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 antitumor 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 for 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.(ABSTRACT TRUNCATED AT 250 WORDS)
O'Brian
C A
CA
Department of Cell Biology, University of Texas M. D. Anderson Cancer Center, Houston 77030.
Ioannides
C G
CG
Ward
N E
NE
Liskamp
R M
RM
D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000037:antagonists & inhibitors; D000818:Animals; D002147:Calmodulin / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D054677:Cyclic Nucleotide Phosphodiesterases, Type 1; D011493:Protein Kinase C / Q000037:antagonists & inhibitors*; D051381:Rats; D013237:Stereoisomerism; D013629:Tamoxifen / Q000494:pharmacology*
D002147:Calmodulin; D013629:Tamoxifen; D011493:Protein Kinase C; D015105:3',5'-Cyclic-AMP Phosphodiesterases; D054677:Cyclic Nucleotide Phosphodiesterases, Type 1