Hedgehog signaling downregulates suppressor of fused through the HIB/SPOP-Crn axis in Drosophila

The Proteasome and Cul3-Dependent Protein Ubiquitination Is Required for Gli Protein-Mediated Activation of Gene Expression in the Hedgehog Pathway

Many cellular processes are regulated by proteasome-mediated protein degradation, including regulation of signaling pathways and gene expression. Among the pathways regulated by the ubiquitin-proteasome system is the Hedgehog pathway and its downstream effectors, the Gli transcription factors. Here we provide evidence that proteasomal activity is necessary for maintaining the activation of the Hedgehog pathway, and this crucial event takes place at the level of Gli proteins. We undertook extensive work to demonstrate the specificity of the observed phenomenon by ruling out the involvement of primary cilium, impaired nuclear import, failed dissociation from Sufu, microtubule stabilization, and stabilization of Gli repressor forms. Moreover, we showed that proteasomal-inhibition-mediated Hedgehog pathway downregulation is not restricted to the NIH-3T3 cell line. We demonstrated, using CRISPR/Ca9 mutagenesis, that neither Gli1, Gli2, nor Gli3 are solely responsible for the Hedgehog pathway downregulation upon proteasome inhibitor treatment, and that Cul3 KO renders the same phenotype. Finally, we report two novel E3 ubiquitin ligases, Btbd9 and Kctd3, known Cul3 interactors, as positive Hedgehog pathway regulators. Our data pave the way for a better understanding of the regulation of gene expression and the Hedgehog signaling pathway.

HIB/SPOP inhibits Ci/Gli-mediated tumorigenesis by modulating the RNA Polymerase II components stabilities

Hedgehog (Hh) signaling mediated by transcription factor Ci/Gli plays a vital role in embryonic development and adult tissue homeostasis in invertebrates and vertebrates, whose dysregulation leads to many human disorders, including cancer. However, till now, cofactors of Ci/Gli which can affect tumorigenesis are not well known. Here, through genetic screen, we find overexpression of active Ci alone is not sufficient to generate tumor-like eye phenotype in Drosophila, however, its overexpression combined with knockdown of hib causes a striking tumor-like big eye phenotype. Mechanistically, HIB/SPOP inhibits Ci/Gli-mediated tumorigenesis by modulating the RNA polymerase II (RNAPII) components Rpb3/Rpb7 stabilities in E3 ligase dependent manner. In addition, Ci/Gli can promote HIB/SPOP-mediated Rpb7/Rpb3 degradation. Taken together, our results indicate Ci/Gli needs to hook up with suitable RNAPII together to achieve the tumor-like eye phenotype and HIB/SPOP plays dual roles through controlling Ci/Gli and Rpb3/Rpb7 protein stabilities to temper Ci/Gli/RNAPII-mediated tumorigenesis.

Dissection of the microRNA Network Regulating Hedgehog Signaling in Drosophila

The evolutionarily conserved Hedgehog (Hh) signaling plays a critical role in embryogenesis and adult tissue homeostasis. Aberrant Hh signaling often leads to various forms of developmental anomalies and cancer. Since altered microRNA (miRNA) expression is associated with developmental defects and tumorigenesis, it is not surprising that several miRNAs have been found to regulate Hh signaling. However, these miRNAs are mainly identified through small-scale in vivo screening or in vitro assays. As miRNAs preferentially reduce target gene expression via the 3' untranslated region, we analyzed the effect of reduced expression of core components of the Hh signaling cascade on downstream signaling activity, and generated a transgenic Drosophila toolbox of in vivo miRNA sensors for core components of Hh signaling, including hh, patched (ptc), smoothened (smo), costal 2 (cos2), fused (fu), Suppressor of fused (Su(fu)), and cubitus interruptus (ci). With these tools in hand, we performed a genome-wide in vivo miRNA overexpression screen in the developing Drosophila wing imaginal disc. Of the twelve miRNAs identified, seven were not previously reported in the in vivo Hh regulatory network. Moreover, these miRNAs may act as general regulators of Hh signaling, as their overexpression disrupts Hh signaling-mediated cyst stem cell maintenance during spermatogenesis. To identify direct targets of these newly discovered miRNAs, we used the miRNA sensor toolbox to show that miR-10 and miR-958 directly target fu and smo, respectively, while the other five miRNAs act through yet-to-be-identified targets other than the seven core components of Hh signaling described above. Importantly, through loss-of-function analysis, we found that endogenous miR-10 and miR-958 target fu and smo, respectively, whereas deletion of the other five miRNAs leads to altered expression of Hh signaling components, suggesting that these seven newly discovered miRNAs regulate Hh signaling in vivo. Given the powerful effects of these miRNAs on Hh signaling, we believe that identifying their bona fide targets of the other five miRNAs will help reveal important new players in the Hh regulatory network.

Engrailed, Suppressor of fused and Roadkill modulate the Drosophila GLI transcription factor Cubitus interruptus at multiple levels

Morphogen gradients need to be robust, but may also need to be tailored for specific tissues. Often this type of regulation is carried out by negative regulators and negative feedback loops. In the Hedgehog (Hh) pathway, activation of patched (ptc) in response to Hh is part of a negative feedback loop limiting the range of the Hh morphogen. Here, we show that in the Drosophila wing imaginal disc two other known Hh targets genes feed back to modulate Hh signaling. First, anterior expression of the transcriptional repressor Engrailed modifies the Hh gradient by attenuating the expression of the Hh pathway transcription factor cubitus interruptus (ci), leading to lower levels of ptc expression. Second, the E-3 ligase Roadkill shifts the competition between the full-length activator and truncated repressor forms of Ci by preferentially targeting full-length Ci for degradation. Finally, we provide evidence that Suppressor of fused, a negative regulator of Hh signaling, has an unexpected positive role, specifically protecting full-length Ci but not the Ci repressor from Roadkill.

Ci/Gli Phosphorylation by the Fused/Ulk Family Kinases

Hedgehog (Hh) signaling culminates in the conversion of the latent transcription factor Cubitus interruptus (Ci)/Gli from a repressor form (Ci^R/Gli^R) into an activator form (Ci^A/Gli^A). While sequential phosphorylation of Ci/Gli by protein kinase A(PKA), glycogen synthase kinase 3 (GSK3), and casein kinase 1 (CK1) is essential for its proteolytic processing that generates Ci^R/Gli^R, sequential phosphorylation of Ci/Gli by the Fused (Fu)/Unc-51 like kinase (Ulk) family kinases Fu/Ulk3/Stk36 and CK1 contributes to the formation of Ci^A/Gli^A. Fu/Ulk3/Stk36-mediated phosphorylation of Ci/Gli is stimulated by Hh, leading to altered interaction between Ci/Gli and the Hh pathway repressor Sufu. Here we describe both in vitro and in vivo assays that determine Ci/Gli phosphorylation by the Fu/Ulk family kinases and its regulation by Hh.

Regulation of Hedgehog Signal Transduction by Ubiquitination and Deubiquitination

The Hedgehog (Hh) family of secreted proteins governs embryonic development and adult tissue homeostasis in species ranging from insects to mammals. Deregulation of Hh pathway activity has been implicated in a wide range of human disorders, including congenital diseases and cancer. Hh exerts its biological influence through a conserved signaling pathway. Binding of Hh to its receptor Patched (Ptc), a twelve-span transmembrane protein, leads to activation of an atypical GPCR family protein and Hh signal transducer Smoothened (Smo), which then signals downstream to activate the latent Cubitus interruptus (Ci)/Gli family of transcription factors. Hh signal transduction is regulated by ubiquitination and deubiquitination at multiple steps along the pathway including regulation of Ptc, Smo and Ci/Gli proteins. Here we review the effect of ubiquitination and deubiquitination on the function of individual Hh pathway components, the E3 ubiquitin ligases and deubiquitinases involved, how ubiquitination and deubiquitination are regulated, and whether the underlying mechanisms are conserved from Drosophila to mammals.

SPOP-PTEN-SUFU axis promotes progression of clear cell renal cell carcinoma via activating SHH and WNT pathway

Although E3 ligase Speckle type BTB/POZ protein (SPOP) promotes tumorigenesis by acting as a key regulatory hub in clear cell renal cell carcinoma (ccRCC), the detailed molecular mechanism remains unclear. Here, we demonstrate that a well-known tumor suppressor, Suppressor of Fused (SUFU), is downregulated by SPOP. Interestingly, this downregulation depends on cullin-3(Cul3)-SPOP E3 ligase, but SUFU is not a direct substrate of SPOP. Phosphatase and tensin homolog (PTEN), a ubiquitinated substrate of SPOP, is involved in SPOP-mediated SUFU reduction. Importantly, inhibition of SUFU leads to elevated SHH and WNT signaling, consequently rescuing the reduced proliferation, migration, and invasion abilities of ccRCC cells caused by SPOP-knockdown. Moreover, combinatorial treatment with SHH and WNT inhibitors shows more effective for suppressing ccRCC cell proliferation and aggressiveness. These findings demonstrate that a novel SPOP–PTEN–SUFU axis promotes ccRCC carcinogenesis by activating SHH and WNT pathway, providing a new treatment strategy for ccRCC.

Positive feedback of SuFu negating protein 1 on Hedgehog signaling promotes colorectal tumor growth

Hedgehog (Hh) signaling plays a critical role in embryogenesis and tissue homeostasis, and its deregulation has been associated with tumor growth. The tumor suppressor SuFu inhibits Hh signaling by preventing the nuclear translocation of Gli and suppressing cell proliferation. Regulation of SuFu activity and stability is key to controlling Hh signaling. Here, we unveil SuFu Negating Protein 1 (SNEP1) as a novel Hh target, that enhances the ubiquitination and proteasomal degradation of SuFu and thus promotes Hh signaling. We further show that the E3 ubiquitin ligase LNX1 plays a critical role in the SNEP1-mediated degradation of SuFu. Accordingly, SNEP1 promotes colorectal cancer (CRC) cell proliferation and tumor growth. High levels of SNEP1 are detected in CRC tissues and are well correlated with poor prognosis in CRC patients. Moreover, SNEP1 overexpression reduces sensitivity to anti-Hh inhibitor in CRC cells. Altogether, our findings demonstrate that SNEP1 acts as a novel feedback regulator of Hh signaling by destabilizing SuFu and promoting tumor growth and anti-Hh resistance.

A functional genomics screen identifying blood cell development genes in Drosophila by undergraduates participating in a course-based research experience

Undergraduate students participating in the UCLA Undergraduate Research Consortium for Functional Genomics (URCFG) have conducted a two-phased screen using RNA interference (RNAi) in combination with fluorescent reporter proteins to identify genes important for hematopoiesis in Drosophila. This screen disrupted the function of approximately 3500 genes and identified 137 candidate genes for which loss of function leads to observable changes in the hematopoietic development. Targeting RNAi to maturing, progenitor, and regulatory cell types identified key subsets that either limit or promote blood cell maturation. Bioinformatic analysis reveals gene enrichment in several previously uncharacterized areas, including RNA processing and export and vesicular trafficking. Lastly, the participation of students in this course-based undergraduate research experience (CURE) correlated with increased learning gains across several areas, as well as increased STEM retention, indicating that authentic, student-driven research in the form of a CURE represents an impactful and enriching pedagogical approach.

Functional and transcriptomic analyses of the NF-Y family provide insights into the defense mechanisms of honeybees under adverse circumstances

As predominant pollinators, honeybees are important for crop production and terrestrial ecosystems. Recently, various environmental stresses have led to large declines in honeybee populations in many regions. The ability of honeybees to respond to these stresses is critical for their survival. However, the details of the stress defense mechanisms of honeybees have remained elusive. Here, we found that the Nuclear Factor Y (NF-Y) family (containing NF-YA, NF-YB, and NF-YC) is a novel stress mediator family that regulates honeybee environmental stress resistance. NF-YA localized in the nucleus, NF-YB accumulated in the cytoplasm, and NF-YC presented in both the nucleus and cytoplasm. NF-YC interacted with NF-YA and NF-YB in vitro and in vivo, and the nuclear import of NF-YB relied on its interaction with NF-YC. We further found that the expression of NF-Y was induced under multiple stress conditions. In addition, NF-Y regulated many stress responses and antioxidant genes at the transcriptome-wide level, and knockdown of NF-Y repressed the expression of stress-inducible genes, particularly LOC108003540 and LOC107994062, under adverse circumstances. Silencing NF-Y lowered honeybee stress resistance by reducing total antioxidant capacity and enhancing oxidative impairment. Collectively, these results indicate that NF-Y plays important roles in stress responses. Our study sheds light on the underlying defense mechanisms of honeybees under environmental stress.

Drosophila hedgehog can act as a morphogen in the absence of regulated Ci processing

Extracellular Hedgehog (Hh) proteins induce transcriptional changes in target cells by inhibiting the proteolytic processing of full-length Drosophila Ci or mammalian Gli proteins to nuclear transcriptional repressors and by activating the full-length Ci or Gli proteins. We used Ci variants expressed at physiological levels to investigate the contributions of these mechanisms to dose-dependent Hh signaling in Drosophila wing imaginal discs. Ci variants that cannot be processed supported a normal pattern of graded target gene activation and the development of adults with normal wing morphology, when supplemented by constitutive Ci repressor, showing that Hh can signal normally in the absence of regulated processing. The processing-resistant Ci variants were also significantly activated in the absence of Hh by elimination of Cos2, likely acting through binding the CORD domain of Ci, or PKA, revealing separate inhibitory roles of these two components in addition to their well-established roles in promoting Ci processing.

Morphogens play a crucial role in determining how cells are organized in developing organisms. These chemical signals act over a wide area, and the amount of signal each cell receives typically initiates a sequence of events that spatially pattern the multiple cells of an organ or tissue. One of the most well-studied groups of morphogens are the hedgehog proteins, which are involved in the development of many animals, ranging from flies to humans.

In fruit flies, hedgehog proteins kickstart a cascade of molecular changes that switch on a set of 'target' genes. They do this by ultimately altering the activity of a protein called cubitus interruptus, which comes in two lengths: a long version called Ci-155 and a short version called Ci-75. When hedgehog is absent, Ci-155 is kept in an inactive state in the cytoplasm, where it is slowly converted into its shorter form, Ci-75: this repressor protein is then able to access the nucleus, where it switches ‘off’ the target genes. However, when a hedgehog signal is present, the processing of Ci into its shorter form is inhibited. Instead, Ci-155 becomes activated by a separate mechanism that allows the long form protein to enter the nucleus and switch ‘on’ the target genes. But it was unclear whether hedgehog requires both of these mechanisms in order to act as a morphogen and regulate the activity of developmental genes.

To answer this question, Little et al. mutated the gene for Ci in the embryo of fruit flies, so that the Ci-155 protein could no longer be processed into Ci-75. Examining the developing wings of these flies revealed that the genes targeted by hedgehog are still activated in the correct pattern. In some parts of the wing, Ci-75 is required to switch off specific sets of genes. But when Little et al. blocked these genes, by adding a gene that constantly produces the Ci repressor in the presence or absence of hedgehog, the adult flies still developed normally structured wings. This suggests that hedgehog does not need to regulate the processing of Ci-155 into Ci-75 in order to perform its developmental role.

Previous work showed that when one of the major mechanisms used by hedgehog to activate Ci-155 is blocked, fruit flies are still able to develop normal wings. Taken together with the findings of Little et al., this suggests that the two mechanisms induced by hedgehog can compensate for each other, and independently regulate the development of the fruit fly wing. These mechanisms, which are also found in humans, have been linked to birth defects and several common types of cancer, and understanding how they work could help the development of new treatments.

SPOP and cancer: a systematic review

The initiation and progression of cancer is dependent on the acquisition of mutations in oncogenes or tumor suppressor genes that ultimately leads to the dysregulation of key regulatory pathways. Though these mutations often occur in direct regulators of such pathways, some may confer tumorigenic potential by indirectly targeting several pathways congruently thereby exerting pleiotropic effects. In recent years, the tumor suppressor gene Speckle Type POZ Protein (SPOP) has gained a lot of attention as it has been found to be altered in a variety of different cancers. SPOP appears to exert pleiotropic tumorigenic effects as multiple different regulatory pathways become dysregulated upon SPOP alterations. SPOP has been identified as an E3 ubiquitin ligase substrate binding subunit of the proteasome complex. Since protein degradation is critical in regulating proper cellular function it is not surprising that the proteasome pathway is often found to be disrupted in cancer. Many studies have now indicated that mutations or changes in the expression of SPOP are one of several underlying reasons of proteasome pathway disruption in different cancers. Ultimately, either SPOP downregulation or mutation promotes stabilization of direct SPOP targets which subsequently promotes cancer through the dysregulation of key regulatory pathways. In this review, we will discuss the current literature on cancer-specific SPOP alterations as well the SPOP targets that are stabilized, and the pathways that are dysregulated, as a result.

SPOP Regulates The Biological Mechanism Of Ovarian Cancer Cells Through The Hh Signaling Pathway

Background

Ovarian cancer is characterized by high metastatic potential and high mortality. More than 80% of primary ovarian malignancies are epithelial ovarian cancers. There is increasing evidence that Speckle-type POZ protein (SPOP) is highly correlated with the development of various types of cancer. However, the effects of SPOP on epithelial ovarian cancer and the associated molecular mechanisms remain unclear.

Materials and methods

We compared SPOP expression between epithelial ovarian cancer tissues and normal ovarian tissues by using immunohistochemical staining. To determine the role of SPOP in epithelial ovarian cancer cells, we overexpressed or knocked down SPOP in the epithelial ovarian cancer cell line OVCAR-3 using lentiviral vectors.

Results

Our results from the present study indicated that SPOP expression was significantly downregulated in human epithelial ovarian cancer and was associated with the FIGO stage and the histopathologic grading of the tumor. The overexpression and knockdown experiments revealed that SPOP inhibited proliferation while promoting apoptosis in ovarian cancer cells. Inhibition of SPOP mis-activated the Hedgehog (Hh) signaling pathway, thereby inhibiting apoptosis in ovarian cancer cells.

Conclusion

SPOP suppresses proliferation and promotes apoptosis in human ovarian cancer cells by inhibiting the Hh signaling pathway, offering the possibility of new approaches for the treatment of ovarian cancer.

Proteostasis in the Hedgehog signaling pathway

The Hedgehog (Hh) signaling pathway is crucial for the development of vertebrate and invertebrate animals alike. Hh ligand binds its receptor Patched (Ptc), allowing the activation of the obligate signal transducer Smoothened (Smo). The levels and localizations of both Ptc and Smo are regulated by ubiquitination, and Smo is under additional regulation by phosphorylation and SUMOylation. Downstream of Smo, the Ci/Gli family of transcription factors regulates the transcriptional responses to Hh. Phosphorylation, ubiquitination and SUMOylation are important for the stability and localization of Ci/Gli proteins and Hh signaling output. Finally, Suppressor of Fused directly regulates Ci/Gli proteins and itself is under proteolytic regulation that is critical for normal Hh signaling.

Hedgehog signalling in the tumourigenesis and metastasis of osteosarcoma, and its potential value in the clinical therapy of osteosarcoma

The Hedgehog (Hh) signalling pathway is involved in cell differentiation, growth and tissue polarity. This pathway is also involved in the progression and invasion of various human cancers. Osteosarcoma, a subtype of bone cancer, is commonly seen in children and adolescents. Typically, pulmonary osteosarcoma metastases are especially difficult to control. In the present paper, we summarise recent studies on the regulation of osteosarcoma progression and metastasis by downregulating Hh signalling. We also summarise the crosstalk between the Hh pathway and other cancer-related pathways in the tumourigenesis of various cancers. We further summarise and highlight the therapeutic value of potential inhibitors of Hh signalling in the clinical therapy of human cancers.

Molecular mechanisms of suppressor of fused in regulating the hedgehog signalling pathway

Highly conserved throughout evolution, the hedgehog (Hh) signalling pathway has been demonstrated to be involved in embryonic development, stem cell maintenance and tissue homeostasis in animals ranging from invertebrates to vertebrates. In the human body, a variety of cancer types are associated with the aberrantly activated Hh signalling pathway. Multiple studies have revealed suppressor of fused (Sufu) as a key negative regulator of this signalling pathway. In vertebrates, Sufu primarily functions as a tumor suppressor factor by interacting with and inhibiting glioma-associated oncogene homologues (GLIs), which are the terminal transcription factors of the Hh signalling pathway and belong to the Kruppel family of zinc finger proteins; by contrast, the regulation of Sufu itself remains relatively unclear. In the present review article, we focus on the effects of Sufu on the Hh signalling pathway in tumourigenesis and the molecular mechanisms underlying the regulation of GLI by Sufu. In addition, the factors modulating the activity of Sufu at post-transcriptional levels are also discussed.

Capping Enzyme mRNA-cap/RNGTT Regulates Hedgehog Pathway Activity by Antagonizing Protein Kinase A

Hedgehog (Hh) signaling plays a pivotal role in animal development and its deregulation in humans causes birth defects and several types of cancer. Protein Kinase A (PKA) modulates Hh signaling activity through phosphorylating the transcription factor Cubitus interruptus (Ci) and G protein coupled receptor (GPCR) family protein Smoothened (Smo) in Drosophila, but how PKA activity is regulated remains elusive. Here, we identify a novel regulator of the Hh pathway, the capping-enzyme mRNA-cap, which positively regulates Hh signaling activity through modulating PKA activity. We provide genetic and biochemical evidence that mRNA-cap inhibits PKA kinase activity to promote Hh signaling. Interestingly, regulation of Hh signaling by mRNA-cap depends on its cytoplasmic capping-enzyme activity. In addition, we show that the mammalian homolog of mRNA-cap, RNGTT, can replace mRNA-cap to play the same function in the Drosophila Hh pathway and that knockdown of Rngtt in cultured mammalian cells compromised Shh pathway activity, suggesting that RNGTT is functionally conserved. Our study makes an unexpected link between the mRNA capping machinery and the Hh signaling pathway, unveils a new facet of Hh signaling regulation, and reveals a potential drug target for modulating Hh signaling activity.

Stability of HIB-Cul3 E3 ligase adaptor HIB Is Regulated by Self-degradation and Availability of Its Substrates

The HIB-Cul3 complex E3 ligase regulates physiological homeostasis through regulating its substrate stability and its activity can be modulated by changing HIB abundance. However, regulation of HIB remains elusive. Here we provide evidence that HIB is degraded through the proteasome by Cul3-mediated polyubiquitination in K48 manner in Drosophila. Strikingly, HIB is targeted for degradation by itself. We further identify that three degrons (⁵²LKSS⁵⁶T, ⁷⁶LDEE⁸⁰S and ¹¹⁷MESQ¹²¹R) and K185 and K198 of HIB are essential for its auto-degradation. Finally, we demonstrate that HIB-Cul3 substrates, Ci and Puc, can effectively protect HIB from HIB-Cul3-mediated degradation. Taken together, our study indicates that there is an exquisite equilibrium between the adaptor and targets to achieve the tight control of the HIB, which is essential for maintaining suitable Hh and JNK signaling. And the mechanism of adaptor self-degradation and reciprocal control of the abundance between adaptor and its substrates is also applied to BTB-Cul3 E3 ligase adaptor dKeap1, dDiablo and dKLHL18.

Deubiquitination of Ci/Gli by Usp7/HAUSP Regulates Hedgehog Signaling

Hedgehog (Hh) signaling plays essential roles in animal development and tissue homeostasis, and its misregulation causes congenital diseases and cancers. Regulation of the ubiquitin/proteasome-mediated proteolysis of Ci/Gli transcription factors is central to Hh signaling, but whether deubiquitinase is involved in this process remains unknown. Here, we show that Hh stimulates the binding of a ubiquitin-specific protease Usp7 to Ci, which positively regulates Hh signaling activity through inhibiting Ci ubiquitination and degradation mediated by both Slimb-Cul1 and Hib-Cul3 E3 ligases. Furthermore, we find that Usp7 forms a complex with GMP-synthetase (GMPS) to promote Hh pathway activity. Finally, we show that the mammalian counterpart of Usp7, HAUSP, positively regulates Hh signaling by modulating Gli ubiquitination and stability. Our findings reveal a conserved mechanism by which Ci/Gli is stabilized by a deubiquitination enzyme and identify Usp7/HUASP as a critical regulator of Hh signaling and potential therapeutic target for Hh-related cancers.

Regulation of Hedgehog signaling by ubiquitination

The Hedgehog (Hh) signaling pathway plays crucial roles both in embryonic development and in adult stem cell function. The timing, duration and location of Hh signaling activity need to be tightly controlled. Abnormalities of Hh signal transduction lead to birth defects or malignant tumors. Recent data point to ubiquitination-related posttranslational modifications of several key Hh pathway components as an important mechanism of regulation of the Hh pathway. Here we review how ubiquitination regulates the localization, stability and activity of the key Hh signaling components.

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

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)/glioma-associated 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.