Inhibition of protein kinase C and calmodulin by the geometric isomers cis- and trans-tamoxifen

The triphenylethylene antiestrogen trans-tamoxifen is an effective antitumor agent used in the treatment of human breast cancer. While the antiestrogenic activity of trans-tamoxifen clearly plays an important role in its tumoricidal action, some of the biological effects of trans-tamoxifen are independent of estrogen. Therapeutic concentrations of trans-tamoxifen inhibit protein kinase C (PKC) and calmodulin-dependent enzymes. PKC and calmodulin play critical roles in growth regulation, and there is evidence that inhibition of PKC and calmodulin by trans-tamoxifen may contribute to the antiumor activity of the drug in vivo. The geometric isomers cis- and trans-tamoxifen have a number of opposing biological activities that have been attributed to their interactions with the estrogen receptor, Cis-tamoxifen is generally estrogenic, whereas trans-tamoxifen is generally antiestrogenic. In this report, we compared the effects of cis- and trans-tamoxifen on PKC activity and on calmodulin-dependent cAMP phosphodiesterase activity. Cis- and trans-tamoxifen inhibited the Ca²⁺- and phosphatidylserine- (PS-) dependent activity of purified rat brain PKC with indistinguishable potencies, but cis-tamoxifen was somewhat more potent than the trans isomer in the inhibition of the Ca²⁺- and PS-independent activity of PKC. In addition, cis-tamoxifen was the more potent isomer in the inhibition of T lymphocyte activation, an event that entails a PKC-requiring signal transduction pathway. A modest preference of the cis isomer was also observed in the inhibition of a calmodulin-dependent cAMP phosphodiesterase. These results suggest a congruence between triphenylethylene binding sites on PKC and on the activated calmodulin–cAMP phosphodiesterase complex. We conclude that the interactions of cis- and trans-tamoxifen with PKC and the activated calmodulin–cAMP phosphodiesterase complex offer a criterion for distinguishing biological effects of triphenylethylenes that are due to interactions with the estrogen receptor from the biological effects resulting from their inhibitory activities against PKC and calmodulin-dependent processes.