ABSTRACT
Four-transmembrane L6 family member 5 (TM4SF5) and its homolog L6, a tumor antigen, form a four-transmembrane L6 family. TM4SF5 expression causes uncontrolled cell proliferation and angiogenesis. Although other genuine transmembrane 4 superfamily (TM4SF) members co-operate with integrins for cell migration, roles of TM4SF5 in the cellular spreading and migration are unknown. Using hepatocarcinoma cell clones that ectopically express TM4SF5, we found that cross talks via an extracellular interaction between TM4SF5 and integrin alpha2 in collagen type I environment inhibited integrin alpha2 functions such as spreading on and migration toward collagen I, which were recovered by suppression of TM4SF5 or structural disturbance of its second extracellular loop using a peptide or mutagenesis. Altogether, the observations suggest that TM4SF5 in hepatocytes negatively regulates integrin alpha2 function via an interaction between the extracellular loop 2 of TM4SF5 and integrin alpha2 during cell spreading on and migration through collagen I environment.
Subject(s)
Integrin alpha2/metabolism , Liver Neoplasms/genetics , Membrane Proteins/metabolism , Antigens, Neoplasm/metabolism , Cell Movement , Clone Cells , Collagen Type I/metabolism , Extracellular Matrix , Hepatocytes/metabolism , Hepatocytes/pathology , Humans , Integrin alpha2/genetics , Liver Neoplasms/metabolism , Liver Neoplasms/pathology , Membrane Proteins/genetics , Neoplasm Invasiveness/genetics , Protein Binding , Protein Structure, Tertiary , Receptor Cross-TalkABSTRACT
Cell adhesion to the extracellular matrix (ECM) can activate signaling via focal adhesion kinase (FAK) leading to dynamic regulation of cellular morphology. Mechanistic basis for the lack of effective intracellular signaling by non-attached epithelial cells is poorly understood. To examine whether signaling in suspended cells is regulated by Fer cytoplasmic tyrosine kinase, we investigated the effect of ectopic Fer expression on signaling in suspended or adherent hepatocytes. We found that ectopic Fer expression in Huh7 hepatocytes in suspension or on non-permissive poly-lysine caused significant phosphorylation of FAK Tyr577, Tyr861, or Tyr925, but not Tyr397 or Tyr576. Fer-mediated FAK phosphorylation in suspended cells was independent of c-Src activity or growth factor stimulation, but dependent of cortactin expression. Consistent with these results, complex formation between FAK, Fer, and cortactin was observed in suspended cells. The Fer-mediated effect correlated with multiple membrane protrusions, even on poly-lysine. Together, these observations suggest that Fer may allow a bypass of anchorage-dependency for intracellular signal transduction in hepatocytes.
