Selective Estrogen Receptor (ER) Modulators Differentially Regulate Phospholipase D Catalytic Activity in ER-Negative Breast Cancer Cells

Abstract

Recent successes in the pharmacotherapeutic treatment of breast cancer are associated with the use of selective estrogen receptor modulators. Two commonly prescribed pharmaceuticals in this class, tamoxifen and raloxifene, have been shown to have effects through estrogen receptor (ER)-independent mechanisms. Hyperactivation of phospholipase D (PLD) in certain tumor-derived cell lines have been reported, and recent findings suggest a role for PLD in transformation and metastasis. In the present study, we compare the effects of tamoxifen and raloxifene on PLD in the ER-positive mammary epithelial cell line MCF-12A, and the ER-negative, highly tumorigenic mammary carcinoma cell line MDA-MB-231. Our data demonstrate that tamoxifen and raloxifene have differential effects on PLD catalytic activity. Tamoxifen stimulates PLD in both ERpositive and -negative cells in vivo, whereas raloxifene inhibits PLD activity in these same cell types. In addition, we show that the active metabolite 4-OH-tamoxifen can be used to pharmacologically discriminate the two isoforms of PLD, through a stimulatory effect on PLD1 and an inhibitory effect on PLD2. Using recombinant PLD1, we show stimulation by tamoxifen requires a factor present in Sf21 insect cells that is not required for inhibition of PLD1 by raloxifene. Furthermore, tamoxifen stimulation and raloxifene inhibition of PLD activities are independent of the amino-terminal portion of PLD1 (amino acids 1-324). Knowledge of the mechanisms of action of these drugs on PLD may provide insights into the pharmacological action of these drugs and the role of PLD in some cancers.

Human Phospholipase D Activity Transiently Regulates Pyrimidine Biosynthesis in Malignant Gliomas

“…10,17,38,47 The use of complementary approaches to assess specificity, including the development of PLD inhibitors with differential chemical scaffolds, will further advance our understanding of the therapeutic value of this target. 37,48 Future experiments will focus on addressing whether similar mechanisms are at work in other relevant disease models.…”

Choline metabolism in malignant transformation

“…Chronic exposure of different non-tumorigenic mammalian cell lines to HePC strongly inhibits PC-PLD activity, which may be related to its antitumour activity 160 . The selective oestrogen receptor modulators tamoxifen and raloxifene, which are commonly used for the chemotherapy of oestrogen receptor-positive breast cancer, were reported to differentially affect PC-PLD activity 161 . Multidrug resistance in human cancer cells was accompanied by increased PLD2 activity in detergent-insoluble glycolipidrich and caveolar membranes 70 .…”

Review of:Estrogen receptor β expression is associated with tamoxifen response in ERα-negative breast carcinoma

“…The size of this cohort has been estimated as being less than 10% of patients with ERa-negative tumors. The reasons for an effect of tamoxifen in ERa-negative tumors have been unclear although some suggestions include false negative assays due to technical issues, or tamoxifen effects via an ER-independent mechanism(s) [2]. Results presented in this paper suggest that tamoxifen may have beneficial effects in ERa-negative but ERb-expressing breast cancer.…”