The Hedgehog signaling pathway as a target for anticancer drug discovery

The Hedgehog (Hh) signaling pathway directs the development of multiple tissues during embryonic development, and contributes to tissue homeostasis in adults. Deficient Hh signaling results in defective embryogenesis; conversely, excessive Hh signaling is associated with an inherited cancer predisposition syndrome (Gorlin Syndrome), and a growing list of sporadic human cancers. It is now clear that multiple components of "The Hh Pathway" can be altered in tumors. The Hhs are morphogens that signal through effectors that are largely unprecedented in drug discovery, with many key concepts derived from studies in Drosophila melanogaster. However, studies in tumor cell lines have recently identified targets that can be exploited for the discovery of human Hh antagonists, with additional targets likely to emerge as the human pathway is further defined. Here, we review basic aspects of Hh signal transduction, with an emphasis on molecular targets for drug discovery. The use of first-generation Hh antagonists such as cyclopamine will also be discussed; such agents remain invaluable in ongoing efforts to validate drug discovery assays and survey tumor lines for Hh dependence. The various types and frequencies of Hh signaling defects in different human tumors will also be reviewed, as will the status of medicinal chemistry efforts to discover novel Hh antagonists. In section VI, we review assays from the literature that could be utilized to discover new Hh antagonists for the treatment of cancer.