All-trans-retinoic acid increased the expression of integrin alpha5beta1 and induced "anoikis" in SMMC-7721 hepatocarcinoma cell.

It is extensively shown that integrin can regulate various cellular functions, including apoptosis, probably by contributing to signal transduction processes through interaction with extracellular matrix (ECM) proteins. In the present study, DNA flow cytometric analysis demonstrated that SMMC-7721 hepatocarcinoma cells treated with 80 microM all-trans-retinoic acid (atRA) showed an increased expression of the integrin alpha5beta1, which was associated with the growth inhibition of the cells. We found that atRA treated cells showed obvious apoptosis. Then, it was postulated that the enhanced content of integrin alpha5beta1 in the absence of ligation with fibronectin (Fn) would stop transducing survival signals, and lead to decreased cell growth and apoptosis. To elucidate this hypothesis, we cultured the atRA treated cell in L-poly-lysine-coated and Fn-coated flask, respectively. The results indicated that Fn binding prevented the cells from apoptosis induced by atRA, in contrast to L-poly-lysine binding. When the transfectant with enhanced expression of integrin alpha5beta1 at the same level of atRA treated cell was cultured in L-poly-lysine-coated flask, apoptosis was triggered. However, apoptotic cell was not detected when those cells were cultured in Fn-coated flask. Meanwhile, culturing the transfectant in the antibody (against integrin alpha5 subunit)-coated flask induced 18.33% of the cells into apoptosis, which is far more than the control group. Our study suggests that increased expression of integrin alpha5beta1 on the surface of SMMC 7721 hepatocarcinoma cell treated by atRA, when unbound to Fn, would stop transducing survival signals to lead to "anoikis", and can be reverted by the interaction of integrin alpha5beta1 with Fn.