A role of fukutin, a gene responsible for Fukuyama type congenital muscular dystrophy, in cancer cells: a possible role to suppress cell proliferation

Involvement of abnormal dystroglycan expression and matriglycan levels in cancer pathogenesis

Dystroglycan (DG) is a glycoprotein composed of two subunits that remain non-covalently bound at the plasma membrane: α-DG, which is extracellular and heavily O-mannosyl glycosylated, and β-DG, an integral transmembrane polypeptide. α-DG is involved in the maintenance of tissue integrity and function in the adult, providing an O-glycosylation-dependent link for cells to their extracellular matrix. β-DG in turn contacts the cytoskeleton via dystrophin and participates in a variety of pathways transmitting extracellular signals to the nucleus. Increasing evidence exists of a pivotal role of DG in the modulation of normal cellular proliferation. In this context, deficiencies in DG glycosylation levels, in particular those affecting the so-called matriglycan structure, have been found in an ample variety of human tumors and cancer-derived cell lines. This occurs together with an underexpression of the DAG1 mRNA and/or its α-DG (core) polypeptide product or, more frequently, with a downregulation of β-DG protein levels. These changes are in general accompanied in tumor cells by a low expression of genes involved in the last steps of the α-DG O-mannosyl glycosylation pathway, namely POMT1/2, POMGNT2, CRPPA, B4GAT1 and LARGE1/2. On the other hand, a series of other genes acting earlier in this pathway are overexpressed in tumor cells, namely DOLK, DPM1/2/3, POMGNT1, B3GALNT2, POMK and FKTN, hence exerting instead a pro-oncogenic role. Finally, downregulation of β-DG, altered β-DG processing and/or impaired β-DG nuclear levels are increasingly found in human tumors and cell lines. It follows that DG itself, particular genes/proteins involved in its glycosylation and/or their interactors in the cell could be useful as biomarkers of certain types of human cancer, and/or as molecular targets of new therapies addressing these neoplasms.

Fukutin regulates tau phosphorylation and synaptic function: Novel properties of fukutin in neurons

Fukutin, a product of the causative gene of Fukuyama congenital muscular dystrophy (FCMD), is known to be responsible for basement membrane formation. Patients with FCMD exhibit not only muscular dystrophy but also central nervous system abnormalities, including polymicrogyria and neurofibrillary tangles (NFTs) in the cerebral cortex. The formation of NFTs cannot be explained by basement membrane disorganization. To determine the involvement of fukutin in the NFT formation, we performed molecular pathological investigations using autopsied human brains and cultured neurons of a cell line (SH-SY5Y). In human brains, NFTs, identified with an antibody against phosphorylated tau (p-tau), were observed in FCMD patients but not age-matched control subjects and were localized in cortical neurons lacking somatic immunoreactivity for glutamic acid decarboxylase (GAD), a marker of inhibitory neurons. In FCMD brains, NFTs were mainly distributed in lesions of polymicrogyria. Immunofluorescence staining revealed the colocalization of immunoreactivities for p-tau and phosphorylated glycogen synthase kinase-3β (GSK-3β), a potential tau kinase, in the somatic cytoplasm of SH-SY5Y cells; both the immunoreactivities were increased by fukutin knockdown and reduced by fukutin overexpression. Western blot analysis using SH-SY5Y cells revealed consistent results. Enzyme-linked immunosorbent assay (ELISA) confirmed the binding affinity of fukutin to tau and GSK-3β in SH-SY5Y cells. In the human brains, the density of GAD-immunoreactive neurons in the frontal cortex was significantly higher in the FCMD group than in the control group. GAD immunoreactivity on Western blots of SH-SY5Y cells was significantly increased by fukutin knockdown. On immunofluorescence staining, immunoreactivities for fukutin and GAD were colocalized in the somatic cytoplasm of the human brains and SH-SY5Y cells, whereas those for fukutin and synaptophysin were colocalized in the neuropil of the human brains and the cytoplasm of SH-SY5Y cells. ELISA confirmed the binding affinity of fukutin to GAD and synaptophysin in SH-SY5Y cells. The present results provide in vivo and in vitro evidence for novel properties of fukutin as follows: (i) there is an inverse relationship between fukutin expression and GSK-3β/tau phosphorylation in neurons; (ii) fukutin binds to GSK-3β and tau; (iii) tau phosphorylation occurs in non-GAD-immunoreactive neurons in FCMD brains; (iv) neuronal GAD expression is upregulated in the absence of fukutin; and (v) fukutin binds to GAD and synaptophysin in presynaptic vesicles of neurons.

Fukutin Protein Participates in Cell Proliferation by Enhancing Cyclin D1 Expression through Binding to the Transcription Factor Activator Protein-1: An In Vitro Study

The causative gene of Fukuyama congenital muscular dystrophy (fukutin) is involved in formation of the basement membrane through glycosylation of alpha-dystroglycan. However, there are other proposed functions that have not been fully understood. Using cultured astrocytes (1321N1), we found nuclear localization of fukutin and a positive relationship between fukutin expression and cell proliferation. Among potential proteins regulating cell proliferation, we focused on cyclin D1, by reverse-transcription polymerase chain reaction, Western blotting, immunocytochemistry, enzyme-linked immunosorbent assay (ELISA), and sandwich ELISA. Expression of cyclin D1 was significantly downregulated by fukutin knockdown and significantly upregulated by fukutin overexpression. Moreover, fukutin was proven to bind to the activator protein-1 (AP-1) binding site of cyclin D1 promoter, as well as the AP-1 component c-Jun. The c-Jun phosphorylation status was not significantly influenced by knockdown or overexpression of fukutin. The present results provide in vitro evidence for a novel function of fukutin, which participates in cell proliferation by enhancing cyclin D1 expression through forming a complex with AP-1. It is likely that fukutin is a potential cofactor of AP-1.

Expression in retinal neurons of fukutin and FKRP, the protein products of two dystroglycanopathy-causative genes

Purpose

Dystroglycanopathies are a heterogeneous group of recessive neuromuscular dystrophies that affect the muscle, brain and retina, and are caused by deficiencies in the O-glycosylation of α-dystroglycan. This post-translational modification is essential for the formation and maintenance of ribbon synapses in the retina. Fukutin and fukutin-related protein (FKRP) are two glycosyltransferases whose deficiency is associated with severe dystroglycanopathies. These enzymes carry out in vitro the addition of a tandem ribitol 5-phosphate moiety to the so-called core M3 phosphotrisaccharide of α-dystroglycan. However, their expression pattern and function in the healthy mammalian retina has not so far been investigated. In this work, we have addressed the expression of the FKTN (fukutin) and FKRP genes in the retina of mammals, and characterized the distribution pattern of their protein products in the adult mouse retina and the 661W photoreceptor cell line.

Methods

By means of reverse transcription (RT)-PCR and immunoblotting, we have studied the expression at the mRNA and protein levels of the fukutin and FKRP genes in different mammalian species, from rodents to humans. Immunofluorescence confocal microscopy analyses were performed to characterize the distribution profile of their protein products in mouse retinal sections and in 661W cultured cells.

Results

Both genes were expressed at the mRNA and protein levels in the neural retina of all mammals studied. Fukutin was present in the cytoplasmic and nuclear fractions in the mouse retina and 661W cells, and accumulated in the endoplasmic reticulum. FKRP was located in the cytoplasmic fraction in the mouse retina and concentrated in the Golgi complex. However, and in contrast to retinal tissue, FKRP additionally accumulated in the nucleus of the 661W photoreceptors.

Conclusions

Our results suggest that fukutin and FKRP not only participate in the synthesis of O-mannosyl glycans added to α-dystroglycan in the endoplasmic reticulum and Golgi complex, but that they could also play a role, that remains to be established, in the nucleus of retinal neurons.

Expression pattern in retinal photoreceptors of POMGnT1, a protein involved in muscle-eye-brain disease

PURPOSE

The POMGNT1 gene, encoding protein O-linked-mannose β-1,2-N-acetylglucosaminyltransferase 1, is associated with muscle-eye-brain disease (MEB) and other dystroglycanopathies. This gene's lack of function or expression causes hypoglycosylation of α-dystroglycan (α-DG) in the muscle and the central nervous system, including the brain and the retina. The ocular symptoms of patients with MEB include retinal degeneration and detachment, glaucoma, and abnormal electroretinogram. Nevertheless, the POMGnT1 expression pattern in the healthy mammalian retina has not yet been investigated. In this work, we address the expression of the POMGNT1 gene in the healthy retina of a variety of mammals and characterize the distribution pattern of this gene in the adult mouse retina and the 661W photoreceptor cell line.

METHODS

Using reverse transcription (RT)-PCR and immunoblotting, we studied POMGNT1 expression at the mRNA and protein levels in various mammalian species, from rodents to humans. Immunofluorescence confocal microscopy analyses were performed to characterize the distribution profile of its protein product in mouse retinal sections and in 661W cultured cells. The intranuclear distribution of POMT1 and POMT2, the two enzymes preceding POMGnT1 in the α-DG O-mannosyl glycosylation pathway, was also analyzed.

RESULTS

POMGNT1 mRNA and its encoded protein were expressed in the neural retina of all mammals studied. POMGnT1 was located in the cytoplasmic fraction in the mouse retina and concentrated in the myoid portion of the photoreceptor inner segments, where the protein colocalized with GM130, a Golgi complex marker. The presence of POMGnT1 in the Golgi complex was also evident in 661W cells. However, and in contrast to retinal tissue, POMGnT1 additionally accumulated in the nucleus of the 661W photoreceptors. Colocalization was found within this organelle between POMGnT1 and POMT1/2, the latter associated with euchromatic regions of the nucleus.

CONCLUSIONS

Our results indicate that POMGnT1 participates not only in the synthesis of O-mannosyl glycans added to α-DG in the Golgi complex but also in the glycosylation of other yet-to-be-identified proteins in the nucleus of mouse photoreceptors.

Fukutin, identified by the Escherichia coli ampicillin secretion trap (CAST) method, participates in tumor progression in gastric cancer

BACKGROUND

Gastric cancer (GC) is the fifth commonest malignancy worldwide and still one of the leading causes of cancer-related death. The aim of this study was to identify a novel prognostic marker or therapeutic target for GC.

METHODS

We analyzed candidate genes from our previous Escherichia coli ampicillin secretion trap (CAST) libraries in detail, and focused on the FKTN gene because it was overexpressed in both GC cell line CAST libraries, MKN-1 and MKN-45.

RESULTS

Quantitative reverse transcriptase PCR analysis of FKTN revealed that FKTN messenger RNA was overexpressed in nine of 28 (32.1 %) GC tissue samples compared with nonneoplastic gastric mucosa. Immunostaining of fukutin showed that 297 of 695 cases (42.7 %) were positive for fukutin. Fukutin-positive GC cases were significantly associated with differentiated histological features, and advanced T grade and N grade. In addition, fukutin expression was observed more frequently in the intestinal phenotype (51 %) of GC than in other phenotypes (37 %) when defined by the expression patterns of mucin 5AC, mucin 6, mucin 2, and CD10. FKTN small interfering RNA treatment decreased GC cell proliferation.

CONCLUSIONS

These results indicate that the expression of fukutin may be a key regulator for progression of GC with the intestinal mucin phenotype.

Muscular dystrophies share pathogenetic mechanisms with muscle sarcomas

Several lines of recent evidence have opened a new debate on the mechanisms underlying the genesis of rhabdomyosarcoma, a pediatric soft tissue tumor with a widespread expression of muscle-specific markers. In particular, it is increasingly evident that the loss of skeletal muscle integrity observed in some mouse models of muscular dystrophy can favor rhabdomyosarcoma formation. This is especially true in old age. Here, we review these experimental findings and focus on the main molecular and cellular events that can dictate the tumorigenic process in dystrophic muscle, such as the loss of structural or regulatory proteins with tumor suppressor activity, the impaired DNA damage response due to oxidative stress, the chronic inflammation and the conflicting signals arising within the degenerated muscle niche.

Post-transcriptional regulation of fukutin in an astrocytoma cell line

Fukutin is the gene responsible for Fukuyama-type congenital muscular dystrophy (FCMD), an autosomal recessive disease associated with central nervous system (CNS) and eye anomalies. Fukutin is involved in basement membrane formation via the glycosylation of α-dystroglycan (α-DG), and hypoglycosylation of α-DG provokes the muscular, CNS and eye lesions of FCMD. Astrocytes play an important role in the pathogenesis of the CNS lesions, but the post-transcriptional regulation of fukutin mRNA has not been elucidated. In this study, we investigated the characteristics of fukutin mRNA using an astrocytoma cell line that expresses fukutin and glycosylated α-DG. The glycosylation of α-DG was considered to be increased by over-expression of fukutin and decreased by knockdown of fukutin. Knockdown of Musashi-1, one of the RNA-binding proteins involved in the regulation of neuronal differentiation, induced a decrease in fukutin mRNA. Immunoprecipitation and ELISA-based RNA-binding assay demonstrated possible binding between fukutin mRNA and Musashi-1 protein. A relationship between fukutin mRNA and vimentin protein was also proposed. In situ hybridization for fukutin mRNA showed a positive cytoplasmic reaction including cytoplasmic processes. From these results, fukutin mRNA is suggested to be a localized mRNA up-regulated by Musashi-1 and to be a component of a mRNA-protein complex which includes Musashi-1 and (presumably) vimentin proteins.

Large induces functional glycans in an O-mannosylation dependent manner and targets GlcNAc terminals on alpha-dystroglycan

Alpha-dystroglycan (α-DG) is a ubiquitously expressed receptor for extracellular matrix proteins and some viruses, and plays a pivotal role in a number of pathological events, including cancer progression, muscular dystrophies, and viral infection. The O-glycans on α-DG are essential for its ligand binding, but the biosynthesis of the functional O-glycans remains obscure. The fact that transient overexpression of LARGE, a putative glycosyltransferase, up-regulates the functional glycans on α-DG to mediate its ligand binding implied that overexpression of LARGE may be a novel strategy to treat disorders with hypoglycosylation of α-DG. In this study, we focus on the effects of stable overexpression of Large on α-DG glycosylation in Chinese hamster ovary (CHO) cell and its glycosylation deficient mutants. Surprisingly, stable overexpression of Large in an O-mannosylation null deficient Lec15.2 CHO cells failed to induce the functional glycans on α-DG. Introducing the wild-type DPM2 cDNA, the deficient gene in the Lec15.2 cells, fully restored the Large-induced functional glycosylation, suggesting that Large induces the functional glycans in a DPM2/O-mannosylation dependent manner. Furthermore, stable overexpression of Large can effectively induce functional glycans on N-linked glycans in the Lec8 cells and ldlD cells growing in Gal deficient media, in both of which circumstances galactosylation are deficient. In addition, supplement of Gal to the ldlD cell culture media significantly reduces the amount of functional glycans induced by Large, suggested that galactosylation suppresses Large to induce the functional glycans. Thus our results revealed a mechanism by which Large competes with galactosyltransferase to target GlcNAc terminals to induce the functional glycans on α-DG.