Steroidogenic Acute Regulatory Protein in Breast Cancer: Mechanistic Insights into Pathogenesis and Therapeutics

Breast cancer (BC), a multifactorial condition, remains one of the most common malignancies in women, in which the majority of BCs are hormone-sensitive and are activated by estrogens, especially 17β-estradiol (E2). Whereas aromatization of androgens to estrogens is achieved by the aromatase enzyme, the steroidogenic acute regulatory (StAR) protein, by mobilizing the transport of intra-mitochondrial cholesterol, plays an indispensable role in E2 biosynthesis. Accumulating evidence indicates that aromatase expression is aberrantly high and analogous in normal and malignant breast tissues, even though endocrine therapy, based on aromatase inhibitors (AIs), has been the mainstay of BC treatment in post-menopausal women. Despite the beneficial effects of AIs, their long-term usage has been associated with undesirable long-term side effects, including endocrine resistance, which is the leading cause of cancer death, warranting an improved therapy for mitigating this devastating disease. Along these lines, we reported that StAR is differentially expressed, along with E2 biosynthesis, in human and mouse cancerous and non-cancerous breast cells and tissues, in which we discovered that StAR is an acetylated protein, in addition to the identification of a number of lysine residues, undergoing acetylation and deacetylation, suggesting the importance of this newly uncovered StAR modification in E2 regulation in mammary tissue. One of the current therapeutic approaches for BC is targeting with histone deacetylase inhibitors (HDACIs), as these epigenetic enzymes control multiple cellular processes, including chromatin remodeling and genomic stability through the dynamic process of acetylation and deacetylation of core histones. Concomitantly, we have demonstrated that several HDACIs, including FDA-approved HDACIs, at therapeutically and clinically relevant doses, alter StAR acetylation patterns and suppress E2 accumulation in both hormone-sensitive human BC and mouse primary cultures of breast tumor epithelial cells. This review provides the molecular insights into breast pathogenesis and its therapeutics, and proposes that a combination therapy involving AI and HDACI, targeting aromatase and StAR, respectively, suppresses intra-tumoral E2 accumulation and limits antagonistic side effects, and these measures are beneficial for the prevention and/or management of hormone-sensitive BC.