Costal2 is no longer able to bind the other three kinases, and so Ci is no longer cleaved and can now enter the nucleus and activate the transcription of Hedgehog target genes (Figure 15–61B). Among the genes activated by Ci is Patched itself; the resulting increase in Patched protein on the cell surface inhibits further Hedgehog signaling—providing another example of negative feedback.
Many gaps remain in our understanding of the Hedgehog signaling pathway. It is not known, for example, how Patched keeps Smoothened inactive and intracellular. As the structure of Patched resembles a transmembrane transporter protein, it has been proposed that it may transport a small molecule into the cell that keeps Smoothened sequestered in vesicles.
Even less is known about the more complex Hedgehog pathway in vertebrate cells. In addition to there being at least three types of vertebrate Hedgehog proteins, there are three Ci-like transcription regulator proteins (Gli1, Gli2, and Gli3) downstream of Smoothened. Gli2 and Gli3 are most similar to Ci in structure and function, and Gli3 has been shown to undergo proteolytic processing like Ci and to act as either a transcriptional repressor or a transcriptional activator. Moreover, in vertebrates, Smoothened, upon activation, becomes localized to the surface of the primary cilium (discussed in Chapter 16), where the Gli proteins are also concentrated, thereby increasing the speed and efficiency of signaling.
Hedgehog signaling can promote cell proliferation, and excessive Hedgehog signaling can lead to cancer. Inactivating mutations in one of the two human Patched genes, for example, which lead to excessive Hedgehog signaling, occur frequently in basal cell carcinoma of the skin, the most common form of cancer in Caucasians. A small molecule called cyclopamine, made by a meadow lily, is being used to treat cancers associated with excessive Hedgehog signaling. It blocks Hedgehog signaling by binding tightly to Smoothened and inhibiting its activity. It was originally identified because it causes severe developmental defects in the progeny of sheep grazing on such lilies; these include the presence of a single central eye (a condition called cyclopia), which is also seen in mice that are deficient in Hedgehog signaling.
The NFκB proteins are latent transcription regulators that are present in most animal cells and are central to many stressful, inflammatory, and innate immune responses. These responses occur as a reaction to infection or injury and help protect stressed multicellular organisms and their cells (discussed in Chapter 24). An excessive or inappropriate inflammatory response in animals can also damage tissue and cause severe pain, and chronic inflammation can lead to cancer; as in the case of Wnt and Hedgehog signaling, excessive NFκB signaling is found in a number of human cancers. NFκB proteins also have important roles during normal animal development: the Drosophila NFκB family member Dorsal, for example, has a crucial role in specifying the dorsal–ventral axis of the developing fly embryo (discussed in Chapter 22).