A unique sialoglycopeptide growth regulator that inhibits mitogenic activity of a phorbol ester tumor promoter.

The ability of a naturally occurring cell surface sialoglycopeptide growth inhibitor to antagonize the induction of DNA synthesis by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was studied with mouse 3T3 cells. The bovine sialoglycopeptide was shown to be a potent antagonist of TPA-induced DNA synthesis in confluent 3T3 cell cultures. Kinetic studies demonstrated that inhibition of TPA-induced DNA synthesis required the addition of the sialoglycopeptide within 15 min of TPA treatment. Addition of the sialoglycopeptide 30 min or longer after the cells were exposed to TPA did not block stimulation of DNA synthesis by TPA. The inhibition of TPA action was shown not to be restricted to DNA synthesis in 3T3 cultured cells since the sialoglycopeptide also inhibited TPA-induced ornithine decarboxylase (ODC, L-ornithine carboxylase, EC 4.1.1.17) activation in suspensions of mouse epidermal and 3T3 cells.