EPINEPHRINE INDUCES BETA-ADRENERGIC DESENSITIZATION AND DIFFERENTIATION OF HL-60 CELLS

The HL-60 cell line was cultured in a serum-free medium and exposed to various concentrations of EPI. The effects on cell growth, differentiation and beta-adrenergic response were followed during the culture period of 72h. Short term exposure (3 min) to EPI (1 nM - 1 mM) in the presence of theophylline (4 mM) caused a dose-dependent increase of cAMP levels with a maximum of 1500% above basal levels. When the cells were exposed to EPI (1 nM - 10 muM) for 72h, a dose-dependent increase of cAMP levels with a maximum of 60% above basal levels. Sustained exposure to EPI generated a dose-dependent desensitization of the beta-adrenergic signal system. After EPI treatment for 72h, IPR (10 muM for 3 min) in the presence of theophylline (4 mM) increased cAMP-levels by only 80% above baseline level (cAMP levels after maintained exosure to EPI), compared to 1080% above unstimulated level in control cells. The alpha-adrenergic receptor blocker PHENT (10 muM) did not affect baseline cAMP level or IPR-dependent cAMP response, but a mixture of EPI and PHENT increased the response to IPR. The HL-60 cell growth was not influenced by EPI. However, after repeated exposure to EPI for 72h a concentration-dependent increase of HL-60 differentiation was demonstrated. Differentiation was not influenced by PHENT. These results suggest a differentiation induction due to a beta-adrenergic-induced cAMP elevation.