SYNERGISTIC MODULATION OF ECTOCA-2+-ATPASE ACTIVITY OF HEPATOMA (LI-7A) CELLS BY EPIDERMAL GROWTH-FACTOR AND CYCLIC-AMP

A mercurial-insensitive ectoATPase, which was more active with CaATP than with MgATP, was induced when human hepatoma (Li-7A) cells were cultured in the presence of epidermal growth factor (EGF) and cholera toxin. Cholera toxin could be replaced by forskolin, 8-Br-cAMP, butyryl-cAMP and dibutyrylc-AMP. Requirement for EGF was specific, but EGF was ineffective if added more than 24 h after the addition of forskolin or cholera toxin. It was concluded that induction of the ectoCa2+-ATPase was a consequence of the synergistic actions of EGF and cyclic AMP. The tyrosine kinase activity of the EGF receptor was essential for the induction of ectoCa2+-ATPase, since enzyme induction was abolished by a tyrosine kinase inhibitor, genistein. Cycloheximide and actinomycin D were also inhibitory to enzyme induction, indicating that enhancement of enzyme activity by EGF and cAMP was not due to post-translational modification. The results of this and previous investigations established that the two ectoATPases of Li-7A cells are under different regulation. © 1990.