Cell-cell dissociation upon epithelial cell scattering requires a step mediated by the proteasome

During development, tissue repair, and tumor metastasis, both cell-cell dissociation and cell migration occur and appear to be intimately linked, such as during epithelial "scattering." Here we show that cell-cell dissociation during scattering induced by hepatocyte growth factor (HGF) or activation of the temperature-sensitive v-Src tyrosine kinase in MDCK cells can be blocked by inhibiting the proteasome with lactacystin and MG132. Although both proteins of the tight junction and the adherens junction redistributed during cell scattering, proteasome inhibitors largely prevented this process, resulting in the stabilization of Triton X-100-insoluble tight junction proteins as well as adherens junction proteins at sites of cell-cell contact. Proteasome inhibition also led to a decrease of E-cadherin turnover in (35)S-labeled cells. In addition, proteasome inhibition partly preserved cell polarity, as determined by the subcellular distribution of Na(+),K(+)-ATPase (basolateral marker) and gp135 (apical marker), and the structure of the subcortical actin ring, both of which are normally disrupted during scattering. However, cells were able to establish focal contacts, and single cell migration toward HGF was unaffected by proteasome inhibition in quantitative assays, indicating that cell-cell dissociation during scattering occurs independently of anchorage-dependent cell migration. Thus, a proteasome-dependent step during scattering induced by HGF and pp60(v-Src) appears to be essential for cell-cell dissociation, disassembly of junctional components, and (at least indirectly) it also plays a role in the loss of protein polarity.