Glucocorticoid Compounds Modify Smoothened Localization and Hedgehog Pathway Activity

Abstract

Summary The Hedgehog signaling pathway is linked to a variety of diseases, notably a range of cancers. The first generation of drug screens identified Smoothened (Smo), a membrane protein essential for signaling, as an attractive drug target. Smo localizes to the primary cilium upon pathway activation, and this transition is critical for the response to Hedgehog ligands. In a high content screen directly monitoring Smo distribution in Hedgehog responsive cells, we identified different glucocorticoids as specific modulators of Smo ciliary accumulation. One class promoted Smo accumulation, conferring cellular hypersensitivity to Hedgehog stimulation. In contrast, a second class inhibited Smo ciliary localization and signaling activity by both wildtype Smo, and mutant forms of Smo, SmoM2 and SmoD473H, that are refractory to previously identified Smo antagonists. These findings point to the potential for developing glucocorticoid-based pharmacological modulation of Smo signaling to treat mutated drug-resistant forms of Smo, an emerging problem in long-term cancer therapy. They also raise a concern about potential crosstalk of glucocorticoid drugs in the Hedgehog pathway, if therapeutic administration exceeds levels associated with on-target transcriptional mechanisms of glucocorticoid action.

Smoothened Goes Molecular: New Pieces in the Hedgehog Signaling Puzzle

“…4B), suggesting that sterol binding by the Smo CRD may be a conserved feature of Hh signaling (69). Glucocorticoids represent an interesting class of Smo modulators as both inhibitors and activators of the Hh pathway have been found with glucocorticoid scaffolds, and budesonide inhibits WT Smo, SmoD473H, and SmoM2 equally well, ideal features for a Smo-targeting drug (70).…”

Novel Insights into the Role of the Smoothened Cysteine Rich Domain in Hedgehog Signalling

“…They could be categorized into two groups: the activatory glucocorticoids and the inhibitory glucocorticoids. As the name suggests activatory glucocorticoids like fluocinolone acetonide (FA) and triamcinolone acetonide (TA) promoted Smo ciliary translocation and downstream Hh signalling [93]. The inhibitory glucocorticoids such as Budesonide (Bud) repressed Smo ciliary translocation and downstream Hh signalling [93].…”

“…In recent times oxysterols and glucocorticoids have been identified as two major groups of small molecules which bind to a site distinct from the cyclopamine binding site in the cavity of the 7TM domains [92][93][94]. Pharmacological and biochemical studies have shown that 20 (S)-hydroxyl cholesterol (20(S)-OHC) acts as an allosteric activator of vertebrate Hh signalling, functioning at the level of Smo [92].…”

“…Glucocorticoids were identified as Smo modulators in high-content cell-based screening experiments [93]. They could be categorized into two groups: the activatory glucocorticoids and the inhibitory glucocorticoids.…”

“…As the name suggests activatory glucocorticoids like fluocinolone acetonide (FA) and triamcinolone acetonide (TA) promoted Smo ciliary translocation and downstream Hh signalling [93]. The inhibitory glucocorticoids such as Budesonide (Bud) repressed Smo ciliary translocation and downstream Hh signalling [93]. Interestingly, although the activatory glucocorticoids were shown to bind in the cavity of the Smo 7TM domains by cyclopamine binding competition assays, the inhibitory glucocorticoids were suggested to bind to a site distinct from the cyclopamine binding site in the 7TM domains [93].…”