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Adrian Salic - One of the best experts on this subject based on the ideXlab platform.

  • a mechanism for vertebrate hedgehog signaling recruitment to cilia and dissociation of SUFU gli protein complexes
    Journal of Cell Biology, 2010
    Co-Authors: Hanna Tukachinsky, Lyle V Lopez, Adrian Salic
    Abstract:

    In vertebrates, Hedgehog (Hh) signaling initiated in primary cilia activates the membrane protein Smoothened (Smo) and leads to activation of Gli proteins, the transcriptional effectors of the pathway. In the absence of signaling, Gli proteins are inhibited by the cytoplasmic protein Suppressor of Fused (SUFU). It is unclear how Hh activates Gli and whether it directly regulates SUFU. We find that Hh stimulation quickly recruits endogenous SUFU–Gli complexes to cilia, suggesting a model in which Smo activates Gli by relieving inhibition by SUFU. In support of this model, we find that Hh causes rapid dissociation of the SUFU–Gli complex, thus allowing Gli to enter the nucleus and activate transcription. Activation of protein kinase A (PKA), an inhibitor of Hh signaling, blocks ciliary localization of SUFU–Gli complexes, which in turn prevents their dissociation by signaling. Our results support a simple mechanism in which Hh signals at vertebrate cilia cause dissociation of inactive SUFU–Gli complexes, a process inhibited by PKA.

  • A mechanism for vertebrate Hedgehog signaling: recruitment to cilia and dissociation of SUFU–Gli protein complexes
    The Journal of cell biology, 2010
    Co-Authors: Hanna Tukachinsky, Lyle V Lopez, Adrian Salic
    Abstract:

    In vertebrates, Hedgehog (Hh) signaling initiated in primary cilia activates the membrane protein Smoothened (Smo) and leads to activation of Gli proteins, the transcriptional effectors of the pathway. In the absence of signaling, Gli proteins are inhibited by the cytoplasmic protein Suppressor of Fused (SUFU). It is unclear how Hh activates Gli and whether it directly regulates SUFU. We find that Hh stimulation quickly recruits endogenous SUFU–Gli complexes to cilia, suggesting a model in which Smo activates Gli by relieving inhibition by SUFU. In support of this model, we find that Hh causes rapid dissociation of the SUFU–Gli complex, thus allowing Gli to enter the nucleus and activate transcription. Activation of protein kinase A (PKA), an inhibitor of Hh signaling, blocks ciliary localization of SUFU–Gli complexes, which in turn prevents their dissociation by signaling. Our results support a simple mechanism in which Hh signals at vertebrate cilia cause dissociation of inactive SUFU–Gli complexes, a process inhibited by PKA.

Yasuhiro Takahashi - One of the best experts on this subject based on the ideXlab platform.

Nao Yokoyama - One of the best experts on this subject based on the ideXlab platform.

Chichung Hui - One of the best experts on this subject based on the ideXlab platform.

  • BCC or not: SUFU keeps it in check
    Oncoscience, 2015
    Co-Authors: Wen-chi Yin, Chichung Hui
    Abstract:

    Basal cell carcinoma (BCC), driven by aberrantly activated HEDGEHOG (HH) pathway, is the most common human malignancy. Current FDA-approved targeted therapy uses Vismodegib to inhibit SMO, a membrane component of the HH pathway. Despite initial impressive tumor regression, the positive clinical response is short-lived in some BCC patients as acquired SMO mutations confer secondary resistance[1]. Clearly, a deeper understanding of the molecular events underlying BCC tumorigenesis is required to devise effective treatments. The activity of SMO is repressed by the HH receptor PTCH1. Upon HH binding, SMO promotes dissociation of GLI transcription factors from the key negative intracellular regulator SUFU, thereby allowing expression of HH target genes[2]. Mutations in PTCH1, SMO, and SUFU, believed to unleash GLI activity, are frequently found in BCC. SUFU, like PTCH1, is a major negative regulator of the HH pathway. We have previously shown that loss of SUFU in mouse keratinocytes promotes Gli2 nuclear localization due to lack of cytoplasmic sequestration, and consequently leads to elevated target gene expression[3]. Surprisingly, unlike Ptch1, inactivation of SUFU alone in the mouse skin does not cause BCC. To identify the key oncogenic events in BCC formation, we performed microarray coupled with Gene Set Enrichment Analysis on Ptch1 and SUFU mutants[4]. The comparative analysis revealed that loss of Ptch1 in keratinocytes led to significant enrichment of gene sets involved in TGF-β signaling and extracellular matrix remodelling, consistent with the tumorigenic phenotype. In contrast, the majority of gene sets uniquely enriched in SUFU knockout keratinocytes are involved in cell cycle control, suggesting a novel role of SUFU in cell cycle regulation. Intriguingly, unlike Ptch1 knockout skin, which showed elevated number of mitotic cells, SUFU knockout skin exhibited normal mitotic count. Furthermore, while DNA damage was found in both mutants, SUFU knockout cells displayed DNA damage-induced G2/M checkpoint cell cycle arrest. These results indicate that Ptch1 knockout cells are able to override the checkpoint and continue proliferation with the unstable genome while SUFU knockouts halt, a key feature likely contributing to their differential cancer phenotypes. Arrest at G2 is typically coupled with accumulation of p53, which activates p21 and 14-3-3σ to sequester mitosis-promoting complex Cyclin-B1/CDK1. Strikingly, p53 protein and p21 transcripts remained low in SUFU mutants despite the arrest. These findings suggest that while both loss of SUFU and Ptch1 result in increased entry into cell cycle and impairment in p53 response to cell cycle-driven DNA damage, SUFU itself may be a positive regulator of cell cycle progression independent of the p53 checkpoint. Upregulation of the major HH pathway effector, Gli2, is a hallmark of BCC and is observed in Ptch1 mouse models. Consistent with our finding that loss of Ptch1 leads to genome instability and evasion of cell cycle checkpoints, Pantazi et al.[5] recently demonstrated that overexpression of GLI2 activator (GLI2sN) in human keratinocytes is sufficient to induce chromosomal aberrations. They also found that GLI2sN overexpression results in suppression of cell cycle regulators p21 and 14-3-3σ, and induction of anti-apoptotic mechanisms. These lines of evidence suggest that GLI2 is likely the major mediator of the malignant transformation induced by the loss of PTCH1 during BCC tumorigenesis. In vitro studies demonstrated that HH signaling can positively regulate cell cycle by promoting the expression of cell cycle regulators (D-type cyclins) and preventing the accumulation of p53. These are consistent with the active mitosis and evasion of cell cycle arrest observed in Ptch1 knockout cells. Our findings suggest that SUFU may also regulate cell cycle. However, it remains unclear why and how the loss of this negative HH pathway regulator causes cell cycle arrest. One possible mechanism is through DNA damage response, which involves the ATM/ATR, CHK1/CHK2, and CDC25C axis to inactivate the Cyclin-B1/CDK1 complex, leading to G2 arrest. Figure 1 Inactivation of Ptch1 and SUFU lead to distinct cellular events in keratinocytes Whether SUFU's cell cycle function is Gli-dependent is also unknown. Although ectopic HH target gene expression was found in both SUFU and Ptch1 mutants, Gli2 protein is significantly reduced in SUFU mutants compared to wildtype, with exclusive nuclear localization. It is possible that a certain threshold of Gli2 activity is required for evasion of cell cycle arrest and tumor surveillance, and that BCC tumorigenesis is stunted in SUFU mutants since the threshold is not achieved. Double knockout of SUFU and Ptch1 may help determine whether SUFU is required for the rapid cell cycle progression induced by loss of Ptch1. In addition, with the recent advances in BioID mass spectrometry[6], identification of SUFU's interactome in keratinocytes may give mechanistic insights into SUFU's involvement in cell cycle regulation. In conclusion, this comparative study of Ptch1 and SUFU mutant mice advanced our understanding of BCC tumorigenesis. Further investigations elucidating the role of SUFU in the cell cycle are warranted for the reason that if SUFU can also function as a positive regulator of the HH pathway, it may represent a potential target for therapeutic intervention of BCC.

  • differential regulation of gli proteins by SUFU in the lung affects pdgf signaling and myofibroblast development
    Developmental Biology, 2014
    Co-Authors: Chuwen Lin, Miaohsueh Chen, Rhodora Gacayan, Chichung Hui, Erica Yao, Hai Song, Paotien Chuang
    Abstract:

    Mammalian Hedgehog (Hh) signaling relies on three Gli transcription factors to mediate Hh responses. This process is controlled in part by a major negative regulator, SUFU, through its effects on Gli protein level, distribution and activity. In this report, we showed that SUFU regulates Gli1 protein levels by antagonizing Numb/Itch. Otherwise, Numb/Itch would induce Gli1 protein degradation. This is in contrast to inhibition of Spop-mediated degradation of Gli2/3 by SUFU. Thus, controlling protein levels of all three Gli genes by SUFU is a conserved mechanism to modulate Hh responses albeit via distinct pathways. These findings in cell-based assays were further validated in vivo. In analyzing how SUFU controls Gli proteins in different tissues, we discovered that loss of SUFU in the lung exerts different effects on Hh target genes. Hh targets Ptch1/Hhip are upregulated in SUFU-deficient lungs, consistent with Hh pathway activation. Surprisingly, protein levels of Hh target Gli1 are reduced. We also found that myofibroblasts are absent from many prospective alveoli of SUFU-deficient lungs. Myofibroblast development is dependent on PDGF signaling. Interestingly, analysis of the Pdgfra promoter revealed a canonical Gli-binding site where Gli1 resides. These studies support a model in which loss of SUFU contributes to compromised Pdgfra activation and disrupts myofibroblast development in the lung. Our work illustrates the unappreciated complexity of Hh responses where distinct Hh targets could respond differently depending on the availability of Gli proteins that control their expression.

  • antagonistic and cooperative actions of kif7 and SUFU define graded intracellular gli activities in hedgehog signaling
    PLOS ONE, 2012
    Co-Authors: Kelvin Law, Xiaoyun Zhang, Vijitha Puviindran, Shigeru Makino, Chichung Hui
    Abstract:

    Graded Hedgehog (Hh) signaling governs the balance of Gli transcriptional activators and repressors to specify diverse ventral cell fates in the spinal cord. It remains unclear how distinct intracellular Gli activity is generated. Here, we demonstrate that SUFU acts universally as a negative regulator of Hh signaling, whereas Kif7 inhibits Gli activity in cooperation with, and independent of, SUFU. Together, they deter naive precursors from acquiring increasingly ventral identity. We show that Kif7 is also required to establish high intracellular Gli activity by antagonizing the SUFU-inhibition of Gli2. Strikingly, by abolishing the negative regulatory action of SUFU, diverse ventral cell fates can be specified in the absence of extracellular Hh signaling. These data suggest that SUFU is the primary regulator of graded Hh signaling and establish that the antagonistic and cooperative actions of Kif7 and SUFU are responsible for setting up distinct Gli activity in ventral cell fate specification.

  • kif7 regulates gli2 through SUFU dependent and independent functions during skin development and tumorigenesis
    Development, 2012
    Co-Authors: Erica Nieuwenhuis, Xiaoyun Zhang, Weilun Nien, Jennifer J R Zhang, Vijitha Puviindran, Brandon J Wainwright, Peter C W Kim, Chichung Hui
    Abstract:

    Abnormal activation of Hedgehog (Hh) signaling leads to basal cell carcinoma (BCC) of the skin, the most common human cancer. Gli2, the major transcriptional activator of Hh signaling, is essential for hair follicle development and its overexpression in epidermis induces BCC formation and maintains tumor growth. Despite its importance in skin development and tumorigenesis, little is known about the molecular regulation of Gli2. SUFU and Kif7 are two evolutionarily conserved regulators of Gli transcription factors. Here, we show that SUFU and Kif7 regulate Gli2 through distinct mechanisms in keratinocytes. SUFU restricts the activity of Gli2 through cytoplasmic sequestration. Kif7 possesses SUFU-dependent and -independent regulatory functions in Hh signaling: while it promotes Hh pathway activity through the dissociation of SUFU-Gli2 complex, it also contributes to the repression of Hh target genes in the absence of SUFU. Deletion of both SUFU and Kif7 in embryonic skin leads to complete loss of follicular fate. Importantly, although inactivation of SUFU or Kif7 alone in adult epidermis cannot promote BCC formation, their simultaneous deletion induces BCC. These studies establish SUFU and Kif7 as crucial components in the regulation of Gli2 localization and activity, and illustrate their overlapping functions in skin development and tumor suppression.

  • suppressor of fused SUFU mediates the effect of parathyroid hormone like hormone pthlh on chondrocyte differentiation in the growth plate
    Journal of Biological Chemistry, 2012
    Co-Authors: Shu-hsuan C. Hsu, Xiaoyun Zhang, Chichung Hui, Steven Y Cheng, Jay S. Wunder, Benjamin A. Alman
    Abstract:

    Growth plate chondrocytes undergo a coordinated process of differentiation, regulating long bone growth. Parathyroid hormone-like hormone (Pthlh) inhibits hypertrophic differentiation in the growth plate chondrocytes and reduces Hedgehog (Hh) signaling. In mice lacking the Hh mediator Suppressor of fused (SUFU), Pthlh treatment resulted in the up-regulation of Hh activity and an increased number of hypertrophic chondrocytes. Furthermore, Pthlh increased SUFU protein levels, and in chondrocytes lacking SUFU, it was unable to process Hh-regulated Gli transcription factors. Pthlh regulates chondrocyte differentiation and Gli activity in a SUFU-dependent manner, with SUFU acting as a molecular switch in its regulation of differentiation.

Takuya Terahata - One of the best experts on this subject based on the ideXlab platform.