Background: The cyclin dependent kinase (CDK) 4/6 inhibitor palbociclib plus endocrine therapy is an effective treatment for patients with estrogen receptor (ER)-positive and HER2-negative breast cancer (BC). Notwithstanding promising clinical outcomes, intrinsic or acquired resistance frequently arises. Given the established role of cancer-associated fibroblasts (CAFs) in the resistance to chemotherapeutics and the implication of the G-protein estrogen coupled receptor (GPER) in mediating stromal functions, we sought to investigate whether CAFs may contribute to palbociclib resistance in BC cells via GPER.
Methods: MCF7 and T47D BC cells, as well as CAFs isolated from breast tumor tissues, were used as model systems. Docking experiments allowed us to evaluate the potential interaction of palbociclib with GPER. Immunoblotting, gene expression arrays, and gene silencing experiments along with 2D and 3D co-culture assays were performed to evaluate the activation of GPER signaling by palbociclib in CAFs. Investigations on large cohorts of BC patients served to assess new potential stromal targets involved in palbociclib resistance.
Results: First, docking studies showed that palbociclib can interact with the binding site of GPER, like its well-known ligands. Next, we found that in CAFs, palbociclib stimulates the main sensors of GPER activation like ERK1/2 activation and c‑Fos up-regulation. To assess the transcriptional changes mediated by palbociclib through GPER in CAFs, we performed a human chemokine gene expression array. Notably, we found that the palbociclib-induced increase of pro-inflammatory genes is no longer evident upon silencing GPER expression. Finally, GPER silencing in CAFs interfered with the capacity of palbociclib to reduce the viability of BC cells in co-culture assays.
Conclusions: Our findings suggest that the activation of GPER signaling in CAFs may contribute to palbociclib resistance in breast cancer cells. On this basis, therapeutic strategies targeting CAFs might enhance the efficacy of palbociclib in BC patients.
