Combining hedgehog signaling inhibition with focal irradiation on reduction of pancreatic cancer metastasis

Abstract

Pancreatic cancer often presents in advanced stages and is unresponsive to conventional treatments. Thus, the need to develop novel treatment strategies for pancreatic cancer has never been greater. Here, we report that combination of focal irradiation with hedgehog (Hh) signaling inhibition exerts better than additive effects on reducing metastases. In an orthotopic model, we found that focal irradiation alone effectively reduced primary tumor growth but did not significantly affect metastasis. We hypothesized that cancer stem cells (CSC) of pancreatic cancer are responsible for the residual tumors following irradiation, which may be regulated by Hh signaling. To test our hypothesis, we showed that tumor metastasis in our model was accompanied by increased expression of CSC cell surface markers as well as Hh target genes. We generated tumor spheres from orthotopic pancreatic and metastatic tumors, which have elevated levels of CSC markers relative to the parental cells and elevated expression of Hh target genes. Irradiation of tumor spheres further elevated CSC cell surface markers and increased Hh target gene expression. Combination of Hh signaling inhibition with radiation had more than additive effects on tumor sphere regeneration in vitro. This phenotype was observed in two independent cell lines. In our orthotopic animal model, focal radiation plus Hh inhibition had more than additive effects on reducing lymph node metastasis. We identified several potential molecules in mediating Hh signaling effects. Taken together, our data provide a rationale for combined use of Hh inhibition with irradiation for clinical treatment of patients with pancreatic cancer.