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Abstract Disclosure: K.S. Young: None. G. Hancock: None. S. Kregel: None. S.W. Fanning: Grant Recipient; Self; Olema Oncology. Although Estrogen Receptor positive (ER+) breast cancer has been successfully targeted in primary tumors, the critical limitations of hormone therapies are ultimately revealed as many women often experience disease recurrence. A subset of these patients harbor a distinctive tyrosine to serine missense mutation at position 537 (Y537S) in ER leading to allele-specific transcriptional programs that enhance metastasis. ER is a ligand-dependent master transcriptional regulator. Probing unique ER structural features can reveal new transcriptional activities. We developed a structurally unconventional Selective Estrogen Receptor Modulator (SERM), T6I-29, to better understand these structure-transcriptional relationships. T6I-29 shows effective anti-tumoral activities in Y537S ESR1 breast cancer cells. To examine the anti-tumoral mechanism of T6I-29 action, we performed RNA-sequencing on ESR1 Y537S mutant breast cancer cells. Compared to other clinically relevant compounds, our drug uniquely and significantly downregulated DKK1, a tumor secreted protein known to modulate the Wnt/β-Catenin and PI3K/Akt pathways. Upregulation of DKK1 correlates with metastatic burden across multiple cancers but has not been evaluated in ESR1 mutant breast cancers. We find that DKK1 levels are significantly elevated in the plasma of 108 ER+ breast cancer patients compared to 100 matched healthy donors. Furthermore, in wild-type (WT) PIK3CA breast cancer cell lines, those with Y537S ESR1 mutation show constitutive expression and secretion of DKK1, while DKK1 expression and secretion is estrogen-dependent in WT breast cancer cells. Intriguingly, WT cells show significant surface bound DKK1, which is secreted into the media upon hormone stimulation and blocked by T6I-29. Understanding the ER-dependence and functional importance of DKK1 secretion offers a novel approach to potentially reduce breast cancer metastatic burden. Presentation: 6/3/2024
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