Mechanism and evolution of cytosolic Hedgehog signal transduction

Hedgehog (Hh) signaling governs many developmental processes by regulating the balance between the repressor (Ci R /Gli R ) and activator (Ci A /Gli A ) forms of Cubitus interruptus (Ci)/gliomaassociated oncogene homolog (Gli) transcription factors. Although much is known about how Ci R /Gli R is controlled, the regulation of Ci A /Gli A remains poorly understood. Here we demonstrate that Casein kinase 1 (CK1) sustains Hh signaling downstream of Costal2 and Suppressor of fused (Sufu) by protecting Ci A from premature degradation. We show that Hh stimulates Ci phosphorylation by CK1 at multiple Ser/Thr-rich degrons to inhibit its recognition by the Hh-induced MATH and BTB domain containing protein (HIB), a substrate receptor for the Cullin 3 family of E3 ubiquitin ligases. In Hh-receiving cells, reduction of CK1 activity accelerated HIB-mediated degradation of Ci A , leading to premature loss of pathway activity. We also provide evidence that Gli A is regulated by CK1 in a similar fashion and that CK1 acts downstream of Sufu to promote Sonic hedgehog signaling. Taken together, our study not only reveals an unanticipated and conserved mechanism by which phosphorylation of Ci/Gli positively regulates Hh signaling but also provides the first evidence, to our knowledge, that substrate recognition by the Cullin 3 family of E3 ubiquitin ligases is negatively regulated by a kinase.

Section: Significancementioning

confidence: 99%

Hedgehog-induced phosphorylation by CK1 sustains the activity of Ci/Gli activator

Shi¹,

Li²,

et al. 2014

“…Activated Smo accumulates within the primary cilium (10,11) and induces the dissociation of Sufu-Gli complexes (4,5), abrogating Gli2/3R formation and promoting the cilium-dependent conversion of full-length Gli proteins (Gli2/ 3FL) into transcriptionally active forms (Gli2/3A) (12,13). Hh signaling coincides with the steady-state accrual of Gli2/3 at the distal tip of the primary cilium (8,14), and Gli2/3A translocate to the nucleus to drive the expression of Ptch1, the constitutively active factor Gli1, and other Hh target genes (1,15).…”

mentioning

confidence: 99%

Arhgap36-dependent activation of Gli transcription factors

Rack¹,

Ni²,

Payumo³

et al. 2014

Hedgehog (Hh) pathway activation and Gli-dependent transcription play critical roles in embryonic patterning, tissue homeostasis, and tumorigenesis. By conducting a genome-scale cDNA overexpression screen, we have identified the Rho GAP family member Arhgap36 as a positive regulator of the Hh pathway in vitro and in vivo. Arhgap36 acts in a Smoothened (Smo)-independent manner to inhibit Gli repressor formation and to promote the activation of full-length Gli proteins. Arhgap36 concurrently induces the accumulation of Gli proteins in the primary cilium, and its ability to induce Gli-dependent transcription requires kinesin family member 3a and intraflagellar transport protein 88, proteins that are essential for ciliogenesis. Arhgap36 also functionally and biochemically interacts with Suppressor of Fused. Transcriptional profiling further reveals that Arhgap36 is overexpressed in murine medulloblastomas that acquire resistance to chemical Smo inhibitors and that ARHGAP36 isoforms capable of Gli activation are upregulated in a subset of human medulloblastomas. Our findings reveal a new mechanism of Gli transcription factor activation and implicate ARHGAP36 dysregulation in the onset and/or progression of GLI-dependent cancers.

“…In mammals, Hh signal transduction depends on the primary cilium, and ciliary accumulation is required for Smo activation (29)(30)(31)(32) (33). Therefore, cilium may function as a signaling center for the Hh pathway in mammals (34). However, the precise mechanism by which Smo cell surface or ciliary accumulation is regulated remains unclear.…”

mentioning

confidence: 99%

Hedgehog-regulated atypical PKC promotes phosphorylation and activation of Smoothened and Cubitus interruptus in Drosophila

Jiang

Liu

Fan

et al. 2014

Significance Hedgehog (Hh) signaling by Smoothened (Smo) is mediated by phosphorylation and cell surface/cilium accumulation, but how the localization of Smo is controlled remains poorly understood. We show that the atypical PKC (aPKC)–partition defective 6 (Par6) complex promotes Hh signaling by phosphorylating Smo and regulating Smo basolateral accumulation in addition to phosphorylating the transcription factor cubitus interruptus (Ci). Our results demonstrate direct involvement of aPKC in Hh signaling beyond its role in cell polarity and suggest that basolateral accumulation of Smo is critical for its activity. Abnormal activation of Smo results in several types of cancers, and Smo can easily acquire drug resistance through mutations. A better understanding of the mechanisms of Smo regulation is critical to developing more effective therapeutic treatments for cancers caused by Smo dysregulation.