Epidermal growth factor controls smooth muscle alpha-isoactin expression in BC3H1 cells

Identification and biological characterization of chicken embryonic cardiac progenitor cells

OBJECTIVES

Many kinds of cardiac progenitor cell populations have been identified, including c-kit(+) , Nkx2.5(+) s and GATA4(+) cells. However, these progenitors have limited ability to differentiate into different cardiac cell types. Recently, a new kind of cardiac progenitor cell named the multipotent Isl1(+) cardiovascular progenitor (MICPs) has been identified, which also expresses Nkx2.5, GATA4, CD34 and Flk1.

MATERIALS AND METHODS

In this study, we have isolated and characterized MICPs from chicken embryonic heart tissues using immunofluorescence and PCR.

RESULTS

Results shown that they express markers of cardiac progenitor cells, with high clonality. They have the ability to self-renew and can give rise to three types of heart cell in vitro.

CONCLUSIONS

Myocytes, smooth muscle cells and endothelial cells. Our work provides evidence for a developmental paradigm of the heart, that endothelial and muscle lineage diversification arises from multipotent cardiac progenitor cells. Existence of these cells provides a new opportunity for myocardial injury repair.

Lysophosphatidic acid and bFGF control different modes in proliferating myoblasts

Myogenic cells provide excellent in vitro models for studying the cell growth and differentiation. In this study we report that lysophosphatidic acid (LPA), a bioactive phospholipid contained in serum, stimulates the growth and inhibits the differentiation of mouse C2C12 myoblast cells, in a distinct manner from basic fibroblast growth factor (bFGF) whose mitotic and anti-differentiation actions have been well investigated. These actions of LPA were both blocked by pertussis toxin, suggesting the involvement of Gi class of G proteins, whereas bFGF acts through receptor tyrosine kinases. Detailed analysis revealed that LPA and bFGF act differently in regulating the myogenic basic helix-loop-helix (bHLH) proteins, the key players in myogenic differentiation process. LPA stimulates the proliferation of undifferentiated myoblasts allowing the continued expression of MyoD, but in contrast, bFGF does so with the MyoD expression suppressed at the mRNA level. Both compounds maintain the myf-5 expression, and suppress the myogenin expression. In addition, while LPA did not inhibit cell-cell contact-induced differentiation, bFGF strongly inhibited this process. Furthermore, LPA and bFGF act cooperatively in their mitogenic and anti-differentiation abilities. These findings indicate that LPA and bFGF differently stimulate intracellular signaling pathways, resulting in proliferating myoblasts each bearing a distinct expression pattern of myogenic bHLH proteins and distinct differentiation potentials in response to cell-cell contact, and illustrate the biological significance of Gi-mediated and tyrosine kinase-mediated signals.

Reduced number and coupling of beta-adrenergic receptors in a modified S49 mouse lymphoma cell line

Long-term culture of S49 wild-type cells in medium containing a high concentration of fetal calf serum leads to a modified (S49m) cell line with a reduced number of beta-adrenergic receptors (R). These S49m cells with a higher rate of proliferation were unable to respond to the beta-adrenergic agonists isoproterenol (ISO) and epinephrine as analysed by measuring adenylate cyclase (ac) activity on purified membranes of these cells. Additionally, no accumulation of cyclic AMP was obtained on S49m intact cells upon stimulation with beta-agonists. Nevertheless, S49m cells were able to respond significantly to the direct activation of the stimulatory guanine nucleotide binding (Gs) protein by aluminium tetrafluoride and sodium fluoride, and to the stimulation of another receptor coupled to the ac system through a Gs protein, by prostaglandin E1 (PGE1). When cloning S49m cells, similar results were obtained upon stimulation with ISO and PGE1 and the cloned cells express the same thy 1.2 and class Id molecules as do S49 cells. The study of S49m cells indicates that they are a beta-adrenergic R-deficient variant distinct from the other variants described for S49 cells.

Control of profilin and actin expression in muscle and nonmuscle cells

Profilin is a small G-actin binding protein implicated in sequestering actin monomers in vivo. We have quantitated profilin and actin expression in human hepatoma HepG-2 cells and in two mouse myogenic cell lines, BC3H1 and C2C12, to determine whether the expression of profilin and the expression of nonmuscle isoactin or total actin are co-regulated. During differentiation of both muscle cell types, profilin and nonmuscle actin expression decrease in a coordinate manner as shown by measurements of steady state mRNA and newly synthesized protein. In human hepatoma HepG-2 cells, the twofold increase in actin synthesis observed after 24 hours of exposure to cytochalasin D did not result in an increase in profilin synthesis. Thus, profilin and actin expression are not co-regulated in all cells. To determine if there is sufficient profilin to sequester a large portion of cellular G-actin, we measured total profilin and G-actin levels in the three cell types. In each case, profilin accounted for less than 10% of the total G-actin on a molar basis. Thus, profilin is not responsible for total G-actin sequestration in these cells. Finally, using poly-L-proline affinity chromatography, we showed that, in the cell types tested, less than 20% of the poly-L-proline purified profilin existed as a complex with G-actin. The profilin in these cells may be interacting with cellular components other than actin.

Density-dependent modulation of vascular smooth muscle alpha-actin biosynthetic processing in differentiated BC3H1 myogenic cells

The expression of vascular smooth muscle (VSM) alpha-actin mRNA during BC3H1 myogenic cell differentiation is specifically stimulated by conditions of high cell density. Non-proteolytic dissociation of cell-cell and cell-matrix contacts in post-confluent cultures of BC3H1 myocytes using EDTA promotes loss of the differentiated morphological phenotype. EDTA-dispersed myocytes exhibit an undifferentiated fibroblastoid appearance and contained reduced levels of both VSM and skeletal alpha-actin mRNA. Muscle alpha-actin mRNA levels in EDTA-dispersed myocytes were not restored to that observed in confluent myocyte preparations by experimental manipulation of cell density conditions. Pulse-labeling techniques using L-[35S]cysteine to identify muscle actin biosynthetic intermediates revealed that EDTA-dispersed myocytes expressed nascent forms of both the VSM and skeletal muscle alpha-actin polypeptide chains. However EDTA-dispersed myocytes were less efficient in the post-translational processing of immature VSM alpha-actin compared to non-dispersed myocytes. Simple cell-to-cell contact may mediate VSM alpha-actin processing efficiency since high-density preparations of EDTA-dispersed myocytes processed more VSM alpha-actin intermediate than myocytes plated at low density. The actin isoform selectivity of the response to modulation of intercellular contacts suggests that actin biosynthesis in BC3H1 myogenic cells involves mechanisms capable of discriminating between different isoform classes of nascent actin polypeptide chains.

The mammalian iris-ciliary complex affects organization and synthesis of cytoskeletal proteins of organ and tissue cultured lens epithelial cells

A water soluble growth inhibitor was isolated from the mammalian ocular iris-ciliary complex. The molecular weight of this protein is 10 kD or lower as determined by ultrafiltration fractionation. The iris-ciliary (IC) complex water soluble protein(s) significantly inhibits synthesis of lower molecular weight proteins of the epithelial cells of the organ cultured mammalian ocular lens. It was also found that this inhibitory effect of IC is mediated via the structural organization of the lens. Monolayer cultures of the lens epithelial cells exposed to IC did not manifest any inhibition of their protein synthesis. Moreover, these tissue cultured lens epithelial (TCLE) cells showed a significant increase in their protein synthetic activities in response to the presence of IC factors in the culture medium. It is postulated that the IC activity is modulated via either the lens capsule, an extracellular matrix, or due to the specific organization of the intact lens. The specific effects of IC on the cytoskeletal organization and synthesis in the organ cultured lens epithelial (OCLE) and TCLE cells were also examined. Both groups, treated with IC factors, manifested significant alterations in their protein synthetic activities and cytoskeletal architecture. The 3H-leucine incorporation experiments showed that alpha-actin and alpha-tubulin synthesis is partially inhibited by IC factors in OCLE cells but vimentin synthesis is not, whereas in TCLE cells all of them showed increased synthesis in response to IC factors. Turnover rates of these proteins in both OCLE and TCLE cells were also computed. The immunofluorescence and microscopic evaluation of OCLE and TCLE cells exposed to IC factors illustrated significant alteration in the cytoarchitecture of the filaments. We demonstrate that an inhibitor(s) molecule of 10 kD or lower size isolated from IC inhibited protein synthesis of OCLE cells and stimulated protein synthesis in TCLE cells. The IC factor also affects the synthesis and organization of cytoskeletal filaments of both the OCLE and TCLE cells.

Transcription of the muscle regulatory gene Myf4 is regulated by serum components, peptide growth factors and signaling pathways involving G proteins

The muscle regulatory protein myogenin accumulates in differentiating muscle cells when the culture medium is depleted for serum. To investigate the regulation of myogenin gene expression, we have isolated and characterized the Myf4 gene which encodes the human homologue of murine myogenin. Serum components, basic FGF (b-FGF), transforming growth factor beta (TGF-beta), and EGF, agents which suppress differentiation of muscle cells in vitro, down-regulate the activity of the Myf4 gene, suggesting that it constitutes a nuclear target for the negative control exerted by these factors. The 5' upstream region containing the Myf4 promoter confers activity to a CAT reporter plasmid in C2C12 myotubes but not in fibroblasts and undifferentiated myoblasts. Unidirectional 5' deletions of the promoter sequence reveal that integral of 200 nucleotides upstream of the transcriptional start site are sufficient for cell type-specific expression. The forced expression of the muscle determining factors, MyoD1, Myf5, and Myf6 and to a lesser degree Myf4, results in the transactivation of the Myf4 promoter in C3H mouse 10T1/2 fibroblasts. Pathways potentially involved in conveying signals from the cell-surface receptors to the Myf4 gene were probed with pertussis- and cholera toxin, forskolin, and cAMP. Dibutyryl-cAMP and compounds that stimulate adenylate cyclase inhibit the endogenous Myf4 gene and the Myf4 promoter in CAT and LacZ reporter constructs. Conversely, pertussis toxin which modifies Gi protein stimulates Myf4 gene expression. In summary, our data provide evidence that the muscle-specific expression of the Myf4 gene is subject to negative control by serum components, growth factors and a cAMP-dependent intracellular mechanism. Positive control is exerted by a pertussis toxin-sensitive pathway that presumably involves G proteins.

Substrate-associated macromolecules promote cytodifferentiation of BC3H1 myogenic cells

Differentiated mouse BC3H1 myogenic cells secrete substrate-associated macro-molecules (SAM) which restrict the proliferation of undifferentiated cells and promote both cell shape changes and expression of predominantly the vascular smooth muscle (VSM)-specific isoform of the contractile protein alpha-actin. While we previously reported that high cell density was required for stimulating maximal expression of VSM alpha-actin in BC3H1 cells (Strauch and Reeser: Journal of Biological Chemistry 264:8345-8355, 1989), the permissive effect of SAM on myoblast cytodifferentiation was not at all dependent on the formation of cell to cell contacts. This observation suggests that biogenesis of an extracellular matrix rather than the formation of physical contacts between cells may be the rate-limiting step for induction of VSM alpha-actin expression at high cell density. The biologically active moieties in SAM that promote cytodifferentiation also are expressed by mouse embryonic fibroblast cell lines and are distinctly different from a class of adheron-like macromolecules released by differentiated BC3H1 myocytes directly into the culture medium. While SAM was cell growth restrictive, reconstituted particulate material (RPM) prepared from myocyte-conditioned medium promoted the adhesion and proliferation of growth-arrested myoblasts. SAM and RPM are composed of different polypeptide subunits which collectively may establish microenvironmental conditions that are permissive for BC3H1 myogenic cell differentiation.

Insulin, epidermal growth factor and fibroblast growth factor elicit distinct patterns of protein tyrosine phosphorylation in BC3H1 cells

The polypeptides which are phosphorylated at tyrosine residues in the murine muscle-like cell line, BC3H1, in response to insulin, epidermal growth factor (EGF) and fibroblast growth factor (FGF) were detected by immunoblotting with antiphosphotyrosine antibodies. Each ligand elicited the tyrosine phosphorylation of a characteristic, largely nonoverlapping set of polypeptide substrates, as classified by subunit Mr, pI, behavior on subcellular fractionation and adsorption to lectin (what germ agglutinin-Sepharose) columns. The dose-response curves for all stimulated tyrosine phosphorylations elicited by a single ligand were superimposable. By contrast, the temporal pattern of the responses elicited by each ligand differed in regard to speed of onset and persistence of the stimulation. Phosphorylation in response to insulin was maximal in a virtually instantaneous fashion and was fully maintained for at least 30 min. The response to EGF increased steadily over the initial 15-60 s to peak values, and fell progressively thereafter. FGF-stimulated phosphorylation was not detectable until 4 min after FGF addition, abruptly rose to maximal within the next 30 s, and declined subsequently. Exposure of BC3H1 cells to active phorbol esters prior to hormone addition altered the response to hormones in a differential fashion. FGF responses were abolished, EGF responses were partially inhibited, whereas the response to insulin was unaffected. Thus, acting on a single cell, insulin, EGF and FGF each mediate the tyrosine phosphorylation of a characteristic, largely nonoverlapping array of polypeptide substrates, indicating that each of these receptor tyrosine kinases exhibits a fundamentally distinct substrate specificity. Differences in the kinetic and regulatory properties of the response to each ligand are also apparent, and reflect the differing regulatory properties of each receptor tyrosine kinase acting in situ.

Aberrant regulation of MyoD1 contributes to the partially defective myogenic phenotype of BC3H1 cells

Two skeletal muscle-specific regulatory factors, myogenin and MyoD1, share extensive homology within a myc similarity region and have each been shown to activate the morphologic and molecular events associated with myogenesis after transfection into nonmyogenic cells. The BC3H1 muscle cell line expresses myogenin and other muscle-specific genes, but does not express MyoD1 during differentiation. BC3H1 cells also do not upregulate alpha-cardiac actin or fast myosin light chain, nor do they form multinucleate myotubes during differentiation. In this study, we examined the basis for the lack of MyoD1 expression in BC3H1 cells and investigated whether their failure to express MyoD1 is responsible for their defects in differentiation. We report that expression of an exogenous MyoD1 cDNA in BC3H1 cells was sufficient to elevate the expression of alpha-cardiac actin and fast myosin light chain, and to convert these cells to a phenotype that forms multinucleate myotubes during differentiation. Whereas myogenin and MyoD1 positively regulated their own expression in transfected 10T1/2 cells, they could not, either alone or in combination, activate MyoD1 expression in BC3H1 cells. Exposure of BC3H1 cells to 5-azacytidine also failed to activate MyoD1 expression or to rescue the cell's ability to fuse. These results suggest that BC3H1 cells may possess a defect that prevents activation of the MyoD1 gene by MyoD1 or myogenin. That an exogenous MyoD1 gene could rescue those aspects of the differentiation program that are defective in BC3H1 cells also suggests that the actions of MyoD1 and myogenin are not entirely redundant and that MyoD1 may be required for activation of the complete repertoire of events associated with myogenesis.

Differential effects of fibroblast growth factor and tumor promoters on the initiation and maintenance of adipocyte differentiation

Fibroblast growth factor (FGF) has been shown to inhibit the differentiation of myogenic and adipogenic cell lines without inducing a proliferative response. We have previously shown that agents capable of activating protein kinase C (PKC), such as FGF and the phorbol ester tetradecanoyl phorbol-13-acetate (TPA), inhibit the differentiation of the adipogenic cell line TA1, as measured by the rapid loss of adipocyte-specific RNAs. We report here that the treatment of fully differentiated TA1 adipocytes with FGF at 10 ng/ml induces the reversal of adipocyte differentiation, even in cells where PKC activity has been down-regulated by TPA pretreatment. In contrast, TPA or lower concentrations of FGF (1 ng/ml), both effective inducers of c-fos RNA in adipocytes, fail to reverse adipocyte differentiation. The adipocytes, however, will extinguish differentiation-specific functions in response to TPA by the addition of a calcium ionophore. Therefore, we propose that there are two FGF-sensitive pathways in TA1 cells: one responsible for the initiation of differentiation (TPA sensitive) and another required for maintenance of the adipose phenotype (TPA insensitive). These results suggest that activation of two distinct signaling pathways--one PKC and calcium dependent, the other FGF activated but PKC independent--are capable of inhibiting the biochemical events responsible for the maintenance of adipocyte differentiation.

The expression of sarcomeric muscle-specific contractile protein genes in BC3H1 cells: BC3H1 cells resemble skeletal myoblasts that are defective for commitment to terminal differentiation

The BC3H1 cell line has been used widely as a model for studying regulation of muscle-related proteins, such as the acetylcholine receptor, myokinase, creatine kinase, and actin. These cells, derived from a nitrosourea-induced mouse brain neoplasm, have some of the morphological characteristics of smooth muscle and have been shown to express the vascular smooth muscle isoform of alpha-actin. To provide further information about the contractile protein phenotype of BC3H1 and to gain additional insights into the possible tissue of origin of these cells, we have examined the expression of a battery of contractile protein genes. During rapid growth, subconfluent BC3H1 cells express the nonmuscle isoform of alpha-tropomyosin (alpha-Tm) and the nonsarcomeric isoforms of myosin heavy and light chains (MHCs and MLCs, respectively), but do not express troponin T(TnT). However, when BC3H1 cells differentiate in response to incubation in serum-deprived medium or upon approaching confluence, they express TnT as well as sarcomeric muscle isoforms of MHC, MLC 2 and 3, alpha-Tm, and alpha-actin. These results suggest that BC3H1 is a skeletal muscle cell line of ectodermal origin that is defective for commitment to terminal differentiation.

Effects of growth factors on myogenic differentiation

It has now been well established that the terminal differentiation of muscle cells in culture is subject to control by hormones and growth factors in the incubation medium. Thus far the most potent and most extensively studied agents are fibroblast growth factor (FGF), the insulinlike growth factors (IGFs), and transforming growth factor-beta (TGF-beta). Independent reports from several laboratories have established that both FGF and TGF-beta are potent inhibitors of differentiation and both appear to act at early stages of commitment to differentiation. Stimulation of differentiation by the IGFs (and by insulin at concentrations in the microgram/ml range) has also been observed and confirmed repeatedly. FGF and IGF are mitogenic for muscle cells, and TGF-beta either has no effect or suppresses cell proliferation, so previous generalizations that mitogens inhibit myogenic differentiation are clearly not valid when results with purified agents in well-defined media are considered. Work with oncogenes and specific toxins is beginning to reveal the mechanisms by which these agents might affect differentiation, and there is reason for optimism that an understanding of the molecular events that control terminal differentiation may be attained in the near future.

Control of differentiation in BC3H1 muscle cells

BC3H1 is a cell line that undergoes a musclelike pattern of differentiation under the appropriate conditions. We have examined the control of the synthesis of proteins characteristic of differentiated muscle in these cells as a function of their position in the cell cycle. We define two positions in the cell cycle where BC3H1 cells can remain stably quiescent. G1d is a restriction point early in the G1 portion of the cell cycle that permits the synthesis of muscle-specific proteins and is probably identical to G0. The second restriction point, G1q, occurs approximately 4 hr later in the G1 portion of the cell cycle and does not permit the synthesis of muscle-specific proteins. Movement of the cells from G1d to G1q occurs when fibroblast growth factor is added to the cells and is reversed when this growth factor is removed. Repression of the synthesis of muscle-specific proteins occurs when fibroblast growth factor is added to cells in G1d. In the case of the muscle form of creatine phosphokinase (M-CPK), the decline in the rate synthesis of this protein is a consequence of a decreased level of its mRNA. By contrast, the repression of alpha-actin synthesis, a protein synthesized only in differentiated cells, appears to be controlled at the translational level. The effect of fibroblast growth factor and other mitogens in these cells require activation of tyrosine kinase(s), but the intracellular targets of these kinases are not known. Studies by others suggest that activation of the ras oncogene can mimic the action of mitogenic polypeptides on these and other muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)

A human airway smooth muscle cell line that retains physiological responsiveness

We report the development of a nontransformed line of human airway smooth muscle cells retaining smooth muscle-specific contractile protein expression and physiological responsiveness to agonists implicated in inflammatory airway diseases. Specific responses to histamine, leukotrienes, bradykinin, platelet-activating factor, substance P, and thromboxane analogues are demonstrated as well as functional coupling to beta-adrenergic receptors. The cell line was characterized using indirect immunofluorescence, as well as electrophoretic separation and immunoblot analysis of smooth muscle-specific actin. Functional responses were assessed by measurements of cytosolic calcium and stimulation of adenosine 3',5'-cyclic monophosphate production. The cells retain their responsiveness over many population doublings and should be a useful model to examine specific receptor-effector mechanisms, as well as the effects of neurohumoral agents on the regulation of airway smooth muscle growth and differentiation.

FGF and EGF act synergistically to induce proliferation in BC3H1 myoblasts

BC3H1 muscle cells proliferate when grown in high concentrations of FBS (20%). Lowering the FBS concentration to 0.5% causes the cells to stop proliferating and is permissive for the morphological and biochemical differentiation of BC3H1 cells. Exposure of differentiated BC3H1 myocytes to high concentrations of serum or to the purified growth factors FGF or TGF-b induced a shutdown of this differentiation program but did not induce cell proliferation (Olson et al., J. Cell Biol., 103:1799-1805, 1986; Lathrop et al., J. Cell Biol., 100:1540-1547, 1985, and J. Cell Biol., 101:2194-2198, 1985). We explored the possibility that BC3H1 cells require factors to act synergistically to induce proliferation. We found that EGF and FGF function in a synergistic fashion to stimulate BC3H1 proliferation. Moreover, the temporal requirement for these growth factors suggest that they are functioning as competence and progression factors for BC3H1 cell proliferation.

Alpha-smooth muscle actin is expressed in a subset of bone marrow stromal cells in normal and pathological conditions

A series of 217 trephine bone marrow biopsies from adult patients and specimens from 16 fetuses and 5 infants were examined for the presence of stromal myoid cells (MCs) using a monoclonal antibody recognizing alpha-smooth muscle actin. In the normal adult bone marrow, stromal cells did not contain alpha-smooth muscle actin, whereas during fetal life, many alpha-smooth muscle actin-containing MCs were connected with vascular sinusoids in the primitive bone marrow. This cell type reappeared in various characteristic distribution patterns in adult bone marrow during different neoplastic and non-neoplastic conditions including metastatic carcinoma, Hodgkin's disease, multiple myeloma, hairy cell leukemia, acute myeloid leukemia (FAB M4, 5, 7) and chronic myelo-proliferative diseases. In general, the appearance of MCs was associated with a slight to pronounced increase in the deposition of reticulin and collagen fibers. We propose that bone marrow MCs represent a distinct subpopulation of fiber-associated or adventitial reticular cells undergoing cytoskeletal remodeling in response to various stimuli.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. II. Two signaling pathways distinguished by pertussis toxin and a potential role for the ras oncogene

In the preceding report (Kelvin, D.J., G. Simard, H.H. Tai, T.P. Yamaguchi, and J.A. Connolly. 1989. J. Cell Biol. 108:159-167) we demonstrated that pertussis toxin (PT) blocked proliferation and induced differentiation in BC3H1 muscle cells. In the present study, we have used PT to examine specific growth factor signaling pathways that may regulate these processes. Inhibition of [3H]thymidine by PT in 20% FBS was reversed in a dose-dependent fashion by purified fibroblast growth factor (FGF). In 0.5% FBS, the normally induced increase in creatine kinase (CK) activity was blocked by FGF in both the presence and absence of PT. Similar results were obtained with purified epidermal growth factor (EGF). We subsequently examined the effect of a family of growth factors linked to inositol lipid hydrolysis and found that thrombin, like FGF, would increase [3H]thymidine incorporation and block CK synthesis. However, PT blocked thymidine incorporation induced by thrombin, and blocked the inhibition of CK turn-on in 0.5% FBS by thrombin. The ras oncogene, a G protein homologue, has previously been shown to block muscle cell differentiation in C2 muscle cells (Olson, E.N., G. Spizz, and M.A. Tainsky. 1987. Mol. Cell. Biol. 7:2104-2111); we have characterized a BC3H1 cell line, BCT31, which we transfected with the val12 oncogenic Harvey ras gene. This cell line did not express CK in response to serum deprivation. Whereas [3H]thymidine incorporation was inhibited by 70-80% by increasing doses of PT in control cells, BCT31 cells were only inhibited by 15-20%. ADP ribosylation studies indicate this PT-insensitivity is not because of the lack of a PT substrate in this cell line. Furthermore, PT could not induce CK expression in BCT31 cells as it did in parental cells. We conclude that there are at least two distinct growth factor pathways that play a key role in regulating proliferation and differentiation in BC3H1 muscle cells, one of which is PT sensitive, and postulate that a G protein is involved in transducing signals from the thrombin receptor. We believe that ras functions in the transduction of growth factor signals in the nonPT-sensitive pathway or downstream from the PT substrate in the second pathway.

Nonuniform behavior of multiple isoactins in the same cell is a cell-dependent phenomenon

The functional significance of multiple isoactins in the same cell is still not understood. To address this question, we examined the response of smooth muscle and cardiac muscle alpha-isoactins to a serial extraction procedure applied to both muscle and nonmuscle cell types. We compared these extraction results with results obtained with the beta- and gamma-nonmuscle actin isoforms from the same cells. In differentiated BC3H1 nonfusing muscle cells (smooth muscle alpha-isoactin), in human rhabdomyosarcoma cells (cardiac alpha-isoactin), and in chick skeletal muscle cells (cardiac alpha-isoactin), different fractions were found selectively enriched in either the nonmuscle or the muscle-specific actin isoforms compared with their relative abundance in whole cell extracts. Conversely, when these same isoactins were examined either in undifferentiated BC3H1 cells or in mouse nonmuscle cells stably transfected with a cardiac alpha-isoactin gene, no enrichment of these isoforms above their relative abundance in whole cell extracts was observed. These results indicate that within the muscle or muscle-like cells examined, the different actin isoforms were either selectively utilized or localized. These results further show that isoactin-specific responses observed were apparently related to the cell type in which they were found and not to differences in inherent physical properties such as solubility of the different isoactins examined.

Growth factors, signaling pathways, and the regulation of proliferation and differentiation in BC3H1 muscle cells. I. A pertussis toxin-sensitive pathway is involved

Cells of the nonfusing muscle cell line BC3H1 stop proliferating and express a family of muscle-specific proteins when the FBS concentration is reduced from 20 to 0.5% (Munson, R., K.L. Caldwell, and L. Glaser. 1982. J. Cell Biol. 92:350-356). Several growth factors have been shown to block differentiation in this cell line. To begin to investigate the potential role of G proteins in signal transducing pathways from these receptors, we have examined the effects of cholera toxin (CT) and pertussis toxin (PT) on proliferation and differentiation in BC3H1 cells. PT specifically ADP ribosylates a protein with an apparent molecular mass of 40 kD in BC3H1 cell membranes, whereas CT specifically ADP ribosylates three proteins of 35-43 kD. When added to exponentially growing cells in 20% FBS, CT and PT inhibited [3H]thymidine incorporation by up to 75% in a dose-dependent fashion. We found the synthesis of creatine kinase (CK) and skeletal muscle myosin light chain was reversibly induced in cells in 20% FBS treated with PT, but no increased synthesis was seen in cells treated with CT or in control cells; Northern analysis indicated this induction was at the level of mRNA. In cells shifted to 0.5% FBS, CT inhibited the normally induced synthesis of CK whereas PT potentiated it by approximately 50%. Forskolin also inhibited growth in 20% FBS and differentiation in 0.5% FBS medium in a dose-dependent fashion. both forskolin and CT elevated cAMP levels compared with control or PT-treated cells, suggesting that CT is blocking proliferation and differentiation by elevating cAMP levels. These results establish that a PT-sensitive pathway is involved in regulating proliferation and differentiation in BC3H1 cells, and we postulate that PT functions by ADP ribosylating a G protein that transduces signals from growth factor receptors in these cells.

Growth factor control of myoepithelial-cell differentiation in cultures of human mammary gland

The relationship between growth and cytodifferentiation was studied in cultured human mammary myoepithelial cells under serum-free culture conditions. Myoepithelial-cell differentiation was monitored by quantifying cells showing immunoreactivity to the muscle isoform of actin; to the membrane glycoprotein common acute lymphoblastic leukemia antigen (CALLA); and to type IV collagen. Growth was quantified either by measuring the actual increase in cell number, or in a more-sensitive assay using immunoreactivity to the cell-proliferation-associated nuclear antigen Ki-67 as a measurement of the number of cells leaving the G0-phase of the cell cycle. The results showed that: (a) Primary cultures of myoepithelial cells on DME-F12 supplemented with cholera toxin (CT) alone resulted in the formation of quiescent cell islets (in the G0-phase of the cell cycle) showing phenotypic traits preserved from the in vivo situation (actin- and CALLA-positive cells with little or no type-IV-collagen immunoreactivity). (b) After addition of epidermal growth factor (EGF), with an ED50 of 1-10 ng/ml, in the presence of CT, the cells entered the G1-phase of the cell cycle, without further increase in cell number. At the same ED50 of EGF, the frequency of CALLA-positive cells decreased, while the number of cells immunoreactive for type IV collagen increased with a maximal effect of EGF seen after 7-11 days. During the same period, the cells remained fully differentiated with respect to actin immunoreactivity. (c) Further addition of insulin (I) to the medium in the presence of EGF and CT resulted in the cells entering an exponential growth phase associated with simultaneous decrease in actin immunoreactivity with a maximal effect of I after 11 days of exposure. The dose-response curve to I was virtually identical for stimulating cell proliferation and for reducing the frequency of actin-immunoreactive cells (ED50 in the range of 30 ng/ml), suggesting that the two processes were controlled by the same initial I-receptor interaction. (d) Some reduction in the number of actin-positive cells was exerted by I-EGF-CT independently of the mitogenic response, but this reduction was further augmented if the cells were allowed to proliferate. (e) Time-course studies of quiescent (G0-phase) cells stimulated to exponential growth revealed that entrance of cells into the G1-phase of the cell cycle preceded the loss of muscle actin filaments. (f) Exponentially growing actin-negative epithelial cells did not resume a myoepithelial phenotype in density-arrested postconfluent cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial smooth muscle cells in vivo: relationship between actin isoform expression and mitogenesis and their modulation by heparin

Quiescent smooth muscle cells (SMC) in normal artery express a pattern of actin isoforms with alpha-smooth muscle (alpha SM) predominance that switches to beta predominance when the cells are proliferating. We have examined the relationship between the change in actin isoforms and entry of SMC into the growth cycle in an in vivo model of SMC proliferation (balloon injured rat carotid artery). alpha SM actin mRNA declined and cytoplasmic (beta + gamma) actin mRNAs increased in early G0/G1 (between 1 and 8 h after injury). In vivo synthesis and in vitro translation experiments demonstrated that functional alpha SM mRNA is decreased 24 h after injury and is proportional to the amount of mRNA present. At 36 h after injury, SMC prepared by enzymatic digestion were sorted into G0/G1 and S/G2 populations; only the SMC committed to proliferate (S/G2 fraction) showed a relative slight decrease in alpha SM actin and, more importantly, a large decrease in alpha SM actin mRNA. A switch from alpha SM predominance to beta predominance was present in the whole SMC population 5 d after injury. To determine if the change in actin isoforms was associated with proliferation, we inhibited SMC proliferation by approximately 80% with heparin, which has previously been shown to block SMC in late G0/G1 and to reduce the growth fraction. The switch in actin mRNAs and synthesis at 24 h was not prevented; however, alpha SM mRNA and protein were reinduced at 5 d in the heparin-treated animals compared to saline-treated controls. These results suggest that in vivo the synthesis of actin isoforms in arterial SMC depends on the mRNA levels and changes after injury in early G0/G1 whether or not the cells subsequently proliferate. The early changes in actin isoforms are not prevented by heparin, but they are eventually reversed if the SMC are kept in the resting state by the heparin treatment.