The small GTPase Rap1 is a modulator of Hedgehog signaling

SGSM2 inhibits thyroid cancer progression by activating RAP1 and enhancing competitive RAS inhibition

Thyroid cancer (TC) is one of the most common malignancies involving the head and neck, and its incidences are increasing every year. Small G protein signaling modulators 2 (SGSM2) belongs to a newly identified protein group that contributes to numerous cancer progression. However, its role in TC remains unknown. The aim of this study was to explore the functions and underlying molecular mechanism of SGSM2 in the progression of thyroid tumorigenesis. Here, we demonstrated that SGSM2 expression was markedly decreased in TC, and that lower SGSM2 expression was potentially related to worse patient prognosis. Meanwhile, the SGSM2 levels were not directly correlated with BRAF or RAS mutations in TC. Based on our functional analysis, ectopic SGSM2 expression strongly prevented cell proliferation, migration, invasion, and tumorigenic activity in TC cells that harbored wild type RAS. Mechanistically, we demonstrated that SGSM2 interacted with Small G protein Ras-associated protein 1(RAP1) and augmented its activity. Activated RAP1 then competitively suppressed RAS activation and thereby downregulated output of MAPK/ERK and PI3K/Akt networks, which are primary contributors of TC. In summary, the present study reports a tumor suppressive role of SGSM2 in TC. Moreover, we revealed the underlying molecular mechanism, thus providing a potential therapeutic target for TCs that harbor wild type RAS.

A schematic model of SGSM2 suppressing the progression of RAS^WT thyroid cancer.

C3G localizes to mother centriole dependent on cenexin, and regulates centrosome duplication and primary cilia length

C3G (RapGEF1) plays a role in cell differentiation and is essential for early embryonic development in mice. In this study, we identify C3G as a centrosomal protein colocalizing with cenexin at the mother centriole in interphase cells. C3G interacts through its catalytic domain with cenexin, and they show interdependence for localization to the centrosome. C3G depletion caused a decrease in cellular cenexin levels. Centrosomal localization is lost as myocytes differentiate to form myotubes. Stable clone of cells depleted of C3G by CRISPR/Cas9 showed the presence of supernumerary centrioles. Overexpression of C3G, or a catalytically active deletion construct inhibited centrosome duplication. Cilia length is longer in C3G knockout cells, and the phenotype could be reverted upon reintroduction of C3G or its catalytic domain. Association of C3G with the basal body is dynamic, decreasing upon serum starvation, and increasing upon reentry into the cell cycle. C3G inhibits cilia formation and length dependent on its catalytic activity. We conclude that C3G inhibits centrosome duplication and maintains ciliary homeostasis, properties that may be important for its role in embryonic development.

Role of miR-337-3p and its target Rap1A in modulating proliferation, invasion, migration and apoptosis of cervical cancer cells

OBJECTIVE

To investigate the role of miR-337-3p targeting Rap1A in modulating proliferation, invasion, migration and apoptosis of cervical cancer cells.

METHODS

The expression levels of miR-337-3p and Rap1A in cervical cancer tissues and normal tissues were evaluated through quantitative Real-time PCR (qRT-PCR) and Western blotting; and correlations of miR-337-3p with clinicopathological characteristics and prognosis of patients were also analyzed. Besides, human cervical cancer cell line HeLa cells were randomly divided into five groups (Mock, NC, miR-337-3p mimic, Rap1A, and miR-337-3p mimic + Rap1A groups). CCK-8 assay was utilized to measure cell proliferation, flow cytometry to evaluate cell apoptosis, and wound-healing and Transwell assays to detect cell migration and invasion.

RESULTS

Cervical cancer tissues presented a significant decrease in miR-337-3p and a remarkable increase in Rap1A protein. Besides, the expression levels of miR-337-3p and Rap1A were closely related to the major clinicopathological characteristics of cervical cancer; and patients with high-miR-337-3p-expression had the higher 5-year survival rate (all p< 0.05). When compared to Mock group, cells in miR-337-3p mimic group were suppressed in proliferation, migration, and invasion, but significantly promoted in apoptosis; meanwhile, cells in the Rap1A group showed changes in a completely opposite trend (all p< 0.05). Moreover, Rap1A can reverse the effect of miR-337-3p mimic on cell proliferation, invasion, migration and apoptosis (all p< 0.05).

CONCLUSION

MiR-337-3p was discovered to be decreased in cervical cancer, and miR-337-3p up-regulation may inhibit the proliferation, migration and invasion and promote the apoptosis of cervical cancer cells via down-regulating Rap1A.

Synaptojanin regulates Hedgehog signalling by modulating phosphatidylinositol 4-phosphate levels

In Hedgehog (Hh) signalling, Hh ligand concentration gradient is effectively translated into a spatially distinct transcriptional program to give precisely controlled context dependent developmental outcomes. In the absence of Hh, the receptor Patched (Ptc) inhibits the signal transducer Smoothened (Smo) by maintaining low phosphatidylinositol 4-phosphate (PI(4)P) levels. Binding of Hh to its receptor Ptc promotes PI(4)P production, which in turn activates Smo. Using wingdiscs of Drosophila melanogaster, this study shows that Synaptojanin (Synj), a dual phosphatase, modulates PI(4)P levels and affects Smo activation, and thereby functions as an additional regulatory step in the Hh pathway. Reducing the levels of Synj in the wing-discs caused enhancement of a Hh dominant gain-of-function Moonrat phenotype in the adult wings. Synj downregulation augmented Hh signalling, which was associated with elevated PI(4)P levels and Smo activation. Synj did not control the absolute pathway activity but rather fine-tuned the response since its downregulation increased expression of decapentaplegic (dpp), a low-threshold target of the pathway while the high-threshold targets remained unaffected. This is the first report that identifies Synj as a negative regulator of Hh signalling, implying its importance and an additional regulatory step in Hh signal transduction.

Cyclase-associated protein 1 (CAP1) is a prenyl-binding partner of Rap1 GTPase

Rap1 proteins are members of the Ras subfamily of small GTPases involved in many biological responses, including adhesion, cell proliferation, and differentiation. Like all small GTPases, they work as molecular allosteric units that are active in signaling only when associated with the proper membrane compartment. Prenylation, occurring in the cytosol, is an enzymatic posttranslational event that anchors small GTPases at the membrane, and prenyl-binding proteins are needed to mask the cytoplasm-exposed lipid during transit to the target membrane. However, several of these proteins still await discovery. In this study, we report that cyclase-associated protein 1 (CAP1) binds Rap1. We found that this binding is GTP-independent, does not involve Rap1's effector domain, and is fully contained in its C-terminal hypervariable region (HVR). Furthermore, Rap1 prenylation was required for high-affinity interactions with CAP1 in a geranylgeranyl-specific manner. The prenyl binding specifically involved CAP1's C-terminal hydrophobic β-sheet domain. We present a combination of experimental and computational approaches, yielding a model whereby the high-affinity binding between Rap1 and CAP1 involves electrostatic and nonpolar side-chain interactions between Rap1's HVR residues, lipid, and CAP1 β-sheet domain. The binding was stabilized by the lipid insertion into the β-solenoid whose interior was occupied by nonpolar side chains. This model was reminiscent of the recently solved structure of the PDEδ-K-Ras complex; accordingly, disruptors of this complex, e.g. deltarasin, blocked the Rap1-CAP1 interaction. These findings indicate that CAP1 is a geranylgeranyl-binding partner of Rap1.

A fixation method to preserve cultured cell cytonemes facilitates mechanistic interrogation of morphogen transport

During development, extracellular cues guiding cell fate determination are provided by morphogens. One mechanism by which morphogens are proposed to traverse extracellular space is by traveling along specialized filopodia called cytonemes. These cellular highways extend between signal-producing and -receiving cells to enable direct morphogen delivery. Although genetic studies support cytoneme involvement in morphogen transport, mechanistic insight into how they are regulated is limited owing to technical challenges associated with performing cell biological analysis of the delicate filopodial structures. Here, we introduce a fixation method whereby cultured cell cytonemes can be preserved for imaging studies, allowing investigation of cytoneme regulation using standard cell biological techniques. Using this method, we examined Hedgehog-containing cytonemes and identified a role for the Hedgehog deployment protein Dispatched in cytoneme stabilization. We demonstrate that Hedgehog and Dispatched colocalize in cytonemes, and that cholesterol-modified Hedgehog acts through Dispatched to increase cytoneme occurrence. Live imaging suggests that this occurs through Dispatched-mediated slowing of cytoneme retraction rates. Dispatched-induced cytoneme modulation was recapitulated in wing imaginal discs of transgenic Drosophila, providing evidence that cultured cell cytoneme analysis is predictive of in vivo functionality.

Summary: A new fixation method for preserving cultured cell cytonemes, used in combination with live cell imaging, reveals that the Hedgehog deployment protein Dispatched promotes cytoneme occurrence by slowing retraction rates.

Rap1A promotes ovarian cancer metastasis via activation of ERK/p38 and notch signaling

As one of the Ras-associated proteins, Rap1A has been linked to cancer initiation and development. However, the precise function of Rap1A in ovarian cancer is still not understood. Here, we show that Rap1A promotes ovarian cancer tumorigenesis and metastasis via stimulating cell proliferation, migration and invasion both in vivo and in vitro. Mechanistic study showed that Rap1A activates extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK) and Notch pathways, leading to the enhanced expression of several epithelial-mesenchymal transition (EMT) markers such as slug, zeb1, vimentin, fibronectin, and MMP9. However, the pretreatment of Rap1A-overexpressing cells with the Notch inhibitor DAPT or ERK inhibitor (U0126) inhibited the up-regulated expression of those molecules. These findings provide the first evidence linking Rap1A with ovarian cancer development through the ERK/p38 and Notch signaling pathways, indicating that Rap1A may be used as a novel diagnostic marker or a therapeutic target for ovarian cancer.

Dataset for phenotypic classification of genetic modifiers of smoothened and Hedgehog

This data article includes supporting information for the research article entitled "The Small GTPase Rap1 is a Modulator of Hedgehog Signaling" [1]. Drosophila wing phenotypes induced by expression of a dominant negative Smoothened (Smo) mutant were cataloged into five distinct classes. Class distributions observed following expression of dominant negative Smo in control and sensitized backgrounds were quantified to serve as references for strength of phenotypic modification. Shifts in class distribution of Hedgehog (Hh) wing phenotypes resulting from introduction of loss-of-function alleles of select Ras family G protein genes and the Hh pathway regulators Fused and Suppressor of Fused are shown.