Fibronectin interaction with alpha 4 beta 1 integrin prevents apoptosis inB cell chronic lymphocytic leukemia: correlation with Bcl-2 and Bax

B cell chronic lymphocytic leukemia (B-CLL) consists of the accumulation of malignant cells that apparently escape normal apoptotic regulation. We hav e studied the role of alpha 4 beta 1 integrin/fibronectin interaction in pr eventing apoptosis of these cells in vitro. B cells from 16 patients showed constant expression of alpha 4 beta 1 and little or no alpha 5 beta 1. B-C LL cells cultured on fibronectin or two previously described fibronectin re combinant fragments (H89 and H0) which contain the ligands for alpha 4 beta l, consistently showed higher viability than control cells cultured on pol y-lysine. The H89 fragment, containing the high affinity ligand CS-1, was t he most efficient substrate with mean cell viability values of 72, 60 and 3 5% at days 2, 5 and 8 of culture, respectively. For control cells these val ues were 40, 27 and 15%, respectively. Parallel cell cycle analysis confirm ed these results. The anti-apoptotic effect required direct contact with im mobilized substrata since it was not observed when using B-CLL conditioned media alone or when clustering alpha 4 beta 1 with specific mAbs in suspens ion. Quantitation of the apoptosis regulatory proteins Bcl-2 and Fax reveal ed that cells cultured on the H89 fragment showed high/moderate levels of B cl-2 (with some interpatient variation) and low levels of Fax resulting in an elevated Bcl-2/Bax ratio. These results indicate that adhesion of B-CLL cells to fibronectin upregulate the Bcl-2/Bax ratio and this may contribute to the anti-apoptotic effect induced via alpha 4 beta 1 integrin.