Characterization of the Placental Alkaline Phosphatase-like (Nagao) Isozyme on the Surface of A431 Human Epidermoid Carcinoma Cells1

A431 human epidermoid carcinoma cells monophenotypically express the placenta! alkaline phosphatase (PLAP)-like enzyme shown by its catalytic and antigenic characteristics, properties which are shared by the Nagao isozyme. More specifically, it is L-leucine sensitive just as is the rare placenta! D-variant of PLAP and the testicular heat-stable enzyme. Collectively, these are all referred to as PLAP-like enzymes. The enzyme was localized to the surface of the plasma mem brane since it was released in an active form by bromelain treatment of cells. The number of molecules per A431 cell was estimated by radioimmunoassay at 7.5 x 105, a value signifi cantly higher than that observed for HeLa TCRC-1 cells (5 x 104) which express the S-variant of PLAP, also referred to as the Regan isozyme. The quantity of the enzyme was increased significantly (10-fold) by treating the cells with modulating agents including sodium butyrate, prednisolone, and hyperosmolar so dium chloride. The identification of a cell line such as A431 with enhanced expression in the amount of the PLAP-like enzyme and which can be further enhanced by modulating agents will facilitate studies of the differences and the similarities between this protein and other variants of PLAP. The A431 cell line now takes its place with other cell lines which are phenotypically restricted in their expression of alkaline phosphatase. Finally, the A431 cell line is also shown here to be a suitable model system for in vivo tumor studies ;>uch as immunolocalization.