The Hedgehog (Hh) morphogenic pathway is a conserved system for regulating development and cell fate. The paracrine Hh signalling pathway has been unambiguously linked to development and aggressiveness of the TNBC subtype. Using single cell transcriptomics we now demonstrate that Hh ligand produced by neoplastic cells leads to Hh pathway activation exclusively in the cancer-associated fibroblast (CAFs) population. Hh-activated CAFs subsequently orchestrate an intricate crosstalk with the adjacent breast cancer (BrCa) compartment via remodelling of the collagen matrix and activation of the FGFR pathway. This expands BrCa subpopulations are enriched for stem cell-like properties, marked by the expression of the mammary stem markers CK6 and ALDH1, specifically at the tumour-stromal interface. Co-culture of TNBC cells with primary CAFs or treatment with recombinant FGF5 in vitro enriches for a cancer stem cell gene signature and markedly increased long-term sphere-forming capacity.
Importantly, treatment of mouse models of TNBC with Hh pathway inhibitors significantly reduces the expression of FGF5 by CAFs and stem cell markers by the neoplastic population. Furthermore, combination therapy of Smoothened inhibitors (SMOi) plus docetaxel chemotherapy dramatically improved survival and reduced metastatic burden in mice bearing patient-derived xenografts (PDXs), compared to single agent arms.
In a phase I clinical trial, we show that the SMOi Sonidegib can be safely administered with docetaxel to patients with metastatic TNBC. Two patients with Hh pathway activation showed evidence of clinical benefit, with one patient experiencing a complete response.
These studies identify the Hh-FGF axis as a novel paracrine mediator of cancer stem cell plasticity and strongly highlight an exciting new therapeutic opportunity targeting both tumour cells and their surrounding signalling support in TNBC.