Spatially resolved HER2 and AR expression in prostate cancer and its implications for HER2 as a distinct therapeutic target.

385 Background: Androgen receptor (AR) target genes are re-expressed during androgen deprivation therapy which stimulates prostate cancer (PC) proliferation. A promising area of investigation focuses on inhibiting androgen-independent signaling pathways—such as human epidermal growth factor receptor 2 (HER2/ ERBB2 )—that promote metastasis. We recently described the role of HER2 in PC and appealed for deeper investigation in diverse populations. HER2 is a well-characterized oncogenic driver in multiple cancer types, including PC. The higher prevalence of HER2 overexpression among Black men with PC has recently come to light, while differences in AR expression and function in Black men has been more well-described. In this study, we utilized spatial transcriptomics to evaluate basal HER2 and AR expression in a racially diverse patient-derived xenograft (PDX) model as well as immunofluorescence to evaluate changes in HER2 and AR induced with HER2 drug targeting in a diverse panel of PC cell lines. Methods: To resolve the spatial dynamics of ERBB2, AR, and other relevant gene expression in PDXs, we utilized the 10x Genomics Xenium In Situ spatial transcriptomics platform. Tissue sections were processed using Xenium’s 377-gene Human Multi-Tissue Cancer panel, augmented with prostate-specific and ERBB2 -related targets. To assess the effects of HER2 drug targeting on HER2 and AR expression, we performed immunofluorescence staining in treated vs. untreated LNCaP, MDA-PCa-2b, and 22Rv1 PC cell lines. Results: Spatial transcriptomic analysis revealed that the ERBB2 and AR transcript burden per tumor area were significantly higher in a PDX developed from a Black patient compared to a PDX developed from a white patient. Immunofluorescence revealed HER2 was abundantly expressed in the cytoplasmic and membranous compartments of all three cell lines, while AR was strongly localized within the nuclei, consistent with its transcriptional activity and confirmed distinct yet partially overlapping subcellular distributions of HER2 and AR. HER drug targeting led to a consistent reduction in HER2 signal intensity across all models. In LNCaP and 22Rv1 cells derived from white patients, HER2 drug targeting markedly diminished HER2 staining, coinciding with a reduction in nuclear AR signal intensity. MDA-PCa-2b cells derived from a Black patient also exhibited decreased HER2 expression following HER2 drug targeting, with a concurrent modest reduction in AR intensity. This result revealed diminished HER–AR colocalization after HER2 targeting, suggesting disruption of HER2–AR signaling interactions. Conclusions: Findings demonstrate that HER2 blockade reduces HER2 expression in PC cells and alters AR nuclear abundance. These results provide evidence for crosstalk between HER2 and AR pathways in PC models and support the hypothesis that HER2 targeting impairs HER2-driven AR signaling dynamics.