Regulation of Id1 protein expression in mouse embryo fibroblasts by the type 1 insulin-like growth factor receptor

The activated type 1 insulin-like growth factor (IGFIR)increases the expression of Id1 proteins in mouseembryo fibroblasts (MEF). Up-regulation depends on afunctional receptor and on multiple pathways originatingfrom different domains of the receptor. In MEF,Id1 protein expression is also up-regulated by serumand certain oncogenes. Signaling through Stat3 playsan important, but not exclusive, role in the up-regulationof Id1 protein levels. In all instances, the increasein Id1 protein expression is paralleled by a correspondingincrease in Id1 promoter activity, as measuredwith a reporter gene.