COOPERATIVE ANTIPROLIFERATIVE EFFECTS OF 8-CHLORO-CYCLIC AMP AND 528 ANTIEPIDERMAL GROWTH-FACTOR RECEPTOR MONOCLONAL-ANTIBODY ON HUMAN CANCER-CELLS

8-Chloro cyclic AMP (8-Cl-cAMP), a site-selective cAMP analogue, is a specific inhibitor of type I cAMP-dependent protein kinase (PKAI) and induces growth inhibition in several human and rodent tumor cell lines , The anti-epidermal growth factor receptor (EGFR) mAb 528 is a blocki ng antibody able to inhibit the in vitro and in vivo growth of several human cancer cell lines that express functional EGFRs, Since enhanced levels of PKAI are generally found in tumor cells and an increase in PKAI expression is induced by transformation through a transforming gr owth factor alpha/EGFR autocrine pathway, we have evaluated whether tr eatment with mAb 528 in combination with 8-Cl-cAMP may have an additiv e or synergistic growth inhibitory effect on human cancer cells, A dos e-dependent inhibition of monolayer cell growth was observed in two hu man colon cancer cell lines (GEO and CBS) and in a human breast cancer cell line (MDA-468) by treatment with either mAb 528 or 8-Cl-cAMP wit h 50% inhibitory concentration of 2-10 mu g/ml or 20-25 mu M, respecti vely, The combined treatment with low noninhibitory doses of mAb 528 ( 0.25 mu g/ml) and with 8-Cl-cAMP had a more than additive growth inhib itory effect with a 3- to 5-fold reduction in the 8-Cl-cAMP 50% inhibi tory concentration in all cell lines tested, This combined treatment w as similarly effective in inhibiting the soft agar cloning efficiency of GEO cells, 8-Cl-cAMP treatment of GEO cells induced a dose-dependen t increase in cell membrane-associated EGPRs with a maximum 3- to 4-fo ld increase within 48-72 h of treatment, These results suggest that a double blockade of the PKAI serine-threonine kinase-dependent and of t he EGFR tyrosine kinase-dependent pathways is potentially useful in ca ncer therapy.