Steroid ligands, the forgotten triggers of nuclear receptor action; implications for acquired resistance to endocrine therapy

Purpose: There is strong epidemiological evidence which indicates that estrogens may not be the sole steroid drivers of breast cancer. We hypothesize that abundant adrenal androgenic steroid precursors, acting via the androgen receptor (AR), promote an endocrine resistant breast cancer phenotype.

Experimental design: AR was evaluated in a primary breast cancer tissue-microarray (n=844). Androstenedione (4AD) levels were evaluated in serum samples (n=42) from hormone receptor positive, post-menopausal breast cancer. Levels of androgens, progesterone and estradiol were quantified using LC-MS/MS in serum from age and grade-matched recurrent and non-recurrent patients (n=6) pre- and post-aromatase inhibitor (AI) therapy (>12 months). Androgen and estrogen receptor signaling pathways activities were analyzed in two independent AI treated cohorts.

Results: AR protein expression was associated with favorable progression-free survival in the total population (Wilcoxon, p<0.001). Breast cancer patients pre-therapy showed decreasing levels of 4AD with age only in the non-recurrent group (p<0.05). LC-MS/MS analysis of an AI sensitive and resistant cohort demonstrated the ability to detect altered levels of steroids in serum of patients pre and post AI-therapy. Transcriptional analysis showed an increased ratio of AR: ER signaling pathway activities in patients failing AI therapy (t-test p<0.05), furthermore, 4AD mediated gene changes associated with acquired AI resistance.

Conclusions: This study highlights the importance of examining the therapeutic consequences of the steroid microenvironment and demonstrable receptor activation using indicative gene expression signatures.