Tamoxifen activates cellular phospholipase C and D and elicits protein kinase C translocation.
The antiestrogen tamoxifen is widely used for endocrine therapy of breast cancer; however, the mechanisms of estrogen receptor-independent interactions of tamoxifen remain ill defined. Here we examine the effect of tamoxifen on the initial steps of cell signal transduction. To this end, phospholipid metabolism and protein kinase C (PKC) translocation were assessed in CCD986SK human mammary fibroblasts treated with tamoxifen. The addition of tamoxifen resulted in dose-dependent and time-dependent increases in the cellular second messengers phosphatidate (PA) and diacylglycerol (DG). On addition of ethanol to the medium, tamoxifen induced the formation of phosphatidylethanol, demonstrating that tamoxifen activates phospholipase D (PLD). Cellular DG also increased in the presence of ethanol, showing that tamoxifen also activates phospholipase C (PLC). In cells prelabeled with choline and ethanolamine, tamoxifen caused increases in choline, phosphorylcholine, ethanolamine and phosphorylethanolamine. Structure-activity relationship studies for activation of PLD revealed that tamoxifen was the most effective, whereas 4-hydroxy tamoxifen was nearly devoid of activity. Phorbol diesters also activated PLD, but estrogen had no influence. Pretreatment of cells with phorbol dibutyrate (PKC down-regulation protocol) blocked phorbol diester- and tamoxifen-induced PLD activity. Exposure of cells to the PKC inhibitor GF 109203X diminished tamoxifen-induced PLD activity. Addition of tamoxifen to cultures elicited selective membrane association of PKC epsilon. We conclude that tamoxifen exerts considerable extra-nuclear influence at the transmembrane signaling level. These events may contribute to effects beyond the scope of estrogen receptor-dependent actions.
Cabot
M C
MC
John Wayne Cancer Institute at Saint John's Hospital and Health Center, Santa Monica, CA 90404, USA.
Zhang
Z
Z
Cao
H
H
Lavie
Y
Y
Giuliano
A E
AE
Han
T Y
TY
Jones
R C
RC
D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D004075:Diglycerides / Q000378:metabolism; D015536:Down-Regulation; D004789:Enzyme Activation; D006801:Humans; D006868:Hydrolysis; D010712:Phosphatidic Acids / Q000378:metabolism; D010713:Phosphatidylcholines / Q000378:metabolism; D010739:Phospholipase D / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013629:Tamoxifen / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000378:metabolism*
D018931:Antineoplastic Agents, Hormonal; D004075:Diglycerides; D010712:Phosphatidic Acids; D010713:Phosphatidylcholines; D013629:Tamoxifen; D011493:Protein Kinase C; D010738:Type C Phospholipases; D010739:Phospholipase D