GX15-070 (obatoclax) induces apoptosis and inhibits cathepsin D- and L-mediated autophagosomal lysis in antiestrogen-resistant breast cancer cells.

Mol Cancer Ther. 2013 Apr;12(4):448-59. 

Schwartz-Roberts JL, Shajahan AN, Cook KL, Wärri A, Abu-Asab M, Clarke R.

Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University School of Medicine, Washington, DC 20057, USA.

 

Abstract:

In estrogen receptor-positive (ER+) breast cancer cells, BCL2 overexpression contributes to antiestrogen resistance. Direct targeting of the antiapoptotic BCL2 members with GX15-070 (obatoclax), a BH3-mimetic currently in clinical development, is an attractive strategy to overcome antiestrogen resistance in some breast cancers. Recently, GX15-070 has been shown to induce both apoptosis and autophagy, yet the underlying cell death mechanisms have yet to be elucidated. Here, we show that GX15-070 is more effective in reducing the cell density of antiestrogen-resistant breast cancer cells versus sensitive cells and that this increased sensitivity of resistant cells to GX15-070 correlates with an accumulation of autophagic vacuoles. Formation of autophagosomes in GX15-070-treated cells was verified by changes in expression of the lipidation of microtubule-associated protein-1 light chain-3 and both confocal and transmission electron microscopy. While GX15-070 treatment promotes autophagic vacuole and autolysosome formation, p62/SQSTM1, a marker for autophagic degradation, levels accumulate. Moreover, GX15-070 exposure leads to a reduction in cathepsin D (CTSD) and L (CTSL1) protein expression that would otherwise digest autolysosome cargo. Thus, GX15-070 has dual roles in promoting cell death: (i) directly inhibiting antiapoptotic BCL2 family members, thereby inducing apoptosis; and (ii) inhibiting downstream CTSD and CTSL1 protein expression to limit the ability of cells to use degraded material to fuel cellular metabolism and restore homeostasis. Our data highlight a new mechanism of GX15-070-induced cell death that could be used to design novel therapeutic interventions for antiestrogen resistant breast cancer.

PMID: 23395885

 

Interpretation:

While endocrine therapy remains the standard of care for ER+ breast cancer, patients often develop endocrine-resistant disease. We demonstrated that targeting prosurvival BCL2 members with the BH3 mimetic, obatoclax, inhibited the growth of ER+ antiestrogen-sensitive and –resistant breast cancer cells. By sequestering antiapoptotic BCL2, breast cancer cells were sensitized to cell death by the antiestrogens, tamoxifen and fulvestrant. Although both the apoptotic and autophagy pathways were activated by obatoclax, we discovered that the final stages of autophagy were halted following obatoclax exposure. Our results showed that obatoclax reduced the protein expression of lysosomal hydrolases that are used to degrade autophagic cargo in the final steps of autophagy. Our observations were both interesting and potentially helpful for antiestrogen therapy: obatoclax sensitized breast cancer cells to antiestrogens and blocked autophagic cargo degradation to induce cell death. These data suggest that anti-BCL2 therapy could enhance the effectiveness of existing antiestrogenic agents and provides another target that can be exploited for the treatment of breast cancer.

Jessica Schwartz-Roberts-1 

Figure 1. A proposed model illustrating how obatoclax alters autophagy in breast cancer cell lines. In healthy cells, autophagosomes fuse with lysosomes to become autolysosomes and the contents are degraded by proteases. In obatoclax-treated cells, the absence of cathepsins results in undigested autolysosomal cargo, leading to disregulated autophagy and cell death. 

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