Autophagy and endocrine resistance in breast cancer

Abstract

The American Cancer Society estimates that over 200,000 new breast cancer cases are diagnosed annually in the USA alone. Of these cases, the majority are invasive breast cancers and almost 70% are estrogen receptor-α positive. Therapies targeting the estrogen receptor-α are widely applied and include selective estrogen receptor modulators such as tamoxifen, a selective estrogen receptor downregulator such as Fulvestrant (Faslodex; FAS, ICI 182,780), or one of the third-generation aromatase inhibitors including letrozole or anastrozole. While these treatments reduce breast cancer mortality, many estrogen receptor-α-positive tumors eventually recur, highlighting the clinical significance of endocrine therapy resistance. The signaling leading to endocrine therapy resistance is poorly understood; however, preclinical studies have established an important role for autophagy in the acquired resistance phenotype. Autophagy is a cellular degradation process initiated in response to stress or nutrient deprivation, which attempts to restore metabolic homeostasis through the catabolic lysis of aggregated proteins, unfolded/misfolded proteins or damaged subcellular organelles. The duality of autophagy, which can be either pro-survival or pro-death, is well known. However, in the context of endocrine therapy resistance in breast cancer, the inhibition of autophagy can potentiate resensitization of previously antiestrogen resistant breast cancer cells. In this article, we discuss the complex and occasionally contradictory roles of autophagy in cancer and in resistance to endocrine therapies in breast cancer.

Keywords::

• 3-methyladenine
• antiestrogen resistance
• aromatase inhibitor
• autophagy
• bafilomycin A1
• breast cancer
• endoplasmic reticulum stress
• fulvestrant
• hydroxychloroquine
• tamoxifen
• unfolded protein response

Financial & competing interests disclosure

Katherine Clark is the recipient of an NIH training grant (grant no. 5-T32-CA009686). This research was supported in part by awards from the Department of Defense Breast Cancer Research Program (BC073977) and from the US Department of Health and Human Services (R01-CA131465; U54-CA149147 and 9XS194 In Silico Research Centers of Excellence) to Robert Clarke. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.