Several studies, including one from our lab, indicate that DNA methylation may be involved in the development of endocrine-resistant breast cancer [
13‐
16]. Therefore, epigenetic therapies that reverse aberrant DNA methylation may prove to be promising therapy for Tamoxifen-resistant disease. However, it is unclear how DNMT inhibitors such as 5-Aza-dC affect methylation and cell behavior, specifically in ERneg, acquired Tamoxifen resistance.
Treatment with 5-Aza-dC inhibited the proliferation of the Tamoxifen-selected, TMX2-28, but not the parental MCF7. TMX2-28 is hypermethylated compared to MCF7 ([
12,
16] and Fig.
2) and was more sensitive to inhibition of DNA methylation by treatment with 5-Aza-dC (Fig.
2b and c) suggesting that epigenetic treatment might be particularly beneficial for women with endocrine-resistant breast cancer. The genes identified as methylated in TMX2-28 and demethylated and potentially re-expressed after treatment with 5-Aza-dC may be good biomarkers of sensitivity to epigenetic therapy. We focused on one of these genes,
TACSTD2, and its product, TROP2, because of its importance as a potential target for treatment.
TACSTD2 is overexpressed in many cancers and has been associated with disease progression, migration, recurrence, and increased proliferation [
27,
33‐
36]. In published cell culture experiments, knockdown of
TACSTD2 inhibits growth in MCF7 (contrary to our findings) and colon cancer cells [
20], fetal rat lung cells (fibroblasts) [
28], fetal lung fibroblasts [
27], cervical cancer cells [
26], and laryngeal carcinoma cells [
30]. However, there also is evidence of decreased
TACSTD2 expression in cancer [
20,
23,
43,
44]. Lin and colleagues [
23] reported that knockdown of
TACSTD2 in lung cancer cells expressing high levels of endogenous
TACSTD2, increased AKT activation and promoted growth. They further showed that promoter methylation is correlated with decreased
TACSTD2 expression in lung cancer cell lines and tumors [
23]. In breast cancer, high levels of membrane TROP2 have been associated with poor prognosis while cytoplasmic staining indicated better survival [
21]. Analysis of 2061 ERpos breast cancers using the Kaplan–Meier Plotter indicates that low levels of
TACSTD2 mRNA are associated with decreased survival (p = 0.018; Additional file
10: Figure S4), while analysis of 801 ERneg breast cancers provides the opposite result: low levels of
TACSTD2 are associated with increased survival (p = 0.083; Additional file
10: Figure S4). [
37] The extent to which
TACSTD2 mRNA levels in these 2862 ERpos and ERneg breast cancers is associated with membrane versus cytoplasmic TROP2 is unknown. Despite the discrepancies, the evidence for over-expression of
TACSTD2 in cancer is sufficiently compelling to support clinical trials targeting membrane TROP2 [
38‐
45].