Abstract
BackgroundStructural requirements for the β1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts. In this study, we examined the reason for the reduced surface expression of β1 integrin in human breast cancer MCF-7 cells compared to normal human breast epithelial (HBE) cells, both of which adhered to collagen type IV.ResultsThe β1 integrin immunoprecipitates from either HBE or MCF-7 cells involved α-actinin while actin coprecipitated with β1 integrin from HBE cells but not from MCF-7 cells. Immunoblotting using the anti-phosphotyrosine (PY) antibody indicated the phosphorylation of β1 integrin at least at tyrosine in both cells. Dephosphorylation of β1 integrin from HBE cells by protein tyrosine phosphatase (PTP), but not by protein serine/threonine phosphatase (PP), caused dissociation of actin from β1 integrin, although dephosphorylation of it from MCF-7 cells by either PTP or PP caused association of the two proteins. In MCF-7 cells β1 integrin coprecipitated doublet of proteins having the Ca2+/calmodulin-dependent protein kinase (CaMK) II activity that was susceptible to KN-62, a specific inhibitor of CaMKII.ConclusionThe results suggest that β1 integrin is tyrosine phosphorylated and links with actin via α-actinin in HBE cells but prevented from linking with actin in MCF-7 cells by phosphorylation at both tyrosine and serine/threonine of β1 integrin which forms a complex with α-actinin and CaMKII. Thus the linkage formation of β1 integrin with actin may be differentially regulated by its tyrosine and serine/threonine phosphorylation in normal HBE cells and breast cancer MCF-7 cells.
Highlights
Structural requirements for the β1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts
The absence of coprecipitation of actin with β1 integrin in MCF-7 cells To determine whether expression of β1 integrin on the cell surface requires its intracellular interaction with the actin cytoskeleton, β1 integrin was immunoprecipitated with the anti-β1 integrin antibody or control IgG from human breast epithelial (HBE) or MCF-7 cells which adhered to collagen IV
To determine whether the failure to coprecipitate actin with β1 integrin was due to the absence of actin expression, whole cell lysates were prepared from HBE or MCF-7 cells, both of which were quiescent and adherent to collagen IV
Summary
Structural requirements for the β1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts. Integrins interact extracellularly with the substratum such as collagens at specific sites called focal adhesions and intracellularly with many actin-binding proteins such as α-actinin, talin, and vinculin, thereby linking these proteins with the actin cytoskeleton. Integrin binds to talin [7], which binds to vinculin [8,9,10], which in turn binds to α-actinin [11,12], which binds to actin. This constitutes a three-protein link between in-. Several amino acids and motifs within the cytoplasmic domain of the β subunit, which are potential phosphorylation sites, have been implicated in the integrin function. The relevance of phosphorylation of these amino acids within the cytoplasmic domain of β1 integrin in its function remains largely unclear
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