Tumor cell adherence to cultured capillary endothelial cells is promoted by activators of protein kinase C

Stimulation of cells with protein kinase C (PKC)-specific activators such as phorbol esters increased in a reversible manner the rate of adherence of 3H]leucine-labelled L1210 cells to cultured bovine cerebral cortex capillary endothelial cells (CEC). This effect was not specific for L1210 cells since 12-O-tetradecanoyl phorbol 13-acetate (TPA) strongly increased the binding of various other tumor cell lines. Phorbol esters increased the rate of L1210 cell adhesion to CEC by enhancing their binding capacity without affecting the apparent affinity of L1210 cells for CEC. This stimulation was specific to the phorbol analogs which activate PKC since it was not effected by 4 alpha-phorbol didecanoate, known to be inactive for PKC. Down-regulation experiments showed that adhesion enhancement was entirely attributable to an effect on tumor cells without contribution of CEC intracellular PKC. PKC inhibitors like staurosporine, sphingosine and H-7 showed strong antagonistic activity towards TPA-induced L1210 cell adherence to CEC (IC50 = 0.5 nM, 160 nM and 10 microM, respectively). Adhesive proteins such as vitronectin, fibrinogen, fibronectin and the tetrapeptide RGDS, an active sequence from their cell-binding domains, exhibited potent, dose-dependent inhibition of PKC-induced tumor cell adhesion.