Differentiation-dependent expression of apolipoprotein A-I in chicken myogenic cells in culture

Dietary lysine imbalance affects muscle proteome in zebrafish (Danio rerio): a comparative 2D-DIGE study

Lysine (Lys) is an indispensable amino acid (AA) and generally the first limiting AA in vegetable protein sources in fish feeds. Inadequate dietary Lys availability may limit protein synthesis, accretion and growth of fish. This experiment aimed to further elucidate the role of Lys imbalance on growth by examining the myotomal muscle proteome of juvenile zebrafish (Danio rerio). Quadruplicate groups of 8 fish were fed either a low-Lys [Lys(-), 1.34 g kg(-1)], medium/control (Lys, 2.47 g kg(-1)) or high-Lys [Lys(+), 4.63 g kg(-1)] diet. Fish growth was monitored from 33 to 49 days post-fertilization (dpf) and trunk myotomal muscle proteome of Lys(-) and Lys(+) treatments were screened by 2D-DIGE and MALDI ToF tandem mass spectrometry. Growth rate was negatively affected by diet Lys(-). Out of 527 ± 11 (mean ± S.E.M.) protein spots detected (∼10-150 kDa and 4-7 pI value), 30 were over-expressed and 22 under-expressed in Lys(-) fish (|fold-change| >1.2, p value <0.05). Higher myosin light chains abundance and other myofibrillar proteins in Lys(-) fish pointed to increased sarcomeric degradation, indicating a higher protein turnover for supplying basal energy-saving metabolism rather than growth and muscle protein accretion. The Lys deficiency also possibly induced a higher feeding activity, reflected in the over-expression of beta enolase and mitochondrial ATP synthase. Contrarily, in the faster growing fish [Lys(+)], over-expression of apolipoprotein A-I, F-actin capping protein and Pdlim7 point to increased energy storage as fat and enhanced muscle growth, particularly by mosaic hyperplasia. Thus using an exploratory approach, this study pinpoints interesting candidates for further elucidating the role of dietary Lys on growth of juvenile fish.

Acute phase lipocalin Ex-FABP is involved in heart development and cell survival

Ex-FABP is an extracellular fatty acid binding protein, expressed during chicken embryo development in cartilage, muscle fibers, and blood granulocytes. Transfection of chondrocytes and myoblasts with anti-sense Ex-FABP cDNA results in inhibition of cell proliferation and apoptosis induction. Ex-FABP expression is dramatically enhanced by inflammatory stimuli and in pathological conditions. In this paper, by in situ whole mount and immunohistochemistry analysis we show that, at early developmental stage, Ex-FABP is diffuse in all tissues of chick embryos. Particularly high level of transcript and protein are expressed in the heart. During acute phase response (APR) induced by endotoxin LPS injection, a marked increase of Ex-FABP mRNA was observed in embryos, highest Ex-FABP expression being in heart and liver. To investigate in vivo the biological role of Ex-FABP, we have directly microinjected chicken embryos with antibody against Ex-FABP. Almost 70% of chicken embryos died and the target tissue was the heart. We detected in heart of the treated embryos a significant increase of apoptotic cells and high level of fatty acids. We propose that the accumulation of fatty acid, specific ligand of Ex-FABP, in the cell microenvironment is responsible of heart cell death, and we suggest that Ex-FABP may act as a survival protein by playing a role as scavenger for fatty acids.

Expression of serum amyloid A in chondrocytes and myoblasts differentiation and inflammation: possible role in cholesterol homeostasis

Serum amyloid A (SAA) is synthesized by the liver during the acute phase. Local expression of SAA mRNA has been reported also in non-liver cells, a potential local source of SAA protein not related to the systemic acute phase response. SAA function has not been established yet. In the present study, we identified SAA as a protein expressed by chondrocytes and myoblasts in response to inflammatory stimula. In both cell systems, SAA mRNA and protein expression is strongly stimulated by bacterial lipopolysaccharide treatment. SAA mRNA expression is also enhanced during terminal differentiation of cells of the chondrogenic and myogenic lineage; mRNA is barely detectable in prechondrogenic cells and is highly expressed in differentiated hyperthrophic chondrocytes. An increased level of SAA mRNA was also observed in vivo when we compared mRNA extracted from tibiae of 10 day embryos, still fully cartilaginous, with tibiae from 18 day embryos, a stage when the endochondral ossification process has already started. p38 activation, a well-known event of the chondrogenesis signaling cascade, controls expression of SAA in cartilage following inflammatory stimuli. SAA secreted by stimulated chondrocytes is associated with cholesterol. Cholesterol is synthesized by the same chondrocytes and is also increased in inflammatory conditions. A role of SAA in cholesterol homeostasis in chondrocytes is proposed.

Cholesterol secretion and homeostasis in chondrocytes: a liver X receptor and retinoid X receptor heterodimer mediates apolipoprotein A1 expression

Cholesterol is required for chondrocyte differentiation and bone formation. Apolipoprotein A1 (apoA-1) plays a major role in lipoprotein clearance and cholesterol redistribution. We report here that apoA-1 is expressed during chondrocyte differentiation in vitro and in vivo. In differentiating chondrocytes, the expression of the liver X receptor (LXR) is modulated and its expression correlates to the expression of apoA-1. The expression of other LXR target genes related to cholesterol homeostasis such as ABCA1 cholesterol transporter and sterol regulatory element-binding protein 1 (SREBP1) is similarly regulated. Small molecule ligands activating either LXR or retinoid X receptor (RXR) lead to a dramatic increase in apoA-1 mRNA and protein expression in cultured chondrocytes. These ligands strongly induce ABCA1 cholesterol transporter expression and effectively mediate cholesterol efflux from hypertrophic chondrocytes. In addition, we report that, in the same cells, the ligands down modulate Serum Amyloid A expression induced by bacterial lipopolysaccharide. Our studies provide evidence that LXR/RXR mediate a fine regulation of cholesterol homeostasis in differentiating chondrocytes.

Inhibition of cell proliferation and induction of apoptosis by ExFABP gene targeting

Ex-FABP, an extracellular fatty acid binding lipocalin, is physiologically expressed by differentiating chicken chondrocytes and myoblasts. Its expression is enhanced after cell treatment with inflammatory stimuli and repressed by anti-inflammatory agents, behaving as an acute phase protein. Chicken liver fragments in culture show enhanced protein expression after bacterial endotoxin treatment. To investigate the biological role of Ex-FABP, we stably transfected proliferating chondrocytes with an expression vector carrying antisense oriented Ex-FABP cDNA. We observed a dramatic loss of cell viability and a strong inhibition of cell proliferation and differentiation. When chondrocytes were transfected with the antisense oriented Ex-FABP cDNA we observed that Ex-FABP down-modulation increased apoptotic cell number. Myoblasts transfected with the same expression vector showed extensive cell death and impaired myotube formation. We suggest that Ex-FABP acts as a constitutive survival protein and that its expression and activation are fundamental to protect chondrocytes from cell death.

Avidin expression during chick chondrocyte and myoblast development in vitro and in vivo: regulation of cell proliferation

Avidin is a major [(35)S]methionine-labeled protein induced by bacterial lipopolysaccharide (LPS) and interleukin 6 (IL-6) in cultured chick embryo myoblasts and chondrocytes. It was identified by N-terminal sequencing of the protein purified from conditioned culture medium of LPS-stimulated myoblasts. In addition, avidin was secreted by unstimulated myoblasts and chondrocytes during in vitro differentiation; maximal expression being observed in differentiated myofibers and hypertrophic chondrocytes. In developing chick embryos, immunohistochemistry revealed avidin in skeletal muscles and growth plate hypertrophic cartilage. Avidin was secreted into culture as a biologically active tetramer. Exogenous avidin added to the medium of proliferating chondrocytes progressively inhibited cell proliferation, whereas addition of avidin to differentiating chondrocytes in suspension allowed full cell differentiation. No toxic effects for the cells were observed in both culture conditions. Western blots of samples from cytosolic extracts using alkaline-phosphatase-conjugated streptavidin showed three biotin-containing proteins. Acetyl-CoA carboxylase was identified by specific antibodies. Based on these data, we propose that avidin binds extracellular biotin and regulates cell proliferation by interfering with fatty acid biosynthesis during terminal cell differentiation and/or in response to inflammatory stimuli.

Large-scale analysis of differential gene expression in the hindlimb muscles and diaphragm of mdx mouse

The mdx mouse is an animal model for Duchenne muscular dystrophy (DMD), which is caused by the absence of dystrophin. Mdx limb muscles substantially compensate for the lack of dystrophin while the diaphragm is affected like DMD skeletal muscles. To understand better the complex cascade of molecular events leading to muscle degeneration and compensatory processes in mdx muscles, we analyzed alterations of gene expression in mdx hindlimb and diaphragm muscles as compared to their normal counterparts. The strategy was based on suppression subtractive hybridization followed by reverse Northern quantitative hybridization. Four subtracted/normalized libraries, containing cDNA clones up- or downregulated in mdx hindlimb muscles or diaphragm, were constructed and a total of 1536 cDNA clones were analyzed. Ninety-three cDNAs were found to be differentially expressed in mdx hindlimb muscles and/or diaphragm. They corresponded to 54 known genes and 39 novel cDNAs. The potential role of the known genes is discussed in the context of the mdx phenotype.

Expression of the extracellular fatty acid binding protein (Ex-FABP) during muscle fiber formation in vivo and in vitro

We report that Ex-FABP, an extracellular protein belonging to the lipocalin family and involved in the extracellular transport of long-chain fatty acids, is expressed in the forming myotubes both in vivo and in vitro. The presence of the protein and of the mRNA was observed in newly formed myotubes at early stages of chick embryo development by immunohistochemistry and by in situ hybridization. At later stages of development myofibers still expressed both the mRNA and the protein. Ex-FABP expression was observed also in the developing myocardium and the muscular layer of large blood vessels. In agreement with these findings, an initial expression of the mRNA and protein secretion by cultured chicken myoblasts were observed only after the onset of myoblast fusion. Double-immunofluorescence staining of these cultured cells revealed that multinucleate myotubes were stained by antibodies directed against both the Ex-FABP and the sarcomeric myosin, whereas immature myotubes and single myoblasts were not. When added to cultured myoblasts, antibodies against the Ex-FABP induced a strong enhancement of the production of the same protein. In all experiments some cell sufferance and a transient impairment of myotube formation were also observed. The finding that the continuous removal of the Ex-FABP from the culture medium of myoblasts, due to the formation of immune complexes, resulted in an overproduction of the protein suggests a feedback (autocrine) control during myotube differentiation and maturation. We propose that the requirement for increased transport and metabolism of free fatty acid released from the membrane phospholipids and storage lipids, mediated by Ex-FABP, may be essential during differentiation of multinucleated myotubes or that an increased local demand of fatty acids and metabolites may act as a local hormone in tissues differentiating and undergoing morphogenesis.

Myogenic conversion of mammalian fibroblasts induced by differentiating muscle cells

Somite-derived skeletal myoblasts are supposed to be the sole source of muscle fibre nuclei during pre- and postnatal development, but evidence is accumulating for unorthodox contributions to muscle fibre nuclei from other cell types. For example, in tissue culture, fibroblasts can fuse with dysgenic myoblasts and restore correct membrane function. We report here the results of a series of experiments investigating this phenomenon and its possible mechanism. 10T1/2 cells, infected with a replication defective retrovirus encoding the bacterial enzyme beta-galactosidase, fused to form beta-galactosidase positive, differentiated myotubes when cocultured with differentiating uninfected C2C12 or primary myogenic cells, but this did not occur when they were cocultured with other cells such as 3T3 fibroblasts or PC12 pheochromocytoma cells. Myogenic conversion ranged from 1 to 10% of the 10T1/2 cell population and required close cell interaction between the different cells types: it was not induced by conditioned medium or extracellular matrix deposited by C2C12 cells. Myogenic conversion was also observed in vivo, after injection of similarly infected 10T1/2 cells into regenerating muscle. Conversion was seen also after coculture of uninfected 10T1/2 cells with primary chick myoblasts, thus demonstrating that it was not dependent upon viral infection and that there is no species or class barrier in this phenomenon. Primary fibroblasts, isolated from different organs of transgenic mice carrying a Lac Z marker under the control of a muscle-specific promoter, restricting beta-galactosidase expression to striated muscle cells, also underwent myogenic conversion, when cocultured with C2C12 myoblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of FGF and FGF receptor genes in human breast cancer

The family of FGF growth factors is involved in several biological processes and might play an important role in tumorigenesis. We have studied the respective expression of 8 of the 9 characterized FGF genes, and of the 4 known FGF receptor genes, in a panel of 10 tumor-cell lines and 103 breast-tumor samples, using RT-PCR and Northern-blot analyses. FGF1 and FGF2 were expressed in almost all samples, while expression of FGF5, FGF6, FGF7, and FGF9 was more restricted. FGFR1, FGFR2 and FGFR4 were expressed at high levels in respectively 22%, 4% and 32% of tumors. FGFR3 expression was not detected. The transcript encoding an FGFR1 isoform with 2 immunoglobulin-like domains was the most prevalent.

Differential response of embryonic and fetal myoblasts to TGF beta: a possible regulatory mechanism of skeletal muscle histogenesis

Embryonic and fetal skeletal myoblasts were grown in culture in the presence of TGF beta. Under the conditions employed, TGF beta inhibited differentiation of fetal but not of embryonic myoblasts. To investigate the possible relevance of these data to skeletal muscle histogenesis in vivo, we studied the proliferation/differentiation state of mesodermal cells in the proximal region of the limb bud at the time of primary fiber formation. BrdU labeling and immunostaining for myosin heavy chains revealed that very few mesodermal cells enter the S phase of the cycle when differentiated primary fibers first appear. However, a few hours later, many cells in S phase surround newly formed muscle fibers, suggesting that the latter may be a source of mitogens for undifferentiated myoblasts. Co-culture experiments supported this hypothesis, showing that medium conditioned by fiber-containing explants can stimulate myoblast proliferation. Taken together these data suggested a possible mechanism for the regulation of muscle fiber formation. The model assumes that fibers form in the proximal region of the limb bud, where TGF beta is known to be present, and BrdU labeling experiments did not reveal cells in S phase. It is conceivable that non-dividing embryonic myoblasts (which do not respond to TGF beta) can undergo differentiation, while fetal myoblasts are inhibited by TGF beta. Once formed, primary fibers may stimulate a new wave of proliferation in fetal myoblasts, in order to expand the pool of cells needed to form secondary fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Pro-opiomelanocortin gene is expressed in post-implantation mouse embryos and enhances growth potential of myogenic cells

We previously showed that the adrenocorticotropin hormone (ACTH) is a mitogen for myoblasts and is present in post-implantation mammalian embryos (Cossu et al. [1989] Dev. Biol. 131:331-336; De Angelis et al. [1992] Dev. Biol. 151:446-458). In this paper, we investigated the expression of the corresponding gene, pro-opiomelanocortin (POMC), by in situ hybridization and polymerase chain reaction. In situ analysis revealed low level expression in the basal layer of 10.5 d.p.c. neural tube and in several discrete areas around the dorsal aorta. By more sensitive Reverse-transcription polymerase chain reaction (RT-PCR) analysis, expression was detected also in developing limb buds and in cultured myogenic cells, but not in fibroblasts. To investigate the possible role of POMC gene expression in myogenesis, we induced its over-expression in proliferating myoblasts. Upon sub-optimal growth conditions, over-expressing cells were found to give rise to clones larger than control cells. The differentiation potential of POMC over-expressing myogenic cells was unchanged.

Characterization of the chicken apolipoprotein A-I gene 5'-flanking region

Apolipoprotein A-I (apoA-I) is a major protein component of plasma high-density lipoprotein in all species studied, and plays an important role in cholesterol homeostasis. In an earlier study, we cloned and structurally characterized the chicken apoA-I gene. In this study, the 5'-flanking region of the chicken apoA-I gene was sequenced and functionally characterized. Sequence analysis of the 510-nucleotide 5' upstream region revealed the presence of TATA and CCAAT boxes. In addition, we identified binding sites for several transcription factors such as Sp1, AP1, and NFI.2. When the 5' fragment was ligated into a promoterless CAT vector and transfected into a chicken hepatocarcinoma cell line (LMH), the bacterial chloramphenicol acetyl transferase (CAT) gene was expressed, suggesting transcriptional regulation is associated with this region. Transfection studies with other 5' deletion constructs revealed that the sequence spanning the region -82 to +87 contained the major transcriptional activity. DNase I footprinting, gel retardation, and Southwestern blot analyses showed that the fragment interacts with nuclear proteins.

Induction of Calbindin-D28K by 1,25-dihydroxyvitamin D3 in cultured chicken intestinal cells

Intestinal cells from chicken embryos were grown in chemically defined, serum-free medium. The majority of cultured cells exhibits an epithelial-like morphology. As demonstrated by indirect immunofluorescence, the epithelial cells, and not the contaminating fibroblasts, express Calbindin-D28K only after 1,25-dihydroxyvitamin D3, the hormonally active form of vitamin D, is added to the culture medium. The highly sensitive reverse transcriptase-polymerase chain reaction shows that both Calbindin-D28K mRNA and the corresponding primary unprocessed transcripts (pre-mRNA) are dramatically increased in cultured intestinal cells treated with 1,25-dihydroxyvitamin D3, thus indicating that Calbindin-D28K is induced by the increased rate of transcription of the corresponding gene.

MyoD, myogenin independent differentiation of primordial myoblasts in mouse somites

The accumulation of two myogenic regulatory proteins, MyoD and myogenin, was investigated by double-immunocytochemistry and correlated with myosin heavy chain expression in different classes of myoblasts in culture and during early myogenesis in vivo. During in vitro differentiation of fetal myoblasts, MyoD-positive cells were detected first, followed by the appearance of cells positive for both MyoD and myogenin and finally by the appearance of differentiated myocytes and myotubes expressing myosin heavy chain (MHC). A similar pattern of expression was observed in cultures of embryonic and satellite cells. In contrast, most myogenic cells isolated from newly formed somites, expressed MHC in the absence of detectable levels of myogenin or MyoD. In vivo, the appearance of both myogenin and MyoD proteins was only detected at 10.5 d postcoitum (d.p.c.), when terminally differentiated muscle cells could already be identified in the myotome. Parasagittal sections of the caudal myotomes of 10.5-d-old embryos showed that expression of contractile proteins preceded the expression of myogenin or MyoD and, when coexpressed, MHC and myogenin did not co-localize within all the cells of the myotome. In the limb bud, however, many myogenin (or MyoD) positive/MHC negative cells could be observed in the proximal region at day 11. During further embryonic development the expression of these proteins remained constant in all the muscle anlagens examined, decreasing to a low level during the late fetal period. Western and Northern analysis confirmed that the myogenin protein could only be detected after 10.5 d.p.c. while the corresponding message was clearly present at 9.5 d.p.c., strongly suggesting a posttranscriptional regulation of myogenin during this stage of embryonic development. These data show that the first myogenic cells which appear in the mouse myotome, and can be cultured from it, accumulate muscle structural proteins in their cytoplasm without expressing detectable levels of myogenin protein (although the message is clearly accumulated). Neither MyoD message or protein are detectable in these cells, which may represent a distinct myogenic population whose role in development remains to be established.

Apolipoprotein A-1 expression is resistant to dimethyl sulfoxide inhibition of myogenic differentiation

Primary cultures of chick embryonic muscle (CEM) were analyzed for the differential expression of a 26-kDa protein during myogenesis. We have identified this 26-kDa protein as apolipoprotein A-1 (Apo A-1), the major protein of serum high density lipoprotein particles. Apo A-1 was expressed in a pattern temporally similar to those of muscle-specific proteins, by myoblasts at very low levels, and by myotubes at high levels. The half-life of Apo A-1 in CEM cell homogenates was 23 min. This fast turnover rate appeared to be due to the secretion of Apo A-1 into the culture medium. To further characterize the relationship of Apo A-1 expression and myogenic differentiation, CEM cultures were treated with dimethyl sulfoxide (DMSO). In the presence of 2% DMSO, myotubes exhibited an atrophied morphology and an inhibition of the synthesis and accumulation of muscle-specific sarcomeric proteins. During recovery from DMSO treatment, the expression and accumulation of muscle-specific proteins returned to high levels. In contrast, the rates of synthesis and secretion of Apo A-1 in control, DMSO-treated, and DMSO-recovered CEM cells were nearly equivalent. These results indicate that the expression of Apo A-1 is not strictly linked to the expression of muscle-specific sarcomeric proteins in skeletal muscle and suggest that independent, or additional regulatory mechanisms exist which modulate Apo A-1 expression during myogenesis.