The classical view of the molecular actions of estrogen is described by its interaction with the intracellular estrogen receptor (ER), the binding of hormone receptor complex to the estrogen response element (ERE) on the DNA and followed by the alterations of gene expressions. Recently it has been reported that membrane estrogen receptor (mER) exist and it is suggested to be G protein linked receptor. In this report we show that under steroid-free culture conditions supplemented with low percentage of charcoal- stripped serum, differential estrogen treatments of human breast cancer MCF7 cells induce different responses of cyclic AMP (cAMP) productions. Treating [2-H-3]adenine-labeled MCF7 cells with 1 nM estrogen for 30 min stimulates cAMP production by measuring the ratio of [H-3]cAMP:Total [H-3]adenine nucleotides (ATP + ADP + cAMP), as determined by column chromatography, when compared with the control. This short-term estrogen treatment also significantly enhanced forskolin stimulated cAMP production when compared with the ratio of cAMP/Total measured in cells stimulated with forskolin alone. Pre-treating MCF7 cells with the same concentration of estrogen for 24 h before the assay, on the contrary, significantly decreased the basal cAMP level and it also suppressed cAMP production stimulated with forskolin when compared with its respective value under short-term estrogen treatment. Estrogen receptor antagonist ICI 182780 abolished both the stimulatory and suppressive effect of estrogen on cAMP synthesis indicating both effects were mediated through ER. Pre-treating cells with pertussis toxin relieved the suppression of cAMP synthesis by chronic estrogen treatment. Our data suggest that estrogen exerts differential effects on the cAMP production in MCF7 cells, involving the activations G alpha(i) and G alpha(s) family of G proteins, depending on the length of time of hormone treatment. (C) 2007 Elsevier Inc. All rights reserved.