Growth arrest specific gene 1 (Gas1) has long been regarded as a cell cycle inhibitor of the G 0 to S phase transition. How GAS1, a GPI-anchored plasma membrane protein, directs intracellular changes without an extracellular ligand or a transmembrane protein partner has been puzzling. A recent series of biochemical and molecular genetic studies assigned the mammalian Hedgehog (HH) growth factor to be a ligand for GAS1 in vitro and in vivo. HH has enjoyed considerable attention for its profound role in embryonic patterning as a classic morphogen, i.e. inducing various cell types in a concentrationdependent manner. GAS1 appears to help transform the HH concentration gradient into its morphogenic activity gradient by acting cooperatively with the HH receptor, the 12-transmembrane protein Patched 1 (PTC1). These findings provoke intriguing thoughts on how HH and GAS1 may coordinate cell proliferation and differentiation to create biological patterns. The role of HH extends to human genetic diseases, stem cell renewal, and cancer growth, and we consider the possibility of GAS1's involvement in these processes as well.Precise integration between cell proliferation and cell type specification during embryogenesis directs the formation and function of each tissue. Integration between these two programs in a given cell is largely determined by intercellular signaling molecules. Here, we review the recent 'unlikely marriage' between two extracellular molecules, GAS1 and HH, and the implications on coordinated cell proliferation and cell type specification.
Gas1 the cell cycle InhIbItorIn 1988, Gas1 was identified as one of six genes that were transcriptionally up-regulated in NIH3T3 cells arrested in cell cycle upon serum starvation, a presumed G 0 arrest. 1 Among the six initially identified growth arrest specific (hence Gas) genes, only Gas1 could suppress cell cycle progression when overexpressed in non-transformed and certain oncogene-transformed NIH3T3 cells. [2][3][4] Because GAS1 overexpressing cells did not incorporate BrdU, it was thought that GAS1 blocked the G 0 to S phase transition. Deletion analysis of GAS1, an outer plasma membrane glycosylphosphatidylinositol (GPI)-linked protein of 37 kDa, concluded that it could induce growth arrest without its GPI anchor, suggesting that it acts through another membrane partner. 5,6 Interestingly, GAS1-induced growth arrest depended on the tumor suppressor gene p53. 7,8 Furthermore, c-Myc and v-Src, powerful proliferation promoting factors, repressed Gas1 expression, suggesting that Gas1 downregulation is an associated step in the promotion of proliferation. 9,10 By contrast, senescent fibroblast cells, though cease proliferating, do not express Gas1. 11 These studies together suggest that GAS1 is a negative cell growth regulator rather than a senescence mediator. However, evidence to demonstrate such a function for Gas1 in an animal model is lacking.