Zoledronic acid inhibits aromatase activity and phosphorylation: Potential mechanism for additive zoledronic acid and letrozole drug interaction

Abstract

Zoledronic acid (ZA), a bisphosphonate originally indicated for use in osteoporosis, has been reported to exert a direct effect on breast cancer cells, although the mechanism of this effect is currently unknown. Data from the ABCSG-12 and ZO-FAST clinical trials suggest that treatment with the combination of ZA and aromatase inhibitors (AI) result in increased disease free survival in breast cancer patients over AI alone. To determine whether the mechanism of this combination involved inhibition of aromatase, AC-1 cells (MCF-7 human breast cancer cells transfected with an aromatase construct) were treated simultaneously with combinations of ZA and AI letrozole for 72 hours. This combination significantly increased inhibition of aromatase activity of AC-1 cells by compared to letrozole alone. Combination treatment of 1nM letrozole and 1μM and 10μM zoledronic acid resulted in an additive drug interaction on inhibiting cell viability, as measured by MTT assay. Treatment with ZA was found to inhibit phosphorylation of aromatase on serine 473. Zoledronic acid was also shown to be more effective in inhibiting cell viability in aromatase transfected AC-1 cells when compared to inhibition of cell viability observed in non-transfected MCF-7. Estradiol was able to partially rescue the effect of 1μM and 10μM ZA on cell viability following treatment for 72 hours, as shown by a shift to the right in the estradiol dose response curve. In conclusion, these results indicate that the combination of ZA and letrozole results in an additive inhibition of cell viability. Furthermore, ZA alone can inhibit aromatase activity through inhibition of serine phosphorylation events important for aromatase enzymatic activity and contributes to inhibition of cell viability.

Zoledronic Acid Reverses the Epithelial–Mesenchymal Transition and Inhibits Self-Renewal of Breast Cancer Cells through Inactivation of NF-κB

“…Whole cell lysates were prepared as previously described (19). For cell fractionation, cells were scraped by rubber policeman in dPBS, washed twice, and centrifuged at 1000 rpm for 5 minutes at 4°C.…”

“…mRNA and promoter expression were measured using qPCR as previously described (19). PCR reactions were amplified for 50 cycles with an annealing temperature of 60°C.…”

Nonhypoxic regulation and role of hypoxia-inducible factor 1 in aromatase inhibitor resistant breast cancer

“…AC1 cells are another set of MCF-7 cells stably transfected with the human placental aromatase gene [ 37 ]. Similar to MCF-7Ca cells, AC1 cells are ER+/HER2-, proliferate in response to estrogen or androstenedione, express the aromatase enzyme and are sensitive to aromatase inhibitors [ 38 ]. AC1 cells, however, were created in our laboratory rather than Dr. Chen’s, and they express higher levels of aromatase (data not shown).…”

Role of Zoledronate in Aromatase Activity Should Be Considered in Future Studies

“…It was previously shown that zoledronate prevented deep bone loss when goserelin was combined with tamoxifen or anastrozole [5]. In a similar study, Schech et al [6] showed that the combination of letrozole and zoledronate caused increased inhibition of aromatase activity compared with letrozole alone. Zoledronate can inhibit the aromatase enzyme by inhibition of serine phosphorylation.…”