Alpha2-adrenergic effect on human breast cancer MCF-7 cells.

(-)Epinephrine (Epi) and (-)Norepinephrine (NEpi) significantly stimulated tritiated Thymidine incorporation in MCF-7 cells at concentrations 10-30pM to 10nM, with an EC50 of 10pM for Epi and 14.2pM for NEpi. To characterize this action, cells were incubated in the presence of NEpi or Epi and different antagonists. The beta-adrenergic antagonist Propanolol showed no effect on the agonist's stimulation, whereas the alpha-adrenergic antagonist Phentolamine, reverted it completely at high concentrations (100 microM). The alpha1-adrenergic antagonist Prazosin (Pra) acted only at high concentrations, while the alpha2-adrenergic antagonist Yohimbine (Yo) reverted the stimulation at an EC50 of 0.11 microM. Likewise, when the cells were incubated in the presence of the specific alpha2-adrenergic agonist Clonidine (Clo), Thymidine incorporation was significantly stimulated at an EC50 of 0.298 pM. Again, the incubation of the cells in the presence of the alpha1-adrenergic antagonist Pra exerted its action at high concentrations, whereas the alpha2-adrenergic antagonist Yo showed a clear reversal of the agonist's enhancement at an EC50 of 0.136 microM. Moreover, Clo caused a clear and significant inhibition of stimulated cAMP levels both in the intracellular and the extracellular fractions. Yo showed a complete reversion of cAMP levels to control values in the presence of Clo, while Pra had the opposite effect. These data suggest that the stimulation provoked in Thymidine incorporation by the agonists Epi, NEpi, and Clo is, at least in part, due to an alpha2-adrenergic mechanism directly on tumoral cells, and that the effect is coupled with inhibition of cAMP levels, as described for this kind of receptors.