Expression of the integrinα ₅ subunit and its mediated cell adhesion in hepatocellular carcinoma

Abstract

Tumor invasion and metastasis are complex processes, requiring the ability of tumor cells to interact with proteins of the extracellular matrix through cell-adhesion molecules on the cell surface. Integrins are heterodimeric membrane glycoproteins, consisting of α and β subunits, which enable cells to recognize adhesive substrates in the extracellular matrix. The roles of the integrinα ₅ β ₁ in tumor invasion are highlighted by finding that some tumor cells have lost or reducedα ₅ β ₁ expression. It therefore functions as a negative signaling regulator. Expression ofα ₅ β ₁ and its mediation of cell adhesion in hepatocellular carcinoma (HCC) have not been elucidated. In surgical specimens of HCC we found, by immunohistochemistry and Northern blot analysis, that theα ₅-positive rates in cancerous tissues were lower than the corresponding rates in non-cancerous tissues. Reduced expression of the integrinα ₅ occurred more frequently in HCC with more malignant phenotypes, such as poor differentiation, large size (more than 10-cm in diameter), absence of capsule and high invasion. Reverse transcription/polymerase chain reaction, a more sensitive assay, was used to detect theα ₅ mRNA level in LCID20, a highly metastatic model of human HCC, and LCID35, a low-metastasis model. The results showed that integrinα ₅ was negative in the former and positive in the latter. Cell adhesion assays showed the maximal percentage inhibition of anti-α ₅ mAb on SMMC 7721 cell adhesion to fibronectin to be 68.9±4.9% at the saturation concentrations of each antibody (200 μg/ml). If anti-α ₅ mAb was combined with anti-β ₁ mAb, the inhibition was 74.1±11.1%. It is concluded that reduced expression of the integrinα ₅ subunit is correlated with more malignant phenotypes of human HCC. Any change in the adhesion of hepatocellular carcinoma cells to fibronectin is mainly dependent upon the function of the integrinα ₅ β ₁.