Molecular pathways: The role of primary cilia in cancer progression and therapeutics with a focus on hedgehog signaling

Abnormal Hedgehog (Hh) pathway activity has been reported in many cancers, including basal cell carcinomas, medulloblastomas, rhabdomyosarcomas, glioblastomas, and breast and prostate cancers. For this reason, the Hh pathway is a flourishing area for development of anticancer drugs such as Hh ligand antagonists (e.g., 5E1 and robotnikinin), Smo inhibitors (e.g., GDC-0449 and IPI-926), and Gli transcriptional activity inhibitors (e.g., GANT58 and GANT61). It is now clear that primary cilia are required for activation of the Hh pathway in normal vertebrate cells. It is in the primary cilium that both positive and negative effectors of the Hh pathway are processed by posttranslational modifications. In many cancers, preliminary results suggest that primary cilia are lost. As drugs that inhibit different steps of the Hh pathway are developed, it will be important to consider howthese drugs will function in the context of primary cilia in the tumor environment. Here, we discuss why some of the Hh inhibitors may be ineffective if primary cilia are lost on cancer cells. Understanding the relationships between clinical inhibitors of the Hh pathway and the presence or absence of primary cilia may turn out to be critical for targeting these therapeutics to the correct population of patients and improving their efficacy. Further work is needed in this area to maximize the potential of these exciting therapeutic targets.