Proteomic signatures identify significantly different inflammatory breast tissue microenvironments depending on mammographic density or estrogen exposure.

Breast density and estrogen exposure are two major risk factors for breast cancer; however, the underlying biological mechanisms remain incompletely understood. Here, we investigated the extracellular proteomic landscape of normal human breast tissue in situ to define the microenvironment associated with these risk factors. Forty-two postmenopausal women with nondense or dense breasts and 19 premenopausal women underwent microdialysis. We quantified 461 inflammatory proteins in breast tissue and matched subcutaneous fat, enabling discrimination between local and systemic alterations. Breast density was assessed using magnetic resonance imaging. Dense breast tissue exhibited a distinct protein signature characterized by immune-related signaling, altered lipid metabolism, and the extracellular presence of intracellular proteins, consistent with cellular stress and immune modulation, with limited changes in angiogenic and extracellular matrix remodeling proteins. In contrast, estrogen-exposed breasts displayed a proteomic profile dominated by pro-inflammatory cytokines, angiogenic factors, extracellular matrix remodeling proteins, and complement activation, indicative of a dynamic and pro-tumorigenic microenvironment. These findings demonstrate that breast density and estrogen exposure are associated with fundamentally distinct breast microenvironments, both permissive for tumor progression. These breast-specific protein signatures provide mechanistic insight into how these risk factors contribute to cancer development and suggest that effective prevention strategies may require differential targeting.