The invasion-suppressor molecule E-cadherin (E-CAD) can be regulated at multiple levels: synthesis, processing and stability of mRNA; synthesis, processing and stability of protein; localization and posttranslational modification of protein; binding to catenins (E-CAD-associated proteins); and size and charge of cell surface glycosaminoglycans. Loss of E-CAD antigen and of E-CAD function in vivo has been observed with cell lines that homogeneously expressed functional E-CAD in vitro. These observations led to the idea that factors in the host may downmodulate E-CAD on the cancer cells, thereby promoting cell invasion. Nude mouse cancers that were homogeneously E-CAD-positive and noninvasive in vitro, formed by epithelioid MDCK or NMuMG cells, stained heterogeneously for E-CAD; such cancers were invasive and metastatic. The in vivo downmodulation appeared to be transient. Ex vivo cultures from primary cancers, as well as from metastases, produced homogeneously E-CAD-positive and noninvasive cells. Downmodulation did not occur when cells were micro-encapsulated and then implanted in the mouse, suggesting a role for immediate cancer cell-host cell contact. Similar in vitro/in vivo/ex vivo experiments with mouse MO4 fibrosarcoma cells, transfected with E-CAD cDNA under the control of a b-actin promotor, showed downregulation at the transcriptional or mRNA stability level. This downregulation was rapidly reversible upon ex vivo culture of the tumor cells. TGF-bl and IGF-I were found, respectively, to downregulate and upregulate the expression or the function of E-CAD. We speculate that IGF-1 restores the function of E-CAD through interaction of the IGF-I tyrosine kinase receptor with the catenin-actin cytoskeletal complex. In human cancers, immunohistochemistry has revealed changes in E-cadherin that agree with the experimental data on transient downmodulation of the invasion-suppressor function of E-cadherin by host factors.