Resistance to
aromatase inhibitors (AIs) involves increased HER2. One mechanism by which HER2 may mediate resistance is through expansion of the tumor initiating cell (
TIC) population. This study investigates whether combining
all-trans retinoic acid (ATRA) and
histone deacetylase inhibitor entinostat (ENT) can inhibit
TICs and HER2 in AI-resistant cells and
tumors. Modulation of cell viability and HER2 expression were assessed in AI-resistant cells treated with ATRA + ENT.
Letrozole-resistant LTLT-Ca cells treated with ATRA + ENT were assayed for changes in
TIC characteristics, such as
TIC markers (BCRP, ALDH, and BMI-1), side population (SP), and mammosphere formation. Xenograft
tumors of MCF-7Ca cells made resistant to
letrozole were treated with ATRA, ATRA +
letrozole, ATRA + ENT, or ATRA + ENT +
letrozole. Resulting
tumors were assayed for changes in
TIC characteristics. Patient samples taken pre- and post-AI treatment were analyzed for changes in ERα and HER2
protein expression. Treatment with ATRA + ENT reduced HER2 expression and viability (P < 0.001) in AI-resistant cells, as well as decreased SP (P < 0.0001), mammosphere formation (P < 0.01), and expression of
TIC molecular markers (P < 0.01) in LTLT-Ca. A reduction in
tumor growth rate was observed in mice treated with ENT + ATRA +
letrozole when compared to mice treated with single agents (P < 0.0001) or ENT + ATRA (P = 0.02). Decreased
TIC characteristics, including mammosphere formation (P < 0.05), were observed in
tumors from the triple combination. An increase in HER2 and downregulation in ERα
protein expression was observed in patients upon resistance to AI (P < 0.005). These studies indicate that the combination of ATRA and ENT inhibits the
TIC population of AI-resistant cells and may be effective in reducing
tumor recurrence.