THU498 Establishing The Link Between Glucocorticoid Signaling And The Circadian Regulator DEC1 In Breast Cancer

Disclosure: M.I. Castro: None. W.S. Ybañez: None. P.D. Bagamasbad: None. Epidemiological studies have linked altered circadian rhythms due to irregular shift work, chronic jet lag, and heightened night-time light exposure to increased breast cancer (BCa) pathogenesis. Dysregulated glucocorticoid (GC; CORT) signaling in BCa may further promote chronodisruption by directly regulating the cyclic expression of clock genes found in the breast. The differentiated embryo-chondrocyte expressed gene 1 (DEC1; also known as STRA13/SHARP2/ BHLHB2) is a transcription factor involved in the regulation of circadian rhythmicity and tumor progression of several cancer types including the breast and associated with hormone receptor signaling in BCa. In this study, we explored the involvement of DEC1 in the crosstalk between GC signaling and the circadian axis in BCa. To establish DEC1 expression pattern in BCa tumor samples, we assessed publicly available RNA-seq datasets from the Genotype-Tissue Expression Project and found no significant difference in DEC1 expression in tumors when compared to normal tissue controls. However, when tumor samples were stratified into molecular subtypes based on transcriptomic data from the Cancer Genome Atlas, DEC1 expression was significantly downregulated in triple-negative BCa (TNBC) in comparison to luminal subtypes. This pattern of expression was reflected in the BCa cell lines where we observed higher DEC1 expression in the estrogen receptor-positive MCF7 cells relative to nontumorigenic triple-negative MCF10A cells and the highly aggressive TNBC MDA-MB-231 cell line. To determine if DEC1 is regulated by GCs in the breast, we treated non-tumorigenic breast epithelial MCF10A cells and the TNBC MDA-MB-231 cells with increasing concentrations of CORT and observed that DEC1 mRNA expression decreases in response to increasing concentrations of CORT. Treatment of MCF10A and MDA-MB-231 cells with GC receptor (GR)- specific agonist RU486 abolished the GC-dependent repression of DEC1 expression and this pattern of DEC1 expression persisted even in the presence of the protein synthesis inhibitor cycloheximide, suggesting that DEC1 is a direct transcriptional target of GR in mammary epithelial cells. Through in silico analysis of publicly available ChIP-sequencing datasets from BCa cell lines, we identified multiple CORT-responsive cis-regulatory elements in the DEC1 locus that could mediate the CORT-dependent repression of DEC1 expression. Lentiviral knockdown of DEC1 in three breast epithelial cell lines was generated and will be used in cancer hallmark assays on proliferation, apoptosis, survival, and migration to assess the functional role of DEC1 in BCa. Taken together, our data will contribute to understanding the combined effect of GC signaling and circadian gene regulation towards BCa progression. Presentation: Thursday, June 15, 2023