Insulin stimulates myogenesis in a rat myoblast line

A serum-free medium for the growth of muscle cells in culture

Rates of cell proliferation essentially equal to those in 10% serum were obtained when Yaffe's L6 myoblasts were incubated in Ham's F-12 medium containing 10(-5) M fetuin, 10(-6) M insulin, and 10(-7) M dexamethasone; we have designated this mixture muscle medium-1 (MM-1). Addition of other growth factors and hormones in various combinations did not increase the proliferation of myoblasts above the rate in MM-1, and neither fetuin nor insulin could be replaced by other growth factors. All glucocorticoids tested (but no other steroid hormones) were active. Fetuins prepared by the rather different procedures of Pedersen, Deutsch, and Spiro were all active, and the active material was heat labile and nondialyzable; this is the first cell culture system in which highly purified Spiro fetuin has been found active. Primary rat myoblasts proliferated more rapidly that fibroblasts in parallel cultures when incubated in MM-1. This simple medium, composed of relatively inexpensive and readily available components, should be useful for the study of muscle cell growth and differentiation.

Effects on induction of tyrosine aminotransferase in fetal mouse liver in vitro of prednisolone, insulin and thyroxine

The hormonal requirements for formation of tyrosine aminotransferase (EC 2.6.1.5) in fetal mouse liver were investigated in organ culture using chemically defined medium. The hormones tested were insulin, thyroxine and prednisolone. Prednisolone alone resulted in a two-fold increase in tyrosine amino-transferase activity in explanted liver in hormone-free medium on day 6, and its effect was dose dependent, but neither insulin nor thyroxine alone induced the enzyme. Addition of prednisolone plus thyroxine and prednisolone plus insulin increased the enzyme activity 1.4- and 1.3-fold, respectively, over that of explants with prednisolone alone. These three hormones together had the greatest effect, causing induction of 1.5-fold more activity than that with prednisolone plus insulin or plus thyroxine. The three hormones were not all needed continuously during the culture period: prednisolone and insulin were required during the early part of cultivation and thyroxine during the later part. The effects of these hormones were blocked by actinomycin D or puromycin, suggesting that these hormones increase de novo synthesis of tyrosine aminotransferase. Phase-contrast microscopy showed that prednisolone stimulated liver epithelial cell outgrowth, probably acting with insulin.

Insulin binding by a cell line (HT 29) derived from human colonic cancer

Insulin binding was demonstrated in cultured HT 29 cells originating from a human colon carcinoma. At 37 degrees and in complete medium, the binding of [125I]insulin (1-4x10-10M) reaches a maximum in 40 min and the cell associated radioactivity remains constant for at least 4 h. No degradation of the hormone is observed under these conditions. The binding is proportional to the number of cells and its pH optimum is 7.8. In the presence of excess insulin 50% of the [125I]insulin is dissociated from the complex after 10 min. At equilibrium, insulin binding is specific: proinsulin is 25 times less potent than native insulin in competing with [125I]insulin and related polypeptide hormones are inactive. Scatchard analysis indicates two classes of binding sites (1400 sites/cell of "high affinity" e.g. 4.7 x 108 M-1, and 20 000 sites of "low affinity" e.g. 4 x 107 M-1). The binding of insulin to this non-target cell shows the same kinetic characteristics and specificity as found for insulin in its target cells, except that HT 29 cells do not degrade the hormone. The problem of the correlation between insulin binding and a biological effect in these cells remains to be elucidated.

Undifferentiated and differentiated L6 myoblast plasma membranes. I: Comparison of the morphology of plasma membrane vesiculation and the factors influencing the vesiculation process

Monolayer cultures of a variety of cell lines can be induced to shed plasma membrane vesicles by exposure to 25 mM formaldehyde--2 mM dithiothreitol. An analysis of the process of membrane shedding can provide a method to probe the general characteristics of the plasma membrane in different cell populations. We have, therefore, analyzed membrane shedding in L6 myoblasts at different stages of differentiation. The results show that the morphology of membrane shedding is comparable in undifferentiated or differentiated L6 myoblasts, as are the factors that influence the process of membrane shedding. L6 myoblasts at various stages of differentiation have similar requirements for metabolic energy, physiological temperature, pH, and monovalent and divalent cations.

Plasma membrane phospholipid, cholesterol and fatty acyl composition of differentiated and undifferentiated L6 myoblasts

The lipid composition of plasma membranes isolated from differentialted and undifferentiated L6 myoblasts have been compared. In general, the plasma membranes of differentiated L6 myoblasts have a higher cholesterol to phospholipid molar ratio than plasma membranes of undifferentiated cells. Differentiated L6 myoblasts have increased relative amounts of phosphatidylethanolamine and phosphatidylcholine ine in their plasma membrane and a decreased relative amount of sphingomyelin when compared with the plasma membranes of undifferentiated myoblasts. In addition, preliminary results show that differentiated L6 myoblasts plasma membrane phospholipid shows differences in the fatty acyl composition, specifically there appears to be relatively more 17:0 and 24:1 and less 16:1 and 18:1 than in plasma membrane phospholipids of undifferentiated L6 myoblasts. These observations indicate that significant changes in plasma membrane lipid composition occur during myoblast differentiation. The role that changes in lipid composition play in control of cellular differentiation, however, remains to be elucidated.

A myogenic cell line with altered serum requirements for differentiation

Dfferentiation properties of a cell line, L84, which originated from a non-fusing clone isolated from the myogenic line L8, are described. In nutritional medium supplemented with 10% serum used routinely with L8 cells, L84 cells continue to proliferate to very high densities and fail to form multinucleated fibres. When grown in medium supplemented with 2% horse serum of 2% horse serum plus 0.1% microng/ml insulin, L84 cells behave very similarly to L8 cells grown in medium supplemented with 10% horse serum: when the cultures reach confluency, proliferation decreases and cells start to fuse and form a dense network of fibres. Large increases in creatine kinase activity and synthesis of myosin are associated with cell fusion. Under conditions in which L84 cells do not fuse the increase in these synthetic activities is not observed, even after extremely high cell densities are reached. The data show that L84 cells retain the programme for their differentiation into muscle fibres. The difference between L84 and its progenitor line L8 lies in the sensitivity to the environmental conditions which trigger the expression of this programme.

Differential effects of inhibitors of cell division upon the growth stimulating activities of insulin and serum in nutritionally depleted human and mouse cells

Cultured mouse and human cells were arrested in their growth by artificially depriving them of phosphate. The quiescent cells could be stimulated to synthesize DNA and to divide by addition to the growth medium of insulin, dialyzed serum and/or the full concentration of phosphate. In order to gain insight into mechanisms by which insulin and serum stimulate growth, the inhibitory effects of antimitotic agents were examined. Of the inhibitors tested, vinblastine and cytocalasin B abolished the growth promoting activity of insulin, while colchicine inhibited the activity of both serum and insulin. The present results suggest that insulin-stimulated growth is meciated by a different path way than serum-stimulated growth and is sensitive to mechanisms that occur at various times prior to insulin addition.

Cell surface changes accompanying myoblast differentiation

Myoblasts are mononucleated cells and associated with differentiation undergo cell fusion and become multinucleated. The current studies have examined cell surface dynamic changes of Concanavalin A lectin receptor mobility and the role of hormones in modulating myoblast differentiation. A uniform distribution of Con-A receptors is observed in undifferentiated cells when reacted with Con-A at 37 degrees C. Cells from differentiating cultures or fully differentiated myotubes reacted similarly at 37 degrees C show a significant redistribution of Con-A into patches, "caps," and endocytic vesicles containing Con-A. If undifferentiated and differentiated cells are first prefixed with glutaraldehyde then reacted with Con-A continuous distribution of Con-A is seen across the cell surface. This suggests redistribution of Con-A and its receptors occurs in differentiated cells reacted with lectin at 37 degrees C. It is further shown that insulin (10 microgram/ml) significantly enhances myoblast differentiation but that this occurs after an apparent stimulation of proliferation. In contrast to insulin, dexamethasone (10 micron and 100 micron) profoundly inhibits myoblast differentiation while having different effects on proliferation; 10 micron dex stimulates cell growth while 100 micron dex suppresses cell proliferation. Lastly, an extracellular filamentous matrix which binds Con-A is observed at the ultrastructural level in high density cultures. No significant redistribution of Con-A is observed on this matrix in distinction to the redistribution observed on the cell membrane in differentiated cells.

Effect of fibroblast growth factor on the division and fusion of bovine myoblasts

The effect of fibroblast growth factor (FGF) on the rate of proliferation and fusion of bovine myoblast has been examined. Addition to the cultures of 0.1 mug-1 mug/ml of FGF stimulates the rate of proliferation and delays the fusion of primary cultures of bovine myoblasts cultured in 10% serum. Final cell densities reached in the presence of 0.1 mug/ml of FGF were fivefold higher than in controls; with 1 mug/ml, they were 10-fold higher. Increases in cell density were paralleled by increases in acetylcholine receptor sites as measured by the binding of 125I-alpha-bungarotoxin. Both fusion and the appearance of acetylcholine receptor sites were delayed in the presence of FGF. Growth hormone, insulin and testosterone, which have been reported to be mitogenic for rat and chick embryo myoblasts, did not have significant effects on DNA synthesis in bovine myoblasts when compared to the FGF. Conversely, FGF did not stimulate the proliferation of chick embryo myoblasts, indicating that it is not active in all vertebrate species.

Gene expression during the differentiation of myogenic cells of the L6 line

The problem of the mechanistic relationship among the different phenotypic expressions in an established myogenic line was approached by blocking cell fusion at different developmental stages, by addition of cytochalasin B. The addition of the drug to cultures at the time when the first two myotubes appeared on the dish, blocked fusion, but did not affect DNA synthesis, expression of myosin, phosphorylase, phosphocreatine kinase, phosphorylase kinase or glycogen synthetase, nor the organization of the elements of the hexagonal lattice. It is concluded that cell fusion is not a prerequisite for the expression of the differentiated phenotype.

Myogenesis of avian striated muscle in vitro: role of collagen in myofiber formation

The complex chemical environment required for the development of the myofiber (myotube) from embryonic avian muscle myoblasts in vitro has been simplified. Myotube formation is shown to occur in the presence of insulin, a low-molecular-weight (fusion) factor obtained from embryo extract, and a collagen substratum. In the absence of collagen, globular structures are formed which are microscopically identical to the globular syncytial structures previously described [de la Haba and Amundsen (1972) Proc. Nat. Acad. Sci. USA 69, 1131-1135]. A high-molecular-weight fraction from embryo extract will also promote myotube formation in the presence of the fusion factor. This fraction is shown to contain (a) collagen, which promotes myotube formation, and (b) an additional factor which also promotes myotube formation, and which we tentatively hypothsize to be an inducer of collagen synthesis by myogenic cells.