Studying the Genomic Role of Cathepsin-D in ER(+) Breast Cancer

Estrogen receptor alpha (ERα), a transcription factor implicated in induction of cell growth in breast cancer, is a therapeutic target that is expressed in >70% of breast tumors. The transcriptional activity of ERα is controlled by ligands and increased through its interaction with co-activators such as the p160/SRC and p300/CBP families. In an attempt to identify the ligand-specific protein complexes involved in transcriptional regulation by ERα, BioID and TurboID screens were performed in two ER+ breast cancer cell lines, T-47D and ZR-75-1. Surprisingly, Cathepsin-D (Cath-D), a lysosomal aspartyl endoproteinase that is an ER target gene, was identified in these screens. Cath-D expression is associated with a poor prognosis and increased metastasis rate in breast cancer irrespective of its catalytic activities {Glondu, 2001 #119}[i]. Cath-D is localized in part to the nucleus where it interacts with TRPS1, a repressor of GATA-mediated transcription and modulator of ERα signaling {Bach, 2015 #117}[ii]. Co-silencing Cath-D and TRPS1 suppressed cell proliferation and inhibited growth under soft agar, suggesting that they cooperate to drive tumorigenesis {Bach, 2015 #117}[ii]. We hypothesized that Cath-D plays genomic as well as non-genomic roles in breast tumor aggressiveness and may alter ERα-mediated transcription. The nuclear localization of Cath-D was confirmed by immunofluorescence using different commercialized antibodies and observed in western blots of chromatin-bound fractions in three different ERα+ breast cancer cell lines, T-47D, ZR-75 and MCF-7. Specificity of the antibodies was confirmed using siRNA-mediated suppression of Cath-D. Moreover, Cath-D was also identified in proximity to TurboID-ERα by LC-MS after chromatin fractionation. The proximity of ERα and Cath-D both in the cytoplasm and nucleus was confirmed by proximity Ligation Assay (PLA) in three ER+ cell lines. Co-immunoprecipitation assays indicated physical interaction of Cath-D with ERα in T-47D cell extracts. Further, Cath-D was detected by ChIP-qPCR on estrogen response elements (EREs) of two ERα target genes, TFF1 and GREB1 in T-47D and ZR-75 cells. These results suggest that Cath-D can interact with ERα on DNA and play genomic roles in ER+ breast cancer cells. [i] Glondu, M., et al. (2001). “A mutated cathepsin-D devoid of its catalytic activity stimulates the growth of cancer cells.” Oncogene20(47): 6920-6929. [ii] Bach, A. S., et al. (2015). “Nuclear cathepsin D enhances TRPS1 transcriptional repressor function to regulate cell cycle progression and transformation in human breast cancer cells.” Oncotarget6(29): 28084-28103.