Inhibition of autophagy stimulate molecular iodine-induced apoptosis in hormone independent breast tumors

Estrogen receptor negative (ER−ve) and p53 mutant breast tumors are highly aggressive and have fewer treatment options. Previously, we showed that molecular Iodine (I2) induces apoptosis in hormone responsive MCF-7 breast cancer cells, and non-apoptotic cell death in ER−ve–p53 mutant MDA-MB231 cells (Shrivastava, 2006). Here we show that I2 (3 μM) treatment enhanced the features of autophagy in MDA-MB231 cells. Since autophagy is a cell survival response to most anti-cancer therapies, we used both in vitro and in vivo systems to determine whether ER−ve mammary tumors could be sensitized to I2-induced apoptosis by inhibiting autophagy. Autophagy inhibition with chloroquine (CQ) and inhibitors for PI3K (3MA, LY294002) and H+/ATPase (baflomycin) resulted in enhanced cell death in I2 treated MDA-MB231 cells. Further, CQ (20 μM) in combination with I2, showed apoptotic features such as increased sub-G1 fraction (∼5-fold), expression of cleaved caspase-9 and -3 compared to I2 treatment alone. Flowcytometry of I2 and CQ co-treated cells revealed increase in mitochondrial membrane permeability (p < 0.01) and translocation of cathepsin D activity to cytosol relative to I2 treatment. For in vivo studies ICRC mice were transplanted subcutaneously with MMTV-induced mammary tumors. A significant reduction in tumor volumes, as measured by MRI, was found in I2 and CQ co-treated mice relative to I2 or vehicle treated mice. These data indicate that inhibition of autophagy renders ER−ve breast tumor cells more sensitive to I2 induced apoptosis. Thus, I2 together with autophagy inhibitor could have a potential tumorostatic role in ER−ve aggressive breast tumors that may be evaluated in future studies.

► Molecular iodine (I2) causes non-apoptotic cell death in MDA-MB231 breast tumor cells.
► Autophagy is activated as a survival mechanism in response to I2 in MDA-MB231.
► Autophagy inhibition sensitizes tumor cells to I2-induced apoptotic cell death.
► Autophagy inhibitor potentiates apoptosis and tumor regressive effects of I2 in mice.