell fate, stemness and cancer

Abstract

The Gli code hypothesis postulates that the three vertebrate Gli transcription factors act together in responding cells to integrate intercellular Hedgehog (Hh) and other signaling inputs, resulting in the regulation of tissue pattern, size and shape. Hh and other inputs are then just ways to modify the Gli code. Recent data confirm this idea and suggest that the Gli code regulates stemness and also tumor progression and metastatic growth, opening exciting possibilities for both regenerative medicine and novel anticancer therapies.

Figure 1

The Hh-Gli pathway and the Gli code in tissue patterning and stem-cell lineages. (a) Representation of the Gli code acting downstream of Hh-Smo signaling. On inhibition of Ptch1 function by the Hh ligands, the repression of Smo by Ptch1 is ended. Smo then orchestrates changes in the Gli code by stabilizing and activating full-length Gli proteins (Gli^A) and blocking the action of inhibitors and the production of Gli repressors (Gli^R), mostly Gli3 in this case. Positive modulators enhance the nuclear localization and the activation of full-length Gli proteins. In the absence of Hh ligands, Ptch1 inhibits Smo and negative modulators sequester full-length Gli proteins in the cytoplasm and target them for cleavage to produce C-terminal truncations (which act as constitutive nuclear repressors) or for complete degradation by the proteasome. Examples of positive and negative modulators are given here and in the text. Note that reception of the Hh signal and the orchestration of the Gli code are proposed to occur in the primary cilium [100,102]. (b) Model of a Hh gradient in a developmental field (top) that grows and becomes patterned through the action of inverse gradients of Gli activators (green) and repressors (red). Different combinations of the Gli code are proposed to activate overlapping sets of targets that result in distinct fates and proliferation/survival rates and thus sizes of tissues and organs. (c) Model of a stem-cell lineage with the activation of stem cells from quiescent cells in the niche by an upsurge of Hh signals. This leads to self-renewal and the production of derived precursors and differentiated cells. The temporal gradient of Hh is interpreted by varying levels of activators and repressors of Gli proteins, leading to a changing Gli code.

Figure 2

Integration of HH-GLI and oncogenic RAS-AKT signaling and a model for the role of positive activating GLI function in tumor progression. (a) Immunolocalization of endogenous GLI1 protein in untreated SK-Mel2 human melanoma cells or in cells treated with AKT and MEK inhibitors [48]. GLI1 protein (green) is mostly nuclear but relocalizes to the cytoplasm in the absence of endogenous RAS-MEK/AKT signaling. The position of the nucleus is shown by DAPI staining (blue) in the right-hand panel. Scale bar = 20 μm. (b) Representation of the peptide-growth factor (PGF)-receptor tyrosine kinase (RTK)-RAS-RAF-MEK and PI3K-AKT pathways (blue ovals), which lead to enhanced nuclear localization of GLI1, downstream of the classical HH pathway (green ovals). Inhibitors are in red. The effect could be direct on the GLI proteins or indirect by repressing GLI inhibitors. In both cases, there would be an enhancement of GLI nuclear levels [48]. (c) Model for the role of GLI activators (GLI^A) in tumor progression. Increasing levels of positive GLI activity (mostly GLI1) induce distinct sets of targets, produce increases in tumor-cell number and induce tumor progression. Increases in GLI activity are brought about by sequential oncogenic hits, such as those activating EGFR, RAS or RAF, those amplifying AKT or those inhibiting the tumor suppressor PTEN. The hypothetical model depicts three scenarios leading to distinct states with different kinetics.

Figure 3

Activating GLI function, tumor progression and cancer stem cells in human gliomas. (a) Model for the progression of human tumors, in which there is an initial expansion of cancer stem cells (green curve), enabling the detection of a stemness signature (doubled arrowed green line) as the cancer stem cells predominate in the tumor population [47]. High-grade tumors (here grade IV tumors) display an expansion of stem cell-derived lineages (red broken line), which dilutes the stem cells and their stemness signature, leading to the killing of the individual. The transcription-factor signature of glioma cancer stem cells [47] is illustrated in the inset. (b) The model in (a) proposes an increase of positive-activating GLI function (mostly GLI1) through the different tumor grades, leading to maximal stem-cell self-renewal and full expansion of derived lineages that can acquire differentiated phenotypes, giving a multiforme character in the case of high-grade gliomas, which are also known as glioblastoma multiforme.

Figure 4

Inhibition of metastatic tumor growth by interference with HH-GLI signaling in vivo. Intravenous injection of lacZ-transduced human melanoma cells into nude mice leads to the metastatic growth of cancer cells in the lungs, here revealed as blue masses after XGal staining. Metastatic growth can be prevented fully by systemic treatment of the injected mice with cyclopamine, which inhibits the HH-GLI pathway and reverts the GLI code to a repressive state (see Ref. [48]). Scale bar = 1 mm.