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Novel off-target effect of tamoxifen--inhibition of acid ceramidase activity in cancer cells.

Biochimica et biophysica acta (2013-08-14)
Samy A F Morad, Jonathan C Levin, Su-Fern Tan, Todd E Fox, David J Feith, Myles C Cabot
ABSTRACT

Acid ceramidase (AC), EC 3.5.1.23, a lysosomal enzyme, catalyzes the hydrolysis of ceramide to constituent sphingoid base, sphingosine, and fatty acid. Because AC regulates the levels of pro-apoptotic ceramide and mitogenic sphingosine-1-phosphate, it is considered an apt target in cancer therapy. The present study reveals, for the first time, that the prominent antiestrogen, tamoxifen, is a pan-effective AC inhibitor in the low, single digit micromolar range, as demonstrated in a wide spectrum of cancer cell types, prostate, pancreatic, colorectal, and breast. Prostate cancer cells were chosen for the detailed investigations. Treatment of intact PC-3 cells with tamoxifen produced time- and dose-dependent inhibition of AC activity. Tamoxifen did not impact cell viability nor did it inhibit AC activity in cell-free assays. In pursuit of mechanism of action, we demonstrate that tamoxifen induced time-, as early as 5min, and dose-dependent, as low as 5μM, increases in lysosomal membrane permeability (LMP), and time- and dose-dependent downregulation of AC protein expression. Assessing various protease inhibitors revealed that a cathepsin B inhibitor blocked tamoxifen-elicited downregulation of AC protein; however, this action failed to restore AC activity unless assayed in a cell-free system at pH4.5. In addition, pretreatment with tamoxifen inhibited PC-3 cell migration. Toremifene, an antiestrogen structurally similar to tamoxifen, was also a potent inhibitor of AC activity. This study reveals a new, off-target action of tamoxifen that may be of benefit to enhance anticancer therapies that either incorporate ceramide or target ceramide metabolism.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Cathepsin B from bovine spleen, lyophilized powder, ≥10 units/mg protein
Sigma-Aldrich
Cathepsin B from human liver, buffered aqueous solution, ≥1,500 units/mg protein (E1%/280)
Sigma-Aldrich
Sphingosine 1-phosphate, ≥95%, powder
Sigma-Aldrich
Sphingosine 1-phosphate, ≥98.0% (TLC)
Sigma-Aldrich
Cathepsin B from human placenta, lyophilized powder, ≥5 units/mg protein
Sigma-Aldrich
Toremifene citrate salt, ≥98% (HPLC)