Smoothened

Gene found in humans and other animals

Smoothened is a protein that in humans is encoded by the SMO gene. Smoothened is a Class Frizzled (Class F) G protein coupled receptor that is a component of the hedgehog signaling pathway and is conserved from flies to humans. It is the molecular target of the natural teratogen cyclopamine. It also is the target of vismodegib, the first hedgehog pathway inhibitor to be approved by the U. S. Food and Drug Administration (FDA). Smoothened (Smo) is a key transmembrane protein that is a key component of the hedgehog signaling pathway, a cell cell communication system critical for embryonic development and adult tissue homeostasis. Mutations in proteins that relay Hh signals between cells cause birth defects and cancer. The protein that carries the Hh signal across the membrane is the oncoprotein and G protein coupled receptor (GPCR) Smoothened (Smo). Smo is regulated by a separate transmembrane receptor for Hh ligands called Patched (Ptc). Ptc itself is a tumor suppressor that keeps the Hh pathway off by inhibiting Smo. The excessive Hh signaling that drives human skin and brain cancer is most frequently caused by inactivating mutations in Ptc or by gain of function mutations in Smo. While direct Smo agonists and antagonists, such as SAG and vismodegib, can bind to and activate or inhibit Smo, how Ptc inhibits Smo endogenously remains a mystery in the field. Currently, Smo is targeted and inhibited directly by a small molecule drug, vismodegib, for the treatment of advanced basal cell cancer, however widespread resistance to this drug has become a prevalent issue. Finding another method to target Smo activity in Hh driven cancers would provide valuable information for novel therapeutics. Identifying these Ptc responsive sites on Smo will help solve a long standing mystery in Hh signaling and suggest new therapeutic strategies to block Smo activity in Hh driven cancers.

Cellular localization plays an essential role in the function of SMO, which anchors to the cell membrane as a 7 pass transmembrane protein. Stimulation of the patched 12 pass transmembrane receptor by the sonic hedgehog ligand leads to translocation of SMO to the primary cilium in vertebrates in a process that involves the exit of patched from the primary cilium, where it normally localizes in its unstimulated state. Vertebrate SMO that is mutated in the domain required for ciliary localisation often cannot contribute to hedgehog pathway activation. Conversely, SMO can become constitutively localized to the primary cilium and potentially activate pathway signaling constitutively as a result of a tryptophan to leucine mutation in the aforementioned domain. In invertebrates like Drosophila, SMO does not organize at cilia and instead is generally translocated to the plasma membrane following hedgehog binding to patched. There is evidence for the existence of an unidentified endogenous ligand that binds SMO and activates it. It is believed that mutations in SMO can mimic the ligand induced conformation of SMO and activate constitutive signal transduction. SMO has been shown to bind Costal 2 and play a role in the localization of the Ci complex and prevention of Ci cleavage. Additionally, it is known that vertebrate SMO contributes to the activation of Gli as a transcription factor via association with ciliary structures such as Evc2, but these mechanisms are not fully understood. It has been proposed that cholesterol activates Smo, and subsequently Hh signaling, by entering the active site through a hydrophobic “oxysterol tunnel,” which can adopt open or closed conformations to allow for activation or inactivation of Smo, respectively, due to allowed sterol binding. Shh would work by inhibiting Ptc, which would increase accessible cholesterol concentrations and allow for the activation of Smo and transmission of the Hh signal. A recent crystal structure has identified two sterol binding sites in Smo, but which site is endogenously regulated by Ptc remains to be determined. The potential sites of regulation include the extracellular cysteine rich domain (CRD) of Smo, as well as a site deep within the transmembrane domain (TMD). Due to the abundance of cholesterol in the plasma membrane (up to 50 mole %), it has also been proposed that Ptc regulates the activity of Smo by controlling cholesterol accessibility specifically within the membrane of the primary cilia, which contains a less abundant, and therefore more readily regulated pool of accessible cholesterol. Typically, upon activation and release of inhibition by Ptc, Smo will relocate to the primary cilia and Ptc will diffuse out of the ciliary membrane. Upon inactivation, Smo no longer becomes concentrated in the ciliary membrane, This hypothesis is supported by methods which can increase or deplete the accessible cholesterol pool, with a subsequent increase or decrease in Hh signaling. This accessible cholesterol pool has been shown to be distinct from the general plasma membrane cholesterol pool in being available for protein interaction and cell uptake. The ciliary membrane has also been shown to contain lower levels of accessible cholesterol due to sequestering of cholesterol by sphingomyelin. In addition to cholesterol’s role as a Hh pathway agonist, it has been shown that cholesterol levels within the ciliary membrane rapidly increase upon treatment with Shh only in the presence of Ptc, further suggesting Ptc regulation of accessible cholesterol as the mechanism behind Smo activation/inhibition. This suggests the hypothesis that Ptc functions by preventing Smo access to cholesterol, and upon Ptc inhibition by Shh, Smo gains access to cholesterol and is subsequently activated, transmitting the Hh signal.

SMO can function as an oncogene. Activating SMO mutations can lead to unregulated activation of the hedgehog pathway and serve as driving mutations for cancers such as medulloblastoma, basal cell carcinoma, pancreatic cancer, and prostate cancer. As such, SMO is an attractive cancer drug target, along with the many hedgehog pathway agonists and antagonists that are known to directly target SMO. For example, oxysterol 20(S) OHC is known to activate vertebrate SMO by binding the cysteine rich domain near its extracellular amino terminal region. In the context of cancer, 20(S) OHC is the target of a proposed anti cancer oxysterol binding inhibitor.