The integrin family of cell surface receptors consists of transmembrane glycoproteins involved in cellular morphology, cytoarchitecture, cell-cell, and cell-extracellular matrix interaction. Changes in integrin receptor expression are associated with malignant transformation. The adhesion promoting activity of several members of the integrin receptors may be modulated. Integrin Associated Proteins and integrin modulating factor that may modulate integrin receptors expression and function have been reported. In this article, we report the identification of a 30-kD protein produced in SiHa cell culture medium that can modulate the expression and function of alpha5beta1 integrin receptor in HeLaS3 cells. The cell adhesion assay clearly demonstrated that HeLaS3 cells grown in a serum-free culture medium of SiHa cells (fresh medium: culture medium = 3:1) stimulated the ligand binding activity of alpha5beta1 receptor to fibronectin in a time-dependent manner, having a peak activity at 72 hours of culture. Immunocytochemical localization showed a very high expression of alpha5beta1 receptor in HeLaS3 cells grown in a SiHa culture medium for 72 hours. The (NH4)2SO4 fractionation demonstrated that proteins present in 80-100% (NH4)2SO4 saturated fraction of serum-free SiHa culture medium have a significant stimulatory effect on the binding of HeLaS3 cells to fibronectin ligand via the alpha5beta1 integrin receptor. High pressure liquid chromatography (HPLC) separation of 80-100% (NH4)2SO4 saturated fraction showed a 30- kD protein in polyacrylamide gel electrophoresis (PAGE) analysis that has a maximum stimulatory effect on the binding of HeLaS3 cells to fibronectin ligand via the alpha5beta1 integrin receptor. In conclusion, our observations indicated that human cervical tumor cells SiHa produce a 30-kD protein that can modulate the expression and function of alpha5beta1 fibronectin integrin receptor of HeLaS3 cells. These findings strengthen the concept that some cellular proteins, also called Integrin Associated Protein, may regulate the integrin receptor expression and function. Studies are in progress to characterize this 30-kD integrin modulating factor and its role in the regulation of integrin receptor function.
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