Bioengineered human bone-mimetic niche compositions regulate breast cancer cell quiescence and therapy response

Abstract Estrogen receptor-positive (ER+) breast cancer (BC) frequently metastasizes to bone, yet understanding how distinct bone compartments influence metastatic progression and endocrine therapy response has been limited by the scarcity of physiologically relevant human multicellular culture platforms. To address this gap, we developed a fully human, 3D bone-mimetic platform that enables modular assembly of engineered osteoblastic (eON), vascularized (eVN), and vascularized osteoblastic (eVON) niches. These niches were generated by perfusion culture of human bone marrow-derived mesenchymal stromal cells (hBM-MSCs) and/or adipose-derived stromal vascular fraction (hAT-SVF) cells within porous ceramic scaffolds. The engineered niches were seeded with an ER+ BC cell line expressing a mutant p27 reporter to track quiescent cell state or with cells from patient-derived xenograft tumor organoids (PDXOs). Using both the ER+ BC cell line and PDXOs, we consistently found that eON enhanced BC proliferation, whereas eVN enriched nuclear NR2F1+ quiescent cells. We further investigated, as a proof of concept, the potential use of our model as a drug testing platform. Treatment with the selective ER degrader fulvestrant reduced cancer cell numbers only in vascularized niches (i.e., eVN and eVON), despite comparable receptor degradation, indicating niche-dependent therapeutic responses. These findings establish a modular human platform for investigating regulation of BC quiescence, proliferation, and endocrine therapy response by different bone niche compositions that in vivo are otherwise anatomically and functionally coupled and cannot be independently interrogated.