Tumor promoter 12-O-tetradecanoylphorbol-13-acetate receptors in normal human transitional epithelial cells

As a prelude to study the promotion with TPA of in vitro transformationof human urothelial cells (HUC) in culture, we characterizedtumor promoter TPA receptors in primary cultures of HUC. [³H]TPAbound specifically to intact living HUC; maximum specific bindingwas attained in 30 min at 37°C. [³H]TPA bound to HUC ina saturable and competitive manner. Scatchard analysis of specificbinding to intact cells displayed a single slope correspondingto an equilibrium dissociation constant (Kd) of 0.56 nM; atsaturation TPA-binding capacity was 2.37 pmol/10⁶ HUC (1.43x 10⁶ sites per cell). [³H]TPA bound specifically and with highaffinity to the particulate fractions of HUC; binding was bothsaturable and reversible. Saturation of the specific bindingof [3H]TPA occurred at 1 nM at 4°C. Scatchard analysis ofspecific binding to the particulate fraction displayed a singleslope corresponding to a Kd of 1.08 nM; at saturation TPA-bindingcapacity was 2.05 pmol/mg protein (750 000 molecules per HUC).[³H]TPA binding was inhibited by the biologically active phorbolester, phorbol didecanoate, whereas inactive phorbol did notcompete for TPA binding. Binding was not affected by sodiumsaccharin, epidermal growth factor, retinoic acid or dexamethasone.[³H]TPA bound specifically to the HUC cytosolic fraction butonly in the presence of calcium and phosphatidylserine. Calcium-activatedand phospholipid-sensitive protein kinase activity was detectedin HUC fractions. These results indicate the presence of high-affinityspecific receptors for TPA in HUC.