Parathyroid Hormone‐Related Protein Inhibition Blocks Triple‐Negative Breast Cancer Expansion in Bone Through Epithelial to Mesenchymal Transition Reversal

Parathyroid hormone‐related protein (PTHrP) plays a major role in skeletal metastasis but its action mechanism has not been fully defined. We previously demonstrated the crucial importance of PTHrP in promoting mammary tumor initiation, growth, and metastasis in a mouse model with a mammary epithelium‐targeted Pthlh gene ablation. We demonstrate here a novel mechanism for bone invasion involving PTHrP induction of epithelial to mesenchymal transition (EMT) and cancer stem cells (CSCs) regulation. Clustered regularly interspaced short palindromic repeats (CRISPR)‐mediated Pthlh gene ablation was used to study EMT markers, phenotype, and invasiveness in two triple‐negative breast cancer (TNBC) cell types (established MDA‐MB‐231 and patient‐derived PT‐TNBC cells). In vitro, Pthlh ablation in TNBC cells reduced EMT markers, mammosphere‐forming ability, and CD44(high)/CD24(low) cells ratio. In vivo, cells were injected intratibially into athymic nude mice, and therapeutic treatment with our anti‐PTHrP blocking antibody was started 2 weeks after skeletal tumors were established. In vivo, compared to control, lytic bone lesion from Pthlh ‐ablated cells decreased significantly over 2 weeks by 27% for MDA‐MB‐231 and by 75% for PT‐TNBC‐injected mice (p < 0.001). Micro‐CT (μCT) analyses also showed that antibody therapy reduced bone lytic volume loss by 52% and 48% for non‐ablated MDA‐MB‐231 and PT‐TNBC, respectively (p < 0.05). Antibody therapy reduced skeletal tumor burden by 45% and 87% for non‐ablated MDA‐MB‐231 and PT‐TNBC, respectively (p < 0.002) and caused a significant decrease of CSC/EMT markers ALDH1, vimentin, and Slug, and an increase in E‐cadherin in bone lesions. We conclude that PTHrP is a targetable EMT molecular driver and suggest that its pharmacological blockade can provide a potential therapeutic approach against established TNBC‐derived skeletal lesions. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.