Novel regulation of Ski protein stability and endosomal sorting by actin cytoskeleton dynamics in hepatocytes

N-butyldeoxynojirimycin (miglustat) ameliorates pulmonary fibrosis through inhibition of nuclear translocation of Smad2/3

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive lung disease. The disease involves excessive accumulation of fibroblasts and myofibroblasts, and myofibroblasts differentiated by pro-fibrotic factors promote the deposition of extracellular matrix proteins such as collagen and fibronectin. Transforming growth factor-β1 is a pro-fibrotic factor that promotes fibroblast-to-myofibroblast differentiation (FMD). Therefore, inhibition of FMD may be an effective strategy for IPF treatment. In this study, we screened the anti-FMD effects of various iminosugars and showed that some compounds, including N-butyldeoxynojirimycin (NB-DNJ, miglustat, an inhibitor of glucosylceramide synthase (GCS)), a clinically approved drug for treating Niemann-Pick disease type C and Gaucher disease type 1, inhibited TGF-β1-induced FMD by inhibiting the nuclear translocation of Smad2/3. N-butyldeoxygalactonojirimycin having GCS inhibitory effect did not attenuate the TGF-β1-induced FMD, suggesting that NB-DNJ exerts the anti-FMD effects by GCS inhibitory effect independent manner. N-butyldeoxynojirimycin did not inhibit TGF-β1-induced Smad2/3 phosphorylation. In a mouse model of bleomycin (BLM)-induced pulmonary fibrosis, intratracheal or oral administration of NB-DNJ at an early fibrotic stage markedly ameliorated lung injury and deterioration of respiratory functions, such as specific airway resistance, tidal volume, and peak expiratory flow. Furthermore, the anti-fibrotic effects of NB-DNJ in the BLM-induced lung injury model were similar to those of pirfenidone and nintedanib, which are clinically approved drugs for the treatment of IPF. These results suggest that NB-DNJ may be effective for IPF treatment.

Quantitative Expression of Key Cancer Markers in the AS-30D Hepatocarcinoma Model

Hepatocellular carcinoma is one of the cancers with the highest mortality rate worldwide. HCC is often diagnosed when the disease is already in an advanced stage, making the discovery and implementation of biomarkers for the disease a critical aim in cancer research. In this study, we aim to quantify the transcript levels of key signaling molecules relevant to different pathways known to participate in tumorigenesis, with special emphasis on those related to cancer hallmarks and epithelial-mesenchymal transition, using as a model the murine transplantable hepatocarcinoma AS-30D. Using qPCR to quantify the mRNA levels of genes involved in tumorigenesis, we found elevated levels for Tgfb1 and Spp1, two master regulators of EMT. A mesenchymal signature profile for AS-30D cells is also supported by the overexpression of genes encoding for molecules known to be associated to aggressiveness and metastatic phenotypes such as Foxm1, C-met, and Inppl1. This study supports the use of the AS-30D cells as an efficient and cost-effective model to study gene expression changes in HCC, especially those associated with the EMT process.

Actin Cytoskeleton and Regulation of TGFβ Signaling: Exploring Their Links

Human tissues, to maintain their architecture and function, respond to injuries by activating intricate biochemical and physical mechanisms that regulates intercellular communication crucial in maintaining tissue homeostasis. Coordination of the communication occurs through the activity of different actin cytoskeletal regulators, physically connected to extracellular matrix through integrins, generating a platform of biochemical and biomechanical signaling that is deregulated in cancer. Among the major pathways, a controller of cellular functions is the cytokine transforming growth factor β (TGFβ), which remains a complex and central signaling network still to be interpreted and explained in cancer progression. Here, we discuss the link between actin dynamics and TGFβ signaling with the aim of exploring their aberrant interaction in cancer.

Fibroblast mechanosensing, SKI and Hippo signaling and the cardiac fibroblast phenotype: Looking beyond TGF-β

Cardiac fibroblast activation to hyper-synthetic myofibroblasts following a pathological stimulus or in response to a substrate with increased stiffness may be a key tipping point for the evolution of cardiac fibrosis. Cardiac fibrosis per se is associated with progressive loss of heart pump function and is a primary contributor to heart failure. While TGF-β is a common cytokine stimulus associated with fibroblast activation, a druggable target to quell this driver of fibrosis has remained an elusive therapeutic goal due to its ubiquitous use by different cell types and also in the signaling complexity associated with SMADs and other effector pathways. More recently, mechanical stimulus of fibroblastic cells has been revealed as a major point of activation; this includes cardiac fibroblasts. Further, the complexity of TGF-β signaling has been offset by the discovery of members of the SKI family of proteins and their inherent anti-fibrotic properties. In this respect, SKI is a protein that may bind a number of TGF-β associated proteins including SMADs, as well as signaling proteins from other pathways, including Hippo. As SKI is also known to directly deactivate cardiac myofibroblasts to fibroblasts, this mode of action is a putative candidate for further study into the amelioration of cardiac fibrosis. Herein we provide a synthesis of this topic and highlight novel candidate pathways to explore in the treatment of cardiac fibrosis.

La plasticidad del hepatocito y su relevancia en la fisiología y la patología hepática

El hígado es uno de los principales órganos encargados de mantener la homeostasis en vertebrados,  además de poseer una gran capacidad regenerativa. El hígado está constituido por diversos tipos celulares que de forma coordinada contribuyen para que el órgano funcione eficientemente. Los hepatocitos representan el tipo celular principal de este órgano y llevan a cabo la mayoría de sus actividades; además, constituyen una población heterogénea de células epiteliales con funciones especializadas en el metabolismo. El fenotipo de los hepatocitos está controlado por diferentes vías de señalización, como la vía del TGFβ/Smads, la ruta Hippo/YAP-TAZ y la vía Wnt/β-catenina, entre otras. Los hepatocitos son células que se encuentran normalmente en un estado quiescente, aunque cuentan con una plasticidad intrínseca que se manifiesta en respuesta a diversos daños en el hígado; así, estas células reactivan su capacidad proliferativa o cambian su fenotipo a través de procesos celulares como la transdiferenciación o la transformación, para contribuir a mantener la homeostasis del órgano en condiciones saludables o desarrollar diversas  patologías.

Increased Purinergic Responses Dependent on P2Y2 Receptors in Hepatocytes from CCl4-Treated Fibrotic Mice

Inflammatory and wound healing responses take place during liver damage, primarily in the parenchymal tissue. It is known that cellular injury elicits an activation of the purinergic signaling, mainly by the P2X7 receptor; however, the role of P2Y receptors in the onset of liver pathology such as fibrosis has not been explored. Hence, we used mice treated with the hepatotoxin CCl₄ to implement a reversible model of liver fibrosis to evaluate the expression and function of the P2Y2 receptor (P2Y2R). Fibrotic livers showed an enhanced expression of P2Y2R that eliminated its zonal distribution. Hepatocytes from CCl₄-treated mice showed an exacerbated ERK-phosphorylated response to the P2Y2R-specific agonist, UTP. Cell proliferation was also enhanced in the fibrotic livers. Hepatic transcriptional analysis by microarrays, upon CCl₄ administration, showed that P2Y2 activation regulated diverse pathways, revealing complex action mechanisms. In conclusion, our data indicate that P2Y2R activation is involved in the onset of the fibrotic damage associated with the reversible phase of the hepatic damage promoted by CCl₄.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

MiR-1908 promotes scar formation post-burn wound healing by suppressing Ski-mediated inflammation and fibroblast proliferation

The cell biological basis for scar formation is mainly via excessive fibroblast proliferation accompanied by hypernomic Col I accumulation and inflammation. The role of miR-1908 in scar formation has not been investigated. In this study, we found that miR-1908 expression was inversely associated with the scar suppressor Ski in normal, burn-wounded, healing and scar dermal tissues in humans. Bioinformatics and luciferase reporter gene assays confirmed that miR-1908 targeted the 3'UTR region of Ski mRNA and suppressed Ski expression. Next, human scar epidermal fibroblasts were isolated and the miR-1908 oligonucleotide mimic and inhibitor were respectively transfected into the cells. Western blot analysis proved that Ski expression was sharply reduced by the miR-1908 mimic. MTT and Cell Counting Kit-8 analyses showed that miR-1908 mimic transfection promoted cell proliferation. Simultaneously, data on real-time qPCR analysis indicated that expression of the fibrotic master gene TGF-β1, Ski-suppressing gene Meox2, Col I and proinflammatory markers IL-1α and TNF-α, were all significantly upregulated. In contrast, the miR-1908 inhibitor had a completely opposite effect on cell proliferation and gene expression. The mimic and inhibitor were locally injected into rats with abdominal burn-wounded scars during a 180-day, post-healing experiment. The miR-1908 mimic injection significantly reduced Ski expression, as well as the area, volume and fibrosis of scars in vivo. And, in contrast, the miR-1908 inhibitor injection had an opposite effect to that of the miR-1908 mimic injection. In conclusion, miR-1908 had a positive role in scar formation by suppressing Ski-mediated inflammation and fibroblast proliferation in vitro and in vivo.

Protein Tyrosine Kinase Fyn Regulates TLR4-Elicited Responses on Mast Cells Controlling the Function of a PP2A-PKCα/β Signaling Node Leading to TNF Secretion

Mast cells produce proinflammatory cytokines in response to TLR4 ligands, but the signaling pathways involved are not fully described. In this study, the participation of the Src family kinase Fyn in the production of TNF after stimulation with LPS was evaluated using bone marrow-derived mast cells from wild-type and Fyn-deficient mice. Fyn(-/-) cells showed higher LPS-induced secretion of preformed and de novo-synthesized TNF. In both cell types, TNF colocalized with vesicle-associated membrane protein (VAMP)3-positive compartments. Addition of LPS provoked coalescence of VAMP3 and its interaction with synaptosomal-associated protein 23; those events were increased in the absence of Fyn. Higher TNF mRNA levels were also observed in Fyn-deficient cells as a result of increased transcription and greater mRNA stability after LPS treatment. Fyn(-/-) cells also showed higher LPS-induced activation of TAK-1 and ERK1/2, whereas IκB kinase and IκB were phosphorylated, even in basal conditions. Increased responsiveness in Fyn(-/-) cells was associated with a lower activity of protein phosphatase 2A (PP2A) and augmented activity of protein kinase C (PKC)α/β, which was dissociated from PP2A and increased its association with the adapter protein neuroblast differentiation-associated protein (AHNAK, desmoyokin). LPS-induced PKCα/β activity was associated with VAMP3 coalescence in WT and Fyn-deficient cells. Reconstitution of MC-deficient Wsh mice with Fyn(-/-) MCs produced greater LPS-dependent production of TNF in the peritoneal cavity. Our data show that Fyn kinase is activated after TLR4 triggering and exerts an important negative control on LPS-dependent TNF production in MCs controlling the inactivation of PP2Ac and activation of PKCα/β necessary for the secretion of TNF by VAMP3(+) carriers.

GPCRs and actin-cytoskeleton dynamics

A multitude of physiological processes regulated by G protein-coupled receptors (GPCRs) signaling are accomplished by the participation of active rearrangements of the cytoskeleton. In general, it is common that a cross talk occurs among networks of microfilaments, microtubules, and intermediate filaments in order to reach specific cell responses. In particular, actin-cytoskeleton dynamics regulate processes such as cell shape, cell division, cell motility, and cell polarization, among others. This chapter describes the current knowledge about the regulation of actin-cytoskeleton dynamic by diverse GPCR signaling pathways, and also includes some protocols combining immunofluorescence and confocal microscopy for the visualization of the different rearrangements of the actin-cytoskeleton. We report how both the S1P-GPCR/G12/13/Rho/ROCK and glucagon-GPCR/Gs/cAMP axes induce differential actin-cytoskeleton rearrangements in epithelial cells. We also show that specific actin-binding molecules, like phalloidin and LifeAct, are very useful to analyze F-actin reorganization by confocal microscopy, and also that both molecules show similar results in fixed cells, whereas the anti-actin antibody is useful to detect both the G- and F-actin, as well as their compartmentalization. Thus, it is highly recommended to utilize different approaches to investigate the regulation of actin dynamics by GPCR signaling, with the aim to get a better picture of the phenomenon under study.

Actin-cytoskeleton polymerization differentially controls the stability of Ski and SnoN co-repressors in normal but not in transformed hepatocytes

BACKGROUND

Ski and SnoN proteins function as transcriptional co-repressors in the TGF-β pathway. They regulate cell proliferation and differentiation, and their aberrant expression results in altered TGF-β signalling, malignant transformation, and alterations in cell proliferation.

METHODS

We carried out a comparative characterization of the endogenous Ski and SnoN protein regulation by TGF-β, cell adhesion disruption and actin-cytoskeleton rearrangements between normal and transformed hepatocytes; we also analyzed Ski and SnoN protein stability, subcellular localization, and how their protein levels impact the TGF-β/Smad-driven gene transcription.

RESULTS

Ski and SnoN protein levels are lower in normal hepatocytes than in hepatoma cells. They exhibit a very short half-life and a nuclear/cytoplasmic distribution in normal hepatocytes opposed to a high stability and restricted nuclear localization in hepatoma cells. Interestingly, while normal cells exhibit a transient TGF-β-induced gene expression, the hepatoma cells are characterized by a strong and sustained TGF-β-induced gene expression. A novel finding is that Ski and SnoN stability is differentially regulated by cell adhesion and cytoskeleton rearrangements in the normal hepatocytes. The inhibition of protein turnover down-regulated both Ski and SnoN co-repressors impacting the kinetic of expression of TGF-β-target genes.

CONCLUSION

Normal regulatory mechanisms controlling Ski and SnoN stability, subcellular localization and expression are altered in hepatocarcinoma cells.

GENERAL SIGNIFICANCE

This work provides evidence that Ski and SnoN protein regulation is far more complex in normal than in transformed cells, since many of the normal regulatory mechanisms are lost in transformed cells.